Appalachian orogenesis: The role of repeated gravitational collapse

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1 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page of XX Geological Society of merica Special Paper ppalachian orogenesis: The role of repeated gravitational collapse T.H. Bell and R. ewman School of Earth Sciences, James Cook University, Townsville, QLD, 4811, ustralia BSTRCT Orogenesis within the ew England ppalachians has classically been regarded as occurring discontinuously even though the collision of plates driving it was essentially continuous for 2 million years from the Taconic through the lleghanian orogenies. Structural, metamorphic, and age data obtained from the cores of porphyroblasts reveal a near continuous history of tectonism that is partitioned within and between outcrops as well as regionally. Very prolonged deformation and metamorphic histories predate the foliation parallel to bedding, and the oblique matrix foliations only reflect brief increments of the uplift path of these rocks back to the earth s surface. The matrix shows none of the structural effects of the path down into the crust. This deepening path is revealed by the sequences of foliations that developed about regionally consistent successions of foliation intersection axis trends preserved within porphyroblasts (FIs). Indirect coupling between plates throughout the period of collision resulted in horizontal shortening accompanied by subvertical foliation development, followed by crustal instability, collapse, and the formation of subhorizontal foliation, repeated over and over until orogenesis ceased. These cycles repeat on time scales as short as 1, to 5, years, but because of partitioning of the deformation, only the weakest rocks preserve much of this history. Shifting directions of relative plate motion every 5 to 3 million years also results in easily deformed rocks being protected by more competent ones, with none of them seeing the total history. Furthermore, if the bulk composition is not suitable for porphyroblast growth, none of this history will be recorded. The recurring role of gravitational collapse and the variable scale of partitioning of this type of deformation is obscured by repeated reactivation of the bedding parallel foliation in multiply deformed rocks containing porphyroblasts in the ew England ppalachians. It is also obscured by the lack of topographic relief relative to total crustal thickness. ITRODUCTIO significant development in tectonics in recent years has been the discovery that foliation inflection/intersection axes preserved within porphyroblasts (FIs) contain quantitative evidence of long histories of deformation and metamorphism that predate the foliation parallel to compositional layering in multiply tectonized rocks. It is possible to distinguish successions of FI trends that can be correlated regionally (Bell et al., 1998; erden, 24), dated (Bell and Welch, 22), and used to determine extended P-T-t paths that are fully integrated with orogenic movement directions and predate all structures preserved within the rock matrix (Kim and Bell, 25). Such work has revealed that 95 deformation is very heterogeneously partitioned through the earth s crust at all scales (Bell et al., 24). It also shows that we cannot readily distinguish multiple generations of foliations that have been reused, reactivated, or formed contemporaneous with the schistosity parallel to compositional layering in the rock matrix (Ham and Bell, 24). Consequently, structural geologists may have been correlating apples with oranges in rocks where there is a well-developed schistosity parallel to bedding, and they have related foliations from outcrop to outcrop across a locality as well as from region to region. Our perspectives on the amount of deformation history gained from structural mapping across the earth s surface are undergoing a fundamental shift as a result of this work.

2 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page of XX 96 T. H. Bell and R. ewman The regional impact of the effects of deformation partitioning associated with the development of subvertical foliations can be distinguished using the distribution of successive generations of FIs shown on a map (e.g., Bell et al., 24). However, the effects of heterogeneous deformation partitioning associated with the development of subhorizontal foliations are much more difficult to distinguish, map, and consider (e.g., Bell and Kim, 24). The reasons for this are: 1. The Earth s surface generally has very limited topographic relief relative to the scale of the crust. 2. The direction of bulk shortening that forms successions of subhorizontal foliations is always vertical; this differs from the direction of subhorizontal shortening forming successions of subvertical foliations, which changes with change in direction of relative plate motion. 3. Subhorizontal schistosities always form in a similar orientation to that in which all previous ones formed. 4. Bedding starts out subhorizontal and is readily reactivated once regional folds have formed during all subsequent deformations (Bell et al., 23). This results in a geologist s seeing very little of the effects of the many various deformations that formed a subhorizontal foliation. The pelitic Wepawaug Formation of the Orange-Milford Belt in the ppalachians of southwest Connecticut contains garnet, staurolite, oligoclase, biotite, and kyanite porphyro blasts preserving inclusion trail geometries that allow us to demonstrate that multiple subvertical and subhorizontal events have formed throughout cadian orogenesis. large-scale fold affecting this region, called herein the Wepawaug Fold, has been modified by multiple episodes of lateral shortening and regional amphibolite facies cadian metamorphism (Dieterich, 1968; ewman, 21). It provides an ideal structure for exploring the role of the intervening episodes of gravitational collapse that elsewhere in the ppalachians have alternated with numerous periods of lateral shortening that produced subvertical foliations. GEOLOGICL SETTIG The Orange-Milford Belt is composed of Early to Middle Palaeozoic (5 35 Ma) metasedimentary and metaigneous rocks and lies within the Connecticut synclinorium that extends north into Massachusetts and Vermont. The synclinorium was interpreted by Rodgers (1985) to be part of the larger Iapetus (Oceanic) Terrane, which also contains the Bronson Hill Belt, the Merrimack Zone, and Taconic allochthons (Fig. 1). This terrane is bounded on the west by the proto-orth merican (Continental) Terrane delineated by Cameron s Line and on the east by the valonian (Continental) Terrane. It preserves a Barrovian prograde sequence, with metamorphic grade increasing from the biotite zone in the east, through the garnet and staurolite zones, to the kyanite zone in the west (Fig. 2). The isograd traces are sublinear and cut across the hinge of the Wepawaug Fold (Fig. 2; Fritts, 1963, 1965a,b), an unusual cuspate or mullioned shaped structure that appears to be plunging to the north-northeast (Fritts, 1962). Calculated peak metamorphic temperatures are 4 C in the lowest-grade rocks in the east and 65 C in kyanite zone rocks in the west, with peak pressures ranging from 7. to 9. kb (gue, 1994). Peak metamorphism in the region is considered to be cadian (around 385 Ma; Palin and Seidemann, 199; Lanzirotti and Hanson, 1996), although there is evidence for reheating during lleghanian orogenesis ( 29 Ma; Clark and Kulp, 1968; rmstrong et al., 197; Moecher et al., 1997). The Wepawaug Formation, an interbedded pelitic and psammitic unit within the core of the Orange-Milford Belt, covers a 7 2 km area (Fig. 2) west of ew Haven (Fig. 1). It contains rare bodies of marble (Hewitt, 1973; Tracy et al., 1983; Palin and Rye, 1992) and local tonalitic igneous intrusives and extrusives (Fritts, 1962, 1963, 1965a,b) and has been correlated with the Waits River and orthfield Formations of Massachusetts and Vermont. It is considered to have a Siluro-Devonian depositional age (Fritts, 1962), although recent U-Pb geochronological studies tentatively suggest an Ordovician age (Lanzirotti, 1995). MTRIX STRUCTURL RELTIOSHIPS macroscopic parasitic fold in the east, which appears to be truncated against the Oronoque member in the west, is shown in Fig. 2. The origin of this truncational like character is uncertain. It has been suggested that in spite of the cuspate geometry, it forms the west limb of a regional fold (Fritts, 1962). Four events in the matrix related to episodes of regional deformation (D 1 to D 4 ) have been interpreted (Dieterich, 1968; ewman, 21) with axial plane structures S 1, S 2, S 3, and S 4 (Fig. 2). Bedding (S ) can only be identified mesoscopically where pelitic and psammitic portions of the Wepawaug Formation are preserved in contact with one another. t low grades, a microscopically distinguishable matrix crenulated cleavage that formed pre-s 1 is locally preserved within microlithons cut by a pervasive bedding-parallel differentiated crenulation cleavage, S 1 (Fig. 3). t the mesoscopic scale, S 1 is folded by D 2 into tight to isoclinal folds, and at the microscopic scale, S 1 is crenulated by S 2. t higher grades, S 1 and S 2 are difficult to distinguish macroscopically due to later folding and prograde matrix coarsening and cannot be spatially correlated across the region. S 2 strikes SSW-E and is the dominant schistosity across the region. It dips subvertically west in the eastern parts of the belt forming an axial plane cleavage to D 2 folds in S 1. third matrix deformation event, D 3, crenulates S 2 throughout the region and forms a gently dipping pervasive matrix fabric, S 3. The intersection lineation of S 2 on S 3 (L 2 3 ) plunges gently to the E or SSW (Fig. 2). Porphyroblasts of garnet, biotite, oligoclase, and staurolite are commonly enveloped by S 2, except in staurolite and kyanite grade samples where S 3 is the dominant pervasive fabric. fourth episode of deformation preserved in

