DOUBLE DATING AGES OF DETRITAL ZIRCON FROM SYNOROGENIC DEPOSITS: HISTORY OF THRUST-BELT EXUMATION, CENTRAL MEXICO

Detrital zircon (DZ) grains from turbidites of the Upper Cretaceous Mineral de Pozos Sandstone, Soyatal Formation and unnamed strata near Jalpan in the fold-thrust belt of Querétaro and San Luis Potosí, central Mexico, were double dated by U-Pb and (U-Th)/He (ZHe) methods. The three samples lie along a published structural cross-section, which indicates exhumation during Cretaceous thrust wedge evolution across the central part of the Sierra Madre Oriental. U-Pb ages of a larger sample set (n = 306) indicate that Cretaceous volcanic rocks of the Cordilleran magmatic arc were the dominant sources for most zircons (33% of grains). DZ U-Pb ages range ~2643-69 Ma and include a dominant component with statistically-overlapping grain ages between ~130 Ma and ~69 Ma. A minor age component (13%) of Jurassic-Early Cretaceous grains ranges ~193-134 Ma.

A subset of the U-Pb dated zircons was double dated; for each sample, 11-17 grains of known U-Pb age were selected for ZHe analysis. Zircon ZHe data reveal at least three major He age groups that correspond to three exhumation events documented in previous studies. One event is represented by eleven zircons with ZHe ages between ~91-85 Ma (26% of analyses), which can be related to thrust movement near Tolimán previously estimated at 85.5 ± 1.5 Ma from an ⁴⁰Ar/³⁹Ar illite age. A second event is represented by four grains with ZHe ages ranging ~80-75 Ma and consistent with previous K-Ar illite-mica dating of the El Doctor fault zone near San Joaquin. A third event indicated by abundant ZHe ages in the range ~63-56 Ma may represent exhumation by out-of-sequence hinterland thrusts, a conclusion supported by evidence from elsewhere in the orogenic wedge. Time-equivalent thrusting on the east flank of the Valles-San Luis Potosí platform, near the eastern end of the structural traverse, was previously estimated at 64 ± 2 Ma (⁴⁰Ar/³⁹Ar age on illite).

We infer that all grains are multi-cycle and cooled during three exhumation events in early Late Cretaceous, Santonian-Campanian, and early Paleogene time. Thus, the dominant population of zircons from the turbidite succession was derived from volcanic rocks of the Cordilleran magmatic arc along western Mexico, indicated by the U-Pb ages, and exhumed during discrete Cretaceous-Paleogene uplift events in the fold-thrust belt, indicated by the ZHe ages.