Three-dimensional fine-scale seismic structure of the Rainbow massif constrained by downward continued multichannel seismic data

Abstract:

We investigate the interaction between magmatism, faulting and hydrothermal circulation at the Rainbow non-transform offset at the slow-spreading Mid-Atlantic Ridge (36°14’N), using active-source seismic data acquired during cruise MGL1309. The multichannel seismic (MCS) data were collected with an 8-km-long hydrophone streamer containing 636 receiver groups spaced at 12.5 m. The inline shot spacing was 37.5 m. We utilize the refraction components of the MCS data to constrain the fine-scale seismic velocity structure of the uppermost 1-2 km of the lithosphere. Twenty profiles are categorized into three groups according to their orientations: parallel or perpendicular to the ridge axis trend, or along the spreading direction. We first migrate the MCS datum from the sea surface to 100 m above the seafloor, so that shallow crustal refractions can be picked as first arrivals at offsets as short as 500 m. Then, we semi-automatically pick first arrivals from all shot gathers, and perform 3-D tomography inversions. Our preliminary results from the group of profiles perpendicular to the ridge axis reveal relatively high velocity bodies (HVB) beneath the Rainbow massif and the Pot of Gold massif on the eastern flank of the rift valley, indicating the uplift of lower crustal materials due to normal faulting. The Pot of Gold massif shows HVBs as shallow as ~100 m below the seafloor (bsf), with Vp=3.5-5 km/s, compared to surrounding crust with lower Vp=2.5-4.5 km/s. In contrast, the Rainbow massif overlies an HVB at larger depths that pinches out at ~500 m bsf. Above and to the sides of the Rainbow HVB, there are low velocity layers up to 2-km thick, suggesting different lithologies and/or pervasive shallow fracturing and alteration. The ongoing study will incorporate the remaining profiles to extend the 3-D velocity model and constrain in detail the shallow variations in structure, lithology, and extent of alteration.