Browsing by Subject "Receiver function"
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Item Dynamics of the eastern edge of the Rio Grande Rift(2013-08) Xia, Yu; Grand, Stephen P.The Western U.S. has experienced widespread extension during the past 10’s of millions of years, largely within the Basin and Range and Rio Grande Rift provinces. Tomography results from previous studies revealed narrow fast seismic velocity anomalies in the mantle on either side of the Rio Grande Rift as well as at the western edge of the Colorado Plateau. The fast mantle anomalies have been interpreted as down-welling that is part of small scale mantle convection at the edge of extending provinces. It was also found that crust was thicker than average ab¬¬ove the possible mantle down-welling, indicating that mantle dynamics may influence crustal flow. We present results from P/S conversion receiver functions using SIEDCAR (Seismic Investigation of Edge Driven Convection Associated with the Rio Grande Rift) data to determine crustal and lithospheric structure beneath the east flank of the Rio Grande Rift. Crustal and lithosphere thickness are estimated using P-to-S and S-to-P receiver functions respectively. Receiver function migration methods were applied to produce images of the crust and lithosphere. The results show variable crustal thickness through the region with an average thickness of 45 km. The crust achieves its maximum thickness of 60km at 105W longitude, between 33.5N and 32.2N latitude. This observation confirms previous receiver function results from Wilson et al, 2005. Body wave tomography (Rocket, 2011; Schmandt and Humphreys, 2010) using similar data to what we used for the receiver function analysis, shows mantle downwelling closely associated with the thickened crust. We believe that the thickened crust might be due to lower crustal flow associated with mantle downwelling or mantle delamination at the edge of the Rio Grande Rift. In this model the sinking mantle pulls the crust downward causing a pressure gradient within the crust thus causing the flow. Our S-P images show signal from the lithosphere-asthenosphere boundary (LAB) with an average LAB thickness of 100 km but with a sharp transition at about 1050 W from 75 km to over 100 km. The region with abnormally thick crust overlies a region where the lithosphere appears to have a break. We interpret our results as showing that lower lithosphere has and is delaminating near the edge of the Great Plains accompanied by lower crustal flow in some places determined by lower crustal viscosity.Item Geophysical investigation of the post Grenville Orogen lithosphere: Texas Gulf Coast(2013-05) Harrington, Tom; Gurrola, Harold; Nagihara, Seiichi; Asquith, George B.Receiver function and Pn tomography were used in conjunction to characterize the seismic velocity structure of the lower crust and upper mantle of the Laurentian Margin of Texas. The velocity interfaces modeled are the Mohoroviçiç Discontinuity (Moho), and the Hales Discontinuity. Observations resulted in crustal thicknesses from 26 to46 kilometers, and a Hales Discontinuity depth from 60to 88 kilometers. Pn tomography provided differential velocity variations of ±0.24 km/s, with the average velocity approximately 8.1 km/s. The Sabine and San Marcos Arches were not imaged in the upper mantle indicating they are crustal features only. A northwest-southeast trending anomaly was observed in observed in receiver function shallow Moho depths, Pn tomography low Moho velocity, and low aeromagnetic data was interpreted to be a relic of strike-slip motion attributed to the Ouachita Rift System. The anomaly crosses the Ouachita Orogenic Front without truncation or offset, inferring that the younger Ouachita Front was a, “thin-skinned,” collision along the Laurentian MarginItem Imaging of the Crust and Moho beneath Oklahoma using Receiver Functions and Pn Tomography; with Emphasis on the Southern Oklahoma Aulacogen(2013-05) Tave, Matthew A.; Gurrola, Harold; Karlsson, Haraldur R.; Asquith, George B.US TransportableArray seismic data over the Southern Oklahoma Aulacogen and surrounding region was downloaded from the Incorporated Research Institutions for Seismology (IRIS) to map lithosphere boundaries including the Mohorovičić discontinuity (Moho) and Hales discontinuity (Hales), along with creating crustal velocity models. Water-level (i.e. prewhitening) deconvolution in the frequency domain was used to filter out the bad stations. The good stations were further improved by cross-correlating and stacking the vertical component of nearby stations (i.e. beamforming) to reduce the need for prewhitening and improve the signal-to-noise ratio in the receiver functions. These stacked move-out corrected receiver functions (SMOCRF) were used to interpret depth and Vp/Vs ratios of features in the lithosphere including