Ultra Low Velocity Zone existence in the high shear velocity region beneath Cocos Plate, Central America, and the Caribbean
Abstract:The lowermost mantle beneath subduction is typically characterized by higher than average shear wave speeds, often with the presence of one or more D” discontinuities. These regions are considered the cooler parts of the convective cycle, in contrast to warmer zones of convective return flow, namely, the vicinity of large low shear velocity provinces (LLSVPs). Ultra-low velocity zones (ULVZs) have been long characterized as related to elevated temperature (and/or chemistry) of LLSVP regions. However, some past work has suggested evidence for ULVZ in the presumed cooler regions. In this study we investigate the region beneath the Cocos Plate, Central America, and the Carribbean for ULVZ using high quality broadband Transportable Array data from EarthScope’s USArray for the presence of ULVZs.
We utilize an ScS-stripping technique that combines a precursor and postcursor to ScS that arise from ULVZ structure, if present. The precursor is a reflection off the top of the ULVZ, while the postcursor is a core-reflection with an added reverberation between the ULVZ top and the core-mantle boundary (CMB). We collected data from deep South American earthquakes recorded in North America and stack data in geographic bins. We find clear evidence for a ULVZ beneath the Gulf of Mexico, but the rest of the study area appears to lack any significant structure. The structure we find is of the order of 100 km wide. The ULVZ properties will be constrained by comparison to predictions from synthetic seismograms.
We explore hypotheses for the origin of a ULVZ in a high shear velocity region. These include mineralogical heterogeneities that convective currents have collected; notable possibilities are accumulated melts from subducted materials, such as ocean crust basalts and banded-iron formation. If water can be transported by subducted slabs to the deep mantle, it can significantly decrease the melting temperature of mantle materials and cause such anomalies. A ULVZ a relatively cold region of the mantle can also be related to products from chemical reactions between the mantle and the core. If ULVZs found in and near LLSVPs are of the same origin as the one found here, then there is the suggestion that convective processes bring material collected outside the LLSVP towards the warmer zones.