Early exhumation of the Frontal Cordillera (Southern Central Andes) and implications for Andean mountain-building at ~33.5°S

Abstract

The Andes are the modern active example of a Cordilleran-type orogen, with mountain-building and crustal thickening within the upper plate of a subduction zone. Despite numerous studies of this emblematic mountain range, several primary traits of this orogeny remain unresolved or poorly documented. The onset of uplift and deformation of the Frontal Cordillera basement culmination of the Southern Central Andes is such an example, even though this structural unit appears as a first-order topographic and geological feature. To solve for this, new (U-Th)/He ages on apatite and zircon from granitoids of the Frontal Cordillera at ~33.5°S are provided here. These data, interpreted as an age-elevation thermochronological profile, imply continuous exhumation initiating well before ~12–14 Ma, and at most by ~22 Ma when considering the youngest zircon grain from the lowermost sample. The age of exhumation onset is then refined to ~20 Ma by combining these results with data on sedimentary provenance from the nearby basins. Such continuous exhumation since ~20 Ma needs to have been sustained by tectonic uplift on an underlying crustal-scale thrust ramp. Such early exhumation and associated uplift of the Frontal Cordillera invalidate the classically proposed east-vergent models of the Andes at this latitude. Additionally, they provide further support to recent views on Andean mountain-building proposing that the Andes at ~33.5°S grew firstly over west-vergent basement structures.

The relationships between tectonics, climate and exhumation in the Central Andes (18–36°S): Evidence from low-temperature thermochronology

“…Deformation in the southern segment was accommodated by two fold-and-thrust belts of opposite vergence located in the Principal Cordillera. Two contrasting views about the onset, the intensity and the partitioning of deformation have been proposed based on either an east-vergent structural model (e.g., Giambiagi and Ramos, 2002;Farías et al, 2010;Giambiagi et al, 2012;Giambiagi et al, 2015a) or a bi-vergent model (Armijo et al, 2010;Riesner et al, 2017;Riesner et al, 2018;Riesner et al, 2019). In the east-vergent model, most of the orogenic shortening has been accommodated by east-vergent thrusts in the hybrid thick-and thin-skinned Aconcagua fold-and-thrust belt, which is located in the eastern Principal Cordillera and separates the modern volcanic arc and the basement culmination of the Frontal Cordillera (Giambiagi et al, 2015a and references therein).…”

Detrital Thermochronology Reveals Major Middle Miocene Exhumation of the Eastern Flank ofthe Andes That Predates the PampeanFlat Slab (33°–33.5°S)

“…Timing of Miocene uplift and exhumation of the Frontal Cordillera and Precordillera and total horizontal shortening based on available data. (1) Lossada et al (2017), (2) Rodríguez et al (2018), (3) Levina et al (2014), (4) Mazzitelli et al (2015), (5) Buelow et al (2018), (6) Giambiagi et al (2003), (7) Riesner et al (2019), (8) Suriano et al (2017), and (9) Walcek and Hoke (2012).…”

“…According to this interpretation, uplift of Frontal Cordillera and Precordillera, and ultimately the Pampean Ranges across the broken foreland basin, occurred in response to the flattening of the slab since circa 9 Ma at the representative latitude of 33°S (Giambiagi et al, 2003; Ramos et al, 2002; Ramos & Folguera, 2009). However, recent work suggests that the Cordón del Plata likely uplifted earlier than previously recognized (Buelow et al, 2018; Hoke et al, 2015; Riesner et al, 2019).…”

“…Many studies invoke past episodes of flattening of the slab on the basis of geochemistry of igneous rocks or spatial patterns of deformation and magmatism (e.g., Isacks, 1988; Kay & Mpodozis, 2002), such that it is often viewed as a principal element of Andean orogenic development. However, recent work in the Pampean flat‐slab points toward major range growth that predates shallowing of the subduction zone (Buelow et al, 2018; Hoke et al, 2015; Lossada et al, 2017; Riesner et al, 2019; Suriano et al, 2017). Moreover, Horton (2018) concludes that changes in plate velocities at the convergent boundary exert a first‐order control on the tectonic regime governing the construction of the Andes, while flat‐slab episodes represent a second‐order driving process.…”

Neogene Retroarc Foreland Basin Evolution, Sediment Provenance, and Magmatism in Response to Flat Slab Subduction, Western Argentina

“…Eastward advance of coarse-grained depo systems across the foreland basin system corresponded unroofing of initial Andean arc rocks and subsequent erosion into Frontal Cordillera igneous basement. Early Miocene thermochronometric cooling ages attest to shortening-related exhumation in the Frontal Cordillera (Hoke, Graber, et al, 2014;Riesner et al, 2019). Erosion of the Frontal Cordilleran further expressed in middle-Miocene deposits to the southwest (Manantiales basin; Mackaman-Lofland et al, 2020: Pinto et al, 2018, southeast (Cachueta basin: Buelow et al, 2018), northern Bermejo basin (Fosdick et al, 2015;Jordan et al, 1996), and northeastern Bermejo basin (Ischigualasto-Villa Unión basin: Lemos-Santos et al, 2019).…”