title: The Role of Magmatism at Rifted Margins
Creators: name: Harkin, Caroline
Publisher: name: University of Liverpool Repository
Year: 2019

description: Rifted margins form by the thinning and stretching of continental lithosphere until it ruptures, forming new oceanic crust and lithosphere, which can be accompanied by decompression melting and the addition of magmatic material. Despite numerous studies on magma-rich margins, we still do not fully understand how and when magmatic features form at rifted margins. To address this I investigate the formation of various magmatic features on three rifted margins, the East Indian margin, the Pelotas margin and the Southeast (SE) Greenland margin, using a range of quantitative techniques. The East Indian margin is an often-cited example of a magma-poor margin; however, some interpretations suggest the transition from exhumed mantle to oceanic crust consists of 9 km thick magmatic crust. Gravity inversion, RDA analysis, subsidence analysis and joint inversion of seismic and gravity data alongside seismic observations, reveal the presence of magma-poor and magma-rich characteristics in the form of exhumed mantle and 9 km thick magmatic crust juxtaposed against each other, resulting from a two-stage breakup. Juxtaposition of end-member characteristics suggests that the use of end-member terminology based on volumes of magma alone is misleading. The Pelotas margin in the South Atlantic shows an extraordinarily thick sequence of seaward dipping reflectors (SDRs), of which the composition and formation is poorly understood. I investigate these SDRs using gravity inversion with a sensitivity to basalt/sediment composition, flexural backstripping and reverse thermal subsidence modelling, joint inversion of seismic and gravity data and seismic observations. I show there are two types of SDRs present on the Pelotas margin, an inner subaerial set of SDRs formed of basalt during pre-breakup intra-continental rifting and an outer set of SDRs formed of a mix of volcaniclastics and basalts during breakup in a subaqueous environment at an embryonic mid-ocean ridge. The SE Greenland margin in the North Atlantic has a broad region of ~15 km thick crust and exhibits strong crustal asymmetry with its conjugate Hatton Bank similar to magma-poor margins. I investigate whether the SE Greenland margin consists of magmatic crust or hyper-extended continental crust sandwiched by magmatic material. Gravity inversion, joint inversion of seismic and gravity data as well as seismic velocity analysis suggest the 15 km thick crust on the SE Greenland margin is magmatic rather than a sandwich of thinned continental crust and magmatic additions. This interpretation requires a sharp continent-ocean boundary, similar to Hatton Bank. Together, these case studies investigate the relationship between magmatism and breakup at rifted margins. The East Indian margin suggests that the use of end-member terminology in the classification of rifted margins is misleading when based only on the magmatic budget. The Pelotas margin shows how extrusive magmatism can record different stages in margin formation. Finally, the SE Greenland margin shows the importance of using quantitative techniques to interpret margin structure and subsequent formation processes.