Fault stepovers link individual fault segments in all the major groups of faults (normal, strike slip, and reverse). In extensional regimes, normal-fault segments can be linked by relay ramps, which have been recognized around the world (see reviews by Faulds and Varga, 1998, and Fossen and Rotevatn, 2016), including, for example, in the United Kingdom (e.g., Peacock and Sanderson, 1994); Canyonlands, United States (e.g., Trudgill and Cartwright, 1994; Commins et al., 2005; Pless, 2014); Basin and Range province in Oregon, United States (Crider, 2001); Mexico (Xu et al., 2011); Spain (Soliva et al., 2006); Italy (Di Bucci et al., 2006; Soliva et al., 2008); western Turkey (Gürboğa, 2014); the North Sea (e.g., Dawers and Underhill, 2000; McLeod et al., 2000); Greenland (Peacock et al., 2000); onshore Africa (e.g., Morley, 2002); and offshore Africa (e.g., Dutton and Trudgill, 2009). Linked normal-fault segments have also been modeled extensively (e.g., Acocella et al., 2005; Hus et al., 2005; Soliva et al., 2006, 2008; Whipp et al., 2016). However, to our knowledge, no relay ramps have been reported or modeled for the Northern Appalachian Basin, for either the Ordovician Trenton and Utica Groups or the Devonian Onondaga Limestone and Marcellus Shale. The lack of reported examples is largely because of a lack of continuous outcrop and, until recently, a lack of 3-D seismic surveys.
Counter Strike 1.6 HD WarField 2013 2018 No Survey
We report in this paper the first identification of both relay ramps and rhombochasms in the northern Appalachian Basin. These newly recognized structures have implications for deciphering the tectonic development of the basin. The relay ramps that occur above the Silurian evaporites of the Salina Group suggest that the deformation associated with these relay ramps developed initially in an extensional environment, not in a compressional environment as commonly envisioned (e.g., Sak et al., 2012; Mount, 2014; Gillespie et al., 2015). Furthermore, the proposed Late Devonian age of the relay ramps suggests that these fault systems were active during the Neoacadian orogeny, at least 50 m.y. before the commonly accepted age of structural development related to the Alleghanian orogeny (e.g., Frey, 1973; Harrison et al., 2004; Sak et al., 2012; Molofsky et al., 2013; Mount, 2014; Gillespie et al., 2015). Our proposed older age of fault initiation signifies that the faults would have been already in existence when oil generation occurred in the northern Appalachian Basin (Jacobi et al., 2012, 2013, 2015, 2018). The faults thus would have been potential conduits for oil and gas migration out of the Devonian black shales up into the higher Devonian sandstones like the Elk and Bradford (Jacobi et al., 2012, 2013).
The importance of information concerning fracture network characteristics, including orientation, timing of development, and fracture aperture and sealing, has been recognized for decades in oil and gas exploration and in other fluid migration studies, such as contaminant migration and geothermal projects (e.g., Christie-Blick and Biddle, 1985; Zoback, 2010; Vignaroli et al., 2013). Recognition of rhombochasms can lead to highly productive fracture plays, since the local transtensional environment that characterizes a rhombochasm results in localized areas of relatively high fracture porosity (e.g., Christie-Blick and Biddle, 1985; Cunningham and Mann, 2007; Zoback, 2010; Mitra and Paul, 2011). The stepover faults and associated fracture networks will be at unexpectedly high angles to the general trends of the orogen-parallel fault systems (e.g., Mitra and Paul, 2011). Localized fracture plays related to Taconic rhombochasms most likely occur in other parts of Appalachian Basin where a component of Taconic strike-slip motion occurred.
The dominant faults in the Mohawk Valley strike north to northeast (Figure 2) and have been regarded as Ordovician Taconic normal faults based on field stratigraphic relationships (see reviews by Bradley and Kidd, 1991; Jacobi and Mitchell, 2018; Jacobi and Ebel, 2019). In the 1980s, eastward subduction-zone models suggested that the normal faults were related to plate flexure and plate subsidence as the Laurentian plate entered the subduction zone during the Taconic Orogeny (e.g., Jacobi, 1981; Rowley and Kidd, 1981; Stanley and Ratcliffe, 1985; Bradley and Kidd, 1991). More-recent models proposed that the faults are related to Taconic retro-arc foreland basin subsidence (e.g., Macdonald et al., 2014, 2017; Jacobi and Mitchell, 2018), although limited eastward subduction during final continent-arc collision also may have taken place.
