Slab Tearing Triggered the Cretaceous Igneous Activity in the Northern Gulf of Mexico Region

摘要

Igneous activity during the Cretaceous (ca. 108 to 65 Ma) in the northern Gulf of Mexico region has been a subject of debate. This igneous system, characteristically derived from the sublithospheric mantle, consists of alkaline basalts predominantly, as well as nepheline syenites, carbonatites, and phonolites. It spans from Arkansas/Mississippi to West Texas. Understanding its mechanism is fundamental to the tectonic and thermal history of the Gulf of Mexico Basin. Competing hypotheses include: (1) It results from the Bermuda hotspot as the North American plate passes over it; (2) edge-driven convection produces melts at the continent-ocean boundary; and (3) reactivation of preexisting lithospheric structures leads to asthenospheric upwelling and melting. While these models explain some observations, there exist unresolved issues. Age distribution does not match predicted pattern of the hotspot model. The edge-driven convection model fails to explain the timing and location of igneous rocks. Enriched geochemical patterns implies the reactivation model is unlikely to be valid. Alternatively, tearing of subducted Farallon slabs at sublithospheric-depth can explain its age distribution pattern and geochemistry. Initiation of a slab tear occurs in the asthenosphere or mantle transition zone beneath Arkansas during the middle Cretaceous, marking the onset of the magmatism in the northern Gulf of Mexico region. Then, it propagates both parallel to and perpendicular to the paleo-trench along the western North American Plate (i.e. southeastward to Jackson Dome in Mississippi, and westward to the Balcones and Trans Pecos regions in Texas). It induces sublithospheric mantle upwelling, followed by decompression melting and magma emplacement. A gap between high P-wave velocity anomalies, currently seen at 300 to 700 km depth beneath southeast United States, may be correlated to the hypothesized Cretaceous slab tear in our model. This model explains the Cretaceous-age sublithosphere-derived igneous rocks at a regional scale, and provides boundary constraints for reconstruction of the Farallon slab back to ca. 110 Ma.