The breakup of East Gondwana : insights from plate modeling, basin analysis, and numerical experiments
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During the Early Cretaceous, East Gondwana began to fragment. Due to temporal proximity to the Cretaceous Normal Superchron and a lack of well resolved seafloor fabric, our understanding of this breakup has historically been limited. A new interpretation of the marine magnetic anomalies preserved within the Somali Basin, provides insight into the motions of the East Gondwana from the Late Jurassic through the Early Mesozoic. When combined and compared with magnetic anomaly interpretations from coeval regional basins, the timing of East Gondwana breakup can be constrained to begin at M15n (135.76 Ma). Within the Enderby Basin, East Antarctica, oceanic and thinned continental crust preserve a record of this rifting. Previous works have suggested that a wide (500 km) domain of thinned continental crust exists between the present-day coastline and a regional, high-amplitude, magnetic anomaly. We offer an alternative interpretation of the Enderby Basin crustal structure, where much of this postulated continental crust is instead thin, proto-ocean crust. This interpretation is based on the lack of isostatically observable crustal thinning throughout the domain, as would be expected for rifted continental blocks. Throughout much of this domain, the crust instead appears to be rugged, thin (<6 km), and of relatively constant thickness, resembling oceanic crust formed at ultraslow/slow ridges. The preferred tectonic interpretation is that, immediately after continental breakup, magmatic production/emplacement was low and formed this proto-ocean domain. A later reorganization of the magmatic system allowed for normal ocean crust to form and is manifest today as a change in crustal structure and thickness and corresponding magnetic anomaly. Numerical modeling experiments were undertaken to investigate potential influences on melt production during passive continental extension. Factors determined to favor delayed magmatic emplacement include: an initial cool lithosphere geotherm, thin crust, rapid extension rates, low mantle potential temperature, and strong crustal rheology. If magmatic emplacement is sufficiently delayed, these factors may influence formation of a magma-poor margin and/or proto-ocean domain. In the Enderby Basin, Permo-Triassic rifting in the Lambert Graben appears to have previously thinning the continental crust. This pre-breakup thinning may be ultimately responsible for the later formation of the observed proto-ocean domain during East Gondwana breakup.