Geometry and kinematics of the Fuegian thrust-fold belt, southernmost Andes

Abstract

Published balanced structural interpretations of the Fuegian thrust‐fold belt (southernmost Andes) fail to address aspects such as basement shortening in the thrust‐fold belt or its structural connection with the central belt of the orogen. We tackle this deficiency by constructing four serial balanced cross sections based on surface geologic information, 2‐D reflection seismic lines, and well logs, which depict the first‐order structural geometry and kinematics from the thrust front to the internal thrust‐fold belt. The interpretation reveals three main tectonostratigraphic packages separated by major detachments: basement (Paleozoic‐Upper Jurassic), Cretaceous, and Paleogene, all variably involved in thrusting along strike. Minimum shortening in the Paleogene cover ranges between 16% and 43%, and significant style changes are observed across and along the thrust wedge. We show that basement thrust sheets have a major role in the construction of the thrust‐fold belt; some of them were emplaced below the thrust front during late Oligocene or early Miocene times. The kinematic evolution of the thrust‐fold belt indicates that the active detachments depended on the existence of high pore pressure to propagate forward. Our model shows that this pore pressure buildup was controlled by overburden, mean rock impermeability, and strain rate; the variable interaction among these factors controlled the propagation and style of the thrust wedge.

Stress Field Evolution of the Southernmost Andean Cordillera From Paleostress Analysis (Argentine Tierra del Fuego)

“…It is difficult to confirm the relationship between tensors and major structures in the field, given that the folds in the thrust‐fold belt are of large wavelengths (a few kilometers), thus exceeding the scale to which one could constrain timing of the small scale fractures studied in our work. However, we can argue that there are no major extensional tectonic features of an age pertaining to the late Cretaceous‐early Miocene evolution of the thrust‐fold belt (e.g., Torres Carbonell & Dimieri, ; Torres Carbonell, Rodríguez Arias, & Atencio, ). Therefore, it is more probable that the tensional tensors formed after the contractional structures, or even simultaneously with them, as explained in (2) below.…”

“…The FTFB propagation toward the foreland during the Late Cretaceous to early Neogene was hindered the buttressing effect of a notable basement promontory called the Río Chico Arch (Torres Carbonell et al, ; Torres Carbonell, Rodríguez Arias, & Atencio, ). This promontory was located in the NE border of the Rocas Verdes basin and was inherited from the Late Jurassic rifting stage (Biddle et al, ; Yrigoyen, ).…”

“…The effect of the interaction between the Río Chico Arch and the FTFB propagation was registered in the stress field variation observed in the studied area (Figure b): the western area of the study zone shows a NE‐SW dominant orientation of the σ 1 , and in the northern sector of Mitre Peninsula, coinciding with the southern border of the Rio Chico Arch, the stress tensor obtained from the fault population analysis shows a main NW‐SE orientation for the average maximum compressive direction (Figure b). Deformation in the FTFB continued until the Miocene through the northward propagation of thrust sheets that molded the Río Chico promontory (Ghiglione et al, ; Ponce et al, ; Torres Carbonell, Rodríguez Arias, & Atencio, ).…”

“…The collision gave rise to stacking and duplexing in the upper crust connected to thin‐skin shortening along the FTFB, deforming the sedimentary cover toward the foreland (e.g., Torres Carbonell & Dimieri, ; Figure c). Shales and fine‐grained sedimentary successions of the back‐arc basin act as detachment levels active during the establishment of the Austral‐Magallanes foreland basin (Torres Carbonell, Rodríguez Arias, & Atencio, , and references therein).…”

“…The FTFB involves Upper Jurassic‐Lower Cretaceous sedimentary rocks deposited in the Rocas Verdes back‐arc basin and in the Austral‐Magallanes foreland basin during the Late Cretaceous‐Neogene (e.g., Calderón et al, ). Sedimentary rocks were detached and imbricated along NE‐ to N‐vergent thrusts (Torres Carbonell, Rodríguez Arias, & Atencio, , and references therein; Figure c). A major left‐lateral strike‐slip fault system, the Magallanes‐Fagnano fault deforms the FTFB (Klepeis, ; Tassone et al, ; Torres Carbonell et al, ; Figure b).…”