Reconstruction of topography and related depositional systems during active thrusting

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B10, PAGES 20,281-20,297, OCTOBER 10, 1994 Recnstructin f tpgraphy and related depsitinal systems during active thrusting Duglas W. Burbank Department f Gelgical Sciences, University f Suthern Califmia, Ls Angeles Jaume Verg6s Departament de Gelgia Dinitmica, Gefisica, i Palentlgia, University f Barcelna, Barcelna, Spain Abstract. Reftable recnstructin f frmer tpgraphy in defrmed regins is cmmnly difficult, due t degradatin f frmer ersinal and depsitinal surfaces. In cntrast mst mdem landscapes, hwever, ancient lcalities can smetimes prvide clearer insights n subsurface gemetries f depsitin, defrmatin, and ersin and n theft variatins thrugh time. In sme exceptinal circumstances, ancient depsitinal sequences are preserved in direct juxtapsitin with the structures that cntrlled their gemetrical and sedimentlgical character. We describe here the evlving tpgraphy and depsitinal respnses caused by the late Ecene grwth f a detachment fld and related thrusts in the suthern Pyrenees. Tpgraphy within these defrming systems can be recnstructed the basis f (1) relief assciated with palevalleys, (2) gemetric relatinships f syntectnic strata with adjacent structures, and (3) relief f hanging walls abve depsitinal r ersinal surfaces f the same age. Onlapping, ffiapping, and verlapping stratigraphic relatinships are interpreted in the cntext f the relative rate f sediment accumulatin versus the rate f uplift f the crest f the fld. In the study area, tw cntrasting fluvial systems prvided sediment the defrming area: a large lngitudinal system, flwing parallel t the fld axes and carrying detritus frm the distant hinterland, and a smaller transverse system that carried lcally derived clasts. During fld grwth, syntectnic sedimentary beds (grwth strata) were prgressively rtated in the frelimb f the fld. Prximal uncnfrmities develped in the frelimb grwth strata, when accumulatin rates were lw. Tpgraphic relief n the backlimb f the grwing fld caused transverse palevalleys (> 150 rn deep) t be incised at high angles t the fld axis. A switch frm incisin t infilling f the palevalleys appears t be cntrlled by relative rates f subsidence, sediment supply and accumulatin, and uplift. During an interval f rapid accumulatin and lw rates f subsidence and uplift, the effects f rising lcal base levels prpagated up the transverse valleys, where they initiated backfilling f the palevalleys. As defrmatin began n an adjacent, mre hinterlandward thrust, waning grwth f the detachment fld permitted depsitinal verlap f its crest, as sedimentatin shifted tward the hinterland. Subsequently, as the new ftwall was flded, lngitudinal rivers filled the space frmerly ccupied by transverse rivers, and a new detachment fld grew in the very shallw (<25 m) subsurface. Althugh similar examples are scarce in the gelgical recrd, the synthesis frm this Pyrenean lcale illustrates hw stratal gemetries, recnstructed fiver patterns, precise stratigraphic ages, and preserved ersinal surfaces can be cmbined t recnstruct evlving tpgraphy during active flding and faulting in terrestrial envirnments. Intrductin It is ften straightfrward t examine presently active tectnic envirnments and t understand the cause-and-effect relatinships amng tectnic activity, evlving tpgraphy, and sedimentatin. Typically it' is much mre difficult t make reliable recnstructins f these interactins in the past. In rder t answer questins cncerning (1) the tpgraphy f the land surface at the time f defrmatin, (2) the curses f rivers with respect t structures, (3) the relative imprtance f lcal streams versus reginally extensive rivers, (4) the rle f nging defrmatin in cntrlling depsitin, and (5) the rates at which uplift, denudatin, and depsitin ccurred, it is necessary t examine Cpyright 1994 by the American Gephysical Unin. Paper number 94JB /94/94JB-(X) the gemetry, psitin, and age f syntectnic depsits, structures, and surfaces. Unfrtunately, ersin frequently bliterates thse strata that were depsited n the flanks f grwing structures, and thus key elements needed fr a tpgraphic recnstructin are lst. In terrestrial strata, mrever, it is cmmnly difficult t determine precise ages fr defrmatin and depsitin, s that recnstructins f rates f past prcesses are ften prly knwn. Reliable recnstructins f sequential changes in tpgraphy in actively defrming envirnments require the fllwing types f data: well-expsed surficial r near-surface structures that develped during depsitin; syntectnic depsits that are directly juxtapsed with thse structures; clearly preserved ersinal and depsitinal surfaces within the syntectnic depsits that define large-scale gemetries f depsitinal units with respect t nearby structures and serve t delineate a sequence f defrmatinal events; extensive tw- and three-dimensinal expsures 20,281

2 20,282 BURBANK AND VERG] S: RECONSTRUCTING TOPOGRAPHY with gd lateral cntinuity t define relief alng the frmer land surface; sedimentlgical data n prvenance and paleflw directins; reliable spatial reference frames within which t define cmparative magnitudes and rates f prcesses; and precise time cntrl n syntectnic depsitin. Few areas display all f these characteristics. In the suthern Pyrenees f nrthern Spain, hwever, there are severalcalities characterized by shallw levels f ersin, excellent expsures, and preexisting magnetstratigraphic chrnlgies f terrestrial depsitin [Burbank et at., 1992]. Here the Palegene depsitinal systems, ersinal surfaces, and related structures can be recnstructed in cnsiderable detail. In this paper, we describe an example frm the eastern blique margin f the Suth-Central Unit (Figure 1) in the vicinity f Oliana [Verg6s and Mu z, 1990]. We fcus n an asymmetric, suth-vergent detachment fld which has a steeply dipping t verturned, suthern limb (herein termed the frelimb) and a less steep limb n the hinterland side f the fld crest (the backlimb). Grwth f the fld cntrlled the river patterns, the cutting and filling f palevalleys, the lcal tpgraphic relief, and the episdic shifting f the depcenter. The detachment fld grew synchrnusly with a much larger fld t the suth (the Oliana anticline) and was fllwed by renewed thrusting alng the Mntsec thrust, which is clser t the hinterland. New magnetic chrnlgies allw us t define rates f aggradatin and related fld grwth, whereas detailed mapping and sedimentlgic studies prvide insights n the interactins between tectnics, sedimentatin, and tpgraphy adjacent the fld. In this study, we use nearly hrizntal, riginal depsitinal surfaces as reference frames in rder t track the prgressive grwth f flds, the rtatin f beds, the depths f incisin, and changes in the relative rates f subsidence. Prvenance studies, palecurrent rientatins, and sedimentlgical characteristics permit us t recnstruct the depsitinal systems and understand their interactins with the defrming flds and thrusts. Our interpretatins yield a fur-stage histry fr this area (Figure 2). In respnse t grwth f a detachment fld, strata fed by rivers flwing parallel t the fld axis accumulate in the fld's frelimb, taper tward the fld, and are cntinuusly rtated by flding (unit 2, Figure 2). At the same time, valleys transverse t the fld axis and up 150 m deep are incised thrugh the erding backlimb f the fld and adjacent hanging wall strata. Because the transverse valleys are graded t the frelimb, when rates f sediment accumulatin markedly increase in the frelimb, the valleys being t fill with lcally derived cnglmerates. As fld grwth stps, defrmatin shifts t a mre hinterlandward thrust, rivers flwing parallel t the thrust trace verlap the lder fld axis (unit 3, Figure 2). Subsequently, mtin n the thrust causes flding f its ftwall, and the depcenter shifts tward a mre distal psitin (unit 4, Figure 2). Tpgraphic relief at each stage,qqu.q O '-...,vet:, Km L t Andrra.',',', Pyrenees 1611 Cc. i Study Area / O Peramla ' gypsum- l. -- I N I Peramla O ' ß Ollana i ø Cnglmeratic units -- Cardna eyapriles Igualada marine marls Mntsec thrust sheet..: :! Sierras Marginales thrust sheets 2 km!, I Figure 1. Simplified gelgic map shwing majr thrusts and stratigraphic units within the Oliana area. The lcatins f the master magnetstratigraphic sectin [frm Burbank et at., 1992] and the studied area (F?gure 3) are shwn. Inset shws a generalized map f the Pyrenees. The study area is lcated astride the eastern blique margin f the Suth Central Unit (SCU).

3 BURBANK AND VERGES: RECONSTRUCTING TOPOGRAPHY 20,283 SE NW Befre 36.6 Ma (36.4 Ma) Sectin A-A' Unit-2 ( cm/kyr),,,,,.. Ecene ':: ii!iiii::... "' "" '" :...' palecene ":' '"'" Oliana... ii;;111!i:::i::;;!!i::iiii!iliiiii!iii-..:'" ' ::""":':'" "":"'"':!i "'"'"' "': Upper anticline,' ii?i Cretace us A, 1250 rn...!::!?:i :i!!;:i!!i!i!iiii!ii'"":'"..,, "'"' iiiiii, " Jurassic Sectin B-B' Unit-2 ( Ma = 6 cm/kyr) axial derived fine sediments lcally derived clasts (_majr palevalley).....,=...m..??f...,e,...:: :::::::::::::::::::::::::::::::: ========================================================!:!:!:i:i:!:!:i:i:i:!:i:i:!:i:!:!:!:!:!:!:!:i:!:!:!:!: '::::::::::::::::::::::::....:.:::;:::: ::::::::::::::::::::::::: ;;:::;:: Sectin A-A' Ma After 35 Ma [ 27% shrtening I Figure 2. Sequential recnstructin f defrmatinal and depsitinal patterns f units 2 t 4 in the hanging wall f the Serres Marginals thrust sheet. Recnstructins are based magnetstratigraphic dates and n grwth strata, bed lengths, and gemetries f preserved ersinal and depsitinal surfaces. (a) Unit 2 is depsited by a lngitudinal river in a develpingrwth syncline. (b) Unit 2 is depsited in the grwth syncline, and the crest f the detachment fld is breached by a transverse palevalley > 150 m deep. (c) Transverse valley begins t fill with lcally derived clasts as the rate f uplift f the detachment fld decreases and accumulatin in the grwth syncline accelerates. (d) Unit 3 verlaps the detachment fld after the palevalley is filled. The Mntsec thrust is reactivated and a megaclast is incrprated the ftwall syncline. (e) Depcenter shifts farther tward the freland as the ftwall cntinues t be uplifted. Fr lcatins f crss sectins A-A' and B-B', see Figure 3.

