A new detrital mica 40Ar/39Ar dating approach for provenance and exhumation of the Eastern Alps

Detrital thermochronology can be used as a tool to quantitatively constrain exhumation rates and its spatial variability from active mountain belts. Commonly used methods for this purpose assume a steady state relationship between tectonic uplift and erosion. However, this assumption does not account for the transitory response of a dynamic orogenic system to changes in the boundary conditions. We propose a different approach that uses the observed detrital age distributions as “markers” of the past exhumation and of the present-day erosion and mixing occurring in a river system. In this paper, we present new 40Ar/39Ar biotite and white mica age distributions for 19 modern river sands from the Eastern Alps north of the Periadriatic line. The results present three main clusters of ages at ~0.5–50, ~60–120, and ~250–350 Ma that record the main orogenic phases in this sector of the Alps. We have applied two numerical methods to the cooling ages to (a) linearly compute the spatial variability of the relative present-day erosion of a set of four detrital mineral samples from drainage basins along the Inn River and (b) quantify the rates of the cooling and erosion in the Tauern Window during Paleocene-Miocene time of the Alpine orogeny. Our results suggest a 0.34–0.84 mm/yr range of exhumation rates for the Tauern Window since the Miocene. Our estimates of exhumation rates of the western Tauern Window are higher than those for the eastern Tauern Window, which is consistent with the previous studies

Basement shear zones development and shortening kinematics in the Ecrins Massif, Western Alps

International audienceIn the Western Alps, Oligocene shortening affected a highly heterogeneous European crust with Liassic half-grabens inherited from the rifting stage and the finite deformation was strongly partitioned between the rigid basement and the weak Jurassic sediments. In the Ecrins massif (Oisans, External Crystalline Massifs, ECM), where the half-grabens are best exposed and preserved, compressional structures within the basement have to date never been described in details. This massif was shortened under moderate metamorphic conditions (250-350°C and 0.1-0.5GPa) and the rheological contrast between the basement and the cover is strong. While the sediments are intensely folded, the cover-basement interface presents apparent open folds underlined by the lower Triassic layers. The basement itself shows a more localised deformation along several brittle-ductile shear zones. We here report new evidences of such brittle-ductile shear zones characterized by anastomosed phyllonitic shear bands rich in phengite and quartz, a low strength material where strain has localized. New detailed maps of reverse shear zones, faults, schistosity and stretching lineations in both the cover and the basement are provided. We show that the Oligocene crustal shortening was mainly E-W to ENE-WSW. Local N-S to NW-SE shortening occurred and was limited to the eastern border of the Ecrins massif, around the Penninic Frontal Thrust, which likely was a sinistral transpressive structure in this area. Finally, new balanced cross-sections show that these basement shear zones have accommodated more than 50% of the Oligocene crustal shortening

Inherited structural controls on fault geometry, architecture and hydrothermal activity: an example from Grimsel Pass, Switzerland

Exhumed faults hosting hydrothermal systems provide direct insight into relationships between faulting and fluid flow, which in turn are valuable for making hydrogeological predictions in blind settings. The Grimsel Breccia Fault (Aar massif, Central Swiss Alps) is a late Neogene, exhumed dextral strike-slip fault with a maximum displacement of 25–45 m, and is associated with both fossil and active hydrothermal circulation. We mapped the fault system and modelled it in three dimensions, using the distinctive hydrothermal mineralisation as well as active thermal fluid discharge (the highest elevation documented in the Alps) to reveal the structural controls on fluid pathway extent and morphology. With progressive uplift and cooling, brittle deformation inherited the mylonitic shear zone network at Grimsel Pass; preconditioning fault geometry into segmented brittle reactivations of ductile shear zones and brittle inter-shear zone linkages. We describe ‘pipe’-like, vertically oriented fluid pathways: (1) within brittle fault linkage zones and (2) through alongstrike- restricted segments of formerly ductile shear zones reactivated by brittle deformation. In both cases, low-permeability mylonitic shear zones that escaped brittle reactivation provide important hydraulic seals. These observations show that fluid flow along brittle fault planes is not planar, but rather highly channelised into sub-vertical flow domains, with important implications for the exploration and exploitation of geothermal energy