ALONG-STRIKE CHANGE IN STRUCTURAL STYLE AND TECTONIC SHORTENING DURING THE DEVELOPMENT OF THE PATAGONIAN RETROARC FOLD-THRUST BELT, SOUTHERN ANDES

The Late Cretaceous closure and inversion of the Late Jurassic age Rocas Verdes back-arc basin (RVB) defines the onset of the Andean orogeny and the development of the Patagonian retroarc fold-thrust belt (FTB) between 50°-54.5° S. New maps and line-balanced cross-sections from three transects across the FTB within a ~150 km2 area near Seno Otway (NW) and Peninsula Brunswick (SE) show a NW-to-SE change from thin- to thick-skinned structural styles and magnitudes of shortening. The nearly 90° bend in the trend of the orogen is attributed to along-strike variation in tectonic shortening within the FTB.

In all hinterland localities chlorite-grade pelitic schists (Paleozoic basement) are imbricated with massive basalt, gabbro, chert and quartzite of the RVB floor, mudstone representing the basin fill (Zapata Fm.), and Jurassic rift volcanics (Tobifera Fm.) (D₁). Toward the foreland several imbricated, foreland-vergent thrust sheets place basement, RVB, and Tobifera Fm. rocks above the Zapata Fm. (D₂). In the footwall, tight, NW-plunging folds that are overturned-to-the north thicken the Zapata Fm. About 20 km along the section toward the northeast Late Cretaceous (~88 Ma) strata of the Magallanes foreland basin overlay the Zapata Fm and are less deformed than the RVB rocks. One basement-involved reverse fault places basement, Tobifera Fm. and RVB rocks above the foreland basin strata (D₃). This fault can be traced for 100 km along strike and probably reflects the inversion of an inherited normal fault. Northeast of this fault near Seno Otway (NW), foreland basin strata are imbricated by thrust faults that sole into a basal detachment. In contrast, in the SE, reverse faults that cut foreland basin strata are steeply dipping (>60°) and are interpreted to reactive Jurassic normal faults. Shortening estimates increase from 15% to 25% from NW to SE. In addition to a shift from thin- to thick-skinned deformation through time that is observed elsewhere in the Patagonian FTB, we document contemporaneous along-strike variability suggesting the kinematics of the FTB are also controlled by the inherited geometry of the RVB. Mechanisms for along-strike kinematic shifts include variation in the location of RVB rift uplifts, thickness of RVB strata, density of inherited normal faults, or the mechanics of forming the Patagonian orocline.