Salt has unusual properties which complicate seismic exploration. A Model study examines some problems encountered in subsalt amplitude analysis and demonstrates that traditional techniques for detecting hydrocarbons and overpressured sediments using amplitude and velocity analysis are severely limited subsalt. Strong mode conversions generated at salt-sediment interfaces intrinsically limit amplitude versus offset techniques by diverting a significant amount of energy away from P-wave reflections. Some of the information lost from P-wave reflections is recouped by examining mode converted reflection amplitudes. Mode conversions are sensitive to different rock properties than P-waves, and those generated at the base of salt bodies are easily identified using acoustic and elastic modeling comparisons. Models demonstrate the use of base of salt converted mode amplitudes to gauge the strength of P-wave reflections for hydrocarbon discrimination. Further, mode converted amplitudes are used in conjunction with P-wave amplitudes from base of salt to detect overpressuring directly beneath salt.

<p> The lateral "corners" where Kamb and Whillans Ice Streams (KIS and WIS) discharge into the Ross Ice Shelf share common geometries and ice mechanical settings. At both corners of the now-stagnant KIS outlet, shear margins of apparently different ages confine regions with a relatively flat, smooth surface expression. These features are called the "Duckfoot" on the northern, right-lateral side and the "Goosefoot" on the other. It has been suggested, on evidence found in ice internal layers, that the flat ice terrains on KIS were afloat in the recent past, at a time when the ice stream grounding line was upstream of its present location. The overdeepening in the bed just upstream of the KIS grounding line supports this view of the past geometry. </p><p> The right-lateral margin at the outlet of the currently active WIS, the location of Subglacial Lake Englehardt (SLE), appears to have many similarities with the right lateral margin of KIS, though with a less developed looking inboard margin. This paper presents a mechanical analysis using surface and bed topography and velocity datasets comparing the Duckfoot flat ice terrain with the terrain around Subglacial Lake Englehardt. At both locations mechanical thinning along shear margins and lows in the bed topography redirects basal water routing towards the features. Here, I consider the history of these features and their role in ice stream variability by comparison of the relict and modern features and via numerical modeling of ice shelf grounding and ungrounding in response to variations in ice flow. </p><p> We propose two scenarios for the development of flat ice terrains/subglacial lakes at the outlets of ice streams. In the first, development of a lake in the hydraulic potential low along a shear margin forces a margin jump as shearing develops along the inboard shore of the margin lake. This thesis presents evidence for an inboard (relative to the main outboard shear margin) zone of shear along the inboard shoreline of SLE, suggesting that subglacial lakes along shear margins are capable of facilitating shear margin jumps. In the second, grounding line advance around a relative low in the bed, creating adjacent margins along the lakeshores, forms a remnant lake. Discerning which of these scenarios is appropriate at the KIS outlet has implications for understanding the history of the ice stream grounding line. </p><p> An ice flow model is used to place these local conditions in a regional context by studying the effect of internal perturbations, such as ice rise stagnation or inward margin jumps, on grounding line position. Bathymetry is important in determining ice stream flow in the ways that might not be otherwise realized in 1-D flow model studies. In the numerical modeling experiments, grounding line advance across the KIS outlet is mediated by the overdeepening in the bed and proceeds not in the direction of ice flow but transverse to flow. This finding adds complexity to both a flowline view of grounding line migration and the theory that grounding lines are unstable in the presence of inward sloping bed topography.</p>

