Extension of the Late Triassic salt into western Peru: lmplications for Andean tectonics and mineral exploration

Abstract

A better understanding of large- and small-scale Andean geological features and timeline can obviously make a difference in exploration. Knowledge of the Andean evolution continues however to be plagued by geological myths and obsolete concepts (such as “compressional tectonic phases” and “ocean-continent collision”), while major key features of this evolution remain underrated. One of these key features is the “Mitu interval”, during which the Peruvian margin was submitted to intense extension starting -240 Ma. These extensional conditions caused intense back-arc rifting along the margin, and a related thick accumulation of sediments (and subordinate volcanic rocks), ranging from terrestrial clastics (the Mitu Group, 240~2 1 5 Ma) to shallow-marine carbonates (the Pucara Group, -21 O-s170 Ma). This rift basin was also the locus of thick salt accumulation approximately during the -21 ~21 0 Ma interval (the Pareni Formation, or Pareni Salt), making that western Peru was occupied by a giant salt basin during part of the Late Triassic (Sempere & Cotrina, 2018; Berrospi et al., 2018). Previous attributions of this major salt unit to the Permian are wrong (Sempere & Cotrina, 2018). Most of the western rim of Amazonian Peru is indeed characterized by abundant salt and other evaporites, as documented both by numerous outcropping diapirs and in seismic information (Sempere & Cotrina, 2018). In the high Andean plateau, where no Permian strata occur, the San Bias salt dome pierces the Pucara limestones southwest of Lake Junin, and gypsum occurs between the Mitu and Pucara groups, being mined at a number of places, such as -16 km WNW of the city of Tarma. Elsewhere in central Peru, effects of salt tectonics can be detected at least along the Cordillera Oriental (Berrospi et al., 2018).\r\nSempere & Cotrina (2018) showed that this salt and related other evaporites must have originally represented a considerable volume (>1 00,000 km3 as a minimum). Halokinesis vigorously developed during the Jurassic and continued through later times, generating a variety of salt-tectonic deformations (which were generally misinterpreted\r\nas resulting from thrust tectonics due to influence of the mainstream compressional paradigm). In this preliminary paper, we address the issue of the western extension of this giant salt basin, and briefly review the implications of the existence of voluminous salt and other evaporites for Andean tectonics and mineral exploration.