T53D-01 INVITED 13:45h
Detailed Fluid/Melt Structure Deduced from Geophysical Field Observations
Geophysical field observations point to the present existence of huge amount of melts in the crust below the Central Andes (e.g. Schilling & Partzsch, 2001). The Central Andes are an ideal place to study melt distributions in a continental crust due to the amount and quality of geophysical, geological, petrological, and geochemical studies within the framework of the Special Research Project 267, Deformation processes in the Andes, including seismic velocity and wave absorption studies, as well as electrical conductivity and gravity observations. Furthermore, laboratory studies and model calculations were performed in conjunction with the field measurements to study the effect of partial melts on the physical properties of rocks. The melt distribution/melt structure were optically determined in the laboratory on quenched samples. Laboratory experiments on the electrical conductivity and elastic properties of partially molten rocks are used in combination with melt structure observations to get a detailed insight into the dependence of physical properties on the melt structure. The interrelation of different properties allows to distinguish between melt isolated in pockets (magma chambers) and interconnected melt along grain boundaries or grain edges. The aim of this contribution is to use the interrelation of different physical properties to get a more detailed insight into the melt distribution of large areas, which have been inferred to be partially molten. The field observations are interpreted with respect to the laboratory experiments, while taking the scaling problem into account. The results deduced from the Andes will be compared to observations from the Tibetan Plateau. Schilling F.R., Partzsch G.M. (2001) Quantifying Partial Melt Portion in the Crust Beneath the Central Andes and the Tibetan Plateau, Physics and Chemistry of the Earth (A), 26, 239-246.
T53D-02 14:00h
Magnetotelluric observations of crustal deformation and flow in Tibet
Geophysical data are a vital source of information regarding the processes at work in active orogens. For example, seismic reflection and magnetotelluric (MT) studies of the Tibetan-Himalayan orogen have revealed zones of partial melting that may delineate regions of rheological weakness. This includes the mid-crustal melt layer in southern Tibet that was imaged with both seismic reflection and MT data, and is associated with the south directed flow that has exhumed the High Himalyan leucogranites. In northern and eastern Tibet, crustal flow has been proposed to account for the observed topography and may be required to maintain a mass balance. A channel of enhanced electrical conductivity has been observed in the Tibetan crust on the INDEPTH data and also in new MT data collected in eastern Tibet. Could this feature represent one such channel of crustal flow? However, geophysical datasets must be carefully interpreted in a number of situations. Firstly, the absence of high conductivity does not exclude the possibility that deformation is occurring through other mechanisms such as creep. Secondly, other minor phases can produce a high electrical conductivity, notably electronic conduction in graphite, sulphides, iron oxides and ionic conduction in aqueous fluids. Finally, there are locations where high electrical conductivity is observed in the absence of a seismic anomalies. If carefully analyzed, and compared and contrasted with other geophysical and geoscientific data, each of these scenarios allows a broader understanding of orogenesis.
T53D-03 14:15h
Composition, Temperature and Metamorphic Facies of the Lower Half of the Thickened Tibetan Crust From the North Kun Lun, Through Bayan Har Into the Qang Tang and Into the Himalaya-South Tibet: Insight on Their Geodynamical Evolution
Wide angle reflection-refraction profiles of P and S-waves were obtained at the NE edge of the Tibetan Plateau of recent postcollisional active convergence tectonics, as well as into the earlier formed Himalaya-South Tibet. From the North Kun Lun through Bayan Har and into the Qang Tang block, the record-sections of six shots and their modelling evidence changes in crustal thickness, layers velocity and also image through fan profiles its internal architecture. Furthermore, recording of P-waves as well as unexpected S-waves gives insight of the lower crust physical state in the domain of block interaction in the northern edge of the Qang Tang and of the Bayan Har. Here, the quality factor Q is estimated in the lower half of the crust by accounting for the differential effect on amplitude-frequency, observed between waves of different penetrations. Attenuation values allow to exclude a significant proportion of partial melt and to estimate the homologous temperature, ratio in-situ to solidus temperatures. The latter depend on the physical conditions being dry, wet or dehydration melting, which are found different among the regions of the northern Bayan Har, northern Qang Tang and we will see also different from the Himalaya-South Tibet. Their in-situ temperatures differ also as estimated from their different Vp with similar felsic composition, highlighted by their low velocity ratio, Vp/Vs. The uncommon joint determination of several parameters: Vp, Vp/Vs, Qp and Qs reveals the composition, the mineralogy and hydration conditions of the lower half of the thickened crust of Tibet and backs up the thickening of the Tibetan crust by tectonic superposition, imbrication of originally normal thickness crusts.
