Alleghanian orogenic-float on the Martic thrust during dextal transpression, central Appalachian Piedmont

Journal of Geodynamics 37 (2004) 613–631 www.elsevier.com/locate/jog

Alleghanian orogenic-float on the Martic thrust during dextral transpression, central Appalachian Piedmont David W. Valentinoa,*, Sameul T. Peavyb, Richard W. Valentinoc b

a Department of Earth Sciences, State University of New York at Oswego, Oswego, NY 13126, USA Department of Geology and Physics, Georgia Southwestern State University, Americus, GA 31709, USA c Department of Geology, Temple University, Philadelphia, PA 19122, USA

Abstract During the Late Paleozoic Alleghanian orogeny, the mid-Atlantic Piedmont experienced transpressional deformation dominated by dextral strke-slip shear zones. The dextral displacement on these shear zones greatly influenced the geographic distribution of lithotectonic units. Transpressional deformation is evident in the Piedmont with the cogenetic development of domes and en-echelon antiforms between many of the shear zones. In the core of the Pennsylvania reentrant, major Alleghanian structures include the dextral Pleasant Grove shear zone and Tucquan-Mine Ridge antiform. Recent field mapping coupled with detailed metamorphic and deformation fabric studies have revealed that a major thrust, the Martic thrust, was also active during this time. Shear bands were identified during petrofabric analysis of the hanging wall rocks to the Martic thrust. The direction of displacement on these shear bands was parallel to the orogen, a direction contrary to earlier studies. Metamorphic mineral assemblages and ceased reaction textures, associated with ductile shear fabrics in the hangingwall rocks, are consistent with lower greenshist facies deformation. This low grade metamorphism, which is generally confined to sheared rocks, overprints the regional upper greenshist- to lower amphibolite-facies assemblages. Structural and magnetic modeling of the hangingwall block has revealed a complex geometry. A model of orogen parallel structural escape, or orogenic float, related to late Paleozoic dextral transpression is employed to explain the late reactivation on this important central Appalachian structure. # 2004 Elsevier Ltd. All rights reserved.

* Corresponding author. E-mail addresses: [email protected] (D.W. Valentino), [email protected] (S.T. Peavy), twvalentino@aol. com (R.W. Valentino). 0264-3707/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jog.2004.02.007

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1. Introduction The scale and regional extent of transpressive strain is related to two tectonic conditions: (1) restraining bend transpression, along transcurrent faults transform plate margins, that results in contractional structural features local thrusts and folds, and conjugate strike-slip faults (Woodcock, 1986); (2) orogen-scale transpression where oblique motion vectors between two tectonic plates result in oblique collision that produces zones of transpressive strike-slip deformation in the hinterland, and belts of thrusting in the foreland (i.e. Woodcock, 1986; Tappionier and Molnar, 1976). Independent of the scale, strain distribution is critical in transpressional deformation zones. Structural domains containing contractional features often occur adjacent to brittle or ductile shear zones that accommodate transcurrent displacement (i.e. Lowell, 1972; Sylvester and Smith, 1976; Hansen, 1989; Bauer and Bidwell, 1990; Rajlich, 1990; Culshaw, 1991). In many examples of transpression, the zones of contraction are described as positiveflower structures (i.e. Gates, 1987). These structures comprise thrust faults with oblique displacement rooted in the strike-slip zone, en-echelon domes forming over or between strike-slip zones and the formation of adjacent thrusts. The distribution of strain and the location of faults can be controlled by the occurrence and attitude of pre-existing structures (Dewey and Burke, 1973), or by the juxtaposition of rock bodies with ductility contrast such as decoupling of cover rocks over a deforming crystalline basement block (Gates et al., 1999). The mid-Atlantic Piedmont (Fig. 1) has been described as the hinterland of the late Paleozoic dextral transpressional Alleghanian orogeny, that impacted the entire Appalachian mobile belt (Maguire et al., 1999; Gates et al., 1999, Valentino et al., 1994, 1995, 1999; Valentino, 1999). In the Piedmont, a complex system of anastomosing ductile shear zones occurs between more rigid blocks of Precambrian crystalline basement, specifically various massifs of Grenville (ca. 1000– 1250 Ma) rock (Crawford and Heorsch, 1984; Wagner and Srogi, 1987). The primary evidence for transpression is the presence of broad, open domes cored by basement blocks, that are located between and are contemporaneous with transcurrent shear zones. A major thrust detachment, known as the Martic thrust (Figs. 1 and 2), resides within the core of the strike-slip zone in the mid-Atlantic region. New data suggests this major detachment accommodated orogenic float (subhorizontal orogen parallel displacement) during dextral transpression.

