Field examples of strike-slip fault terminations in Mongolia and their tectonic significance

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1 ß TECTONICS, VOL. 18, NO. 3, PAGES , JUNE 1999 Field examples of strike-slip fault terminations in Mongolia and their tectonic significance Amgalan Bayasgalan and James Jackson Bullard Laboratories, University of Cambridge, Cambridge, England Jean-Fran5ois Ritz and Sebastien Carretier Laboratoire de Gdophysique et Tectonique, Universitd Sciences et Techniques de Languedoc Montpellier, France Abstract. Deformation at the ends of large intracontinental strike-slip faults that do not simply link other major structures often involves rotations about a vertical axis. We use earthquake slip vectors, surface rupture in earthquakes, and geomorphology to examine the ends of three major strike-slip faults in Mongolia. In these places a simple pattern is seen, consisting of a thrust fault on one side, with a displacement that decreases away from the strike-slip fault, consistent with local rotational deformation. Strike-slip faults that terminate in this way allow the style of faulting to change spatially within a deforming area, for example, from dominantly strike-slip to dominantly dip-slip, while still accommodating the overall deformation required by larger-scale regional motions. Such a change in fault style should also be accompanied by a change in the rotation rate about a vertical axis, which may be detected paleomagnetically. The kind of strike-slip fault termination described here may have consequences for how large strike-slip faults evolve and grow and for the variation in displacement along their length. 1. Introduction The essential feature of active tectonics on the conti- nents is that the deformation is usually distributed over wide areas and not restricted to the relatively narrow zones that define most plate boundaries in the oceans [e.g., McKenzie, 1972; Molnar and Tapponnier, 1975]. Within these wide deforming regions some of the largest earthquakes occur on strike-slip faults that are several tens, or even hundreds, of kilometers in length [e.g., Molnar and Tapponnier, 1975; Molnar and Deng, 1984; Jackson and McKenzie, 1984]. As a consequence of the deformation being distributed on the continents, these strike-slip faults do not, in general, simply link with other major structures at their ends, in the way that Copyright 1999 by the American Geophysical Union. Paper number 1999TC /99/1999TC true transform faults join ridge or trench segments on narrow plate boundaries. What, then, happens at the ends of these major intracontinental strike-slip faults? Some large strike-slip faults appear simply to end against uncleforming regions, such as the large eastwest faults in northeastern Iran that abut against stable Afghanistan [Jackson and McKenzie, 1984] or those in the eastern Gobi-Altay of Mongolia that appear to die out in the relatively flat and featureless Gobi Desert [Baljinn tam et al., 1993; Cunningham et al., 1997]. Others end in regions of diffuse deformation with a different structural style, such as the strike-slip faults of the northern Aegean Sea, which end in a region of normal faulting in central Greece [Ta tmaz et al., 1991], or those of eastern and northern Tibet, which end in the distributed thrusting of the Longmen Shah and Nan Shah [Molnar and Lyon-Caen, 1989; Meyer et al., 1998]. In a general way, it has been recognized for some time that large intracontinental strike-slip faults, if they are not simply transform faults bounding rigid blocks, are likely to be associated with rotations about a vertical axis [e.g., Freund, 1970, 1974; Gaffunkel, 1974]. Thus the east-west left-lateral faults of eastern Iran can accommodate a north-south right-lateral shear between Iran and stable Afghanistan if the faults rotate clockwise [Jackson and McKenzie, 1984; Jackson et al., 1995]. This geometry is virtually identical to the proposed clockwise x oration of the left-lateral strike-slip faults in eastern Tibet, which can then accommodate a right-lateral shear between Tibet and southwest China [England and Molnar, 1990; Holt et al., 1991]. In the Aegean Sea [Ta tmaz et al., 1991; Jackson et al., 1992] and in the northern part of the South Island of New Zealand [Lamb and Bibby, 1989; Little and Roberts, 1997], paleomagnetic declination show that rapid rotations occur in the regions of distributed extension or shortening at the ends of large strike-slip fault systems. In all these cases the links between the strike-slip faulting, the rotations about a vertical axis, and the overall velocity field in the deforming region are, in principle, understood. However, this understanding is only in outline. Little attention has been given to what structures 394

2 BAYASGALAN ET AL.: STRIKE-SLIP FAULT TERMINATIONS 395 are actually found in the field where the strike-slip faults end or interact or to how those structures move. That is the subject of this paper. We will examine the structures where strike-slip faults end or interact in two regiona of Mongolia (Figure 1). We begin with a description of the coseismic ruptures and geomorphology associated with the 1967 Mogod earthquakes, for which seismological constraints on the faulting processes are also available. We then look at the coseismic ruptures of the 1957 Gobi-Altay earthquake (M,0 8.0) and show that the coseismic fault patterns that we see in both of these earthquakes are reflected also in the geomorphology of the eastern Gobi- Altay range, on other active fault systems that have not ruptured in modern-day earthquakes. We also look at the structures associated with an en echelon step between two large strike-slip faults in the Gobi-Altay. Finally, we consider the significance of the patterns we see for the larger-scale regional tectonics and for fault growth. The approach we use is first to describe the observations and then to discuss simple ways in which the faults we see can move and contribute to the regional deformation. This is essentially a kinematic point of view, which ignores questions concerning the dynamics or forces responsible for the deformation in the first place. Those questions, though valid and interesting, are beyond the scope of this paper, which thus separates the question of how the faults move from that of why they do so in any particular way, in the same manner in which it is possible to describe the relative velocities of plates without addressing the forces that drive them. 2. Field Examples in Mongolia 2.1. Tectonic Setting The active deformation of western Mongolia is dominated by distributed right-lateral and thrust faulting in the NW trending mountains of the Mongolian Altay and by distributed left-lateral and thrust faulting in the E-W ranges of the Gobi-Altay (Figure 1). Both ranges have been the sites of large (M,0 8) earthquakes in this century and display many late Quaternary fault scarps [Baljinnyam et al., 1993, Cunningham et al., 1996a, b]. In addition, large earthquakes have also occurred in this century on an E-W left-lateral strike-slip fault at Bulnay (in 1905, M,0~8.0) and on a N-S right-lateral fault at Mogod (in 1967, M,0 7.1). The arrangement of these fault systems (Figure 1) resembles a parallelogram of conjugate faults [Huang and Chen, 1986] surrounding the high dome of Hangay, which is the site of significant Quaternary volcanism. The overall deformation of this part of central Asia is likely to involve roughly 90øE 95øE 100øE 105øE 90øE 95øE 100øE 105øE Figure 1. Tectonic setting of western Mongolia, showing how the system of NW-SE right-lateral faults in the Mongolian Altay and the E-W left-lateral faults of the Gobi Altay can achieve N-S shortening in this region [e.g., England and Molnar, 1997] by rotating anticlockwise and clockwise, respectively [Schlupp, 1996].

