Cordilleran Metamorphic Core Complexes Research Articles

Cenozoic structural evolution of the Catalina metamorphic core complex and reassembly of Laramide reverse faults, southeastern Arizona, USA

AbstractThis study investigates the Late Cretaceous through mid-Cenozoic structural evolution of the Catalina core complex and adjacent areas by integrating new geologic mapping, structural analysis, and geochronologic data. Multiple generations of normal faults associated with mid-Cenozoic extensional deformation cut across older reverse faults that formed during the Laramide orogeny. A proposed stepwise, cross-sectional structural reconstruction of mid-Cenozoic extension satisfies surface geologic and reflection seismologic constraints, balances, and indicates that detachment faults played no role in the formation of the core complex and Laramide reverse faults represent major thick-skinned structures.The orientations of the oldest synextensional strata, pre-shortening normal faults, and pre-Cenozoic strata unaffected by Laramide compression indicate that rocks across most of the study area were steeply tilted east since the mid-Cenozoic. Crosscutting relations between faults and synextensional strata reveal that sequential generations of primarily down-to-the-west, mid-Cenozoic normal faults produced the net eastward tilting of ∼60°. Restorations of the balanced cross section demonstrate that Cenozoic normal faults were originally steeply dipping and resulted in an estimated 59 km or 120% extension across the study area. Representative segments of those gently dipping faults are exposed at shallow, intermediate (∼5–10 km), and deep structural levels (∼10–20 km), as distinguished by the nature of deformation in the exhumed footwall, and these segments all restore to high angles, which indicates that they were not listric. Offset on major normal faults does not exceed 11 km, as opposed to tens of kilometers of offset commonly ascribed to “detachment” faults in most interpretations of this and other Cordilleran metamorphic core complexes. Once mid-Cenozoic extension is restored, reverse faults with moderate to steep original dips bound basement-cored uplifts that exhibit significant involvement of basement rocks. Net vertical uplift from all reverse faults is estimated to be 9.4 km, and estimated total shortening was 12 km or 20%. This magnitude of uplift is consistent with the vast exposure of metamorphosed and foliated cover strata in the northeastern and eastern Santa Catalina and Rincon Mountains and with the distribution of subsequently dismembered mid-Cenozoic erosion surfaces along the San Pedro Valley. New and existing geochronologic data constrain the timing of offset on local reverse faults to ca. 75–54 Ma.The thick-skinned style of Laramide shortening in the area is consistent with the structure of surrounding locales. Because detachment faults do not appear to have resulted in the formation of the Catalina core complex, other extensional systems that have been interpreted within the context of detachments may require further structural analyses including identification of crosscutting relations between generations of normal faults and palinspastic reconstructions.

Thermochronology of cordilleran metamorphic core complexes: example of Song Chay massif in Northern Vietnam

Based on the reconstruction of the thermal evolution of the Song-Chai granitoid massif (Northern Vietnam) the long-term existence of granitoid magma at deep levels of the Earths crust (H = 15-20 km, t ~ 20-50 Ma) is established. Geodynamic analysis and mathematical modeling of thermal history of the granitoid batholith cooling shows that the magmatic chamber should be considered as thermal trap on the lower level of the earths crust, preserving residual granite melts for a long time. Activation of the magmatic chamber occurs in post-collisional strike-slip tectonics zones and is associated by tectonic exhumation of large segments of the earths crust. Ultimately, this leads to the transformation of the batholith into Cordilleran type metamorphic core complexes, emplacement of residual rare-metal melts and the formation of commercial deposits.

Eocene exhumation and extensional basin formation in the Copper Mountains, Nevada, USA