<p> This study uses DC Electrical Resistivity Tomography (ERT) to delineate hydrostratigraphic units within the lower South Platte River alluvial aquifer. The geophysical investigation was conducted at Tamarack Ranch State Wildlife Area in northeastern Colorado, where the South Platte River is artificially recharged via pumping to surface recharge ponds and groundwater flow through the underlying unconfined alluvial aquifer system. Twenty-seven ERT profiles collected within a 4.2 km² study area on the south bank of the South Platte River define 3 different electrostratigraphic units. The ERT data was correlated with drilling logs and laboratory resistivity measurements to develop a hydrostratigraphic model and confining bedrock surface map. Results indicate 7-25 m thick eolian sand deposits (50-800 ohm-m) serve as infiltration zones and do not readily store groundwater. These eolian deposits form up to 15 m high sand hills in the southern half of the study area, and underlie recharge ponds that are used as water sources for artificial recharge of the river. The underlying alluvium (20-3890 ohm-m) varies from 10-70 m thick and functions as the primary groundwater storage unit. A 10-20 m thick intermittent conductive zone (25-80 ohm-m) occurs within the upper part of the alluvial layer that underlies the sand hills, and is interpreted to be caused by clay deposits that potentially influence initial groundwater flow paths emanating from the recharge ponds. The alluvium is underlain by highly conductive siltstone and claystone bedrock formations (1-60 ohm-m) that confine the aquifer system. The bedrock surface is complexly eroded (1055-1110 m.a.s.l.) and is characterized by prominent large-scale paleo-topographic lows (at typical scales of 700 m wide, 35-40 m deep and 700 m wide, 20-25 m deep) that occur on the northern bank of an incised paleo-channel. These features are interpreted to represent a paleo-topographic surface formed by groundwater outflow in the form of piping and sapping networks. The rugged bedrock topography establishes a previously unrecognized first order control on groundwater flowpaths within the unconfined alluvial aquifer.</p>

An integrated vertical incidence to wide-angle seismic data set has been used to develop a consistent migrated seismic reflection image and seismic velocity model of the Brooks Range fold and thrust belt in north central Alaska. Common midpoint (CMP) reflection data image the principal structures comprising the Brooks Range: the Endicott Mountains allochthon (EMA), the crustal scale Doonerak duplex, the master detachment, a 1.0-1.5 sec thick zone of lower crustal reflectivity just above the crust-mantle boundary, and a complex crustal root. The master detachment separates the crust into units which have been uplifted and deformed in the fold and thrust belt from those which have not. Least squares inversion of both reflected and refracted travel time data produced a velocity model consistent with CMP images of the Brooks Range, Bouguer gravity data, and seismic velocities determined from petrophysical data. Maximum crustal thickness in the range is 49 km, in an asymmetric root located under the EMA. At the root, an offset in lower crustal reflectivity is observed along with two deep zones of reflections north of the root. These reflections are interpreted as a Moho offset of some 5 km, resulting from subduction of the Brooks Range lower crust northward beneath the North Slope. The seismic reflection data, velocity data, and surface geologic constraints are used to identify the boundaries of major structural assemblages in the Brooks Range and restore 3 interpretations of the range to their pre-Jurassic configurations. Minimum shortening estimates derived above the basal decollement for the 3 models approximate 500-600 km of Mesozoic-Recent shortening. The amount of sub-decollement shortening may be as little as that now observed, 50-65 km, or may be comparable to the 500-600 km observed for upper-intermediate crustal rocks. Proximity of the continental subduction zone to the crustal scale Doonerak duplex suggests that the development of the fold and thrust belt has been at least partially controlled by the lower crust/mantle subduction.

Geochemical and petrologic tools were deployed to investigate the redox state and water content of the earth's upper mantle. Study results are discussed in the context of their linkages to the atmospheric oxygen level, hydrospheric water budget and lithospheric evolution of continents. Because the partitioning of V is redox-sensitive and otherwise similar to that of Sc which is not redox sensitive, the V/Sc ratios of basalts of different ages act as a natural recorder of the redox states of the upper mantle. Through a comparison between global mid-ocean ridge basalts and Archean basalts, the fO2 of the upper mantle was inferred to have changed by no more than 0.3 log units since Archean. Combined with results from a thermodynamic model simulating the redox reactions of volcanic gases, this observation argues against the idea that the increase in oxygen in the atmosphere &sim;2.3 billion years ago was caused by redox transition in the upper mantle. Through a geochemical and petrologic study at the Feather River Ophiolite (in northern California), global water recycling rates at subduction zones were estimated based on reconstructed serpentinization depths for the oceanic lithospheric mantle. Within uncertainties, the estimated water recycling rates roughly match global volcanic dewatering rates, which suggest the hydrospheric water storage is current at steady-state. Based on water contents measured in mantle xenoliths from the Colorado Plateau and vicinity, the idea that the lithospheric mantle beneath the western North America was rehydrated by the dewatering of the flat-subducting Farallon slab is confirmed. As predicted by an updated flow law for olivine aggregates, hydration might have weakened the basal lithosphere beneath the Colorado Plateau and thus induced lithospheric thinning by &sim;15 km as a result of basal erosion. Extrapolation of the flow law to thick, cratonic lithosphere further suggests lithospheric thinning of much larger extents can occur if enough water was introduced during hydration. If so, subduction-induced hydration might have played an important role in regulating the lithospheric evolution of continents.

