T23A-01
Do the great Himalayan earthquakes occur on the plane of detachment?
The most accepted conceptual tectonic models of the Himalayan Seismic Belt (HSB) suggest that below the Main Central Thrust (MCT) lies the Basement Thrust Front (BTF), a 'ramp', the geometrical asperity that accumulates the tectonic stress due to collision tectonics, and the great earthquakes occur on the basement thrust/or on the plane of detachment that separates the Indian shield and the Himalayan sedimentary wedge. Some authors named it Main Himalayan Thrust (MHT). These models are based on the teleseismic data and geological/geophysical evidences. The microearthquake data and a relook into the source processes of the great earthquakes in the Himalaya do not support these models. The four known great (M~8.0) earthquakes in the Himalaya, from west to east, the 1905 Kangra, 1934 Bihar, 1897 Shillong and the 1950 Assam, occurred by different tectonic processes, and possibly none can be explained by the conceptual tectonic models of the HSB; each occurred in its own unique tectonic environment.
T23A-02
Structure and Dynamics of Tien Shan From Integrative Modeling of the new GPS, Gravity and Seismic Data
The Tien Shan range is the longest and most active mountain belt in Central Asia. The lithosphere deformations result from a fast convergence rate between India and Eurasia, whilst Tien Shan absorbs almost half of the ongoing convergence. This situation is absolutely unique for intracontinental mountain belts. The mechanism, which controls crustal shortening in this area, remains up to now unclear. For example, a striking disagreement exists between the shortening rates of ~20 mm/yr (from GPS measurements) and ~10 mm/yr (from seismic moments of large crustal earthquakes). We analyse jointly the gravity data from the new satellite missions CHAMP and GRACE, regional GPS observations and new seismic data to model structure of the crust and upper mantle and to identify the style and intensity of the tectonic processes, which are responsible for strong crustal deformations in the region. As a basis for the geodynamic modelling we use the new 3D seismic model of the Tien Shan lithosphere (Vinnik et al., 2004,2006), which is based on a joint inversion of P and S receiver functions for almost 40 local broadband seismograph stations. The new gravity, GPS and seismic data have been jointly analysed to construct a dynamic model of Tien Shan. The obtained results can be summarized in the following points: (1) We detect a very strong deflection of the Tien Shan lithosphere from isostatic equilibrium. The model of pure crustal shortening does not work in the studied area. The best fit of the modelling results is found for the model according to which the Tarim plate partially underthrusts Tien Shan; (2) It is necessary to assume partial detachment of the lithosphere if the present convergence rate was kept during all history of Tien Shan; (3) Large density-velocity anomalies in the upper mantle are found in the central part of the studied area. These anomalies could be a result of magmatic underplating during the initial stage of tectonic evolution; (4) The weak lithosphere, which is a result of magmatic intrusion, could be the factor that provoked growth of mountains after collision of India and Eurasia.
T23A-03
Structural And Litological Inversion Using Gravity And Mangetic Data Integrated With Geological Infomation
Basin geometry and structure can be derived from the inversion of gravity and magnetic data. Using some newly developed techniques these inversions can also discriminate between different lithologies. Magnetic data can be inverted for magnetic basement and for intermediate layers of volcanic material. Gravity data can be inverted for structures which produce density contrasts in basins. Combining these results, basin structure can be explored to determine depth to basement, to high density structures such as carbonates, and to high susceptibility structures such as volcanics. New high resolution data sets for gravity and magnetic data have been merged and now cover all of Southeast Asia. These data allow us to extend our interpretation to all offshore basins and some onshore as well. We use the structural interpretations described above along with enhancements of gravity, magnetic and digital elevation model data. Combining these interpretations and enhancements with published data in a GIS environment allows us to constrain additional interpretations of tectonic development, and sedimentary facies and structures in basins. Examples are shown for basins around the South China Sea. In addition and example from the Tampico area of Mexico will be presented.
T23A-04
Basin formation due to cooling after magmatic activity
Significant topographic relief is identified in the basement surface of many sedimentary basins, albeit hidden by the sedimentary sequences. We discuss the possibility of formation of wide and deep sedimentary basins by thermal subsidence caused by magmatic intrusions into the crustal and mantle lithosphere. Magmatism may heat the lithosphere substantially independent of its origin, be it due to deep sources (e.g. mantle plumes) or decompression melting in high strain environments. Thermal expansion of the rocks of the lithosphere will cause uplift of the Earth's surface and erosion will bring the surface back to sea level. The subsequent cooling of the lihosphere creates the basin. Thermal sag basins are usually attributed to the heating of the crustal and mantle lithosphere due to extensional events, but here we focus on the role of heat brought into the crust and mantle lithosphere by magma. A characteristic feature of such basins is that relatively little faulting is expected in association with the subsidence. We find evidence from the Danish-Norwegian Basin in the North Sea area for significant magmatic intrusion into the crust during the Carboniferous to Permian. The subsequent Triassic subsidence shows almost no faulting, which indicates that the subsidence was not caused by tectonic stresses. The amount of magmatic rocks (>40-100,000 km3) presently detected in the crust is sufficient to explain the observed subsidence as the thermal relaxation to the magmatic event together with the isostatic response to the sedimentary load. As such there is no need for invoking extensional stresses for explaining the Danish- Norwegian Basin.
