ALBERT

Beschreibung: 〈span〉〈div〉Summary〈/div〉Magnitudes of differential stress in the lithosphere, especially in the crust, are still disputed. Earthquake-based stress drop estimates indicate median values 〈 10 MPa whereas the lateral variation of gravitational potential energy per unit area (〈span〉GPE〈/span〉) across significant relief indicates stress magnitudes of ca. 100 MPa in average across a 100 km thick lithosphere between the Indian lowland and the Tibetan plateau. These standard 〈span〉GPE〈/span〉-based stress estimates correspond to membrane stresses, because they are associated with a deformation that is uniform with depth. We show here with new analytical results that lateral variations in 〈span〉GPE〈/span〉 can also cause bending moments and related bending stresses of several hundreds of MPa. Furthermore, we perform two-dimensional thermo-mechanical numerical simulations (1) to evaluate estimates for membrane and bending stresses based on 〈span〉GPE〈/span〉 variations, (2) to quantify minimum crustal stress magnitudes that are required to maintain the topographic relief between Indian lowland and Tibetan plateau for ca. 10 Ma and (3) to quantify the corresponding relative contribution of crustal strength to the total lithospheric strength. The numerical model includes viscoelastoplastic deformation, gravity and heat transfer. The model configuration is based on density fields from the CRUST1.0 data set and from a geophysically and petrologically constrained density model based on 〈span〉in situ〈/span〉 field campaigns. The numerical results indicate that values of differential stress in the upper crust must be 〉 ca. 180 MPa, corresponding to a friction angle of ca. 10°, to maintain the topographic relief between lowland and plateau for 〉 10 Ma. The relative contribution of crustal strength to total lithospheric strength varies considerably laterally. In the region between lowland and plateau and inside the plateau the depth-integrated crustal strength is approximately equal to the depth-integrated strength of the mantle lithosphere. Simple analytical formulae predicting the lateral variation of depth-integrated stresses agree with numerically calculated stress fields, which show both the accuracy of the numerical results and the applicability of simple, rheology-independent, analytical predictions to highly variable, rheology-dependent, stress fields. Our results indicate that (1) crustal strength can be locally equal to mantle lithosphere strength and that (2) crustal stresses must be at least one order of magnitude larger than median stress drops in order to support the plateau relief over a duration of ca. 10 Ma.〈/span〉

Beschreibung: According to general relativity, a clock experiencing a shift in the gravitational potential U will measure a frequency change given by f / f    U / c 2 . The best clocks are optical clocks. After about 7 hr of integration they reach stabilities of f / f  ~ 10 –18 and can be used to detect changes in the gravitational potential that correspond to vertical displacements of the centimetre level. At this level of performance, ground-based atomic clock networks emerge as a tool that is complementary to existing technology for monitoring a wide range of geophysical processes by directly measuring changes in the gravitational potential. Vertical changes of the clock's position due to magmatic, post-seismic or tidal deformations can result in measurable variations in the clock tick rate. We illustrate the geopotential change arising due to an inflating magma chamber using the Mogi model and apply it to the Etna volcano. Its effect on an observer on the Earth's surface can be divided into two different terms: one purely due to uplift (free-air gradient) and one due to the redistribution of matter. Thus, with the centimetre-level precision of current clocks it is already possible to monitor volcanoes. The matter redistribution term is estimated to be 3 orders of magnitude smaller than the uplift term. Additionally, clocks can be compared over distances of thousands of kilometres over short periods of time, which improves our ability to monitor periodic effects with long wavelength like the solid Earth tide.

Beschreibung: How convergent systems distribute strain among frontal thrusts is a major concern regarding seismic hazard assessment. Along the 2500 km Himalayan arc, the seismic behavior of the Bhutan region is unknown, because it corresponds to the only portion of the arc where no evidence of major earthquakes has been reported. This can be due either to the fact that no active tectonic studies have been conducted or to continental shortening being absorbed by the Shillong plateau 150 km farther south. Analyzing offset fluvial terraces in south-central Bhutan shows that two major earthquakes ruptured the Himalayan frontal thrust during the last millennium, and that a comparable rate of Holocene deformation (~20 mm/yr) is accommodated across the Himalaya in Bhutan as in central Nepal. Thus, the propensity for great earthquakes in Bhutan is similar to what is observed in neighboring portions of the Himalaya arc. This in turn suggests that the shortening process beneath the Shillong plateau has little effect on how strain accumulates within the Bhutanese Himalaya.

Beschreibung: Our knowledge and understanding of the 3-D lithospheric structure of the Himalayas and the Tibetan Plateau is still challenging although numerous geophysical studies have been performed in the region. The GOCE satellite mission has the ambitious goal of mapping Earth's gravity field with unprecedented precision (i.e. an accuracy of 1–2 mGal for a spatial resolution of 100 km) to observe the lithosphere and upper-mantle structure. Consequently, it gives new insights in the lithospheric structure beneath the Himalayas and the Tibetan Plateau. Indeed, the GOCE gravity data now allow us to develop a new strategy for joint gravimetry-seismology inversion. Combined with teleseismic data over a large region in a joint inversion scheme, they will lead to lithospheric velocity-density models constrained in two complementary ways. We apply this joint inversion scheme to the Hi-CLIMB (Himalayan–Tibetan Continental Lithosphere during Mountain Building) seismological network which was deployed in South Tibet and the Himalayas for a 3-yr period. The large size of the network, the high quality of the seismological data and the new GOCE gravity data set allow us to image the entire lithosphere of this active area in an innovative way. We image 3-D low velocity and density structures in the middle crust that fit the location of discontinuous low S -velocity zones revealed by receiver functions in previous geophysical studies. In the deeper parts of our velocity model we image a positive anomaly interpreted to be the heterogenous Indian lithosphere vertically descending beneath the centre of the Tibetan Plateau.

Beschreibung: The current understanding of the Himalayan lithosphere stems mostly from cross-sections through the range at the longitude of the Kathmandu Basin. In this paper we laterally extend the analyses of structures and rheology along the Nepal Himalayas between the Pokhara valley and the Arun river. We take advantage of available information and a new data set including gravity measurements and a receiver function profile. It appears that the geometry of the Moho inferred from seismological profiles and long-wavelength gravity anomalies does not exhibit major East–West variations within the 350-km-wide study area. Using thermomechanical modelling, we show that the northward deepening of the Moho observed along profiles perpendicular to the main thrust faults can be interpreted simply as the bending of a strong India Plate. This result suggests a gradual mechanical decoupling between the crust and the mantle, leading to a northward decrease of the effective elastic thickness of the Indian lithosphere from ~75 km to ~25 km beneath the Ganga Basin and the Tibetan Plateau, respectively. Our results also confirm (partially) eclogitized lower Indian crust beneath southern Tibet. At shorter wavelengths, the observed gravity profiles exhibit some small lateral variations that can be interpreted in terms of east–west variations of the thickness of subsurface geological structures such as the Ganga Basin and the Tethyan Sedimentary Sequence.