No abstract

Primary and pseudosecondary fluid inclusions occur in oscillatory-and sector-zoned omphacite in eclo gitic veins from the Monviso ophiolitic complex in the Western Alps. The inclusions contain aqueous brines and daughter crystals of halite, sylvite, calcite, dolomite, albite, anhydrite and/or gypsum, barite, baddeleyite, ru tile, sphene, Fe oxides, pyrite and monazite. This daugh ter mineral suite indicates high solubilites of Na,

The contact between the western border of the Tauern Window and the overlying Austroalpine nappes (Austria and Italy) is marked by a structural, petrologic, and geochronologic discontinuity that developed in the early stages of unroofing of the window. Within the window, pressure‐temperature‐time paths (J. Selverstone, 1985) indicate that significant ductile thinning occurred shortly after the cessation of thrusting (Paleocene ‐ Eocene). Sense‐of‐shear indicators (rotated porphyroblasts, asymmetric augen and pressure shadows, S‐C fabrics) associated with a west plunging stretching lineation indicate that this thinning occurred in response to top‐to‐the‐west low‐angle normal shear during metamorphism. The same sense of normal shear is present in the Mesozoic members of the Austroalpine sequence west of the window. In these rocks, however, west directed shear occurred at low temperatures (<300°C) after the thermal peak of metamorphism. Structural data and the regional geometry imply that the western end of the Tauern Window is a low‐angle normal fault (Brenner Line) that juxtaposed brittlely deformed rocks of the Austroalpine nappes against more ductilely deformed rocks from the window. Biotite K/Ar and Rb/Sr data suggest a Miocene age for final movement on the Brenner Line. These data indicate a prolonged history of east‐west extension in the Eastern Alps that affected all crustal levels (early ductile thinning of lower crust followed by mid‐to‐upper‐crustal low‐angle normal faulting). A model is presented that relates the young east‐west extension to displacement transfer from dextral movement on the Periadriatic Lineament. In this model, the Brenner Line and, by analogy, the Simplon Line can be thought of as detachment faults that tectonically unroofed the Tauern Window and Lepontine thermal highs, respectively.

No abstract

No abstract

Ductile shearing in the core of the Tauern Window, Austria, transformed metagranodiorite into Si-undersaturated garnet-chlorite-staurolite schist at a depth of c. 35-40 km during the Alpine orogeny. Four distinct zones have been recognized extending from the wallrock into the centre of the shear zone: Zone I-unaltered metagranodiorite with subordinate amphibolite; Zone 11-biotite-white mica-garnet schist; Zone 111-biotite-phengite schist; Zone IV-quartz-absent, garnet-chlorite-staurolite schist with garnets up to 10 cm across. Whole-rock analyses show a dramatic decrease in SiO, from >65 wt% in Zone I to <35 wt% in Zone IV; Ca, Na, and Sr also decrease across the shear zone, whereas Al, Ti, Fe, Mg, P, Cr, Ni, Zn, and Rb all increase towards Zone IV. Mass-balance calculations indicate that shearing was accompanied by up to 60% volume loss near the centre of the shear zone. Comparison of the Tauern Window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in Mg, Fe, and Ti is typical for volume-loss shear zones, but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour.In the samples studied here, volume loss appears to have resulted from channellized fluid flow during shearing, producing time-integrated fluid fluxes of 210s cm3 cm-, in Zone IV. This large volume of, fluid may have originated, in part, from dehydration of flysch camed beneath the metagranodiorites during Eocene movement on the North Penninic subduction zone. Development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating.

No abstract

The Salmon River suture zone in west central Idaho juxtaposes volcanic arc rocks of the Wallowa terrane directly against cratonic North America. Detailed metamorphic studies along a 10 km traverse perpendicular to the suture indicate that the arc and two crystalline fragments thrust upon it each record different pressure-temperature (P-T) histories. From lowest to highest structural level: the Wallowa terrane shows only subgreenschist metamorphism, the Rapid River plate (RRP) records unroofing and cooling from ~ 8 kbar and 550øC to 6 kbar and 475ø-500 ø C, and the Pollock Mountain plate (PMP) shows evidence for polymetamorphism and records burial and heating paths to final equilibration conditions of 9-11 kbar and 600 ø-625 ø C. At-At hornblende ages combined with the P-T data suggest that currently exposed levels of the RRP and P MP were juxtaposed against one another at 15-20 km depth at or prior to 118 Ma, indicating that 10-20 km of uplift, and hence also the onset of collision-related metamorphism, occurred before ~ 118 Ma. Correlation of the metamorphic and age data with geometric constraints from the initial Sr 0.706 line and the dimensions of the RRP and PMP permit construction of large-scale retrodeformable sections of the west side of the suture from Late Jurassic through Late Cretaceous time. The abrupt nature of the Sr 0.706 line implies that the arc-continent boundary extends vertically through most of the crust, which requires sharp downwarping of the arc lithosphere in order to account for the P MP metamorphic data. Narrow zoned overgrowths on PMP garnets record this burial event and require initially rapid (_> 3 km/m.y.) uplift rates in order to be preserved. We suggest that the onset of rapid uplift resulted from the separation of the negatively buoyant lithospheric root from the downwarped arc, allowing buoyant rise of fragments of thickened crust. Detachment of the root is suggested to change the environment of crustal shortening from one in which footwalls of thrusts or shear zones sink to one in which Paper number 91TC02418. 0278-7407/92/91TC-02418510.00 hanging walls rise. This mechanism represents an alternative to cessation of shortening or onset of tectonic denudation as an explanation for the transition from burial to uplift of high-pressure metamorphic terrains. Subsequent uplift appears to have been slow and to have occurred in a hinged fashion such that mineral and whole rock ages decrease systematically towards the suture zone. The consumption of lithosphere during _> 40 km of shortening between two crustal blocks implies that the Salmon River suture is the trace of an intracontinental subduction zone. Burial and collision apparently began before about 130 Ma, and thus any precollision strike-slip faulting or tectonic escape of intervening terranes was likely accomplished in Jurassic and earliest Cretaceous time. and the craton [e.g., Davis et al., 1978]. In west central Idaho, however, these terranes, which include Mesozoic melange, marginal slope-andrise sequences, and perhaps even...