Abstract Within extended orogens, records that reflect the driving processes and dynamics of early extension are often overprinted by subsequent orogenic collapse. The Copper Mountains of northeastern Nevada preserve an exceptional record of hinterland extensional deformation and high-elevation basin formation, but current geochronology and thermochronology are insufficient to relate this to broader structural trends in the region. This extension occurred concurrent with volcanism commonly attributed to Farallon slab removal. We combine thermochronology of both synextensional hanging-wall strata and footwall rocks to comprehensively evaluate the precise timing and style of this deformation. Specifically, we apply (U-Th)/(He-Pb) double dating of minerals extracted from Eocene–Oligocene Copper Basin strata with multi-mineral (U-Th)/He and 40Ar/39Ar thermochronology of rocks sampled across an ∼20 km transect of the Copper Mountains. We integrate basement and detrital thermochronology records to comprehensively evaluate the timing and rates of hinterland extension and basin sedimentation. Cooling and U-Pb crystallization ages show the Coffeepot Stock, which spans the width of the Copper Mountains, was emplaced at ca. 109–108 Ma, and then cooled through the 40Ar/39Ar muscovite and biotite closure temperatures by ca. 90 Ma, the zircon (U-Th)/He closure temperature between ca. 90 and 70 Ma, and the apatite (U-Th)/He closure temperature between 43 and 40 Ma. Detrital apatite and zircon (U-Th)/(He-Pb) double dating of late Eocene fluvial and lacustrine strata of the Dead Horse Formation and early Oligocene fluvial strata of the Meadow Fork Formation, both deposited in Copper Basin, shows that Early Cretaceous age detrital grains have a cooling history that is analogous to proximal intrusive rocks of the Coffeepot Stock. At ca. 38 Ma, cooling and depositional ages for Copper Basin strata reveal rapid exhumation of proximal source terranes (cooling rate of ∼37 °C/m.y.); in these terranes, 8–12 km of slip along the low-angle Copper Creek normal fault exhumed the Coffeepot Stock in the footwall. Late Eocene–early Oligocene slip along this fault and an upper fault splay, the Meadow Fork fault, created a half graben that accommodated ∼1.4 km of volcaniclastic strata, including ∼20 m of lacustrine strata that preserve the renowned Copper Basin flora. Single-crystal sanidine 40Ar/39Ar geochronology of interbedded tuffs in Copper Basin constrains the onset of rapid exhumation to 38.0 ± 0.9 Ma, indicating that surface-breaching extensional deformation was coincident with intense proximal volcanism. Coarse-grained syndeformational sediments of the Oligocene Meadow Fork Formation were deposited just prior to formation of an extensive regional Oligocene–Miocene unconformity and represent one of the most complete hinterland stratigraphic records of this time. We interpret this history of rapid late Eocene exhumation across the Copper Mountains, coeval volcanism, and subsequent unconformity formation to reflect dynamic and thermal effects associated with Farallon slab removal. The final phase of extension is recorded by late, high-angle normal faults that cut and rotate the early middle Miocene Jarbidge Rhyolite sequence, deposited unconformably in the hanging wall. These results provide an independent record of episodic Paleogene to Miocene exhumation documented in Cordilleran metamorphic core complexes and establish that substantial extension occurred locally in the hinterland prior to province-wide Miocene extensional break-up.

Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

Abstract. High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range Province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada), suggest the presence of two distinct rheological transitions in the middle crust: (1) the brittle–ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear, and (2) the localized–distributed transition, or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths.In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. The LDT is not always exhumed, or it may be obscured by later deformation, but in the Whipple Mountains, it can be directly observed where high-strain mylonites captured from the middle crust depart from the brittle detachment along a mylonitic front.

The effects of lower crustal strength and preexisting midcrustal shear zones on the formation of continental core complexes and low-angle normal faults

To investigate the formation of core complexes and low-angle normal faults, we devise thermo-mechanical simulations on a simplified wedge-like orogenic hinterland that has initial topography, Moho relief, and a preexisting mid-crustal shear zone that can accommodate shear at very low angles (<20°). We mainly vary the strength of the lower crust and the frictional strength of the preexisting mid-crustal shear zone. We find that the strength of the lower crust, and the existence and strength of a preexisting shear zone significantly affect the formation and evolution of core complexes. With increasing lower crustal strength, we recognize varying extensional features with decreasing exhumation rate: these are characterized by bivergent metamorphic massifs, classic Cordilleran metamorphic core complexes, multiple consecutive core complexes (or boudinage structures), and a flexural core complex underlined by a large subsurface low-angle detachment fault with a small convex curvature. Topographic loading and mantle buoyancy forces, together with divergent boundaries, drive a regional lower crustal flow that leads to the exhumation of the lower crust where intensive upper crustal faulting induces strong unloading. The detachment fault is a decoupling zone that accommodates large displacement and accumulates sustained shear strain at very low angle between upper and lower crust. Though the regional stress is largely Andersonian, we find non-Andersonian stress in regions adjacent to the preexisting shear zone and those with high topographic gradient. Our new models provide a view that is generally consistent with geological and geophysical observations on how core complexes form and evolve.