3 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page of XX ppalachian orogenesis 97 ew York Cameron's Line Vermont Massachusets Connecticut Rowe Hartford Basin Connecticut Valley Synclinorium Hawley Belt Waterbury Dome ew Hampshire Bronson Hill nticlinorium Pelham Dome Willimantic Dome Central Maine Terrane valon Terrane Merrimack Zone Putnam-ashoba Zone Rhode Island arragansett Basin 5km Figure 1. Regional tectonic map of the ew England ppalachian Orogenic Belt illustrating the main lithotectonic terranes discussed in the text (modified from Moecher et al., 1997). The Orange-Milford Belt (black in box ) is located in the southeast zone of the Connecticut Valley Synclinorium and is bound in the west by the East Derby Fault and unconformably overlain in the east by Triassic sediments of the Hartford Basin. Cameron s Line (CL) is a tectonic boundary separating Taconic metamorphism in the west from cadian metamorphism in the east. PD Pelham Dome; WD Waterbury Dome (a Taconic allochthon); WLD Willimantic Dome. the matrix (D 4 ) produced kinks with subvertically dipping axial planes (S 4 ) that strike approximately east-west (Fig. 2). FOLITIOS PRESERVED I PORPHYROBLSTS Differentiated Crenulation Cleavages Differentiated crenulation cleavages developed in the matrix prior to porphyroblast development are commonly preserved as helicitic inclusion trails inside garnet porphyroblasts (Figs. 4 and 4B). Crenulations in the included microlithons are most commonly defined by quartz and ilmenite with the included differentiated crenulation cleavage seams defined predominantly by ilmenite. The crenulations originated as asymmetric matrix microstructures that were overgrown by garnet porphyroblasts during subsequent deformations. Sigmoidal, Staircase, and Spiral Geometries The most common inclusion trail geometry preserved in garnet-grade samples of the Wepawaug Formation is sigmoidal and resulted from a single phase of porphyroblast growth. Complex inclusion trails, including staircase and spiral geometries, are interpreted to be the product of one or more phases of syn-tectonic growth (Figs. 4E 4H; Bell and Johnson, 1989). FI MESUREMETS foliation intersection or inflection axis in porphyroblasts (FI) is measured for a sample by observing the orientation of the switch in inclusion trail asymmetry within porphyroblasts (clockwise or anticlockwise) from a series of vertically oriented thin

4 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page of XX 98 T. H. Bell and R. ewman Triassic and Jurassic Rocks of the Hartford Basion (TR) C T S 2 Wepawaug Formation (DSw) Maltby LakesMetavolcanics (Om) Oronoque Member (Derby Hill Schist) (Odo) + + +llingtown Metadiabase (Oa) Derby Hill Schist (Od) Metamorphic isograd " " " n = 194 S 3 TR S 2 S 3 S 4 L 2 3 km East Derby Fault Wepawaug Fold DSw Om Odo Oa n = 9 S 4 n = 17 L 1 2 ky st gt bt " Od n = 9 L 2 3 Od Odo no outcrop kyanite staurolite garnet n = 121 F EDF B DSw Vertical= Horizontal Om Odo Oa Od S 1 S 2 S 3 S 4 Isograd n = 26

