Cenozoic Tectonic History of the Sierra de Perija, Venezuela- Colombia, and Adjacent Basins

The four major Cenozoic tectonic phases in the Sierra de Perija and adjacent basins are the early Eocene tectonic phase, the middle Eocene Caribbean orogeny, the late Oligocene phase, and the late Miocene to present Andean orogeny. Ages of unconformities associated with particularly rapid regional uplift during these phases are early Eocene (53 m.y.), middle Eocene (45 m.y.), late Oligocene (25 m.y.), and Pliocene (3 m.y.). Northwest-southeast compression may have commenced in the Perija and the Maracaibo Basin as early as the early Eocene. By the early Eocene the Macoa-Totumo arch had begun to to form during intense alpine-type folding and thrusting to the east in Falcon and Lara. During the late Oligocene phase, the Palmar area was uplifted and the most important structural features for hydrocarbon accumulation in the Maracaibo Basin developed. The late Oligocene phase initiated a basement block tectonic style that culminated during the Pliocene in the northwest thrusting of the Santa Marta massif, Sierra de Perija, and Venezuelan Andes over the adjacent basins. The main uplift of the Sierra de Perija occurred during the late Miocene-Pliocene Andean orogeny. Right-lateral oblique-slip movement of 90-100 km on the Oca fault and the left-lateral oblique-slip movement of 100 km on the Santa Marta fault were caused by late Tertiary overthrusting in the Sierra de Perija and Santa Marta massif. The northwest-southeast shortening that uplifted the Santa Marta massif, Sierra de Perija, and Venezuelan Andes us related to Caribbean-North Andean convergence along the South Caribbean marginal fault. During the Pliocene the Panama volcanic arc collided with South America. The North Andean block became detached from the South American plate and is being wedged slowly to the north between the rapidly converging Nazca, Caribbean, and South American plates. The convergence of the three plates has produced rapid subduction at the Colombia trench (6.4±0.7cm/yr; 088°±7°), slow subduction at the South Caribbean marginal fault (1.7±0.7cm/yr; 128°±24°), and the right-lateral shear (1.0±0.2cm/yr; 235°±5°) on the Bonoco and East Andean fault systems

Bilayer Quantum Hall Systems at nuT = 1: Coulomb Drag and the Transition from Weak to Strong Interlayer Coupling

Measurements revealing anomalously large frictional drag at the transition between the weakly and strongly coupled regimes of a bilayer two-dimensional electron system at total Landau level filling factor nuT = 1 are reported. This result suggests the existence of fluctuations, either static or dynamic, near the phase boundary separating the quantized Hall state at small layer separations from the compressible state at larger separations. Interestingly, the anomalies in drag seem to persist to larger layer separations than does interlayer phase coherence as detected in tunneling

Double layer two-dimensional electron systems: Probing the transition from weak to strong coupling with Coulomb drag

Frictional drag measurements revealing anomalously large dissipation at the transition between the weakly- and strongly-coupled regimes of a bilayer two-dimensional electron system at total Landau level filling factor $\nu_T =1$ are reported. This result suggests the existence of fluctuations, either static or dynamic, near the phase boundary separating the quantized Hall state at small layer separations from the compressible state at larger separations. Interestingly, the anomalies in drag seem to persist to larger layer separations than does interlayer phase coherence as detected in tunneling.Comment: 4 pages, 4 figure

Tectonic Development of Panama, Costa Rica, and the Colombian Andes: Constraints from Global Positioning System Geodetic Studies and Gravity

Global Positioning System (GPS) measurements suggest the existence of a rigid Panama- Costa Rica microplate that is moving northward relative to the stable Caribbean Plate. Northward motion of South America relative to the Caribbean plate is independently suggested by the April 1991 Costa Rica earthquake, active folding in the North Panama deformed belt, and a south-dipping Wadati-Benioff zone beneath Panama. Panama may also be continuing to collide eastward with the northern Andes. Rapid subduction is occurring at the Middle America (72mm/yr), Ecuador (70mm/yr), and Colombia (50mm/yr) trenches. The northern Andes are moving northeastward relative to stable South America. Preliminary GPS results also suggest Caribbean-North Andean convergence and an independent North Nazca plate. About 6 Ma the Panama-Choco island arc collided with the northwestern margin of South America, eventually forming a land bridge between the Americas; closed the Pacific-Caribbean seaway, changing ocean circulation patterns and perhaps the world\u27s climate; folded the East Panama deformed belt; and uplifted the Eastern Cordillera of Colombia. An interpretation of the paleo-Romeral suture in southern Colombia as a low-angle fault dipping to the west into the lower crust under the Cordillera Occidental is compatable with seismic velocity and gravity data. During the Late Cretaceous the Western Cordillera oceanic terrain was obducted eastward on the fault system over continental crust