T23A-05
Hydrothermal precious-metal deposits related to graben-calderas of the Sierra Madre Occidental
The Sierra Madre Occidental (SMO) covers the NW portion of Mexico and it is the host for several important precious metal mine operations, such as Tayoltita, Cienega, Topia, Fresnillo, Zacatecas, Guanajuato and Bolaños, just to mention a few. The southern part of the Basin and Range extension affected also NW Mexico and formed NW- to NE-trending normal faults that bound many large grabens, which are particularly long and deep in the southern SMO. Both graben formation and mid-Tertiary silicic volcanic activity coincided in space and time, particularly for the 38-23 Ma period, the Ignimbrite Flare-up event, but this activity dates back to Eocene and was as young as Miocene. This volcanism included large rhyolitic domes, too. At the southern SMO, the vents of this silicic volcanism are related to graben's master faults and we have named them graben-calderas. Evidences include large pyroclastic dikes and post-ignimbrite aligned rhyolitic domes and lava dikes. All these features were found along the graben-caldera walls or on the graben's shoulders. Some of these vents are related to gold and silver hydrothermal mineralization. In most cases a paleo-lake filled the graben-caldera for a period of time, either during the ignimbrite emplacement or after it. Some of the graben-caldera ignimbrites were deposited in subaqueous environments and post-ignimbrite rhyolitic domes and dikes were intruded in non-consolidated water-saturated tuffs or sedimentary deposits. This lacustrine environment provided the necessary water for the hydrothermal system. The combination of all these factors in space and time, grabens+volcanism+water, resulted in the development of precious-metal hydrothermal ore deposits. Bolaños mine in the Bolaños graben represent our case-study, but we have confirmed the same tectono-volcanic-lake relationship at other mine-districts along the SMO. We conclude that locating the fissural vents of the silicic ignimbrites by means of just geologic mapping is be the best and most economical first approach to locate the associated hydrothermal mineralization.
T23A-06
Uplift of continental crustal blocks adjacent to the Rancheria Basin-Guasare area: the effects of Maastrichtian-Paleocene collision along the southern Caribbean plate boundary
In the Rancheria basin (RB) and Guasare area (GA), Maastrichtian-Paleocene synorogenic strata overlie the Aptian-Campanian carbonate platform. Nowadays, RB is bounded to the west by metamorphic-and-igneous cored Santa Marta massif, where Upper Cretaceous strata overlie unconformably pre-Cretaceous rocks. The eastern boundary of the RB is the Perija range that includes volcaniclastic and sedimentary rocks of Jurassic and Cretaceous age in the hanging-wall of a NW-verging, low-angle dipping thrust belt. The GA is on the eastern foothills of the Perija range and corresponds to the western boundary of the Maracaibo basin. Strata architecture, seismic reflectors, gravity, provenance, and paleocurrent analyses carried out in those basins constrain the timing and style of uplift of Santa Marta massif and Perija range, which are linked with tectonism along the southern Caribbean plate. Maastrichtian-Paleocene strata thicken eastward up to 2.2 km in the RB, and this succession includes (in stratigraphic order): foram-rich calcareous mudstone, oyster-pelecypod rich carbonate-siliciclastic strata, coal- bearing mudstones and feldspar-lithic-rich fluvial sandstones. Internal disconformities and truncations of seismic reflectors are identified to the west of the RB, but there are not major thrust faults at this part of the basin to explain such unconformities and truncations. In Early Paleocene, carbonates developed better to the west of the RB, whereas mixed carbonate-siliciclastic deposition continued toward the east of the RB. In early Late Paleocene, influx of terrigenous material (key grains=metamorphic, microcline and garnet fragments) derived from the Santa Marta massif increased to the west, but to the east of the RB and GA carbonate-siliciclastic and carbonate deposition continued, respectively. In mid-Late Paleocene, diachronous eastward advance of paralic/deltaic environments, tropical humid climate, and high subsidence rates favored production and preservation of peat in RB and GA. In the late Late Paleocene, inversion along a buried graben system under the Perija range explain supply toward RB and GA of micritic, volcanic, and sedimentary rock fragments, and the record of a thinner Upper Paleocene strata in the GA than in the RB. Tectonic subsidence in the RB was mainly related to pivoting of the Santa Marta massif as result of collision of the Maracaibo continental sub-plate with the southern margin of the Caribbean oceanic plate. This model explains the generation of accommodation space in the RB without faulting, denudation of upper crustal material of the Santa Marta massif, early capture of terrigenous detritus in the RB that favored carbonate deposition in the GA, the mechanism of initial inversion of the Perija range, and the present positive gravity anomaly under the Santa Marta massif.