T53D-04 14:30h
Co-Anatectic Crustal Failure in the Absence of Geophysically Detectable Partial Melt
The India-Asia collision's syntaxial terminations are loci of active deformation and vigorous exhumation. Quantitative constraints on tectonic processes operating in the syntaxes are now available from a large suite of studies conducted at the Nanga Parbat and Namche Barwa massifs by several research groups. In the western syntaxis in particular, deployment of seismic and magnetotelluric arrays at Nanga Parbat has provided an image of crustal rheologies and the distribution of fluid phases. Surface constraints from structural surveying reveal major deformation localization via two conjugate shear zones whose crustal-scale characteristics are imaged by the geophysics. Extensive geochronological constraints from multiple syn-deformation shear zone granitoids indicate a prolonged history (steady-state?) of weakening and failure. In contrast to the southern-Tibet portions of the collision (where INDEPTH geophysical imaging has been interpreted to suggest crustal weakening coupled with extensive partial-melt accumulation), at Nanga Parbat no extensive partial-melt candidate is discernible in the geophysical data. Instead, these data suggest the presence of anomalously hot but dry crust beneath a region that thermochronologic and petrologic data show has experienced rapid and extensive exhumation during the Neogene. Observed shear zone-hosted granitoids result from anatexis that at any instant is volumetrically trivial for the massif as a whole. It is unclear what role geophysically "invisible" subsurface melt might play in weakening. Three-dimensional numerical experiments indicate that positive mechanical-thermal feedback associated with localized exhumation is sufficient to produce melt lozenges and that further melt-enhanced weakening is not required for "aneurysm" behavior. That the Namche Barwa massif shares a remarkably similar geologic history with Nanga Parbat despite significant differences in tectonic setting underscores the importance of vigorous exhumation in active tectonic systems.
T53D-05 14:45h
Syntectonic Melt Distribution in Deep Crust Inferred From Residual Granulites: Implications for the Rheology of Orogenic Crust
Anatectic systems are heterogeneous, nonlinear and characterized by multiphase flow. Each process contributing to melt extraction has a characteristic length and time scale, and it is the nonlinear interactions and feedback among them that give rise to the patterning observed in lower crust. Melting and melt ascent and emplacement modify the physical properties of the crust and generate viscosity contrasts, which lead to a heterogeneous response and localization. The characteristic length scale of deformation is important, since reducing the length scale for the same velocity of deformation increases the strain rate (e.g. whole crust to orogenic channel to shear zone). Whether patterns of localization are stable with increasing strain and what effect localization has on rheology are poorly understood. Geophysical imaging suggests 6-20 vol.% interconnected melt in the crust of active orogens, numerical modeling of the transition from coupled doubly-vergent wedge structure to plateaux formation requires a viscosity drop of up to 4 orders of magnitude, and deformation experiments on rock undergoing melting indicate a 4-10-fold drop in strength due to wetting of most grain boundaries as melt volume approaches 10%. The magnitude of weakening accompanying melting suggests deformation dominantly by melt-assisted diffusion creep with melt segregation and extraction rather than magma behavior (i.e. bulk rheology of anatectic crust is solid-dominated rather than melt-dominated). However, fertile rocks generate 10-50 vol.% melt (at 1 GPa, 1,173K), which suggests melt is extracted to maintain a solid-dominated rheology. Melting occurs at multiphase grain boundaries around hydrate phases, whereas plutons represent 1,000-10,000 km3 of crystallized magma; this requires focusing the flow of segregated melt to channels that allow ascent through subsolidus crust. Studies of residual granulites suggest that melt has migrated from grain boundaries to networks of (leucosome-filled) structures to ascent conduits (now steeply-inclined rod or tabular granites). However, the common assumption that leucosome vol. equals melt vol. is precluded by the mineralogy and chemistry of leucosomes, which indicates both cumulate (early-crystallized solids) and fractionated (late-crystallized residual liquids) varieties. At the grain scale, location of melt is controlled by fabric and strain. At outcrop in residual granulites leucosome occurs in fabric parallel and transverse stromata, along foliation planes and in dilation/shear bands, forming networks analogous to ideal deformation band networks. Leucosome networks commonly are elongate parallel to lineation, so melt flow is inferred to have been primarily in the plane of the foliation and along the lineation to developing dilatant structures, and through the network of structures to ascent conduits (commonly dikes). Dike emplacement occurs along a preferred direction independent of anisotropy, suggesting stress control, which with the macroscopic fracture-like discontinuities characterizes the process of formation as a fracture phenomenon. Petrographic continuity of leucosome with granite in dikes suggests a once continuous melt-bearing network, and indicates that material in leucosomes and dikes underwent final crystallization at the same time. Blunt dike tips and zigzag propagation paths point to ductile fracture as the mode of formation, probably by pore growth and coalescence of melt pockets. The coupling between fracture formation and mass transfer appears to be significant for fracturing in crustal environments above the solidus, where a large amount of plastic strain may accumulate before fracture and where dislocation-mediated ductile fracture may be expected.