Jurassic Barrovian metamorphism in a western U.S. Cordilleran metamorphic core complex, Funeral Mountains, California

Large-magnitude retroarc shortening of Cretaceous age is well documented in the Sevier orogenic belt of the western United States, and has been associated with eastward Franciscan subduction that began in the Middle–Late Jurassic, but evidence for major Late Jurassic retroarc shortening has been lacking. Here we report new Lu-Hf garnet geochronology, pressure-temperature ( P - T ) paths, and 40 Ar/ 39 Ar thermochronology data that document tectonic burial of Late Jurassic age in a Barrovian metamorphic terrain, the Funeral Mountains metamorphic core complex, California, located in the hinterland of the Sevier orogenic belt. The P - T paths determined from growth-zoned garnets in upper greenschist facies pelitic schist show steep P - T trajectories consistent with metamorphism during thrust loading. The age of thrust loading is constrained by a five-point Lu-Hf garnet isochron to be 158.2 ± 2.6 Ma (2σ). Partial exhumation, recorded in 146–153 Ma 40 Ar/ 39 Ar muscovite cooling ages, closely followed garnet growth. Late Jurassic Barrovian metamorphism has not been previously recognized in Cordilleran metamorphic core complexes, possibly due to being obscured by Late Cretaceous to Tertiary deformation, magmatism, and metamorphism. This study finds that the period of large-magnitude crustal shortening in the retroarc extended into the Late Jurassic, and may have closely followed the formation of the coherent orogenic system associated with east-dipping Franciscan subduction.

Effects of permeability fields on fluid, heat, and oxygen isotope transport in extensional detachment systems

AbstractField studies of Cordilleran metamorphic core complexes indicate that meteoric fluids permeated the upper crust down to the detachment shear zone and interacted with highly deformed and recrystallized (mylonitic) rocks. The presence of fluids in the brittle/ductile transition zone is recorded in the oxygen and hydrogen stable isotope compositions of the mylonites and may play an important role in the thermomechanical evolution of the detachment shear zone. Geochemical data show that fluid flow in the brittle upper crust is primarily controlled by the large‐scale fault‐zone architecture. We conduct continuum‐scale (i.e., large‐scale, partial bounce‐back) lattice‐Boltzman fluid, heat, and oxygen isotope transport simulations of an idealized cross section of a metamorphic core complex. The simulations investigate the effects of crust and fault permeability fields as well as buoyancy‐driven flow on two‐way coupled fluid and heat transfer and resultant exchange of oxygen isotopes between meteoric fluid and rock. Results show that fluid migration to middle to lower crustal levels is fault controlled and depends primarily on the permeability contrast between the fault zone and the crustal rocks. High fault/crust permeability ratios lead to channelized flow in the fault and shear zones, while lower ratios allow leakage of the fluids from the fault into the crust. Buoyancy affects mainly flow patterns (more upward directed) and, to a lesser extent, temperature distributions (disturbance of the geothermal field by ~25°C). Channelized fluid flow in the shear zone leads to strong vertical and horizontal thermal gradients, comparable to field observations. The oxygen isotope results showδ18O depletion concentrated along the fault and shear zones, similar to field data.

Structural evolution of the Gonzha block (Argun-Idermeg superterrane of the Central Asian orogenic belt)

The structural data and available geochronological constraints suggest that the Gonzha tectonic block located in the northeastern part of the Argun-Idermeg superterranes, Central Asian orogenic belt, is considered as an analogue of Cordilleran metamorphic core complexes of western Transbaikalia. These metamorphic core complexes, as well as the Gonzha block, may have resulted from collapse of a Late Mesozoic orogen after closure of the Mongolia-Okhotsk paleo-ocean basin.

Time–temperature–fluid evolution of migmatite dome crystallization: Coupled U–Pb age, Ti thermometry, and O isotopic ion microprobe depth profiling of zircon and monazite

