Extensional basin evolution in the Cordillera Blanca, Peru: Stratigraphic and isotopic records of detachment faulting and orogenic collapse in the Andean hinterland

Abstract

Sedimentologic, provenance, oxygen isotope, and 40Ar/39Ar results provide insights into late Cenozoic evolution of an extensional hinterland basin in the Peruvian Andes. The 5–6.5 km Cordillera Blanca composes the glaciated footwall of a low‐angle normal fault parallel to active contractional structures in the Andean fold‐thrust belt. The ∼200 km long, WSW dipping (19°–36°) Cordillera Blanca detachment fault accommodated >12–15 km of dip‐slip displacement, inducing subsidence of the hanging‐wall supradetachment basin, which is filled by ∼1300 m of conglomerate, sandstone, siltstone, and limited carbonate of the upper Miocene‐Pliocene Lloclla Formation. Lithofacies associations are attributed to lacustrine fan‐delta to proximal, stream‐dominated alluvial fan sedimentation. Provenance data record footwall unroofing of Jurassic through Miocene volcanic and sedimentary rocks during NW directed, axial dispersal of sediment. Provenance data record the appearance of footwall‐derived granite clasts from the upper Miocene Cordillera Blanca batholith and a change to transverse, WSW directed transport. Variations in the character and preservation of basin fill suggest along‐strike propagation of the fault through time. Initial extension and subsidence is constrained to the latest Miocene by an 40Ar/39Ar biotite age of 5.4 ± 0.1 Ma for a basal tuff in the Lloclla Formation. Estimation of paleolake water composition shows very negative δ18O(VSMOW) values (−13.6‰ to−18.2‰), indicating that high elevations comparable to modern were already attained in the Cordillera Blanca during earliest normal faulting and basin evolution. These results lend support to models suggesting that substantial shortening, crustal thickening, and surface uplift were necessary precursor conditions for the generation of the Cordillera Blanca detachment fault.

Cenozoic Evolution of Hinterland Basins in the Andes and Tibet

“…Reverse slip on steep, basement-involved faults generated isolated ranges bounded by individual flexural basins in the Puna plateau (Schwab, 1985;Hongn etal.,2007)andDuba-LunpolaregionofcentralTibet (Leeder et al, 1988). Hinterland extension produced rift and supradetachment basins in locations such as the Peruvian Andes (McNulty and Farber, 2002;Giovanni et al, 2010) and southernmost Tibet (Yin, 2000;Garzione et al, 2003). Additional examples of fault-induced and flexural subsidence help explain basin evolution in transtensional and transpressional hinterland settings, notably in central Tibet (Taylor et al, 2003, this volume) and Ecuador (Hungerb€ uhler et al, 2002;Winkler et al, 2005).…”

“…21.3c) is situated in the hanging wall of the active, 20-45 west-dipping Cordillera Blanca normal fault Schwartz, 1988;Petford and Atherton, 1992;McNulty and Farber, 2002). The >1.3 km-thick succession was deposited on folded Jurassic-Cretaceous strata and lower Cenozoic arc volcanic rocks (Giovanni et al, 2010). Basin initiation roughly coincided with late Miocene flattening of the subducted Nazca slab and cessation of arc magmatism at 2-14 S (Hampel, 2002;McNulty and Farber, 2002).…”

Mantle-derived helium in hot springs of the Cordillera Blanca, Peru: Implications for mantle-to-crust fluid transfer in a flat-slab subduction setting

“…Ma that predates extension in the area, the formation of the Cordillera Blanca detachment fault, and unroofing of the CB massif (Giovanni et al, 2010). Regionally, this period of activity is interpreted to represent the waning stages of arc magmatism during the transition to flat-slab subduction.…”

Possible threshold controls on sediment grain properties of Peruvian coastal river basins

“…1a) where mean annual water discharge reaches a value of almost 80 m 3 s −1 , but where the size of the D 50 is low. We relate this to the possible supply-limited state of this stream, conditioned by the orogen-parallel valley of the Rio Santa between the Cordillera Blanca and the Cordillera Negra, which has acted as a subsiding graben for 5.4 Myr (Giovanni et al, 2010;Margirier et al, 2015) and which might thus have operated as a sediment trap. This interpretation is also consistent with the low 10 Be-based catchment-averaged denudation rates measured for the Rio Santa basin, as noted by Reber et al (2017).…”