Showing papers on "Mohorovičić discontinuity published in 1988"

A geophysical study of the Earth's crust in central Virginia: Implications for Appalachian crustal structure

Abstract: A regional seismic reflection line (I-64) across the Virginia Piedmont has provided a stacked section suitable for an integrated interpretation of geophysical data in the region. A highly reflective upper crust, an allochthonous Blue Ridge Province, underlying thrust sheets including the Blue Ridge master decollement, and a basal decollement at a depth of about 9 km (3 s) are confirmed on the seismic data. Immediately east of the Blue Ridge Province, Appalachian structures plunge to as much as 12 km (4 s) depth. The Evington Group, Hardware terrane, and Chopawamsic metavolcanic rocks (Carolina terrane) crop out in the Piedmont Province, and numerous eastward dipping reflections originate from these rocks in the subsurface. These eastward dipping reflectors overlie a gently west dipping (10°–15°), highly reflective zone that varies in depth from 1.5 s (4.5 km) beneath the Goochland terrane to 4 s (12 km) beneath the rocks of the Evington Group. Some of the overlying eastward dipping reflections apparently root in this zone. The zone may include decollement surfaces along which the overlying rocks were transported. Relatively few reflections originate from within autochthonous Grenville basement at the western end of the profile. The Goochland granulite terrane is interpreted to be a westward thrust nappe structure that has overridden a portion of the Chopawamsic metavolcanic rocks. A broad zone of east dipping (20°–45°) reflections bounds the Goochland terrane on the east. These reflections may originate from deformation zones and continue to Moho depths. They appear to be correlative with similar events seen on other Appalachian lines. The pervasiveness of the zone of east dipping events on other seismic reflection lines and the continuity of the adjacent Piedmont gravity high suggest continuity of crustal features along the length of the Appalachians. A major conclusion of this study is that crustal thinning is responsible for the main components of the gravity field in Virginia, that is, the Appalachian gravity gradient and the Piedmont gravity high. The crust thins from about 52 km beneath the Appalachian mountains to about 35 km beneath Richmond, Virginia, and then rethickens by up to 10 km beneath the zone of east dipping reflections (mylonites?) east of Richmond. The I-64 seismic data also contain a sequence of reflections at about 9–12 s, indicative of lower crustal layering; the base of this zone of reflections coincides almost exactly with the Mohorovicic discontinuity interpreted from earlier refraction work. The layering extends about 70 km west from Richmond, Virginia, and is interpreted as a lower crustal transition zone that is believed to persist across most of Virginia.

Quasi-continental oceanic structure beneath the Arabian Fan sediments from observed surface-wave dispersion studies

Abstract: The fundamental and higher modes of surface waves generated by 31 earthquakes and recorded at seismographic stations along the western margins of India and Pakistan (Trivandrum, Kodaikanal, Goa, Bombay, Poona, New Delhi, Nillore, and Quetta) are used to estimate the crustal structure beneath the Arabian Fan sediments. The sedimentary thickness is determined from the observed higher mode data. The observed dispersion data suggest an increase in crustal thickness northward, from an approximately 16 km crustal thickness at the southern tip of India (Trivandrum) to an approximately 28 km crustal thickness at the regions of 20°N and above latitude, with an overlying 6 km sedimentary thickness. This gradual increase in crustal thickness in the northward direction and the attaining of quasi-continental oceanic (transition from continent to ocean) structure beneath the Arabian Fan sediments suggest that the Mohorovicic discontinuity may have resulted from a change in crystal structure due to increase of pressure and not a phase change. The same material exists beneath the Moho, and it does not represent the boundary between two different materials. The transition has given rise to crustal thickening in the northward direction. Another possible explanation is that the increase in hydrostatic pressure due to the load exerted by a large sedimentary column together with horizontal pressure caused by the collision of Indian and Eurasian plates has given rise to an increase in temperature near the Moho. Because of the thermal blanketing effect of this large sedimentary column, an inferred rise in temperature may have either changed the upper mantle into material with crustal-like velocity or may have given rise to metamorphism of earlier existing sedimentary rocks. An inferred high temperature near the Moho depth beneath the Arabian Fan sediments is in close agreement with the high attenuating zone at the shallow depth (30 to 45 km) as determined by Singh (1987).

Deep crustal reflections in Australia 1957-1973 — II. Crustal models

Abstract: SUMMARY In a companion paper, we described experiments conducted by the Australian Bureau of Mineral Resources (BMR) between 1957 and 1973 to record deep crustal seismic reflections. At 24 of the 46 sites investigated, events were recorded at times up to 10-13 s. At some sites, only one or two shots were recorded; where more comprehensive investigations were made, the events were consistent with deep reflections, though they lacked continuity in many cases. The results were sufficiently encouraging that we have now analysed the events on the hypothesis that they represented deep crustal reflectors, including the Moho. In 1976, BMR introduced digital recording, and successfully carried out several long deep crustal profiles since then. This paper analyses only the recordings made with analogue equipment prior to 1973. In a few places, the reflection structure could be compared with nearby refraction traverse results; some events correlated well with first or second-order discontinuities in the refraction velocity-depth curves, while for others there was no correspondence. It is difficult to generalize as to which types of refractors give rise to reflections. The refraction surveys show that the Moho is not always a boundary with a sharp change in velocity across it. In some areas in Queensland the deepest reflector correlates well with the refracting Moho. For reflection shots not near refraction surveys, we converted travel times to depths using simplified velocity models based on refraction surveys in similar geological environments, and on isostatic balance considerations using gravity and elevation data. In central Australia, if the deepest reflector is the Moho, a relatively thin and light crust is implied. However comparison with refraction models makes it seem probable that a heavier, thicker crust could exist, with the Moho being deeper than the deepest reflector. Also in the Tasman Geosyncline, it is probable that the Moho is deeper than the deepest reflector.