Fluid infiltration of high-grade terrains is commonly difficult to track; however, the use of secondary ionization mass spectrometry (SIMS) depth profiling analysis of zircon in high-grade metamorphic rocks allows acquisition of micron-scale U–Pb geochronology, Ti in zircon thermometry, and oxygen isotope measurements that can be used to decipher temporal and physicochemical records of orogens. This investigation focuses on a migmatite dome in a Cordilleran metamorphic core complex that records a long history of partial melting, deformation, and fluid flow during crustal thickening and/or rapid orogenic collapse. SIMS depth-profiling analyses of the outermost rims of zircon and monazite from samples representing a range of crustal levels elucidate the nature of the deep crustal environment during the final stages of extension and orogenic collapse from 58 to 50 Ma, following a major episode of granitoid emplacement and partial melt crystallization. The results from ultrathin (0.25 to 5.00 µm) outer rims of zircon demonstrate that fluid infiltration driven by amphibolite facies metamorphic devolatilization and/or igneous crystallization persisted at deep crustal levels until just before the time that the rocks were rapidly exhumed to upper crustal levels. Collectively, the results indicate that metamorphic fluids continued to infiltrate the gneiss dome from ca. 58 to 50 Ma. The overlap in ages from the zircon rims and monazite with argon cooling ages suggests that the dome cooled rapidly from > 650 °C to 300 °C from 51–47 Ma. The results further demonstrate that the evolution of high-grade terrains, including the fluid history, can be revealed in unprecedented detail through the combination of depth profiling analyses of U–Pb age, Ti thermometry, and O isotopes.

THE KAKOPETROS AND RAVDOUCHA IRON-OXIDE DEPOSITS, WESTERN CRETE, GREECE:FLUID TRANSPORT AND MINERALIZATION WITHIN A DETACHMENT ZONE

Small iron deposits at Kakopetros and Ravdoucha in western Crete are hosted by an extensional detachment zone at the roof of the high-pressure–low-temperature metamorphic core complex known as the Phyllite-Quartzite unit. The iron oxides occur in a brecciated layer of phyllite, quartzite, and marble up to tens of meters thick. They fill fractures and vugs in the breccia and partly impregnate the marble. The iron oxides, which were formerly mined in open pits, are predominantly composed of goethite and subordinate oxyhydroxides of the manganomelane group. The field relationships and microstructures indicate that precipitation of the iron-oxide minerals was related to fluid flow focussed along the detachment fault. δ 18 O values of goethite indicate crystallization at low temperatures (31°–40°C) and at a shallow depth of about 1 km. Microscopic investigations show that the deposition of iron oxides was syntectonic and occurred during deformation in the uppermost crust. Similar iron oxides are reported from low-angle brittle detachment horizons in the Cordilleran metamorphic core complexes of North America and suggest that small iron- and manganese-oxide deposits of this type may be a characteristic feature of detachment zones.

40Ar-39Ar ages of the Louzidian-Dachengzi ductile shear zone near Chifeng, Inner Mongolia and their tectonic significance

The Louzidian normal fault occurs as the eastern detachment fault of the Kalaqin metamorphic core complex. Field observations and microstructural analyses reveal that the Louzidian-Dachengzi ductile shear zone developed in its lower-plate was genetically related to sinistral strike-slips and extensional faulting. Two samples from this ductile shear zone yield 40Ar-39Ar plateau ages of 133 Ma (Bi) and 126 Ma (Kp), which are concordant with their isochron ages. The plateau age of 133 Ma (Bi) records the formation age of the ductile shear zone. The inconsistent relationship between the earlier strike-slip ductile shear zone and the later normal fault makes the Kalaqin Quasi-metamorphic core complex distinctive from Cordilleran metamorphic core complex. These ages provide important geochronological data for putting constraints on the formation age and genesis of such ductile shear zones.

Large‐magnitude extension along metamorphic core complexes of western Arizona and southeastern California: Evaluation with paleomagnetism

Cordilleran metamorphic core complexes (MCCs) and their associated structures (detachment faults and mylonites) are the products of large‐magnitude Cenozoic extension. Paleomagnetic data from the Harquahala, Harcuvar, Buckskin, and Whipple MCCs of western Arizona and southeastern California (∼1900 samples; 191 of 278 sites accepted for inclusion in group and grand means) allow us to estimate the magnitude of footwall tilting associated with extension. All MCC crystalline rocks sampled are in the footwalls of associated brittle detachment faults. Rocks sampled include synkinematic, Miocene intrusions (dikes and plutons), Late Cretaceous granites, and Proterozoic host rocks. These rocks (with different levels of success at the site level) yield dual polarity, high unblocking temperature, and high to moderate coercivity magnetizations. Demagnetization behavior reveals that those sites with well‐grouped magnetizations and high laboratory unblocking temperatures and/or high to moderate coercivities have characteristic remanent magnetizations carried by either magnetite, with minor hematite, or by hematite, with minor magnetite. Results of baked contact tests show that these footwall rocks possess thermoremanent magnetizations or high‐temperature thermochemical remanent magnetizations acquired early in their cooling history, either before or during extension. Our overall interpretation of the paleomagnetic data is that during Cenozoic extension, MCC footwalls tilted (flattened) only a modest amount (&lt;15°) in the direction of tectonic transport. We also interpret Harcuvar and Whipple MCC paleomagnetic data to indicate tectonic tilting (steepening) of the mylonitic zone to form a back dipping mylonitic front. The Whipple‐Bullard‐Eagle Eye detachment fault system and associated mylonites are interpreted to have originated with a low‐angle dip (10° to 25° NE). We relate the small amount of footwall flattening in the direction of tectonic transport and the steepening of the back dipping mylonitic front to the buoyant rise of exhumed footwalls according to rolling‐hinge/isostatic uplift models. Our interpretation refutes the widespread applicability of models that predict MCCs to represent strongly tilted crustal blocks, originally bounded by high/moderate‐angle normal faults. Additionally, a fold test of Harquahala MCC data yielded “negative” results. This suggests that antiformal footwall arches represent the oldest structures of MCCs that developed prior to, or during mylonitization of footwall rocks (before acquisition of the remanent magnetizations carried by these rocks).