the upper, middle, and lower crustal layers as well as the Moho and Hales discontinuity, and related these to the geology of Southern Oklahoma Aulacogen and its subsequent features. Also the data was used to pick Pn arrival times at each station and then complete straight ray Pn tomography to better see the geologic features expressed on the Moho. The Moho depth along the Southern Oklahoma Aulacogen correlates well with the previous geological and geophysical work done from the literature with deep signature beneath the Amarillo-Wichita uplift, and general shallowing trend from west to east across the region. There is evidence of an iron depleted Moho beneath the aulacogen. The Pn tomography results show the Southern Oklahoma Aulacogen represented along the Moho by a linear low-velocity feature. The lower velocity along the uplift may be due to an iron depleted area of old Moho that has depressed the mantle and thus increased Pn travel time. My recommendation is to follow-up with a longer time period study over a the entire area of the central to west continent thus incorporating larger offsets between earthquake and event as well as many more ray path directions over the entire area.Item Mapping the Rivera and Cocos subduction zone(2013-12) Suhardja, Sandy Kurniawan; Grand, Stephen P.The crust and upper mantle seismic structure beneath southwestern Mexico was investigated using several techniques including teleseismic tomography using 3D raytracing, a joint tomographic inversion of teleseismic and regional data that included relocation of regional seismicity, and a P to S converted wave study. The data used in these studies came from a broadband seismic deployment called MARS. The seismic deployment lasted 1.5 years from January 2006 to June 2007 and the stations covered much of Jalisco and Colima states as well as the western part of Michoacan states. At depth less than 50 km, P-wave receiver function images show a clear dipping slow velocity anomaly above a fast velocity layer. The slow anomaly convertor seen in receiver functions is directly above a fast dipping seismic anomaly seen in regional tomography results. The slow velocity with high Vp/Vs ratio is interpreted as a high pore fluid pressure zone within the upper layer of subducting oceanic crust. Regional seismicity was located using the double difference technique and then relocated in a tomography inversion. The seismicity is located very close to the slow dipping boundary to depths of 30-35 km and thus along the plate interface between the subducted and overlying plate. Deeper events are below the slow layer and thus are intraplate. Receiver function results also show a weaker continental Moho signal above the dipping slab that I interpret as a region of mantle serpentinization in the mantle wedge. Inland of the subduction zone, a clear Moho is observed with a maximum thickness of near 42 km although it thins to near 36 km depth towards the north approaching the Tepic-Zacoalco Rift. Using H-K analysis to examine Vp/Vs ratios in the crust, I find a band of very high Vp/Vs along the Jalisco Volcanic lineament as well as beneath the Michoacan-Guanajuato volcanic field. These observations suggest the continental crust is warm and possibly partially molten over broad areas associated with these two magmatic regions and not just locally beneath the volcanoes. I also found seismicity associated with the Jalisco Volcanic Lineament but it was trenchward of the volcanoes. This may indicate extension in this region is part of the explanation for this magmatic activity. At depths below 100 km, the tomography results show clear fast anomalies, about 0.3 km/s faster than the reference model, dipping to the northeast that I interpret as the subducting Rivera and Cocos plates. Tomography models show that the Rivera slab is dipping much steeper than the Cocos plate at depth. Below 150 km depth, the Rivera plate shows an almost vertical dip supporting the interpretation that the slab has steepened through time beneath Jalisco leading to a coastward migration of young volcanism with mixed geochemical signatures. The location of the young volcanism of the Jalisco Volcanic Lineament is just at the edge of the steeply dipping slab seen in the tomography. The magmatism is thus likely a nascent arc. The models also display evidence of a gap between the Rivera and Cocos plates that increases in width with depth marking the boundary between the two plates. The gap lies just to the west of Colima graben and allows asthenosphere to rise above the plates feeding Colima volcano. Another interesting finding from this study is a possibility of a slab tear along the western edge of the Cocos plate at a depth of about 50 km extending 60 km horizontally. The tear is coincident with a lack of seismicity in this region although there are events below and above the tear.