On the use of regional joint sets as trajectories of paleostress fields during the development of the Appalachian Plateau, New York

Abstract

To compare the orientation and development of jointing with the orientation and magnitude of finite strain recorded in the Upper Devonian rocks of the Appalachian plateau, New York, we mapped systematic joint sets on an area of 20,000 km2. In this area, Wedel [1932] mapped folds with limb dips of less than a degree and axes that change strike by 30° from 090° in the east to 060° in the west. We observed two different cross‐strike joint sets that maintain their approximate cross‐strike position from east to west. Yet, in detail the angle between the sets is 18°±2° in the east and 30°±4° in the west. In many outcrops one joint set parallels the direction of maximum compressive strain (εƒ) as recorded by deformed fossils, whereas the other joint set never parallels εƒ. Rare calcite‐filled joints are oriented parallel to the direction of εƒ. In addition, the calcite‐filled joints both cut and are cut by solution cleavage. These observations suggest that the joint set paralleling εƒ formed during the deformation event represented by the deformed fossils. The joint set that does not parallel εƒ somehow reflects a deformational event other than that producing fossil distortion, as suggested by strain relaxation experiments. In an outcrop of the Machias formation, where the direction of εƒ is 15° from the trace of a cross‐strike joint set, the tensor average from 14 subsurface strain relaxation tests shows in situ maximum comprehensive strain (εoc = 10−4) parallel to cross‐strike joints but not the direction of εƒ. Strain relaxation is coaxial with a fabric anisotropy indicated by sonic velocity tests. Our idea is that the orientations of cross‐strike joints parallel to εoc were controlled by the same rock property that causes strain relaxation on overcoring, whereas the orientations of the cross‐strike joints parallel to εƒ were controlled by the stress field causing the fossil distortion. Placing these structural data in a regional context allowed us to construct a dynamic and kinematic model for the structural evolution of the New York plateau. The model indicates variation in both boundry conditions and material behavior through a series of four distinct deformational events, which begin prior to lithification and end in the Recent. Thus our analysis suggests that the structural features we have used represent a set of highly sensitive tools for investigating the deformational history of the Appalachian foreland.

Theoretical analysis of cross‐joint geometries and their classification

“…As opening-mode fractures, cross joints propagate normal to local least principal stress, and thus the planar central portion of a cross joint (i.e., the portion unaffected by local stress perturbations) may be used to construct regional stress trajectories in a manner similar to other joint sets [e.g., Muller and Pollard, 1977;Engelder and Geiser, 1980;Engelder, 1982] of theoretical cross-joint trajectories that best match the observed geometries (Figure 10). We have selected two field examples to demonstrate the application of the methodology.…”

Innovative Methodology for Detection of Fracture-Controlled Sweet Spots in the Northern Appalachian Basin

“…These thrusts indicate that the northward dipping limb of 42 the Tully (see following section) is a fault-modified structure. The thrusts are consistent with a rollover fold/blind ramping thrust model for the locally northerly dip of the Tully.The NE-strike is surprising, since Engelder and Geiser (1980) showed generally ENE-striking -strike-parallel‖ fractures in the region, and to the south E-to ENE-striking structures are the norm for easterly trends. Thus, the NE-striking faults (and NE-striking FIDs and fracture set in the same area) may not be related to the ENE-striking strike-parallel fractures related to a farfield Alleghanian stress field.…”

“…However, a confusing and often contradictory array of publications and opinions make deciphering the character of the fractures and fracturing history from the published literature extremely difficult. In general, three main systematic fracture sets were thought occur in the Appalachian Plateau of NYS: Set I is orthogonal to Alleghanian folds (cross-fold, or cross-strike, joints), Set II is approximately parallel to the Alleghanian folds (fold-parallel, or strike-parallel, joints) and Set III maintains an orientation of about 060 o across the Appalachian Plateau of NYS (e.g., Parker, 1942;Nickelsen and Hough, 1967;Engelder and Geiser, 1980). Set I consists of two fracture sets, Ia and Ib, with orientations less than 30 o apart.…”

“…At both RDJ 74 and RDJ 74A+B, the fracture frequency of the northerly-striking fracture sets is about an order of magnitude greater than that of the ENEstriking set. Similarly, at site 1B in Figure 3.1-9 [TR #1], the northerly-striking fractures have nearly an order of magnitude greater frequency that the easterly striking fractures.The E-and ENE-striking fractures have been referred to as the strike-parallel (fold-parallel) fracture set (e.g., Engelder and Geiser, 1980). Typically, these fractures were thought to abut the NNW-striking set, and were therefore thought to be younger than the NNW-striking set (e.g., Engelder and Geiser, 1980).…”

“…Set Ib fractures fan across NYS, generally orthogonal to the arc of Alleghanian fold traces (e.g., Engelder and Geiser, 1980). Although Engelder and Geiser (1980) believed that Set 1a fractures -show no evidence of...rotation [and] the strike of the joints maintains parallelism for 100 km before abruptly rotating about 20 o to the east‖ (p. 6334), Engelder and Geiser (1980) also stated that -in general, the mean orientations of... Ia rotate counterclockwise from east to west...‖ (p. 6323).The proposed relationships between sets 1a and 1b have often appeared to be complicated and even contradictory. Parker (1942) believed that sets 1a and 1b were a conjugate shear pair, but Nickelsen and Hough (1967) suggested that sets Ia and Ib were not a conjugate shear pair because of a lack of evidence for shear and inconsistency in the fracture sets that form the conjugate pair.…”