Browsing by Subject "Moho"
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Item Characterization of the Southern High Plains by seismic, gravity, and topographic analysis(2010-12) Hoemberg, Jeffrey; Gurrola, Harold; Zhou, Hua-wei; Leverington, David; Asquith, George B.A topographic database was downloaded from the USGS and applied to several wavelength filtering processes. Seismic data were collected over a period of nine years at seven seismic stations deployed by Texas Tech University across the Southern High Plains, and all available additional data were downloaded from an addition 12 stations in the US Array. This dataset was processed by deconvolution with a subsequent application of an array processing algorithm and then stacked. The processed topographic and seismic data were analyzed in conjunction with gravity data acquired by Ozyavas to observe the presence of shallow crustal structures, midcrustal discontinuities, and the Moho in an attempt to relate these to subtle modern topographic relief across the southern High Plains. Stacks produced from the receiver functions identified five distinct boundaries at: 10km, 18km, Moho, 85km, and 170km. Each boundary was cataloged and mapped spatially. A hinge line trending southwest-northeast observed in the 18km discontinuity lies collinear and appears to have influence over the expression of the Caprock Escarpment. Similarly, an observed ridge in the Moho topography lies collinear to the Matador Arch and may implicate a trend of crustal weakness. The deeper discontinuities at 85km and 170km have structures that mimic features observed in the filtered gravity datasets. Gravity profiles were then reconstructed from Ozyavas, 2004, along three transects of the South Plains and resulting subsurface structure to 200 kilometers was approximated using the discontinuities from seismic as tie points through each line. These cross sections were useful for verification that the inferred layers from the discontinuities were valid results based on observed gravity data.Item The Northeastern Gulf of Mexico : volcanic or passive margin? : seismic implications of the Gulf of Mexico Basin opening project(2013-05) Duncan, Mark Hamilton; Christeson, Gail L.; Van Avendonk, Harm J. A.The Gulf of Mexico Basin Opening project (GUMBO) is a study of the lithological composition and structural evolution of the Gulf of Mexico (GoM) that uses Ocean Bottom Seismometer (OBS) data from four transects in the Northern GoM. I examine 39 OBS shot records in the easternmost transect for shear wave arrivals and pick shear wave travel times from the 11 usable records. I then carry out a tomographic inversion of seismic refraction travel times. I use the resulting shear-wave velocity model in conjunction with a previously constructed P-wave model to examine the relationship between Vp and Vs. I compare velocities in the sediment and basement with empirical velocities from previous studies for the purpose of constraining lithological composition below the transect and make an interpretation of the structural evolution of the eastern GoM. The seismic velocities for crust landward of the Florida Escarpment are consistent with normal continental crust. Seaward of the Escarpment, velocities in the upper oceanic crust are anomalously high (Vp = 6.5 – 7 km/sec; Vs = 4.0 – 4.6 km/sec). A possible explanation for this observation is that GoM basalt formation consisted of basaltic sheet flows, forming oceanic crust that does not contain the vesicularity and lower seismic velocities found in typical pillow basalts. Increased magnesium and iron content could also account for these high velocities. Seismic refraction and reflection data provide a means of investigating the nature of the Moho in the northeastern GoM. I use a finite difference method to generate synthetic record sections for data from eight instruments that are part of the two easternmost GUMBO seismic lines (lines 3 & 4). I then vary the thickness of the Moho in these synthetic models and compare the results with the original receiver gather to examine the effects this variability has on amplitudes. The data from the instruments chosen for these two lines are representative of continental and transitional crust. The finite difference models indicate that the Moho beneath GUMBO 3 is ~1500 m thick based on the onset and amplitudes of PmP arrivals. All five instruments display consistent results. The instruments along GUMBO 4 suggest a Moho almost twice as thick as GUMBO 3 on the landward end of the transect that grades into a Moho of similar thickness (1750 m) in the deep water GoM. The three instruments used to model the Moho in this area show that the Moho ranges from ~1750 to 3500 m in thickness. The sharper boundary beneath continental crust in GUMBO Line 3 supports other evidence that suggests magmatic underplating and volcanism in the northern GoM during the mid-Jurassic. The thicker Moho seen on the landward end of GUMBO Line 4 that is overlain by continental crust was likely unaffected by GoM rifting. Therefore, the Moho beneath the Florida Platform might be as old as the Suwannee Terrane, and complex Moho structure is not uncommon for ancient continental crust.Item Seismic analysis of the Rocky Mountain Front and Great Plains using transportable array data(2011-05) Castille, Kristopher A.; Gurrola, Harold; Zhou, Hua-wei; Asquith, George B.USArray seismic data along the Rocky Mountain Front and Great Plains was downloaded from the Incorporated Research Institutions for Seismology (IRIS) to map lithosphere boundaries including the Mohorovičić discontinuity (Moho), Hales discontinuity (Hales), and lithosphere-asthenosphere boundary (LAB). 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) and simultaneously deconvolved 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 Moho, Hales discontinuity, and the LAB, and related these to the geology of the Rocky Mountain Front and Great Plains. The Moho depth across the Rocky Mountain Front and Great Plains correlates well with the previous geological and geophysical work done from the literature with deep signatures along the Rocky Mountain Front, southern granite-rhyolite belt, and Black Hills area, and shallow signature along the southern portion of the Rio Grande Rift. Crustal Vp/Vs ratios are mostly uniform throughout the study area at 1.73 to 1.76, which is average for continental crust. The Hales discontinuity is 80 to 90 km for most of the study area. This correlates well with the original discovery made by A.L. Hales in 1969. Deeper regions on the Hales may be related to orogenic events from the past. It appears that Precambrian features are preserved in the mantle between the Moho and Hales discontinuity. The lithosphere-asthenosphere boundary can be divided into a shallow northern half and deep southern half in the study area. Vp/Vs ratios for each interval do not vary much throughout the entire study area. However, Vp/Vs ratios mapped for the Hales discontinuity show higher values above the Yavapai-Mazatzal transition zone, which is interpreted to be the boundary of this accreted Proterozoic province. Skeptical of the lithosphere-asthenosphere boundary results due to crustal reverberations affecting the quality of data that can occur with P-to-S imaging. Recommend following up with S-to-P receiver functions.