Regional Structural and Geophysical Studies of Interaction and Overlap—Primary and secondary basement controls on thrust sheet geometries

Abstract

Active buttressing of thin-skinned structures by simultaneous thick-skinned structures is a primary basement control for thrust belt geometries. Stratigraphie variations in the sedimentary cover are a form of secondary basement control that can explain many changes in thrust geometry better than primary controls. Thin-skinned structural style changes related to secondary basement control lack the precise temporal and geographic proximities that provide the critical evidence for primary control. Secondary controls, however, are ubiquitous throughout orogenic belts, not just near present basement uplifts, and they provide a powerful mechanism to understand regional and local changes in thrust geometries. Secondary controls are derived from the downwarp behavior of the basement surface when loaded by sediments. Variable subsidence produces different stacking sequences of lithic units (and structural lithic units for later deformation). When deformed by thrusting, lateral ramps and changes in duplex and imbricate geometries result from the regionally variable stratigraphie package being folded and faulted.An unexposed Snake River fault zone has been proposed previously to explain apparent offsets of stratigraphie and structural trends across the Snake River Plain. This feature is often considered to involve foreland basement rocks as well, and to be part of a foreland structure that affected the thrust belt's evolution. On a palinspastic map, isopachs and many facies boundaries south of the plain trend northeast-southwest. Once these stratigraphie variations within thrust sheets of the Idaho-Wyoming-Utah thrust belt are understood, the need for any such fault zone is removed because correlative structures across the plain carry different stratigraphie sections.Basement asperities as the cause of thrust ramps are examined in two field examples in which basement and sedimentary strata are involved in the same thrust sheet at the surface. In both the Ogden, Utah, and the Mountain City, Tennessee, areas, the presence of basement seems to have no effect on the structural geometries developed above them, although stratigraphie layering does. As a result, basement faulting in general seems a less likely direct cause for thrust ramp locations than does stratigraphie control. The inference is that, because different stratigraphie packages vary at different rates and in different directions, they are the most likely control on the areal and vertical locations of thrust ramp positions.

Active Shortening Simultaneous to Normal Faulting Based on GNSS, Geophysical, and Geological Data: The Seismogenic Ventas de Zafarraya Fault (Betic Cordillera, Southern Spain)

“…This boundary could be related to a high Bouguer gravity anomaly located in Málaga (Figure 1b), associated with high‐density ultramafic rocks embedded in the crust of the Internal Zones (Pedrera et al., 2020; Torné et al., 1992). The fact that the boundaries of this body coincide with the trace of the compressional front would suggest it acts as a buttress (Woodward, 1988). Furthermore, together with the differences between the External and Internal Zones, this body could cause the heterogeneity of the front.…”

“…The geometry of these thrust systems will also depend on the crustal structure beneath the detachment, determining the formation of lateral and oblique ramps (Boyer, 1995; Hinsch et al., 2002; Mitra, 1997; Wiltschko & Eastman, 1983). In fact, one of the most important control mechanisms for the geometry of thrust systems is active buttressing (Woodward, 1988). The migration of extensional systems is another factor to consider in gravitational collapse and thrusting phenomena.…”