From Crustal Thickening to Orogen‐Parallel Escape: The 120‐Myr‐Long HT‐LP Evolution Recorded by Titanite in the Paleozoic Famatinian Backarc, NW Argentina

Abstract

Exposed sections of accretionary orogens allow reconstruction of their tectonic evolution. Most commonly, orogens are characterized by two-dimensional shortening perpendicular to the orogenic front. We describe the midcrustal section of the backarc of the early Paleozoic Famatinian accretionary orogen, exposed in the Sierra de Quilmes. Here crustal deformation evolved from a typical two-dimensional shortening with tectonic transport toward the west, to a noncoaxial constrictional strain with a southward tectonic transport parallel to the orogen. During the early phase of deformation, high-temperature and low-pressure (HT-LP) metamorphic complexes were juxtaposed by west directed thrusting on remarkably thick shear zones forming a thrust duplex. Deformation of the buried footwall complex continued after the exhumed hanging wall ceased to deform. We suggest that the thermally weakened footwall complex responded by initiating a phase of south verging thrusting, parallel to the orogen, associated with strong constriction, associated with L-tectonites, and sheath folds. This late phase of deformation defines a noncoaxial constrictional regime characterized by simultaneous east-west and vertical shortening and strong north-south, orogen-parallel stretching. Titanite ages and Zr-in-titanite thermometry demonstrate that this backarc remained above 700°C for 120 Myr between 500 and 380 Ma. Combined with regional geology, the new data suggest that west verging thrusting interrupted an early, back-arc extensional phase, and lasted from~470 to 440 Ma and that footwall constriction and south verging thrusting continued for another 40 to 60 Ma. The Famatinian backarc exposed in Sierra de Quilmes thus is an example of how shortening and orogenic growth in a hot orogen was counterbalanced by lateral flow.

Protracted Thermal Evolution of a Migmatitic Terrane as Revealed by Multiple Geochronometers From the Retro‐Arc of the Early Paleozoic Famatinian Orogen in NW Argentina

“…This was confirmed by Farias et al. (2020) who investigated titanite from footwall samples and concluded that this retro‐arc region “remained above 700°C for 120 Myr between 500 and 380 Ma” some 60 Myr longer than the hanging wall. In this paper, we expand on these finding by investigating the complementary temporal and thermal record of zircon, monazite and titanite of several samples from the footwall.…”

“…The most extensive dating work of rocks of the Agua del Sapo complex was the U‐Pb dating of titanites by Farias et al. (2020). They found widely variable ranges of dates.…”

Petrostructural and Geochronological Constraints on Devonian Extension‐Shortening Cycle in the Chinese Altai: Implications for Retreating‐Advancing Subduction

“…Accordingly, it can be concluded that the main melting process took place at least between 430 and 410 Ma. Such age spreads of the melting process are common in hot orogens worldwide, including the Lachlan orogenic belt in Australia (Collins, 2002a, 2002b; Foster & Gray, 2000; White & Chappell, 1988) and the Famatinian Orogen in Argentina (Farias et al., 2020; Wolfram et al., 2019). By contrast, the timing of D1 B phase cannot be precisely determined by the current geochronological data.…”

Tectonic Transport Directions, Shear Senses and Deformation Temperatures Indicated by Quartz c-Axis Fabrics and Microstructures in a NW-SE Transect across the Moine and Sgurr Beag Thrust Sheets, Caledonian Orogen of Northern Scotland

“…Transport directions can, of course, change both spatially and temporally, and may reflect complex flow patterns (particularly under ductile conditions in the mid-lower crust) and strain path partitioning at a wide range of scales [7][8][9][10][11][12][13]. Additionally, orogen-scale arcuate lineation patterns are commonly interpreted as indicating transpressional deformation associated with variable components of orogen-perpendicular shortening and orogen parallel shearing (e.g., Burg et al, [14]; Xypolias et al, [15]).…”