Relaxation of continental lithosphere: An explanation for Late Cretaceous reactivation of the Sabine Uplift of Louisiana‐Texas

With few unconstrained assumptions, a simple quantitative model for flexural isostasy between two crustal blocks of different thickness that subsequently undergoes partial relaxation of accumulated stress can explain reactivation of the Sabine Uplift in the mid‐Cretaceous. Thin salt over the Sabine Uplift indicates that it was a positive area in the Middle Jurassic but began to subside during the Late Jurassic. During the Late Jurassic and Early Cretaceous, the Sabine Uplift had no topographic expression and was the center of a large, flat bottomed basin covering most of east Texas and north Louisiana. Reactivation of the Sabine Uplift in the mid‐Cretaceous caused a minimum of 150 m of uplift and extensive erosion of Lower Cretaceous rocks. During the Late Cretaceous and Tertiary, the Sabine Uplift resumed subsiding but at a slower rate than the adjacent basins. Since the mid‐Cretaceous, the Sabine Uplift has risen more than 1 km relative to the East Texas Basin. We suggest that this structural history is consistent with the following simple quantitative model for flexural isostasy. During formation of the Gulf of Mexico in the Late Triassic‐Early Jurassic by rifting and extension of the lithosphere, brittle deformation of the crust created small‐scale wavelength of tens of kilometers, lateral variations in crustal thickness. Initially, the newly formed margin was in point‐wise Airy isostatic equilibrium. Thus synrift and/or early postrift subsidence was characterized by a series of rift valleys or half grabens separated by uplifts. However, as the margin cooled and contracted, the lithosphere became stronger, and subsequent loading was regionally compensated. Thus postrift subsidence was characterized by a broad regional downwarp. If, at some later point in time, the rigid portion of the lithosphere was weakened or relaxed, then lateral density variations would have been recompensated at shorter wavelengths. Thus areas of little or no crustal extension would have been uplifted and areas of high extension would have experienced additional subsidence. Gravity and total tectonic subsidence interpretations agree that the Sabine Uplift region is underlain by thicker crust than the adjacent East Texas and North Louisiana basins. Possible mechanisms for relaxation of continental lithosphere are thermal rejuvenation, horizontal in‐plane stress, viscous relaxation, and extensional faulting.