published_or_final_version / abstract / Earth Sciences / Doctoral / Doctor of Philosophy

published_or_final_version / Geography and Geology / Master / Master of Arts

The-west-dipping, north-northwest-trending Black Peak fault and associated monoclines which form the western flank of the Marathon dome, are well exposed near Del Norte Gap. Field mapping shows that the dip of the Black Peak fault increases downward from zero to 80 degrees. A northeast-trending right lateral fault cuts the hanging wall of the Black Peak fault at Del Norte Gap. The Black Peak fault has greater displacement south of the gap than north of the gap. Several north and northwest-trending normal faults cut the Cochran Mountains. The folding and faulting took place after the deposition of the upper Boquillas Limestone and before the deposition of Quaternary gravels. Vertical uplift of the Marathon dome during the Laramide orogeny produced the Black Peak fault and associated features. Normal faults later cut the area. The structural features near Del Norte Gap correspond well to previously described analytical and experimental configuration of features produced by differential vertical movement of basement blocks and previously described examples of vertical tectonics. / text

As a tool in discriminating basic rocks from different tectonic settings, a type of diagram was developed that employs three ratios of trace elements. The diagram separates basic rocks formed in mid-ocean ridge, intra-plate, and volcanic arc settings. It can be used to differentiate basalts from marginal basin, forearc, and arc rift zone settings. A second application of this type of diagram, employing major elements, distinguishes tholeiitic, calcalkaline, and boninitic series volcanic rocks. The southern part of the Foothills terrane, western Sierra Nevada, California, is composed chiefly of Jurassic-Triassic(?) metavolcanic and metasedimentary rocks of lower greenschist grade. Major tectonism affecting the terrane, associated with the Late Jurassic-Early Cretaceous Nevadan orogeny, was sinistral transpression with shearing along the Bear Mountains and Melones fault zones. The line of slip in high shear strain regions is approximated by the modal stretching lineation, which is at a rake of approximately 45° SE to the general shear zone orientation, suggesting sub-equal components of strike slip and dip slip. The sense of shear from kinematic indicators is consistently east side to the northwest. The terrane hosts three types of syngenetic massive sulfide deposits: Cyprus-type Cu deposits, Kuroko-type Zn-Cu-Pb deposits, and Besshi-type Cu-Zn deposits. The Cyprus-type deposits lie at the top of a Triassic(?) tholeiitic - basalt sequence in the lower Penon Blanco Formation. The deposits are part of an ophiolitic sequence that appears to have formed in an open-ocean spreading center environment. Felsic lava facies host the Kuroko-type deposits at the top of the Middle to Late Jurassic upper Gopher Ridge Formation, a dominantly bimodal sequence of meta-rhyolitic lavas and tuffs and meta-basaltic lavas. The tectonic setting appears to have been an arc-rift zone that formed during the transition from arc volcanism forming the lower Gopher Ridge Formation to younger basinal sedimentation forming the Mariposa Formation. The Besshi-type deposits are sediment-hosted in the Late Jurassic Mariposa Formation. They appear to have formed in the median part of a long linear basin between rifted arc segments. The inferred tectonic setting of the sulfide deposits was an early back-arc or interarc basin, which may have been related to transtensional tectonics.

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Thesis (M.Sc.)--University of the Witwatersrand, Faculty of Science, 1991 / This study documents and interprets the stratigraphy and sedimentary environments of the upper Johannesburg and Turffontein Subgroups of the Witwatersrand Supergroup on St. Helena Gold Mine. These data are used to construct a tectono-stratigraphic framework and determine the general distribution of economic mineralization. [Abbreviated abstract. Open document to view full version]

