Isolated epeirogenic mountains in supra-subduction systems, the case of north patagonian massif, Argentina

Abstract

Supra-sbduction mountain belt systems can have fore-, intra- and back arc expressions that include thin or thick skinned thrust-and-fold belts, and titled fault blocks.The extent to wich the supra-subduction orogenic system spreads over the continental plate, is mainly controlled (in a simplified system) by the plates coupling systems and by relative buoyancyand rheology. The most important subducted plate constraints are buoyancy-related. Nevertheless, given the uniform composition of oceanic plates, the buoyancy differences will depend on the total thickness of the ocean plate, lithosphere thickness, subduction rate and convergence angle. On the other hand, the continental plate constraints are mainly related to rheological parameters, and depend on the crustal and lithosphere thickness profile, and the lateral thermal heterogeneity due to the different geologic histories of accreted terrains. This paper focuses on lower crust granulites xenoliths, and considers the end case of a supra-subduction system with a laterally heterogeneous continental crust, consisting on a young thin (hot) back arc continental crust that includes a thick (cold) block of older continental crust (Northpatagonian Massif). It further considers the buoyant behaviourof old-thick-cold block with respect to the surrounding back-arc crust as aresponse the subduction driven upper mantle dynamics, during upper Eocene-lower Oligocene. Surface and sub-surface geology show striking differences between the Northpatagonian Massif and its surrounding crust. The masif is surrounding by basis of different ages (Fig.1), such as the Neuquén basin to the North-west, the Colorado basin to the North-east; the Cañadón Asfalto basin to the South an the Ñirihuau basin to the West. These basins show deformation, by thrusting and folding (thick and thin skinned, titled fault blocks). Instead, the massif consists of early and late Paleozoic metamorphic complexes intruded by Ordovician, Devonian, Carboniferous, Permian and Triassic plutonic rocks (Pankhurst et al 2006), large Triassic and Jurassic volcanic rhyolitic complexes (Rapela et al 2005), few Triassic sediments and a thin sedimentary cover that overlay an extended planation surface developed from the late Jurassic to early Cretaceous time, that include Jurassic, upper Cretaceous and Tertiary sediments (the Maastrichtian-Danian sediments are of marine nature). Most of south-eastern side of the Massif is covered by Oligocene plateau basalts that were erupted from the top of the Massif onto the surroundinglower land. Deformation in the massif is restricted to faulting ( mostly strike and slip), no titled block faults, and the thin sedimentary cover shows no folding. Nevertheless, the marine Maastrichtian-Danian sedimentsstand sub-horizontal with no deformation at1200 meters on the top of the massif with respect to the 400-500 meters that these same sediments stand deformed by faulting and folding at the surrounding basins. The differencial uplift history of the Massif with respect to the surrounding crust and the Andes, is constrained by: a) the Maastrichtian-Danian marine sediments, b) the Oligocene plateau basalts, c) the Eocene-Oligocene-early Miocene Ñirihuau basin and its western side (it separates the Massif from the Andes), and d) the upper mantle and lower crust xenoliths. By the end of the Cretaceous time, the Northpatagonian Massif was a planation surface below sea level at the back arc of the Andes, while the Farallón plate was subducting benesth the South American plate. A drastic change occured in the Paleogenereaching its climax at the Pligocene. The Andes at this latitud were flooded with the development of the fore arc, arc and back arc Ñirihuau basin, under an extensional regime (Cazau et al 1989), and the Northpatagonian Massif wassubject to uplift (with no internal deformation), so fast that by early Oligocene, the Somon Cura basaltic plateau lavas spilled from the top of the Massif at 1200 meters flowed down to the surroundig country rock at 400-500 meters. Upper mantle xenoliths along Patagonia from Santa Cruz to Neuquén show minwralogy and Os-Re TDM Mantle Depletion Ages that can be considered to be lithosphere formation ages that suggesta young thin back-arc lithosphere (spinel facies) formed from a convecting mantle that surroundolder and thick (garnet facies) depleted lithospheres beneath the Northpatagonian and Deseado Massif (Schilling et al 2008, Schilling y Tassara, 2008). Our work concentrates on the lower crust granulites xenoliths from Paso de Indios (Fig. 1) and shows the granulites from the lower crust around the massif, are garnet free, and develops olivine coronas along contacts of orthopiroxene and clinopyroxene with respect to plagioclase, suggesting a thin crust and a clockwise TP evolution trend. We have dated the cristallization ages of zircon in these granulites by U-Pb geochonology using SHRIMP (Sensitive High Resolution Ion Microprobe Technology) at Beijin. The preliminary results indicates ages of 200 to 154 MA (Castro and Aragón unpublished). This new data suggest that the growth processes of the lower crust around the Massif are also quite recent.The different mountain building processes of vertical movement of the Northpatagonian Massif with respect to fold and trust belts and tilted fault blocks of the surrounding crust can be modelled (with the constraint that the Massif uplift y coeval to the Oligocene extentional regime), by three different processes: a) regional uplift by Hot spot, b) mantle upwelling by Slab roll back, c) Isostatic rebound by detachment of the massif rootsfrom the subducted plate by the development of a slab window. Uplift by hot spot emplacement is related to the removal of lithosphere by the hot spot and the isostatic reequilibration of the remaining lithosphere. The relative small volumes of magmatism of the Somon Cura basalt plateau, the lack of a rifting stage, and the fact that uplift is concentreated on the massif rather than a ragional scale seem to be restrictive conditions for this hypotesis. Uplift by slab roll back, is related to a subduction rate increase and the consequent subduction angle increase, with the development of mantle upwelling at the subducted plate front. Paleomagnetic studies for this segment (42ºS) of the South America plateare not conclusive for convergence rates (Pardo Casas and Molnar 1987), but indicate low angle convergence for the Farallón-South America plates, and the presence of the triple joint Farrallón-Aluk-South America. Oligocene magmatism shows a strong OIB signature in the Coastal belt concluding a slab window development (Muñoz et al, 2000). This OIB signature is also present to the east, well within the continental crust , in the El Maiten Volcanic Belt and as far as the Somun Cura basaltic plateau. Uplift by detachment of the roots of the massif from the subducted slab by the development of a slab window, is supported by the OIB signature of the Oligocene magmatism all across the South American plate at this latitude, and the presence of the triple joint Farrallón-Aluk-South America plates during the Paleogene at this latitude. The different rheological behaviour of the thick-old lithosphere of the massif with respect with the thin-young surrounding lithosphere also agree with the hypotesis of an isotopic uplift of the massif as it detached from the subducted plate.