High Fidelity Record of Orogenic Exhumation and Basin Subsidence Patterns Through Integrated Isotopic Analysis: Cacheuta Basin, South-Central Andes

Abstract

Retroarc foreland basins in contractional arc settings contain evidence of temporal and spatial variations in magmatic activity, deformation, and exhumation. The integration of sedimentologic and stratigraphic analyses with detrital zircon U-Pb geochronology and detrital apatite (U-Th)/He thermochronology provides excellent constraints on depositional architecture, provenance, timing and rate of basin subsidence, and nature and episodicity of orogenic exhumation.

Neogene synorogenic strata of the Cacheuta basin southwest of Mendoza record episodic sediment flux produced during eastward thrust propagation. Initial basin subsidence is constrained as Early Miocene (19.20 +0.26 Ma) by U/Pb geochronology, which provides excellent chronological control (maximum depositional ages) through the basin. Sediment influx and depositional rate vary systematically upsection, with coarse clastic pulses reflecting thrust propagation and magmatic episodes represented by rapid accumulation of tuffaceous sediment.

Detrital apatite thermochronology records the initiation of basin inversion at ~4-5 Ma. Detrital apatite (U-Th)/He ages in strata younger than ~9.7 Ma match syndepositional zircon ages, and apatite ages in strata older than ~9.7 M progressively decrease downward, consistently younger than volcanic tuff U/Pb ages. The oldest strata yields a distinct cluster of closure ages at ~4-5 Ma, reflecting exhumation during eastward propagation of the thrust system.

Integration of high resoluton detrital geochronology and thermochronology provides unprecedented constraints on the timing of basin subsidence and orogenic exhumation. Detrital apatite (U-Th)/He thermochronology corroborate U-Pb detrital zircon depositional ages when the strata have not been deeply buried. In strata buried to greater than ~3 km, the thermochronometry constrains the rate and timing of basin inversion and exhumation.

This technique has direct application to exploration in frontier basins where the thermal history and hydrocarbon charge risk are incompletely understood. The subsidence history of basins with few wells can be characterized in both contractional and extensional environments based on orogenic exhumation-basin subsidence patterns revealed by integrated detrital apatite (U-Th)/He thermochronology and U-Pb zircon geochronology. When combined with potential fields geophysical data, basin screening for charge risk can be performed in the pre-seismic exploration phase.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018