Caden James Howlett

Quantification of the timing and mechanisms of crustal thickening in Cordilleran orogenic hinterlands remains challenging. We address this in the Andes using whole‐rock geochemistry and zircon petrochronology from a >70 Myr volcanic record in the Tinguiririca valley of Chile (∼35S), which captures a pivotal transition from extensional to contractional deformation. Zircon (Sm, Gd, Dy)/Yb ratios generally track trends in whole‐rock geochemical crustal thickness (paleomohometry) models across this record, with Dy/Yb showing the most consistent correspondence. Our results indicate modest crustal thinning coeval with Late Cretaceous normal faulting and oblique plate convergence. Eocene and Early Miocene samples yield similar crustal thicknesses (∼35 km), suggesting limited local net attenuation during proposed intra‐arc rifting (Abanico basin). The crust then thickened by ∼10 km to near‐modern values (∼46 km) during the Neogene; Quaternary estimates match modern geophysical constraints, validating the paleomohometry approach. Thickening was most rapid between ca. 17 and 13 Ma, coinciding with accelerated retroarc shortening and plate convergence. Additional zircon geochronologic and geochemical proxies (Th/U, phosphorus content, and oxygen fugacity) indicate enhanced Miocene crustal melting and magmatic productivity. Along‐strike comparisons reveal north‐to‐south younging and decreasing magnitude of thickening, mirroring trends in retroarc shortening. These findings indicate that underthrusting of foreland lithosphere during retroarc shortening drove crustal thickening, magmatic flare‐up, and generation of orogenic topography in the southern Central Andes.

The Frontal Cordillera is a first‐order geologic feature of the southern central Andes, hosting the highest hinterland topography above the modern Pampean flat‐slab segment. The timing of Frontal Cordillera exhumation is important for testing models of Andean tectonics, yet large latitudinal gaps exist between structural and thermochronological constraints for the region. We conducted a thermochronometric study using a 4.4 km age‐elevation transect along the northeast ridge of Cerro Mercedario, the highest peak in the La Ramada massif at ∼32°S. Zircon (U‐Th)/He dates indicate partial resetting, supporting a limited magnitude of exhumation in even the most extreme Andean topography. Single grain apatite (U‐Th‐Sm)/He dates range from 8.5 ± 0.9 to 35.8 ± 3.6 Ma, with median dates of ∼10.5 to ∼15.7 Ma with increasing elevation. Integrated with geologic mapping and thermal history modeling, these data suggest Early to Middle Miocene exhumation along the Santa Cruz and Espinacito faults concomitant with uplift of the La Ramada massif. New apatite helium data from the Cordillera del Tigre segment of the Frontal Cordillera are partially reset and preferred modeling interpretations suggest exhumation ca. 11–9 Ma, coeval with shortening in the eastward adjacent Precordillera. These data add to accumulating regional evidence for out‐of‐sequence deformation during the Miocene, consistent with internal (hinterland) growth of a subcritical orogenic wedge contemporaneous with surface uplift and crustal thickening in the south‐central Andes.

We investigate rifting during continental collision in southern Tibet by testing kinematic models for two classes of rifts: Tibetan rifts are defined as >150 km in length and crosscut the Lhasa Terrane, and Gangdese rifts are <150 km long and isolated within the high topography of the Gangdese Range. Discerning rift kinematics is a crucial step towards understanding rift behavior and evolution that has been historically limited. We evaluate spatiotemporal trends in fault displacement and extension onset in the Tangra Yumco (TYC) rift and several nearby Gangdese rifts and examine how contraction and rift exhumation relate to evolution of the Gangdese drainage divide. Igneous U-Pb and zircon (U-Th)/He (ZHe) results indicate rift footwall crystallization between ~59-49 Ma and cooling between ~60-4 Ma, respectively, with ZHe ages correlating with sample latitude. Samples from Gangdese latitudes (~29.4-29.8°N) yield predominantly Oligocene-early Miocene ages, whereas samples north of ~29.8°N yield both late Miocene-Pliocene ages and Paleocene-Eocene ages. Thermal history models indicate two-stage cooling, with initially slow cooling followed by accelerated cooling during late Miocene-Pliocene time. From spatial distributions of ZHe ages we interpret: (1) ~28-16 Ma ages from Gangdese latitudes reflect exhumation along contractional structures, (2) ~8-4 Ma ages reflect rift-related exhumation, and (3) ~60-48 Ma ages indicate these samples experienced lesser rift exhumation. Our data are consistent with a segment linkage evolution model for the TYC rift, with interactions between rifts and contractional structures likely influencing the evolution of topography and location of the Gangdese drainage divide since Miocene time

Apatite fission-track and zircon (U-Th/He) data sets.

Apatite fission-track and zircon (U-Th/He) data sets.

