Age and geodymamic setting of Jurassic arc volcanism in southern coastal Peru

Abstract

The westem margin of the South American Plate is the Earth's longest lived active continental margin, and hence serves as the type locality for active margin orogenic processes. The Tertiary evolution of the margin has been studied in detail (Dewey and Lamb, 1992), whereas the Mesozoic and older history is still poorly constrained. Our study aims to reconstruct the late Triassic(?)- Jurassic evolution of the southern Peruvian margin and to constrain the age, chemistry, and palaeogeographic setting of volcanic-sedimentary arc sequences in the Arequipa Terrane. The Triassic-Jurassic transition in southern Peru is characterized by a cessation of continental extension (the "Mitu" rift) and renewed subduction related magmatism along the present-day coastline and inland region of the Arequipa massif, south of 12'S. Several continuous rock sections proximal to the coast of the Arequipa Terrane are interpreted to have originated in a continental are setting, which possibly commenced in the later Triassic (?) to Jurassic (Sempere et al., 2002). We speculate that the extensional geodynamic scenario which formed the Mitu Rift, may have terminated during renewed (orthogonal) subduction, giving rise to active margin magmatism. Understanding the timing and nature of Jurassic arc magmatism will also shed light on the reasons for the lack of Jurassic rocks in the 'craton-free zone', north of 12'S (north of Lima). Our project commenced with a detailed investigation of the Jurassic sedimentary and volcanic rocks along the Peruvian coastline between 15'S and 18.5'S The distribution and depositional ages of Jurassic are sequences are only tentatively constrained by inaccurate radiometric age data, and sparse fossil data (Martinez et al, unpublished) We will derive an accurate chronestratigraphic framework for the are sequences using the UPb and Ar Ar dating techniques. Furthermore, a detailed description of the facies distribution of both volcanic and sedimentary rocks, combined with a geochemical study, will allow us to distinguish between back-ure and continental arc volcanism, and hence constrain the tectonic and paleogeographic evolution of the region.