2. Geologic setting The Pennsylvania salient in the mid-Atlantic region is host to a regionally extensive geologic structure known at the Martic line, or Martic overthrust (Figs. 1 and 2). The Martic thrust separates Cambrian metacarbonate and metasiliclastic units (Antietam-Harpers, Conestoga and Ledger Formations) of the Laurentian passive margin (Rodgers, 1968) from siliciclastic metasediments (Octoraro and Peters Creek Formations) of the Iapetan rift-to-drift transition (Gates and Valetino, 1991; Valentino et al., 1994; Kasellas and Glover, 1997; Valentino and Gates, 1995). Early debates over the geologic significance of the Martic contact focused on depositional verses a structural origin (Knopf and Jonas, 1929; Mackin, 1962). Although the Martic line was first discussed by Stose and Jonas (1939) to be a regional thrust fault, careful geologic mapping and petrofabric analysis by Cloos and Heitanen (1941) and the field studies and synthesis of Wise

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Fig. 1. Compiled geologic map with major structures, including the Martic Line, for the mid-Atlantic Piedmont. The inset map shows the location in the Appalachian orogen, and the area of Fig. 2 is shown by the black dashed line. (sources: Cloos and Heitanen, 1941; Knopf and Jonas, 1929; Alcock, 1994).

(1970) clearly demonstrated the Martic line’s structural importance as related to the metamorphic history. These researchers deciphered a complex history of movement along the Martic line dominated by a series of thrust slices (Cloos and Heitanen, 1941), or duplex, and polyphase thrusting (Wise, 1970). The thrusting was attributed to hinterland deformation during the Ordovician Taconic orogeny (Lapham and Bassett, 1964) in the Appalachians. Cloos and Heitanen (1941), who primarily focused on study of the footwall to the Martic ‘‘overthrust’’, showed that regional progressive metamorphism post-dated the development of a thrust duplex. Additionally, Wise (1970) argued that the structure evolved from an early brittle thrust that was later folded and incorporated into a zone of subhorizontal regional ductile flow. A generally accepted model for the Martic thrust involves northwest directed displacement (movement perpendicular to the general grain of the orogen) that emplaced the Piedmont siliciclastic metamorphic rocks on top of the Cambrian metacarbonates in a northwesterly direction (i.e. Freedman et al., 1964; Wise, 1970; Crawford and Crawford, 1980; Muller and Chapin, 1984; Wagner and Srogi, 1987). Due to the length of the Martic contact spanning more than 200 km, different segments of the contact preserve different deformation history, as first emphasized by Wise (1970). The main segment of the Martic line, and one of the least linear segments, is located within hinge region of

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Fig. 2. (A) Detailed bedrock geology map of the area immediately south of the Martic Line. Magnetic survey lines (Line 1–3) are shown in the gray lines, and the cross section lines, A–A0 and B–B0 , are represented by the end points. Formation contact north of the Martic Line were compiled from Cloos and Heitanen (1941); (B) Cross section A–A0 that traces through the Martic Forge; (C) Cross section B–B0 that traces through the eastern segment of the Martic Line.

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the Mine Ridge-Tucquan antiform. This antiform evolved as a basement cored crustal-scale dome (Freedman et al., 1964; Wise, 1970; Valentino, 1999) located between two late Paleozoic transcurrent dextral shear zones (Valentino, 1990, 1999; Valentino et al., 1994; Valentino and Gates, 2001). The footwall to the Martic thrust contains metamorphosed Cambrian metacarbonate and minor metaclastic units. The hangingwall contains pelitic and semi-pelitic schist units of the Octoraro Formation. Recent mapping, and metamorphic and structural petrography revealed the conditions of deformation and movement history for the Martic thrust in the Tucquan block. Modeling of new magnetic data helped to constrain the form of the detachment surface and three-dimensional shape of the hanging wall block.

3. Geology of the Martic thrust 3.1. Footwall The footwall to the Martic thrust is underlain primarily by the Conestoga Formation. Locally the Conestoga Formation is made up of interlayered white marble (few cm to few dm thick) with gray to silver-gray mica-rich marble. Cloos and Heitanen (1941) mapped slivers of metasandstone (Antietam-Harpers Formation undivided) and dolomitic marble (Ledger Formation) within the Conestoga Formation (Fig. 2). They interpreted these slivers and lenses to be stacked thrust slices. 3.2. Hangingwall The Martic thrust hanging wall includes members of the Octoraro Formation (Fig. 2B and C). Generally the outcrop belts of the Octoraro Formation members parallel the trend of the Tucquan antiform with a strike ranging between 055 and 070 and dipping shallowly northwest or southeast depending on the location on the antiform. Locally the Martic thrust truncates some of the Octoraro Formation members. Fig. 2 shows the structural position between the members and the descriptions that follow provide specific information for subdivision justification. 3.3. Hanging wall rock units Geologic mapping in the hanging wall block of the Martic thrust delineated numerous mapscale lithologies (members) within the Octoraro Formation (Valentino, 1999; Valentino et al., 1994). Some of these units have been described in earlier field guides (Valentino, 1990, 1994). The following subsections present the formal description of Octoraro Formation members that occur in closest proximity to the Martic thrust. The delineation of the lithology variation for the Octoraro Formation was critical to understanding the structure of the hangingwall block, and modeling the magnetic data. 3.3.1. Bowery Run schist The Bowery Run schist (‘‘obr’’ on Fig. 2) is a muscovite-quartz schist with minor albite, and variable occurrence of garnet and biotite. Typically this rock unit is a tan- to silver-schist with very straight foliation. Rare thin metasandstone layers (< 10 cm) occur within the schist, and

