A Temporal and spatial changes in shortening direction during progressive deformation of the central Appalachian fold-and-thrust belt: Evidence from joints and veins EVANS, Mark A., Department of Physics and Earth Sciences, Central Connecticut State University, New Britain, CT ABSTRACT: In a deforming fold-and-thrust belt, structures do not form instantaneously across the belt, but instead are related to the passage of a deformation front, which starts in the hinterland and progresses toward the foreland. Therefore, structures in one part of the belt are not necessarily time-correlative with those in another part. Changes in the regional stress field during the development of the orogen will result in changes the orientation of the structures being formed progressively toward the foreland. In addition, structures associated with later deformation events (and different stress fields) may overprint earlier- formed structures. The Late Paleozoic central Appalachian fold-and-thrust belt in West Virginia and northern Virginia is rich in syntectonic extensional joints and veins (mineral filled fractures) that, along with other mesoscale structures, record the shortening history for the orogen. Joint and vein orientations, cross cutting relationships, and vein mineral paragenesis are used to constrain the timing of structure formation. The earliest joints are pre-folding ENE-striking and NNW-striking sets that contain multiple stages of mineralization. They are common in the Pendleton Co., WV area, but are rare to the north. They are interpreted to be related to an early NNW-directed Alleghanian shortening event, possibly the formation of the Southern Appalachians. These structures are followed by very common pre-to syn-folding NE-striking and NW- striking sets that formed with the forelandward growth of folds associated with the development of the underlying Cambro-Ordovician duplex. These sets contain multiple stages of mineralization. The NW set exhibits a progressive swing in orientation to a more westerly strike from east to west across the Valley & Ridge, and reflects a change in shortening direction over time. Syn- to post-folding, rarely mineralized WNW-striking joints are found primarily in the Wills Mt. and Allegheny Front area, and are interpreted to be associated with the emplacement of the Wills Mt. duplex. Finally, post-folding, rarely mineralized EW-striking joints and veins overprint pre-existing structures throughout the Valley & Ridge, and are interpreted to be related to a late Alleghanian E-W shortening event associated with out-of-sequence thrusting in the Blue Ridge. OBSERVATIONS, SUMMARY and QUESTIONS ● Joint and vein fracture sets in the Valley and Ridge province are defined by strike orientation, vein mineral paragenesis, cross cutting relationships and unfolding relationships. ● The fracture sets are pre-folding (NNW and ENE sets), pre- folding to early syn-folding (NW and WNW sets along with strike sets), and post –folding (EW set). ● The curvature of the NW set strikes across the region indicates that shortening direction changed during folding of the Valley and Ridge province. ● The ENE, NNW, and EW fracture sets appear to be associated with a regional stress field, while the NW and WNW fracture sets were driven by the advancing deformation front. ● The earliest NNW set apparently records a NNW directed shortening. This shortening may also be recorded in joints and veins in the Great Valley. Is this the Southern Appalachian shortening event? ● Is the North Mountain thrust fault a relay thrust of the Saint Clair thrust fault? ● What deformation events are recorded by the brittle fractures in the Great Valley? Figure 1. Location map of the study area. Shown are structure stations where joint and vein data were collected. AR-CM is the Adams Run Cacapon Mountain anticlinorium. INTRODUCTION: The central Appalachian fold-and-thrust belt was deformed during the Late Paleozoic Alleghanian orogeny associated with the collision of Laurentia and Gondwana. The belt is characterized by three major thrust systems : 1) the Blue Ridge thrust system, 2) The North Mountain Thrust system, and 3) the Lower Carbonate Duplex that deforms the overlying Paleozoic cover rock sequence. In order to investigate how the shortening direction changed over time, joint and vein (mineral filled fracture) data was collected throughout the region. The data include: Joint and vein orientation Unfolding relationships Vein mineral paragenesis Cross-cutting relationships In addition, fluid Inclusion microthermometry of vein minerals. Provides data on the temperature and pressure conditions during vein formation. These data are used to establish a regional sequence of joint and vein formation that may be related to a changing shortening direction over time, as well as the forelandward development of fold and thrust structures. Figure 6. Deformation sequence for cross section A-A’ (Fig.1) showing the progressive shortening of the Paleozoic section. Sample sites are placed in their approximate restored stratigraphic positions. Heavy double-ended bars represent range of overburdens determined from fluid inclusion data, single bars are actual overburden