A 3-D Gravity Tectonic Study of Ita Mai Tai Guyot: An Uncompensated Seamount in the East Mariana Basin

Ita Mai Tai is a large, locally uncompensated seamount on the eastern edge of the East Mariana Basin. A large positive gravity anomaly of 254 mgal characterizes the summit and a low of -69 mgal, the surrounding moat. Using polygonal prisms to approximate the bathymetry, the observed gravity was inverted to calculate an average density of 2.59 g/ems for the seamount. Observed-calculated gravity residuals are reduced by including the flanking sedimentary basins and a dense volcanic conduit. The drill sites from DSDP Legs 20 and 89 describe a volcanic edifice formed in the Aptian!Albian on Jmassic/Cretaceous crust. The volcanism is recorded in volcanoclastic and epiclastic deposits in the basins nearby. The guyot was covered initially by a succession of reefal and lagoonal sediments followed by a thick mantling of pelagic sediments after it subsided. Gravity models that adequately match the calculated and observed data sets for Ita Mai Tai show little crustal thickening, suggesting that Ita Mai Tai is almost completely locally uncompensated

The Extended Tracking Network and Indications of Baseline Precision and Accuracy in the North Andes

The CASA UNO Global Positioning System (GPS) experiment (January-February 1988) included an extended tracking network which covered three continents in addition to the network of scientific interest in Central and South America. The repeatability of long baselines (400-1000 km) in South America is improved by up to a factor of two in the horizontal vector baseline components by using tracking stations in the Pacific and Europe to supplement stations in North America. In every case but one, the differences between the mean solutions obtained using different tracking networks was equal to or smaller than day-to-day rms repeatabilities for the same baselines. The mean solutions obtained by using tracking stations in North America and the Pacific agreed at the 2-3 millimeter level with those using tracking stations in North America and Europe. The agreement of the extended tracking network solutions suggests that a broad distribution of tracking stations provides better geometric constraints on the satellite orbits and that solutions are not sensitive to changes in tracking network configuration when an extended network is used. A comparison of the results from the North Andes and a baseline in North America suggests that the use of a geometrically strong extended tracking network is most importanwt hen the network of interest is far from North America

Seismic evidence for blind thrusting of the northwestern flank of the Venezuelan Andes

Surface geology and seismic and well data from the northwestern flank of the Venezuelan Andes indicate overthrusting of Andean basement rocks toward the adjacent Maracaibo Basin along a blind thrust fault. The frontal monocline is interpreted as the forelimb of a northwestward verging fault-related fold deformed over a crustal-scarle map. The Andean block has been thrust 20 km to the northwest and uplifted 10 km on a ramp that dips about 20o-30o southeastward. The thrust fault ramps up through crystalline basement rocks to a decollement horizon within the shaly units of the Cretaceous Colon-Mito Juan formations. Backthrusts in the monocline produce a wedge geometry and reduce the amount of blind slip required on the decollement northwest of the Andes. The rigid Andean uplift was caused by northwest-southeast compressive tectonic forces related to the convergence of the Caribbean plate, the Panama volcanic arc, and northwestern South America. The thick (up to 6 km) molasse deposits accumulated in the foredeep basin indicate that the Venezuelan Andes started to rise as early as the early Miocene. However, a late Miocene intramolasse unconformity marks the beginning of the formation of the monocline and the greatest uplift. The crustal-scale fault-related fold model may explain structural features seen in other areas of basement-involved foreland deformation

A compact high-flux cold atom beam source

We report on an efficient and compact high-flux Cs atom beam source based on a retro-reflected two-dimensional magneto-optical trap (2D MOT). We realize an effective pushing field component by tilting the 2D MOT collimators towards a separate three-dimensional magneto-optical trap (3D MOT) in ultra-high vacuum. This technique significantly improved 3D MOT loading rates to greater than $8 \times 10^9$ atoms/s using only 20 mW of total laser power for the source. When operating below saturation, we achieve a maximum efficiency of $6.2 \times 10^{11}$ atoms/s/W