Rheological control on the initial geometry of the Raft River detachment fault and shear zone, western United States

The strain, exhumation history, and field orientation of a well‐exposed shear zone and detachment fault in the Raft River Mountains of northwestern Utah, a Cordilleran metamorphic core complex, have been studied to determine the kinematics of ductile shearing and initial orientations of the shear zone and detachment fault. Mapping and strain and kinematic analysis indicate that the top‐to‐the‐east Raft River shear zone initially developed parallel to an unconformity separating Archean rocks from overlying Proterozoic quartzite and schist for at least 24 km in the shear direction. Experimental rock deformation data from lithologies similar to the Archean and Proterozoic rocks suggest the unconformity represented a significant rheological boundary at the deformation temperatures; the base of the shear zone was localized along the boundary between relatively weak quartzite above and stronger monzogranite below. An extensive thermochronological database is used to reconstruct the position of the basement unconformity in temperature‐lateral distance coordinates. The initial average dip of the shear zone and basement unconformity is estimated between 7° and 30°, assuming subhorizontal isotherms and geothermal gradients of 20°–40°C/km. The east dip of the unconformity at the onset of Miocene extension is interpreted to have resulted from late Eocene unroofing and flexure beneath a top‐to‐the‐WNW extensional shear zone in the western Raft River, Grouse Creek, and Albion Mountains. The observations from the Raft River shear zone suggest that the orientation of some midcrustal shear zones may not reflect the predicted orientation for ductile faults according to ductile failure criteria but, rather, the orientation of rheological boundaries along which deformation is localized. Furthermore, detachment faults that are superimposed on mylonite during progressive displacement and footwall unroofing may use an inherited mechanical anisotropy from the mylonite, and their orientations may not reflect the predicted orientation of shear fractures in isotropic rock. The common parallelism between detachment faults and mylonitic foliation may indicate a mechanical and kinematic preference for localization of throughgoing brittle faults parallel to preexisting mylonitic foliation. Because of this preference, studies restricted to detachment faults which lack footwall mylonite or restricted to structural levels between the breakaway and mylonitic front have more bearing on the question of the initial dip of normal‐sense shear fractures (faults) within the seismogenic crust.

The Louzidian Normal Fault near Chifeng, Inner Mongolia: Master Fault of a Quasi-Metamorphic Core Complex

A special metamorphic core complex underlain by a low-angle strike-slip ductile shear zone is present near Chifeng in eastern Inner Mongolia, northern China. The geology of the study area is similar to that of several Cordilleran metamorphic core complexes, but contrasts in significant ways as well. A major ESE-dipping normal fault, the Louzidian Range frontal fault, formed during Late Cretaceous extension. This fault separates a crystalline footwall locally containing mylonitic basement gneisses and granitic rocks (0 to >3 km thick) from a non-metamorphic hanging wall that is distended by normal faults. However, the shear sense of the underlying mylonitic shear zone, a low-angle strike-slip zone, is not compatible with the Louzidian fault. It may be related to a pre-Cretaceous regional sinistral strike-slip event rather than the Late Cretaceous regional crustal extension common throughout eastern China. Pre-existing mylonitic fabric anisotropy appears to have controlled the development of the Louzidian normal fault. Chloritic breccias locally developed along the fault indicate that it cut deeply into the crust of northern China.

