Integrated geophysical methods to examine lower crust of southern High Plains
The high plains of west Texas have a more complex basement structure (Figure 2.7) compared to its flat and featureless surface (Budnik, 1989). The Matador arch, an east-west-trending structural zone that separates the Palo Duro and Midland basins, is a basement-involved faulted and folded belt (Ye et al., 1996). The southern Oklahoma aulacogen that has been described as a failed rift was initiated during Cambrian extension. Subsequently, a wide early Paleozoic basin developed over the initial rift. During the late Paleozoic, fault-bounded basins and uplifts developed in cormection with the Ouachita orogeny, which altered sedimentation patterns. These structures formed a predominantly NW-SE alignment (Coffman et al., 1986). The Amarillo uplift is a narrow basement positive element that trends west-northwest across the Texas Panhandle, to the north of Palo Duro basin. The Anadarko basin trends west-northwest and on its southwest margin is separated from the Wichita-Amarillo uplift by a fault zone with up to 12 km of structural relief (Evans, 1979; Luza et al., 1987). The Permian basin of west Texas is located in the foreland of the late Paleozoic Marathon-Ouachita orogenic belt. The Permain Basin can be characterized as a broken foreland with several prominent, fault-baunded, basement-involved uplifts (Drobek et al., 2002).
The gravity profiles suggest that mafic intrusions, uplifts, lower crustal upwarps and shallower layer in the upper mantle are the combined reasons for high gravity anomalies in the region and low gravity anomalies are the combined results of basins, low density rocks of Debaca and Panhandle Terrane, downwarps in the upper mantle layer in conjunction with deeper layer in the upper mantle. The gravity increase in the EW direction in regional scale seems related to a rise in the layer in the upper mantle. The upwaps in the boundary between upper and lower crust are obvious in the north south extending gravity profiles adjacent to the Southern Oklahoma aulacogen might be (?) related to the Late Paleozoic deformation in the foreland of Ouachita orogeny that culminated in Ancestral Rocky mountain orogeny and crustal shortening in the study area. Receiver functions obtained from seismic stations in Lubbock and Albuquerque show that Moho is deep beneath the seismic station of Lubbock around 56 km and crust is around 35 km beneath the seismic station in Albuquerque. Gravity models (A-A', BB', C-C) with a rift pillow pinching out towards High plains and getting smaller towards north make our gravity models consistent with the seismic data. Gravity models without a rift pillow shows that Moho gets deeper beneath the seismic station in Albuquerque and gets shallower towards high plains which does not fit our seismic data.