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Marine geophysical study of cyclic sedimentation and shallow sill intrusion in the floor of the Central Gulf of California

Abstract

A new marine geophysical study in the central Gulf of California provides new insights into uppercrustal processes associated with the transition from continental rifting to seafloor spreading. This research is based on the collection and processing of multibeam bathymetry, 3.5 kHz profiles, and two multichannel seismic (MCS) reflection grids collected throughout the central Gulf of California. One high-resolution MCS profile collected across the Southern Trough of Guaymas Basin shows alternations of seismically transparent sedimentary units and horizontally layered strata with high-amplitude internal reflectors. Correlation with DSDP drilling results reveals that reflector alternations are due to cyclical changes between diatomaceous mud turbidites and mud turbidites rich in terrigenous clastics driven by glacial/interglacial cyclicity. This correlation is also supported by the spatial extent of the seismic units, specifically the distance off-axis at which they intercept an intrusion-sediment complex that spreads away from the axis at ̃23 km/Myr. Seismic stratigraphy also shows that during glacials, accumulation rates in the southwest part of Guaymas Basin increased significantly, filling the axial trough. During interglacials, greater aridity and higher sea-level reduced the delivery of terrigneous clastics and axial extension outpaced sedimentation, resulting in regeneration of the axial rift relief. Throughout the central Gulf, analysis of MCS and 3.5 kHz profiles and multibeam bathymetry reveals extensive evidence of shallow, young magmatic intrusions into unconsolidated hemipelagic muds blanketing axial troughs, off-axis oceanic crust, and thinned continental crust. Multiple sills 1-2 km in diameter have a characteristic concave-upwards profile are most common as off-axis intrusions into unconsolidated hemipelagic muds flooring the Guaymas, Carmen, and Farallon Basins, and within the sediment cover of subsided continental crust at the basins' margins. Integration of geophysical datasets suggests that concave-upwards sills have circular to elliptical three-dimensional geometry. Plotting concave upwards sill width against emplacement depth reveals an overall linear trend (deeper = wider sill) and modeling results suggest emplacement depth as a main control on sill structure. MCS and 3.5 kHz profiles also show extensive evidence of hydrothermal fluid flow and hydrocarbon generation above multiple sills including pipe -like features, pockmarks, acoustic turbidity, bright spots, negative polarities, and eye-like structures interpreted as hydrothermal deposits

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