5 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page of XX ppalachian orogenesis 99 Figure 2. () Lithological map and structural dataset of the Orange-Milford Belt, southwest Connecticut, ew England ppalachians (rea in Fig. 1). Outcrop pattern modified from Fritts (1963, 1965a,b) and Rodgers (1985) after structural mapping (March ugust 1997). Structural data plotted on equal area stereonets. General strike of S 1 (not shown), S 2, and S 3 is SSW-E. S 2 is the dominant fabric throughout the belt, tight to isoclinally folding S 1 in the east and becoming the pervasive fabric as metamorphic grade increases to the west. S 2 and S 3 fold axes plunge gently ( 15 ) to the north. S 3 folds the earlier fabrics, and S 4 forms localized centimeter-scale kink folds (striking W-E) of the earlier fabrics. Metamorphic grades are biotite (bt), garnet (gt), staurolite (st), and kyanite (ky) separated by field isograds delimited by field and microstructural analysis. (B) Possible west-east cross section along the line - in. The Wepawaug Fold is shown in this interpretation as a large open syncline that has been modified by D 1, D 2, D 3, and D 4 matrix deformations. The prominent variation in thickness of both the Maltby Lakes Metavolcanics and the llingtown Metadiabase (Om and Oa, respectively) across the Orange-Milford Belt possibly result from the primary intrusive morphology. (C) Stereo nets showing poles to S 2, S 3, and S 4 and the plunges of intersection lineations L 1 2, L 2 3, and fold axes F S 2 2C4 S 2 S 1 S <1 sured by observing the asymmetry switch in differently dipping thin sections cut with strike parallel to the FItrend (Fig. 7 in Bell et al., 1992). The plunge lies between the asymmetry switch when looking down and is recorded as an angle between and 9. Samples with FItrends changing from the core to the rim of porphyroblasts can be measured and provide vital relative timing information that allows the succession of FIs to be determined (Bell et al., 1998). Differentiated crenulation cleavage and microlithons preserving crenulated cleavage are commonly preserved inside porphyroblasts. The intersection lineation between the crenulated and crenulation cleavages can also be measured. It is referred to as a pseudo-fi because it did not form during porphyroblast growth. The curvature of the differentiated cleavage itself defines the FI, not the curvature of the preporphyroblast crenulations preserved within the quartz rich or Q-domains that formed during a deformation that occurred before the event that accompanied porphyroblast development. Where possible, the pseudo-fi was measured because it can potentially be utilized as a structural indicator akin to a FI, with the proviso that it may have been rotated by deformation occurring before porphyroblast growth during the same or a previous event (e.g., Bell and Bruce, 26). 1mm Figure 3. Photomicrograph of biotite grade phyllite illustrating the interrelationship between successive foliations. In this sample S 1 is the dominant foliation (subvertical) and is defined by differentiated layers of chloritic/micaceous material and quartz rich microlithons. Rarely, S 1 microlithons preserve evidence of an earlier matrix fabric, S 1 (inset box). This fabric is only preserved in the lowest biotite grade samples and is not regionally correlatable, and therefore is not assigned a chronological S number. Where preserved, S 1 is crenulated in a sinistral sense looking north into S 1 (i.e., top to the west). S 2 characteristically crenulates S 1 and is most obvious in the chloritic/micaceous layers. Sample 2C4. Orientation of vertical thin section in top left corner, scale in top right corner. Magnification of image 4, plane polarized light. sections observed consistently in the one direction around the compass (Bell et al., 1998). For example, Fig. 5 contains a simple spiral with a clockwise curvature from core to rim in the 4 section but an anticlockwise curvature in the 36 section. This switch in asymmetry takes place across the FI trend. The principle is expanded in Figs. 5B and 5C. FI plunges can be mea- RESULTS total of 87 FI and pseudo-fi trends were determined from 65 samples of schistose Wepawaug Formation (Table 1, Fig. 6). Fifty-four samples contain a single FI, one contains a pseudo-fi overprinted by a single FI, another contains two single unrelated FIs differentiated by different lithology types in the same outcrop (C117), and eight preserve more than one FI in a core/rim or core/median/rim relationship (C7, C3, C81, C87, C88, C116, C123, and C139B). One sample (C1) preserves a pseudo-fi overprinted by a core and a rim FI. Matrix intersection lineations preserved within porphyroblast strain shadows were measured in five samples (C1, C11, C87, C15, and C111). Seven samples preserve FIs measured from staurolite, biotite, and oligoclase porphyroblasts (Table 2). total of 22 FI plunges were measured from 17 samples (Table 1). FI plunges tend to be subhorizontal (e.g., Bell and Hickey, 1997; Bell et al., 1997); a spread of sample locations was selected across the regional structure to determine whether the FIs were subhorizontally plunging in this region or varied across the structure. For determining plunges, samples were

6 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page 1 1 of XX 1 T. H. Bell and R. ewman C44, 4S 95 B C 1mm C117, D E 1mm C116, 13 F 1mm G C172, 12 H 1mm