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distinguish this unit from other pelitic schist units in the Octoraro Formation. The metasandstone layers contain mostly quartz and microcline with accessory muscovite, chlorite and biotite. The Bowery Run schist defines a narrow belt along the southern segment of the Martic thrust (Fig. 2C). Eastward thinning and pinch-out of the Bowery Run schist is inferred to be structural truncation along the thrust. 3.3.2. Stewart’s Run schist The Octoraro Formation contains a belt of muscovite-chlorite-plagioclase schist (‘‘osr’’ on Fig. 2) on the southern limb of the Tucquan-Mine Ridge antiform. Plagioclase average modal abundance is approximately 30%, however, local variation in the abundance and fine grain size of chlorite and muscovite causes this unit to have a phyllite texture in places. Near the Martic thrust the Stewart’s Run schist resides structurally above the Bowery Run schist across a sharp contact, and locally the Stewart’s Run schist contains upward of 5% modal magnetite. 3.3.3. Tucquan Creek schist. The Tucquan Creek schist is muscovite-chlorite schist with little or no metamorphic plagioclase (‘‘otc’’ on Fig. 2), and covers a vast region along the axis of the Mine Ridge-Tucquan antiform. Garnet and/or chloritoid occur as common accessory minerals, and kyanite occurs locally near the antiform crest. The garnets are euhedral, contain straight to slightly curved inclusion trails, and up to 1 cm in diameter (Fig. 3A). The Tucquan Creek schist is structurally below the Bowery Run schist, but also structurally overlies the Conestoga Formation along the Martic thrust in the east. Fischer et al. (1979) reported on pelitic schist in the Piedmont with anomalously low magnetic signature, and they suggested that it contains a higher than usual amount of aluminum. Portions of the Tucquan Creek schist corresponds to the proposed high-Al schist. The metamorphic mineral assemblages garnet-chlorite; garnet-chloritoid-chlorite; kyanite-chloritoidchlorite, and the lack of biotite are consistent with a high-Al bulk composition.

Fig. 3. Photomicrographs of garnets from the Octoraro Formation showing the evidence for retrogression associated with ductile shearing. (A) Euhedral garnet (gt) with inclusion trails from a location outside the ductile shear zones that occur near the Martic Line. (B) Garnet (gt) fragments with abundant secondary chlorite (chl) from a location within the sheared rocks near the Martic Line.

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3.3.4. Peque Creek schist The Peque Creek schist overlies the Tucquan Creek schist to the north (‘‘opc’’ on Fig. 2), and contains silver to gray coarse-grained muscovite schist with abundant metamorphic plagioclase and garnet. The plagioclase is often a major component of the rock making up 5–15% modally. With a sharp to gradational contact, the Peque Creek and Tucquan Creek schists are distinctly different based on the occurrence and abundance of plagioclase. Garnets in this unit are euhedral to anhedral depending on the proximity to the Martic thrust. Fig. 3B shows an example of fractured garnets with secondary chlorite from the region a few kilometers south of the Martic thrust, however, southern parts of the Peque Creek schist contain more euhedral garnets. 3.3.5. Martic Forge schist A thin unit (50–250 m) that overlies the Peque Creek schist, contains abundant plagioclase, microcline and biotite (‘‘omf’’ on Fig. 2). Due to the abundance of quartz and feldspar the Martic Forge schist has preserved compositional layering that may represent original sedimentary compositional variation. Slight variations in the mica content define layering on the scale of tens of centimeters. Although the dominant schistosity in the Martic Forge schist dips very shallowly, the compositional layering dips steeply toward the north (Fig. 2B). The Martic Forge schist forms a narrow belt along the northern segment of the Martic thrust and is the primary rock unit at the type location at Martic Forge. The lower section of this unit contains more muscovite and chlorite near the contact with the underlying Conestoga Formation.

4. Deformation, metamorphism, and magnetic modeling All the units in the Octoraro Formation and the footwall units contain evidence for multiple phases of deformation and metamorphism as described by earlier workers (Cloos and Heitanen, 1941; Freedman et al., 1964;Wise, 1970; Valentino et al., 1994). In a relative sense, the oldest schistosity in the Octoraro Formation (S1) is defined by alternating layers of mica-rich and quartz-rich domains that are observed in thin-sections, and rarely observed at outcrops. In some cases, these compositional layers may represent primary transposed compositional layering (sedimentary) as described for the Martic Forge schist above. The S1 schistosity is best observed in the hinges of F2 isoclinal folds that are associated with the regional second generation (S2) schistosity (Freedman et al., 1964; Wise, 1970; Valentino, 1994, 1999), but within the Bowery Run schist, it is sufficiently preserved in outcrop, and it dips northwest. Most of the Octoraro Formation is dominated by this second generation (S2) schistosity. The S1 schistosity is associated with the lower greenschist facies metamorphic mineral assemblage muscovite-chlorite-albite in most places where it is preserved. However, the S2 schistosity is associated with a number of mid- to upper-greenschist facies metamorphic mineral assemblages: (1) garnet-biotite-muscovite; (2) garnet-muscovite-chlorite; (3) garnet-chloritoid-chlorite; and (4) kyanite-chloritoid-chlorite. Except for the Bowery Run schist, where only the garnet-biotitemuscovite assemblage dominates, all three assemblages occur in the other members of the Octoraro Formation. The S2 schistosity is warped into a broad arch, locally known as the Mine Ridge-Tucquan antiform (Figs. 1, 2 and 4). This fold dominates the regional structural grain of this part of the

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Fig. 4. Lower hemisphere sterogram with a contour diagram of the poles to the regional schistosity (S2) in the Octoraro Formation across the axis of the Tucquan-Mine Ridge antiform.