4 ß... ß. ß.. 20,284 BURBANK AND VERGI S' RECONSTRUCTING TOPOGRAPHY can be defined by the height differences between the depsitinal surfaces and preserved ersinal tpgraphy nearby. Gelgic Setting The Suth Central Unit (SCU) [Sdguret, 1972] cmprises an imbricate system f emergent thrusts (frm nrth t suth: the Bixls, Mntsec, and Serres Marginals thrusts, Figure 1) which display an verall piggyback sequence f thrusting during latest Cretaceus t early Ecene times. Subsequent suthward transprt ver the freland depsits is recrded by middle Ecene t Oligcene syntectnic strata [Vergds and Burbank, 1994]. Ceval with this suthward transprt, lder hinterland thrusts were reactivated, in bth the central [Martinez-Pe a and Pcv[, 1988; Mu z, 1992; Puigdefdbregas et al., 1992] and eastern parts f the SCU (Figure 1) [Burbank et al., 1992; Verggs and Muaz, 1990]. The study area is lcated alng the eastern bundary f the SCU, where a set f blique thrusts merge nrthward int a single thrust: the Segre thrust. The Oliana anticline is a 15-kmlng, dubly plunging structure lcated in the ftwall f and blique t the Segre thrust. Frmed by stacking tw slices f Palegene strata, the anticline is an allchthnustructure that was transprted 10 km t the suth acrss a decllement lcalized within the upper Ecene Cardna salt [Vergds and Mu z, 1990; Vergds et al., 1991]. T the nrthwest f the Oliana anticline and ceval with its grwth, synrgenicnglmerates that verlap and are cut by thrusts demnstrate that the Meszic rcks f the SCU in the hanging wall f the Segre thrust are defrmed by break-back thrusts (i.e., a sequence f thrusting that becmes yunger tward the hinterland) and related flds. Details f the thrust gemetry, sequencing, age, and rates f shrtening are given by Vergds and Muhz [1990] and Burbank et al. [ 1992]. The present study describes the landscapevlutin assciated with grwth f a detachment fld within the Setres Marginals thrust sheets and later shrtening n the Mntsec thrust. Bth the fld and the hanging walls f these thrusts include Meszic and Palegene strata (Figures 3 and 4). Methdlgy The study area was mapped in detail n aerial phtgraphs and tpgraphic maps at scales f -1:16,000 and 1:25,000, respec- ::::::::::::::::::::::: unit-4 :.:. r i * 60 * 7BB' x,,,, 31' 35' /,Fig. 2BB'.e. 30:,, Fig. 7AA' unit-2 500m i... Fig. 2AA' & 4 aaa= e Magnetic sectin rn Cretaceus limest Megaclast... Ersinal base f Palevalley < === Trend f Majr Palevalley Figure 3. Gelgic map f the syntectnic strata (units 1-4) assciated with the Mntsec and Serres Marginals thrust sheets (shaded). Grwth synclines, as depicted by strike line mapping, ccur in the ftwall f the Mntsec thrust and in the frelimb f the detachment fld in the Setres Marginals hanging wall. The bases f the palevalleys and the breached crest f the detachment fld are shwn. The large arrw depicts the trend f the nw-erded palcvalley acrss the detachment fld. Lcatins f the magnetic sectin and varius crss sectins are shwn.

5 ... BURBANK AND VERG] S: RECONSTRUCTING TOPOGRAPHY 20,285 SE Oliana anticline Serres Marginals T i'' Mntsec detachment thrust I unit-3 thrusts fld... unit-1! un -4 u. nit-2 _[... NW iiil '"!"' '"'""'"'" ' Palecene Figure 4. Structural crss sectin between the backlimb f the Oliana anticline and the Mntsec thrust sheet. Majr structures within the Serres Marginals hanging wall, including the detachment fld, and the gemetries f the syntectnicnglmerates (units 1-4) define a break back (hinterlandward) sequence f thrusting. Differences in the amunt f flding f the cnglmerates and the underlying, lder strata result frm inheritance f previusly frmed structures. See Figure 3 fr lcatin f sectin. fively. Sectins were measured and described thrugh each f the majr depsitinal units, and data n facies, palecurrents, and prvenance were cllected. The present gemetry f frmer ersinal and depsitinal surfaces was recrded and later restred t a predefrmatinal psitin with respect bedding surfaces that were assumed t have been essentially hrizntal at the time f depsitin. Samples fr palemagnetic studies were cllected where fine grained strata were sufficiently abundant and sectins late Ecene interval f break-back thrusting. Previusly cmpleted magnetstratigraphic studies [Burbank et al., 1992], when cmpared with the new time scale f Cande and Kent [1992], indicate that break-back thrusting and grwth f the Oliana anticline ccurred during latest Ecene times between and 34.0 Ma. In this paper, we fcus n units 2 and 3 which were depsited and defrmed during the reactivatin f flds and thrusts in the Serres Marginals and Mntsec thrust sheets. were sufficiently lng. These samples were prcessed thrugh multiple levels f thermal demagnetizatin: a prcedure used Unit 2 successfully in previus studies t create magnetstratigraphies in crrelative strata n the flanks f the Oliana anticline (Figure 3). (Fr details f the magnetstratigraphic prcedures, see Burbank et al. [1992].) Interpretatin f the grwth strata is based in part n mdels that predict stratal gemetries as a functin f the relative rates f sediment accumulatin versus crestal uplift (Figure 5). Crestal uplift is defined as the structural uplift plus r minus any Unit 2 was depsited bth n the frelimb f the detachment fld and acrss the intervening terrain between the fld crest and the Mntsec thrust, including the backlimb f the detachment fld (Figures 2, 3, and 4). Expsed within the fld at present and in the ftwall t the Mntsec thrust are upper Cretaceus sandstne (Aren Frmatin), Palecene red beds and lacustrine limestnes (Garumnian Frmatin), and a thin sequence f lwer depsitin r ersin, respectively, ver the crestal area f a fld. Ecene nummulitic marls, limestnes, sandstnes. These Crestal uplift thus defines changes in psitin f the highest surface f the fld and is differentiated frm bedrck uplift [England and Mlnar, 1990]. The reference level fr measuring crestal uplift is either the psitin f crrelative flded strata in the adjacent syncline r the base f the syntectnic strata in the adjacent basin. Althugh here we use crestal uplift rates, ne culd als interpret nlap-fflap gemetries in the cntext f uplift rates f a site n the flanks f a fld. stratigraphic units vary significantly in their resistance t ersin, and these differences have played imprtant rles in cntrlling the tpgraphy during defrmatin. Tday, thin fins f lacustrine limestne, the mst resistant lithlgy, stand m abve the adjacent slpes and demarcate the margins f paletpgraphic high spts in the landscape. Thse strata which predate the late Ecene defrmatinal events are mre strngly flded than are the syndefrmatinal cnglmerates that mantle them. This enhanced flding indicates that at the initiatin f late Ecene depsitin, mderate Syntectnic Depsitin defrmatin had already ccurred during early and/r middle The upper Ecene t lwer Oligcene stratigraphic successin Ecene flding [Verggs, 1993]. [Burbank et al., 1992] is dminated by fur, primarily cnglmeratic sequences that are interleaved with varius thrusts and de- Unit 2 is preserved in three lcalities within the study area. The thickest accumulatin ccurs in a grwth syncline in the frmed by the Oliana anticline (Figures 3 and 4). Because the frelimb f the detachment fld situated between a Serres ldest cnglmerate (unit 1) is cut by the utermst Serres Marginals thrust, and the yungest cnglmerate (unit 4) is nly slightly defrmed by the yungest (Mntsec) thrust, these cn- Marginals thrust imbricate t the SE and the fld crest (Figures 3, 4, and 6). The ther ccurrences are within tw palcvalleys, 50 t 150 m deep, that were erded in the backlimb f the glmerates and their assciated finer grained strata bracket the detachment fld and in the Mntsec ftwall. All f these