3 396 BAYASGALAN ET AL.: STRIKE-SLIP FAULT TERMINATIONS N-S shortening [e.g., England and Molnar, 1997], perhaps as a consequence of the India-Asia collision to the south. Shortening of this orientation can be achieved by counterclockwise rotation of the Mongolian Altay and clockwise rotation of the Gobi-Altay [Schlupp, 1996], as both ranges rotate away from the direction of maximum shortening, in the manner suggested for conjugate strike-slip faults by Freund [1970]. Late Tertiary counterclockwise rotation of the Mongolian Altay is suggested also by paleomagnetic declinations [Thomas et al., 1998], discussed in section The 1967 Mogod Earthquakes, Central Mongolia The main surface ruptures produced by the two Mogod earthquakes of January 5 (Mw 7.1) and January 20 (M 0 6.4), 1967, consist of ~20 km of N-S right-lateral strike-slip faulting (heavy line in area b in Figure 2) and ~12 km of thrust faulting with a NW-SE strike, dipping NE (heavy toothed line in area c in Figure 2) [Baljinnyam et al., 1993]. The N-S strike-slip ruptures (area b) are clear and continuous along most of their length, with an average horizontal slip of ~1.5 m and a maximum of ~3.2 m, from 48ø10 N to their junction with the NW-SE thrust fault ruptures (area c) in the south [Baljinnyam et al., 1993]. Along most of their length these strike-slip ruptures usually have a vertical component as well, up on the eastern side. Other N-S ruptures (thin lines in area a in Figure 2) were found the location of this subevent at the northern end of the main strike-slip ruptures (area b), the locations of the first (M 06.5) and third (M 0 6.6) subevents are such that they can account for the ruptures in areas a and c, respectively. A relocation of the principal aftershock (670120) relative to the mainshock puts it ~10 km to the SE of the first subevent. The locations and mech- anisms of the mainshock subevents and the principal aftershock are shown in Figure 2. Our seismological analysis supports the suggestion of Natsag-Y 'm et al. [1971] and Baljinnyam et al. [1993] that most of the surface faulting occurred in the mainshock. The mechanism and slip vector of the first subevent is well constrained by first-motion polarities, but those of the second and third subevents, whose waveforms interfere with those of the first subevent, are inevitably less well determined. The second subevent is shown in Figure 2 with a component of shortening, consistent with an uplifted eastern side along most of the rupture in area b. The slip vector of the third subevent is shown as pure thrust faulting, consistent with the lack of observed strike-slip component on the thrust fault ruptures in area c [Baljinnyam et al., 1993]. The best determined mechanism is that of the principal aftershock on January 20, whose slip vector azimuth is well constrained to ø, close to that shown for the third subevent of the mainshock. In summary, it is clear that the slip vectors on the thrusts (220 ø for the third mainshock subevent and 211 ø for the aftershock) are different from that of the strike-slip faulting in the north of the main strike-slip ruptures (area b), following the western side of the range front east of Mogod for first mainshock subevent (182ø), but the apparent difference in slip vectors between the second (strike slip, a distance of 15 km as a series of disconnected fissures 204 ø) and third (thrust, 220 ø) mainshock subevents is and fractures with vertical offsets of less than 0.4 m, sometimes arranged as left stepping tensional cracks, suggesting right-lateral strike slip. Other minor cracks occurred in the valley floor (area d). The NW-SE thrust not well resolved. For the purposes of this paper our main interest is in the thrust fault surface ruptures (area c), which follow the ridge of Tfileet Uul SE from the southern end of fault ruptures (area c) show a maximum vertical compo- the N-S strike-slip fault (Figure 2). These ruptures are nent of m near their junction with the strike-slip faulting and decrease in height toward the SE, reaching typical values of 2-3 m in the central part and eventually dying out at the SE end [Natsag-Y 'm et al., 1971; Baljinnyam et al., 1993]. P waveforms of the main shock on January 5 show that rupture occurred in three subevents, the first two having strike-slip mechanisms and the third having a substantial thrust component [Huang and Chen, 1986]. We reanalyzed both the P and $H waveforms of the mainshock and the largest aftershock of January 20. Our analysis will be reported in detail elsewhere, but our results broadly confirm Huang and Chen's [1986] conclusions, including their suggestion that the secclose to the ridge crest on the SW side, even crossing over the crest toward the SE end, such that the scarp sometimes faces uphill. It is clear from this pattern that repeated slip on this particular rupture surface cannot have produced the present topography [Baljinnyam et al., 1993]. However, we believe that the Tiileet Uul ridge was produced by motion on similar, subparallel surfaces to that which moved in 1967, for two reasons. First, the elevation of the Tiileet Uul ridge and the displacement on the 1967 surface ruptures die out together toward the SE, approximately coincidently (Figure 3). Second, the total height, or relief, of the topographic step associated with the ridge dies out to the SE in the same way. This is indicated in Figure 3, which shows ond and third subevents occurred south of the first. the profile of the ridge crest (profile b-b ) and parallel The largest subevent of the mainshock was the sec- profiles (profiles a-a and c-c ) 3 and 4 km SW and NE ond (Mw 6.8) and is likely to have been responsible of the ridge crest, respectively. The difference between for the ruptures in area b of Figure 2. By positioning profiles b-b and a-a and between profiles b-b and c-

4 BAYASGALAN ET AL.- STRIKE-SLIP FAULT TERMINATIONS ø O0'E 103 ø 15'E! 48 ø 15'N 48 ø 15'N 48 ø 00'N 48 ø 00'N ::: i':' ½.-'.-:.-"'" - km 103 ø 00'E 103 ø 15'E Figure 2. Summary map of the 1967 Mogod earthquakes (see Figure 1 for location). The main surface fault ruptures (areas b and c) are shown in heavy lines and are taken from Baljinnyam et al. [1993]. The three mainshock subevents (marked 1, 2, and 3) and the mechanism of the principal aftershock (670120) are taken from our analysis of the P and $H waveforms (to be published elsewhere). Slip vectors are shown with large white arrows. All aftershocks were located relative to the mainshock by the method of Jackson and Fitch [1979], and the pattern was placed geographically by locating the second mainshock subevent at the northern end of the main continuoustrike-slip surface ruptures (area b).

5 398 BAYASGALAN ET AL.: STRIKE-SLIP FAULT TERMINATIONS A -" "-'.---" :_.,....-.,...,?.,:._...-..,._.-:-.._...-_.,,-:.... [ """ "" '= ' ii' z..-.' "'.'---.:: '"'- :' i.-'":----' ':-...- :- '-' '- '.,_ :... "" '"'"' : :,..._.._...,.. ',.... ; :i""'" :"-"' ': : ' "- i'"':':':':'"=' ' '"' ':'=':'"-"; ='½"---'-...--"i '?;!;!!, -:...: ':'* ;a:;... '.-,-----,, --,...,,...,--,,?'.-..=?"'----':-.----'.',?.'-m. *--?.- ':'. e '-". ; '-'; (m) 2000 ß ;., -. '":'=":'"' :'"'": '"-'-:::::..,.'"' -,½' /-'... b-b' B (m) (in) (m) Figure 3. (a) Perspective view of the Tiileet Uul ridge, looking NW. (b) Three topographic profiles from NW (left side) to SE (right side) along the ridge crest (profile b-b') and parallel to the ridge on SW (profile a-a') and NE (profile c-c') sides. (c) The relief along the Tiileet Uul ridge. The differences between profiles a-a' and b-b' and between profiles b-b' and c-c' are likely to approximate the cumulative displacement on the thrust faults responsible for the ridge's morphology. The point where the N-S strike-slip fault joins the Tiileet Uul ridge is marked as ss. The thrust ruptures die out in the SE near X. Numbers below the zero line are the vertical scarp heights reported at various points by Baljinnyam et al [1993].