Piedmont. The antiform was interpreted to have formed by vertical uplift of the Mine Ridge Grenvillian basement block by Wise (1970), and interpreted to have formed as a transpressional dome between two dextral shear zones by Valentino (1999). A plot of the poles to S2 for the hanging wall to the Martic thrust shows the gentle warp in the S2 schistosity across the antiform crest and also suggests a fold interference pattern (Fig. 4). Steeply dipping dextral shear zones cross-cut the Octoraro Formation and contain a third generation schistosity (S3). Although mostly confined to the shear zones, S3 also occurs as a steeply dipping, northeast striking, weeklydeveloped crenulation cleavage across the Tucquan antiform (Freedman et al., 1964; Valentino et al., 1994; Valentino, 1999). 4.1. Structures near Martic Line The Octoraro Formation close to the Martic Line contains evidence for a third phase of deformation and metamorphism (M3 and D3). Discrete shear zones that range from a few centimeters to a meter thick cross cut the S2 schistosity at a low angle (Fig. 5A). These shear zones contain mineral lineations including mica streaks and quartz rods (Fig. 5B). The mineral lineations plunge moderate to gently and trend between 045 o and 075 o (Fig. 6). The sense of shear was examined at 34 localities using outcrop data, rock slabs and thin sections. Along the segment of the Martic thrust south of the Mine Ridge basement massif (Fig. 2), the schistosity of the D3 shear zones dip toward the southeast, mineral lineations plunge gently to moderately ENE, (Fig. 6A) and the relative shear sense is oblique left-lateral. Where the Martic thrust traces over the axis of the Mine Ridge-Tucquan antiform, the dominant (S2) schistosity dips northwest and southeast defining a broad, open antiform (Fig. 4), but small shear bands and

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cross cutting shear zones dip southward. Moderate to shallowly plunging mineral lineations trend ENE-WSW. Shear sense analysis reveals that the Octoraro Formation was displaced east-northeastward over the Cambrian metacarbonates (Fig. 2). The trace of the Martic thrust across the antiform axis abruptly bends to the WSW. The small shear zones and discrete shear bands, that cross cut the dominant (S2) schistosity in the Octoraro Formation, dip shallowly toward the SE with mineral lineations that trend mostly toward the southwest (Fig. 6C). Shear sense analysis along this segment of the Martic thrust revealed displacement of the Octoraro Formation (hanging wall) toward northeast (Fig. 5A). This shear sense could be interpreted as a low angle oblique-sinistral displacement.

Fig. 5. (A) Photograph of a rock slab from the Octoraro Formation that contains two shear bands that cut across the S2 schistosity. The accompanying photomicrograph shows one of the shear bands (chl-chlorite; ctd-chloritoid; mumuscovite; qt-quartz). The view is toward the northwest at a subvertical surface. (B) Photograph of an outcrop surface that is parallel the shear bands shown Fig. 5A. The view is looking directly down at a subhorizontal surface. Lineations on the outcrop surface are defined by fragmented garnet crystals with chlorite pressure fringes.

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4.2. Shear-related metamorphic overprint Within the small cross cutting shear zones and shear bands (Figs. 5 and 6), quartz and plagioclase are deformed ductilely and brittlely respectively. Quartz exhibits undulatory extinction, core-mantle structures, and dynamic recrystallization. On the contrary, grains of albite contain abundant fractures, and the margins of grains are highly jagged. Interpreting the texture and relative ductility of the minerals, metamorphic grade during shearing was lower greenschist facies. The brittle behavior of plagioclase and the ductility of quartz, brackets the thermal conditions to 350–450  C (Koch et al., 1980; Tullis and Yund, 1987). Fig. 7 shows false color X-ray images of garnets from the Peque Creek schist inside and outside the cross-cutting late shear zones near the Martic thrust. Concordant chemical zoning in FeO and MnO with minor chemical alteration along fractures is apparent in garnets not seriously impacted

Fig. 6. Lower hemisphere stereograms of structures in the Octoraro Formation near the Martic Line. (A) Contour diagram of poles to shear bands with accompanying mineral lineation poles from the eastern segment of the Martic Line near the East Branch Octoraro Creek. (B) Contour diagram of poles to shear bands with accompanying mineral lineation poles from the central segment of the Martic Line near Bowery and Stewart’s Run. (C) Plot of poles to the regional schistosity (S2) and the shear bands with accompanying mineral lineation poles from the western segment of the Martic Line near Martic Forge.