7 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page of XX ppalachian orogenesis 11 Figure 4. Photomicrographs and associated line diagrams illustrating inclusion trail geometries (S i ) preserved in Wepawaug Formation porphyroblastic mineral phases. Strike of vertical thin section shown by single barbed arrow in upper left corner in dips S at 4 ; the strike is shown by a single barbed arrow). Scale bar in lower left corner. () Inclusion trails defining a differentiated crenulation cleavage that predates porphyroblast growth. The axis is called a preporphyroblast crenulation axis, or pseudo-fi. Sample C44. Partially crossed polarized light, 4 magnification. (B) Line drawing of. (C) Sigmoidal inclusion trail geometry. Sample C117. Partially crossed polarized light, 4 magnification. (D) Line drawing of C. (E) Staircase inclusion trail geometry. Sample C116. Plane polarized light, 4 magnification. (F) Line drawing of E. (G) Spiral inclusion trail geometry. Sample C172. Partially crossed polarized light, 4 magnification. (H) Line drawing of G. further selected on the basis of a large garnet density (more than five per thin section) and on the basis that enough sample remained, after cutting the minimum of eight vertical thin sections to determine the FI trend, to make several differently dipping sections striking parallel to this trend. Five maxima are recognizable in the FI trends oriented W-SSE, SSW- E, WSW-EE, ESW-EW, and W-SE (Fig. 7). The multiaxis plot shows similar maxima, excluding the W-SSE peak. The single axis plot shows sets of maxima similar to the total plot, with a further peak oriented SW-E. U 2 test, as described by Bell et al. (1998), confirms that the distribution of total FI data is nonrandom with a value of.224; this is higher than the critical value at the 95% confidence level of.187, which means that we can reject the hypothesis that this is a random distribution. Multiaxis Porphyroblasts and Relative Timing Ten of the 65 samples preserve multiple FIs, and this allows the relative timing between the 55 samples containing a single FI to be constrained (Tables 1 and 3; e.g., Bell et al., 1998). The following successions are apparent: 1. W-SE oriented core FIs succeeded by SW-E to WSW-EE rim FIs (samples C7, C88, C123) or a W-SSE rim FI (sample C81) 2. WSW-EE oriented pseudo-fi and core FIs changing to a WW-ESE oriented rim FI (sample C116) or a SSW-E rim FI (sample C1) 3. W-E core FI succeeded by a WW-ESE median FI (sample C87) 4. WW-ESE core or median FIs succeeded by W- SSE rim FIs (samples C3, C87, C139B) 5. W-SSE median FI followed by a SSW-E rim FI (sample C3) 6. W-SSE and SSW-E oriented rim FIs that are either continuous with matrix microstructures or succeeded by SSW-E to SW-E matrix intersection lineations that are continuous into strain shadows surrounding porphyroblasts (samples C1, C146, and C163). FIs continuous with matrix microstructures are oriented W-SSE to SW-E. The consistency of the succession of FI trends obtained from these samples suggests that those containing single FIs can be divided into a series of sets that match this succession (see later). 33 o 36 o FI 4 o B 28 o 6 o 24 o 1 o 22 o FI 18 o 15 o 5 FI C thin section planes Figure 5. () lthough an FI trend is measured for a sample using all the porphyroblasts intersected by each vertical thin section, the principle of measurement is shown using a 3D sketch drawn of a simple spiral. (B) Sketch illustrating the principal with a fold preserved in an outcrop. The geologists to either side have no idea of the plunge direction. However, the geologist in the center does. (C) Precise measurement of FI made by cutting sections 1 apart and constraining the asymmetry switch within 1.

8 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page of XX 12 T. H. Bell and R. ewman Continuity of Inclusion Trails with Matrix Foliations Sample Inner FI Trend Plunge Trend Plunge FI Trend C C C C C2 8 2 C C C C C C C C C77B C C87 C88 C9 C94 C95 C99 C1 C14B C15 C16 C17 C111 C112 C116 C117i C117ii C123 C124 C126 C127 C128 C129 C13 C131 C132 C133 C134 C139B C14B C141 C142 C144 C146 C147 C15 C151 C152 C154 C155 C157 C162 C163 C17B C171 C171B C172 C179 C182 C183 C187 C193 PPC Median Outer FI Trend Plunge Matrix Trend Continuity between inclusion trails and matrix foliations is a powerful constraint on the timing of inclusion trail formation because continuity and similar microstructures within and outside of porphyroblasts can be used to constrain the growth of these large crystals in relation to preserved episodes of matrix deformation (Johnson and Vernon, 1995; Bell et al., 1998). Samples preserving FIs oriented approximately SW-E contain inclusion trails that are either truncated by or continuous with matrix microstructures allowing a split of single- and multi-fi data in this orientation. Those truncated by matrix fabrics are commonly characterized by fine-grained inclusions of quartz and acicular ilmenite that do not reflect the predominantly coarse-grained interlayered quartz and mica matrix fabrics observed. This is interpreted as evidence of earlier garnet growth that included mineral phases prior to extensive matrix recrystallization and grain coarsening during prograde metamorphism. Samples preserving FIs defined by foliations that are continuous with matrix fabrics are interpreted to have formed relatively late in the matrix tectonometamorphic history (synchronous with matrix intersection lineations in approximately the same orientation). This is reinforced by the observation that the inclusion trails preserved are commonly coarse-grained quartz and ilmenite, reflecting the observed matrix microstructure (samples C1, C14B, C142, C146, C147, C155, C163, C171B, and C193). Biotite inclusion trails are not continuous with matrix S 2, being truncated by S 2 anastomosing around the porphyroblasts (sample C21). The inclusion trails preserved in these samples are also characterized by fine-grained inclusions of quartz and ilmenite that do not reflect the current coarse-grained appearance of the matrix mineralogy. Oligoclase inclusion trails are discontinuous with matrix S 2 when the FI is oriented SW-E to W-E (samples C3 and C21) or continuous with matrix S 2 when the FI is oriented S (sample C163; Figs. 8 and 8B). Staurolite FIs oriented S to SW-E are defined by inclusion trails continuous with matrix S 2 (samples C81, C87, and C88; Figs. 8C and 8D). DIVISIO ITO SETS The pattern of relative timing of FIs in samples containing different trends in porphyroblast cores versus rims or cores versus medians versus rims and the five peaks in the rose diagram (Fig. 7) allow us to propose that all the FIs Table 1. Garnet FI trends and plunges (with their respective ranges) measured from schistose Wepawaug Formation samples collected across the Wepawaug Fold. Samples preserving preporphyroblast crenulation axes (pseudo-fi) have a separate column because they are distinguished by texture and only preserved in the cores of porphyroblasts. Multi-FI sample FIs are referred to as either inner FI, median FI, or outer FI based on their relative location inside the porphyroblasts. Single-FI samples are presented in the inner FI column. Matrix intersection lineations (in the matrix column) were also measured where clearly preserved either in strain shadows adjacent to porphyroblasts or in immediate proximity to porphyroblasts. Trends relative to true north, plunges relative to horizontal. (C117i pelitic lithology, C117ii psammitic lithology).

