Joel S. Watkins

Geology and tectonic evolution of a juvenile accretionary terrane along a truncated convergent margin: Synthesis of results from Leg 66 of the Deep Sea Drilling Project, southern Mexico

Drilling results from the Pacific margin of southern Mexico indicate that this region is characterized by a Neogene accretionary wedge, progressively emplaced against older, tectonically truncated continental crust. Accretion has occurred by both offscraping of sediments at the base of the trench slope and underplating of sediments at depth beneath the accretionary wedge and the continental crust. Mass-balance and incremental-uplift studies suggest that about one-third of the incoming sediment is subducted beneath the leading edge of the continental crust. Piston and drill cores indicate that the trench is sand dominated and flanked by slopes covered principally by mud. A large submarine canyon bypasses sediment past the shelf and inner trench slope. The volume of sediment bypassed to the trench and adjacent lower and outer slope equals 5 to 6 times that deposited on the shelf, upper slope, and mid-slope. The muddy inner slope is characterized by foraminiferan-free mud below the calcite compensation depth (CCD) and foraminiferan-bearing mud above the CCD. The upper slope accumulates laminated mud within the oxygen minimum zone. The shelf is covered by sand and mud. Quartzofeldspathic sand compositions in the Leg 66 area reflect sources in the crystalline basement complex exposed along the coast. Structural fabrics of Leg 66 cores from offscraped and overlying slope deposits show zones of inconsistent dip, stratal disruption, and scaly mudstone, characteristics of many melange-wedges exposed on land. Deformation transgresses the boundary between the offscraped and slope deposits, demonstrating tectonic incorporation of the slope sediments into the accretionary wedge. The rate of deformation of the slope deposits diminishes rapidly landward from the trench. Deposits overlying the continental crust show dip patterns due to mesoscopic folding, as well as local spaced cleavage and faulting, but they show no stratal disruption or scaly mudstone Oblique-slip faulting predominates between the accreted wedge and the continental crust and may reflect decoupling of these two basement types. Frozen sediment, probably bearing gas hydrate, was recovered above a bottom-simulating reflector at two sites.

The velocity structure of the lunar crust

Seismic refraction data, obtained at the Apollo 14 and 16 sites, when combined with other lunar seismic data, allow a compressional wave velocity profile of the lunar near-surface and crust to be derived. The regolith, although variable in thickness over the lunar surface, possesses surprisingly similar seismic properties. Underlying the regolith at both the Apollo 14 Fra Mauro site and the Apollo 16 Descartes site is low-velocity brecciated material or impact derived debris. Key features of the lunar seismic velocity profile are: (i) velocity increases from 100–300 m s−1 in the upper 100 m to ∼ 4 km s−1 at 5 km depth, (ii) a more gradual increase from ∼ 4 km s−1 to ∼ 6 km s−1 at 25 km depth, (iii) a discontinuity at a depth of 25 km and (iv) a constant value of ∼ 7 km s−1 at depths from 25 km to about 60 km. The exact details of the velocity variation in the upper 5 to 10 km of the Moon cannot yet be resolved but self-compression of rock powders cannot duplicate the observed magnitude of the velocity change and the steep velocity-depth gradient. Other textural or compositional changes must be important in the upper 5 km of the Moon. The only serious candidates for the lower lunar crust are anorthositic or gabbroic rocks.

Deep seismic reflection results from the Gulf of Mexico

Deep sounding seismic reflection data show undeformed reflectors at depths down to 11 kilometers beneath the continental rise and abyssal plain and 7 kilometers in basins of the lower slope. Weak reflectors are visible beneath the salt of the Sigsbee Scarp and within salt ridges separating the lower slope basins.