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by the late shearing (Fig. 7A and B). Chlorite replaced garnet along fractures (Fig. 3B), and the it clearly truncates the growth zoning in the garnet (Figs. 7A and B). Small anhedral garnets with no apparent FeO and MnO concentration zoning occur in highly sheared rocks. Secondary chlorite pressure fringes are commonly associated with these garnets and they define a mineral elongation lineation (Fig. 5B). These small anhedral garnets, with no obvious chemical zoning, are interpreted to be fragments of once larger garnets, like those shown in Fig. 3. Formation of secondary chlorite at the expense of primary garnet is consistent with lower greenschist facies metamorphic conditions. 4.3. Magnetic data collection and processing Magnetic susceptibility data was collected from the hangingwall and footwall rocks across the Martic thrust (Table 1). A significant magnetic susceptibility contrast exists between the Conestoga Formation (footwall) and the Octoraro Formation units (hangingwall). Additionally, there is a wide range of magnetic susceptibility readings for the schist units within the Octoraro Formation. Based on the contrast, magnetic field surveys were conducted along three lines that cross the Martic thrust. The objective of these surveys was to generate models from the data that would provide geometric information on the hangingwall block, possibly identify unit distribution in the hangingwall that is not apparent from field data due to limited exposure, and provide dip information for the thrust at depth. Magnetic data were collected over a four-hour period on

Fig. 7. False-color X-ray images of garnets from the Octoraro Formation. These images were obtained using the Cameca electron microprobe at Virginia Polytechnic Institute. Both images show the relative composition of either FeO. (A) Garnet with minor cracks filled with chlorite that truncates the garnet FeO zoning. This garnet is typical of garnets in the Octoraro Formation away from the Martic Line. (B) Small anhedral garnet with well formed chlorite pressure fringes. Small anhedral garnets are typically found in the area of Octoraro Formation near the Martic Line. See text for details.

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Table 1 Data used to determine the magnetic susceptibility for rock units in the study area. Note the highly magnetic susceptibility for some samples from the Stewart’s Run and Tucquan schists of the Octoraro Formation. Portions of these rock units contain substantial magnetite (2-5 modal percent). Averages of the susceptibility measurements were used in the modeling Rock units

Volume w/in Ave. resistance Resistance Change in Susceptibility Density Comment chamber (cc) (w/sample) (empty) resistance (cgs) (g/cm3)

Antietam-Harpers 13.2 Antietam-Harpers 14.3 Bowery Run 34.7

10 900 10 900 10 868.5

10 900 10 900 10 900

0 0 31.5

0 0 0.00011

2.91 2.52 2.73

Bowery Run

27.5

10 887

10 900

13

0.000045

2.85

Bowery Run

16.1

10 880

10 900

20

0.00007

2.89

Chickies Fm.

17.5

10 840

10 900

60

0.00021

2.64

Chickies Fm.

16.1

10 895

10 900

5

0.000017

2.76

Conestoga Fm. Conestoga Fm. Felsic basement Felsic basement Martic Forge Martic Forge Peque Creek Peque Creek Peque Creek Stewart’s Run

20.9 14.4 13.4 11.6 14.4 15.5 18.7 16.5 14.4 27.4

10 900 10 900 10 900 10 900 9815 10 900 10 020 10 475 10 900 10 895

10 900 10 900 10 900 10 900 10 900 10 900 10 900 10 900 10 900 10 900

0 0 0 0 1085 0 880 425 0 5

0 0 0 0 0.0038 0 0.0031 0.0015 0 0.000017

2.91 2.89 2.49 2.72 2.8 2.51 2.71 2.8 2.89 2.38

Stewart’s Run Stewart’s Run Stewart’s Run Tucquan Creek Tucquan Creek Tucquan Creek

24.7 25.7 23.7 16.4 17.5 17.5

9945 9772.5 10 900 9971 10 900 10 360

10 900 10 900 10 900 10 900 10 900 10 900

955 1127.5 0 929 0 540

0.0033 0.004 0 0.0033 0 0.0019

2.51 2.64 3.46 2.89 2.89 2.7

Non-magnetic Non-magnetic Minimal magnetism Very low magnetism Very low magnetism Minimal magnetism Very low magnetism Non-magnetic Non-magnetic Non-magnetic Non-magnetic Highly magnetic Non-magnetic Highly magnetic Magnetic Non-magnetic Very low magnetism Highly magnetic Highly magnetic Non-magnetic Highly magnetic Non-magnetic Magnetic

20 August 1999 using a pair of proton precession magnetometers (see Fig. 2 for location of survey lines). The survey lines correspond to rural roads that cross roughly perpendicular to the strike of the structure. Data were collected at 100 m increments with one magnetometer while the second magnetometer remained at a fixed point near the beginning of the line. The fixed magnetometer recorded the magnetic field at 1 min intervals throughout each survey to account for the diurnal variation in the magnetic field. The location of some survey points were adjusted to avoid magnetic noise from buried metal associated with the road (such as culverts, guard-rails, and gas, water, and power lines). The magnetic data collected along each line was compared to the fixed station data to ascertain if there were any large changes in the magnetic field during the course of

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Fig. 8. Magnetic models that cross the Martic Line. The magnetic survey lines are shown on Figs. 1 and 2A. The upper portion of each model shows the observed magnetic data (intermediate gray line with black points), the calculated magnetic signature based on the models in the lower panel (light gray line), and the error between the calculated and observed data (heavy black line). The lower panels are simple models for the Octoraro Formation based on average susceptibility for various units. These models were generated to demonstrate the possible geometry of the hangingwall block and depth to the thrust; however, the internal details of the hangwall block are not supported by the geologic mapping. (A) Line 1; (B) Line 2; (C) Line 3.