Thermobarometric constraints on the tectonothermal evolution of the East Humboldt Range metamorphic core complex, Nevada

The East Humboldt Range of Nevada provides a record of deep-crustal tectonic processes in a classic Cordilleran metamorphic core complex located in the hinterland of the Late Cretaceous to early Tertiary Sevier orogenic belt. New constraints reported here on the metamorphic history of this terrane suggest an overall clockwise pressure-temperature-time ( P-T-t ) path that began with deep tectonic burial and metamorphism at kyanite + staurolite + garnet grade before Late Cretaceous time (possibly in Late Jurassic time?). Subsequently, higher-temperature Late Cretaceous peak metamorphism overprinted this event, resulting in widespread partial melting and leucogranite injection contemporaneous with emplacement of a large-scale recumbent fold (the Winchell Lake nappe). A new 207Pb*/206Pb* date of 84.8 ± 2.8 Ma on syntectonic leucogranite from the hinge zone of this fold constrains the age of this major phase of tectonism. Metamorphism at this time probably reached the second sillimanite isograd, at least at deep structural levels, with peak P-T conditions of 800 °C and >9 kbar. High-grade conditions persisted during extensional tectonic denudation throughout much of Tertiary time. In conjunction with previously published work, the petrologic and thermobarometric results reported here for the northern East Humboldt Range delineate a steeply decompressional P-T trend that extends from ∼9 kbar and 800 °C to 5 kbar and 630 °C. In the light of decompressional reaction textures, microstructural evidence, and previously published thermochronometric results, we interpret this trend as a P-T-t path for Late Cretaceous to Oligocene time. At least 2 kbar of this decompression (equivalent to at least 7 km of denudation) occurred in Late Cretaceous to early Tertiary time. This interpretation supports the idea that tectonic exhumation of deep-crustal rocks in northeastern Nevada began during or immediately after the closing stages of the Sevier orogeny. Finally, the steepness of the proposed P-T-t path implies a thermal evolution from a colder to a much hotter geotherm, a circumstance that probably requires plastic thinning of the lower plate in addition to brittle attenuation and removal of the upper plate during Tertiary extension.

Chronology of polyphase extension in the Windermere Hills, northeast Nevada

Fission-track and 40 Ar/ 39 Ar dating and chemical correlation of volcanic strata exposed in the Windermere Hills and northern Pequop Mountains, northeast Nevada, indicate a protracted, polyphase history of Tertiary (late Eocene‐late Miocene) extension along the northern margin of a major Cordilleran metamorphic core complex. Early extension is recorded by a west-tilted half graben filled with early Oligocene (34.79 ± 0.18‐39.18 ± 0.12 Ma) sedimentary rocks in the eastern Windermere Hills above the lowangle Black Mountain detachment fault. The early Oligocene half graben conformably overlies a widespread suite of late Eocene (39.18 ± 0.12‐ 40.38 ± 0.06 Ma) calc-alkaline volcanic rocks, reflecting a temporal link between early extension at a high structural level and the end of the ignimbrite flare-up. These strata are cut by east-west‐striking normal faults, which are exposed along, and parallel to, the northern margin of the metamorphic complex. Available age data (e.g., between 14.93 ± 0.08 and 34.79 ± 0.18 Ma) permit the interpretation that the east-west‐ striking faults formed at the same time as, or after, large-magnitude unroofing of highgrade rocks. We interpret the east-west‐ striking faults to accommodate differential uplift of greenschist-grade metamorphic rocks in the upper crust, above a lateral ramp in a west-northwest‐directed mylonitic shear zone. Subsequent extension in the Windermere Hills is defined by deep, rapidly filled half grabens of middle Miocene (<7.42 ± 2.0 to 14.93 ± 0.08 Ma) age that unconformably overlie older faults and synextensional deposits. These are the youngest half grabens in the region and are inferred to be initiated by extensional stresses imparted to the base of the lithosphere by a laterally spreading mantle plume (e.g., the Yellowstone hotspot) located in southeastern Oregon at this time.