9 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page of XX ppalachian orogenesis 13 C7 Metamorphic isograd Wepawaug Fold axis (inferred) Sample location Lithological contact Single FI PPC Inner FI Median FI Outer FI T C193 C77B EDF DSw Om Oa Odo Od -1-2 C187 C2 C182 C183-3 Vertical = Horizontal -3 C179 C172 C171/B C14B C111 C155 C147 C144 C15 C17 C14B C1 C11 C152 C151 C146 C142 C141 C157 C154 C15 East Derby Fault C163 C162 C3 C72 C46 C7 C81 C17 C21 C3 ky st grt bt C99 C133 C132 C13 C16 C95 C94 C134 C131 C139B C88 C48 C126 C44 C127 C129 C117 C87 C57 C54 C9 C123 C128 C124 C112 C116 C1 no outcrop km Figure 6. Sample localities preserving porphyroblast FI/pseudo-FI data and their spatial relationship to the Wepawaug Fold axis. ll samples are from the pelitic Wepawaug Formation. The trace of the Wepawaug Fold axis is inferred from macroscopic lithological relationships and is more deformed than implied by this diagram (ewman, 21). Metamorphic zones superimposed on the diagram are biotite (bt), garnet (grt), staurolite (st), and kyanite (ky) and are deformed by superposed folding (ewman, 21). Two inset boxes illustrate FI/pseudo-FI orientations in areas of high density for clarity. possible cross section is also inset.

10 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page of XX 14 T. H. Bell and R. ewman Sample C3 C1 C21 C81 C87 C88 C163 Trend 35 5ß Staurolite 2 1 ß ß 5 5ß Plunge Biotite Oligoclase Trend Plunge Trend Plunge 15 5ß 75 5ß 8 1 ß 175 5ß 1 ß 1 ß Table 2. FI trends and plunges (with their respective ranges) from mineral phases other than garnet measured from schistose Wepawaug Formation samples collected across the Wepawaug Fold. Samples preserve single FIs for each mineral phase. Trends relative to true north, plunges relative to horizontal. can be divided into a succession of five sets (Fig. 9; the approach is described in detail in Bell et al., 1998; Yeh and Bell, 24). The observed pattern indicates an early garnet pseudo- FI and FI set oriented W-SE (Set ), followed by a garnet (plus rare oligoclase and biotite) pseudo-fi/fi set oriented WSW-EE (Set 1) and then WW-ESE (Set 2). The foliations defined by the inclusion trails preserved within these porphyroblasts are always truncated by the matrix foliations. These were followed by a W-SSE (Set 3) trending set of FI preserved within garnet (plus rare oligoclase) porphyroblasts defined by inclusion trails that locally are continuous with foliations in the matrix. The last formed SSW-E (Set 4; Fig. 9) trending set of FI are preserved within garnet and staurolite porphyroblasts and are defined by inclusion trails defining foliations that are continuous with those in the matrix. FI PLUGES FI plunges were measured from several samples where the criteria outlined above were met (Fig. 1). Significantly, the plunges are mostly gentle and always #3. ITERPRETTIO D DISCUSSIO Lack of Rotation of Porphyroblasts The succession of five FI sets suggested by samples containing FIs that changed trend from core to rim or core to median to rim, plus the nonrandom clustering of all FIs around five peaks on a rose diagram, has considerable significance. It indicates for FI Set 1 onward that the porphyroblasts did not rotate during deformation that accompanied their growth or that occurred subsequently when the younger sets developed (c.f., erden, 1995, 24). The curvature of inclusion trails ranges up to 18 in each FI set and in some cases even more (e.g., Figs. 4G and 4H). If the porphyroblasts had rotated up to 18 in each FI set, the FI plunges would have ranged from to 9 rather than cluster close to the horizontal as shown in Fig. 1. Furthermore, a random distribution of FI trends relative to the succession of FIs would have been obtained by the time FI Set 3 had developed (Fig. 11). The development of FI Set 4 would have spread this much farther (e.g., Fig. 14 in Bell and Kim, 24; Fig. 14 in Ham and Bell; 24, and Fig. 1 in Bell et al., 25). Gently Plunging FIs and Their Significance The foliations in porphyroblasts range from predominantly steep (Figs. 4 to 4F), to flexed (Figs. 4C and 4D) or crenulated (Figs. 4E, 4F, 8, and 8B upper central porphyroblast) about predominantly subhorizontal axial planes and locally steep ones (Figs. 8 and 8B), to shallow dipping (Figs. 8C and 8D). Therefore, the gently plunging FIs in Fig. 1 reveal a very significant geometric relationship. For all the FIs, where measured, to have gentle plunges, the alternating deformation forming them must predominantly involve subhorizontal axial plane structures because only these consistently generate subhorizontal inflections or intersections on foliations of any other orientation for the range of FI trends observed (Fig. 12). Porphyroblast growth over steeply dipping foliations occurred during deformation events generating subhorizontal axial planes or foliations (Figs. 4C and 4D), and this near orthogonal relationship accords well with the models for syntectonic porphyroblast growth described by Bell and Hayward (1991). Similar behavior would be expected for porphyroblasts that overgrew a subhorizontal foliation during a deformation producing a subvertical foliation (Figs. 8C and 8D). The succession of FItrends from Sets through 4 records five periods of FIdevelopment. Each of these five periods of FI development potentially involved several foliation-producing events; for example, the spiral inclusion trail geometry in Figs. 4G and 4H requires four foliations to form successively about the one FI where the inclusion trail geometry results from the overprinting of successive foliations (Bell and Johnson, 1989). During this history of overprinting deformations, the FIs remained gently plunging (Fig. 1). Consequently, the history of orogenesis recorded by these rocks consists of deformations with subhorizontal axial planes, or producing subhorizontal foliations, alternating with deformations producing subvertical foliations.