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the survey. After correction for diurnal variation, the residual magnetic data were then smoothed using a 5-point moving-averaged filter (Fig. 8) before modeling was performed. 4.3.1. Modeling the magnetic field Two-dimensional modeling of the smoothed, residual magnetic data was performed along the average azimuth of each profile using the GM-SYS modeling program (Northwest Geophysical Associates, Inc.). Information on the magnetic susceptibilities was derived from measurements made with a Model MS-3 magnetic susceptibility bridge on representative samples of geologic units in the area (Table 1). Susceptibility values range from 0.0 cgs for felsic basement rocks and metasedimentary units north of the Martic Line, to a high of 0.004 cgs for the Stewart’s Run schist, which contains abundant magnetite locally. Other units with high susceptibilities were the Tuquan Creek and Pequa Creek schists, with a range of from 0.0015 to 0.0038 cgs. The only metasedimentary unit in the footwall with significant susceptibility is the Chickies Formation (quartzite), with values ranging from 0.000017 to 0.00021 cgs. Results of the modeling are shown in Fig. 8. The data was modeled by the emplacement of structures of magnetic material with susceptibilities ranging from 0.001 to 0.004 cgs in the near surface. The rocks of the footwall were modeled with magnetic susceptibility rock of 0.00 cgs based on the measurements from the Conestoga Formation. The one exception is a susceptibility of 0.002 cgs used to model a body of Chickies Formation in the footwall in survey line 1 (Fig. 8A). Variability of the magnetic susceptibility (Table 1) of the Octoraro Formation units is well demonstrated with these models; however, it must be pointed out that these models are nonunique. 4.3.2. Magnetic modeling results The model of survey line 1 (Fig. 8A) was best fit with a concave upward shape for the Martic thrust, and an antiform of high susceptibility rock in the hangingwall. Direct observation of northwest-dipping compositional layering in the Octoraro Formation near the Martic thrust, and southeast-dipping layering away from the thrust (Fig. 2C) supports the modeled map-scale antiform internal to the hangingwall. As well, the presence of rock with high magnetic susceptibility is supported by the presence of magnetite-bearing schist in the Sterwart’s Run unit in the vicinity of survey line 1. The model of survey line 2 (Fig. 8B) was developed using bodies of rock with magnetic susceptibilities of 0.001 and 0.002 cgs for the hangingwall, and 0.00 cgs for the footwall. The large shift (upward of 300 nT) in magnetic value differences along the survey line were modeled using a warped thrust contact. There is no observed surface data that supports this model. The model of survey line 3 (Fig. 8C) was developed using a southeast-dipping, narrow, wedge-shape rock body for the hangingwall with susceptibilities of 0.001–0.004 cgs. Again the footwall was modeled with susceptibility of 0.00 cgs. The overall shape of the hangingwall agrees with the shape of the hangingwall developed from the structural data. There is considerable lithologic variability in the Octoraro Formation in this region including the presence of magnetite-bearing rock units (Fig. 2A). As well, the range of observed magnetic susceptibility readings is consistent with those used in the model. At this time, the details of the model are not directly supported by detailed surface data due to limitations in bedrock exposure. However, the internal structure of the hangingwall in the magnetic model (Fig. 8C) may provide more insight into the internal unit variation in the Octoraro Formation.

