Annia K. Fayon

Isothermal decompression, partial melting and exhumation of deep continental crust

Abstract Decompression of deep, hot continental crust is the primary mechanism of crustal melting, with major consequences for the geodynamics of orogens. Decompression within thickened continental crust may be initiated by processes driven from above (erosion, tectonic denudation) and/or below (crust/lithosphere thinning, buoyant rise of deep crust). On a larger scale, decompression of subducted continental crust may add material, including melt, to the overlying, non-subducting plate. This mechanism has the potential to produce large amounts of melt because fertile material is continually conveyed into the mantle, where it eventually buoyantly ascends and melts. Decompression-driven melting of continental crust may account for the high melt fractions (≥20 vol.%) and great thickness (20–30 km) inferred for the partially molten layer in orogenic crust. When high melt volumes are present in the crust and/or the thickness of the partially molten layer is large, the subsequent thermo-mechanical evolution of orogens is strongly influenced by lateral (channel) and vertical (buoyant) crustal flow. For both lateral and vertical flow, the presence of melt decouples deep crust from upper crust, and continental crust from mantle lithosphere. A major consequence of vertical crustal flow is the generation of migmatite-cored gneiss domes that riddle most orogens. High-grade rocks in many domes record pressure-temperature-time (P-T-t) paths indicating near-isothermal decompression followed by cooling from T > 700 °C to T < 350 °C in <2–5 Ma. Diapiric ascent of partially molten crust accounts for the decompression rate and magnitude required to maintain a near-isothermal path. We propose that gneiss domes are a signature of decompression and crustal melting, and are therefore fundamental structures for understanding the thermo-mechanical evolution of continental crust during orogeny.

Effects of plate convergence obliquity on timing and mechanisms of exhumation of a mid-crustal terrain, the Central Anatolian Crystalline Complex

Abstract Apatite fission-track (FT) ages from the Central Anatolian Crystalline Complex (CACC), a microcontinent within the Turkish segment of the Alpine–Himalayan orogen, vary dramatically from north to south. This variation correlates with differences in the obliquity of convergence of the continental fragment relative to the Pontide belt (for the northern CACC) and the Tauride belt (for the southern CACC). The northern CACC was deformed and metamorphosed during Late Cretaceous orogen-normal collision and was exhumed from the mid-crust to shallow crustal levels ( 50 Ma to the present. In contrast, the southern CACC (Nigde Massif) was initially deformed and metamorphosed during Late Cretaceous contraction, but subsequently developed as a metamorphic core complex in a wrench-dominated regime and was exhumed to

Timing of deformation and exhumation in the western Idaho shear zone, McCall, Idaho

The western Idaho shear zone is one of several Cretaceous high-strain zones in the Cordillera that are thought to have been associated with the northward translation and/or docking of terranes presently in British Columbia. Located in west-central Idaho, this zone of intense deformation consists of a mid-crustal exposure of a lithospheric-scale, intra-arc dextral shear zone that overprints the Salmon River suture zone along the western edge of the Idaho batholith. U/Pb zircon geochronology constrains the main phase of deformation to between ca. 105 and 90 Ma. Cessation of movement on the shear zone occurred by 90 Ma, as determined by dating of the syntectonic Payette River tonalite and a crosscutting pegmatite dike in the Little Goose Creek complex. The 40Ar/39Ar thermochronology indicates that the shear zone passed through both the hornblende (~550 °C) and biotite (~325 °C) closure temperatures between 85 and 70 Ma. The 40Ar/39Ar biotite dates from an outcrop-scale, crosscutting shear zone are indistinguishable from that of the host rock, indicating that deformation occurred above the closure temperature of biotite. Apatite fission-track analysis suggests that exhumation to shallow crustal levels occurred ca. 40 Ma during mid-Tertiary regional exhumation or renewed tectonic activity along the Salmon River suture zone. Taken together, the 40Ar/39Ar results and apatite fission-track analyses indicate a two-stage uplift history for the western Idaho shear zone. Overall, the geochronology indicates that dextral transpressional movement on the western Idaho shear zone was temporally distinct from the Early Cretaceous suturing event. Additionally, the first stage of exhumation recorded by the western Idaho shear zone immediately followed transpressional deformation. Cessation of displacement on the western Idaho shear zone by ca. 90 Ma indicates that the exhumation did not solely occur as a result of ductile deformation on the shear zone itself. Moreover, dextral strike-slip movement on the western Idaho shear zone had also ceased by 90 Ma, indicating that terrane translation models for the Cordillera can only use the western Idaho shear zone to accommodate northward translation up to ca. 90 Ma. Lastly, the timing of movement on the western Idaho shear zone and contractional deformation recorded in the Insular terrane suggests a correlation between these events. This hypothesis implies that the deformation recorded in the western Idaho shear zone may have been linked to the oblique collision of the Insular superterrane with North America.

