[1] We estimate the variation in strength of the Eastern Himalayan-Tibetan lithosphere from 2D gravity and topography coherence functions calculated with the Thomson-Slepian multitaper technique. The coherence shows evidence of structural weakness of the plate in the N-S direction, aligned with the direction of maximum compression. The Tibetan plateau is characterized by an effective elastic thickness (Te) that is much lower than would be expected from its large crustal thickness and the additional strength contributed by a coherently folded mantle inferred from shear wave splitting. The seismogenic thickness indicates that only the upper crust is contributing to the elastic strength and not the mantle. The predominant weakness of the lithosphere in the N-S direction revealed from the anisotropy in the coherence function suggests that the mantle may not be directly coupled to crustal deformation beneath the Tibetan plateau.

Late Ordovician volcanic rocks of the English Lake District typically have a magnetic remanence dominated by a single characteristic component. Previous investigations have interpreted this remanence as both of primary (pre-folding) and secondary origin. Palaeomagnetic field tests have been conducted on (a) andesite blocks from an autobrecciated lava top, (b) andesite blocks in mass-flow breccias, and (c) fault-blocks tilted during Ordovician caldera collapse to establish the time of remanence acquisition. All three tests show that the lavas retain a magnetization acquired during initial cooling: magnetizations of the breccias are coherent within clasts and random between clasts, whilst magnetizations of the tilted fault blocks converge with better that 95 % confidence when corrected for the effects of caldera collapse. In contrast, the volcaniclastic sedimentary and pyroclastic rocks possess an Ordovician secondary remanence acquired after strata had been tilted by volcano-tectonic subsidence. A distributed sample of 65 andesite and basalt sheets through the Borrowdale Volcanic Group has a mean remanence direction D/I = 341.9/-48.9°( α 95 = 4.0°) yielding a positive fold test and a palaeomagnetic pole at 12.7°E, 4.3°S (dp/dm = 3.5/5.3°). A progressive steepening of the palaeofield direction is recorded during emplacement of the Borrowdale Volcanic Group (~I = -39°to I = -51°) which continued into the interval of volcanotectonic overprinting (I = -62°); the equivalent motion of Eastern Avalonia is ~20°into higher southerly latitudes.Both the Eycott and Borrowdale volcanic groups exhibit uniform normal polarity throughout. Correlation with the geomagnetic time scale for the Ordovician restores the broad correlation between the two groups by constraining their emplacement and partial overprinting to a single long normal polarity chron occupying the Nemagraptus gracilis and earlier part of the Diplograptus multidens biozones (late Llandeilo and early Caradoc). All the volcanism, therefore, occurred within a period of no more than ~5 Ma. The palaeomagnetic evidence confirms that the Borrowdale Volcanic Group was affected by both syn-volcanic deformation (caldera collapse) and regional compressive deformation prior to deposition of the (late Ordovician-Silurian) Windermere Supergroup. The succession of primary and secondary Ordovician palaeomagnetic poles from the Lake District inlier defines an anticlockwise apparent polar wander (APW) loop with the apex correlating with 'soft' closure of the Iapetus Ocean and late Ordovician deformation. The APW paths from Avalonia and Baltica converge at this point as subduction ceased and the arc subsided beneath the sea after mid-Caradoc times.

[1] We present measurements of the mechanical strength of the lithosphere of the South Indian Peninsular Shield (SIPS). Values of Effective Elastic Thickness (Te) were obtained from azimuthally averaged coherence measurements g 2 (jkj) between Bouguer gravity and topography using a multitaper method (MTM) and a mirrored periodogram method (MPM). Values of Te from MTM (11 -16 km) are consistently lower than those from MPM (20-31 km) and are contained within the seismogenic thickness (Ts), substantiating earlier measurements in shield regions that have been tectonically inactive for a long time. The 2D coherence function g 2 (k) calculated with the MTM reveals little mechanical anisotropy in the SIPS. We conduct analyses of different provinces within the SIPS and show how the results correlate with maximum horizontal stress orientations (s Hmax ) and heat flow measurements. Local processes appear to influence the strength of the SIPS lithosphere more profoundly than processes occurring at the plate boundary.

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Initialization of model parameters is crucial in the conventional 1D inversion of DC electrical data, since a poor guess may result in undesired parameter estimations. In the present work, we investigate the performance of neural networks in the direct inversion of DC sounding data, without the need of a priori information. We introduce a two-step network approach where the first network identifies the curve type, followed by the model parameter estimation using the second network. This approach provides the flexibility to accommodate all the characteristic sounding curve types with a wide range of resistivity and thickness. Here we realize a three layer feed-forward neural network with fast back propagation learning algorithms performing well. The basic data sets for training and testing were simulated on the basis of available deep resistivity sounding (DRS) data from the crystalline terrains of south India. The optimum network parameters and performance were decided as a function of the testing error convergence with respect to the network training error. On adequate training, the final weights simulate faithfully to recover resistivity and thickness on new data. The small discrepancies noticed, however, are well within the resolvability of resistivity sounding curve interpretations.