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5. Structural and tectonic implications The Martic thrust is a geologically significant structural boundary in the mid-Atlantic region of the Appalachian orogen because it divides mostly metasediments of the Cambrain-Ordovician stable platform margin (Rodgers, 1968), from metasiliciclastic rocks associated with Paleoproterozoic Iapetan rift -to-drift tectonics (Gates and Valentino, 1991; Valentino and Gates, 1995). Juxtaposition of these lithotectonic sequences, with the rift-to-drift sequence residing structurally above the passive margin sequence, requires structural inversion of the Iapetan passive margin. In the mid-Atlantic Piedmont, there is sufficient evidence to conclude that the rocks were subjected to the Ordovician Taconian orogeny and the Carboniferous Alleghanian orogeny, albeit, to different extents. The regionally extensive metamorphism, and accompanying deformation, is attributed to the Taconian orogeny (Lapham and Bassett, 1964; Rodgers, 1968; Wise, 1970; Crawford and Crawford, 1980; Wagner and Srogi, 1987). As described in this paper, some rock fabrics associated with the Martic thrust appear to be late in the structural history. But, the juxtaposition of the passive margin (Conestoga Formation and associated minor units) and riftto-drift sequences (Octoraro Formation) must predate this late shearing because both sequences were impacted by the regional deformation (S1) and metamorphism (Cloos and Heitanen, 1941; Valentino, 1999). In fact, there is apparent continuity of the regional metamorphic zones and isograds (Valentino and Gates, 2001) in the region of the Martic thrust suggesting that any late movement on the fault was dominantly strike-parallel. In addition to foreland directed thrusting, as documented by all previous researchers (Cloos and Heitanen, 1941; Freedman et al., 1964; Wise, 1970), the data presented herein supports a phase of later orogen-parallel displacement, or orogenic-float on the Martic thrust. A number of large steeply dipping dextral shear zones occur in the western Piedmont (Valentino et al., 1994, 1995; Valentino, 1999). These shear zones cross cut the presumed Taconian structures and metamorphic zones, and separate major lithotectonic units (Figs. 1 and 10). The Peters Creek Formation, which has been interpreted to be Paleoproterozoic rift-related metasediments (Gates and Valentino, 1991; Valentino and Gates, 1995), is divided from the Octoraro Formation by the Pleasant Grove-Huntingdon Valley ductile shear zone (Valentino et al., 1994; Krol et al., 1999). Metamorphic overprint (M3) associated with this shear zone was documented to be lower greenschist facies (Valentino, 1999), and the overprint associated with deformation along the Martic thrust is indistinguishable, and most likely related to the same low-grade thermal event. The other option would be an independent low-grade metamorphic episode preserved only in the Martic thrust, but such an event would be unique for the western Piedmont and require an unnecessarily complex geologic history. Although the metamorphic overprint for the Martic thrust has yet to be dated directly, white micas in the Pleasant Grove shear zone (northern Maryland) were dated by Krol et al. (1999) using 40Ar/39Ar technique. They concluded that the micas in the shear zone record late Paleozoic ( 311 Ma) metamorphism that was associated with the Alleghanian orogeny. Along strike to the northeast (Fig. 1), the Pleasant Grove shear zone traces through the area discussed herein, and into the Huntingdon Valley shear zone in eastern Pennsylvania (Valentino et al., 1994). The metamorphic overprint documented by Krol et al. (1999), occurs within the segment of the Pleasant Grove shear zone in the western Piedmont of Pennsylvania. The age of this segment of the shear zone has not been directly constrained using geochronology, but the deformation and

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metamorphism was interpreted to be Alleghanian based on style of deformation and cross-cutting relationships (Valentino et al., 1994; Valentino, 1999; Gates et al., 1999). In addition to the metamorphic overprints associated with the Martic thrust and Pleasant Grove shear zone being indistinguishable, both structures cut across the regional Taconian structures and metamorphic zones in the western Piedmont (Valentino, 1999; Valentino and Gates, 2001). From these relationships, it is reasonable to conclude that both structures are related chronologically, and therefore, the latest displacement on the Martic thrust records the local effects of the Alleghanian orogeny. It is also reasonable to conclude that the Martic thrust and Pleasant Grove zone are related geometrically since they occur in close proximity, and both record orogen-parallel displacement. Previous studies in the western Piedmont concluded that the Tucquan antiform developed as a crustal-scale tranpressional dome between the Pleasant Grove and Lancaster Valley dextral shear zones (Valentino, 1990, 1999). The Tucquan antiform is approximately 30 km across at the widest point (immediately west of the Susquehanna River), and prior to post-Paleozoic exhumation, the antiform structural relief is estimated to have been about 7–8 km above the current level of erosion (Valentino, 1999; Valentino and Gates, 2001). To the east, the antiform width decreases to less than 10 km and the estimated structural relief would have been considerably less than at the widest part of the fold (Fig. 1). The main segment of the Martic thrust occurs entirely within the Tucquan antiform, and the late displacement documented herein, would have accommodated mass movement from the part of the antiform with the greatest relief toward the region of the antiform with lower structural relief. The magnetic models portray a complex dip pattern on the Martic thrust (Figs. 8 and 9), especially where the thrust traces across the Tucquan antiform hinge region (Fig. 8B). Structure contours for the thrust constructed from the magnetic models show dip on the thrust in the antiform hinge region to be locally westerly and consistent with east-directed displacement (Figs. 9 and 10). The modeled warp in the thrust occurs in the region where the eastern segment abruptly

Fig. 9. Structure contour map for the geometry of the Martic contact at depth based on the magnetic models shown in Fig. 8. Note that irregularity in the structure contour lines near Line 2.

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Fig. 10. Schematic structural model showing orogen parallel thrusting on the Martic Line, and associated with late Alleghanian dextral shearing between the Octoraro and Peters Creek Formations.

traces northwesterly. Another possible interpretation for the apparent anomaly in the thrust, is that the hangingwall is broken by multiple late thrust zones, with one tracing southwest from the eastern segment through the region where the modeled thrust is warped (Figs. 8C and 9), and the other including the western segment of the Martic line (Fig. 2). This idea is supported by a ductile thrust internal to the Octoraro Foramtion in the region of the Susquehanna River directly along strike (Valentino and Gates, 2001). The magnetic models (Fig. 8), and structure contour map (Fig. 9) produced from the models may support the continuation of these thrusts toward the northeast, where they may merge or even cut the Martic thrust.