6 BAYASGALAN ET AL.: STRIKE-SLIP FAULT TERMINATIONS øE 100øE 101øE 102øE 103øE... "-' ' ' "'" ::*: *'-";% "... : ":-' ---' - : :' --'-"ø-": ' '. '::. A'::'*:-"-;';.... '..":,:'::::; ':. ::.-..' ;-::: :.; ' ' '.'. : *: : :!... m` : : :...: :: :: C:.? ' :,::::::,-,.**:.;.:,;. ;'.. i....,... :::*' "'":"* '" *::'*: "'"' '"'... '-*,*: '"'&:: :"-*- '... ::':*:... '::' :':':'* ' +... "'... ':"" ':-":... - ' -"' """*':""i :::': ';"*'*- *'i"*...,::..--?,.---.::,.:...,-:***., :"-" J4.,:;iii:: i;i,...::;,,,;.,:::}i,,,';i** : :,,:...- ::,.:..:;:;';;:... :?",-...:,.,...- "'" '... :':"-"-,-":a -L.,_:,...-.-,,:..!..h""-:-'"--:':":... :.:.'".;.;:: :: ::::.::-:: ".";'"::::'*,::i' -' -;.:ii!(i? ;i. *,., :,,: -*.:..;... ::* ::::*::.*:...' ;":*':: : ::-'-'"....:."i '","-...'"'m...-.',. ;...-' i½*.'.*:'.f**f;, -"-'-*'"'"'q..---: '?;:..:: o,;..-..'¾.%r'::.j'"!4i:' J :... [ : :.:..::'-'-."-.'":... : ½.:..,, 'ff..... o.... :.,:.....-: ::.::..::.,... :.:..::::.....:..: :.:::..:: , :::..:... ` k :`...`. e..:. :;:`2.. :..:: :`..: :.:*: ::%:.:... :...:...:...`....:...>..: : :::: : ::.. :.`.. : :.:; :.:.:. :...::x::.-..- :,::,.½.?....'...., ß '.,..':.":' -, -" :':'... ::i:: iii ::.::;i.:.;::i:i:.;:...:'..:...' -ii. iõ! i ½i!i:i ':.'.:½:-::L..-'.-:. "':- : :i5:::. '... ß :{::. ' ' :::.:-;:.:-..i 44 øn, ' ' ':' øn 99øE 100øE 101 øe 102øE 103øE Figure 4. Summary map of the eastern Gobi-Altay of Mongolia, showing the extent of the 1957 earthquake ruptures along Ih Bogd and Baga Bogd (solid lines) and the en echelon relationship with the faulting along Artz Bogd (dashed lines). T marks the Toromhon thrust. East of Artz Bogd, there are no other obviously active structures. c, which reach a maximum where the N-S strike-slip faulting joins the ridge, is likely to represent the cumulative topography on the ridge thrust system, and this too dies away, from a maximum at the join with the strike-slip fault to the SE, coincident with the 1967 surface ruptures. The 1967 ruptures end close to where the ridge crest elevation and the relief begin to decrease most rapidly (marked X in Figure 3c). In summary, the N-S Mogod strike-slip fault ends in a thrust (or reverse fault) on one side, the displacement on which dies away over a distance of ~15 km. The slip vector azimuth on the thrust is not parallel to the strike-slip fault, but as the strike-slip fault may have a shortening component, we cannot be certain that the slip vectors on the two faults are different The Eastern End of the 1957 Ruptures on the Bogd Fault, Eastern Gobi-Altay The Gobi-Altay Mountains of Mongolia consist mainly of E-W trending left-lateral strike-slip faults, most of which have a reverse component of slip that is responsible for uplift of the elongate ranges. In this section and sections we are concerned with the east- ern end of the Gobi-Altay fault system, where faulting along the Ih Bogd and Baga Bogd massifs dies out near longitude 102øE but another fault then continues the deformation farther east along the north side of Artz Bogd (Figure 4). The Baga Bogd and Artz Bogd faults are in an en echelon right-stepping arrangement. The eastern ends of both faults show features that resemble the geometry and morphology seen at Mogod. The 1957 Gobi-Altay earthquake (M o~8.0) was one of the largest earthquakes in Asia this century [Molnar and Deng, 1984] and was responsible for ~260 km of left-lateral strike-slip surface fault rupture. The surface faulting has been reported in detail by Florensov and $olonenko [1963], Baljinnyam ei al. [1993], and Kurushin et al. [1997]. Here we are concerned with the eastern end of the 1957 earthquake faulting near ( lziyt Uul (Figures 5 and 6), where the ESE trending leftlateral strike-slip motion along the north side of Baga Bogd changes from a relatively localized and simple rupture with typical horizontal offsets of 3-4 m (which characterizes much of the length of the main 1957 strike-slip fault) to a more complicated system of ruptures with apparently smaller horizontal offsets (probably < I m but difficult to measure) [Kurushin et al., 1997]. The easternmost 1957 ruptures are N-S trending thrust faults, vergent both to the east and west, which decrease in vertical displacement southwards over a distance of ~7 km from a maximum value of ~1.2 m at the eastern end of the strike-slip fault to ~0.1 m near Huts Uul [Kurushin et al., 1997]. At their northern end the thrusts formed in 1957 follow subdued preexisting scarps typically 2-3 m high, presumably related to earlier slip events. At their southern end, where displacements are less, there is no clear evidence for preexisting scarps. The slip vector on these thrusts is not well constrained. In 1997 we observed that both north and northeast of Huts Uul (Figures 5 and 6), the scarps included some right stepping en echelon mole tracks, implying a possible component of right-lateral motion as well as the

7 400 BAYASGALAN ET AL- STRIKE-SLIP FAULT TERMINATIONS 101ø45'E 102ø00'E /// /"'"' /I"/ 5!on I 44ø50'N nu't'... :... 44o40 'N 0 BOG o 44ø40'N. I 101ø45'E I 102o00'E Figure 5. Map of the faulting at the eastern end of Baga Bogd and the western end of Artz Bogd, in the eastern Gobi-Altay of Mongolia (see Figure 4 for location). The 1957 earthquake ruptures occur mainly on the north and northeast side of Baga Bogd and are from Kurushin et al. [1997]. The thin lines in this region are part of the main strike-slip ruptures but have a vertical component also, with the south side up (marked u) relative to the north (marked d) In 1996 we observed a small segment of thrust faulting, 2 km long with a 30 cm high scarp, at Taragt (marked T), which also moved in The approximate slip vector azimuth (marked SS) on the strike-slip fault is relative to the north side. Faulting on the south side of Baga Bogd and northern side of Artz Bogd are from our own field observations. Shaded areas are elongate anticlinal ridges. Circled letters and numbers are structures discussed in the text. shortening. The slip vector azimuth on these thrusts may therefore be slightly oblique to the slip vector on the strike-slip fault (thought to be ø) [Baljinnyam e! al., 1993; Kurushin et al., 1997]. parallel with the faulting at Mogod is nonetheless obvious. The strike-slip fault ends with thrusting on one side, whose displacement dies away from the strike-slip fault. The N-S thrust ruptures formed in 1957 are in almost fiat alluvial fan deposits 1-2 km east of the N-S range 2.4. The Toromhon Thrust, Eastern front marking the east end of Baga Bogd, which, if it is an older thrust fault, is severely eroded. To some extent, the N-S gradient in vertical displacement seen in Gobi-Altay For comparison with the east end of Baga Bogd and for later discussion, we note at this point one other the 1957 thrust ruptures is mirrored in the t..opography place where the 1957 earthquake ruptures are associated in this region, with the highest elevation (Olziyt Uul; with N-S thrust faulting. This location is at Toromhon, 2058 m high and 5 km west of the thrusts) near the at the eastern end of the Ih Bogd massif (Figures 4 strike-slip fault and a gentle slope to the south. How- and 7), in a saddle between that mountain and the ever, the origin of this geomorphology is ambiguous; next big massif of Baga Bogd. In the 1957 earthquake the ESE strike-slip ruptures are associated nearly ev- the Toromhon thrusts had substantial surface displaceerywhere with a south-side-up reverse component of slip ments of 3-5 m over 15 km of their ~20 km length from [Kurushin et al., 1997], which could also produce such the strike-slip fault [Kurushin et al., 1997]. In its NNE a slope. Thus, although the association of the 1957 thrust ruptures with the overall geomorphology is not clear, the trending northern part a right-lateral component of slip was observed, but in its SSE trending southern part it had a left-lateral component, so its overall slip vector

