| Summary: | The tectonic mechanism driving Cretaceous magmatism in the Gulf of Mexico (GoM) region is debated. This magmatism postdates GoM seafloor spreading by 40 Myr, initiated at 108 Ma and lasted through the Cretaceous. Spanning Texas to Mississippi it consists of igneous rocks with geochemical signatures pointing to a sub-lithospheric mantle origin. Hypotheses for this magmatism include: (1) the Bermuda hotspot; (2) edge-driven convection; (3) lithospheric reactivation; and (4) deep, low-angle subduction. My research shows none are fully satisfactory and that GoM magmatism should be correlated to other Cretaceous – Eocene kimberlites and lamproites from Arkansas to the Northwest Territories. They are located 1000+ km inboard from, and aligned sub-parallel to, the western margin of North America (NA). I propose that the Farallon slabs stagnated in the mantle transition zone in the Early Cretaceous, and generated sporadic, dense, low-degree partial melts by dehydration and decarbonation. As the slabs penetrated the lower mantle, instabilities at slab edges caused upwelling that brought alkali-rich carbonatitic melts to the base of the lithosphere. Subsequently, the NA lithosphere with varying thickness, discontinuities, and compositions interacted with the rising melt, producing a variety of magmatic rocks. This model connects intraplate magmatism with slab stagnation, and provides a critical constraint on the Cretaceous NA history. Following the Laramide orogeny, Cenozoic extension dominated the western US, forming the Rio Grande rift (RGR). Kinematics of the extension is critical to evaluate tectonic models. While the N-trending, right-stepping RGR largely shows orthogonal E-W extension, the NW-striking, oblique Tusas segment preserves W- and SW-trending slip directions. While a multi-directional extension model is possible, a continuous E-W extension with reactivation of pre-existing weakness can alternatively explain slip direction variations. Initial extension on reactivated faults was recorded by W-trending slickenlines. Subsequently, extension was re-oriented to SW-trending, i.e., pure dip-slip, due to local stress rotation across heterogeneities. Then, the Embudo accommodation zone began to accommodate E-W extension, causing diffuse, SSW-directed slip on the Tusas segment. The early extension along the Tusas segment was abandoned once the Embudo transfer fault formed. This study highlights the significance of obliquity and inherited heterogeneity in the kinematic evolution of rifts. Earth and Atmospheric Sciences, Department of
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