Fission track analysis of the footwall of the Catalina detachment fault, Arizona: Tectonic denudation, magmatism, and erosion

New apatite and zircon fission track ages obtained from the footwall of the Catalina metamorphic core complex record a complicated cooling history associated with mid-Tertiary extension. Zircon fission track ages record the progressive unroofing of the Catalina metamorphic core complex along the Catalina detachment fault. Zircon fission track ages range from 31.9 to 19.4 Ma, generally decrease in the hanging-wall slip direction, and yield slip rates along the Catalina detachment fault ranging from 1.2 to 12 km Myr−1. In contrast, apatite fission track ages increase in the hanging wall slip direction. Samples from the main range of the Santa Catalina Mountains yield apatite fission track ages of 20.5 to 14.6 Ma; samples from the Santa Catalina Mountain forerange, located closer to the detachment fault, yield apatite fission track ages of 21.4 to 18.8 Ma. Rapid cooling (40° to 60 °C Myr−1) related to detachment faulting is best recorded by zircon fission track ages and higher-temperature thermochronometers in the main range and by nearly concordant zircon and apatite fission track ages in the forerange. Slower cooling (3°–7°C Myr−1) of the footwall is recorded by shortened mean confined fission track lengths (<14 μm) and is related to erosional unroofing. Approximately 2 km of late-Tertiary erosion played a significant role in the unroofing of the footwall of the Catalina metamorphic core complex in contrast to metamorphic core complexes in western Arizona, where detachment faulting is the dominant unroofing mechanism.

Exhumation of orogenic crust: Diapiric ascent versus low-angle normal faulting

Many high-grade metamorphic terrains record isothermal decompression, implying rapid exhumation or heat input during decompression. These terrains commonly contain gneiss domes that are spatially and temporally associated with low-angle normal faults such as those bounding metamorphic core complexes. To understand the thermomechanical relationship of gneiss domes and core complexes, we use twodimensional numerical modeling to evaluate exhumation and cooling rates resulting from diapiric ascent of partially-molten crust, a proposed mechanism for gneiss dome formation, versus exhumation of orogenic crust by low-angle normal faulting, the primary unroofi ng mechanism in metamorphic core complexes. Pressure-temperaturetime paths calculated for vertical ascent rates of 2–20 km/m.y. show that isothermal decompression is possible for rocks within a diapir. In contrast, paths calculated for rocks in the footwall of low-angle normal faults show that cooling occurs as rocks are brought closer to Earth’s surface, and the rate at which these rocks cool is controlled by fault dip, displacement rate, and amount of displacement. The amount of heat loss per unit time during decompression increases with fault dip. For exhumation of rocks in the footwall of a very low-angle fault (~10°), decompression paths occur with little cooling (quasi-isothermal), but the shallow dip of the fault does not allow signifi cant decompression. Low-angle normal faulting alone cannot result in isothermal decompression: The presence of gneiss domes in core complexes requires an additional exhumation process, such as diapiric ascent or a more structurally and thermally complex evolution of the detachment system. In the late stages of exhumation, once the rocks have risen to depths of <15 km and experience rapid cooling, detachment faulting may be the primary mechanism of the fi nal unroofi ng and juxtaposition of formerly deep rocks and upper crustal rocks.

Low‐temperature thermochronologic record of Eocene migmatite dome emplacement and late Cenozoic landscape development, Shuswap core complex, British Columbia

Exhumed mid-to-lower crustal rocks offer an opportunity to determine the mechanisms, conditions, timing, and consequences of the ascent of hot rocks from deep to shallow crustal levels. We used results of low-T thermochronology (zircon and apatite (U-Th)/He, apatite fission track) to document the very shallow emplacement ( 1800 m) have preserved Eocene fission-track ages and evidence of rapid cooling (≥60°C/Myr). This Eocene cooling event corresponds to rapid exhumation by upward flow of partially molten crust and final exhumation by detachment faulting. Samples collected below 1800 m in elevation display a wide range of apatite fission track ages (43–15 Ma) and track length distributions that reflect prolonged residence in the apatite partial annealing zone. These age-elevation relations imply that the dome rocks reached the near surface (<2 km) during initial upward flow and tectonic exhumation in the Eocene and that little erosion of the Eocene surface has occurred since that time. Thermal modeling of the lowest elevation samples (≤ ~600 m) and intrasample apatite (U-Th)/He age variations reveal enhanced erosion and relief production at the onset of continental glaciations at ~3 Ma. Our work illustrates the dynamic links between deep and shallow crustal processes and the evolution of topography in a deeply incised hot orogen.