Geology of metasedimentary rocks and Late Cretaceous deformation history in the northern Valhalla complex, British Columbia

The Valhalla complex, a Cordilleran metamorphic core complex, is a domal culmination made up of gently dipping interlayered sheets of igneous and supracrustal rocks that were deformed and metamorphosed in the Middle Jurassic and Late Cretaceous, and exhumed by extensional faults in the Eocene. Mapping, fabric, and metamorphic studies of predominantly metasedimentary rocks in Valhalla and Passmore domes in the northern part of the complex, together with published geochronological data, reveal a significant Late Cretaceous tectonic history. This includes extensive magmatism, the culmination of upper amphibolite facies metamorphism (approx. 800°C and 8 GPa), migmatization, development of a dominant penetrative transposition foliation, and localization of strain on ductile thrust faults termed the Gwillim Creek shear zones. The Valhalla assemblage, a package of metasedimentary rocks in Valhalla and Passmore domes, comprises a heterogeneous sequence of pelitic schist, marble, calc-silicate gneiss, psammitic gneiss, metaconglomerate, quartzite, amphibolite gneiss, and ultramafic rocks. Based on the presence of distinct laterally continuous marker units and similar lithologic ordering, we propose that the Valhalla assemblage is correlative with part of the Palaeozoic North American stratigraphic succession. If this is correct, then the Valhalla assemblage represents an inverted sequence of strata that has been thinned by as much as 60%; thinning may have occurred during Late Cretaceous transposition foliation development. The Gwillim Creek shear zones, originally mapped in a restricted locality in Gwillim Creek, were found to merge into one broad, ductile shear zone beneath Valhalla dome and extend throughout the entire Valhalla complex. The general style and timing of Late Cretaceous deformation in the Valhalla complex is characteristic of that found throughout the Shuswap complex in a belt of rocks that were at mid-crustal levels during the Cretaceous. This zone is thought to have accommodated Cretaceous - Early Tertiary shortening in the eastern Cordillera, and is the ductile equivalent of the higher level Rocky Mountain thrust belt to the east.

Intracrustal subduction and gravity currents in the deep crust: Sm-Nd, Ar-Ar, and thermobarometric constraints from the Skagit Gneiss Complex, Washington

Isotopic and thermobarometric (pressure-temperature, P-T) data on a gneissic quartz diorite, combined with previous U-Pb ages and P-T data on wall-rock metapelites, define a precise P-T time series for a portion of the Skagit Gneiss Complex, a deep crustal crystalline complex within a zone of intracontinental collision. Concordant U-Pb and Sm-Nd ages on zircon indicate crystallization at 68 Ma, which preceded metamorphism of nearby pelitic wall rocks (garnet core assemblages) and the pluton (plagioclase-hornblende coronas on garnet) at ≈800–900 MPa, 700–800 °C. Following nearly isothermal decompression and sillimanite-cordierite metamorphism of wall rocks (garnet rims plus matrix) at 300–500 MPa, 650–700 °C, a Sm-Nd mineral isochron including two garnet separates records an initial phase of cooling of the complex at 60 Ma. Slow cooling (≈10 °C/m.y.) extends from approximately 60 Ma to 50 Ma. Hornblende and biotite Ar-Ar ages of 47 Ma and 45 Ma, respectively, define a second unroofing event; cooling rates during this period were approximately 100 °C/m.y. Late Cretaceous deep burial followed by two distinct unroofing episodes is also observed in some Cordilleran metamorphic core complexes and may suggest a common origin. We propose that regional shortening in the Cordilleran interior is accommodated by a process of intracrustal subduction, i.e., the upper crust is forced downward into a crustal asthenosphere, resulting in a cold, buoyant root. Heating and weakening of the root cause the formation of gravity currents, effective viscosity being in the range of 10^(16)–10^(19) Pa s, spreading outward below nonfluid upper crust. Spreading of the gravity current, which does not necessarily contribute to crustal thinning, is driven by the small density contrast between the root and its mid-crustal surroundings. Hence unroofing is neither a response to overthick crust nor controlled by the orientation of principal stress axes in the upper crust and upper mantle. The root then cools slowly until final unroofing to near-surface levels, which may occur via either extension or shortening with consequent erosion.

Large-magnitude extensional deformation in the South Mountains metamorphic core complex, Arizona