Improved 3-D models of P-wave velocity and density are presented for better understanding of volcano-tectonic processes around the Island of Hawaii. The summit and upper rift zones of Kilauea are underlain by high-velocity and positive-density anomalies, indicative of magma intrusives dominated by dikes and melt-rich olivine cumulates. Seismicity is clustered at the seaward edge of this body, indicating that the cumulate body pushes the flank outward above a frictional decollement. The intrusive rocks along Kilauea's and Mauna Loa's rift zones are not continuous along their lengths, suggesting that eruptions along the lower rift zones could be fed vertically from the mantle, rather than downrift from the summit reservoirs. Mauna Loa's southeast flank is underlain by an anomalously large volume of intrusive materials that lacks the distinctive positive density anomaly observed above active rift zones. Therefore, this cumulate body is probably now cold and solidified, representing an ancient rift zone. Similar to Kilauea, earthquakes are concentrated along the boundary of this body, but here accommodate seaward motion of the adjacent flank rather than the cumulate body. Mauna Loa also appears to have a buried northwest rift zone, overlying the older flanks of Hualalai and Mauna Kea. Both Hualalai and Mauna Kea show south trending high-velocity and density features, also indicative of buried rift zones. High- and low-velocity anomalies beneath Loihi seamount are interpreted to indicate the presence of intrusive cumulates within the volcanic edifice and oceanic crust, and partial melt within the upper mantle, respectively. Low velocities beneath the Hilina and Kao'iki fault zones are attributed to thick piles of volcaniclastic sediments deposited on the submarine flanks. In contrast, the submarine outer bench of Kilauea is marked by anomalously high-velocity materials, possibly evidence for a buried seamount that may impede outward spreading of the flank today.

The Caribbean-South America diffuse plate boundary is characterized by tectonic transpression with oblique convergence. In northwestern Venezuela, the underthrusting of the Caribbean Plate beneath northwestern South America, and the tectonic escape of the Maracaibo block complicate the boundary. The BOLIVAR project acquired onshore-offshore refraction and marine reflection data along the 450 km profile 70W, which extends from the Venezuela Basin, on the Caribbean plate, to the Maracaibo block, in the diffuse boundary zone. A 2-D velocity model was generated from wide-angle data, and it shows good correlation with the reflection data analyzed. We present evidence consistent with the underthrusting of the 15 km Caribbean plateau; however, we propose that the landward extent and depth of the oceanic crust are substantially less than previously suggested. The model also indicates the presence of an 8 km crustal thinning located to the north of the Oca-Ancon Fault and the inverted Falcon Basin.