8 BAYASGALAN ET AL.- STRIKE-SLIP FAULT TERMINATIONS 401 E102'2' I E102'3' I N44'48' N44'48'!ziyt Uul N44'47' N44'47' N44'46' N44'46' N44'45' N44'45' N44'44' N44'44' E102'2' E102'3' E102'4' E102'5' E102'6' Figure 6. Air photo mozaic of the eastern end of the 1957 Gobi-Altay earthquake ruptures (see Figure 5). The arrow marked SS is the approximate slip vector azimuth on the strike-slip fault. The vertical offset on the thrusts (h) is in meters [from Kurushin et al., 1997]. may be approximately E-W, as is inferred for the main strike-slip fault [Baljinngtam et al., 1993; Kurushin et al., 1997] The Eastern End of Artz Bogd, Eastern Gobi-Altay The morphology of the steep, linear range front bound. ing the north side of Artz Bogd resembles in many ways the range front followed by the main 1957 Gobi-Altay strike-slip earthquake fault, but it lacks any continuous expression of Holocene surface rupture. At one place (at longitude 102ø31 E) we found unequivocal evidence of a left-lateral offset, where a beheaded stream was displaced 24 m at the range front. Thus we assume that the range front is created by a left-lateral fault with a reverse component. The E-W range front ends abruptly at longitude 102ø4TE, where it joins a N-S trending system of thrust faults (Figure 8). The reverse nature of the easternmosthrust in Figure 8a is revealed in the contorted white sandstones and conglomerates of Cretaceous age [Devgtatkin, 1970] in the footwall adjacento an exposed gouge zone (at latitude 44ø26.15 N) and in the anticlinal ridge formed in volcanic rocks in the hanging wall. The topography associated with the main N-S

9 402 BAYASGALAN ET AL.' STRIKE-SLIP FAULT TERMINATIONS 'E 101 ø00'e 11ey of Lakes 45o00'N 45ø002q Bogd fault Toromho n ß 1 v ' 44ø50'N 44ø50'N 100o45'E 101ø00'E Figure 7. Map of the 1957 earthquake faulting (heavy lines) in the Toromhon region, based on Kurushin e! al. [1997]. See Figure 4 for location. Strike-slip offsets are shown in meters; scarp heights (h) on thrust faults are also in meters. The sense of vertical motion on the strike-slip fault is marked up (u) or down (d). thrust dies away to the south over ~40 km, such that the highest elevation (2100 m) is near its northern end, close to the join with the E-W trending strike-slip fault, and at its southern end the hanging wall merges with the surrounding desert (1300 m). This particular aspect of the geomorphology is much clearer here than at the eastern end of the 1957 earthquake ruptures (Figure 8), with the thrust continuing south beyond any uplift associated with a reverse component on the E-W strikeslip fault. Thus we assume that displacement on the N-S thrust system decreases away from the strike-slip fault. We could not find any clear evidence of strike-slip movement on the N-S thrusts and so cannot constrain their slip vector. However, this geometry is similar to that at Mogod (Figure 2) and at the eastern end of the 1957 ruptures (Figure 5), although the thrust is longer in this case. East of the N-S thrusts in Figure 8a, the landscape is essentially fiat, and the Artz Bogd structures appear to be the most eastern active faults of the Gobi-Altay. In summary, the strike-slip faults at Mogod, eastern Baga Bogd (the 1957 Gobi-Altay earthquake fault), and Artz Bogd all end in thrust faults on one side, with displacements on the thrusts decreasing away from the strike-slip fault The Overlap Region Between the Baga Bogd and Arts Bogd Faults So far, the thrusts we have described have been at a high angle, almost perpendicular, to the major strikeslip faults. Along the Ih Bogd-Baga Bogd-Artz Bogd ranges other active thrusts or reverse faults also occur that are subparallel to the main strike-slip faults. Several of these thrusts have been described by Florensov and $olonenko [1963], Baljinnyam e! al. [1993], and Kurushin et al. [1997]. They are likely to be related to the accommodation of the shortening component of motion that accompanies the strike-slip in these ranges and to

10 BAYASGALAN ET AL' STRIKE-SLIP FAULT TERMINATIONS 403 N44-40 '_ E1 EIp3': N44'30'-- --N44O20 ' ' Kilometers ' a) --N44O10 ' E1 N44ø40 ' g EI02'10'! EIO '! N'44'30' N44ø30 ' N44'20' b) El02' E102' 10' E102'20' E102'30' E102'40 N44'10' - Figure 8, (a) Topographic map of the eastern end of the Artz Bogd fault system. (b) Russian KOSMOS satellite image of the eastern end of the Artz Bogd range. See Figure 4 for location.

11 .. ß BAYASGALAN ET AL.- STRIKE-SLIP FAULT TERMINATIONS Figure 9. (top) SPOT-P (( )CNES 1992, distribution by SPOT IMAGE) imageof the "foreberg" scarps (F1 and F2) and anticlinal ridges (A1 and A2) north of the western end of Artz Bogd (see Figure 8). (bottom) Detailed air photo showing the drainage pattern on the asymmetric anticline ridge (A1) developed in the alluvial fans and muds. This anticline is steepest on its southern side and presumably covers a thrust dipping north. the development of the "flower" like structural features ures 5 and 9) ~15-20 m high, developed in alluvial fan discussed by Cunningham et al. [1997] and Bayasgalan deposits. On the southern sides of the scarps the fans et al. [1999]. are uplifted, incised, and tilted. These scarps resemble Abundant evidence of active shortening parallel to the "forebergs" described along the Ih Bogd-Baga Bogd the ranges is found in the region where the Baga Bogd ranges by Florensov and Solonenko [1963], Baljinnyam range dies out in the east as the Artz Bogd range be- et al. [1993], and Kurushin et al. [1997], and they repregins ~15 km to the south, in an en echelon overlapping sent south dipping thrusts or reverse faults which come arrangement (Figures 5 and 9). At the western end close to and perhaps break the surface (as others did in of Artz Bogd, north of the range front, there are two 1957). Nearby, three asymmetric anticlines (A2-A4 in steep north facing scarps (marked F1 and F2 in Fig- Figures 5 and 9), all steeper on their northern sides, also