Paleomagnetic data are used to test controversial aspects of Cordilleran metamorphic core complexes, including the original dip of extensional structures, origin of the mylonitic front, and applicability of rolling-hinge models. We obtained paleomagnetic data (115 sites, 82 accepted for analysis) from the weakly deformed interior of a synkinematic, footwall intrusive suite and Proterozoic footwall rocks of the South Mountains metamorphic core complex, central Arizona. These rocks yield dual polarity, high unblocking temperature, and high to moderate coercivity magnetizations. Positive baked contact tests indicate that footwall rocks possess primary thermoremanent magnetizations (TRMs) or high-temperature thermochemical remanent magnetizations (TCRMs) acquired early in their cooling history and during ductile and brittle extensional deformation of structurally higher rocks. This is consistent with thermochronologic data indicating rapid synkinematic cooling from crystallization through the range of laboratory unblocking temperatures for the magnetic mineralogy of these rocks (between about 22 and 17.5 Ma). Paleomagnetic data are considered as two populations based on the structural asymmetry of the South Mountains metamorphic core complex: (1) a front side characterized by northeast-dipping (∼10) mylonitic fabrics and brittle extensional structures, and (2) a back-dipping side characterized by rollover of the mylonitic zone to form a southwest- or back-dipping (∼15) mylonitic front. Comparison of paleomagnetic data from these two sides suggests that the back-dipping mylonitic front was synkinematically tilted about 10 down-to-the-southwest. The data support a folded shear zone hypothesis for origin of the mylonitic front and the interpretation that footwall rocks possess primary, Miocene-age TRMs or TCRMs. A second regional fold test involved data from sites on both flanks of the topographically prominent northeast-trending mountain range–scale antiform. The negative result from this fold test demonstrates that this structure formed early in the extensional history and prior to magnetization acquisition by the plutons. We obtained a well-grouped footwall grand mean from 62 front-side and 20 back-dipping site means ( N = 82, D = 1.0, I = 51.7, k = 41.8, α 95 = 2.5). We calculated this grand mean with the assumption that front-side sites have remained structurally untilted, whereas back-dipping side sites require removal of 10 of southwest dip. This grand mean is statistically indistinguishable (95% confidence level) from time-averaged Miocene expected directions. We thus conclude that the current gentle dip of front-side mylonites and detachment faults is original. Therefore, both ductile and brittle extensional deformations of the South Mountains metamorphic core complex were accommodated along low-angle structures (dip of ≤15). Our interpretation refutes the widespread applicability of models that predict metamorphic core complexes to represent tilted crustal blocks originally bounded by moderate-angle normal faults and does not support rolling-hinge models of metamorphic core complex evolution that require a moderate-angle ramp.

Constrictional strain in a non-coaxial shear zone: implications for fold and rock fabric development, central mojave metamorphic core complex, california

Finite strain and fold analyses of footwall mylonites in the central Mojave metamorphic core complex (CMMCC) reveal two distinct deformation paths that generated non-coaxial constrictional strain. The first path, recorded in the Waterman Hills, accomplishes constrictional strain through the formation of L-tectonites at the grain scale. The second path, recorded in the Mitchel Range, involves a combination of plane strain at the grain scale and Y-axis shortening through syn-mylonitic folding. The ductile shear zone in the Waterman Hills is approximately 500 m thick and is developed entirely within a Miocene syn-kinematic granodiorite. Strain magnitude decreases structurally downward. The mylonites contain abundant NE-directed non-coaxial microstructures yet are L-tectonites. R f/ø analysis of quartz-ribbon grain shapes indicate prolate strains ( K=6) and bulk chemical data suggest that mylonitization was isochemical or involved volume loss. Therefore, ductile shearing in the Waterman Hills involved true constrictional strain at the grain scale. Immediately to the southeast in the Mitchel Range, the lateral continuation of the shear zone is more than 1000 m thick and primarily involves lithologically heterogeneous pre-Tertiary basement. The mylonitic fabric comprises a well-developed foliation and NE-trending stretching lineation. R f/ø strain analysis was applied to grain shapes of cataclasized and strongly altered garnet porphyroclasts in a peraluminous granite. In contrast to the Waterman Hills, finite strains in these L- S-tectonites approximate plane strain ( K = 1.1). However, the mylonitic fabric is strongly folded about axes uniformly oriented subparallel to the stretching lineation. The coaxial folds range from open to isoclinal. We interpret the folds to have nucleated with axes parallel to the stretching lineation during mylonitization and to have accomplished Y-axis shortening in the shear zone. Macroscopic folds of the brittle detachment spatially correspond to open folds in the ductile shear zone, suggesting that Y-axis shortening continued after the fault zone had reached the brittle regime. Corrugations and folds oriented subparallel to the transport direction are characteristic features of fault zones in Cordilleran metamorphic core complexes. Therefore, constrictional strain may be common in core complexes. Strain compatibility with upper plate rocks in core complexes may be maintained by Y-axis shortening along conjugate strike slip faults. A nearly uniaxial stress field ( σ 3 < σ 2 ≈ σ 1) is consistent with most of the structures formed in Basin and Range extension and could generate the observed constrictional strain.