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The Kinematics and History of Brittle Deformation in the Petersburg Granite

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The Kinematics and History of Brittle Deformation in the Petersburg Granite W&M ScholarWorks Undergraduate Honors Theses Theses, Dissertations, & Master Projects 5-2010 The Kinematics and History of Brittle Deformation in the Petersburg Granite James K. McCulla College of William and Mary Follow this and additional works at: https://scholarworks.wm.edu/honorstheses Part of the Geology Commons Recommended Citation McCulla, James K., "The Kinematics and History of Brittle Deformation in the Petersburg Granite" (2010). Undergraduate Honors Theses. Paper 451. https://scholarworks.wm.edu/honorstheses/451 This Honors Thesis is brought to you for free and open access by the Theses, Dissertations, & Master Projects at W&M ScholarWorks. It has been accepted for inclusion in Undergraduate Honors Theses by an authorized administrator of W&M ScholarWorks. For more information, please contact [email protected]. The Kinematics and History of Brittle Deformation in the Petersburg Granite Richmond, Virginia James McCulla May 16, 2010 James McCulla Abstract The Petersburg granite forms a large Carboniferous pluton in the eastern Piedmont of Virginia and is well exposed in the James River along the Fall Zone in Richmond. These expansive outcrops were studied to characterize the fracture geometry and understand the kinematic history of brittle deformation in the Petersburg granite. Previous workers have suggested some fractures in the Richmond area are radial fractures associated with the 35 Ma Chesapeake Bay impact crater in eastern Virginia. We mapped fractures at five locations along an 8 km transect of the James River. The Petersburg granite in Richmond is cut by two dominant fracture sets. The older set strikes NE to ENE, dips steeply to the north, and commonly displays gently plunging mineralized slickensides that record dextral slip. P- and T-axes for these shear fractures are clustered and consistent with a subhorizontal σ1 oriented WNW-ESE. Quartz, muscovite, and biotite indicate mineralization and slip occurred under greenschist facies conditions. The younger set strikes NNW to NNE, is subvertical, and rarely mineralized. Cross cutting relations and surface ornamentation indicate the younger set are extension fractures, although rare fractures were reactivated as reverse faults. We interpret the older set to have formed during WNW-directed contraction in the Alleghanian orogeny. The orientation of the younger set records E-W extension and is parallel to extensional faults in the Richmond Triassic basin, exposed 15 km to the west. Reactivation of these extension fractures may have occurred under the late Cenozoic compressional stress field. Regardless, there is no tenable evidence linking fractures in the Petersburg granite to deformation associated with the Chesapeake Bay impact crater. 1. Introduction James McCulla To a structural geologist, sets of fractures in rocks offer a great opportunity to learn about those rocks’ stress history. Fractures, the product of brittle deformation, form when stress on a rock exceeds the rock strength and the rock breaks along a creating joints and faults. Fractures and joints are related; joints record small amounts of movement at right angles to the fracture plane. These planar features on the register as lines on the rock surface. Fractures can record the orientation of stresses active upon the rock during its deformation; this applies especially if fractures have any lateral movement during the fracturing event. These shear fractures commonly form lineations on the fracture surfaces, recording the general orientation of stresses during deformation. Workers can then compare this information with knowledge of the rock’s age to determine what actually might have caused the fractures to form; if geochronology is not available, fractures and fractures sets can often serve as relative dating tools. If one rock formation is fractured while an adjacent is not, the adjacent rocks are younger than those fractured. The Peterburg Granite is a large batholith in east-central Virginia (Fig. 1) consisting of K-feldspar, quartz, and plagioclase with minor amounts of biotite and muscovite and is characterized by several readily examinable fracture sets, suggestive of extensive deformation. The granite is 330 Ma in age (Wright et al., 1975) and intruded during the Alleghanian orogeny (Gates & Glover, 1989). Possible causes of fractures in the Peterburg Granite include fracturing during the actual orogenic event, Mesozoic rifting that led to the opening of the Atlantic Ocean, and radiating and concentric fractures created by from the Eocene Chesapeake Bay Impact Structure. James McCulla Structural data were collected from the Peterburg Granite exposed in Richmond, Virginia; these