6 20,286 BURBANK AND VERGI S: RECONSTRUCTING TOPOGRAPHY ^. GROWTH PRE-GROWTH STRATA STRATA OVERLAP FOLD CREST ii:. ':... : DETACHMENT OFFLAP!:i:: ::i:i::... :.::i:i: FOLD Eisbacher et al., 1974] parallel t the traces f the lcal thrusts and fld axes. At the base f the successin n the flded frelimb, prly bedded, unsrted, mnlithlgic breccias f angular clasts f Garumnian limestnes in jig-saw-like gemetries are present in places. In ther sites, crudely bedded, subangular t subrunded cnglmerates with large clasts f Garumnian limestnes as much as 1 m in diameter ccur. Age: The results frm 16 class I and II magnetic sites fllwing stepwise thermal demagnetizatin indicate that at least three B. UPLIFT < ACCUMULATION C. UPLIFT >ACCUMULA TION and prbably six magnetic reversals are encmpassed by unit 2 (Figure 8). In the cntext f the previusly determined OVERLAP, THEN OFFLAP OFFLAP, ONLAP, THEN OVERLAP magnetstratigraphic framewrk fr units 2 and 3 n the SE flank f the Oliana anticline [Burbank et al., 1992], this reversal pattern can be interpreted (Figure 8) t indicate that unit 2 was depsited between ~36.7 and 35.0 Ma [Cande and Kent, 1992]. These age limits yield mean cmpacted accumulatin rates f ~16 cm/kyr. Gemetrical relatinships: Whereas tabular bedding geme- UPLIFT < ACCUMULATION, UPLIFT > ACCUMULATION, tries are displayed thrughut much f its extent, beds within unit THEN THEN D. UPLIFT> ACCUMULATION E. UPLIFT < ACCUMULATION 2 clearly taper and are increasingly defrmed tward bth the SE and the NW (Figure 4). Althugh the depsitinal cntacts f unit 2 with the underlying strata t the SE are nt well expsed, Figure 5. (a) Nmenclature fr a detachment fld and grwth flding f the beds and clear thinning f unit 2 tward the SE strata depsiteduring flding. (b)-(e) Mdels f verlap, ffiap, indicate that the backlimb f the Oliana anticline was steepening and nlap. Predictable gemetries f synteetnie strata result during depsitin. Because the cnglmeratic beds prgressively frm cntrasts in the relative rates f erestal uplift versus ceval nlap the flanks f the grwing Oliana anticline, they indicate rates f accumulatin. Crestal uplift is measured with respect t that the relative rate f flank uplift was less than the average rate either (1) the base f the synteetnie strat adjacent t the fld r (~16 cm/kyr) f sediment accumulatin (Figure 5), althugh the (2) the psitin f crrelative marker beds fund in bth the crestal uplift rate f the anticline was likely t have been anticline and the adjacent syncline. When rates f accumulatin cnsiderably higher. are cnsistently greater than the rate f crestal uplift, verlap will ccur (Figure 5b), whereas lwer rates f accumulatin versus T the NW, unit 2 is juxtapsed against and incrprated uplift lead t ffiap (Figure 5c). Reversals in the relative the frelimb f the detachment fld (Figure 6). Clear nlapping f the cnglmerates with the pre-ecene strata is seen alng magnitude f these rates causes a switch in the beddingemetry much f the expsed cntact. During grwth f this anticlinal (Figure 5d). Onlap (Figure 5e) ccurs fllwing ffiap and a fld, frmerly subhrizntal depsitinal surfaces within unit 2 change t mre rapid accumulatin rates. were prgressively rtated, such that sme lder surfaces are presently verturned in the prximal parts f the grwth syncline. A clear hinge line separates (1) the central area f unit 2 ccurrences lie between the hanging wall f the Mntsec thrust depsitin in which stratal thicknesses are generafly cnstant and and the backlimb f the Oliana anticline which was als grwing beds are tabular frm (2) the regin adjacent the anticfine in during depsitin (Figures 1 and 4). which stratal thicknesses abruptly taper tward the anticline. Grwth syncline. Unit 2 is at least 250 m thick within the Because these tapered strata are lateral cntinuatins f adjacent axis f the grwth syncline (Figure 7), but tapers rapidly tward tabular strata, cmprise well runded clasts, are largely fluvial in the detachment fld and mre gradually tward the backlimb f rigin, and cntain basement clasts even in many sites clse t the Oliana anticline. the fld, they represent part f an axial fluvial system flwing Sedimentlgy: Sedimentatin here is dminated by belts f parallel t the fld axis and can be cnsidered t have had amalgamated channelized cnglmerate separated by thin (1-5 subhrizntal upper surfaces at the time f depsitin (with m), red siltstne and sandstne. Althugh mst cnglmeratic respec t the NW-SE crss sectin), rather than being slping beds are cm thick, individual beds can attain 5 m in transverse fan surfaces. thickness. The beds are bradly tabular but display small (1-4 m), Recnstructed histry f defrmatin and depsitin: Our inset channels. The cnglmerates are clast supprted, cntain interpretatin f the magnetic data implies that sediment-accumumany pebbly lenses, and uncmmnly display planar crss latin rates varied strngly during depsitin f unit 2. In cmbedding. The clasts are well runded and are heterlithic. parisn t relative uplift rates n the flank f the fld, high rates Althugh dminated by clasts f Meszi carbnates that culd f aggradatin (~25 cm/kyr) during the initial phases f depsibe lcally derived, unit 2 within the grwth syncline als cntains tin prmted nlap f the detachment frelimb. The prgresbasement clasts derived frm the Pyrenean "Axial Zne" >30 km sively rtated bedding surfaces f the upper 65% f unit 2 and the t the nrth. The cnglmerates appear t be f whlly lcal gemetry f their cntacts define the grwth f the detachment rigin nly in lcalities within a few meters f the grwing fld and related ersin and depsitin. They demnstrate that detachment fld. Palccurrent directins (N = 113) based n during a 1-m.y.-lng interval, the fld's frelimb prgressively clast imbricatins, crss bedding, and numerus elngate "gut- steepened and verturned in respnse t cntinued shrtening. As ters" n the base f many cnglmerate units (Figure 8) indicate the anticfine grew, its crestal area and thse pregrwth strata n that flw, acrss much f its extent, was t the SW, i.e., a its backlimb (i.e., all strata that were nt manfled by syntectnic lngitudinal r axial flw directin [Burbank and Beck, 1991; sediments) were subjected t ersin.

7 BURBANK AND VERGES: RECONSTRUCTING TOPOGRAPHY 20,287?... :... : ::C-r-e'Sf:i:::::: 't-:- :'"" i:i '... ii:.i..d.ei:af.eh.ment-f-.!d 'ih. ' ':':i i?" ; ß.....:':;:-Ga'.:mm'ni-ani:.,'m.e's:ne... i ::.: :... '-'- : ": :: ";'":;;... ::: :: ::::: ' ::;':"'" :;... :;:: : ;: ':7' :' :: ::.::A :.:. :.. '::: ß ::' : :' ' ": :... ::U.nit:-- -:-.:-. :'"' ::' ::3':'.:.< :: : :' ::: :' :":':".,... - ::.: t.-. - :: :. '" :.: : :...;.:.:.: :: : : :.':.::. - -': ::::::::::::::::::::::::::::::: :,...-- : ":t' ;: ' ' ''.. ; '::::: -..'.:.:..:" "?:... ; : ":?:'.'- '"' - - t'..:...:::.: :..:::.::...;;...: : - :: ?:......:..... :J. : : :<. :.... ' '""' ' :: : ,. : :. _ : :> ':: ' ': :......:.. ;.. :' '".7 ' ".:L ß.,:.....,....;:. :.:..:.... ;.-- : ;.:...? :.?. :... :.: Figure 6. Overview f the upper i00 m f unit 2 in the grwth syncline and the verlying cnglmerates f units 3 and 4. Prgressive tapering and steepening f each stratigraphic packet in unit 2 delineates the grwth syncline adjacent the flded frelimb f Garumnian limestnes (visible n the right skyline). Alng the length f the detachment fld fr a strike distance f psitin f these valleys is strngly crrelated with lcal 1-2 km, variable amunts f ersin have resulted in cntrasting structure, whereby high pints n the frmer landscape cntact relatinships with unit 2. In regins where the Garumnian crrespnd with resistant pregrwth strata, and palevalleys frm limestnes were nt successfully breached, the ldest syntectnic in weaker beds and alng hanging wall synclines. Minimum strata rest upn these limestnes in the frelimb f the fld and have been verturned alng with this limb (Figures 6 and 7,. prfile A-A'). In areas where ersin incised thrugh the fld crest (Figure 7, prfile B-B'), an angular uncnfrmity ccurs within unit 2. The cnglmerates abve the uncnfrmity are nt depths f the palevalleys can be measured directly frm the presently preserved fill within the valleys and by the gemetry f the valley margins. The relative tpgraphic psitin f each palevalley and stratigraphic psitins with their fills can be estimated thrugh cmparisn t a cmmn reference level: in verturned, but they display a very prnunced angular discrdance with the pregrwth strata (Figure 7, prfile B-B'). We interprethe uncnfrmity surface t represent a cntinuatin SE NW f the ersinal surface defining the base f the palevalley t the nrth. Breaching f the detachment fld des nt appear t have been cntrlled by bvius weaknesses in the expsed pre- Ecene strata. Alignment f the breached anticline with a palevalley riented at a high angle t the fld crest (Figure 3) sug- A-A' 780 m gests that greater ersin was assciated with cncentrated flw m alng this transverse valley. Thus despite the verall imprtance 'f lngitudinal transprt and depsitin, higher gradient 200 m transverse drainage systems and their ersive capabilities can Uncnfrrnity. _..- strngly influence gemetries within the syntectnic strata. B-B'. 620 m Cntrasting ersinal and depsitinal histries are assciated axially derived fine sediments with unbreached versus breached prtins f the detachment fld. magnetic Where the crest is unbreached, the well-expsed, upper 100 m f Prjected 520 m sectin -,,... I 1 syntectnic strata (Figure 6) appear t be cntinuusly defrmed 1 and display similar tapers and angularelatinships amng successive cnglmeratic beds (Figure 7A). In cntrast, ersinal breaching f the resistant limestnes frming the crest and frelimb f the anticline permits accelerated ersin f the fld's cre and subsequent lwering f the depsitinal surface. This results in a truncatin f bth the pregrwth and syngrwth strata in the midst f the main phase f anticlinal grwth, creating tw wedges f fanning grwth strata that are superpsed alng an angular discrdance within the grwth syncline. The yunger wedge is inset farther tward the hinterland (Figure 7B). Palevalley depsits. Tw palevalleys were incised at high angles t the crest f the detachment fld and stretched frm there t the Mntsec thrust sheet (Figures 3, 9, and 10). The Figure 7. Simplified crss sectins f unit 2 in the grwth syncline. The lcatin f the magnetic sectin is prjected int the plane f sectins. Fllwing early nlap f syntectnicnglmerates n preexisting and slwly defrming ftwall tpgraphy, cntinuus flding f the unbreached anticline led t an verturned, wedge-shaped prgressive uncnfrmity (A-A'). Ersinal breaching f the detachment fld (B-B') alng the trend f the majr hanging wall palevalley causes truncatin f the lder nlapping, prgressively flded cnglmerates. See Figure 3 fr sectin lcatins....:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:...::!:i:i:!:i:i:!:!:!:i:!:!:i:!:i:i:i:!:i:i:i:i:i:i:i:i:!:i:

8 20,288 BURBANK AND VERGI S: RECONSTRUCTING TOPOGRAPHY VGP Latitude Height (m) Lithlgy ( ,-,,, - - I I ' I I S ',._'._'. ß / N3 '. '. m R , _----:-- 50 I 0. Nrmal Class I Unit 2 MPS in Grwth ine R2 Plarity_ -e-errr Class n II Latitude Reversed Oliana MPTS õ MPS ' m Figure 8. Lithstratigraphy, palecurrents, magnetic plarity stratigraphy (MPS), and crrelatin fr unit 2 in the grwth syncline. Paleflw was dminantly t the SW. Tw magnetznes (in gray) are nly defined by single class I sites. Crrelatin f the MPS frm unit 2 with the Oliana MPS frm the sutheast flank f the Oliana anticline [frm Burbank et al., 1992] and with the magnetic time scale [Cande and Kent, 1992] is shwn. Shaded areas representhe stratigraphic range n units 2 and 3. this case, the base f the verlying cnglmerate (unit 3) which frm it by ne f the limestne fins (Figures 9 and 10). Cut int was depsited acrss all f the utcrps f unit 2 in the study Palecene siltstnes and flred by additinal beds f resistant area. limestnes, the valley is at least 1 km wide and is -140 m deep. The smaller palevalley was incised -50 m int a syncline cred The western valley wall is vegetated and prly expsed, and the by lwer Ecene sandstnes. Bth margins f the ~ 1.3-km-wide eastern limit f the palevalley has been remved by ersin. palevalley are defined by fins f resistant freshwater Palecene Flw indicatrs in the valley fill (see belw) and the spatial limestne (Figures 3, 9, and 10). In fact these frmed tpgraphic alignment f the majr palevalley with the breached crest f the barriers n the landscape that generally cnfined the fluvial sys- detachment fld define the -N-S trend f the palevalley acrss tem within the palevalley. A majr palevalley was situated the backlimb f the detachment fld (Figure 3). Given the geimmediately t the east f the minr ne (Figure 3) and is divided metrical necessity that the palevalleys were cnnected with the Figure 9a. Overview t the nrth f the majr (fight) and minr (left) palevalleys in unit 2 with the lcatins f sectins A, B, and C shwn. The flded strata f unit 3 and the megaclast f Cretaceus limestne verlie unit 2 and are strngly flded by the Mntsec verthrust. A small, nnthrusted remnant f unit 4 is visible n the left, where it verlies the Mntsec thrust. In the fregrund, Garumnian limestnes at the crest f the Serres Marginals anticline bund the grwth syncline (flded strata visible in lwer left).

9 .: BURBANK AND VERGl S: RECONSTRUCTING TOPOGRAPHY 20,289 Figure 9b. Rapid lateral facies changes in the majr palevalley are displayed by the abrupthinning and fining tward the suthwest f the cnglmerate beds. In cntrast, the "passing cnglmerate" and the verlying basal carse-grained bed f unit 3 taper tward the nrtheast, prbably due t syndepsitinal uplift alng the Mntsec thrust. Lcatin f sectin B (Figure 10) is shwn. A cne f talus breccia ccurs alng the flank f the Cretaceus megaclast n the left.... :; :.....::. ß Syncline - arurnnian limestne. -. :i..;?.: :.: <"' "*"--' ;! ;-:-* " ':" '"'"'"'"- ':'":""'"' :..-:.;....:..:.:" Figure 9c. View parallel t the structural fxnt (t the SW) that emphasizes the rle f the resistant Garumnian limestnes in defining the gemetry f the minr palevalley; the detachment fld in the basal beds f unit 3 abve less defrmed strata f unit 2; and the strngly verturned ftwall syncline cntaining units 2 and 3 beneath the Mntsec thrust. grwth syncline, we use the magnetichrnlgy f unit 2 in the grwth syncline and the age and relative stratigraphic psitin f the unit 2- unit 3 bundary t estimate the ages f the undated palevalley ffils. Thus the strata in the majr and minr palevalleys are estimated t range fxm Ma and Ma, respectively. Minr palevalley sedimentlgy: The strata f unit 2 within the smaller palevalley are generally tabular bdies, but their sedimentlgy cntrasts with that f the ceval cnglmerates in the grwth syncline. N clasts f Axial Zne rigin were recgmst tpgraphically cnstrained part f the fill display eastwards flw directins (Figures 10 and 11), whereas higher up in the fill, palecurrents appear t swing t the suth. In the lwer parts f verlying unit 3, -25 m stratigraphically higher, palecurrent directins are t the $W, and clasts f Axial Zne rck types are cmmn. The lwer 15 m f the fill are characterized by 0.2- t 2-mthick beds f subangular t subrunded pebble-t-bulder cnglmerates displaying planar, abrupt cntacts between depsitinal units (Figure 11a). Clasts are cmmnly >10 cm in nized within the palevalley, and all f the rck types present are diameter, and sme bulders f-1-m diameter are present. als fund in nearby thrust sheets. On the basis f imbricafins, Althugh many f the beds are matrix-rich and inversely graded gutters, grves, and crss beds, palecurrents in the lwest, (at least in their lwer parts), they are als clast supprte. The

10 20,290 BURBANK AND VERGI S' RECONSTRUCTING TOPOGRAPHY SW Ill 200 NE Cnglmerate" - - levall Y... Sectin A Ecene Zne f ressive Rtatin Sectin C Uncnfrmi Sectin B Red -..?'.....'-..\?? Unit 2 Cnglmerate with Lcal Prvenance [-'.'.' Siltstne and Mudstne f Unit 2 I%,':': _':.,_t Talus & Prximal Breccias ii:!!ii!!i!!i!ii?!:iii!iii!i Unit 3 Cnglmerate, including Axial Zne dasts r' ':"'::::"' ": :-'ii Garumnian Freshwater Limestne [iiiii!iiiiiiiiiii!iii!iiii Palecene Redbeds ß V/l/-/_...,... _,, r p :.'.0..:..:'.(;_ '.: :'"' Palevalley f Unit 2 /eo Palecurrents,n M, nr Figure 10. Tpgraphy, schematic stratigraphic gemetries, palecurrent directins, clast surce areas, and sectin lcatins fr units 2 and 3 in the majr and minr palcvalleys. Sectin is apprximately perpendicular t the rientatin f the palcvalleys. See Figure 3 fr lcatin. middle and lwer parts f many units lack internal structure and are disrganized, whereas the tps f several units shw cnsistent clast imbricatins. Gemetrical relatinships: Prir t the initiatin f filling, the palevalley was -50 m deep and 1 km wide. The strata f unit 2 within the minr valley passively nlap the beveled lwer The upper half f the palcvalley fill is characterized by mre Ecene strata. This erded surface displays a steplike gemetry channelized beds with scurs and grves up t 50 cm deep alng their bases (Figure 11 a). Mst beds are cm thick and rarely exceed 1 m. Internal structure, including parallel and with inclined treads up t I00 m wide and risers f 5-15 m that crrespnd either with lithlgical breaks r with abrupt changes in bedding attitudes in the pregrwth strata. Dips f the strata planar crss bedding, is cmmn, especially in the sandiet units. 'within the palevalley fill are shallw and unifrm, suggesting Cnglmeratic beds are supprted by subangular t subrunded little syndepsitinal tilting. clasts, and bth cnglmerates and sandstnes typically fine Majr palevalley sedimentlgy: The majr palevalley is upward. The tpmst cnglmerate beds vertp the cnfining situated adjacent t and east f the minr palevalley (Figures 3, limestne fin t the east and spread laterally int the large palc- 9, and I0). In cntrast the laterally cnsistent strata f the mivalley t the east (Figure I0). nr valley, the infilling f the majr valley is characterized by The matrix-rich and carsening upward character f the early marked lateral changes in bth facies and sediment-accumulatin depsits f the minr palcvalley fill are interpreted as rates acrsshrt distances. The facies changes are clearly visible representing hypercncentrated flws [Smith, 1988] and in utcrp, where acrss a lateral distance f-400 m, there is a nnchesive mass flws [Nemec and Steel, 1984]. Subangular rapid fining in grain size frm nearly cntinuus cnglmerates c!asts f lcal rck types suggest that these flws were t >70% medium grained sandstnes siltstnes (Figure 9). t/ansprted nly shrt distances frm nearby hanging walls. Tw sectins were measured in the valley fill: ne in carse- These early flws were funneled eastward within the cnfined margins f the palcvalley. The cnsistent imbricatins fund n the upper surfaces f sme flws suggest thathey were fiuvially grained strata the center f the valley and the ther in much finergrained strata near the eastern valley wall. The strata in the valley center (Figures I0 and 11b) are dmirewrked. As the valley bradened upward, fluvial depsitin nated by amalgamated, clast-supprted cnglmerates with typiß. frm shallw rivers became mre cmmn, and flw d rectmns cal bed thickness f-50 cm. Less than 5% f the beds have mean rtated tward the suth, i.e., transverse t the structural grain grain sizes smaller than 2-5 mm. The cnglmerates are prly (Figures 10 and 11 a). srted, have undulatry t mderately scured bases, and cm-