The Mesozoic-Cenozoic tectonic evolution of western Venezuela occurred in five major stages; (1) Jurassic extension was superposed on peneplaned Late Paleozoic folded-belts. NNE-trending half-grabens developed in a back-arc setting and were filled with continental red-beds and volcanics. (2) A thin, southwardly onlapping, Cretaceous passive margin sequence was related to the opening of the Atlantic. Carbonates dominate the lower portion of the section but the upper Cretaceous is mainly clastic. (3) A Late Cretaceous ? - Tertiary foredeep evolved from an A-Type subduction which advanced from the west and from the north across northern Venezuela, and was the result of interactions between the subducted Pacific-Caribbean Plates and the overriding South American Plate. The peak of deformation occurred at the end of the Middle Eocene, at which time the Sierra de Perija, its southwestern extension and the Internal Caribbean folded-belts, emerged as positive structures. During this compressional event, the Jurassic half-grabens were partially inverted and doubly-vergent basement-involved thrusts branched from a major decollement underlying the Andean active margin (Santa Marta block). (4) Eastward progression of the Sierra de Perija persisted during the Oligocene - Middle Miocene, at which time the Santander Massif was uplifted. North of the Oca Fault and farther east, a series of half-grabens were formed from a right-stepping relay system of right-lateral strike-slip faults superimposed on the Internal Caribbean Folded-belt (Falcon area). (5) The NW-vergent Venezuelan Andes emerged as the main structure during the Late Miocene - Pliocene and were probably associated with a deeper decollement than the one related to the Perijas. The Andes succeeded the Perijas as the main sediment source and separated the foredeep into distinct sub-basins; The Maracaibo Basin to the north and the Barinas-Apure Basin to the south. Coeval with the Andean deformation are two inversion phases affecting the transtensional Falcon structures that are responsible for the present configuration of the Falcon Anticlinorium.

Real Earth media is anelastic, which affects both kinematics and dynamics of propagating waves. Waves are attenuated and dispersed. If anelastic effects are neglected, inversion and migration can yield erroneous results. The anelastic effects, on propagating waves, in real rocks can be well described by a viscoelastic model. Hence, viscoelastic wave propagation simulation is a well suited base for a realistic inversion algorithm derived through the adjoint state technique. We have developed a finite-difference simulator to model wave propagation in viscoelastic media. The viscoelastic scheme is only slightly more expensive than analogous elastic schemes. This thesis also presents a method for modeling of constant Q as a function of frequency based on an explicit closed formula for calculation of the parameter fields. The $\tau$-p (intercept time-slowness) domain permits economical modeling and inversion of 3-D wave propagation in layered media.

Over 1,000 km of high resolution seismic profiles and nearly 200 piston cores, vibracores, and geotechnical boring descriptions were interpreted to document the evolution of the Trinity/Sabine incised valley system. Long-term sea level fluctuations (20 ky duration and 50 m amplitude) produced coeval coastal plain and fluvial terrace sequences, as sea level fell from the $\partial\sp{18}$O stage 5e highstand $\approx$120 kyBP to the stage 2 lowstand $\approx$20 kyBP. The geometry of the offlapping coastal sequences matches well the shape of $\partial\sp{18}$O curves. The stage 5e condensed section/downlap surface is regionally correlatable, and ties to $\partial\sp{18}$O analysis of wells at the Louisiana shelf margin. The Trinity/Sabine incised valley began forming $\approx$110 kyBP, and was continually reoccupied as sea level fell, leaving a complex architecture of incised fluvial terraces along the valley margin. Short-term sea level changes (less than a thousand years duration and 5 to 10 m in amplitude) influenced valley-fill deposition during the stage 1 transgression. Parasequences comprised of linked upper bay facies and tidal facies aggrade to base level during periods of sea level stillstand. Rapid sea level rises are manifested as flooding surfaces which bound these parasequences. Parasequences within the Trinity/Sabine valley are observed at $-$36 m (PS I), $-$29 m (PS II), $-$20 m (PS III), and $-$14 m (PS IV). Tentative ages for the parasequences are PS I $>$9,200 yBP, PS II 9,200 to 8,600 yBP, PS III 8,600 to 6,800 yBP, and PS IV 6,800 to 2,800 yBP. The rate of sea level rise affected valley-fill facies architecture by controlling accommodation. Rapid, large sea level rises enhance deposition and preservation of lower bay and tidal facies. Steps in the bayline surface may reflect periods of bayhead delta progradation. The large shelf sand banks (e.g., Sabine and Heald Banks) formed during stillstands, but were extensively reworked and then isolated during subsequent sea level rise.