Along-strike variation in crustal shortening and kinematic evolution of the base of a retroarc fold-and-thrust belt: Magallanes, Chile 53°S-54°S

Abstract

The Late Cretaceous closure of the Late Jurassic-Early Cretaceous Rocas Verdes basin resulted in the development of the Patagonian fold-and-thrust belt and Magallanes foreland basin between 50°S and 54.5°S. New geologic maps, structural data, and two retrodeformed, line-balanced cross sections from the Magallanes region of Chile (53°S-54°S) constrain the kinematic evolution and along-strike correlations of deformation that occurred at the base of the fold-and-thrust belt near the brittle-ductile transition. The stratigraphic architecture of the predecessor basin controlled the position of regional décollement levels. During the initial stage of closure (Albian-Campanian), the floor of the Rocas Verdes basin was imbricated and thrust onto the continental margin to form a regional décollement within Jurassic-Lower Cretaceous shale. Continued shortening resulted in the deepening of the décollement to a ductile shear zone that formed <1 km below the basement-cover contact. Ductile polyphase folding accommodated basement shortening and was mechanically decoupled from the overlying fold-and-thrust belt below the lower décollement. Ramps that cut Jurassic volcanic deposits linked the lower and upper décollements and transferred displacement into the nascent foreland basin. A second stage of shortening, characterized by basement-involved reverse faults that cut the early décollements, reflects complete closure of the Rocas Verdes basin by the Maastrichtian-Eocene. Coniacian to Eocene shortening estimates indicate a northwestsoutheast increase from 26% to 37% over 100 km along strike and are consistent with regional models of the Patagonian fold-andthrust belt. The results provide an important example of the kinematic evolution of the base of a retroarc fold-and-thrust belt in an "Andean-style" orogen.

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

“…This kind of response of preexisting and new normal faults upon flexural loading in foreland basins has been documented in many other similar geological settings [ Bradley and Kidd , ; Agarwal et al ., ; Londoño and Lorenzo , ; von Hartmann et al ., ], including the South Malvinas basin which is laterally linked to the Austral‐Magallanes basin [ Platt and Philip , ; Bry et al ., ]. It is also interesting to point out, regarding interpretations of inversion of normal faults published elsewhere [ Betka et al ., ], that only one possible inverted normal fault, with a displacement of a few hundred meters, was recognized within our data set (section C‐C′), suggesting no major influence of inversion tectonics during shortening in the Fuegian thrust‐fold belt.…”

Using detrital zircon U‐Pb ages to calculate Late Cretaceous sedimentation rates in the Magallanes‐Austral basin, Patagonia

“…Closure of the Patagonian sector of the Rocas Verdes basin began in Early Cretaceous time (Fildani & Hessler, ; Malkowski et al ., ; Betka et al ., ) as subduction began along the western margin of South America. This caused obduction of the Sarmiento ophiolite complex and growth of the Andean volcanic arc and fold‐thrust belt (Wilson, ; Fildani & Hessler, ; Fildani et al ., ; Calderón et al ., ; Torres‐Carbonell & Dimieri, ).…”

Tectonic controls on the Maastrichtian-Danian transgression in the Magallanes-Austral foreland basin (Chile): Implications for the growth of the Southern Patagonian Andes

“…Shoaling during the transition between the Tres Pasos-Fuentes to Dorotea-Rocallosa formations within a normal regressive context can be ascribed to increases in sedimentation rates that outpaced accommodation (by subsidence), the aftermath of accentuated erosion in the hinterland. The results of structural studies suggest an orogen-wide basement-involved faulting event (thick-skinned) manifested by crustal basement shortening and internal deformation (e.g., Fosdick et al, 2011;Betka et al, 2015). Altogether, the fold-thrust-belt evolution and sedimentary records in the basin suggest a phase of mainly internal shortening and thickening of the wedge, with limited forelandward advance of the deformation front (Fig.…”

“…In the Ultima Esperanza Province, it is inferred by the Tenerife thrusting (with a basal décollement on the argillaceous Zapata Formation) between 74-27 Ma (Fosdick et al, 2011), which continued exhuming the Tobífera Formation and incorporated Upper Cretaceous foredeep deposits into the fold-thrust belt. In the Magallanes Province, Betka et al (2015) suggested a phase of out-of-sequence thrusting and basement-involved reverse faulting beginning in the Maastrichtian and extending to the Paleogene. The deformation and cooling ages of ophiolitic blocks in the hinterland around 74-72 Ma (Rapalini et al, 2008) together with progradation and depositional system shoaling (Tres Pasos and Dorotea formations) (Covault et al, 2009;Romans et al, 2009;Schwartz and Graham, 2015;Schwartz et al, 2017;Gutiérrez et al, 2017) may also reflect deformation in the fold-thrust belt.…”

“…12b), accompanied by considerable erosion of the wedge at least during the Maastrichtian-Danian transition (Sial et al, 2001). Upper Cretaceous foredeep deposits are incorporated into the fold-thrust belt in the Ultima Esperanza and Magallanes Provinces (e.g., Tenerife Thrust of Fosdick et al, 2011;Betka et al, 2015) while thrusting of Maastrichtian-Danian rocks with brittle-ductile deformation under Journal Pre-proof submarine conditions occurred in Tierra del Fuego (e.g., D1 structures of Torres Carbonell and Olivero, 2019). The forelandward transfer of deformation is evidenced in the sedimentary record by the upward replacement of basement-derived detritus by chert and sedimentary lithics throughout the Maastrichtian-Paleocene strata, portraying a recycled-orogen trend in the Chorrillo Chico Formation (Fig.…”