By integrating high-precision, U-Pb zircon geochronology with zircon and whole rock/ tuff geochemistry, the tectonic history of an active margin can be assessed independently from strata preserved in sedimentary basins. This approach is particularly useful in regions where subsequent active tectonism has resulted in extensive igneous intrusion, tectonic dissection, or structural deformation of crustal suites, obscuring the record of previous geologic events. In this work I present 603 U-Pb zircon age coupled with zircon geochemical analyses collected from Sensitive High Resolution Ion Microprobe (SHRIMP) microanalyses, 15 U-Pb zircon Thermal Ionization Mass Spectrometry (TIMS) ages, and 109 U-Pb Laser Ablation-Inductively Coupled Plasma Mass Spectrometer (LA-ICPMS), and 41 whole rock geochemical analyses to assess 1) the utility and limitations of U-Pb zircon tuff geochronology when estimating absolute depositional ages of strata for chronostratigraphy, 2) the petrogenesis of volcanic tuffs from zircon U-Pb age and rare earth element (REE) compositions, and whole rock major and trace element concentrations, 3) advantages and limitations of geochronologic techniques (TIMS vs. SHRIMP vs. LAICPMS) and 4) the interaction between plate margin magmatism, orogenic deformation, and sedimentation into a tectonically active basin.
Abstract To advance tectonic models of plateau growth in response to the India‐Asia collision, parameters such as the timing, mechanism (s), and extent of Cenozoic deposition and deformation throughout the eastern Tibetan Plateau need to be determined. To better understand these parameters, we examine the Mula basin, located in the southern Yidun terrane, using field data, detrital zircon geochronology, εHf zircon isotopic values, and thin section petrology. The Mula basin is a NW‐SE elongate (~28 km long and 5–8 km wide) exposure of nonmarine strata ~1,000 m thick, deposited in alluvial environments. The basin is bound to the northeast by a thrust fault (327, 34°NE) that places Triassic Daocheng Pluton and Triassic Yidun Group rocks on top of the nonmarine strata. The western boundary is defined by an unconformable contact between the overlying nonmarine strata and Triassic Daocheng Pluton and Yidun Group rock. An intrabasin thrust fault, parallel to the basin‐bounding fault, places older nonmarine strata on top of younger nonmarine strata, illustrating postdepositional deformation. Provenance results indicate that sediment influx originated primarily from localized drainage catchments and lacked major, well‐organized throughgoing systems. A maximum depositional age of 45.5 ± 0.5 Ma, based on the weighted mean average of the youngest detrital zircon population (s), demonstrates an Eocene or younger age for strata. We interpret the Mula basin to have developed in a contractional deformational regime driven by a far‐field, upper‐crustal response associated with the transition from an Andean style margin to a continent‐continent collisional margin as India impinged upon Eurasia.
The Salmon River suture zone of western Idaho (USA) records mid-crustal metamorphism and deformation associated with orogenesis during Mesozoic accretion of volcanic arc terranes to western Laurentia. We present petrographic and microstructural observations, garnet geochemistry, pressure-temperature isochemical phase diagrams, and Sm-Nd garnet and U-Pb zircon ages to investigate the timing and conditions of metamorphism in the Salmon River suture zone. The Salmon River suture zone is comprised of three thrust sheets: from east to west, the amphibolite facies Pollock Mountain plate, upper greenschist to amphibolite facies Rapid River plate, and greenschist facies Heavens Gate plate. The Pollock Mountain plate was isothermally loaded from 6 to >8 kbar at ∼700 °C between 141 and 124 Ma during northwest-southeast crustal shortening. The underlying Rapid River plate was isothermally loaded from 7 to ∼10 kbar at 600–650 °C during ca. 124–112 Ma metamorphism, which is contemporaneous with late- to post-peak metamorphism and ca. 118 Ma exhumation of the overlying Pollock Mountain plate. In the Rapid River plate, thrust sheet emplacement induced high-strain ductile deformation and led to regional development of linear-planar fabrics. The 206Pb/238U zircon ages for syndeformational to postdeformational magmatism record ca. 117 Ma or younger juxtaposition of the two plates on the southeast-dipping Pollock Mountain thrust fault. Coeval 124–112 Ma metamorphism of the Rapid River plate, ca. 118 Ma exhumation of the Pollock Mountain plate, and ca. 117 Ma or younger movement along the Pollock Mountain fault suggest that metamorphism of the Rapid River plate was possibly driven in part by thrust juxtaposition and loading along the Pollock Mountain fault. In this context, we interpret that metamorphism records diachronous thrust stacking during prolonged (>30 m.y.) accretionary orogenesis in western Idaho.