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Seismic refraction data collected in 1985 by the USGS were used in this study to derive the P and S velocity structure of the crust beneath the Gulf of Maine. The data quality differs among instruments and is affected by surficial lateral heterogeneities, a ringy source signature and reverberations. Velocity models of the crust were computed by one-dimensional raytracing and by wavefield continuation. Pg arrivals were modeled using both techniques to derive the P velocity of the upper 5-15 km of the crust and give very similar results. Strong Sg arrivals were also observed, and computed S amplitudes generated from P-S conversion for different scenarios show that the observed S wave is generated at the basement top. Two small sediment basins are indicated in the Central Plutonic Zone and two faults are suggested in the Fault Zone and the Central Plutonic Zone, respectively. Beneath the sediments the layering is uniform with dips of less than 2° and a fairly laterally homogeneous velocity structure, in spite of lateral variations in reflectivity. P and S velocities increase from 5.3 and 2.8 km/s, respectively, at the basement to 6.4 and 3.7 km/s at 10 km depth. A laterally discontinuous low velocity zone is indicated at 6-10 km depth which might be caused by laccolithic granitic intrusions. However, magnetic and gravity data do not show indications for felsic intrusions where the low velocity zones are observed. Velocity differences among some instruments suggest anisotropy in the upper 6 km of the crust, as observed in onshore Maine. These instruments indicate velocities parallel to the structural grain of the Appalachians of 6.1-6.4 km/s and velocities transverse to the grain of 5.8-6.1 km/s in the depth range 2-6 km. Cashes Ledge granite, a site of an intense magnetic high, has a reduced velocity compared to surrounding rocks and might extend to at least 10 km depth. Poisson's ratio for the upper crust ranges from 0.23-0.26. To derive the velocity structure of the middle and lower crust, wide-angle reflections interpreted to be PmP and SmS were modeled by one-dimensional raytracing. In addition synthetic seismograms were computed using the WKBJ method to constrain possible middle and lower crust velocity models by their PmP and SmS amplitudes. Recorded PmP and SmS wide-angle reflections have quite different amplitudes and travel-times among instruments suggesting a heterogeneous lower crust. The crust below 10 km depth has an average P velocity of 6.5-6.8 km/s and an average S velocity of 3.7-3.9 km/s. Most instruments indicate a Poisson's ratio of around 0.25 between 10 km depth and Moho and one instrument suggests a Poisson's ratio of 0.28. Hence, the middle and/or lower crust under the Gulf of Maine is heterogeneous and represents average crust modified by mafic intrusions, probably during Mesozoic extension. Moho depth is indicated between 30 and 37 km depth. Wide-angle reflections coming from 28 km depth as indicated by two instruments are interpreted to come from the top of a lower crustal intrusion. This interpretation is supported by an observed mismatch between the models giving a thickness of 28 km and the reflection data. Although it represents a different geological terrane, the velocity and thickness of the crust beneath the central Gulf of Maine is very similar that onshore Maine. / Graduation date: 1991

The Whittier fault forms the central part of a fault system extending from the East Montebello fault at Whittier Narrows to the Elsinore fault, which is traced as far as the Mexican border. The Whittier fault forms a restraining bend in this fault system, resulting in uplift of the Puente Hills. The northwestern part of the Whittier fault in the Whittier oil field in the eastern Los Angeles basin strikes approximately N65��W and dips 70-75�� northeast. The fault is near the range front of the Puente Hills northwest of Turnbull Canyon, and within the Puente Hills to the southeast. The central reach of the Whittier fault had normal separation in the Relizian and Luisian stages of the middle Miocene. From the Mohnian through Repettian stages of the late Miocene and early Pliocene, little, if any, offset occurred until the initiation of reverse offset in the Venturian stage of the late Pliocene. A component of right-lateral strike-slip may have been added near the end of the Pliocene, coinciding with the formation of the Elsinore fault. The Workman Hill and Whittier Heights faults may have formed in the late Pliocene to early Pleistocene, coinciding with the possible initiation of strike-slip on the Whittier fault. The present sense of slip on the Whittier fault southeast of the study area is nearly pure right-lateral strike-slip, with a slip rate of 2-3 mm/yr. The northwestern part of the Whittier fault has a component of reverse slip of approximately 1 mm/yr. The amount of strike-slip on this part of the fault was not determined by this study. The Rideout Heights, 304, and 184 low-amplitude anticlines formed in the Whittier oil field area in the late Miocene and early Pliocene. The Rideout Heights anticline is a southwest-verging fault-propagation fold trending northwesterly from the mouth of Turnbull Canyon through the Rideout Heights area. Strata are overturned in the southwest limb of the fold, and normally dipping in the northeast limb; the fold has been cut along its hinge by the Whittier fault. The 304 and 184 anticlines are north-verging and appear to be beddingplane shear folds in the northeast limb of the La Habra syncline. Recent strike-slip on the Whittier fault may have reactivated the 184 anticline, causing uplift of the footwall block south of Turnbull Canyon. North of Turnbull Canyon, the Whittier fault is at the range front with no evidence of Quaternary footwall uplift. The 304 anticline could be a fault-propagation fold from a previously-unknown southwest dipping blind reverse fault south of the Whittier fault; uplift on this fold could also be the cause of footwall uplift south of Turnbull Canyon. Active fault traces, possibly strike-slip, are on or near the Whittier fault south of Turnbull Canyon, but to the north, recent offsets appear to be northeast of the Whittier fault in the Puente Hills. These offsets may represent an attempt of the Whittier fault to straighten itself by bypassing the restraining bend at Turnbull Canyon. so, this movement is too recent to offset conglomerate beds more than a few tens of meters. / Graduation date: 1998