Geophysical characterization of the crustal structure of the Central Bransfield Basin around Orca Volcano: evidence for a rapidly evolving back-arc rift

Abstract In 2019 we collected gravity and magnetic measurements, in conjunction with multibeam swath bathymetry and multi-channel seismic reflection profiling, to characterize the neovolcanic zone around Orca Volcano in the Central Bransfield Basin, Antarctica. New analysis of mafic dredge samples shows that the caldera is composed of rocks with a wide range of measured susceptibilities. Multi-channel seismic reflection profiles across Orca Volcano showed the presence of several reflectors under the caldera, characterized by strong amplitudes and phase-reversed polarity. Gravity and magnetic data also show coincident anomalies. Our 2D forward modelling shows that these anomalies are the result of a shallow magma accumulation zone underlain by a low-density, low-susceptibility volume. We also observe that the axial ridge to the south-west of the volcano is characterized by a narrow zone of low density and high susceptibility, consistent with a focused rift axis. To the north-east of the volcano, the potential field data show that the axial rift is left-lateral offset and is somewhat broader. An asymmetric pattern of decreasing density and susceptibility of igneous rocks across the rift is consistent with a pattern of extension that has only recently evolved into a narrow rift and the mid-ocean ridge basalt-like compositions seen in dredge samples. These results are consistent with rifting that is rapidly transitioning to seafloor spreading.