YANG, KENN-MING, and STEVEN L. DOROBEK, Department of Geology, Texas A&M University, College Station, TX

ABSTRACT: Lithospheric Heterogeneity and its Effects on Late Paleozoic Synorogenic Foreland Tectonics in the Permian Basin, Texas and New Mexico

The Permian basin of West Texas and southern New Mexico is located in the foreland area of the Ouachita-Marathon orogenic belt. The basin was segmented into several sub-basins by uplifts during late Paleozoic orogeny. The north-south trending Central Basin Platform (CBP) strikes at high angles to the front of the orogenic belt and separates the Delaware basin to the west from the Midland basin to the east. The Ozona Arch is an easterly extension of the southern CBP and separates the Midland basin from the ValVerde basin. The Val Verde basin was the foredeep in front of the Marathon orogenic belt during Mississippian to early Permian time. In this study, restored synorogenic strata in the basinal areas were used for flexural analyses across the foreland. Results of the flexural analyse were used to constrain variations in the mechanical properties of foreland lithosphere that might have affected the complex synorogenic tectonics across the Permian basin.

The restored synorogenic geometry of the Val Verde basin has a typical flexural profile in cross-section. However, the basin becomes narrower and deeper toward its western end where it is bordered by the southernmost part of the CBP. Values for the rigidity of lithosphere beneath the Val Verde basin (estimated by comparing best-fit theoretical curves to the observed synorogenic profiles) decrease from 10{23} N-m at the eastern end to 10{21} N-m at the western end of the basin.

Flexural analyses also were done along E-W trending cross-sections that extend from the Delaware basin, across the CBP, and into the Midland basin. The CBP was treated as a distributed tectonic load that produced

lithospheric flexure of the adjacent basinal areas. The distributed load was used to calculate a static profile of the deflected lithosphere across the area. Best estimates for rigidity were obtained by comparing the observed synorogenic basin geometries with the model profiles. Calculated rigidities decrease toward the southern part of the Permian basin, but are similar to rigidity estimates from the Val Verde basin.

Our flexural analyses indicate that the lithosphere near the southernmost part of the CBP has the lowest rigidity in the Permian basin. Here, the CBP is characterized by the maximum amount of vertical uplift along the platform margin and by the greatest crustal shortening across the platform. This area of apparently weak lithosphere also is adjacent to the sharply curved salient of the Marathon fold-and-thrust belt. Coincidence of the sharp bend in the Marathon orogenic belt and maximum crustal deformation in the foreland area suggests a mechanical and causal relationship. The arcuate geometry of the Marathon foredeep around the prominent Marathon orogenic belt may have produced very high bending stress that contributed to the lower flexural rigidity in this area.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.