6. Conclusions The Martic thrust is a major structure in the mid-Atlantic Piemdont that separates rocks of the Cambrian-Ordovician passive margin from the Late Proterozoic rift-to-drift siliclastic metasediments. Juxtaposition of these rocks units, and structural inversion of the Iapetan passive margin across the Martic thrust was most likely accomplished during the Taconian orogeny. But, data presented in this paper suggest that the Martic thrust was last active during Alleghanian dextral tranpression. The thrust accommodated orogen-parallel escape similar to the orogenicfloat proposed for other regions of the Piedmont (Gates et al., 1999). The reactivation of the Martic thrust during the Alleghanian orogeny was possibly due to the major variations in the local structural relief along the length of one of the largest transpressional antiforms in the orogen.

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Acknowledgements Funds for the geologic mapping were provided by the Pennsylvania Geological Survey from 1989–1996. Garnet false-color X-ray images were obtained at the Electron Microprobe Laboratory at Virginia Polytechnic Institute and State University. We thank Robert Tracy for help in the study of the Octoraro Formation garnets. Partial support for this research was provided through the Faculty Development Grant at the State University of New York at Oswego. The thoughtful review and comments from Lee Slater, Alexander Gates, David Thomas, and one anonymous reviewer were greatly appreciated. References Alcock, J., 1994. the discordant Doe run thrust: implications for stratigraphy and structure in the Glenarm supergroup, southeastern Pennsylvania Piedmont. Geological Society of America Bulletin 106, 932–941. Bauer, R.L., Bidwell, M.E., 1990. Contrast in the response to dextral transpression across the Quetico-Wawa subprovince boundary in northeastern Minnesota. Canadian Journal of Earth Sciences 27, 1521–1535. Cloos, E., Heitanen, A.M., 1941. Geology of the ‘‘Martic overthrust’’ and the Glenarm series in Pennsylvania and Maryland, Geological Society of America, Special Paper 35. Crawford, M.L., Crawford, W.A., 1980. Metamorphic and tectonic history of the Pennsylvania Piedmont. Journal of the Geological Society of London 137, 311–320. Crawford, W.A., Hoersch, A.L., 1984. The geology of the Honey Brook Upland, southeastern Pennsylvania, In: Bartholomew, M.J. (Ed.), Geological Society of America, Special Paper 194, pp.111–125. Culshaw, N., 1991. Post-collisional oblique convergence along the Thelon tectonic zone, north of Bathurst Fault, NWT, Canada. Journal of Structural Geology 13, 501–516. Dewey, J., Burke, K., 1973. Tibeten, Variscan and Precambrian basement reactivation: products of continental collision. Journal of Geology 81, 683–692. Freedman, J., Wise, D.U., Bentley, R.D., 1964. Patterns of folded folds in the Appalachian Piedmont along Susquehanna River. Geological Society of America Bulletin 75, 621–638. Fischer, G.W., Higgins, M.W., Zeitz, I., 1979. Geological interpretations of aeromagnetic maps of the crystalline rocks in the Appalachians, northern Virginia to New Jersey, Maryland Geological Survey Report of Investigations 32. Gates, A.E., 1987. Transpressional dome formation in the southwest Virginia Piedmont. American Journal of Science 287, 927–949. Gates, A.E., Muller, P.D., Krol, M.A., 1999. Alleghanian transpressional orogenic-float in the Baltimore terrane, central Appalachian Piedmont, In: Valentino, D.W., Gates, A.E. (Eds.),The Mid-Atlantic Piedmont: Tectonic Missing Link of the Appalachians, Geological Society of America, Special Paper 330, pp. 127–139. Gates, A.E., Valentino, D.W., 1991. Late Proterozoic rift control on the shape of the Appalachians: the Pennsylvania reentrant. Journal of Geology 99, 863–872. Hansen, V.L., 1989. Structural and kinematic evolution of the Teslin suture zone, Yukon: record of an ancient transpressional margin. Journal of Structural Geology 11, 717–733. Kasselas, G., Glover, L. III., 1997. Late Proterozoic cover rocks in the Blue Ridge of northern Virginia: Do they include a terrane boundary? In: Gates, A.E., Glover, L., III (Eds.), Central and Southern Appalachian Suters: Results from the EDGE Project and Related Studies, Geological Society of America, Special Paper 314. Koch, P.S., Christie, J.M., George, R.P., 1980. Flow law of ‘‘wet’’ quartzite in the a-quartz field. EOS 61, 376. Knopf, E.B., Jonas, A.I., 1929. Geology of the McCalls Ferry-Quarryville District, Pennsylvania, United States Geological Survey Bulletin 799. Krol, M.A., Muller, P.D., Idleman, B.D., 1999. Late Paleozoic deformation within the Pleasant Grove shear zone, Maryland: results from 40Ar/39Ar dating of white mica, In: Valentino, D.W., Gates, A.E. (Eds.), The Mid-Atlantic Piedmont: Tectonic Missing Link of the Appalachians, Geological Society of America, Special Paper 330, pp. 93– 112.

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