Crustal structure of continental fragments
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Continental fragments and microcontinents are blocks of continental crust rifted off of passive margins. These relatively unthinned regions (compared to the surrounding crust) offer a perplexing conundrum as to their tectonic history: why are these blocks relatively undeformed compared to the surrounding regions on the passive margin? In this report we review the crustal structure as revealed from deep crustal seismic studies of modern continental fragments and microcontinents. From this review, it is clear that magmatic underplating or plume and LIP emplacement are not essential to isolating continental blocks during rifting. Many continental fragments have thick crusts (> 20 km) with thin layers of overlying sediments, while other continental fragments are severely thinned (~10 km thick) and exhibit horst and graben topography in the upper crust. The wide variability in crustal thickness and structure of continental fragments and microcontinents suggests that many different tectonic processes can explain these features. Initial widespread intra-continental rifting, active upwelling from back-arc spreading, plume-induced rift jumping, inherited weaknesses in ancient suture zones, and shifting extension directions can all contribute to localizing deformation in the surrounding basins, thus separating continental fragments from the mainland.