Is erosion important to the structural and petrological evolution of mountain belts? The nature of active metamorphic massifs colocated with deep gorges in the syntaxes at each end of the Himalayan range, together with the magnitude of erosional fluxes that occur in these regions, leads us to concur with suggestions that erosion plays an integral role in collisional dynamics. At multiple scales, erosion exerts an influence on a par with such fundamental phenomena as crustal thickening and extensional collapse. Erosion can mediate the development and distribution of both deformation and metamorphic facies, accommodate crustal convergence, and locally instigate high-grade metamorphism and melting.

Abstract. Within the syntaxial bends of the India-Asia collision the Himalaya terminate abruptly in a pair of metamorphic massifs. Nanga Parbat in the west and Namche Barwa in the east are actively deforming antiformal domes which expose Quaternary metamorphic rocks and granites. The massifs are transected by major Himalayan rivers (Indus and Tsangpo) and are loci of deep and rapid exhumation. On the basis of velocity and attenuation tomography and microseismic, magnetotelluric, geochronological, petrological, structural, and geomorphic data we have collected at Nanga Parbat we propose a model in which this intense metamorphic and structural reworking of crustal lithosphere is a consequence of strain focusing caused by significant erosion within deep gorges cut by the Indus and Tsangpo as these rivers turn sharply toward the foreland and exit their host syntaxes. The localization of this phenomenon at the terminations of the Himalayan arc owes its origin to both regional and local feedbacks between erosion and tectonics.

Microorganisms capable of actively solubilizing and precipitating gold appear to play a larger role in the biogeochemical cycling of gold than previously believed. Recent research suggests that bacteria and archaea are involved in every step of the biogeochemical cycle of gold, from the formation of primary mineralization in hydrothermal and deep subsurface systems to its solubilization, dispersion and re-concentration as secondary gold under surface conditions. Enzymatically catalysed precipitation of gold has been observed in thermophilic and hyperthermophilic bacteria and archaea (for example, Thermotoga maritime, Pyrobaculum islandicum), and their activity led to the formation of gold-and silver-bearing sinters in New Zealand's hot spring systems. Sulphatereducing bacteria (SRB), for example, Desulfovibrio sp., may be involved in the formation of goldbearing sulphide minerals in deep subsurface environments; over geological timescales this may contribute to the formation of economic deposits. Iron-and sulphur-oxidizing bacteria (for example, Acidothiobacillus ferrooxidans, A. thiooxidans) are known to breakdown gold-hosting sulphide minerals in zones of primary mineralization, and release associated gold in the process. These and other bacteria (for example, actinobacteria) produce thiosulphate, which is known to oxidize gold and form stable, transportable complexes. Other microbial processes, for example, excretion of amino acids and cyanide, may control gold solubilization in auriferous top-and rhizosphere soils. A number of bacteria and archaea are capable of actively catalysing the precipitation of toxic gold(I/ III) complexes. Reductive precipitation of these complexes may improve survival rates of bacterial populations that are capable of (1) detoxifying the immediate cell environment by detecting, excreting and reducing gold complexes, possibly using P-type ATPase efflux pumps as well as membrane vesicles (for example, Salmonella enterica, Cupriavidus (Ralstonia) metallidurans, Plectonema boryanum); (2) gaining metabolic energy by utilizing gold-complexing ligands (for example, thiosulphate by A. ferrooxidans) or (3) using gold as metal centre in enzymes (Micrococcus luteus). C. metallidurans containing biofilms were detected on gold grains from two Australian sites, indicating that gold bioaccumulation may lead to gold biomineralization by forming secondary 'bacterioform' gold. Formation of secondary octahedral gold crystals from gold(III) chloride solution, was promoted by a cyanobacterium (P. boryanum) via an amorphous gold(I) sulphide intermediate. 'Bacterioform' gold and secondary gold crystals are common in quartz pebble conglomerates (QPC), where they are often associated with bituminous organic matter possibly derived from cyanobacteria. This may suggest that cyanobacteria have played a role in the formation of the Witwatersrand QPC, the world's largest gold deposit.