The faults associated with the graben apparently totally ceased motion by the end of Ordovician since the Cherokee unconformity, which approximates the base of the Silurian in the Mohawk Valley region (e.g., Swezey, 2002), appears generally undisturbed above the graben (Figure 13). The Cherokee unconformity marks the end of the Taconic tectophase of the Taconic Orogeny (Figure 3; e.g., Ettensohn and Brett, 2002; Ettensohn, 2004). In other 3-D surveys, the faults have different timings of final cessation, and a few penetrate the entire Devonian section (Jacobi, 2011, 2012). These variable times of fault cessation inferred from the seismic sections are consistent collectively with (1) the Late Ordovician faulting in the Mohawk Valley determined from the age of breccias and growth fault sections in the Upper Ordovician Black River, Trenton and Utica section (e.g., Bradley and Kidd, 1991; Jacobi and Mitchell, 2002); (2) the inferred reactivated faulting that affected the Silurian and Devonian units (Jacobi and Smith, 2000, for a review see Jacobi and Mitchell, 2018; Jacobi and Ebel, 2019); and (3) highly compartmentalized effects of (multiple) hydrothermal circulation events (e.g., Smith, 2006; Marner et al., 2008; Jacobi et al., 2018; Hunt, 2020).
Structures associated with Hoffmans Fault in Wolf Hollow (Figure 15) indicate that both strike-slip and dip-slip motion occurred on Hoffmans Fault. These structures include (1) drag folds in the down-dropped Utica and Lorraine Groups against the flat-lying carbonate succession of the Little Falls Dolostone to Trenton Group, (2) heavily veined shale, (3) a thin zone of scaly cleaved melange, (4) a heavily fractured zone, and (5) a small number of faults with minor throw (on the order of a centimeter). The steeply dipping veins that strike collinearly with the strike of the fault display stepovers with both restraining bends (Figure 15A) and releasing bends (Figure 15B) that suggest both left- and right-lateral motion along the fault zone. An approximately 5-cm drag fold with a vertically plunging axis and an axial surface parallel to the veins also indicates left-lateral motion (Figure 15C). Dip-slip motion is inferred from downdip plunging slickenfibers (Figure 15C), which is consistent with observed thickening of the Trenton and Utica succession on the eastern, downthrown side of this fault (Jacobi and Mitchell, 2018, figure 5 therein).
If the Neoacadian fault model proposed herein is correct, then the faults began developing before oil and gas generation. Based on the sediment infills, the age of faulting ranged from circa 380 to 370 Ma (Jacobi et al., 2012, 2013, 2018). Subsidence curves suggest that oil generation in the Marcellus Formation in the area of the 3-D seismic survey began circa 360 Ma, with peak oil generation between circa 360 and 310 Ma (Jacobi et al., 2012, 2013, 2018). Gas generation followed the oil generation from circa 360 to 270 Ma. This model is confirmed by bitumen-filled veins associated with the faults in core (e.g., Jacobi et al., 2018). These faults thus were most likely conduits for oil and gas migration away from the Marcellus Formation; the stratigraphically higher sandstones such as the Devonian Elk and Bradford sandstones were probably charged in this manner.
Careful analyses of thickness variations of reflector intervals across the faults in 3-D surveys can reveal the timing and sense of dip-slip motion of the suite of faults in the target area. Additionally, small thickness variations in particular reflector intervals at fault stepovers can indicate the timing and sense of motion for a component of strike-slip faulting, as shown above for the Mohawk Valley region. Comparison of the thickness variations across the faults in the seismic survey will allow determination of the pervasiveness of a particular motion history in the target area.
Structures in the northern Appalachian Basin generally have been ascribed to compressional tectonics related to the Alleghanian orogeny. Relay ramps implying local extensional stress conditions and rhombochasms implying strike-slip motion have not been reported in the northern Appalachian Basin. However, we have recognized relay ramps in a 3-D seismic survey in the Appalachian Basin of western Pennsylvania. The relay ramps are observed in structure maps of the Devonian Onondaga Limestone (which underlies the Marcellus Formation).
(A) Time-structure map on the top of Proterozoic basement in eastern New York State based on three-dimensional seismic surveys. Approximate location is within the eastern white-dashed box in Figure 1. Color ramp for depth is green and orange = high, blue = low. Red line indicates the generalized trace of the main series of linked northeast-striking faults. Dashed black line indicates approximate northwestern extent of significant downwarping related to the grabens. Both the black dashed line and the red line display right steps in the fault system. Brown hachured lines indicate fault traces picked on each inline and crossline (dip-slip component of fault motion is down on the hachured side). Easterly striking fault traces are indicated by red and white arrows (see text for discussion of red and white arrows). Yellow arrows indicate deepest parts of the grabens, which are located near right steps of the fault system. This geometrical relationship is especially evident in the southwestern two grabens (no. 1 and no. 2). (B) Simplified schematic diagram of (A) that displays the relationships among the fault segments, fault right steps, and deepest extents of the grabens (blue ellipses). The black dashed line indicates the approximate outline of the southeastern side of the grabens. The right steps of the main fault (in red) and associated deepest parts of the grabens are interpreted to represent rhombochasms that developed in a right-lateral locally transtensional regime. (C) Simplified schematic diagram showing the development of a rhombochasm at the right step of a right-lateral strike-slip fault. 2ff7e9595c
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