Bruce P. Luyendyk

Experimental Approach to the Paleocirculation of the Oceanic Surface Waters

Experimental investigations of the pattern of Middle Cretaceous oceanic surface currents in the Northern Hemisphere have been made, using planetary vorticity models developed by Von Arx. The main features of the ocean configuration are an open Tethys seaway and Central American region, an Atlantic Ocean approximately 50 percent of its present size, a Pacific Ocean about 25 percent larger than it is at present, and the ancient Tethys Ocean. Two zonal wind profiles have been considered in the experiments, a “glacial” profile simulating the zonal profile in existence today and a “nonglacial” profile with the westerlies belt about 10° farther north than the glacial profile. Paleoclimatic data support the existence of the non-glacial situation in the Middle Cretaceous, and these results are considered most relevant. The features of this experiment which differ significantly from the present-day oceanic surface circulation are the circum-global flow of the Tethys current, the flow of the Gulf Stream into the Labrador Sea, and cross-polar flow of water from the Atlantic into the North Pacific. In another experiment, the Tethys region was closed off near Malaysia in order to simulate the obstruction of the Tethys current. The Central American region was left open. Weak and variable currents are seen in the Tethys (or ancient Mediterranean), and the circum-global current is absent. North Atlantic circulation occurs from Greenland down to the equator. Atlantic and Pacific waters exchange through the Isthmus of Panama. A net inflow of Pacific water through this gap into the Atlantic maintains the cross-polar flow of “warmer” waters from the Atlantic to the Pacific. Geologic evidence of the first land connections in the Isthmus region can be correlated with the formation of the Labrador Current (3.4 m.y. ago and 3 m.y. ago, respectively). It is proposed that cessation of net inflow of Pacific water into the Atlantic caused cross-polar flow to stop. This, coupled with a deteriorating mean global temperature, was sufficient to trigger the formation of ice in the Arctic region.

Rifting History of the Woodlark Basin in the Southwest Pacific

Marine geophysical evidence has been obtained for the rate and history of sea-floor spreading in the Woodlark Basin. The eastern part of this basin is presently separating from the Australian Plate at over 4 cm per yr in a northerly direction. The western part of the basin is not presently spreading. This spreading rift marks the southern boundary of the Solomons Plate which is bounded by subduction zones in the north and east (the New Britain and northern Solomons Trenches, respectively) and in the west by a combination strike-slip rifting (dip-slip) boundary in eastern Papua (New Guinea). A vector triangle solution near the Solomons Trench-Woodlark Rift triple point gives underthrusting of the Solomons Plate beneath the northern Solomon Trench in a northeasterly direction at about 11 cm per yr. The Woodlark Basin began opening as a sphenochasm, with a pole near the tip of eastern Papua about 20 m.y. B.P. This was caused by left-lateral shear in the region induced by a change in the relative motion pole of the Australia and Pacific Plates. The basin opened only a few degrees at this time, then stopped. Rifting in the entire basin resumed about 3 m.y. B.P., based on magnetic anomaly data. About 1 m.y. B. P., the spreading center in the western basin shifted to the Woodlark Rise.

Dips of Downgoing Lithospheric Plates Beneath Island Arcs

Dips of downgoing lithospheric plates were measured by inspecting published vertical sections of relocated hypocenters beneath Pacific and Indian island arcs. For any one arc the maximum dip of the slab in a particular section, as determined by the hypocenter configuration, is inversely proportional to the distance from the section to the pole of relative motion of the two plates involved. Considering several oceanic arcs, a common inverse relationship exists between the dip and the relative rate of convergence of the plates at the location of the measurement. This dip-rate relationship may be a manifestation of the heavier downgoing plate that is sinking to an equilibrium (vertical) position.

Shallow Structure of the New Hebrides Island Arc

Marine geological and geophysical studies of the New Hebrides island arc have been made to study (a) the present development of lithospheric plate boundaries, (b) evidence for creation of oceanic crust behind the frontal arc in interarc basins, and (c) evidence for reversal of the arc from east-facing to the present-day west-facing orientation. The arc system is bisected between 13° and 15° S. by the east-west Hazel Holme Fracture Zone which connects the trench and a north-south—trending spreading center (Nova Rise) on the Fiji Plateau near 174° E. The crust on the plateau south of the fracture zone is very young. Narrow interarc basins are present but youthful, south of about 18° S. North of the Hazel Holme Fracture Zone, interarc basins are less well developed and apparently even younger. Most of the Fiji Plateau has apparently been formed by spreading from the Nova Rise rather than within interarc basins associated with the New Hebrides. The tectonics of the central region of the arc system, immediately south of 15° S., appears to be governed by the transform section of the Hazel Holme Fracture Zone and by subduction of the D9Entrecasteaux Fracture Zone into the trench rather than by interarc spreading. In this region, the western and eastern chains of the New Hebrides group have been recently uplifted and tilted toward one another, creating a sedimentary basin. Most data do not support the idea that the eastern island belt in this region, including Maewo and Pentecost islands, is an ancient remnant of an east-facing arc system. These islands have been uplifted only very recently and later than the western islands. Therefore, any east-facing subduction phase must have ceased recently and occurred after subduction beneath the western islands. We suggest instead that the eastern island belt represents an interarc basin floor or a frontal arc uplifted behind the volcanic line.

Central North Atlantic Plate Motions over the Last 40 Million Years

The relative motion vector for the North American and African plates has been determined from detailed charting of the trend of the Atlantis fracture zone for over 1000 kilometers in the central North Atlantic near 30�N and from identification of marine magnetic anomalies and deep-sea drilling results. The vector (pole) is located at 52.5�N, 34�W and has a magnitude (opening rate) of 5.7 x 10-7 degree per year. Major changes in either the pole location or the opening rate are not evident for the last 40 million years.

Geophysical study of the northwest African margin off Morocco

Abstract Six geophysical profiles were taken over the northwestern African margin between the Canary Islands and Morocco. A magnetic smooth zone occupies the upper continental rise landward of lineated sea-floor spreading anomalies of Late Jurassic to Early Cretaceous age. This smooth zone is believed due in part to a Late Jurassic uniform polarity interval; its seaward boundary is correlated with a basement high and/or abrupt landward thickening of sediments. Spreading may have started over 200 × 10 6 yr ago at a rate of 0·55 cm/yr and accelerated 150 to 160 × 10 6 yr at the smooth zone boundary. No counterpart of the east coast magnetic slope anomaly was observed off Morocco while the distance between the shelf break and the smooth zone boundary is also less here than off northeastern America. This difference suggests a shift in the position of the accreting plate boundary from west to east, about 200 × 10 6 yr ago, after the formation of the slope anomaly. Gravity data show that the Moroccan margin is largely in isostatic equilibrium, and that the zone of transition from oceanic to continental crustal thickness is wider off the Canaries (400 km) than off Morocco (100–150 km). The sediment section of the seismic reflection profiles includes an upper acoustically transparent unit (usually restricted to the upper rise) a middle stratified unit, and a lower transparent unit. The upper unit is probably pelagic calcareous ooze. Other lithologies are unknown but the stratified unit may in part include an Early Tertiary unconformity. Sections of the shelf and slope exhibit deformation that have probably been caused by the Alpine orogeny.