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Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1981. / Microfiche copy available in Archives and Science. / Bibliography: leaves 229-245. / by Peter David Efran Goreau. / Sc.D.

The mode of tectonics by which early Archean proto-continents were deformed was investigated in the Pilbara Craton in Western Australia, which has not been substantially tectonically deformed since ~3.2 Ga. The craton consists of a unique dome and keel structure where vertical, low-grade metamorphism basaltic greenstone keels surround large granitic (TTG) domes. The dominant model for 3.5-3.2 Ga deformation in the Pilbara is gravity-driven vertical tectonics, or partial convective overturn in a hot crust. In this model, the granitic bodies rose upward as solid-state diapirs, and the greenstones "sagducted" downward around the granitic bodies. Australian scientists acquired deep seismic reflection data crossing a granitoid-greenstone boundary. Their processing did not image the geologically mapped steep dip of the boundary because standard methods limit the maximum dip. A 37-km section of these data were reprocessed using 2D Kirchhoff prestack depth migration to include vertical dips. The western half of the migrated section images a granitoid dome with weak to no reflectivity that extends deeper than 4 km. The eastern half images 2-3 km of layered volcanic rocks of the Fortescue Group overlying the greenstones. Seismic velocity models created using travel-time tomography suggest a thin weathering layer overlying slightly fractured crystalline rocks. These fractures close within 200-300 m depth, and velocity reaches bedrock speeds consistent with expected values of granitoids to the west and volcanic rocks of the Fortescue Group to the east. The best migrated image contains several reflections with dips (~45-55˚) cross-cutting each other from both directions at the location of the expected granitoid-greenstone boundary. This strongly suggests the presence of steep dips in the upper ~1.5 km but does not provide a definitive image. This inconclusive result is due to strong surface-wave noise, the crooked 2D seismic line, and the 3D nature of the geologic boundary at the seismic line. A very small seismic velocity gradient within the crystalline bedrock limits the maximum depth to which vertical features can be imaged. / Master of Science / The Pilbara craton is one of the few exposed and intact pieces of continents that were formed ~3.2 billion years ago. This research analyzes how these early land masses were deformed. There are two methods by which early land masses evolved—vertical tectonics (a more rudimentary, gravity-driven form of plate movement) or horizontal tectonics (which is closer to modern-day tectonics and requires many stages of deformation). This area has a unique dome-and-keel structure where greenstones (metamorphosed volcanics) are vertically wrapped around large granitic domes. Studying the vertical features of the greenstones will allow us to ascertain how tectonics evolved in the area. A seismic survey was conducted in 2018 in the area. These data were reprocessed to include steep dips to extract the exact location of the steeply dipping boundary between the dome and keel structure at depth. The resulting image contains inconclusive evidence due to the physical limitations of the geology and the sharp bend in the seismic line. Further studies need to be done to determine if the Pilbara Craton was formed by vertical tectonics.

Approximately 9.5 km of seismic reflection data over the western margin of the Sanford Triassic Basin in North Carolina were acquired by Chevron, Inc. in 1972. These data enabled a comparison of the relative effectiveness of dynamite and vibroseis sources to be made. A comparison of 3-fold and 12-fold dynamite data, as well as 24-fold vibroseis data with a relatively limited bandwidth, indicate that frequency content of the source used in seismic surveys over Triassic basins is more critical in determining the quality of the stacked data than subsurface coverage. The use of an automatic line drawing routine proved useful in delineating the subsurface geometry of the basin. Available well data were used in conjunction with the seismic data to locate the Cumnock (coal-bearing) Formation within the basin and construct a geologic section across the study area. Strong reflections from the bottom of the basin were recorded. Reflections from beneath the basin ( > 0.7 sec) are attributed to meta volcanics and mylonites of the Carolina Slate Belt. / M.S.