12 BAYASGALAN ET AL.: STRIKE-SLIP FAULT TERMINATIONS 405 b) q Figure 10. (a) Plan view of a set of E-W left-lateral faults that accommodates N-S right-lateral shear by rotating clockwise. This principle operates in northeast Iran and eastern Tibet. Note the overlap or shortening implied by the hatched regions at the eastern ends of the faults. (b) Cartoon summary of the coseismic faulting and slip vectors at Mogod. Large solid arrows are slip vector azimuths relative to the side of the strikeslip fault marked "fixed". The curved arrow shows the sense of rotation within the hanging wall of the thrusts. See text. occur in alluvium and valley fill [see also Cunningham et al., 1997]. These, too, representhe surface expression of buried south dipping reverse faults. The left stepping en echelon arrangement of these thrust scarps and anticlines along the front of Artz Bogd is consistent with a left-lateral component of motion. Fault scarps from north dipping reverse faults occur on the south side of Baga Bogd (at B1 in Figure 5 and farther west) and are an important feature on the south side of the Ih Bogd massif also [see Florensov and Solonenko, 1963; Baljinnyam et al., 1993; Kurushin et al., 1997]. In contrast, along Artz Bogd, reverse faults and folds are found only in this region of the overlap with Baga Bogd and not farther east. The intensity of the shortening in the region of the overlap between Baga Bogd and Artz Bogd suggests that it may be related to the interaction between the two strike-slip systems. In section 3 we discuss a possible explanation for this behavior. 3. Kinematics and Rotations The principal features at the ends of the Mogod, Baga Bogd, and Artz Bogd strike-slip faults are thrusts whose displacements die away with distance from the strikeslip faults. Such a gradient in displacement along a thrust fault implies a relative rotation between its hanging wall and footwall blocks about a vertical axis. This is not surprising, as we have already indicated that we expect intracontinental strike-slip faults to be associated with rotations. Thus the schematic cartoon in Figure 10a shows the plan view of a deforming zone that accommodates N-S right-lateral shear against a rigid 9 boundary by left-lateral strike slip on E-W faults that rotate clockwise. This is essentially the geometry suggested for NE Iran [Jackson and McKenzie, 1984] and eastern Tibet [England and Molnar, 1990; Holt et al., 1991]. It allows the E-W faults to terminate in triangular regions of shortening at the eastern ends of the rotating blocks. The shortening can then be accomole modated by thrust faults whose displacements decrease with distance from the strike-slip faults. However, the geometry in Figure 10a implies the same slip vector at the ends of the strike-slip faults as that on the thrusts, which may not be what we always see. Figure 10b shows a sketch of the situation at Mogod. The slip vector on the thrust is virtually perpendicular to the fault, with no strike-slip component, implying a relative rotation between footwall and hanging wall about a pole near the SE end of the thrust fault. As mentioned in section 2.2, there is some uncertainty about the slip vector on the main strike-slip ruptures, but it may be close to pure strike slip and thus different from that on the thrust. If the slip vectors are truly different, then they require something else to be happening: either substantial internal deformation of the rotating fault-bounded blocks or perhaps accommodation of a shortening component on other thrusts subparallel to the strike-slip faults. At the ends of the Baga Bogd and Artz Bogd strike-slip faults the slip vectors on the thrusts are not well constrained. We mention this potential difficulty with the slip vectors because the cartoon in Figure 10a should not be taken too literally and because it has later consequences (Figure 11). Thus, at the ends of the strike-slip faults that we have described, we believe that relative rotations of the fault-bounded blocks are a likely consequence of the geometry we see, regardless of whether those blocks can be regarded as internally rigid (i.e., whether the slip vectors on the thrusts and strike-slip faults are the same). However, as Figures I and 10a make clear, these rotations cannot really be considered in isolation; they are part of the way in which the faulting accommodates the regional deformation. On a larger scale, what these geometries really allow is a change in rotation rates between adjacent regions: in the case of Figure 10a, be- tween the deforming zone on th%:ii West and the rigid region in the east. This may be e' ecially important if the style of the regional deformation changes from one dominated by strike-slip faulting to one dominated by thrusting. Figure 11 illustrates this concept. Figure 11a shows a broad deforming zone between two plates, plates A and B, that are converging in a N-S direction. In the west, shortening is achieved by thrust faults that do not rotate about a vertical axis, whereas in the east it is achieved by right-lateral strike-slip faults and thrusts that rotate counterclockwise, as is illustrated in Figure 1lb. The junction between the two faulting styles is achieved by rotating thrust faults at

13 406 BAYASGALAN ET AL.- STRIKE-SLIP FAULT TERMINATIONS b) V V&V& &.....v,,,/..v v a, 'V v... v XX_X PLATE B v v Figure 11. (a) Plan view of a deforming region between two converging rigid plates. Plate B moves north relative to plate A. The convergence is achieved by parallel thrust faults that do not rotate in the west but by rotating strike-slip faults in the east. The rotating, or pivoting, thrust faults at the ends of the strike-slip faults allow the transition to be made between the rotating and nonrotating domains. (b) Close-up sketch of the rotating thrust and strike-slip faults. If the slip vectors on the thrusts and strike-slip faults are not identical, then the rotating blocks cannot be rigid but must deform internally. This problem is reduced if the strikeslip faults have a shortening component as well (see text). the ends of the strike-slip faults. Thus, although the strike-slip faults end, the deformation associated with their rotating thrusts does not but continues west into the region of nonrotating thrusts. We chose to illustrate Figure 11 with right-lateral faults because its configuration resembles that at the northern end of the NW-S E right-lateral strike-slip faults of the Mongolian Altay (Figure 12). These strike-slip faults turn into E-W thrusts at their NW ends, and the topography suggests that the thrusts die out toward the west. West of.,,88øe, the intense strike-slip faulting of the Mongolian Altay changes to a pattern dominated by E-W thrusts of the NE Tien Shan. Shortening in a N-S direction can be a( hieved everywhere, but the strike-slip faults (and the thrusts at their ends) must ro- tate anticlockwise, whereas the thrusts in the west need not. This suggestion is supported by the limited palcomagnetic data available. Thomas et al. [1998] found no significant rotation of palcomagnetic declinations in Oligocene and Miocene sediments in the Zaisan basin (Z in Figure 12) but a 35ø4-15 ø counterclockwise rotation of upper Miocene-middle Pliocene sediments in the Chuya depression (C in Figure 12). In terms of the velocity field describing the overall deformation in Figure 11, there are two principal differences between the west and east parts: (1) in the east there is an anticlockwise angular velocity (or vertical vorticity) which is missing in the west, and (2) the strike-slip faulting in the east requires significant gradients in the E-W velocity along strike in that part of the zone. This sideways "expulsion" of material from the converging zone follows because strike-slip faults conserve surface area, whereas thrusts do not. These lateral velocity gradients can be reduced if the strike-slip faults also have a reverse or shortening component [see McKenzie and Jackson, 1983, 1986], and it is presumably such shortening that is responsible for the high mountains of the Mongolian Altay (Figure 12). The component of shortening may be localized on the strikeslip faults, thus allowing them and the thrusts at their ends to have the same slip vector, but it could also be partitioned onto thrusts parallel to the strike-slip faults instead. Either way, some shortening of the blocks bounded by the strike-slip faults must occur if lateral velocity gradients are to be reduced. Such shortening is likely to be most noticeable where the parallel strikeslip faults are close together, and herein might lie an explanation for the intense shortening perpendicular to the strike-slip faults seen in the en echelon step between Baga Bogd and Artz Bogd (Figures 5 and 9). In this narrow step the shortening must then be more localized (Figure 13), but the effect is general and may be the origin of the oblique slip vector and high topography associated with the strike-slip fault systems of both the Mongolian and Gobi Altay ranges. Thus the principal significance of the fault terminations we describe is that they allow the style of faulting to change spatially, while still achieving the overall regional deformation, as is illustrated in Figures 11 and 12. The change could be between dominantly strike slip and dominantly dip slip (Figures 11 and 12) or between any two sets of conjugate strike-slip faults that rotate in opposite directions [e.g., Freund, 1970], such as the Mongolian Altay and the Gobi-Altay (Figure 1; though in this particular case we have no detailed structural evidence to support this suggestion). In either case the change in structural style should be accompanied by a change in the rotation rate (or sense) about a vertical axis.