11 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page of XX ppalachian orogenesis 15 Total FIs Figure 7. Equal area rose plots of FI and pseudo-fi trends measured from biotite, garnet, staurolite, and oligoclase porphyroblasts from the Wepawaug Formation of the Orange-Milford Belt. () ll FI and pseudo-fi. (B) Multiaxis samples. (C) Single-axis samples. The plot of all FI/pseudo-FI data () does not illustrate separable peaks in orientation, but that of the multiaxis data (B) does, with definite maxima oriented WSW-EE, ESW-EW and W-SSE. The single axis dataset (C) shows a large spread in data; however, correlation in orientation with the multiaxis samples differentiates the data into discernible frequency maxima. Peaks are even more pronounced when textural criteria for separation of the data are introduced, such as continuity with matrix microstructures and textural similarities. = 87 Max. % = 11.5 B = 21 Max. % = 14.3 C Multi FIs Single FIs The partitioning of deformation across an outcrop (Bell et al., 1998) as well as regionally (Bell et al., 24) during the development of a long succession of subhorizontal and subvertical foliations generally results in none of the rock recording the full deformation history, the bulk recording very little, and a very small portion recording some. This can be understood by closely examining Fig. 13, which shows a progression of overprinting zones of deformation that are spatially partitioned. The portion of rock within which the full history is present diminishes rapidly with each successive subvertical and subhorizontal deformation, so much so that after four deformations about one FI trend (Fig. 13D), only a tiny portion has recorded all the history (a staircase succession on the left and a spiral succession on the right a mixture between these is also possible). Once the effects of changing the direction of horizontal bulk shortening are added, which of course generates the FI succession, it becomes very apparent that no one portion of rock feels the effects of all of the deformation history. Furthermore, Fig. 13D shows that along the same zone of deformation, 36 of apparent rotation of a foliation can be preserved at the same time that 18 of apparent rotation forms in a second location, 9 (not shown) in a third, and no porphyroblasts grow at a fourth where a preexisting foliation is simply reused. The Significance of Multiply Repeated Subhorizontal Foliations Multiply repeated subhorizontal foliations have extraordinary significance for orogenesis. Such foliations can develop in one of four ways: = 66 Max. % = 12.1 Deformation Partitioning and the Imposition of lternating Subvertical and Subhorizontal Foliations 1. Through crustal extension associated with the development of a new plate boundary by pulling crust apart (Wernicke, 1985). 2. Through crustal extension caused by rollback (migration of the trench toward the subducting plate) (Meulenkamp et al., 1988; Boronkay and Doutsos, 1994). 3. Extrusion combined with almost steady-state gravity collapse during postsuturing continent collision (e.g., Beaumont et al., 21) 4. Through gravitational collapse of an overthickened orogen that became unable to support the topographic or density head extruded by crustal shortening dominating erosion and isostasy (Bell and Johnson, 1989, 1992)

12 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page of XX 16 T. H. Bell and R. ewman 13 1mm C163 S 2 biotite inclusions S 2 S i S i S i S 2 B S i S 2 C mm transposed S 2 S 2 recrystallized quartz S 3 S 3 S i C D S 2

13 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page of XX ppalachian orogenesis 17 Sample PPC Inner FIM edian FI Outer FIM atrix Trend Plunge Trend Plunge Trend Trend Plunge Trend C7 145ß 5ß 5 1ß C ß ß 2ß 65ß 15ß 1ß 1ß 1 1ß 3ß 1ß 35ß 5ß C3 1ß 1ß 155 5ß 3 1ß C ß 17 2ß C87 95ß 5ß 115ß 5ß 15ß 5ß 15ß 15ß C88 145ß 15ß 55 5ß 3ß 1ß C ß 2ß 2ß 15 5ß 1ß 3ß C ß 1ß 1ß 7 1ß 1ß 1ß C139B ß 16 1ß Table 3. Garnet multi-fi trends and plunges (with their respective ranges) measured from schistose Wepawaug Formation samples collected across the Wepawaug Fold. Trends relative to true north, plunges relative to horizontal. Pseudo-FI preporphyroblast crenulation axis. Matrix matrix intersection lineation, measured where clearly preserved either in strain shadows adjacent to porphyroblasts or in immediate proximity to porphyroblasts. The first of these mechanisms is discounted for most of ppalachian orogenesis, because the overall environment was one of plate collision. This is supported by the development of the progression of subvertical foliations (overprinting subhorizontal ones) that define the succession of FItrends. The second of these mechanisms could potentially be a possibility because Meulenkamp et al. (1988) and Boronkay and Doutsos (1994) have shown that the egean was affected by four phases of shortening during the 13 million years of orogenic history when this arc was dominated by the effects of rollback. The third mechanism is possible if periods of horizontal shortening with sufficient intensity to develop a vertical foliation could be interspersed with this process. The fourth mechanism is certainly possible because correlation of these rocks with those in Vermont and orth Central Massachusetts suggests that this history continued for a period spanning at least 75 million years (Bell and Welch, 22; Bell et al., 24) during which time collision of at least one island arc, possibly a midocean ridge and a microcontinent occurred with orth merica. This history will be dramatically extended when regions dominated by Taconic or lleghanian portions of the orogeny are considered in the same detail that those dominated by the cadian have been examined. Significantly, support for these processes is being found in the margins of orogens where alternations of uplift and subsidence... imply alternations of lateral shortening and extension (e.g., Hosseini- Barzi and Talbot, 23). Indeed, many of the refold geometries seen in the external parts of thrust belts on the margins of orogens could result from such processes (e.g., Mitra and Yonkee, 1985). Topographic (Or Density) Head versus Rollback-Driven Gravitational Collapse and Spreading Both gravitational collapse of an orogen alone, as conceived by Bell and Johnson (1989), or rollback-driven gravitational col- lapse can occur without cessation of the collisional plate motion that provides the overall driving force for orogenesis and could generate the subhorizontally dipping foliations recorded by porphyroblast growth accompanying lengthy orogenesis (Hayward, 1992; dshead-bell and Bell, 1999; Bell et al., 23, 24). Consequently, it is worth examining gravitational collapse and rollback driven gravitational collapse in some detail to determine whether any differences can be distinguished in their effects on the orogenic record left in the hotter parts of orogens where porphyroblasts grow. Geometrically, gravitational collapse and spreading can take place above a basal detachment while plate convergence continues (Figs C). This would result in the development of a subhorizontal foliation that is variably partitioned through higher levels of the crust above a basal detachment, below which a subvertical foliation continues to develop (Fig. 14B; e.g., Bell and Johnson, 1989, 1992). Significantly, the upper part of the portion of the model undergoing collapse in Fig. 14B produces extensional structures (shear sense shown with arrows) that form synchronously with structures in the lower part of this portion, which all structural geologists interpret as compressional; these are thrust geometries as shown in Fig. 14B (shear sense shown with arrows). Episodically, horizontal shortening will dominate when the topographic (or density) head decreases resulting in vertical foliations developing above the basal detachment as well as below (Fig. 14C). These processes will occur whether or not continental collision takes place. Geometrically, rollback driven gravitational collapse will predominantly develop subhorizontal foliations during orogenesis. However, it could also develop subvertical foliations if there was variable coupling between the two plates (Figs. 14D 14F). The distribution of earthquakes above modern day subduction zones suggests that the coupling between converging plates is Figure 8. () and (B): Oligoclase feldspar porphyroblasts in a graphite-rich kyanite grade sample preserving inclusion trails (inclusion trails dashed lines) that are continuous into the pervasive matrix foliation (S 2 non-dashed lines). The included material defines an earlier foliation that is longer preserved in the matrix. Strike of vertical thin section shown by single barbed arrow in upper left corner, scale in upper left corner. Sample C163. Plane polarized light. (C) and (D): Partially retrograded staurolite porphyroblast with inclusion trails continuous with matrix S 2 preserved in strain shadows around the porphyroblast. Included fabric is dashed; matrix fabrics are solid lines. Staurolite in darker grey, sericite alteration in lighter grey. S 2 is subsequently transposed by a later fabric, S 3. Strike of vertical thin section shown by single barbed arrow in upper left corner, scale in upper right corner. Sample C87. Plane polarized light.

