Areas Of Crustal Thinning Research Articles

Deep crustal structure of the area of intersection between the Shackleton Fracture Zone and the West Scotia Ridge (Drake Passage, Antarctica)

The Shackleton Fracture Zone, which forms the boundary between the Antarctic and Scotia plates in the Drake Passage, is characterized by a present-day left-lateral motion. The West Scotia Ridge, an extinct spreading centre, formed the oceanic crust of the western Scotia Plate and intersects the Shackleton Fracture Zone in a complex deformed area. Multichannel seismic, gravity, magnetic and multibeam swath bathymetry data were acquired during the ANTPAC 97/98 cruise with the Spanish vessel B/O HESPERIDES in the area of intersection of these two tectonic features. The new data reveal its asymmetrical deep crustal structure, which developed as a consequence of the overprinting of extensional and contractional deformation events. The main seismic features of the crust of the Scotia and Antarctic plates are deep dipping braided reflectors, which may be a consequence of an initial stretching deformation related to the Shackleton Fracture Zone. At present, this fracture zone is characterized by thickened oceanic crust, deformed by reverse and transcurrent faults and locally bounded by areas of crustal thinning. The present morphology of the West Scotia Ridge has the characteristics of slow spreading centres, with a central valley bounded by two elongated highs. However, its structure reveals that after spreading ended, there was a NW–SE contractional deformation event, with a thrust of about 40 km of slip that resulted in asymmetrical crustal thickening. Incipient subduction initiates subparallel to the strike of the spreading centre. This tectonic event may be related to an episode of closure of South America and the Antarctic Peninsula, probably of Pliocene age. We conclude that in oceanic domains, areas with a weak crust (fracture zones and spreading centres) constitute the most sensitive regions for analysing the regional tectonic evolution since deformation events are better recorded there than in normal oceanic crust.

Seismic crustal structure of Galicia Continental Margin, NW Iberian Peninsula

The Galicia margin is one of the most intensively surveyed continental margins. However, little is known about its deep structure between 11° and 9° W. In this work, we present the interpretation of a W‐E 425 km long survey section, using near‐vertical seismic reflection, coincident refraction/wide angle seismic reflection and gravity data. Our results show that the crustal thickness varies from 30 km inland to 9 km near the ocean‐continent boundary, with two areas of crustal thinning. The main thinning of the crust is located below the Deep Galicia Margin. The crust under the Galicia Interior Basin is also thinned, reaching a total thickness of 19 km. The Galicia Bank corresponds to a thicker area, reaching 22 km.

Global petrologic variations on the Moon: A ternary‐diagram approach

A ternary‐diagram approach for determination of global petrologic variations on the lunar surface is presented that incorporates valuable improvements in our previous method of using geochemical variation diagrams. This new approach uses a ternary diagram that is subdivided into equally spaced segments along each of its three sides and has a triangular area in its center. The segments are assigned distinct colors that cover the range of the visible spectrum. The apexes are assigned the three primary colors, and the center triangle, which represents equal proportions of the three apexes, is assigned the color gray. The apexes are assigned the average Fe and (Th/Ti)c (ratio normalized to chondrites) compositions of KREEP/Mg‐suite rocks, mare basalts, and ferroan anorthosites. The composition of each picture element (pixel) within these Apollo orbital gamma ray databases is used to determine its position within the ternary diagram. The color corresponding to this ternary position is then placed on a classification map at that pixel's position within the orbital databases. Error databases produced for the Fe and (Th/Ti)c data were used to exclude pixels having high errors. The resultant classification map shows spatial transitions among petrologic units, allows direct determination of the relative proportions of each end‐member composition in a pixel, and increases the geologic interpretability of these data over that of the individual elemental databases. The classification units correspond remarkably well to observed geologic units, when we consider the spatial resolution (100 km) of the gamma ray spectrometer. Our results are as follows: (1) The highlands contain large areas of relatively pure ferroan anorthosite. (2) The average composition of the upper lunar crust is represented by an “anorthositic gabbro” composition, which supports the “magma‐ocean” hypothesis for lunar crustal genesis. (3) KREEP/Mg‐suite rocks are a minor fraction of the upper lunar crust. (4) Within the farside highlands, areas of KREEP/Mg‐suite rocks coincide mostly with areas of crustal thinning, which are probably areas of KREEP basalt extrusions or localized excavations of Mg‐suite rocks or KREEP‐rich rocks. (5) Portions of the east limb and farside highlands have considerable amounts of a mafic, chondritic Th/Ti component (like mare basalt) whose occurrences coincide with mapped concentrations of light plains that display dark‐halo craters; the presence of this component supports the hypothesis that mare volcanism had occurred within this highland region before the end of final heavy bombardment.

Development of the Porcupine Seabight: use of magnetic data to show the direct relationship between early oceanic and continental structures

The Porcupine Seabight is a broad, composite rectilinear embayment in the margin of the Irish continental shelf which seems to be floored at least in the southern part by oceanic crust. There is a geometrically straightforward relationship between areas of crustal thinning, rifting, and the position of probable 'oceanic crust'. It is proposed that an important crustal unit here termed the Porcupine Segment includes the Porcupine Bank and the western end of the Slyne Ridge. Interpretation of a new magnetic map indicates that opening in the Porcupine Seabight appears to have taken place along a set of transform faults which 'root' in an older dextral shear fault set oriented at about 130°. Three major zones along the longitudinal axis of the Seabight relate to the geometry of magnetic stripes in the subjacent basement and can be used, in conjunction with opening related to a distant pole of rotation, to explain the overall present physical configuration of the basin. The Porcupine Seabight probably opened no earlier than about 170 Ma, following the establishment of a Jurassic volcanic province in the vicinity of the southern Seabight elbow.

A Model for Flood Basalt Vulcanism

The question of whether basaltic rocks in continental flood basalt provinces are primary magmas or whether they are descended in general from picritic parent magmas is reviewed. It is suggested that the latter is more likely to be correct on the evidence of phase relations and the relative rareness of mantle materials with appropriate Fe/Mg ratios. Major element variations in the residual liquids of fractional and equilibrium crystallization of basaltic magmas are modelled for a variety of crystallizing assemblages. It is concluded that crystallization of olivine, clinopyroxene, and plagioclase has a marked effect on buffering chemical change in many important elements. It is this effect which accounts for the apparent uniformity of large volumes of flood basalts, not, as has sometimes been supposed, a series of implausible coincidences in the amount of material fractionated from each magma batch. It is further argued that much of the variation seen in basalts may be imposed by polybaric fractionation operating throughout crustal depths, that is at pressures up to at least 12 kb. Parental picritic magmas rising from the mantle reach the surface in exceptional areas of crustal thinning. More usually, however, it is suggested that they intrude the base of the crust as a series of sills which differentiate into upper gabbroic and lower ultramafic portions. Much of the 'low pressure' fractionation of basaltic magmas may take place in this deep crustal sill complex and evolved liquids are transmitted to the surface as their density becomes sufficiently low. This implies that in areas of flood vulcanism a potentially large new contribution to the crust is made by underplating, the volumes of concealed cumulates being at least as large as the amount of erupted surface lava.