14 BAYASGALAN ET AL.' STRIKE-SLIP FAULT TERMINATIONS 407 U.! u.! Z Z Z U.! LtJ U.! LLI Z Z

15 408 BAYASGALAN ET AL.: STRIKE-SLIP FAULT TERMINATIONS a) Baga Bogd Artz Bogd its great length (~260 km). One possibility is that it increased in length incrementally with each earthquake, with the relatively small growth increments expected from theoretical models [Cowie and $cholz, 1992]. Alternatively, it may have grown, perhaps more quickly, by merging originally disconnected parallel segments along strike (some of which may have been inherited from previous deformation, as suggested by Cunningham et al. [1997]), each of which ended in thrust faults. In the latter case it would leave behind clues to its ori- gin in the form of: (1) older thrust faults roughly perpendicular to the trend of the strike-slip fault and (2) variations in total strike-slip offset along the fault, as the linking sections between the original strike-slip segments will be younger and lack the displacement accub) mulated by the rotating thrust faults. Could this be the significance of the Toromhon thrust (Figures 4 and 7)? Figure 13. Cartoons to illustrate the regions of the shortening (hatched areas) at the ends of and in the Was it formed as part of a thrust system at the end of an Ih Bogd strike-slip segment, abandoned when the strikeoverlap region between the Baga Bogd and Artz Bogd slip fault propagated east to join a Baga Bogd segment strike-slip faults, showing (a) before rotation and (b) farther along strike, and then partially reactivated in after rotation. the 1957 earthquake? Reactivation of preexisting faults is a common occurrence, and a consequence of slip on the Toromhon thrust system (pointed out by Kurushin 4. Implications for Fault growth et al. [1997]) is a variation in the coseismic strike-slip component on the main 1957 fault, with a larger strike- The simple ideas presented here may have impli- slip displacement to the west of Toromhon than to the cations for the lengthening of strike-slip faults along east. Whether such variation is mirrored in the total strike. Thrust faults that localize the deformation at cumulative offset along strike is not known. Propagathe end of strike-slip faults prevent those faults from in- tion of strike-slip faults by linking segments along strike creasing in length, as the strike-slip displacement is con- is favored by the stress perturbation caused by strikeverted into shortening and topography. However, thrust systems tend to evolve by propagating new thrusts into their footwalls, and many such examples have been described in central Asia [e.g., Avouac et al., 1993; Meyer et al., 1998]. When this happens, the strike-slip fault may lengthen to utilize the new thrust at its end, thus slip rupture [see, e.g., Cowie, 1998] and in this case would lead to the notably irregular shape of the final strike-slip fault which is the characteristic of the eastern Gobi-Altay (Figure 14). Meyer et al. [1998] suggest a similar evolution for the major strike-slip faults in the NE Tibetan plateau. abandoning the older one, and in this way the strike-slip fault may grow in discrete stages. It is also possible that growth in this manner will allow the strike-slip fault to 5. Discussion eventually meet and merge with another strike-slip fault that was originally beyond its termination. In this way the strike-slip system could increase its length relatively This study was motivated by the observation that many strike-slip faults on the continents do not link continuous plate boundary like structures in the way quickly by merger, rather than by steady increments, that transform faults do in the oceans. In such cases which is a process that has been observed in dip-slip faults [e.g., Trudgill and Cartwright, 1994; Dawers and Anders, 1995; Jackson et al., 1996]. The present activity on the 1957 Gobi-Altay earthquake fault probably began no more than 5 Myr ago [Kurushin et al., 1997]. It appears to have a relatively where the strike-slip faults terminate rather than continue as a plate boundary, it is very likely that they are involved in rotations about a vertical axis, as Freund [1970, 1974] realized some time ago. The strike-slip terminations we have discussed in Mongolia are probably quite common, and similar examples have been low average slip rate (< 2 mm/yr) and a consequently described elsewhere, notably in Asia (discussed below). long recurrence interval for major earthquakes (probably the order of thousands of years) [see Ritz et al., 1995]. Thus an interesting puzzle about the Gobi-Altay earthquake fault (and many other long strike-slip faults However, we emphasize that they are not the only ones that occur, and others are certainly possible. Freund [1974] points out that strike-slip faults can terminate by splaying into multiple strike-slip strands, as happens in in Asia) is in what manner and how quickly it achieved New Zealand [Freund, 1971; Little and Roberts, 1997]

16 BAYASGALAN ET AL.: STRIKE-SLIP FAULT TERMINATIONS 409 Figure 14. Topography of the Gurvan Bogd mountains (comprising Ih Bogd, Baga Bogd, and Artz Bogd) in the eastern Gobi-Altay and their associated strike-slip and thrust faults. We speculate that Ih Bogd and Baga Bogd faults were once separate strike-slip segments (solid lines) that later joined up (dashed line) and that may yet join up with Artz Bogd (dotted line). or by abutting against another strike-slip fault, as the the great 1951 Beng Co earthquake (M,o~8) in Tibet Garlock fault does against the San Andreas in Cali- terminated in the extensional Gulu graben [Armijo et fornia. Both these schemes will involve rotations [e.g., al., 1986, 1989]. Finally, in the Aegean Sea a series of Gaffunkel, 1974]. Nonrotational terminations are also subparallel NE-SW right-lateral strike-slip faults termipossible in some circumstances, for example, where a nate in the SW against a series of normal faults that strike-slip fault forms a boundary between two extend- rotate clockwise [Tagtmaz el al., 1991]. ing regions in which the extension is spread over dif- In summary, although strike-slip terminations of the ferent distances or achieved by normal faults tilting in type we have described in Mongolia are not the only different directions on either side of the strike-slip fault ones that can occur, the fact that they are recogniz- [e.g., Davis and Burchfiel, 1973; Stewart, 1980]. able in a number of places suggests they are important. The strike-slip terminations we have described in Their tectonic significance is likely to be in allowing the Mongolia involve thrust faults whose displacement de- regional deformation to continue to be accommodated creases with distance from the strike-slip fault. Simi- while the faulting changes in style, either from one domlar features have been described from other places in inated by strike slip to one dominated by dip slip (e.g., Asia. Thus the 1967 Dasht-e-Bayaz and 1997 Zirkuh between the NE Tien Shan and the Mongolian Altay) earthquakes in Iran both involved strike-slip faults that or from right-lateral to left-lateral conjugate strike-slip terminated in thrusts whose topographic expression de- sets (e.g., between the Mongolian Altay and the Gobicreases with distance from the strike-slip fault [Jackson Altay). The main consequence of such a change in fault and Fitch, [1979]; Berberian et al. [1999]. Burchild et style will be a change in the rotation rates about a veral. [1991], Zhang et al. [1991], and Meyer et al. [1998] tical axis, which may be revealed by paleomagnetic decall describe similar terminations to the major strike-slip lination data (depending on the rates). In terms of the faults in NE Tibet. The Dzhungarian fault in the Tien overall velocity field representing the deformation, the Shan is a major NW trending strike-slip fault that ter- change in faulting will be revealed by a spatial change minates in E-W thrusts that apparently die out to the in the component of vertical vorticity. west [Tapponnier and Molnar, 1979]. The principles we Finally, we point out that by concentrating on kinehave discussed are not restricted to areas of shortening matics and geometry, we have ignored dynamical quesand may also be expected where strike-slip faults occur tions such as the relation between rotations in the upper in regions of extension. Thus the principal aftershock of crust and deformation in the deeper mantle lithosphere. the 1976 Tang Shan earthquake (M o7.8) in NE China We have consistently restricted ourselves to the question involved normal faulting at the end of the main strike- of how can these faults move and achieve the regional slip rupture [N5b lek et al., 1987]. Strike-slip rupture in deformation. With more detailed examples it may be