14 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page of XX FI Set B Set 1 C Set 2 = 1 Class Interval = 1ß Max. Percentage = km = 27 Class Interval = 1ß Max. Percentage = km 1 = 21 Class Interval = 1ß Max. Percentage = km T ky st gt bt no outcrop no outcrop no outcrop D Set 3 E Set 4 = 14 Class Interval = 1ß Max. Percentage = km = 16 Class Interval = 1ß Max. Percentage = km Legend: Sample location Lithological contact 1 FI trend FI plunge direction and value Garnet FI 1 2 Garnet PPC 3 Oligoclase feldspar FI Biotite FI no outcrop no outcrop Staurolite FI Figure 9. Division of Wepawaug Formation FI and pseudo-fi data into sets based on orientation correlation with multi-fi samples, continuity with matrix microstructures, and textural features. () Set oriented W-SE. ll inclusion trail geometries are truncated by matrix microstructures. (B) Set 1 oriented WSW-EE. ll inclusion trail geometries are truncated by matrix microstructures. (C) Set 2 oriented WW-ESE. ll inclusion trail geometries are truncated by matrix microstructures. (D) Set 3 oriented W-SSE. (E) Set 4 oriented SSW-E. Inclusion trail geometries are commonly continuous with matrix fabrics.

15 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page of XX ppalachian orogenesis 19 FI B FI 1 C FI 2 C124 C77B C88 C1 C1 C15 C123 C111 C154 C16 C21 C21 C116 C123 C1 C116 n=2 n=11 n=1 D FI 3 E FI 4 C129 C1 C155 n=4 C172 C163 C163* C187 n=4 C129 C46 Figure 1. Lower hemisphere equal area plots of FI trend and plunges. Sample selection criteria for determining axis plunges were: (1) enough sample remaining to make several differently dipping sections striking parallel to the axis trend, (2) a high density of porphyroblasts ( 5 per thin section), and (3) sample location (for complete fold belt representation). The number of samples is noted underneath each respective plot. () Pseudo-FIs. (B) Inner FIs. (C) Outer FIs. (D) Single FIs. C163* denotes the trend and plunge of a staurolite FI. partial or discontinuous (Byrne et al., 1988; Wang and Suyehiro, 1999; Hsu, 21). The commencement of rollback, that is, migration of the trench toward the subducting plate, could initiate gravitational collapse as shown in Fig. 14E. Recommencement of coupling between the two plates could reinitiate bulk horizontal shortening as shown in Fig. 14F. Consequently, alternating development of subhorizontal and subvertical foliations is possible during plate collision whether or not it is accompanied by rollback. s rollback continued, extensive sedimentation would take place in the fore arc allowing sediments forming synchronous with orogeny to be affected by subsequent orogenesis when coupling reoccurred. If a slab of continental crust or an island arc was transported in on the plate that was rolling back, the relative rates of migration of the trench versus plate motion would affect what took place. If rollback was faster than plate motion, then the processes described immediately above would continue. If plate motion was faster than rollback, then continental collision would occur, and successions of subvertical and subhorizontal foliations could be expected to form in a more regular manner than prior to arrival of that crust. If decoupling of the newly arrived continental crust from the subducting plate took place then the pattern would revert to that expected for oceanic crust colliding with continental during rollback. Multiple developments of successive subvertical and subhorizontal foliations can occur whether rollon (Figs C), rollback (Figs. 14D 14F) or no migration of the trench occurred. Potentially, if rollback dominated, one might expect the number of compressional events recorded by porphyroblasts to be much lower than that if it had not. Such orogens would preserve a dominance of subhorizontal foliations within porphyroblasts. This would occur because porphyroblasts preferentially grow at the start of deformation for events where the foliation being crenulated lies at a high angle to the deforming event (Bell and Hayward, 1991; Bell et al., 24). Therefore, porphyroblasts would

