Potential Field Evidence for a Volcanic Rifted Margin along the Texas-Louisiana Gulf of Mexico Transitional Lithosphere

Volcanic rifted passive margins (VRMs) are characterized by the eruption of large quantities of lavas, seaward-dipping reflectors consisting of a mixture of volcanic and sedimentary deposits, and high velocity lower crust; VRMs form at some transitions between oceanic and continental crusts. Because the transitional crust of the Gulf of Mexico passive margin is buried beneath thick sediments (including seismically opaque salt) the nature of this transitional crust is poorly known. Because of the abundance of dense and magnetic mafic igneous rock, gravity and magnetics are powerful tools for identifying VRMs where other techniques cannot. Regional aeromagnetic data indicates that a large amplitude magnetic maximum over land parallels the Texas-Louisiana coastline from the Mexican border to Lafayette, LA. The Bouguer gravity anomaly map is less distinctive with either a small amplitude, positive anomaly or no significant anomaly. More distinctive on the Bouguer gravity anomaly map is a gravity maximum 150 km seaward of the magnetic anomaly which also parallels the coastline. There is a corresponding magnetic maximum, however the width and amplitude is smaller than the land-based magnetic anomaly. To quantify the sources of both the magnetic and gravity anomalies, a 2.5-D model was constructed along a profile from Mesoproterozoic crust of the Llano uplift to probable oceanic crust in the Gulf of Mexico. Constrained by results from geological and seismic refraction studies, the best fitting model indicates that a highly magnetized/dense layer up to 24 km and 240 km wide can explain the observed magnetic and gravity anomalies. The preferred model indicates that the transition from the unextended craton to the oceanic crust consists of two zones: 1) nonmagnetic inner zone of gradually thinning crust and 2) a highly magnetic and dense outer zone of seaward-thinning crust corresponding to a VRM similar to that along the Vøring margin of Norway.