17 410 BAYASGALAN ET AL.: STRIKE-SLIP FAULT TERMINATIONS possible to address other questions such as whether the angle between the strike-slip fault and its terminating thrust is important or what controls the length of that thrust. At the moment, and with such limited data, these questions are beyond our scope. 6. Conclusions Deformation at the ends of large intracontinental strike-slip faults that do not simply link other major structures is likely to involve some sort of rotation about a vertical axis. In Mongolia we can see a particular pattern at the end of some strike-slip faults that consists of thrust faulting on one side, with a displacement that dies away with distance from the strike-slip fault, suggesting relative rotation of the footwall and hanging wall blocks of the thrust about a vertical axis. The tectonic significance of strike-slip terminations of this type is that they allow a dramatic change in the style of faulting, for example, from dominantly strike slip to dominantly dip slip or from left-lateral to right-lateral strike slip, while still allowing the regional deformation to be accommodated. In such cases the change in fault style will be accompanied by a variation in the rotation rate about a vertical axis, which may be detectable paleomagnetically, as in the junction between the Mongolian Altay and the NE Tien Shan. Strike-slip terminations of this type may also have consequences for the development and growth of ma- jor strike-slip systems. Strike-slip faults can lengthen when the thrusts at their ends propagate new thrusts into their footwalls, leaving behind abandoned older thrusts, which may occasionally be reactivated in earthquakes. Such behavior will lead to variations in cumulative strike-slip displacement along the major faults and may also allow them to merge with other strikeslip faults farther along strike to create longer faults, initially with rather irregular outlines in plan view. Acknowledgments. We thank Trinity College and Amerada Hess for supporting our work and providing a studentship for A. Bayasgalan while he was at Cambridge. A grant from the Royal Society helped with fieldwork, and one from NERC (GR9/02922) helped with remote sensing. Collaboration between Cambridge and Montpellier was enhanced through the Alliance Programme of the British Council and Minist[re des Affaires Etrang[res. We are grateful to P. Molnar, D. McKenzie, and D. Schwartz for careful reviews that greatly improved this paper. Cambridge Earth Sciences publication ES5554. References Armijo, R., P. Tapponnier, J.L. Mercier, and H. Tonglin, Quaternary extension in southern Tibet: Field observations and tectonic implications, J. Geophys. Res., 91, 13,803-13,872, Armijo, R., P. Tapponnier, andh. Tonglin, Late Cenozoic right-lateral strike-slip faulting in southern Tibet, J. Geophys. Res., 9 1, , Avouac, J.-P., P. Tapponnier, M. Bai, H. You, and G. Wang, Active thrusting and folding along the northeastern Tien Shan, and rotation of Tarim relative to Dzungariaand Kazakhstan, J. Geophys. Res., 98, , Baljinnyam, I., et al., Ruptures of major earthquakes and active deformation in Mongolia and its surroundings, Mere. GeoL $oc. Am., 181, pp. 62, Bayasgalan, A., J.A. Jackson, J.-F. Ritz, and S. Carretier, "Forebergs", flower structures, and the development of large intracontinental strike-slip faults: The Gurvan Bogd fault system in Mongolia, J. $truct. GeoL, in press, Berberian, M., J.A. Jackson, M. Qorashi, M.M. Khatib, K. Priestley, M. Talebian, and M. Ghafuri-Ashtiani, The 10 May 1997 Zirkuh (Qa'enat) earthquake (Mw 7.1): Faulting along the Sistan suture zone of eastern Iran, Geophys. J. lnt., 136, , Burchfiel, B.C., Z. Peizhen, W. Yipeng, Z. Weiqi, S. Fangrain, D. Qidong, P. Molnar, and L. Royden, Geology of the Haiyuan t ault zone, Ningxia-Hui autono- mous region, China, and its relation to the evolution of the northeast margin of the Tibetan plateau, Tectonics, 10, , Chen, W.-P., and H. Kao, Seismotecton- ics of Asia: Some recent progress, in The Tectonic Evolution of Asia, edited by A. Yin and T.M. Harrison, pp , Cambridge Univ. Press, New York, Cowie, P.A., A healing-reloading feedback control on the growth rate of seismogenic faults, J. Struct. Geol., 20, , Cowie, P.A., and C.H. Scholz, Growth of faults by accumulation of seismic slip, J. Geophys. Res., 97, 11,085-11,095, Cunningham, W.D., B.F. Windley, D. Dorjnamjaa, G. Badamgarav, and M. Saandar, Late Cenozoic transpressionin southwestern Mongolia and the Gobi-Altai- Tien Shah connection, Earth Planet. $ci. Lett., 1 0, 67-82, 1996a. Cunningham, W.D., B.F. Windley, D. Dorjnamjaa, G. Badamgarav, and M. Saandar, Structural transect across the Mon- golian Altai; Active transpressional mountain building in central Asia, Tectonics, 15, , 1996b. Cunningham, W.D., B.F. Windley, L.A. Owen, T. Barry, D. Dorjnamjaa, and J. Badamgarav, Geometry and style of partitioned deformation within a late Cenozoic transpressional zone in the east- ern Gobi Altay Mountains, Mongolia, Tectonics, 277, , Davis, G.A., and B.C. Burchfiel, Garlock Fault: An intracontinental transform structure, southern California, Bull, Geol. Soc. Am., &l, , Dawers, N.H., and M.H. Anders, Displacement-length scaling and fault linkage, J. $truct. Geol., 17, , Delvaux, D., K. Theunissnen, R. van der Meer, and N. Berzin, Dynamics and paleostress of the Cenozoic Kurai-Chuga depression of Gorny Altai (south Siberia) Tectonic and climatic control (in Russian), Russ. Geol. Geophys., 36, 26-45, Devyatkin, E.V., Geology of the Cenozoic of western Mongolia (in Russian), in Geology of the Mesozoic and Cenozoic of Western Mongolia, pp , Nauka, Moscow, England, P., and P. Molnar, Right-lateral shear and rotation as the explanation for strike-slip faulting in eastern Tibet, Nature, 3, , England, P., and P. Molnar, The field of crustal velocity in Asia calculated from Quaternary rates of slip on faults, G eophys. J. Int., 130, , Florensov, N.A., and V.P. Solonenko (eds.), The Gobi-Altay Earthquake (in Russian), 391 pp., Akad. Nauk, Moscow, (English translation, 424 pp., Isr. Program for Sci. Transl., Jerusalem, 1965.)

