Foreland basin stratigraphic control on thrust belt evolution

The link between orogenic activity and foreland basin stratigraphy is well established; however, potential controls by foreland basin stratigraphy on thrust belt architecture have not been fully evaluated. Mechanical properties of typical foreland basin stratigraphic successions influence the structural development of fold-thrust belts in predictable ways. Fundamental features of foreland basins include the onset of rapid subsidence and deposition of a coarsening-upward sedimentary succession. In the lower part of this succession are finegrained, distal foreland basin deposits. Enlargement of the orogenic wedge through frontal accretion incorporates the foreland basin strata into the thrust belt, and distal foreland basin depositional units may be preferentially exploited as a thrust detachment zone, resulting in multiple detachment levels. We propose that foreland basin stratigraphic architecture has significant influence on the structural development of thrust belts and that, by extension, processes that influence foreland basin sedimentation may ultimately influence orogenic evolution far removed in time and space. INTRODUCTION Frontal parts of most fold-thrust belts consist of structurally cannibalized foreland basin successions. Whereas robust explanations for patterns of foreland basin sedimentation and stratigraphy have been developed (e.g., Quinlan and Beaumont, 1984; Allen et al., 1991; Sinclair, 1997; Liu et al., 2005), and numerous structural studies demonstrate the importance of mechanical properties of the stratigraphic column in controlling deformation within a fold-thrust belt (e.g., Woodward and Rutherford, 1989; Pfiffner, 1993), the connection between foreland stratigraphy and structural architecture has not been systematically explored. In this study we synthesize data from several foreland basins and fold-thrust belts to demonstrate the relationship between stratigraphic architecture of the basin and structural style of the adjacent fold-thrust belt, highlighting the formation of intermediate-level detachments (i.e., regional detachments that are at a higher structural level than the basal décollement). We find that detachments preferentially form in fine-grained, distal foreland basin deposits. The presence of multiple detachments within thrust belts affects structural development, including partially decoupled shortening and crustal thickening without deep exhumation (Ruh et al., 2012). The implication of this relationship is that characteristic patterns of foreland basin sedimentation that are products of orogenic evolution may in turn have significant control over orogenic evolution. FORELAND BASIN STRATIGRAPHIC ARCHITECTURE Foreland basins include wedge-top, foredeep, forebulge, and backbulge depozones that develop as a result of isostatic adjustment of the lithosphere during loading by the adjacent fold-thrust belt (DeCelles and Giles, 1996). Synthetic stratigraphic sequences have been generated based on the horizontal migration of these depozones during orogenic growth (e.g., Quinlan and Beaumont, 1984; Flemings and Jordan, 1989). At the cratonward edge of a uniformly advancing foreland basin system, limited subsidence and sedimentation initially occur in the back-bulge depozone, which typically consists of fine-grained shallow-marine to nonmarine sediment (Flemings and Jordan, 1989; DeCelles and Giles, 1996). Although net sediment accumulation in a back-bulge depozone is at least an order of magnitude less than that of the foredeep, back-bulge depozones are roughly twice the width of the foredeep (DeCelles and Giles, 1996). In some instances, the record of sedimentation in the back-bulge depozone is eroded by passage of the forebulge (Crampton and Allen, 1995). The transition from forebulge to distal foredeep deposition is characterized by accelerating subsidence and limited sediment supply (Flemings and Jordan, 1989). Distal foredeep sedimentation often represents maximum water depths and comprises fine-grained clastic material or carbonate pelagic sediments (Sinclair, 1997). Following deposition of the distal foredeep unit, sediment supply and caliber increase and water depth decreases as the proximal foredeep migrates toward the craton. DETACHMENT FORMATION AND DEFORMATION MECHANISMS The observed structural geometry in fold-thrust belts is often used to divide the stratigraphic succession into structural lithic units that tend to deform as a mechanically coherent package (Currie et al., 1962; Woodward and Rutherford, 1989). Separating these structural lithic units are mechanically incompetent (low shear strength) stratigraphic layers that contain detachments. The central premise of this study is that fine-grained, distal foreland basin deposits (back-bulge to distal foredeep depozone) frequently contain detachments and tend to separate structural lithic units in fold-thrust belts. In addition to their mechanical incompetence, finegrained, low-permeability, distal foreland units are commonly overpressured as a result of disequilibrium compaction following rapid burial by synorogenic units (Swarbrick and Osborne, 1998). Effective stresses are thought to approach yield stress several tens of kilometers into the foreland ahead of the leading thrust ramp (van der Pluijm et al., 1997). Rocks ahead of the deformation front undergo internal strain before bulk failure, manifested as plastic hardening and then plastic softening for Coulomb materials (Mitra et al., 1984). For incompetent layers, this plastic softening extends significantly farther into the foreland and can include small amounts of layer-parallel displacement, which accommodates plastic deformation and pressure solution in the competent layers (Mitra et al., 1984; Yonkee and Weil, 2010). The formation of a new thrust sheet in the foreland basin depends on the initial shear failure of a relatively competent layer within the foreland basin succession. Once the competent layer fails, the shear propagates until it intersects an