The “Laramide-style” uplifts of North America—characterized by blocks of Proterozoic-Archean basement that were exhumed along reverse faults within the Cordilleran foreland basin—are widely interpreted to be a result of flat-slab subduction of oceanic lithosphere beneath the continent. Despite this general consensus, the causal mechanisms of basement-cored uplifts remain unclear. Assessment of the hotly debated geodynamic models that relate flat-slab subduction and upper crustal deformation hinge on the availability of accurate estimates for the timing of exhumation of Laramide uplifts. A major problem with current models is that they do not incorporate timing constraints from the Laramide region of central Montana. This region represents the northernmost extent of Laramide deformation and timing constraints are critically needed to enhance our understanding of how stress is transmitted inboard during flat-slab subduction. We present the first low-temperature thermochronological ages from the Little Belt Mountains (LBM) of central Montana, which is the northernmost Laramide-style uplift with exposed basement gneisses. Apatite fission track ages ranging from ca. 90-70 Ma suggest that the core of the LBM was exhumed through the 120-60°C apatite partial annealing zone in the Late Cretaceous. These ages corroborate recent low-T thermochronology and sedimentological work that propose an earlier than previously recognized onset of Laramide deformation in southwestern Montana and eastern Idaho (>80 Ma), but additional data are needed to reduce the uncertainty between ages and ascertain the exhumation history of the LBM. To this end, we will integrate new low-temperature thermochronology (<150°C) and associated thermal history models in order to constrain the Cretaceous tectonic evolution of the LBM and the extent of Laramide deformation in the western USA.

Metamorphic core complexes (MCCs) are a product of crustal extension, but their dynamics are still debated. Early research suggests that the formation of MCCs in the western United States was due to gravitational collapse of crust that had been thickened during Cordilleran orogenesis. However, the instability of overthickened crust alone cannot explain the diachronous formation of core complexes with a strong spatial dependency, as there was relatively uniform crustal thickness along strike of the Cordillera. For this reason, there is an interest in what role other lithospheric processes (such as subducted slab removal) play in the evolution of MCCs. We investigate the role of such processes by determining the temporal relation between magmatism and extension in the Anaconda MCC (AMCC) of western Montana. Geologic mapping, zircon U‐Pb geochronology, and zircon (U‐Th)/He thermochronology reveal that the initiation of extension in the AMCC in the Eocene (∼53 Ma) began at least 3 Myr after the emplacement of voluminous Paleocene two‐mica plutons. We interpret that the AMCC is an example of a core complex that was primed for extension by magmatic thermal weakening and suggest that foundering of the Farallon flat slab and the onset of the ignimbrite flareup in western Montana was responsible for the initiation of AMCC extension. An updated compilation of MCC cooling ages and Cenozoic volcanic activity across the western United States supports previous interpretations that the removal of Farallon oceanic lithosphere likely initiated MCC exhumation in some regions.

Despite the widespread occurrence and economic significance of gold placer deposits, modern provenance studies of placer sediments remain largely qualitative. This study applies detrital zircon (DZ) geochronology to determine the source of zircon in placer deposits. We then evaluate the provenance of the zircon to assess whether the gold might have been derived from the same sources, thereby providing a case study of the use of DZ geochronology applied to placers. We present a new set of DZ U-Pb ages (n = 1058) and Lu-Hf (n = 61) isotopic data from four placer deposit samples collected from the Pioneer District of western Montana (USA). Each of the four samples yielded similar age spectra, with a range of U-Pb ages between 3000 and 25 Ma. We interpret that ≥250 Ma zircons were recycled from the Mesoproterozoic Belt Supergroup, Paleozoic–Mesozoic sedimentary rocks, and the Upper Creta­ceous–Paleocene Beaverhead Group. Our 237 DZ U-Pb ages ≤250 Ma reveal two prominent age-probability peaks centered at ca. 69 Ma and ca. 26 Ma, which we interpret to record first-cycle derivation from the Royal stock and nearby Dillon Volcanics, respectively. We evaluate these data using an inverse Monte Carlo DZ unmixing model that calculates relative contributions from plausible source units, determining a 12% contribution from the Royal stock and a 43% contribution from the Beaverhead Group. A current absence of the Beaverhead Group in the hypothesized source region suggests complete erosion of the unit into the placer-bearing basin. Detrital zircon geochronology, Hf isotopic data, and the unmixing mod­eling results offer the first zircon-based support for previous interpretations that the Late Cretaceous Royal stock precipitated gold along its contact with overlying Proterozoic–Mesozoic sedimentary strata. Subsequent exhuma­tion and erosion of the lode source led to gold deposition in the Anaconda metamorphic core complex supradetachment basin during the late Oligo­cene–late Miocene. The worldwide occurrence of gold placer deposits with unknown source areas provides abundant opportunity to apply these tech­niques elsewhere.