Crustal structure beneath the Indochina peninsula from teleseismic receiver functions

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95076/1/grl27455.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/95076/2/grl27455-sup-0012-ts01.pd

Crustal Formation on a Spreading Ridge Above a Mantle Plume: Receiver Function Imaging of the Icelandic Crust

Iceland sits astride a mid-ocean ridge underlain by a {mantle} hotspot. The interplay of these two geological processes has the potential to generate a complex and laterally variable crustal structure. The thickness of the Icelandic crust is a long running and controversial debate, with estimates ranging from a "thin'' 20 km crust to a "thick'' 40 km crust. We present new images of the first order seismic discontinuity structure of the Icelandic crust based on a joint inversion of receiver function and ambient noise derived surface wave dispersion data. Inversion results are validated through comparison to receiver functions multi-phase common conversion point stacks across the densely instrumented Northern Volcanic Zone. We find a multi-layered crustal structure consisting of a 6-10 km deep upper crust underlain by either one or two discontinuities. The shallower discontinuity is found at depths of ~20 km throughout Iceland. The deeper discontinuity is only present in some regions, defining the base of a lens-like lower layer with maximum depths of 44 km above the center of the mantle plume. Either of these two discontinuities could be interpreted as the seismic Moho, providing an explanation why previous estimates of crustal thickness have diverged. Such structure may form via underplating of a pre-existing oceanic crust as has been hypothesized in other ocean island plume settings. However we demonstrate with a simple petrological model that variability in seismic discontinuity structure can also be understood as a consequence of compositional variation in melts generated with distance from the plume center

Gas flow through shallow sediments - A case study using passive and active seismic field data

Offshore Quaternary sediment sequences are generally weak and unconsolidated, and fluid flow through these rock formations is hard to predict or model. Surface flow observations, such as gas seeps, often appear random and may be controlled by the architecture and facies distribution within the shallow strata. Using geophysical data acquired before and after a sub-seabed blowout from a 4700m-deep interval in a North Sea hydrocarbon exploration well, we investigate the nature of subsurface gas flow behaviour in shallow sediment sequences. The underground gas blow-out lasted for a period of almost one year, and when the flow from the deep gas reservoir was stopped, we observed that fluids continued to flow in the shallow subsurface and probably continue to this day, almost 30 years later. During the underground blowout phase, passive seismic data revealed episodes of up to 30 min with high seismicity followed by quieter periods of several hours. Time-lapse seismic data revealed that gas had migrated into several shallow sand layers, and that this migration had continued over for at least 25 years. The seismic data also indicated that gas entered into a shallow tunnel valley complex approximately 1–2 years after the blowout, and at a later stage migrated further. The 3D seismic data also shows indications of gas leakage outside the well bore of the relief well, drilled through the overburden sediments 1.2 km away from the main well. This implies that wells drilled through weak overburden rocks can weaken the formations potentially creating vertical pathways for gas migration. The observations from this gas migration case history are used to gain more general insights into the flow of buoyant gases, especially CO2, in shallow unconsolidated sediment sequences