depths determined from fluid inclusion isochore intersection method. Data from calcite are given as blue lines, data from quartz are black lines. Dashed line is interpreted extent of overburden at each time step. Stage b – NNW Set Stage c - NW Set Stage d - NW Set Stage e - WNW Set Stage f - WNW Set Late-Stage f - EW Set Figure 9. Series of paleogeographic maps corresponding to deformation stages in Figure 6. Each stage shows areas that are experiencing erosion or deposition related to forelandward emplacement of thrust systems. Arrows show average shortening direction and how it rotates during deformation. ‘Bow ties’ represent average strike of extensional fractures during each stage. Load resulting from emplacement of Blue Ridge thrust system Load resulting from emplacement of North Mountain thrust system JOINT AND VEIN SEQUENCE (Cont.) Cross-Cutting Relationships Conclusive age relationships between joints and veins are rare. However, some consistent patterns are found. EW-striking joints consistently abut against and/or hook into NW joints EW-striking veins cut across NW-striking veins EW-striking veins are also found to cut across NE-striking veins NE-striking joints were commonly, but not exclusively, found to abut NW Figure 4. Examples of vein fill. All photomicrographs taken through crossed polars, Scale bar is 1 mm. (A) Prismatic quartz. (B) Blocky quartz. (C) Fibrous calcite with curved crystal fibers. (D) Elongate blocky calcite with crack-seal texture. (E) Partly recrystallized calcite. (F) Sheared blocky calcite, followed by elongate blocky calcite. Figure 5. Examples of vein mineral paragenesis. (A) Quartz and chlorite (qtz/chl) followed by qtz1 and by later qtz2. (B) Highly twinned calcite (cc1) followed by less twinned cc2 and later qtz1. (C) qtz/chl followed by qtz1. (D) Close-up of boundary between qtz/chl and qtz1. JOINT AND VEIN SEQUENCE Unfolding Relationships Joint and vein sets were defined by their orientation and relationship to bedding. Most cross-fold joints and veins cross the fold axis at a high angle, cut cleanly across multiple beds, and are subvertical. The NW set (i.e., NW-striking) that has poles that fall on, or clockwise of, the average fold axis on the stereonet (Fig. 2) is the most common. Upon unfolding, the poles all become subvertical. WNW-striking joints and veins are found only in the western part of the study area. Their poles do not change orientation significantly upon unfolding. EW striking joints and veins have poles that are anticlockwise of the average fold axis. Upon unfolding, the poles become more diffuse. Three sets of strike fractures are defined after unfolding. Common NE-striking joints and veins broadly cluster on the normal to average bedding trend (Fig. 2). Less common ENE-striking joints and veins cluster approximately 10° anti-clockwise of north-south Poles to a NNW-striking set cluster approximately 10° anticlockwise of east-west. Figure 2. Example of how joint and vein sets were determined. Vein Mineral paragenesis Vein mineral paragenesis was used to constrain the timing of vein formation. Early- formed veins, contain multiple episodes of vein minerals (e.g., Fig 5) some of which may be deformed; while later-formed veins contain either one or maybe two episodes of mineralization (e.g., Fig. 4) or no mineralization at all. NW-striking veins may contain up to three or four episodes of mineralization and commonly exhibit shear and recrystallization WNW-striking veins may contain several episodes of mineralization EW-striking veins are usually not mineralized, but may contain one, or less commonly two, episodes of mineralization. All three sets of strike veins may have multiple episodes of mineralization Chlorite is only found in ENE and NE sets. Figure 3. Examples of lower-hemisphere equal area stereonets with regional joint and vein data by stratigraphic level. A) Allegheny Front Braillier through Hampshire Fms. B) Patterson Creek anticline, Tonoloway and Helderberg Fms. C) Adams Run anticline, Tonoloway and Helderberg Fms. A B C Figure 7. Orientation of Cross- fold joints and veins from each fracture set shown at the structural station where they are found. Note: NNW and ENE sets are uncommon and concentrated in the southern part of the Valley and Ridge. NW set fracture strikes curve across the valley and ridge indicating a pre-folding to early syn folding change in shortening direction. WNW set fractures are concentrated in the western part of the Valley and Ridge. EW fractures are uncommon, but are concentrated in the western part of the Valley and Ridge. Figure 8. palinspastic reconstruction of the central Appalachians based on three balanced cross-sections across the region, including A- A’ (Figs 1 and 6). Also includes are major present day structures (shaded) and the location missing stratigraphic section due to the emplacement of the North Mountain Thrust Sheet. Preliminary joint and vein data from the Great Valley of Virginia also shown. Stereonets show poles to fractures rotated to bedding horizontal. So, all joints and veins are interpreted to be pre-folding. Pre-folding Syn-folding Post- folding Load resulting from emplacement of Wills Mountain thrust system