11 ..... BURBANK AND VERGI S' RECONSTRUCTING TOPOGRAPHY 20,291 A. Sad. B. Sad. C. Sad. Structures Structures Structures & & & Ht. Lith- Grain Pale- Ht. Lith- Grain Pale- Ht. Lith- Grain (m) lgy size currents (m) lgy size currents (m) lgy size currents 45 4O 4O 35 4O if' Unit F 35 3O..:...' - - 3O 25 '_L ; Unit D 25 2O - it C 2O '- ß. :..-': I L_,, Unit a 10,.,.,-..-.:,:::..., c Unit '0' Cnglmerate Overbank Siltstne ::::: Sandstne Lacustrine Mudstne Gutter 11 Palecurrents Imbricatin [ 'Carsening-up Matrix-Supprted Cnglmerate Oversized Clasts Uncnfrmity Ripples Palesl.. Crss bedding --- Wavy Bedding Rtlets ' ' Burrws ' Fining-up _ Parallel Bedding Figure 11. Stratigraphiclumns f unit 2 in the minr and majr palevalleys. (See Figure 10 fr lcatins.) (a) Stratigraphy f small palevalley. Nte upward rtatin f palecurrents frm east t suth. (b) Stratigraphy f the central part f the majr palevalley. (c) Stratigraphy f the marginal facies f the majr palevalley. Labeled carse-grained units can be crrelated t stratal grupings the central valley and display rapid thinning tward the valley margin. prise subangular t subrunded clasts f lcal rck types. Althugh sme beds carsen r fine upward, mst f them are ungraded. A few beds are matrix-rich and disrganized. Crss bedding at scales f cm is present in sme pebbly units, and cnsistent clast imbricatins are fund in many beds, in part due t the presence f many discidal clasts. Clusters f large (-50 cm in diameter) bulders frm the centers f lbate depsitinal units that display lw-angle crss beds dipping away frm the central bulders. At the panramic scale, distinctive hrizntal breaks are visible within the vertical successin f cnglmerates and can be traced laterally int fine grained units (Figure 9). At the detailed utcrp scale, these breaks are defined by finer grained cnglmerates that are used t delineate bundaries between majr alepsitinal units (Figure 1 lb). The laterally equivalent strata near the western valley wall display a sharply cntrasting character. These beds (Figures 10 and 11 c) are predminantly siltstne, fine-t-medium sandstne, and mudstne. Cnglmerates ccur as thin packets f individual r amalgamated beds, and each gruping f cnglmerates can be traced laterally t ceval thicker and carser units in the valley center. Calcareus palesls are cmmnly present, and burrwing and rt traces are almst ubiquitus in the massive, fine

12 20,292 BURBANK AND VERGI S: RECONSTRUCTING TOPOGRAPHY grained units. A small prprtin f the siltstnes are laminated, and thin (15 cm) arenaceus limestnes ccur several times in the upper part f the sectin. The tpmst part f the valley fill belw the base f unit 3 is a 20- t 25-m-thick successin f thin (<40 cm) sandstnes (20% f ttal thickness) and yellw-range and pink siltstnes. These strata are extensively mttled and burrwed, and they cntain abundant rtlets and pedgenically altered sandstnes. This fine-grained sequence drapes acrss the tp f bth palcvalleys and may als crrespnd with a zne lacking utcrps at the unit 2/3 bundary in the grwth syncline. The sedimentlgical data indicate an alluvial fan f lcal rigin filled the medial part f the majr palcvalley. Persistent imbricatins, lw-matrix cntent, large lateral extent f beds, and limited stratal thicknessesuggesthat depsitin ccurred frm uncnfined fluvial flws, sme f which are likely t have been hypercncentrated [Smith, 1988]. The bulder clusters are interpreted as bar cres arund which lw-angle accretin ccurred as the bar expanded [Hein, 1984]. Alng the western valley margin, the fine-grained nature f the strata, abundant biturbatin and sil develpment, and ccasinal thin limestnes suggest relatively slw depsitin n a lw-gradient fldplain. Rapidly thinning cnglmeratic units, 1- cnglmerate (the "passing cnglmerate"; Figures 9B and 10) that crsses the tp f the minr palcvalley and can be traced tward the center f the majr palcvalley. Cmparisn f these markers shws that there was a clear-cut shift f the zne f rapid accumulatin frm the valley center tward its margin. Belw unit A, a much greater thickness f strata accumulated in the central part f the palcvalley. Part f this is simply due t passive infilling f an erded landscape. Traceable beds which thin tward the valley margin, hwever, indicate that there was als greater subsidence in the valley center. This trend cntinues up thrugh unit F, during which time there is 2-3 times mre accumulatin in the valley center than near its western margin (Figures 10 and 11). This interval cincides with the previusly described prgressive rtatin f the basal strata alng the valley wall, such that the cntrasts in accumulatin prbably result frm differential uplift f the valley wall during flding f the Serres Marginales hanging wall. Neither the bserved 7 ø taper that results frm thinning f grups f beds nr the rapid change frm a tabular bed gemetry in the valley center t a strngly tapering ne tward the margin are cnsistent with the expected bedding gemetry and tpgraphic shape f an unperturbed alluvial fan [Wallace, 1978; West, 1991]. It therefre appears that flding at this scale can 3 m thick, encrached int this marginal envirnment infre- have dramatic effects n sediment accumulatin acrss shrt quently. Given the clse prximity f these tw sectins (-400 m), the abrupt facies changes between them indicate that the lateral margin f an alluvial fan is preserved here (Figures 9 and 10). Highly energetic depsitinal envirnments are present in the valley center, whereas pnded and tranquil depsitin characterizes the regin between the fan and the valley wall. The extensive palcsl-dminated successin that caps unit 2 suggests that an interval f quiescence prevailed prir t the initiatin f depsitin f unit 3. The absence f cnglmerates fur- (~400 m) distances and that differential subsidence cnfined the fan t the valley center during the early part f the valley filling. Between unit F and the "passing cnglmerate", the ppsite accumulatin trend prevails: there is mre accumulatin tward the valley margin than in its center (Figure 10). In the lwer part f this sequence, there appear t be nearly equal accumulatin rates, but in the later stages during predminantly finer grained depsitin, the valley-margin rates appear t be 2-4 times greater than in the center. This suggests that, during the later stages f unit 2 depsitin, defrmatin was shifting t a mre hindward ther suggests that the drainage system was being rerganized, lcatin, i.e., the Mntsec thrust [Burbank et al., 1992], and bepssibly in respnse t the initial transfer f thrust defrmatin t the mre hindward Mntsec thrust. ginning t defrm the prximal part f the basin adjacent the thrust. Gemetrical relatinships: Near the western margin f the majr palcvalley, the basal strata f unit 2 have been rtated during depsitin. Over a stratigraphic span f 20 m, bedding dips decrease frm vertical t -55 ø t the NW. These beds were Unit 3 Unit 3 in the study area was depsited abve unit 2 frm the depsited cevally with sme f the syntectnic strata in the nrth limb f the Oliana anticline t the trace f the Mntsec frelimb f the grwing detachment fld. The rtated beds can be thrust. It als cvers with little defrmatin the crest f the deinterpreted as a wedge-shaped "prgressive uncnfrmity" [Riba, 1976] that pens tward the hinterland. This back-tilting is interpreted t result frm flexural slip flding during tightening f the syncline in the pre-ecene strata which underlie the margin f the majr palcvalley. Unifrm dips f beds abve the rtated strata suggest that differential flding slwed r ended midway thrugh the filling f the palcvalley. Given that the palcvalley represents at least 150 m f incisin near the trace f the Mntsec thrust, it is nt surprising that cnsiderable ersin als ccurred acrss the crest f the grwing detachment fld where traversed by the palcvalley. During this tachment fld. Depsitin f unit 3 appears t cincide with mtin n the Mntsec thrust, which placed unit 3 in a ftwall psitin with respect t the active thrust. The depsitinal and defrmatinal histry f unit 3 and its cntrasts with the underlying units are influenced by its structural psitin and the way in which defrmatin was prpagated thrugh the ftwall. Sedimentlgy. Mst f the expsures f unit 3 ccur in steep, mstly inaccessible cliffs fr which we have n detailed sedimentlgy. Nnetheless, sme striking cntrasts with unit 2 are apparent. Mst imprtantly, rather than deriving slely frm lcal surce areas, the cnglmerate clasts indicate significant incisin, uplift f the cre f the detachment fld was apparently derivatin frm the Axial Zne. The inference that these shuld t rapid t permit permanent accumulatin f syntectnic strata within the palcvalley. Because individual strata and grupings f strata can be physically traced frm the palevalley's western margin tward its then belng t lngitudinally flwing rivers is brne ut by palccurrents in the basal parts f unit 3 (Figure 10) which clearly shw transprt the SW, parallel t the structural frnt. Thus, in cntrast t the slping transverse surfaces in the center, it is pssible t cmpare lateral changes in stratigraphic palcvalleys f unit 2, the depsitinal surface f this unit shuld thicknesses and apparent sediment-accumulatin rates. Fur reference hrizns serve t delineate these rate changes: the base f the palcvalley; the base f "unit A"; the tp f "unit F", and the be essentially hrizntal in sectins transverse t structural frnt. The cnglmerates f unit 3 are clast-supprted, display brad bedding (1-3 m), and cntain uncmmn, but well-bedded, fine