T23A-07
Crust to Upper Mantle Echoes of the Black Sea Opening and Seismotectonic Consequences on the NW Inland
The paper aims at revealing some tectonic and geodynamic imprints of the Black Sea opening mainly inferred from the potential fields analysis. DSS lines and seismic tomography are added in order to strengthen the interpretation and/or deepen the depth of investigation. Crust structure and dynamics (1) The presence of the oceanic crust in the central part of the basin is well reflected in the geomagnetic anomaly pattern. (2) Unlike some previous hypotheses postulating the existence of an unique rifting, up to date filtering techniques pointed out an unexpected pattern of the gravity and geomagnetic anomalies, trending almost perpendicular each-other within eastern and western basin, thus advocating for a distinct opening of the W and E Black Sea. (3) Correlation with the magnetostratigraphic scale revealed a geomagnetic reversal and seems to indicate a later opening of the eastern basin; off-shore seismics confirm the model by showing a slight overthrusting of the E Pontides over W Pontides. (4) It seems that the W Black Sea opening split the Moesian Plate into several slivers by creating/reactivating older faults trending north-westward. (5) Crust expelled by the Black Sea opening accommodated in various circumstances: (i) East Carpathians it met the inclined outer flank of the TTZ and came into an oblique subduction to which specific peculiarities of the South Harghita Mts. volcanism might be associated; (ii) South Carpathians, crustal slivers facing the vertical contact of the Intra-alpine sub-plate could not subduct, but went into a lithosphere buckling to which the lowest gravity low on the Romanian territory, located in front and not beneath the highest mountains in Romania, seems to be related; (iii) within the bending area of East Carpathians, where three tectonic plates met each other, the speed excess provided by the W Black Sea opening created an unstable triple junction. Upper mantle echoes Fingerprints of the Black Sea opening are well reflected in some recent seismic tomography. The tomography shows in depth extension of some previously mentioned faults into the upper mantle. It seems that W Black Sea rifting affected not only the lithosphere, but part of the upper mantle has been also expelled towards Carpathians. A higher velocity body extending to about 300 km in depth is associated to the Vrancea active seismic zone. Seismotectonic considerations Looking at the seismicity of the SE Carpathians foreland two aspects should be stressed: (i) the unusual seismicity of the Moesian Platform; (ii) the intermediate-depth earthquakes within the bending area of East Carpathians (Vrancea zone) that might be hardly associated to a subduction process. After the Black Sea opening ended, the drift of the NW inland towards Carpathians went on due to the active rifting within SW Arabian Plate. As a rule, lithosphere slivers within the foreland advance together towards Carpathians kept by friction. However, sometimes, they relatively slip each to other thus generating earthquakes along their wedges. An alternate mechanism for the unusual intermediate-depth seismicity within Vrancea zone was proposed: the unstable triple junction. It seems that the Black Sea opening provided the necessary speed excess to the MoP for generating an unstable transform-transform-compression triple junction within Vrancea area. The penetration of the central lithospheric segment of the triple junction into the hotter upper mantle would be followed by thermo- baric accommodation phenomena (e.g. thermal stress, phase-transform processes, dehydrations) to which intermediate-depth seismicity might be related.
T23A-08
Beam-formed Receiver Function Analysis From the Southern Sierra Nevada, CA: A Moho Hole??
Single station receiver functions in the southern Sierra Nevada, California, USA show considerable variation in Moho conversion intensity. Specifically, in the range's western foothills, a region termed "the Moho Hole" (e.g. Zandt et al, 2004), the Moho appears to be completely absent on single station receiver functions. I explore the possibility that signal-generated reverberations have sufficiently obscured the seismograms to disguise a Moho P-S conversion. Using data from the 1997 Sierra Paradox Experiment, seismograms from 3 stations spaced ~20 km apart are slant-stacked to make the radial, transverse, and vertical components used in deconvolution. By stacking before deconvolution, signals from the Moho and intra-crustal discontinuities become more pronounced while reflections off of structure near individual receivers destructively interfere with one another. Receiver functions for these 3 station beams compare favorably with post-deconvolution stacking of the receiver functions calculation of their constituent stations. Of paramount interest is the use of this technique in the region interpreted as the "Moho Hole". Here, beam formed receiver functions better constrain the extent and geometry of the small amplitude Moho conversion than previous studies. In fact it seems that this region is part of a larger- scale (~100 km) west-northwestward trend of weakening P-S signal. This dimming is spatially correlated with a thickening crust, seen in increasing P to P-S lag times. I propose that the features are cogenetic: products of long-wavelength crutal down-flexure as the result of viscous coupling to downwelling delaminated lower lithosphere. Investigating this response is important to understanding the mechanism and kinematics of Sierran uplift and the response of upper crustal material to delamination events.