ALBERT

Notes: We present new images of the lower crust and Moho beneath the Valencia Trough—a young rift basin in the western Mediterranean. These images were obtained from a two-ship, wide-aperture reflection experiment and show several features not distinguishable on previously available conventional single-ship reflection profiles.The Moho, which was previously only seen intermittently, can now be confidently traced throughout the basin. We have constructed a present-day depth-to-Moho map and estimated the degree of crustal thinning for the whole basin. Crustal thinning is at a maximum in the centre of the basin, where β values reach 3.15 ± 0.25. At the margins of the basin the β value decreases to 1.5 ± 0.1.The reflective character of the lower crust and Moho is different beneath different parts of the basin. We have been able to correlate these differences with the amount of stretching. We therefore interpret the variations of the observed lower crustal reflectivity as having been caused by the most recent (Neogene) stretching event that opened the Valencia Trough. Along the Iberian margin there is well-developed lower crustal reflectivity consisting of 1-2s two-way time (TWT) of 1-4 km long, near-horizontal reflectors underlain by a more continuous, although not significantly stronger, reflector interpreted to be the reflection Moho. Offshore, this lower crustal reflective unit thins rapidly, such that it is undetectable 40-50 km from the coastline where the crust has been stretched by a factor of 1.7 ± 0.1. As the lower crustal reflectivity becomes undetectable the reflection Moho becomes a robust, continuous event. Where β exceeds 2.4 ± 0.2, however, the Moho is a weak event and difficult to trace. We infer that either the extension itself or associated melting significantly weakened or even destroyed the lower crustal reflectivity in the centre of the basin and enhanced the Moho where extension was moderate.The Balearic margin is somewhat anomalous in that there appears to have been flexural loading of the crust due to thrusting and folding that occurred at the same time as extension in the Valencia Trough. The lower crust shows evidence of weak, but locally variable lower crustal reflectivity. It is possible that the lower crustal reflectivity was preserved simply because the Moho was flexed downward and so decompression, and hence melting, of the upper mantle was restricted. This suggests that the melting itself rather than the extension is the primary mechanism of lower crustal modification.

Notes: Diapiric structures showing striking spatial periodicity have been widely observed at shallow levels on single and multichannel seismic reflection profiles within Indus fan sediments. The study area is some 200 to 400 km from the mouth of the Indus at the fan's northern margin in the region of the Murray Ridge. Neighbouring diapirs are generally associated with the same, acoustically transparent layer. They form at depths less than 500 in below the seafloor and have the form of gentle waves on the upper surface of this ‘parent’ layer. The acoustic appearance of the features may be controlled by the presence of methane, which has been widely reported in the area.The diapirs are formed by gravitational instability of overpressured muds within the rapidly deposited sedimentary sequence. Simple 2-D, two-layer gravity instability modelling of diapir wavelengths and growth rates has determined values of approximately 104 to 106 for the ratio of equivalent viscosity of the overburden sediments to that of the diapiric layer; and a maximum equivalent viscosity of 1011 to 1012 Pa s for the diapiric layer itself.