Tectonic control on the evolution of Maastrichtian-Paleogene synorogenic sequences of the Fuegian thrust fold belt, Argentina

TORRES CARBONELL, PABLO J.; OLIVERO, EDUARDO B.; DIMIERI, LUIS V.

Mar del Plata, Argentina

Congreso; GEOSUR 2010 International Geological Congress on the Southern Hemisphere; 2010

Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Italia; Instituto de Geofísica "Daniel A. Valencio", Argentina; Dipartamento di Scienze Geologiche, Università di Urbino, Italia

A geological study of the Atlantic coast of Tierra del Fuego between the punta Gruesa (54º 21’ S; 66° 38.5’ W) and the río Policarpo (54º 39’ S; 65º 30’ W) (Figure 1), and other sectors of the Fuegian Andes, allowed us to define the stratigraphy and sedimentology of synorogenic successions from the Austral basin, and their genetic relations with the geometry and kinematics of the Fuegian thrust fold belt.We define seven sequences between the Maastrichtian and the Miocene, bounded by syntectonic unconformities: Maastrichtian-Danian (180 to 800 m), Paleocene (50 to 370 m), Ypresian (450 to 650 m), Lutetian (80 m), upper Lutetian-Priabonian (1200 m), Oligocene (1600 to 200 m) and uppermost Oligocene-Miocene (200 m). These successions are composed of marine sedimentites, mostly deposited by gravity flows below the storm-wave base. The paleocurrent directions and the petrography indicate sediment provenance areas in the volcanic arc along the Pacific margin of the Andes, and in the core of the Fuegian Andes, the former dominant between the Maastrichtian and the Lutetian, and the latter since the late Lutetian.The detailed mapping of the Fuegian thrust fold belt structures allowed us to construct two balanced cross-sections that depict their subsurface geometries. The southern cross-section shows main décollements at the base of the Cretaceous and above the Maastrichtian-Danian, and thrust-related folding of the Cretaceous-Miocene sedimentary cover. The total shortening in that cross-section is of 41.8 km. The northern cross-section has a main décollement at the base of the Paleocene-Ypresian rocks, and minor ones in Bartonian-Priabonian levels. This section shows thrust-related folding of the Paleocene-Miocene sedimentary cover, with a total shortening of 17.8 km.By combining both cross-sections, estimating their location before the development of the Neogene Fagnano transform system (Figure 2), six contractional stages are defined in relation to the evolution of the thrust-fold belt, age-constrained by the biostratigraphy of the synorogenic successions recognized: Df1 (Danian) with low percentages of layer parallel shortening in the foreland; Df2 (Ypresian) with development of thrust-related folding in a forward thrust-sequence and shortenings between 7 and 18.8 km (21%); Df3 (Lutetian) with development of out-of-sequence structures and a shortening of 6.6 km (7.3%); Df4 and Df5 (Oligocene) related to backthrusting with a shortening of 13.6 km (15.2%); Df6LC and Df6PG (latest Oligocene-Miocene), the last contractional stages recorded in the eastern Fuegian thrust-fold belt, comprising thrust-related folding within the belt with a shortening of 2.8 km (3.1%), and in the leading edge of deformation with a shortening of 10.5 km (11.6%).The thrust-fold belt reveals an episodic evolution that can be analyzed in terms of the Coulomb wedge theory, obtaining a model with three main stages: a critical wedge during the Danian to Ypresian (stages Df1 and Df2) with forward directed thrusting and a progressively diminishing taper angle, a subcritical wedge between the Lutetian and the ‘mid’ Oligocene (Df3 a Df5) with development of out-of-sequence structures and backthrusts and a tendence to attain a critical taper, and a critical wedge during the latest Oligocene to early Miocene (Df6PG) with foreland displacement of the thrust wedge and propagation of the basal décollement to shallower levels.The Austral foreland basin system evolved as a single depocenter (foredeep) during deposition of the Maastrichtian-Danian, Paleocene and Ypresian sequences, the latter posibly also accumulated in depocenters atop active structures (wedge-top). Between the Lutetian and the Miocene, sedimentation occured in two depocentres: the wedge-top and the foredeep. During the Oligocene this segmentation of the basin resulted in a thicker succession within the wedge-top, which distinguishes it from classic tectonostratigraphic models. During the Miocene, cessation of contractional deformation in the thrust-fold belt was simultaneous with the development of the last foredeep of the basin.