incompetent layer already near or at yield stress to form a new detachment and thrust sheet (Eisenstadt and De Paor, 1987; Davis and Engelder, 1985). In thrust belts with rheological layering, intermediate-level detachments may develop within these incompetent layers in addition to the basal décollement. This phenomenon is often revealed at the front of a thrust belt where the basal décollement ramps up to an intermediate-level detachment at a higher stratigraphic level (time, T = 1; Fig. 1). With continued shortening, the basal décollement will return to a lower structural position and a duplex system may form at depth, isolating the intermediate-level detachment as a roof thrust in a classic duplex system (Mitra, 1986) (T = 2; Fig. 1). In the following we show that these intermediate-level detachments are commonly initiated in distal foreland deposits. CASE STUDIES The first stage (T = 1) in the kinematic sequence described here and illustrated in Figure 1 is exemplified by the Main Frontal thrust in the Himalayan thrust belt, which ramps up to the base of Subhimalayan synorogenic sedimentary rocks that represent distal foreland basin sedimentation (DeCelles et al., 1998; Mugnier et al., 1999). The Ouachita orogenic system (southern United States) provides an example of the second stage in the kinematic sequence (T = 2; Fig. 1), where an intermediate-level detachment in the distal foreland basin units, the Tesnus GEOLOGY, July 2015; v. 43; no. 7; p. 1–4 | doi:10.1130/G36597.1 | Published online XX Month 2015 © 2015 eological Society of A erica. For permission to copy, contact [email protected]. 2 www.gsapubs.org | Volume 43 | Number 7 | GEOLOGY Formation and Stanley Group, was folded by later deformation resulting from slip along the basal décollement and duplex formation (Chapman and McCarty, 2013). In both the Himalayan and Ouachita examples, the formation of an intermediate-level detachment in a fold-thrust belt and the deposition of foreland basin sediments are temporally related to the same orogenic event. However, some foreland basins contain several stratigraphic units vertically and along strike that represent distal foreland sedimentation at different times and locations associated with multiple orogenic events. For example, the central Appalachians (eastern United States) underwent at least three major orogenic events during the Paleozoic, including the Taconic, Acadian, and Alleghanian orogenies (Hatcher et al., 1989), each of which produced clastic wedges with the characteristic stratigraphic architecture of foreland basin systems (Ettensohn, 1994) (Fig. 2). This is a complication to the simple model described above, wherein a single foreland depozone is exploited as an intermediate-level detachment in the same orogenic event, but it presents an opportunity to test the correlation between foreland basin depozone and fold-thrust belt architecture. We predict that the distal foreland units will be preferentially exploited as intermediate-level detachments in composite foreland basins. Three regional detachments exist in the central Appalachian foldthrust belt: a basal décollement in the Cambrian Waynesboro Formation and two detachments at intermediate structural levels in the middle Ordovician Martinsburg Group and lower Devonian shale including the Marcellus Formation (Perry, 1978) (Fig. 2). Detachments in the Martinsburg Group and lower Devonian shale were active during the Alleghanian orogeny, and occur within shale-rich, distal foredeep units deposited during the older Taconic and Acadian orogenies, respectively (Ettensohn, 1994). In some places in the central Appalachians, both the Devonian and Ordovician distal foredeep detachments are present, resulting in multiple stacked duplexes (Mitra, 1986). Similar multidetachment patterns are observed in the Canadian Rocky Mountains and the central Andes. Numerous bedding-parallel detachments are present within the Alberta thrust belt, the most prominent of which are located within the Mississippian Banff Formation, the Jurassic Fernie Group, the middle Cretaceous Alberta Group, and the upper Cretaceous Bearpaw Formation (Spratt and Lawton, 1996). Except for the detachment in the Banff Formation, all of these detachments correspond to shale intervals at the base of a foreland basin sequence (Cant and Stockmal, 1989). In the Andes, an intermediate-level detachment is present in the Devonian Los Monos and Icla Formations (McQuarrie, 2002), which are distal foreland deposits related to Paleozoic terrane accretion events (Isaacson and Díaz-Martínez, 1995). DISCUSSION AND CONCLUSIONS To demonstrate the relationship between distal foreland depozones and intermediate-level detachments, we plot major detachment positions and depozones for several well-known thrust belts (Fig. 3). There is a correlation between the position of intermediate-level detachments and distal foreland depozones (distal foredeep to backbulge) (Fig. 3). The range of possible detachment positions in fold-thrust belts is not limited to distal foreland deposits. Prominent exceptions to this relationship are the Pyrenees (Spain) and Zagros (Iran) thrust belts, which are dominated by multiple detachments in evaporites that predate their respective foreland basin systems (Vergés et al., 1992; Sherkati et al., 2006). The presence of evaporite alone, however, is not inconsistent with the model described. For example, a regional intermediate-level detachment occurs within evaporites of the Arapien Formation in the western U.S. Cordillera (DeCelles and Coogan, 2006), possibly deposited in a back-bulge depozone. Similarly, the structurally highest regional detachment in the Zagros occurs in Miocene evaporites (O’Brien, 1957; Sherkati et al., 2006), which were deposited in the distal foreland basin system (Fakhari et al., 2008). Accepting the premise that thrust belt architecture is in many instances linked to a characteristic foreland stratigraphy, we can begin to explore potential new relationships between foreland basins and orogenic growth. 10 km Frontal Himalaya