Seismic stratigraphy of the western Venezuela Basin

Abstract Multi-channel seismic reflection records from the central Venezuela Basin reveal planar reflections within the upper half of the oceanic crust as well as within the overlying sediment layer. The high resolution of the reflection data allows a division of the sediment section into three seismic intervals bounded by strong laterally persistent amplitude peaks which can be traced throughout much of the basin. The underlying crustal reflection zone is divided into two seismic intervals based on an angular relationship between the reflections which constitute each zone. Deep-sea drilling permits local determination of age and lithology of the seismic intervals of the sediment section and the top of the uppermost crustal interval. The lateral continuity of high-amplitude peaks that separate the sedimentary seismic intervals suggests that the boundaries are for the most part time-synchronous throughout the Venezuela Basin. Thinning and local pinch-outs of the deeper sedimentary seismic intervals in several parts of the basin contrast with the continuity in thickness of the shallower sedimentary seismic intervals. This suggests that the deeper intervals were affected more than shallower intervals by bottom currents which scoured and redistributed sediment from Cretaceous to Eocene time, but which became much weaker in post-Eocene or Early Oligocene time. Perhaps the weakening of bottom currents in Late Eocene or Early Oligocene was related to the development of the Lesser Antilles island arc and the isthmus of Panama in Eocene time. Planar seismic reflections within the high-velocity crust of the Venezuela Basin extend 1.5 sec below the base of the sediment section. These planar reflections extend tens of kilometers laterally. The upper interval of this crustal section consists of fairly level internal reflections compared to the lower interval with more steeply dipping reflections. The boundary between these two seismic intervals is not a distinct reflection, but rather a zone of apparent merging of the two intervals. The reflections of the lowest seismic interval become lost in the noise at a depth approximately equivalent to the top of the region in the lower Venezuela Basin crust with refraction velocities of 7.2 km/sec.

Apollo 14 Active Seismic Experiment

Explosion seismic refraction data indicate that the lunar near-surface rocks at the Apollo 14 site consist of a regolith 8.5 meters thick and characterized by a compressional wave velocity of 104 meters per second. The regolith is underlain by a layer with a compressional wave velocity of 299 meters per second. The thickness of this layer, which we interpret to be the Fra Mauro Formation, is between 16 and 76 meters. The layer immediately beneath this has a velocity greater than 370 meters per second. We found no evidence of permafrost.

Deep seismic reflection results from the gulf of Mexico: part I.

Deep sounding seismic reflection data show undeformed reflectors at depths down to 11 kilometers beneath the continental rise and abyssal plain and 7 kilometers in basins of the lower slope. Weak reflectors are visible beneath the salt of the Sigsbee Scarp and within salt ridges separating the lower slope basins.

Interpretation of multi-channel seismic reflection records from the Gulf of Mexico

Abstract New multi-channel seismic reflection data illuminate details of the structure, stratigraphy and geologic history beneath the abyssal plain of the Gulf of Mexico. These data show a thick sedimentary section lying on an irregular acoustic basement thought to be oceanic crust formed in early Mesozoic time. Six seismic units within the sedimentary section are defined on the basis of reflection characteristics and basin-wide continuity. One unit containing the salt (Jurassic?) that feeds the Campeche-Sigsbee Salt Dome Province can be traced northward toward the Sigsbee Escarpment but pinches out against the base of the Campeche Escarpment. The salt layer places limitations on the location or age of a plate boundary between North America and Africa-South America which has been suggested to have been active in the Gulf in Jurassic time. The four units lying above the salt reflect an extended period of pelagic sedimentation followed by mid-Tertiary-Pleistocene turbidite sedimentation.

Gravity Investigation of the Depth of Source of the Piedmont Gravity Gradient in Davidson County, North Carolina

The Piedmont gravity gradient is perhaps the most striking feature of the gravity field of the southeastern United States. It extends more or less linearly through the Piedmont, separating positive gravity values to the southeast from negative gravity values to the northeast. Analysis of data from a detailed gravity survey in central North Carolina indicates that the source of the anomaly is located within the upper crust. The best model that we were able to obtain consisted of a block 7.6 km thick located at the surface of the ground. The block has a density contrast of +0.213 gm-cm −3 relative to rock to the northwest. The block may be bounded on the northwest by a vertical interface. The higher density rocks causing the anomaly lie within the Carolina Slate Belt.

Fine-scale seismic stratigraphy in the western North Atlantic

Seismic reflection profiles from the lower continental rise off Cape Hatteras illustrate the importance of cycle-to-cycle correlation in seismic stratigraphy. The reflection data reveal that horizons β and A are unconformities with truncations of 120 m or less in the study area. These unconformities probably result from erosion, which suggests that bottom currents were active as early as middle Cretaceous time. The reflection geometries and amplitudes also show that the seaward termination of reflector X may be a facies transition from the seaward-prograding rise sediments to abyssal plain turbidites.

A Seismic Refraction System for Lunar Use

A miniaturized seismic refraction system has been constructed for possible use on early manned lunar landings. The detection system consists of three geophones, a three-channel amplifier, a geophone calibrator, and a logarithmic compressor system. A grenade launching device and an astronaut-held thumper staff are used as the sources of seismic energy. Seismic energy from the explosive sources is detected, amplified, logarithmically compressed, converted to digital form, and formatted for real-time transmission to earth. Total weight of the seismic detection system and ancillary electronics, exclusive of the explosive sources, is 6.25 lb.