Intrusive and depositional constraints on the Cretaceous tectonic history of the southern Blue Mountains, eastern Oregon

We present an integrated study of the postcollisional (post–Late Jurassic) history of the Blue Mountains province (Oregon and Idaho, USA) using constraints from Cretaceous igneous and sedimentary rocks. The Blue Mountains province consists of the Wallowa and Olds Ferry arcs, separated by forearc accretionary material of the Baker terrane. Four plutons (Lookout Mountain, Pedro Mountain, Amelia, Tureman Ranch) intrude along or near the Connor Creek fault, which separates the Izee and Baker terranes. High-precision U-Pb zircon ages indicate 129.4–123.8 Ma crystallization ages and exhibit a north-northeast–younging trend of the magmatism. The 40Ar/39Ar analyses on biotite and hornblende indicate very rapid (<1 m.y.) cooling below biotite closure temperature (∼350 °C) for the plutons. The (U-Th)/He zircon analyses were done on a series of regional plutons, including the Lookout Mountain and Tureman Ranch plutons, and indicate a middle Cretaceous age of cooling through ∼200 °C. Sr, Nd, and Pb isotope geochemistry on the four studied plutons confirms that the Izee terrane is on Olds Ferry terrane basement. We also present data from detrital zircons from Late Cretaceous sedimentary rocks at Dixie Butte, Oregon. These detrital zircons record only Paleozoic–Mesozoic ages with only juvenile Hf isotopic compositions, indicating derivation from juvenile accreted terrane lithosphere. Although the Blue Mountains province is juxtaposed against cratonic North America along the western Idaho shear zone, it shows trends in magmatism, cooling, and sediment deposition that differ from the adjacent part of North America and are consistent with a more southern position for terranes of this province at the time of their accretion. We therefore propose a tectonic history involving moderate northward translation of the Blue Mountains province along the western Idaho shear zone in the middle Cretaceous.

Cooling and exhumation of the southern Idaho batholith

We conducted a (U-Th)/He zircon thermochronology study of the southern part of the Idaho batholith (central Idaho, USA) to constrain cooling through ∼200 °C and exhumation of the batholith. Samples were collected adjacent to the Idaho-Oregon (IDOR) seismic transect and at localities where U-Pb zircon, geochemical, and fabric analyses were conducted. The rocks affected by the western Idaho shear zone and associated border zone suite of the batholith cooled through the closure temperature for He in zircon prior to ca. 60 Ma, before or during emplacement of the voluminous Atlanta lobe. In contrast, the Atlanta lobe (Atlanta peraluminous suite) records a relatively constant cooling rate, in which the (U-Th)/He zircon ages are systematically ∼30 m.y. younger than the U-Pb zircon ages. We interpret this data to reflect post-magmatic isobaric cooling with little or no unroofing. The only deviation from a smooth regional cooling pattern occurs near Sawtooth Valley, where samples from the Sawtooth Range on the west side of the valley show distinctly younger ages than those from the White Cloud Peaks to the east. We interpret this difference to reflect recent cooling and exhumation associated with extensional deformation. The regionally consistent pattern of cooling and hence exhumation indicates that the current exposure level of the Idaho batholith was <5 km deep (assuming a geothermal gradient of 40 °C/km) at 50 Ma during the initiation of Challis magmatism. Our data are consistent with the existence of a crustal plateau during formation of the Atlanta lobe of the Idaho batholith.

Exploring the relationship of scratch resistance, hardness, and other physical properties of minerals using Mohs scale minerals

The Mohs scale is enshrined in geoscience curricula as a simple and effective tool for identifying minerals and understanding the influence of crystal structure and chemistry on physical properties; e.g., hardness. Measuring scratch resistance is different from measuring hardness, however, because scratching involves components of loading and shearing, whereas “absolute” hardness is measured by the response of a material to vertical loading (indentation). Although it is not practical for most undergraduate classes to do indentation hardness testing, students can evaluate tabulated quantitative hardness data and compare these data with their own determination of relative scratch resistance. To help students better understand physical properties of minerals, and in particular the concept of mineral hardness, we present an example exercise based on recent systematic measurements of the hardness of Mohs scale minerals using indentation techniques. This exercise allows students to explore the differences in hardness among minerals of the Mohs scale, enhancing their understanding of the Mohs scale itself as well as the chemical and physical factors that influence mineral hardness. The exercise is most appropriate for Earth materials and mineralogy classes, but can be adapted for students with different levels of expertise, including introductory physical science students.

Doing time: Apatite fission track analysis in undergraduate geoscience courses

Geologic time is a central concept in Earth science teaching and research, but undergraduate students are seldom involved in doing time: measuring ‘ages’ of rocks or minerals. Integration of apatite fission track (AFT) analysis in geoscience classes is an accessible way to involve students in determining dates for geologic events. AFT analysis is an active-learning, inquiry-based technique in which students can visualize and understand concepts such as radioactive decay and its relationship to geologic ages, and the method and its applications can be a basis for cross-discipline instruction within a physical sciences curriculum. Fission-track analysis exercises can be incorporated into a variety of physical science classes or can be run as a separate workshop on geologic time or tectonics. We present a teaching module for incorporation of AFT analysis into general physical geology or historical geology lecture and/or laboratory. This exercise involves counting fission tracks using images from prepared samples (apatite grains already separated from a rock) with known uranium content. The data collected by students can then be used to demonstrate a variety of concepts, from radioactive decay to time-temperature paths and rates of geologic processes. With this module, undergraduates learn problem-solving techniques and experience a hands-on, quantitative approach to geologic time, rates of geologic processes, and the scientific method in general.