COMPARISON OF THICK VERSUS THIN-SKINNED STYLES OF LARAMIDE THRUSTING IN THE SOUTHERN ROCKIES AND SIERRA MADRE ORIENTAL FOLD-THRUST BELTS

In order to evaluate the differences and similarities between the Sierra Madre Oriental fold-thrust belt (SMO) and southern Rocky Mountains (SRM) we integrated geologic field mapping, well data, and structural restorations with previously published geochronologic data. The SMO exhibits alternating salients and recesses, which lie westward of a linear front. The salients are underlain by thick Jurassic evaporite deposits while recesses contain sparse and thinly bedded evaporite strata. Salients of the SMO display a thin-skinned structural style of shortening due to the presence of a structurally shallow and weak evaporite-floored detachment. Thrust faults sole into the evaporite horizon, which is regionally extensive in the salients. Recesses and the linear front display a thick-skinned style of deformation similar to the SRM. Basement uplifts involve Gulf of Mexico-related rift basins, suggesting that rift faults were reactivated during shortening and therefore locally control the location and trend of the SMO. Alternatively, the linear front may be controlled by a parallel basement ridge suggesting an eastern barrier of thicker crust that prevented further eastward displacement of the SMO thrusts.

In contrast to the SMO, the SRM is linear and dominantly thick-skinned. The trend of basement uplifts is interpreted to be controlled by the pre-existing Proterozoic basement structures. The lack of pronounced oroclines in the SRM is likely due to the absence of evaporites that are discontinuous, but present in the SMO. Despite the structural differences between the SRM and SMO, both share similar timing. Thermal and thermochronolgic data from the SMO indicates that basement rocks were exhumed ca. 40 to 35 Ma. Geochronologic data from the SRM suggests that cessation of shortening occurred from Middle Eocene to Early Oligocene (ca. 37 to 29 Ma), which overlaps with exhumation in the SMO.

The similarities in timing of deformation in the SRM and SMO suggest to us that deformation in both regions share a common mechanism. While evaporites contribute to a thinner-skinned deformation style in the SMO, both exhibit linear thrust fronts possibly controlled by preexisting basement features, albeit of very different age. This highlights the influence of the pre-existing geologic architecture on fold-thrust-belt evolution.