ABSTRACT. The mechanics and petrological signature of a collisional mountain belt can be significantly influenced by topographic and erosional effects at the scale of large river gorges. The geomorphic influence on crustal scale processes arises from the effects of both stress localization due to existing topography, and also erosional removal of advected crustal mass. The shear stress concentration and normal stress amplification due to topographic gradients and loads divert strain away from existing topographic loads, while concentrating strain into topographic gaps. Efficient erosional removal of material within topographic gaps with widths of at least the thickness of the brittle crustal layer results in differential advection of crustal material. Concentrated exhumation within a gap leads to thermal thinning of the upper brittle layer of the crust, removing the highest strength part of the continental crust and significantly reducing the integrated crustal strength beneath the topographic gap. A rheological weak spot, triggered by efficient incision, grows in intensity as strain becomes increasingly concentrated within the weak region. The growth of extreme topography of an isolated massif requires that the process of creation of the massif is related to the weakening process and can result from the velocity pattern produced by erosionalrheological coupling. As a result, distinctive thermal/mechanical regions develop within the crust in response to these river-influenced velocity patterns and these regions impose a characteristic signature on material advecting through. The signal is one in which the region of highest topography is bracketed by two high-strain zones between which concentrated advection produces lozenges of sillimanite and dry melt stability approximately 20 kilometers beneath the summit. Above these lozenges is a thermal/mechanical boundary layer containing an active hydrothermal system driven by steep thermal, topographic and mechanical gradients. These thermal mechanical regions are fixed with respect to a crustal reference frame. Passage of rock beneath and through these regions under these conditions produces the distinctive petrology and structure of mantled gneiss domes and is recorded within the moving petrological reference frame. Such erosional-rheological coupling can explain the occurrence of some high-grade gneiss domes in ancient collisional belts as well as the presence of active metamorphic massifs at both ends of the Himalayan orogen.

No abstract

No abstract

No abstract

Schists exposed in the central Southern Alps, New Zealand, 1–5 km east of the Alpine fault zone, have been rapidly uplifted during the late Cenozoic. A regionally consistent sequence of mesoscopic structures discordant to the schistosity is recognized extending backward in time from recent brittle displacements of deglaciated surfaces through structures exhibiting brittle‐ductile transitional behaviour to ductile deformational features. Kinematic analysis of these structures gives consistent principal subhorizontal shortening directions similar to the present day principal horizontal shortening direction, indicating their relation to late Cenozoic uplift of the Southern Alps. Analysis of deformed veins suggests a minimum shortening strain of about 50% perpendicular to the foliation. The veins crosscut upright mesoscopic to macroscopic folds which commonly develop high‐strain zones on their limbs. Within such zones a strong stretching lineation plunges gently SW, approximately perpendicular to the stretching lineation in the mylonites along the Alpine fault. The folds and high‐strain zones do not appear to be related to late Cenozoic uplift but may have originated during an earlier phase of dominantly strike‐slip motion. Fluid inclusion studies give depth‐temperature estimates of around 3 km and 285°C for the development of the brittle structures and 6–8 km at 310°–350°C for the brittle‐ductile transition. The deformed veins show evidence of extensive fluid infiltration during the later phases; data from fluid inclusions and metamorphic assemblages give an estimate of 15–20 km and 400°–450°C for their deformation. These data, combined with uplift rates determined by other studies, allow the construction of a depth‐temperature‐time path which indicates nearly isothermal decompression associated with rapid uplift. A shallow brittle‐ductile transition (6–8 km) is consistent with numerical modeling of thermal and mechanical behavior of the crust during rapid uplift associated with continental collision and with the presence of high heat flow in the area. Because of the small temperature change over a large amount of decompression, pressure may be an important factor in controlling the depth of the brittle‐ductile transition. The marked temperature drop above the brittle‐ductile transition is thought to be enhanced by the influence of convection as an effective cooling mechanism.