Browsing by Subject "Central Andes"
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Item Cenozoic deformation history of the Andean plateau in southern Peru : stratigraphic, structural, and geochronologic constraints(2015-08) Perez, Nicholas David; Horton, Brian K., 1970; Gulick, Sean; McQuarrie, Nadine; Steel, Ron; Stockli, Daniel FCenozoic shortening in the central Andes of southern Peru was accommodated by thin- and thick-skinned deformation that governed hinterland/foreland basin dynamics, the timing and location of exhumation, and development of modern high topography. A new line length balanced cross section estimates 130 km of shortening (38%) across the Eastern Cordillera and Subandean Zone. I propose the location of a pre-Andean graben in the Eastern Cordillera, and a kinematic model that links selectively inverted basement-involved normal faults to shallow detachments that accommodate thin-skinned deformation across the orogen. New U-Pb zircon geochronology from synrift deposits establishes Triassic age deposition, and suggests compartmentalized rift basins were filled with local Eastern Cordillera sediment sources. Eocene exhumation in the Eastern Cordillera represents reactivation of Triassic normal faults and the onset of Andean deformation. In-sequence deformation was transferred from the Eastern Cordillera to the Altiplano by the thin-skinned Central Andean Backthrust Belt and induced flexural subsidence in the Ayaviri hinterland basin beginning at ~30 Ma. Facies analyses, sediment provenance, geochronology, and structural mapping define multiple phases of basin reorganization that are temporally correlative with motion along basin margin thrust faults. Major middle Miocene reorganization of the Ayaviri basin is linked to ~17 Ma out-of-sequence thrust fault motion in the Western Cordillera. Oligocene-Miocene hinterland basin evolution in the northern Altiplano was driven by thrust tectonics. U-Pb detrital zircon geochronology from Cretaceous through Cenozoic stratigraphy in hinterland and foreland basins record distinct provenance differences since the Cretaceous. This is the detrital record of either an inherited structural high in the Eastern Cordillera that predated Eocene shortening and created two depocenters with distinct provenance, or lateral provenance variations across a large retroarc foreland basin. Existing K/Ar, ⁴⁰Ar/³⁹Ar, and new zircon (U-Th)/He thermochronology suggest Eocene-Oligocene exhumation in the Eastern Cordillera was synchronous ~400 km along strike. New apatite (U-Th)/He data from the Eastern Cordillera demonstrate a change to localized, diachronous exhumation and uplift events in the Miocene-Pliocene. Apatite (U-Th)/He thermochronology demonstrates onset of deformation in the Subandean Zone by ~15 Ma, after shortening and exhumation in the Eastern and Western Cordillera ceased.Item Cenozoic evolution of a fragmented foreland basin, Altiplano plateau, southern Peru(2012-05) Fitch, Justin David; Horton, Brian K., 1970-Debate persists on the timing, magnitude and style of crustal shortening, uplift and basin evolution in the Andes. Many studies suggest that the central Andes, including the Altiplano plateau, were gradually uplifted as a result of protracted Cenozoic retroarc shortening. However, recent isotopic studies conclude that the Andes instead rose in pulses, with the most significant event occurring at 10-6 Ma. Many researchers attribute these rapid pulses of uplift to lower lithosphere delamination events. A better understanding of the history of Cenozoic crustal shortening is essential for determination of the mechanism(s) of Andean uplift. The well-exposed Cenozoic San Jerónimo Group was studied in the Ayaviri basin of the northern Altiplano in southern Peru. The 3-5 km-thick succession is situated at 3900-4800 m elevation, between the Western Cordillera magmatic arc and the Eastern Cordillera fold-thrust-belt. New detrital zircon U-Pb geochronological results from four sandstones and one reworked tuff in the San Jerónimo succession show large age populations indicative of syndepositional volcanism between approximately 38 and 27 Ma. A 1600-m-thick magnetostratigraphic section further constrains the depositional timing and accumulation rate of the upper portion of the succession. Sedimentological observations show a rapid transition from cross-stratified braided-fluvial sandstones to proximal channel-fill and alluvial-fan conglomerates at ~30 Ma. Paleocurrent measurements show important temporal and spatial variations in sediment dispersal patterns while conglomerate clast counts show an upsection transition from almost exclusively volcanic input to increasing contributions of clastic, quartzite, and limestone detritus. The corresponding shifts in depositional environment and sediment provenance are attributed to the activation of new thrust structures in close proximity to the basin, namely the Pucapuca-Sorapata fault system, indicating the presence of an eastward advancing fold-thrust belt dating to at least 38 Ma and reaching the Ayaviri basin within the northern Altiplano plateau at ~30 Ma.