16 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page of XX 11 T. H. Bell and R. ewman FI 1 FI 3 FI FI B FI 2 Figure 11. The effects of rotation of FI by up to 9 either way about successive FI sets 1, 2, and 3 in (), (B), and (C) respectively (the portions marked in grey show the portion of the stereonet over which the FI would range). lthough only 9 of rotation is shown, the amount of curvature of inclusion trails ranged up to 18 with both clockwise and anticlockwise apparent rotations around each FI. We have not bothered to include the rotational effects of development of FI 2 on FI 1, the whole stereo would have been covered by the development of FI 3. This does not include any rotational effects of FI 3 on porphyroblasts containing FIs 1 and 2 or any rotational effects of FI 4 on the preceding sets. Clearly, if rotation of the porphyroblasts had occurred, we would not get the simple distribution of shallowly plunging FIs that we observe. C preferentially grow in horizontal bulk shortening events in a rollback environment (Fig. 14F). Within the ppalachians, porphyroblasts have overgrown subhorizontal and subvertical foliations in roughly equal proportions (e.g., Fig. 1 in Bell et al., 23; Fig. 11 in Ham and Bell, 24; Fig. 8 in Bell et al., 25), suggesting that rollback was not a significant process during the period of orogenesis from prior to 424 to 34 million years ago (Bell and Welch, 22). ppalachian Orogenesis intersection of steep with subhorizontal planes intersection of steep with steep planes Figure 12. Stereonet showing how the intersection of a subhorizontal plane with any other steeply dipping plane produces a gentle plunge (circles). It also shows how the intersections of steeply dipping planes have mainly steep plunges. They only range to gentle plunges when the intersecting steeply dipping planes are very close in strike and mainly have opposite dips. The ppalachians are classically regarded as having undergone three phases of orogenesis that occurred during the Taconian (Ordovician), cadian (Devonian), and lleghanian (Permo-Carboniferous). Furthermore, these phases of orogenesis have been interpreted as resulting respectively from the collision with the orth merican craton of an island arc (Stanley and Ratcliffe, 1985), an island arc or microcontinent, and a continent (Mosher, 1983). Dating of successive FI sets in Vermont has revealed that orogenesis was far more incessant than this with deformation and metamorphism being near continuous from well before 424 Ma to 34 Ma in SE Vermont (Bell and Welch, 22); that is, from the Taconic through nearly to the lleghanian. Dating and FI studies in other parts of the orogen support this and show deformation continues into the lleghanian (Lisowiec, 25; Rich, 25). The same succession of FItrends observed in SW Connecticut is preserved in Central and SE Vermont and orth-central Massachusetts (Figs. 15 and 16), but the numbers of FIs are differently proportioned between sets (e.g., Bell et al., 24). This suggests

17 GS_sp414_chapter_6.qxd 6/27/6 1:15 PM Page of XX ppalachian orogenesis 111 that over this period of time the deformation associated with orogenesis was very heterogeneous and inhomogeneously partitioned on a regional scale throughout the ew England ppalachians (e.g., Bell et al., 24). Heterogeneous partitioning of the deformation also occurs at outcrop scale with no single rock preserving all the history of deformation that the region has been through. That is, no single sample contains all five FI sets. Fig. 13 shows that this is a natural consequence of a deformation history of progressive bulk inhomogeneous shortening. Furthermore, preservation of this history is only possible where porphyroblast growth has occurred throughout that history and Fig. 13 shows this would be exceedingly rare due to both the effect of bulk composition on the timing of porphyroblast growth, as the temperature and pressure of the rock mass increase, and the partitioning of the deformation itself. The Path through the Crust X Y Y X Y Y X B C Structural geologists have generally regarded the sequence of foliations preserved in the matrix of a rock as revealing most of the deformation history that the rock has undergone. Quantitative work over the past decade on the foliation microstructures preserved in porphyroblasts has revealed that this is not the case in the ew England ppalachians. In most rocks, garnet porphyroblast growth predates all the matrix foliations now preserved in the matrix other than bedding (Bell et al., 1998, 23, 24, 25; Ham and Bell, 24). In our experience, all regionally metamorphosed porphyroblastic rocks in this region contain a schistosity parallel to bedding or compositional layering. In general, most geologists have interpreted this schistosity to result from the first deformation. However, the quantitative work with porphyroblasts described herein, as well as that undertaken across ew England, reveals that a large number of deformations predate this foliation (e.g., Bell et al., 1998, 24). Reactivation or shear along the bedding occurs during every deformation on at least one limb of a regional fold (Bell et al., 23, 25; Ham and Bell, 24) rotating preexisting foliations into parallelism with the compositional layering. Garnet containing FI Sets through to the early stages of FI Set 4 track the descent into the metamorphic pile (Kim and Bell, 25; Bell, Cihan, Evans, and Welch, unpublished data). However, the matrix foliations oblique to bedding within these porphyroblastic rocks appear to result from ascent through the metamorphic pile (Kim and Bell, 25). That is, the development of such foliations generally tracks the beginning of the decrease in pressure with the very weakest foliations postdating the youngest phases of porphyroblast growth. Significantly, no one rock that we have examined contains all five FIs of the succession recorded herein in SW Connecticut. or has any such sample been found elsewhere across ew England. Fig. 13 shows for one FI trend just how little of a rock experiences all the deformation history in just four events, where the deformation is partitioned on an outcrop scale. This effect is compounded when the effects of juxtaposition of rock types of different competency due to the switching of foliation development from subvertical to subhorizontal is Figure 13. Schematic cross sections showing the influence of the partitioned development of deformation during the formation of successive subhorizontal and subvertical foliations in limiting the portions of rock that see any of the full deformation history. () The partitioned development of zones of deformation within which a subhorizontal foliation develops with a clockwise (top-to-the-right) shear sense. The rocks had previously been affected by a pervasive subvertical foliation, which has not been shown. The scale is such that the macrolithons (e.g., X) between the spaced anastomosing zones (Y) of more intense deformation are virtually undeformed, with most of the deformation, involving a range of scales of partitioning, taking place within the zones labeled Y. If porphyroblast growth occurred for the first time in these rocks, it would occur within finer-scale zones of progressive shortening within the zones labeled Y. The differentiation asymmetry preserved within such porphyroblasts is that shown schematically on the right by stylized staircases and spirals. (B) Partitioned development of subvertical of zones of deformation within which a subvertical foliation develops with an anticlockwise shear (right side up) on the left and clockwise shear (left side up) on the right. (C) Partitioned development of subhorizontal of zones of deformation within which a subhorizontal foliation develops with a clockwise shear (top to the right). (D) Partitioned development of subvertical of zones of deformation within which a subvertical foliation develops with an anticlockwise shear (right side up) on the left and clockwise shear (left side up) on the right. D