13 BURBANK AND VERGI S' RECONSTRUCTING TOPOGRAPHY 20,293 sandstne interbeds. The clasts f unit 3 are generally runded t appears that the strata f these beds thicken n each flank f the subrunded, suggesting they are farther traveled than mst f the anticline and thin ver its crest, whereas the underlying siltstnes clasts in the prximal palevalleys f unit 2. In areas clse t the and palesls clearly thicken alng the fld axis. Significantly, Mntsec thrust, the successins f cnglmerates and intervening the underlying "passing cnglmerate" the tp f unit 2 is nt fine-grained siltstnes in unit 3 display mre tabular gemetries similarly flded (Figure 10). This discrdant flding suggests that and cver brader areas than d the underlying strata within the during the early stages f Mntsec thrusting, a lcalized palevalleys. The ttal stratigraphic thickness f unit 3 thins detachment prpagated at very shallw depths within the finesuthward tward the Oliana anticline, and the generally even grained units, and a detachment fld f small amplitude (-15 m) prprtins f very carse- and fine-grained depsitinal units in develped abve it. prximal areas are replaced by increasingly dminant cnglmeratic strata tward the suth. Unit 4 One remarkable aspect f unit 3 is the incrpratin f a megaclast f Cretaceus limestne (Figure 9). This very large Unit 4 verlies unit 3 thrughut the study area and is blck (-350 m lng and > 100 m thick) apparently slid frm the preserved in steep cliffs that are generally inaccessible. On all hanging wall t the ftwall f the Mntsec thrust. The base f sides f the Oliana anticline, unit 4 is extensively preserved and this megaclast is nt expsed, but abutting its margins are cnes represents the last majr depsitinal event in this regin. The f unsrted, angular clasts f lcally derived mnlithlgic magnetic data [Burbank et al., 1992] indicate that initial breccias that are incrprated int ill-defined beds dipping depsitin f unit 4 began at Ma. When sedimentsteeply away frm the blck. At higher stratigraphic levels, the accumulatin rates frm the underlying strat are extraplated t megaclast is nlapped and vertpped by tabular units f the tp f this unit, they suggest that it may span 3-4 m.y. [Vergds cnglmerates and siltstnes that display n significant thickness and Burbank, 1994]. changes as they apprach the depsitinal cntact with the blck. Sedimentlgy. This unit cntains abundant clasts that were Because it ccurs near the base f unit 3 at a time when derived frm the Axial Zne and cnsists almst entirely f clastrenewed mtin n the Mntsec thrust was just beginning supprted, runded, and ften well-imbricated cnglmerates. [Burbank et al., 1992], it appears likely that prir t this latest Mst f the carse-grained depsitinal units are 1-3 m thick and phase f defrmatin, there were at least several hundred meters display a nrmally graded t ungraded character. In cntrast f residual tpgraphy n the Mntsec hanging wall as a result f the underlying units, there are very few fine-grained intervals middle and early Ecene mtin n this thrust [Vergds and preserved within unit 4 (<_5%). Where there are thin units f in- Mu qz, 1990]. Arund the megaclast's margins, talus cnes tervening siltstnes, hwever, lng gutters are very cmmn in develped, and later, bth these cnes and the megaclast were the base f the verlying cnglmerates. In cmbinatin with passively envelped by the aggrading beds f unit 3. clast imbricatins, these gutters prvide unambiguus paleflw Age. Unit 3 has nt been directly dated in the study area. directins tward the SW. Therefre unit 4 was depsited frm Magnetic dating f related strata n the sutheastern limb f the rivers flwing parallel t the structural grain and carrying far- Oliana anticline [Burbank et al., 1992], in cmbinatin with the traveled clasts int the freland. new age limits placed n unit 2 (Figure 8), indicates that unit 3 Where unit 4 is preserve directly adjacent the Mntsec ranges in age frm t Ma. On the sutheast limb f hanging wall, small, steeply inclined cnes f breccia abut the the Oliana antidine, m accumulated during this interval lwer part f the Mntsec escarpment. There is n evidence that at a mean rate f cm/kyr. Only -150 m f unit 3 are pre- unit 4 vertpped the Mntsec hanging wall in this transect. In served in the present study area t the nrthwest f the anticline. fact, the tpgraphic relief n the Mntsec hanging wall was This yields a mean rate f-15 cm/kyr, which is essentially un- much greater than the thickness f unit 4, which was represented changed frm the mean rate f accumulatin f unit 2. by a steep aprn f talus breccias banked against the lwer Gemetrical relatinships. At the large crss-sectinal scale, slpes f the hanging wall. Althugh the ttal amunt f the thinning and nlap f unit 3 suthward n t the nrth flank tpgraphy inherited frm middle and early Ecene defrmatin Oliana anticline (Figure 3) clearly indicate this large anticline is unknwn, it is likely t represent abut half f the present was still grwing at this time. In prximal areas adjacent t the relief, given the limited amunt f late Ecene Mntsec Mntsec thrust, the strata f unit 3 have been strngly flded shrtening [Vergds and Burbank, 1994]. (Figure 9) int a ftwall syncline. Small thrust faults ccur Gemetrical relatinships. Because it is the stratigraphically alng sme f the cntacts between the megaclast and adjacent and tpgraphically highest syntectnic unit, unit 4 is mre discnglmerate beds. During flding, the thelgical cntrasts be- sected than the lder depsitinal units, and its cmplete gemetween the megaclast and the surrunding strata prbably caused try is mre difficult t describe. Nnetheless, its prximal strata the mre rigid blck t glide alng minr ut-f-syncline thrusts. are nly very slightly defrmed by the Mntsec thrust which On the nrth side f the megaclast, strata f unit 3 are steeply gently flds them in several places. This absence f strng dedipping and display a fannin gemetry ranging at present frm frmatin indicates that majr mtin n the Mntsec had termidips f 20 ø verturned t nrth t dips f 65 ø t the suth. This nated when depsitin f unit 4 cmmenced. syndepsitinal wedge-shaped gemetry in unit 3 is interpreted t Unit 4 thickens away frm the Mntsec hanging wall tward result frm flding and uplift f the Mntsec ftwall. Gentle the suth and SE (Figure 2) and then thins as it appraches the warping f unit 3 as it passes ver the crest f the detachment backlimb f the Oliana anticline. Our tectnic recnstructins fld suggests that this anticline and the hanging wall syncline t suggest that mst f the thrusting and flding within the anticline its nrth (Figures 2 and 4) cntinued t tighten in respnse t itself ceased at this time [Burbank et al., 1992]. Nnetheless, the Mntsec thrusting. thinning f unit 4 adjacent the Oliana anticline indicates that Directly abve the limestne fin that defines the eastern sme tpgraphy persisted acrss its structural crest, where tday margin f the minr palevalley (Figure 9a and 9c), the basal ersin f the weakly resistant marls in the cre f the fld have cnglmeratic strata f unit 3 are flded int an anticline. It caused a strng tpgraphic inversin.

14 , 20,294 BURBANK AND VERGI S' RECONSTRUCTING TOPOGRAPHY Discussin The grwth f the Oliana anticline and the ceval defnnatin f structures related t the Serres Marginals and Mntsec thrust sheets representhe fundamental tectnic cntrls n bth depsitin and tpgraphy in the study area. Tday, the Segre River cuts directly thrugh the strutrurally highest part f the Oliana anticline. During defrmatin, hwever, this structure presented an blique barrier t flw f the ancestral Segre River and caused it t divert t either flank f the grwing fld (Figure 12). Alng the sutheastern limb f the Oliana anticline, lw tpgraphy persisted suthwards int the undefrmed freland, and palecurrent indicatrs [Burbank et al., 1992] suggest a fanning f flw directins nce the fiver passed the nrtheasternse f the anticfine, which cntrlled the entry pint f the river int the freland. On the nrthwestern limb f the anticline where we have fcused the present study, the ancestral Segre was channeled alng the suthwestrending structural mat, represented by an irregular synfrm between the Mntsec hanging wall highlands and the Oliana anticline (Figure 2). The prgressively fanning dips and the tapered gemetry f strata within the grwth syncline f unit 2 prvide clear evidence f syndepsitinal rtatin f the frelimb f a grwing detachment fld. During early stages f depsitin and fld grwth, rapid accumulatin primarily prmted nlap f inherited tpgraphy. The rates f vertical uplift are interpreted t have been slwer than the accumulatin rates (-25 cm/kyr) at this stage. During the succeeding stage f depsitin ( Ma), accumulatin rates in the grwth syncline were slw (-6 c:n/kyr), finer grained strata were depsited, and the palevalleys attained their maximum depths as they incised acrss the crest f the detachment fld. Aggradatin in the grwth syncline accelerated t ~14 cm/kyr between 35.7 and 35.0 Ma. Cntinuus tapering f strata tward the syncline's margin, cmbined with the absence f clear evidence fr either strng verlap r fflap with respect Figure 12. Cmparisn f the majr structural and depsitinal the fld crest, suggests that vertical rates f flank uplift and elements in the vicinity (a) f the Oliana anticline and (b) f fluvial depsitin were nearly equivalent alng the frelimb f Wheeler Ridge, suthern Califrnia. The absence f a majr fiver the detachment fld. Thus during depsitin f unit 2, the at Wheeler Ridge, cmbined with mre rapid uplift f the antilngitudinally flwing river was cnfined between the Oliana an- cline and impinging thrusts, cntrl the large-scale differences in ticline and the detachment fld. Despite the nging uplift at the depsitinal gemetries between these sites. The transverse rates f mm/yr in the flds defining the margins f this valleys and smaller-scale drainage patterns at Wheeler Ridge trugh, n significantpgraphy was created within the depsi- prvide reasnable analgs fr the palcvalleys in Setres tinal plain. Marginals anticline. The psitining f the palevalleys prvides insights n the rles played by lithlgy and structure in cntrlling incisin, river. It appears that detrital sediment prductin fxm the small, whereas the infilling f the palevalley reveals aspects f the steeply inclined, and largely limestne-dminated lcal catchtpgraphy, defrmatin, and surce areas in the Serres ments was verwhelmed by the cntributins f the axial rivers. Marginals hanging wall. Nt unexpectedly, flding and uplift f The incisin f the larger transverse palcvalley thrugh the variably resistant bedrck strata lead t accentuated ersin f crest f the grwing Serres Marginals anticline caused a lcally weaker beds. The minr palevalley is enclsed by a resistant imprtant uncnfrmity t be generated within unit 2 in the limestne and is lcalized alng the axis f a sandstne- and grwth syncline. This resulted in recgnizable alng-strike marl-cred fld. These same resistant limestnes als delineate variatins in the gemetric relatinship between syntectnic the highest preserved pint alng the nly preserved margin f strata and the flded pre-ecene strata. The deeper incisin that the majr palevalley, which is incised int less resistant red ccurred alng the majr palcvalley caused juxtapsitin f unit beds. 2 with erded limestne in the backlimb f the fld (sectin B-B', The palcvalleys representhe nly significantransversele- Figure 7), rather than verlapping the frelimb, as is seen farther ments in the evlving landscape that are still preserved. These t the west (sectin A-A', Figure 7). valleys are riented perpendicular t the structural trend and The switch fxm incisin t depsitin alng the palcvalleys served as cnduits carrying lcally derived detritus t the lngi- ccurred while the detachment fld was grwing and the strata f tudinally riented, larger river (sectin B-B", Figure 2). The unit 2 were filling the grwth syncline. We assume that because vlumetrically small cntributin (<10%; Figures 2, 3, and 4) f the transverse valleys were tributaries t lngitudinal river in the the infilled transverse valleys t the ttal fill represented by units grwth syncline (Figures 2 and 12a), the psitin f the tp f the 2-4 indicates the clear dminance f depsitin frm the axial fill in the syncline represented the lcal base level fr the

