The Steinmann Trinity revisited: mantle exhumation and magmatism along an ocean-continent transition: the Platta nappe, eastern Switzerland

Abstract

The close association of serpentinites, basalts and radiolarites, later known as the Steinmann Trinity, was clearly described by Steinmann from the south Pennine Arosa zone and its southern prolongation, the Platta nappe of the eastern Swiss Alps. This classical 'ophiolite' is distinctly different from typical fast-spreading ridge associations and can be compared with the transitional crust occurring along non-volcanic passive continental margins in present-day oceans. It includes serpentinized peridotites that we interpret as subcontinental mantle rocks, which were exhumed along low-angle detachment faults and locally overlain by extensional allochthons of continental crust, minor gabbroic intrusions, tholeiitic pillow lavas and flows and a succession of oceanic sediments. The serpentinized peridotites record deformation at falling temperatures during extension leading to final exposure of the mantle rocks at the sea floor and their inclusion in tectonosedimentary breccias (ophicalcites). Field relationships, and mineral-chemical and radiometric data show that the gabbros intruded already serpentinized mantle rocks at shallow depth 161Ma ago. They are Mg gabbros, Fe gabbros and Fe-Ti gabbros, cut by dioritic pegmatoid veins and albitite dykes, which originated by differentiation from the same parental magma. All gabbros show the same metamorphic evolution, i.e. intrusion at relatively low pressure, oceanic hydration at elevated temperature and a subsequent static oceanic alteration. The pillow lavas stratigraphically overlie the exhumed mantle rocks and the tectono-sedimentary breccias related to the exhumation of both mantle rocks and gabbros. However, both gabbros and pillow basalts are characterized by eNd values typical for an asthenospheric mid-ocean ridge-type source of the melts. They may be the products of a steady process that combined extensional deformation with magma generation and emplacement. They appear to document the onset of sea-floor spreading across an exhumed subcontinental mantle during the earliest phases of a slow-spreading ridge.

Unravelling the interaction between tectonic and sedimentary processes during lithospheric thinning in the Alpine Tethys margins

“…Froitzheim and Eberli 1990;Reston et al 2004);and; (3) mantle unroofing associated with serpentinization (e.g. Pickup et al 1996;Skelton and Valley 2000;Desmurs et al 2001). Based on this close link between onshore and offshore studies, new questions emerged and alternative models on the mechanisms leading to lithospheric thinning have been proposed.…”

Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study

“…Clasts include fault rocks (cataclastic/altered basement and indurated black gouges), continental basement rocks, pre-rift sediments and, in the areas floored by exhumed mantle lithosphere, even mantle rocks (Figs. 7e and f; Desmurs et al, 2001). Clast composition is generally closely related to the lithological composition of the neighboring exhumed basement.…”

“…Further away from the allochthons, clasts are sourced from footwall lithologies including fault rocks (cataclastic basement, black gouges), continental or mantle basement rocks (Figs. 7e and f; Desmurs et al, 2001). Downsection, these breccias become progressively more massive and less polymictic, grading into the so-called "tectono-sedimentary breccia layer" (Manatschal et al, 2006).…”

Reconstructing the Alps–Carpathians–Dinarides as a key to understanding switches in subduction polarity, slab gaps and surface motion

“…Hyper-extended, magma-poor margins and oceancontinent transitions like those preserved in the Alps were probably a general feature of Alpine Tethys in Cenozoic time and were characterized by thick, depleted subcontinental mantle underlying thin extensional allochthons of continental crust (Lemoine et al 1987;Desmurs et al 2001). The neutral to negative buoyancy of such old transitional crust (70-90 My old at the onset of Alpine Tethyan subduction) perhaps explains why continental lithosphere makes up almost half of the total subducted material in the Alpine-Mediterranean domain as estimated by comparing plate reconstructions with teleseismic tomographic images down to the mantle transitional zone (Handy et al 2010).…”