data included orientations of fractures, slickenlines, and dikes throughout the granite. Fracture and slickenline data, as well as crosscutting relations, were analyzed in an attempt to characterize extension fractures from shear fractures, and the shear fractures were in turn analyzed to determine the sense of shear and the implied orientation of principal stress during deformation. All of this information was used to determine an overall deformation history of the Petersburg Granite, which was unique to the Richmond, VA area and the first of its kind. This deformation timeline can be compared to other deformation histories of rocks in the area to help create an overall timeline of deformation and learn about the geologic history of eastern North America. 2. Fracture Geometry There are two types of fractures: shear and extension. It is relatively simple to determine the difference between shear and extension fractures if obvious indicators like slickenlines are present; these give a “sense of shear” and point towards shearing as the deformation mechanism. Extension fractures can be indicated by small, parallel cracks at an angle to the orientation of the fracture and are often filled in with post-fracturing mineralization like quartz. If such indicators are not present, other methods can be used to determine what types of forces are responsible for the deformation. For example, Reidel shear structures are formed in the early stages of development of a shear zone and can be easily identified in field samples (Katz et al., 2002). Fractures like these are only found in shear environments and form in sets at certain angles to the principal stress; they can be used to differentiate between extension and shear fractures. James McCulla It is also possible to examine a fracture set in order to determine the direction from which stresses were applied during the actual deformation of the rock. Computer programs like Stereonet and FaultKin have computerized this process, but other manual methods exist wherein a worker plots great circles and lines on a 3-D stereographic projection in order to determine σ1 and σ3 for a given fault population. σ1 is usually oriented between 30º and 45º from the fault; visualizing fracture orientations in a fracture set using stereographic projections enables one to estimate σ1 using this fact (Marshak & Mitra, 1988). Marshak and Mitra (1988) also outline another method for determining σ1 using stereographic projections for larger fault populations (i.e. >100 faults or fractures). This method is useful only for fractures with numerous lineations indicating slip direction. Fracture planes are plotted on an equal area stereographic projection and compared to another plot of the lineations for the fractures. Most slips for a fracture population should point away from the σ1 direction, allowing one to determine the principal stress orientation by transposing the lineations projection over that of the fractures (Fig. 2). 3. Geologic Setting The Piedmont geologic province is situated in the center of Virginia and is bounded by the Fall Zone on the east (and the Coastal Plain beyond that) and the Blue Ridge on the west. Petersburg Granite is the main member of the Fall Zone in the Richmond area and has been studied by geologists as far back as the late 1800s, and Watson (1906) documented two dominant sets of joints, one striking to the NE and dipping subvertically and one striking to the NW and dipping subvertically. Watson (1906) observed two secondary sets, also dipping subvertically: one striking to the east James McCulla and one striking to the north. He noted that any joints not dipping subvertically dipped between 20º and 82º in all compass directions. Watson (1906) concluded these secondary joints were products of weathering and exfoliation. Bloomer (1939) presents more data regarding the Peterburg Granite. According to Bloomer, rocks of the Peterburg Granite extend from Hanover County in Virginia into northern North Carolina. He observed evidence for a Carboniferous (now Mississippian) age based on knowledge of orogenies in the east coast of the United States and on the presence of Peterburg Granite boulders in Triassic basins in the Richmond area. Bloomer (1939) notes some high-angle faulting in the western edge of the exposed Peterburg Granite about 25 km to the west of Richmond but does not supply more structural data or hypotheses about deformation. Wright et al. (1975) attempted to date the Peterburg Granite in addition to commenting on the emplacement of the pluton and the age of the pluton in relation to other igneous bodies in the eastern United States. Zircons from the Peterburg Granite in the Richmond area were dated to 330 ± 8 Ma, but their results were not completely concordant; they attribute the discordant ages of about 25 Ma to weathering-related lead leaching. An age
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