Browsing by Subject "Rifted margins"
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Item From rifting to collision : the evolution of the Taiwan Mountain Belt(2013-05) Lester, William Ryan; McIntosh, Kirk D.; Lavier, Luc LouisArc-continent collisions are believed to be an important mechanism for the growth of continents. Taiwan is one of the modern day examples of this process, and as such, it is an ideal natural laboratories to investigate the uncertain behavior of continental crust during collision. The obliquity of collision between the northern South China Sea (SCS) rifted margin and Luzon arc in the Manila trench subduction zone allows for glimpses into different temporal stages of collision at different spatial locations, from the mature mountain-belt in central-northern Taiwan to the 'pre-collision' rifted margin and subduction zone south of Taiwan. Recently acquired seismic reflection and wide-angle seismic refraction data document the crustal-scale structure of the mountain belt through these different stages. These data reveal a wide rifted margin near Taiwan with half-graben rift basins along the continental shelf and a broad distal margin consisting of highly-extended continental crust modified by post-rift magmatism. Magmatic features in the distal margin include sills in the post-rift sediments, intruded crust, and a high-velocity lower crustal layer that likely represents mafic magmatism. Post-rift magmatism may have been induced by thermal erosion of lithospheric mantle following breakup and the onset of seafloor spreading. Geophysical profiles across the early-stage collision offshore southern Taiwan show evidence the thin crust of the distal margin is subducting at the Manila trench and structurally underplating the growing orogenic wedge ahead of the encroaching continental shelf. Subduction of the distal margin may induce a pre-collision flexural response along the continental shelf as suggested by a recently active major rift fault and a geodynamic model of collision. The weak rift faults may be inverted during the subsequent collision with the continental shelf. These findings support a multi-phase collision model where the early growth of the mountain belt is driven in part by underplating of the accretionary prism by crustal blocks from the distal margin. The wedge is subsequently uplift and deformed during a collision with the continental shelf that involves both thin-skinned and thick-skinned structural styles. This model highlights the importance of rifting styles on mountain-building.Item Mesozoic rifting along the eastern seaboard of North America : insights from the seismic velocity structure of the Newfoundland margin and the northern Gulf of Mexico(2014-12) Eddy, Drew Richard; Van Avendonk, Harm J. A.Passive margins along the eastern seaboard of North America formed during early Mesozoic continental rifting and seafloor spreading, tectonic processes that are not fully understood. Seismic refraction and reflection data at the northeastern and north-central Gulf of Mexico and the Grand Banks of Newfoundland, Canada, are used to interpret the deep seismic velocity structure of sediments, crust, and mantle. These interpretations allow for a better understanding of continental rifting, mantle upwelling, magmatism, and seafloor spreading. Magma-poor rifting of the Newfoundland-Iberian margin developed a wide continent-ocean transition zone (COT). I present an analysis of 2-D marine seismic refraction and reflection data from the SCREECH project, including a shear velocity model to constrain the composition of the Newfoundland COT. Comparing SCREECH Line 2 Vp/Vs ratios with depth to potential lithologies supports a COT comprised of hyperextended continental crust and serpentinized mantle. Reconstructions of the opening of the Gulf of Mexico basin are impeded by a lack of seafloor magnetic anomalies and an abundance of sediments that obscure acquisition of seismic refraction datasets. Accordingly, the roles of mantle upwelling, magmatism, and lithospheric extension in this small ocean basin are poorly known. I present new 2-D marine seismic refraction data from the U.S. Gulf of Mexico collected during the 2010 GUMBO project. Rifting in the eastern Gulf of Mexico developed above a zone of anomalously high mantle potential temperatures that led to abundant magmatism. Syn-rift basins in continental crust, high velocity lower crust, a narrow zone of crustal thinning, and seaward-dipping reflectors support this interpretation. Oceanic crust here is thick despite slow seafloor-spreading rates, implying continuation of a thermal anomaly after rifting. In the north-central Gulf of Mexico, transitional crust is consistently thin (~10 km) across a wide zone. Velocity-depth comparisons, asymmetry of the north-central Gulf with the Yucatán margin, and dating of onshore xenoliths support either stretched and magmatically intruded continental crust or a multi-stage episode of seafloor spreading with ridge jumps. I contend that although tectonic inheritance may ultimately influence the location of a passive margin, the rifting process is largely controlled by mantle potential temperature and upwelling rate.Item The evolution of hyperextended rifted margins : linking variations on the width, asymmetry, and strain distribution to lithospheric strength and geodynamic processes(2015-12) Svartman Dias, Anna Eliza; Lavier, Luc Louis; Hayman, Nicholas W.; Van Avendonk, Harm; Stockli, Daniel; Buck, Roger WThe goal of the work presented here is to improve the understanding of the processes controlling the styles of rifting. A large focus is on the structural and thermo-mechanical evolution of magma-poor margins. Applying parameters values that encompass those inferred from Atlantic margins to geodynamic numerical experiments of lithospheric extension successfully reproduce the variety of crustal thicknesses, widths and asymmetries observed at those margins. The results are grouped into four end members of margins for varying initial lithospheric strength and extension rates. The first two end members are narrow and asymmetric, and narrow and near-symmetric conjugate margins. The other two are wide extensional systems that evolve into asymmetric conjugate margins with one side <100 km wide, and the other >100-300 km wide; and highly asymmetric conjugate margins wherein the wide conjugate is 200 km to > 350 km across. All margins described above form by depth-dependent stretching, and polyphase sequential faulting, including detachment faults. In addition to different distributions of thinning, total crustal thinning across conjugate margins is not always balanced by an equal magnitude of distributed plastic deformation within the lithospheric mantle. The unbalanced thinning is associated with small-scale convection developed in the later stages of extension in the models. Mantle rheology and the continuous weakening of the lithosphere dominate the evolution of narrow systems. The formation of the wide asymmetric systems occurs due to deformation migration – diachronous rifting - wherein neither the upper nor the lower crustal deformation remains fix. Such extension is controlled by both the crustal and mantle rheology, and the initial lithospheric strength is preserved throughout most of the margin evolution. Two effects of bending stress at adjacent areas – one strengthening the fault, and the other weakening the rift flank – may contribute to the deformation migration. The change in curvature may help localize new faults near the rift-flank inflexion point. Despite the simplified lithosphere initial configuration assumed, the evolving extension results in complex rifted margins. Making further predictions of subsidence and thermal histories of margins will require integrating geodynamic modeling results with kinematic subsidence and heat flow studies in order to develop tectono-sedimentary models in closer agreement with observations.