Distribution and timing of lithospheric breakup across the Gulf of Mexico: The role of seaward-dipping reflectors, spreading propagators, and crustal shear zones Open Access

We summarize evidence suggesting that magmatic accreted crust (subaerially accreted crust and submarine accreted oceanic crust) underlies a much larger portion of the Gulf of Mexico basin than has been appreciated previously. This conclusion suggests that traditional models of the Jurassic tectonic development of the basin, with wide areas of thinned continental crust underlying the salt basins, require significant modification. Using an updated compilation of long-offset, deep-penetrating offshore and reprocessed onshore seismic reflection profiles, we produced a new plate kinematic interpretation for the Gulf of Mexico linked to a process-based understanding of key tectonic events, their timing, and the distribution and structure of crustal types and pre-salt sediments observed across the Gulf of Mexico. The near-onshore and offshore Gulf of Mexico region is interpreted to be underlain by accreted magmatic crust formed during two phases of seafloor spreading: (1) an older rim of subaerial seafloor spreading marked by seaward-dipping reflectors that grade laterally into thin, accreted crust of an enigmatic nature overlain by an undeformed pre-salt sedimentary succession, and (2) younger production of more normal submarine Penrose crust. Continental breakup was diachronous, initiating at 200−190 Ma and becoming younger to the east, and marked by easterly trending extensional propagators preserved as basin systems along the western margin of Florida: the Mississippi Salt Basin, Apalachicola Basin, and Tampa Embayment. These propagators formed successively from north to south and west to east as the Gulf of Mexico spreading system adjusted to Yucatan rotation, before the spreading axis shifted southward into the Florida Straits. Phase 1 breakup initiated north of the present coast along the Houston magnetic anomaly, with little local evidence for upper-crustal faulting. Any crustal thinning there would thus have been a consequence of lower-crustal, depth-dependent continental extension. Regionally, unextended continental crust may be evidence of exploitation of preexisting Alleghanian-Ouachita weaknesses, of which the western continuation of the Suwannee shear zone is a prime candidate. Between phase 1 breakup (200−190 Ma) and 169 Ma, Yucatan migrated southeastward with South America (Gondwana) and rotated ∼15° counterclockwise. This gradual southward shift of Gulf of Mexico accretion may have resulted from the region’s extension axis encountering rheological strength barriers related to the Central Atlantic and proto−Caribbean Ocean margins. Each successive line of breakup was characterized by an initial phase of subaerial extrusions and development of seaward-dipping reflectors. Evidence suggests that these extension systems in the eastern Gulf of Mexico occurred in a widening and propagating basin network below global sea level, where continental sediments were deposited in subaerial and/or lacustrine environments and ultimately capped by evaporites. In phase 2, between 169 Ma and 140 Ma, Yucatan rotated an additional ∼52° counterclockwise. Evaporites started forming in Bajocian (169 Ma) time during transient connection(s) to the global ocean. Fully marine conditions were established in Callovian (164 Ma) time as rotation continued, resulting in submarine accretion of Penrose crust. A major implication of this work is confirmation that prolific hydrocarbon systems can develop on “oceanic” (accreted) crust if ambient depositional environments are favorable.