An animated tectonic reconstruction of southwestern North America since 36 Ma

Abstract

We present tectonic reconstructions and an accompanying animation of deformation across the North America–Pacific plate boundary since 36 Ma. Intraplate deformation of southwestern North America was obtained through synthesis of kinematic data (amount, timing, and direction of displacement) along three main transects through the northern (40°N), central (36°N– 37°N), and southern (34°N) portions of the Basin and Range province. We combined these transects with first-order plate boundary constraints from the San Andreas fault and other areas west of the Basin and Range. Extension and strike-slip deformation in all areas were sequentially restored over 2 m.y. (0–18 Ma) to 6 m.y. (18–36 Ma) time intervals using a script written for the ArcGIS program. Regions where the kinematics are known constrain adjacent areas where the kinematics are not well defined. The process of sequential restoration highlighted misalignments, overlaps, or large gaps in each incremental step, particularly in the areas between data transects, which remain problematic. Hence, the value of the reconstructions lies primarily in highlighting questions that might not otherwise be recognized, and thus they should be viewed more as a tool for investigation than as a final product. The new sequential reconstructions show that compatible slip along the entire northsouth extent of the inland right-lateral shear zone from the Gulf of California to the northern Walker Lane is supported by available data and that the east limit of active shear has migrated westward with respect to North America since ca. 10 Ma. The reconstructions also highlight new problems regarding strain-compatible extension east and west of the Sierra Nevada– Great Valley block and strain-compatible deformation between southern Arizona and the Mexican Basin and Range. Our results show ~235 km of extension oriented ~N78°W in both the northern (50% extension) and central (200% extension) parts of the Basin and Range. Following the initiation of east-west to southwest-northeast extension at 15–25 Ma (depending on longitude), a significant portion of right-lateral shear associated with the growing Pacific– North America transform jumped into the continent at 10–12 Ma, totaling ~100 km oriented N25°W, for an average of ~1 cm/yr since that time.

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