15 BURBANK AND VERGI S' RECONSTRUCTING TOPOGRAPHY 20,295 transverse palevalleys. If the tp f the fill is raised abve the supprts a mdel suggesting that during late stages f defrmalevel f the incised bedrck f the palevalley, sediments will tin, hrizntal shrtening is ften superseded by vertical uplift aggrade within the palevalley. If, n the ther hand, the depsi- driven primarily by flding. This phenmenn can generate cntinal surface in the grwth syncline is lw with respec t the siderable tpgraphic relief late in the defrmatinal sequence. bedrck f the palevalley and if the transverse sediment supply At Oliana, this thickening and uplift shift the lngitudinally is nt high, then cntinued incisin f the transverse valley will flwing river tward the freland (Figure 2), where it impinges ccur. Depsitin r ersin in the palevalleys can therefre be n the nrthern flank f the Oliana anticline. interpreted in the cntext f changes in (1) the balance between Cmparisn f the recnstructed gemetries f the Oliana the rate f sediment supply t the grwth syncline frm regin with thse assciated with presently active structures lngitudinal rivers and the rate f subsidence f the syncline and serves t emphasize the imprtance f relative rates f tectnic, (2) the rate f crestal uplift f the detachment fld versus sedi- ersinal, and depsitinal prcesses in cntrlling the evlutin ment supply and stream pwer in the transverse palevalleys. In f the landscape. In a structural psitin analgus t the Oliana the cntext f these cnceptual interactins, incisin in the trans- anticline with respect the Ebr basin during the latest Ecene, verse valley is prmted by high rates f subsidence in the Wheeler Ridge in the suthern San Jaquin valley f Califrnia grwth syncline and f crestal uplift in the fld, lw rates f sed- represents the utermst majr fld [Medwedeff, 1992] in an aciment supply t the transverse valley and f accumulatin in the tive fld-and-thrust belt that brders a large depsitinal basin. grwth syncline, and high stream pwer in the transverse valley. Wheeler Ridge is an eastward plunging anticline that dives belw In the study area, the deepest incisin f the palevalley and the the mdern depsitinal plain at its eastern end (Figure 12b). Its develpment f an uncnfrmity in the grwth syncline near the hanging wall has been cut by transverse streams that are lcalized muth f the transverse valley appear t have ccurred when by transverse faults [Medwedeff, 1992]. Where rates f hanging sediment-accumulatin rates were at their lwest levels (Figures wall uplift have exceeded rates f incisin f the anticline, frmer 2, 7, and 8). Aggradatin and backfilling in the palevalleys are water gaps have been cnverted t wind gaps. On the distal, interpreted t be prmted by rapid accumulatin in the grwth nrthern flank f Wheeler Ridge, shrt-radius (generally <1 km), syncline, high rates f sediment supply t the transverse valley, lcally surced alluvial fans are being actively defrmed in their decreases in subsidence in the grwth syncline and/r f uplift in prximal parts by the grwing fld. T the suth f the fld crest, the crestal regin f the detachment fld, and decreases in stream the depsitinal plain between the fld and the Pleit thrust is pwer in the transverse valley. In the cntext f the magnetic prgressively incrprated int the rapidly uplifting backlimb f chrnlgy (Figure 8), the dubling f accumulatin rates after the grwing anticline, where it is attacked by ersinal prcesses Ma and a relative slwing f crestal uplift rates in the de- The emergence f Wheeler Ridge abve the depsitinal plain tachment fld appear t crrespnd with the switch frm incisin has strngly perturbed the previus depsitinal systems. Small, t infilling in the larger palevalley. transverse streams that had fed int the site f the present wind The minimum tpgraphic relief that can be dcumented n gap have been diverted t the east, where they jin thers and are the larger palevalley is -150 m, which represents the either fcused int the water gap r are diverted arund the stratigraphic separatin f the base and the tp f the palevalley plunging nse f the anticline. The tpgraphy n the large fill. The average slpe f the preserved valley wall is abut-20 ø. alluvial fans near t Wheeler Ridge suggests that they als have During the ~l-m.y.-lng interval when the palevalley was been deflected arund the grwing anticline. On the nrthem side filling, ersin f the interfluves wuld have reduced the riginal f the anticline, which is dwnstream with respect the reginal relief n the palevalley t that bservable tday. drainage system, tw subreginal alluvial fan systems cnverge In the case f unit 2, nearly all f the hanging wall tpgraphy (Figure 12b) tward a zne f reduced accumulatin: a develped during grwth f the detachment fld and during "depsitinal shadw zne." T the suth f Wheeler Ridge, the tightening f the previusly frmed syncline which was inherited active Pleit thrust uplifts the mre prximal parts f the thrust frm earlier defrmatin in the Ecene (Figure 2). In cntrast, belt and causes transverse streams t incise int its hanging wall. much f the tpgraphic relief f the Mntsec hanging wall is At tw scales, Wheeler Ridge displays gemetries that are likely t have been inherited. The size f the megaclast within the analgus with thse recnstructed at Oliana. At the small scale, basal part f unit 3 suggests a minimum f several hundred me- the transverse valleys in Wheeler Ridge are cmparable t the ters f hanging wall relief during early reactivatin f the Oliana palevalleys during stages in which lcal uplift rates f Mntsec thrust. The inclined talus cnes f unit 4 that are aligned the fld crest surpassed the accumulatin rates n the ftwall. alng the lwer part f the Mntsec hanging wall indicate that a At the large scale, Wheeler Ridge presents a majr tpgraphic minimum f 500 m f relief abve the depsitinal plain existed barrier that deflects majr drainage systems arund it. Similarly, at the end f thrusting, during which time -1 km f shrtening the Oliana anticline acted as a divisive barrier t the ancestral and nearly equal amunts f hanging wall uplift tk place. Segre River (Figure 12a) which was diverted either int the brad Sequential recnstructins acrss the defrming Serres freland t its sutheast which lacked many tpgraphic barriers Marginals and Mntsec thrust sheets (Figure 2) indicate that r int the structural mat alng its nrthwest flank (analgus t between 35 Ma and 33 Ma, -750 m f shrtening (27%) ccurred the separatin between the Pleit Thrust and Wheeler Ridge). between the pinning pints n the Mntsec thrust and the There are several imprtant differences between Wheeler innermst Serres Marginals thrust [Verggs, 1993]. After-35.6 Ridge and Oliana. First, the majr drainage is nt pen t the Ma, the Mntsec ftwall was differentially tilted and uplifted, west, because the uplifted Pleit hanging wall and Wheeler Ridge such that the prximal regins were raised 250 m with respect t intersect tward the west and their mutual uplift rates exceed the ceval, frmerly hrizntal surfaces nly 1 km away. This uplift rate f aggradatin. Cnsequently, Wheeler Ridge cannt divert a was accmplished primarily thrugh flding. Bth preexisting majr fiver system t either flank, as happens at Oliana. Secnd, flds in the pretectnic strata and newly depsited strata f units whereas the depsitinal area abve the submerged eastern fld 2 and 3 were tightened int chevrn gemetries displaying -60 ø nse prbably prvides an excellent mdel fr verlapping interlimb angles (Figure 2) [Ramsay, 1974]. This late stage uplift syntectnic gemetries (Figure 5b), fr much f the length f