Velocity anisotropy is present at a point in a medium if the seismic velocity in one direction in general differs from that in another direction. The problems associated with the determination of subsurface velocity in anisotropic media by the use of surface seismic reflection data are analyzed. Previous studies of anisotropy in exploration seismology required bore-hole data as well as surface data to detect the presence of velocity anisotropy. Three special types of wave propagation are of interest in reflection seismology, in addition to the general case. The theory of isotropic media is commonly utilized in exploration seismology. Elliptical anisotropy has been the method for handling anisotropic media in the past. The theory of transversely isotropic media is studied in detail since this is a reasonable anisotropy model for exploration use. Layered periodic isotropic structures are considered because of the relationships between the elastic coefficients that yield transverse isotropy in the limiting case for which the isotropic lavers are thin in comparison to the wavelength of a propagating disturbance. Synthetic common-depth-point reflection seismic traces were generated for a uniformly anisotropic halfspace, a model with seismic velocity increasing linearly with depth, velocity increasing stepwise with depth, a buried anisotropic interval in an otherwise isotropic section, and models characterized by the dip varying continuously with depth. Correlation methods (velocity analysis) are developed for the determination of rms velocity vs. two-way reflection time for both isotropic and anisotropic (transversely isotropic) media. These methods are applied to the models discussed above for varying amounts of anisotropy for each model. When the surfaces defined by the velocity analysis correlation matrices are integrated to determine the volume under the surface, it is possible to determine within about one percent the degree of anisotropy in a uniformly anisotropic medium. In a medium of varying anisotropy, it does not appear possible to obtain the same degree of accuracy as for the uniform case. Two isotropic dipping layer models were studied to determine the effects of dip on velocity analysis. The effects of dip are such that the analysis methods yield erroneous results for dips in excess of about 10-12 degrees for the models studied. Random noise degrades the velocity analysis (i.e., the magnitudes of the correlation peaks), but does not affect the accuracy of the results. Lateral velocity gradients appear to have no discernible effects on a velocity analysis for the models studied. Results of this study indicate that the compressional wave data normally used in reflection seismic work may not be useful for the detection of velocity anisotropy. Shear wave (SV) data, on the other hand, are ideally suited to this purpose. Hmvever, the necessity of shear wave data for the detection of anisotropy may limit these methods strictly to land use. This study indicates that the probability of detecting anisotropy by using surface methods is sufficiently high to warrant field testing. / Ph. D.

Reflection seismic data were acquired (by CONOCO, Inc.) which targeted known channel interruption of an upper Pennsylvanian coal seam (Herrin #6) in the Illinois basin. The data were reprocessed and interpreted by the Regional Geophysics Laboratory, Virginia Tech. Conventional geophysical techniques involving field acquisition and data processing were modified to enhance and maintain high frequency content in the signal bandwidth. Single sweep processing was employed to increase spatial sampling density and reduce low pass filtering associated with the array response. Whitening of the signal bandwidth was accomplished using Vibroseis whitening (VSW) and stretched automatic gain control (SAGC). A zero-phase wavelet-shaping filter was used to optimize the waveform length allowing a thinner depositional sequence to be resolved. The high resolution data acquisition and processing led to an interpreted section which shows cyclic deposition in a deltaic environment. Complex channel development interrupted underlying sediments including the Herrin coal seam complex. Contrary to previous interpretations of channel development in the study area by Chapman and others (1981), and Nelson (1983), the channel has been interpreted as having bimodal structure leaving an"island" of undisturbed deposits. Channel activity affects the younger Pennsylvanian sediments and also the unconsolidated Pleistocene till. A limit to the eastern migration of channel development affecting the the Pennsylvanian sediments considered in this study can be identified by the abrupt change in event characteristics. / Master of Science

Large amplitude seismic reflections from within the Catoctin Formation of central Virginia are interpreted to originate from acoustically thin beds of interlayered metabasalts and metasediments. Large acoustic impedance contrasts exist between epidotised layers ( epidosites and volcanic breccia) and non-epidotised layers (greenstones and phyllites) within the Catoctin Formation. Acoustic impedance contrasts also exist between greenstones (metabasalts) and phyllites (metasediments). Constructive interference of small amplitude reflections from thin beds result in large amplitude, reverberating reflections. Thin bed reflections that approximate the first derivative of the source wavelet constructively interfere to give even larger amplitude reflections than those originating by conventional tuning. Computer modeling based on two geologic sections of thin beds of epidosites interlayered with greenstones and of greenstones interlayered with phyllites and epidosites indicates that large amplitude reflections result from constructive interference of thin bed reflections. / Master of Science

