PHANEROZOIC BRITTLE TECTONICS IN THE SOUTH AMERICAN CONTINENTAL PLATFORM, SOUTHEAST BRAZIL: NEW INSIGHTS FROM FISSION TRACK STUDIES ON APATITE IN REACTIVATED FAULT ZONES

Abstract

Apatite Fission Track Method (FTM) studies were performed on samples coming from two different fault domains in the Precambrian basement of southeast Brazil in order to evaluate subsequent Phanerozoic tectonic movements in that part of the South American Platform. The samples studied were collected along brittle faults in the Mantiqueira mountain range and in the Jundiai upland plain (Braganca Paulista and Extrema), approximately 100 km northeast of Sao Paulo, Brazil. The results of paleostress analysis, as well as the presence of as pseudotachylyte material in the reactivated fault zones, indicates a rapid strain rate and high frictional temperature along these faults. The recognition of deformation related to this brittle tectonic regime is of key importance for the reconstruction of Paleozoic and Mesozoic tectonic evolution of the South American Platform. The causal effects of these tectonic readjustments of cratonic rocks during the breakup of West-Gondwana during the Cretaceous and also in younger geological history, are some of the key parameters for understanding thePhanerozoic evolution of the Mantiqueira mountain range. Our FTM data shows the oldest regional thermal histories are recorded in the Jundiai upland plain beginning in the Upper Triassic (~190 Ma) at 50 o C, indicating a rapid cooling that is coincident with the tectonic subsidence of the Parana Basin and, probably, the uplift and preservation of Gondwana surface. These data also show slow linear heating (between 50 to 90 o C) of the southeastern Brazilian margin up to the Lower Cretaceous (~120 Ma). This phenomenon could either be related to migration of the Trindade plume, or extensional/compressional movements. At ~120 Ma a structural inversion occurred and the previous slow linear heating was replaced by slow linear cooling (from 90 to 25 o C) that has persisted up to the present time. Locally, in the younger fault domain, in the Mantiqueira Range near Extrema, fission tracks began to be recorded at ~ 110 Ma ago and were, since then, submitted to a slow linear cooling very similar to that observed for two country rock samples and the samples collected in the older faulted rocks (Braganca Paulista). Then considering the thermal histories of all the samples studied here, there is an agreement about a pervasive thermal event taking place ~110 Ma ago followed by a linear cooling up to the present, related to regional erosion. These processes are considered to be related to the South-Atlantic opening, with widespread extensional tectonic movements along the southeastern Brazilian coast.