Abstract
SUMMARY The southern boundary of the Cayman Trough in the Caribbean is marked by the Swan Islands transform fault (SITF), which also represents the ocean–continent transition of the Honduras continental margin. This is one of the few places globally where a transform continental margin is currently active. The CAYSEIS experiment acquired an ∼165-km-long seismic refraction and gravity profile (P01) running across this transform margin, and along the ridge-axis of the Mid-Cayman Spreading Centre (MCSC) to the north. This profile reveals not only the crustal structure of an actively evolving transform continental margin, that juxtaposes Mesozoic-age continental crust to the south against zero-age ultraslow spread oceanic crust to the north, but also the nature of the crust and uppermost mantle beneath the ridge-transform intersection (RTI). The traveltimes of arrivals recorded by ocean-bottom seismographs (OBSs) deployed along-profile have been inverse and forward modelled, in combination with gravity modelling, to reveal an ∼25-km-thick continental crust that has been continuously thinned over a distance of ∼65 km to ∼10 km adjacent to the SITF, where it is juxtaposed against ∼3–4-km-thick oceanic crust. This thinning is primarily accommodated within the lower crust. Since Moho reflections are only sparsely observed, and, even then, only by a few OBSs located on the continental margin, the 7.5 km s–1 velocity contour is used as a proxy to locate the crust–mantle boundary along-profile. Along the MCSC, the crust–mantle boundary appears to be a transition zone, at least at the seismic wavelengths used for CAYSEIS data acquisition. Although the traveltime inversion only directly constrains the upper crust at the SITF, gravity modelling suggests that it is underlain by a higher density (>3000 kg m–3) region spanning the width (∼15 km) of its bathymetric expression, that may reflect a broad region of metasomatism, mantle hydration or melt-depleted lithospheric mantle. At the MCSC ridge-axis to the north, the oceanic crust appears to be forming in zones, where each zone is defined by the volume of its magma supply. The ridge tip adjacent to the SITF is currently in a magma rich phase of accretion. However, there is no evidence for melt leakage into the transform zone. The width and crustal structure of the SITF suggests its motion is currently predominantly orthogonal to spreading. Comparison to CAYSEIS Profile P04, located to the west and running across-margin and through 10 Ma MCSC oceanic crust, suggests that, at about this time, motion along the SITF had a left-lateral transtensional component, that accounts for its apparently broad seabed appearance westwards.
Highlights
Large-scale transform faults and fracture zones are observed to segment passive rifted margins globally, but such transform ocean-continental margins are infrequently studied and are much less well understood than their rifted equivalents
Ray-trace check As a further means to test which of the P-wave inversion and density models is a more likely representation of the true crustal structure beneath the active transform margin and Swan Islands transform fault (SITF)/ridge-transform intersection, the P-wave inversion model was converted into a node-specified model with distinct layer boundaries imposed (Fig. 9a)
Oceanic crustal accretion at the Mid-Cayman Spreading Centre The modelling of all CAYSEIS profiles conducted to date (Harding et al, 2017; Van Avendonk et al, 2017; Grevemeyer et al, 2018a; Peirce et al, 2019b) concludes that oceanic crustal formation at the MCSC is dependent on temporal fluctuations in magma supply, where magma rich accretion swaps to magma poor spreading over a time frame of several million years, and where magma may arrive in pulses to specific locations along the spreading axis, rather than consistently along the entire segment length (Harding et al, 2017)
Summary
Large-scale transform faults and fracture zones are observed to segment passive rifted margins globally, but such transform ocean-continental margins are infrequently studied and are much less well understood than their rifted equivalents. Gravity and magnetic data (ten Brink et al, 2002) suggest that 2-3 km-thick oceanic crust, formed at the adjacent ultraslow spreading Mid-Cayman Spreading Centre (MCSC – full rate of 15-17 mm y-1 – Holcombe and Sharman, 1973; Dick et al, 2003; DeMets et al, 2007), is juxtaposed against 20-30 km-thick continental crust, making this an ideal location to study how these margins evolve. 3. DATA ACQUISITION The CAYSEIS (Crustal accretion and transform margin evolution at ultraslow spreading rates) project acquired active-source seismic, gravity and swath bathymetry data along six profiles (called Profiles P01-P06) in the Cayman Trough (Fig. 2 – FS Meteor cruise M115 – Grevemeyer et al, 2016; Peirce, 2015), together with passive seismic.
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