S U M M A R YWe analyse data from seismic stations surrounding the Alboran Sea between Spain and North Africa to constrain variations of the lithosphere-asthenosphere boundary (LAB) in the region. The technique used is the receiver function technique, which uses S-to-P converted teleseismic waves at the LAB below the seismic stations. We confirm previous data suggesting a shallow (60-90 km) LAB beneath the Iberian Peninsula and we observe a similarly shallow LAB beneath the Alboran Sea where the lithosphere becomes progressively thinner towards the east. A deeper LAB (90-100 km) is observed beneath the Betics, the south of Portugal and Morocco. The structure of the LAB in the entire region does not seem to show any indication of subduction related features. We also observe good P receiver function signals from the seismic discontinuities at 410 and 660 km depth which do not indicate any upper-mantle anomaly beneath the entire region. This is in agreement with the sparse seismic activity in the mantle transition zone suggesting the presence of only weak and regionally confined anomalies.
This paper proposes eight single-element controlled oscillators based on Differential Voltage Current Conveyors. These circuits employ all-grounded passive components and provide current and voltage outputs, simultaneously. Some of the circuits provide independent oscillation and frequency control. Sensitivity analysis of the proposed circuits is also presented. Performances of the proposed oscillators are investigated using circuit simulations, and it is found that their results are in good agreement with theory.
Cambay Rift Zone (CRZ) is an ancient rift that had last witnessed volcanism and rifting activity 68-65 million years ago, when the Indian plate migrated over the Deccan plume, potentially signifying the presence of a shallow LAB. Here we apply converted wave techniques to a teleseismic earthquake dataset to image the mantle below the CRZ. We place the Lithosphere-Asthenosphere Boundary (LAB) beneath this rift at a depth where a ∼10% drop occurs in shear wave velocity. The seismic velocity structure indicates that the lithosphere thickness varies from ∼60 km beneath the rift to ∼110 km beneath rift walls. Furthermore, the upper mantle discontinuities (410 and 660 km discontinuities) are found to be thinner by ∼2 sec (i.e. ∼20km) with delayed 410-km discontinuity and normal 660-km discontinuities. We infer this to be caused by a thermal anomaly in the upper mantle. The surface heat flow at CRZ is high. A geothermal gradient extrapolated from the surface heat flow intercepts CO2-bearing mantle peridotite solidus at 60 km, and thus could signal the presence of small amounts of carbonatite-type magma. We suggest that the CRZ might become reactivated by the generation of small pockets of melt at the LAB, which is being triggered by a thermal anomaly in the upper mantle beneath this currently inactive rift.
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