T53D-06 15:00h
Non-uniform Extensional Processes Influenced by Fluid and Melt Distributions Below the Great Basin-Colorado Plateau Transition Zone, Utah, Revealed Through Electrical Conductivity Structure
Electrical conductivity provides independent understanding of deep hydration, thermal regime, fluidization/melting, lithospheric-scale fabric and faulting, and economic resource controls. Since the early 1970's, regional conductivity surveys have shown a first-order partitioning of current activity in the Great Basin province, with the eastern and western margins being more anomalous w.r.t. a relatively quiescent Great Basin interior, in keeping with other indicators. The lower crust throughout the region is electrically conductive corresponding to a small fraction (less than 0.5%) of hypersaline fluids and silicic melts, thus implying weak rheology, but this is especially apparent in the active eastern Great Basin. The thermal profile of the central province lies near the ACMA geotherm below ~75 km, and the upper mantle there appears horizontally isotropic and only weakly hydrated at most. In contrast, eastern Great Basin upper mantle appears substantially hotter, with significant probable melting and an abrupt, non-uniform vs depth transition eastward to the stable Colorado Plateau. Within the transition zone itself, conductivity structures ressembling low angle detachments soling into a concentrated lower crustal conductor appear, with the latter interpreted to reflect ponded melts and exsolved fluids from basaltic underplating. Crustal-scale, steeply dipping conductive fault zones also appear in the province and may represent those where major deep earthquakes nucleate. Pre-Late Cenozoic heritage is revealed in detailed study of the Carlin Trend gold province, with a family of structures attributed to deep source rocks, Eocene intrusion, stratal deformation and alteration/graphitization. Lower crustal fabric inherited from the Proterozoic continental margin still appears to influence some deep electrical interconnection of fluids and melts today.
T53D-07 15:15h
Crustal flow in the India-Asia collision constrained by earthquakes
Continent-continent collision is one of the fundamental processes in the plate tectonic cycle and is critical to understanding crustal evolution. The India-Asia collision provides the opportunity to investigate the mechanics of continent-continent collision in a modern setting avoiding many of the uncertainties inherent in studying ancient mountain belts. A fundamental problem in understanding the India-Asia collision is what drives deformation in the Tibetan Plateau. Seismic activity provides one of the best constraints on the stresses responsible for mountain building and how that stress is distributed. We have examined earthquakes distributed throughout the Indian foreland, the Greater Himalaya and the Tibetan Plateau. We focus on earthquakes with well-controlled depths, and use the Harvard Centroid Moment Tensor (CMT) catalog to map brittle deformation and stress orientations in the region. We find that deep earthquakes within the Indian Plate dip beneath an aseismic wedge. This aseismic wedge extends north from beneath the Himalaya to the Tibetan Plateau. In most of Tibet earthquakes are restricted to the upper 35 km of the crust until the Kunlun fault is reached. At the Kunlun fault seismicity extends to depths as great as 50 km. The region between the Himalaya and the Kunlun fault deforms by transtension. This coincidence of transtension with a thick aseismic lower crust is consistent with the lower crust beneath Tibet deforming as a plastically flowing channel.