Sonic wave methods can be used to provide information on the engineering properties of rocks. The advantages of such techniques include minimal sample preparation, fast site preparation for field tests, reproducible and nondestructive tests, and capability for large scale testing. During this research an extensive review of the most widely accepted engineering rock classification systems was undertaken and their principles, advantages and disadvantages are presented in detail. The mathematical equations describing wave propagation through elastic and viscoelastic media are analyzed in order to determine the dynamic parameters most likely related to static properties. A detailed description of the instrumentation and experimental procedures used for sonic testing is presented. Based on the experimental data, a correlation between the most characteristic static and dynamic properties was established. These relationships can be used to modify existing engineering rock classification systems, by appropriately substituting static properties by the much easier to measure, in the field and in the laboratory, sonic wave parameters. Furthermore, a classification scheme was developed, incorporating information pertaining to the static modulus of elasticity and frequency of joints from sonic wave information. / Master of Science

Approximately 37 km of predominantly 24-fold P-wave Vibroseis data and 16 km of 24-fold SH-wave Vibroseis data were acquired in the southern portion of the folded Appalachians near the Bane Dome in Giles County, Virginia. Data processing techniques included the application of newly developed methods for crossdip removal as well as the determination of statics solutions in the case of time variant shifts within the data traces. Minimum-phase filter deconvolution was also applied for the removal of reverberating energy and multiples recorded on the SH—wave lines. V_p/V_s ratios were used to aid in the determination of lithologies in the absence of bore-hole data. Interpreted thickening of the Lower Cambrian to Upper Precambrian sequence beneath the Bane Dome appears to represent Eocambrian rifting. Faults generated at that time may now be reactivated by the present stress regime, causing earthquake activity in this area. Interpretation of the seismic data supports a duplex structure proposed for the Paleozoic rocks of the Bane Dome Complex within the Narrows thrust sheet of southwestern Virginia. / Ph. D.

The largest earthquakes on Earth take place on the megathrusts of subduction zones, but the slip behavior of megathrusts is variable. This thesis considers why by conducting local, regional and global studies of the interrelationships between the structure and seismogenic behavior of subduction zones. New marine geophysical data collected from the collision zone between the Louisville Ridge seamount chain with the Tonga-Kermadec trench constrain overthrusting and subducting plate structure. Mo'unga seamount is identified beneath the outer-forearc, which calibrates the association of residual bathymetric anomalies and subducting relief, implies an E-W geometry for the subducted ridge and suggests the 200 km wide Louisville seismic gap is modulated by the sediment filled flexural moat. Spectral averaging is then applied along the Tonga-Kermadec margin and along strike variations in overthrusting plate structure are verified by wide-angle seismic transects. The remnant Tonga-Ridge occupies the inner fore-arc and residual free-air gravity anomalies constrain its latitudinal extent (north of 30.5°S), width (110±20 km) and strike (~005° south of 25°S). Plate tectonic reconstructions suggest the Lau Ridge is unmodified by subduction related erosion, <200 km of the Tonga Ridge has been eroded, and neither ridge ever occupied the southern Kermadec arc. Crustal thickness variations are thus inherited, reflecting the Cenozoic tectonic evolution of the Tonga-Kermadec-Hikurangi margin. Spectral averaging is finally applied to all subduction zones on Earth. Part one develops a global catalogue of subducting relief, which is compared with seismological and geodetic inferences of fault-slip behavior. Most seamounts are aseismic, relatively undeformed and observations are not consistent with mechanical models proposing full-decapitation. Aseismic ridges are also associated with megathrust complexity, but are of a larger wavelength and contrasting mode of isostatic compensation. Part two shows almost all intra-margin along-strike transitions in seismogenic behavior are related to pre-existing crustal structure. A paired forearc anomaly is interpreted consisting of a trench-parallel ridge landward of the deep-sea-terrace basin. The ridge crest correlates with the down-dip limit of coseismic slip and strong interplate coupling, the up- dip limit of tremor epicentres, and is interpreted as defining the boundary between the velocity-weakening and seismogenic portion of the subduction interface and the down-dip frictional transition zone. Paired anomalies may be attributed to unrecovered interseismic elastic strain, the preferential subduction erosion of the outer-forearc and/or underplating beneath the inner forearc.