Evolución tectono-estratigráfica durante el triásico superior de la región de Los Menucos, macizo norpatagónico nor-occidental, provincia de Río Negro

Abstract

Two sets of volcano-sedimentary rocks of different ages, one of the Permian-Early Triassic age corresponding to the Los Menucos Complex and another of probable Late Triassic age, were identified in the Los Menucos region, in the northern sector of the North Patagonian Massif. The lithological, stratigraphical and sedimentological characteristics of the provisionally assigned to the Upper Triassic rocks that crop out in the vicinity of Llancaqueo farm, 6 km towards the NE of Los Menucos, will be first presented. There is a volcanic and volcaniclastic succession with at least 368 m thick, which overlies an angular and erosive unconformity developed on the Los Menucos Complex. The lower section of the succession is characterized by deposits corresponding to alluvial fan systems, high-energy braided rivers and sinuous rivers with floodplains, while the upper section is represented by a syn-sedimentary magmatism composed of mesosilicic to basic lava flows associated with subvolcanic bodies, and riodacitic ignimbrites. The informal denomination of sucesión volcanosedimentaria Llancaqueo was proposed to separate the volcano-sedimentary sequence of the Llancaqueo-type area from the rocks belonging to the Los Menucos Complex. In a regional context, the possible Upper Triassic age and the lithological characteristics of the sucesión volcano-sedimentaria Llancaqueo, suggest the search for correlations with the pyroclastic, volcanic and epiclastic sequences belonging to the Precuyano Cycle, or lateral equivalent within the Neuquén Basin. Second, the main structures that affect the Llancaqueo volcano-sedimentary succession in the area surrounding the Llancaqueo farm and in the areas of La Laja and La Mina hills, in the vicinity of Los Menucos, were characterized. Three first and second-order fault domains were recognized, with strikes: 1) E-W with WNW-ESE and ENE-WSW variations, sub-vertical and mostly dextral or, to a lesser extent, leftlateral kinematics; 2) NE-SW to N-S and dextral or dextral-normal or occasionally sinestral kinematics and 3) NW-SE to WNW-ESE, high angle and sinestral or sinestral-normal kinematics, or low angle and reverse motion. In addition, meso and megascopic folds trending NW-SE to WNW-ESE and NE-SW are developed both in the rocks of the Los Menucos Complex and in the Llancaqueo volcano-sedimentary succession. The integration of the geometry of the faults and folds at their different scales and the kinematic analysis of the major faults from the study of minor faults, allowed establishing two successive deformation phases (D1 and D2). D1, of late triassic - early jurassic age, would be linked to a transtensive regime with NNE-SSO to NE-SW extension directions and WNW-ESE to NW-SE shortening. Within the framework of this phase and actively controlled by the structure, the basin containing the Llancaqueo deposits would have been developed. On the other hand, D2, of possible Late Jurassic - Early Cretaceous age, would have a transpressive character related to a general NE-SW shortening and would have generated reactivations of the previously generated faults. In the regional context, the stratigraphic and structural characteristics of the study area and the northwestern sector of the Northpatagonian Massif, in general, are very similar to the tectonics described in the Neuquén basin for Triassic-Cretaceous times. In this sense, both in the Neuquén basin and in the area of the north-western Northpatagonian Massif, a generalized crustal extension regime in a NE-SW direction would have dominated, in the first instance, during the Late Triassic – Early Jurassic, equivalent to what is shown here is proposed as deformation phase D1. Then, during the Upper Jurassic – Lower Cretaceous, a transpressive deformation event would have occurred, comparable to the D2 deformation phase (proposed in this thesis) that generated the reactivation of the previously formed structures.