18 BAYASGALAN ET AL.: STRIKE-SLIP FAULT TERMINATIONS 411 Freund, R., Rotation of strike-slip faults in Sistan, southeast Iran, J. Geol., 78, , Freund, R., The Hope Fault: A strike slip fault in New Zealand, N.Z. Geol. Surv. Bull., 86, 49 pp., Freund, R., Kinematics of transform and transcurrent faults, Tectonophys., 21, , Garfunkel, Z., Cenozoic tectonic history of the Mojave desert California, and its relation to adjacent regions, Bull. Geol. Soc. Am., 85, , Holt, W.E., J.F. Ni, T.C. Wallace, and A.J. Haines, The active tectonics of the Eastern Himalayan syntaxis and surrounding regions, J. Geophys. Res. 96, 14,595-14,632, Huang, J., and W.-P. Chen, Source mechanisms of the Mogod earthquake sequence of 1967 and the event of 1974 July 4 in Mongolia, Geophys. J. R. astr. Soc., 8, , Jackson, J.A., and T.J. Fitch, Seismotectonic implications of relocated aftershock sequences in I_ran and Turkey, Geophys. J. R. astr. Soc., 57, , Jackson, J.A., and D.P. McKenzie, Active tectonics of the Alpine-Himalayan Belt between western Turkey and Pakistan, Geophys. J. R. Astron. Soc. 77, , Jackson, J., J. Haines, and W. Holt, The horizontal velocity field in the deforming Aegean sea region determined from the moment tensors of earthquakes, J. Geophys. Res., 97, 17,657-17,684, Jackson, J., J. Haines, and W. Holt, The accommodation of Arabia-Eurasia plate convergence in Iran, J. Geophys. Res., 100, 15,205-15,219, Jackson, J.A., R. Norris, and J. Youngson, The structural evolution of active fault and fold systems in central Otago, New Zealand: Evidence revealed by drainage patterns, J. Struct. Geol., 18, , Khilko, S.D., R.A. Kurushin, V.M. Kochetov, I. Balzhinnyam, and D. Monkoo, Strong earthquakes, paleoseismogeological and macroseismic data (in Russian), in Earthquakes and the bases oj seismogenic zoning oj Mongolia, Transactions 41, The Joint Soviet-Mongolian Scientific Geological Research Expedition, pp , Nauka, Moscow, Kurushin, R.A., A. Bayasgalan, M. lziybat, B. Enhtuvshin, P. Molnar, Ch. Bayarsayhan, K. Hudnut, and J. Lin, The surface rupture of the 1957 Gobi-Altay, Mongolia, earthquake, Spec. Pap. Geol. Soc. Am., 320, pp. 143, Lamb, S.H., and H.M. Bibby, The last 25 Ma of rotational deformation in part of the New Zealand plate boundary zone, J. Struct. Geol., 11, , Little, T.A., and A.P. Roberts, Distribution and mechanism of Neogene to present-day vertical axis rotations, Pacitic-Australian plate boundary zone, Sout} Island, New Zealand, J. Geophys. Res., 102, 20,447-20,468, McKenzie, D., Active tectonics of the Mediterranean Region, Geophys. J. R. astr. Soc., 30, , McKenzie, D., and J.A. Jackson, The relationship between strain rates, crustal thickening, paleomagnetism, finite strain and fault movements within a deforming zone, Earth Planet. Sci. Left., 65, , (Correction, Earth Planet. Sci. Left., 70, 444, 1984.) McKenzie, D., and J.A. Jackson, A block model of distributed deformation by faulting, J. Geol. Soc. Lond., 1 3, , ray-exposure dates: Application to the Bogd fault, Gobi-Altay, Mongolia, Geology, 23, , Schlupp, A., N6otectonique de la Mongolie occidentale analysge a partir de donn es de terrain, sismologiques et satellitaires, Ph.D. thesis, Univ. Louis Pastuer, Strasbourg, France, Stewart, J.H., Regional tilt patterns of late Cenozoic basin-range fault blocks, western United States, Bull. Geol. Soc. Am., 91, , Tapponnier, P., and P. Molnar, Active faulting and Cenozoic tectonics of the Tien Shan, Mongolia and Baykal Regions, J. Geophys. Res., 8, , Taymaz, T., J. Jackson, and D. McKenzie, Active tectonics of the north and central Aegean Sea, Geophys. J. Int., 106, , Thomas, J.C., R. Lanza, A. Kazansky, C.V.S. Zykin, N. Semakov, D. Mitrokhin and D. Delvaux, Paleomagnetic study of Cenozoic sediments in the Siberian A1- tai and the Zaisan basin, central Asia: Evidence for heterogeneous vertical axis Meyer, B., P. Tapponnier, L. Bourjot, rotations during the Tertiary (abstract), F. Metivier, Y. Gaudemer, G. Peltzer, in Active Tectonic Continental Basins, G. Shunmin, and C. Zhitai, Crustal edited by N.L. Dobretsov, J. Klerkx, thickening in Gansu-Qinghai, lithospheric mantle subduction, and oblique, strikeand N.A. Logatchev, pp. 67, Univ. Gent, Gent, Belgium, slip controlled growth of the Tibet plateau, Trudgill, B., and J. Cartwright, Relay- Geophys. J. Int., 135, 1-47, Molnar, P., and Q. Deng, Faulting associated with large earthquakes and the average rate of deformation in central and eastern Asia, J. Geophys. Res., 89, , Molnar, P., and H. Lyon-Caen, Fault plane solutions of earthquakes and active tec- tonics of the Tibetan plateau and its margins, Geophys. J. Int., 99, , Molnar, P., and P. Tapponnier, Cenozoic tectonics of Asia: Effects of a conti- ramp forms and normal-fault linkages, Canyonlands National Park, Utah, Bull. Geol. Soc. Am., 106, , Zhang, P., B.C. Burchfiel, P. Molnar, Z. Weiqi, J. Dechen, D. Qidong, W. Yipeng, L. Royden, and S. Fangrain, Amount and style of Late Cenozoic deformation in the Liupin Shan area, Ningxia autonomous region, China, Tectonics, 10, , A. Bayasgalan and J. Jackson, Univernental collision, Science, 189, , Nab lek, J., W.-P. Chen, and H. Ye, The Tangshan earthquake sequence and its sity of Cambridge, Bullard Laboratories, Madingley Road, Cambridge CB3 0EZ, England. ( bayas@esc.cam.ac.uk; jackson@esc.cam.ac.uk) imphcations for the evolution of the North S. Carretier and J.-F. Ritz, Labo- China basin, J. Geophys. Res., 92, 12,615- ratoire de G6ophysique et Tectonique, 12,628, Natsag-Y'tim, L., I. Baljiunyam, and D. Monhoo, Mongolian earthquakes (in Russian), in Seismic Regionalization of Ulan Bator, pp , Nauka, Moscow, Ritz, J.-F., E.T. Brown, D.L. Bourl s, H. Philip, A. Schlupp, G.M. Raisbeck, F. Yiou, and B. Enkhtuvshin, Slip rates along active faults estimated with cosmic- Universit6 Sciences et Techniques de Languedoc, Montpellier Cedex, France. ( ritz@dstu.univ-montp2.fr; Sebastien. C arret ier@ dst u.univ- mont p 2.fr ) (received November 18, 1998; revised February 16, 1999; accepted February 19, 1999.)