Basal Subduction Tectonic Erosion (STE), Butter Mélanges, and the Construction and Exhumation of HP-UHP Belts: The Alps Example and Some Comparisons

Abstract

Studies of high-pressure-ultrahigh-pressure (HP-UHP) belts stand to benefit from having a better idea of how subduction constructs them. Their exhumation, too, merits a closer look at epeirogenic processes. Progress in both these matters now appears possible and their relevance tested, using the Alps, the world's most comprehensively studied such belt, as the prime example. Its comparative youth means that less of the potentially pertinent surface detail has been lost by erosion or burial. Much evidence suggests that subduction downbend is actually not a flexure but is a seismically evident, escalator-like, through-plate step-faulting process. Especially where the subducting plate is younger than about 90 Ma, this gives it the property (STE) of incorporating upper-plate material in the step-angles, thus moving forward the position of the downbend surface that it has carved in the upper plate rapidly, and at a shallow angle for hundreds of kilometers. This yields a nominally "flat" interface profile, above which, at the shallow end of the system, the upper-plate crust will have been chilled and perhaps had its lower part removed, thus providing an origin for basement thrust sheets and (as subducted imbricate slices) those seen in HP-UHP belts. Seismology shows that "flat" interface profiles often pass through an inflection nearer the trench. This means that STE is "taking a second cut" and then disgorges the step-angle material along the succeeding flat part of the interface. When a STE-thinned margin imbricates, this diagnostic material appears as a fluid-overpressured tectonic mélange "buttered" (no shearing at the contact) onto the underside of each. Included floaters (up to >5 km in size) exhibit many lithologies, even slickensided, in an ex-oceanic, usually pelitic matrix that protected them from deformation. This "butter" is a focus of attention in HP-UHP belts, especially for its mafic-ultramafic floaters, while the water in its matrix seems important for reaction kinetics, for eventual epeirogenic action, and even to flux partial melting. Subduction of imbricate slices, already cooled by STE beneath them, and their successive lodgement beneath the "roof" and angle of the distant downbend profile carved by STE into the hanging wall, builds a subcreted triangular section extending far down the back wall, much further than hitherto thought possible. So this part exhumes the most, breaking through the "roof" above as a fault-line prone to strike-slip (Insubric, Møre-Trøndelag, Median Tectonic Line). In the Alps, and following extensive STE from the north, successive imbrication marked by Cretaceous flysch sequences transposed the paleogeographic order, so the Piemont ophiolites were from an oceanic-crusted northern forearc, not a separate ocean. Such transpositions cannot happen during collision; they require too much transport. During Eocene collision, the old block-and-basin structure in the European autochthon was epeirogenically rejuvenated (external massifs, Tauern window) by deep-crust heating and petrological density effects caused by the overthrusting, ultimately obstructing surface closure and causing back-thrusting. At an intermediate stage (early Oligocene) Adria, with the Austroalpine "roof" still attached, moved >200 km to WNW, creating the Western Alps as a tectonically distinct entity. Unroofing was caused by epeirogenic action at depth, not by "extensional collapse." Brief comparisons with six other HP-UHP belts—Kokchetav, Dabie-Sulu, Franciscan, Sanbagawa, Maksyutov, and the Western Gneiss Region—are encouraging. In the latter case, Norway and Greenland must have been closely juxtaposed by collision, not separated by the present shelves built shortly thereafter, their present epeirogenic characteristics being consistent with such construction. Exhumation of the assemblage, mainly attributable to self-reheating of the deepest parts, which in most cases may not even now be exposed, cannot begin until cooling by ongoing subduction stops (collision) or markedly slows. The resultant volume increase (and incipient melting?) at such depths may provide much of the required buoyancy, but also pushes the plates apart at depth and imposes late-stage extension upon the superincumbent material. To the extent that this volume increase is laterally constrained at depth, it must enhance exhumation rate and attenuate the steeply subducted slices. The subducted assemblage has been kept so cool by continuing subduction that not until this reheating becomes sufficiently established can zircons within it begin to provide a record. The subsolidus overgrowths they then develop record the progress of heating and cooling during exhumation; they do not even record the moment of collision and provide no insight as to the constructional history—a function reserved for low-closure-temperature techniques that record the chilling by STE. That history, in the case of the Alps, from the onset of STE to the onset of active exhumation, occupied a period of at least 70 m.y. and is unlikely to have taken much less in comparable orogens.