Long-Period Magnetotelluric (MT) Investigation of the Southern Oklahoma Aulacogen; New Insights Into Past Episodes of Rifting

Long-period MT data were collected at 13 stations using LEMI instruments along a profile across the Southern Oklahoma Aulacogen (SOA) in Oklahoma, USA, in order to investigate the structure and modification of the lithosphere beneath the rift. The SOA has a multi-phase tectonic history beginning with plume related triple-junction rifting in the Cambrian, which was then followed by structural inversion as part of the Ancestral Rocky Mountain (ARM) orogeny. The data was processed and converted into impedance data for a period range of 1-10,000 seconds and was inverted using a 2-D non-linear conjugate gradient inversion algorithm. The 2-D resistivity model contains two low resistivity anomalies that can be related to the tectonic evolution of the SOA, lithosphere enrichment, and mid-lithosphere discontinuities (MLDs). The first is located in the crust and upper subcontinental lithospheric mantle, and the second begins at a depth of 100 km and extends to the base of the lithosphere and over the entire model space. We attribute the first anomaly to a combination of hydrogen enrichment of mantle minerals and the possible presence of conductive phases like graphite and sulfides deposited during rifting. During ARM deformation the anomaly was further enhanced by grain size reduction and the graphite phase being smeared and interconnected. We attribute the lower lithosphere conductor to a 10 km thick phlogopite layer that is potentially fluorine enriched and is coincident with a seismic MLD at 100 km. This layer is then underlain by a lithospheric keel of hydrogen enriched mantle minerals. Given that the voluminous tholeiitic affinity magmatism that occurred during rifting requires a generally thin lithosphere, we suggest the lower conductor marks the location where the lithosphere was potentially delaminated during rifting and replaced by plume material. We also suggest the earlier modification of the lithosphere during rifting likely metasomatically weakened the region, priming the SOA for localizing ARM deformation. Given prior suggestions that ARM uplifts localized along paleo-rifts, this provides a possible mechanism for the discrete and isolated nature of the uplifts. Finally, the possibility of fluorine enrichment of phlogopite could be one mechanism for explaining the variable electrical responses of MLDs.