16 20,296 BURBANK AND VERGI S: RECONSTRUCTING TOPOGRAPHY Wheeler Ridge, the rate f crestal uplift (3.2 m/kyr) is cnsiderably greater than the rate f sediment accumulatin (1.8 m/kyr) adjacent t the anticline [Medwedeff, 1992]. As a result, rather than develping gradually tapering syntectnic beds that are incrprated and recgnizable in the frelimb as is seen in the detachment fld at Oliana, syntectnic strata at Wheeler Ridge pinch abruptly againsthe frelimb and display such strng fflap (Figure 5c) that they can nt clearly be seen as incrprated int the grwing fld. Third, the presence f readily erded marl in the cre f the Oliana anticline inhibits recgnitin, n the distal side f the fld, f fans with lcally derived clasts that wuld be analgus t thse at Wheeler Ridge. When bth the differences and similarities between Oliana and Wheeler Ridge are cnsidered, they define a spectrum f respnses t active cntractinal defrmatin. At Wheeler Ridge, an ergdic substitutin f space fr time serves t delineate likely During filling f the transverse palevalleys, lcal surce areas are almst exclusively represented. 5. When aggradatin is rapid in cmparisn t uplift, grwing structures and any preexisting tpgraphy can be nlapped r verlapped by syntectnic strata. At Oliana, accumulatin rates f -25 cm/kyr are sufficiently rapid t create strng nlap. Near equality in rates f accumulatin and uplift lead t cnspicuus thinning and flding f syntectnic strata int a grwth syncline (prgresiv e unc nfrmity). 6. Large-scale gemetries f strata within grwth synclines als depend n the ersin f the crestal area f the adjacent fld. When the crest remains intact, cntinuus depsitin and flding are ften recrded by the syntectnic strata in the frelimb. In cntrast, when the crest is breached, secndary uncnfrmities can develp within the grwth synclines due t lwered transverse gradients alng palevalleys, especially during times f lw respnses f the landscape t varying rates f uplift and aggrada- accumulatin and rapid subsidence in the syncline. tin. These spatially variable rates prduce depsitinal and tpgraphic gemetries that mimic many f thse that result frm the temprally varying rates and prcesses, such as thse defined at Oliana. The mdern envirnment f Wheeler Ridge serves t define mre clearly surficial relatinships between structure, tpgraphy, and sedimentatin, whereas the ancient setting f Oliana permits ne t view the depsitinal, ersinal, and structural gemetries that are presently bscured beneath the Earth's surface at Wheeler Ridge. 7. Very similar structural gemetries can be fund thrugh a cmparisn f the Oliana area with the presently defrming Wheeler Ridge in suthern Califrnia. This mdern analg f a plunging anticline impinged upn by hindward thrust sheets displays many f the same features that have been recnstructed fr the Oliana area, including structurally cntrlled transverse palevalleys and structurally diverted drainages. The main differences between these tw settings result frm the mre rapid rates f crestal uplift at Wheeler Ridge which results in prnunced fflap f many syntectnic strata and in the develpment f pr- Cnclusins nunced tpgraphic barriers t flw. Whereas mdern analgs can display unambiguus mdern gemrphic relatinships, the Depsitinal and tpgraphic patterns at Oliana develped in respnse t thrusting, grwth f a detachment fld, anticlinal uprecrd f their previus defrmatinal and depsitinal histry is ften bscured in the subsurface. Althugh gemrphic prcesses lift, and cntinued defrmatin f previusly flded structures and tpgraphy at ancient sites can nly be recnstructed, rather during late Ecene times. Our analysi shws the fllwing: 1. Alng the defrmatinal frnt, there exists an interplay between lngitudinally flwing rivers frm distant surces and than bserved, stratigraphic and structural expsures prvide the pprtunity t dcumenthe tempral evlutin f the structure and the landscapes related t its defrmatin. transversely flwing rivers f lcal rigin. In regins where the erding hanging walls cmprise resistant strata and are character- Acknwledgments. This research was partially supprted by grants ized by relatively small catchments, the transverse cntributin t EAR and frm the Natinal Science Fundatin and the sedimentary recrd will be diminished. Where there is high grants frm the Petrleum Research Fund (ACS-PRF 20591, 17625, and tpgraphy n hanging walls inherited frm earlier defrmatin, 23881) t D.W.B. J.V. received supprt fr this study frm the Subprgrama de Perfeccinamient para Dctres y Tecn61gs (1990) hwever, large landslide blcks can tumble int ftwall synclines. and DGICYT prjects PB and PB Discussins with A. Meigs and J. Friedmann have served t imprve this paper significantly. 2. Flding assciated with bth the frelimbs and backlimbs f Very thrugh and helpful reviews by J. K. Snw, D. J. Harbr, and M. grwing structures causes related thinning and defrmatin f Ellis greatly imprved this manuscript. syntectnic strata. The resulting wedge-shaped gemetries f syntectnic strata pen bth tward the freland and the hinterland and recrd fld grwth. References 3. When flding and crestal uplift are rapid in cmparisn t Burbank, D.W. and R.A. Beck, Mdels f aggradatin versus the rate f aggradatin, relief f the hanging wall abve the prgradatin in the Himalayan freland, Gel. Rundsch., 80, , depsitinal plain increases, transverse valleys becme accentuated, strng fflap ccurs in depsitinal znes, and prximal uncnfrmities may develp as lder syntectnic strata are flded, uplifted, and expsed t ersin. 4. Transverse palevalleys are lcalized by the structure and lithlgy int which they are incised. The cres f flds and Burbank, D.W., J. Vergts, and J.A. Mufiz, Ceval hindward- and frward-imbricating thrusting in the central suthern Pyrenees: Timing and rates f shrtening and depsitin, Gel. Sc. Am. Bull., 104, 1-18, Cande, S.C., and D.V. Kent, A new gemagnetic plarity time scale fr weakly resistant strata prvide favred pathways. At Oliana, rethe Late Cretaceus and Cenzic, J. Gephys. Res.,97, 13,917-13,953, sistant limestnes uphld the valley walls, and synclinal flds Eisbacher, G.H., M.A. Carrigy, and R.B. Campbell, Paledrainage have been explited by the palevalleys. When crestal uplift rates pattern and late rgenic basins f the Canadian Crdillera, in are nly a few millimeters per year, palevalley incisin can Tectnics and Sedimentatin, edited by W.R. Dickinsn, Sc. Ecn. smetimes keep pace with uplift. The tpgraphic relief n such Palentl. Mineral. Spec. Publ., 22, , palevalleys at Oliana is > 150 m. The transitin frm incisin t England, P., and P. Mlnar, Surface uplift, uplift f rcks, and depsitin within a palevalley is apparently cntrlled by accelerating rates f aggradatin in cmparisn t hanging wall uplift. exhumatin f rcks, Gelgy, 18, , Hein, F.J., Deep-sea and fluvial braided channel cnglmerates: a

17 BURBANK AND VERGI S: RECONSTRUCTING TOPOGRAPHY 20,297 cmparisn f tw case studies, in Sedimentlgy f Gravels and Verg6s, J., Estudi gelbgic del versant sud del Pirineu riental i central: Cnglmerates, edited by E.H. Kster and R.J. Steel, Mere.. Can. Sc. Evluci6 cinem tica en 3D, Ph.D. thesis, 203 pp., Univ. f Barcelna, Pet. Gel., 10, 33-50, Martinez-Pefia, M.B., and J. Pcvf, E1 amrtiguamient frntal de la Verg6s, J. and D.W. Burbank, Ecene-Oligcene thrusting and basin estmctura de la cberta surpirenaica y su relaci6n cn el anticlinal de cnfiguratin in the eastem and central Pyrenees (Spain), in Tertiary Barbastr-Balaguer, Acta Gel. Hisp., 23, 81-94, Basins f Spain, edited by P.F. Friend and C.J. Dabri, Cambridge Medwedeff, D.A., Gemetry and kinematics f an active, laterally University Press, New Yrk, in press, prpagating wedge thrust, Wheeler Ridge, Califrnia, in Structural Verg6s, J. and Mufiz, J.A., Thrust sequences in the suthem central Gelgy f FM and Thrust Belts, edited by S. Mitra and G.W. Fisher, Pyrenees, Bull. Sc. Gel. Fr.,8, , pp. 3-28, Jhns Hpkins University Press, Baltimre, Md.,1992. Verg6s, J., J.A. Mufiz, and A. Martinez, Suth Pyrenean fld-and-thrust Mufiz, J.A., Evlutin f a cntinental cllisin belt: ECORS-Pyrenees belt: Rle f freland evapritic levels in thrust gemetry, in Thrust crustal balanced sectin, in Thrust Tectnics, edited by K.R. McClay, Tectnics, edited by K.R. McClay, pp , Chapman and Hall, pp , Chapman and Hall, Lndn, Lndn Nemec, W. and R.J. Steel, Alluvial and castal cnglmerates: their Wallace, R.E., Gemetry and rates f change f fault-generated range significant features and sme cmments n gravelly mass-flw frnt, nrth-central Nevada, U.S. Gel. Surv. J. Res., , depsits, in Sedimentlgy f Gravels and Cnglmerates, edited by E.H. Kster and R.J. Steel, Mem.. Can. Sc. Pet. Gel, 10, 1-32, West, R.B., Tectnic gemrphlgy, landfrm mdelling, and sil chrnlgy f alluvial fans f the Tejn embayment, suthemmst San Jaquin Valley, Califmia, M.Sc. thesis, 148 pp., Univ. f Calif., Santa Barbara, Puigdef bregas, J., J.A. Mufiz, and J. Verges, Thrusting and freland basin evlutin in the Suthem Pyrenees, in Thrust Tectnics, edited by K.R. McClay, pp , Chapman and Hall, Lndn, Ramsay, J.G., Develpment f chevrn flds, Gel. Sc. Am. Bull., 85, , Riba, O., Syntectnic uncnfrmities f the Alt Cardener, Spanish Pyrenees: A genetic interpretatin, Sediment. Gel., 15, , S6guret, M., l tude tectnique des nappes et s6fies d6cll6es de la partie centrale du versant sud des Pyr6n6es, Publ. Univ. Sci. Tech. Langeduc, Sgr. Gel. Struct.,2, 1-155, Smith, G.A., Sedimentlgy f prximal t distal vlcanclastic dispersed acrss and active fldbelt: Ellensburg Frmatin (late Micene), central Washingtn, Sedimentlgy, 35, , D. W. Burbank, Department f Gelgical Sciences, University f Suthern Califrnia, Ls Angeles, CA ( Internet: burbank@cda.usc.edu) J. Verg6s, Departament de Gelgia Din&mica, Geffsica, i Palentlgia, University f Barcelna, Barcelna, Spain. ( Internet: jaumev natura.ge.ub. es) (Received April 23, 1993; revised January 5, 1994; accepted February 14, 1994.)