Oceanic Character of Sub-Salt Crust in the NW Gulf of Mexico (GOM) Using Seismic Refraction and Reflection Data

Abstract:

Significant renewed interest in the geological development of the NW GOM is exemplified by the acquisition of academic seismic refraction and oil industry seismic reflection data. There is agreement that the GOM formed by Jurassic separation of North America and Yucatan, but disagreements remain on the distribution and timing of extended continental versus oceanic crust. Van Avendonk et al. (Geology, v43, 2015) interpreted seismic refraction data from the 2010 “GUMBO” expedition as rifted continental crust thinned by large-scale extensional faulting and syn-rift magmatism beneath the NW GOM. However, seismic reflection evidence for this extension is non-existent, and diagnostic fault-controlled syn-rift packages are not resolved. A very different interpretation of basement type and basin evolution is possible by applying geological process linked to hyper-extended margin formation to the same data. We note: 1) Base salt and Moho interfaces are well imaged; top basement is not resolved. We interpret a pre-salt sedimentary sequence 5-10 km thick, with velocities up to 6 km/s; high velocities in this sequence likely relate to greenschist-facies metamorphism associated with early high heat flow and deep burial. 2) Velocities of 6-8 km/s characterize crystalline basement but do not uniquely determine crustal type (i.e., velocity does not equate to rock type). Lateral variations (0-8 km) in crustal thickness are consistent with slow/ultra-slow seafloor spreading. 3) The undeformed base salt reflector and pre-salt sediment sequence imply a post-kinematic setting and a substantial delay between breakup and Callovian salt deposition. 4) Liassic Central Atlantic breakup is kinematically linked to the GOM and related SDR magmatism. Inboard SDRs, observed on both conjugate margins of the GOM, imply outboard oceanic crust.

Together, these observations are consistent with regional sub-salt basement of early-mid Jurassic slow/ultra-slow spreading oceanic crust, associated with minor normal faulting. This contrasts with a model of stretched continental crust that gradually thins from 14 km at the Texas coast to 6 km with strong lateral heterogeneities associated with magmatic intrusions in the ultradeep GOM.