Testing models of Laramide orogenic initiation by investigation of Late Cretaceous magmatic-tectonic evolution of the central Mojave sector of the California arc

The Mojave Desert region is in a critical position for assessing models of Laramide orogenesis, which is hypothesized to have initiated as one or more seamounts subducted beneath the Cretaceous continental margin. Geochronological and geochemical characteristics of Late Cretaceous magmatic products provide the opportunity to test the validity of Laramide orogenic models. Laramide-aged plutons are exposed along a transect across the Cordilleran Mesozoic magmatic system from Joshua Tree National Park in the Eastern Transverse Ranges eastward into the central Mojave Desert. A transect at latitude ∼33.5°N to 34.5°N includes: (1) the large upper-crustal Late Cretaceous Cadiz Valley batholith, (2) a thick section of Proterozoic to Jurassic host rocks, (3) Late Cretaceous stock to pluton-sized bodies at mesozonal depths, and (4) a Jurassic to Late Cretaceous midcrustal sheeted complex emplaced at ∼20 km depth that transitions into a migmatite complex truncated along the San Andreas fault. This magmatic section is structurally correlative with the Big Bear Lake intrusive suite in the San Bernardino Mountains and similar sheeted rocks recovered in the Cajon Pass Deep Scientific Drillhole. Zircon U-Pb geochronology of 12 samples via secondary ionization mass spectrometry (SIMS) (six from the Cadiz Valley batholith and six from the Cajon Pass Deep Scientific Drillhole) indicates that all Cretaceous igneous units investigated were intruded between 83 and 74 Ma, and Cajon Pass samples include a Jurassic age component. A compilation of new and published SIMS geochronological data demonstrates that voluminous magmatism in the Eastern Transverse Ranges and central Mojave Desert was continuous throughout the period suggested for the intersection and flat-slab subduction of the Shatsky Rise conjugate deep into the interior of western North America. Whole-rock major-element, trace-element, and isotope geochemistry data from samples from a suite of 106 igneous rocks represent the breadth of Late Cretaceous units in the transect. Geochemistry indicates an origin in a subduction environment and intrusion into a crust thick enough to generate residual garnet. The lack of significant deflections of compositional characteristics and isotopic ratios in igneous products through space and time argues against a delamination event prior to 74 Ma. We argue that Late Cretaceous plutonism from the Eastern Transverse Ranges to the central Mojave Desert represents subduction zone arc magmatism that persisted until ca. 74 Ma. This interpretation is inconsistent with the proposed timing of the docking of the Shatsky Rise conjugate with the margin of western North America, particularly models in which the leading edge of the Shatsky Rise was beneath Wyoming at 74 Ma. Alternatively, the timing of cessation of plutonism precedes the timing of the passage of the Hess Rise conjugate beneath western North America at ca. 70–65 Ma. The presence, geochemical composition, and age of arc products in the Eastern Transverse Ranges and central Mojave Desert region must be accounted for in any tectonic model of the transition from Sevier to Laramide orogenesis.Published versio

The Transition from Sevier to Laramide Orogeny Captured in Upper-Plate Magmatic Structures, Eastern Transverse Ranges, CA

The onset of the Laramide orogeny is of great tectonic significance to the geologic history of the US, but the timing and nature of the shift between Sevier and Laramide tectonics remains enigmatic. The eastern Transverse Ranges of southern California provide the opportunity to observe the effects of Laramide tectonics on the mid-crust. Wide Canyon is a north/south-trending canyon in the northern Little San Bernardino Mountains of the eastern Transverse Ranges. Al-in-hornblende thermobarometry of Needy et al. (2009) yields a projected paleodepth depth of ~20 km for Wide Canyon where Cretaceous granitoids intrude metamorphic country rock of Proterozoic age in a regional NW-trending antiform. U/Pb geochronology of five igneous samples from Wide Canyon reveals two zircon growth events at 88-84 Ma (32 analyses) and 76-72 Ma (30 analyses). Granite, granodiorite and gabbro samples contain zircons with 88-84 Ma cores and 76-72 Ma rims. One granite sample yielded a unimodal age of 74.1 ± 1.6 Ma. U and Th concentrations among 88-84 Ma zircon cores span three and four orders of magnitude in concentration, respectively, as would be expected from samples that range from granite to gabbro. In contrast, 76-72 Ma zircon rims yield U-Th concentrations that are within an order of magnitude. Syndeformational structures such as a regional synmagmatic shear zone, melt-filled parasitic folds and a melt-filled field area-scale antiform are constrained by the reported 76-72 Ma zircon ages. Shear sense indicators in the southwest-dipping shear zone (S/C fabrics, sigma and delta clasts and asymmetric boudins) yield strong top-to-the-northeast kinematics. The bimodal age distributions coincide with the shift from Sevier to Laramide tectonics during the Late Cretaceous. 88-84 Ma zircon ages are interpreted as a pulse of arc-magmatism during the Sevier Orogeny. Mafic rocks of synchronous age found in the arc-derived Teutonia batholith bear compositional similarities to mafic rocks of Wide Canyon, indicative of similar magmatic sources from 88-84 Ma. Zircon ages of 76-72 Ma zircon ages are interpreted as recording a shearing event, the emplacement of some granite bodies, and widespread zircon overgrowth. Top-to-the-northeast kinematics in the southwest-dipping synmagmatic regional shear zone are interpreted as backthrusting during northwest/southeast regional contraction that is well constrained by U/Pb zircon geochronology. This contraction is interpreted as the onset of Laramide tectonics between 76-72 Ma. The timing for the onset of flat slab subduction in the Mojave section of the Cordilleran arc is much later than is presumed by proposed tectonic models.Two zircon growth events are observed in igneous rocks of Wide Canyon at 88-84 Ma and 76-72 Ma. Bimodal zircon ages, are widespread in granite, granodiorite and gabbro across the field area, exhibit 88-84 Ma cores and 76-72 Ma rims. One granite sample yielded a unimodal age population from 76-72 Ma. Trace element concentrations among 88-84 Ma zircon cores are highly diverse, whereas 76-72 Ma zircon rims yield much more homogeneous trace element geochemistry. Syndeformational structures such as a regional synmagmatic shear zone, melt-filled parasitic folds and a melt-filled field-area-scale antiform are constrained by the reported 76-72 Ma zircon ages. Shear sense indicators in the southeast-dipping shear zone (S/C fabrics, sigma and delta clasts and asymmetric boudins) yield strong top-to-the-northeast kinematics across the synmagmatic shear zone. The bimodal age distributions coincide with the shift from Sevier to Laramide tectonics during the Late Cretaceous. 88-84 Ma zircon ages are interpreted as a record of a pulse of arc-magmatism during the Sevier Orogeny. Mafic rocks of synchronous age found in the Teutonia batholith bear compositional similarities to mafic rocks of Wide Canyon, implying that initial compositions from 88-84 Ma emplacement are preserved despite the later zircon-growth event. Zircon ages of 76-72 Ma zircon ages are interpreted as recording a shearing event, the emplacement of some granite bodies, and widespread zircon overgrowth. Top-to-the-northeast kinematics in the southwest-dipping synmagmatic regional shear zone are interpreted as backthrusting during northwest/southeast regional contraction that is well constrained by U/Pb zircon geochronology. This contraction is interpreted as the onset of Laramide tectonics between 76-72 Ma. This precise observation of the onset of flat slab subduction in the Mojave section of the Cordilleran arc must be taken into account by future tectonic modelers