Donna L. Whitney

Gneiss domes and orogeny

Many gneiss domes record positive feedback between decompression and partial melting of orogenic middle crust. Exhumed orogens are riddled with gneiss domes cored by migmatites that underwent dehydration melting during decompression. The decreasing buoyancy associated with increasing melt fraction drives further decompression at near-isothermal conditions as the partially molten crust rises diapirically. This combination of processes may explain the generation and retention of large volumes of crustally derived melt recorded in many deep-seated migmatite terranes and inferred for active orogens. In exhumed orogens, the signature of the rapid ascent of partially molten crust is a gneiss dome cored by migmatite ± granite. The large volume of material involved in the vertical transfer of partially molten crust indicates that the formation of gneiss domes is an efficient mechanism for heat advection during orogenesis.

High-resolution records of the late Paleocene thermal maximum and circum-Caribbean volcanism: Is there a causal link?

Two recently drilled Caribbean sites contain expanded sedimentary records of the late Paleocene thermal maximum, a dramatic global warming event that occurred at ca. 55 Ma. The records document significant environmental changes, including deep-water oxygen deficiency and a mass extinction of deep-sea fauna, intertwined with evidence for a major episode of explosive volcanism. We postulate that this volcanism initiated a reordering of ocean circulation that resulted in rapid global warming and dramatic changes in the Earth’s environment.

Continental and oceanic core complexes

Core-complex formation driven by lithospheric extension is a first-order process of heat and mass transfer in the Earth. Core-complex structures have been recognized in the continents, at slow- and ultraslow-spreading mid-ocean ridges, and at continental rifted margins; in each of these settings, extension has driven the exhumation of deep crust and/or upper mantle. The style of extension and the magnitude of core-complex exhumation are determined fundamentally by rheology: (1) Coupling between brittle and ductile layers regulates fault patterns in the brittle layer; and (2) viscosity of the flowing layer is controlled dominantly by the synextension geotherm and the presence or absence of melt. The pressure-temperature-time-fluid-deformation history of core complexes, investigated via field- and modeling-based studies, reveals the magnitude, rate, and mechanisms of advection of heat and material from deep to shallow levels, as well as the consequences for the chemical and physical evolution of the lithosphere, including the role of core-complex development in crustal differentiation, global element cycles, and ore formation. In this review, we provide a survey of ∼40 yr of core-complex literature, discuss processes and questions relevant to the formation and evolution of core complexes in continental and oceanic settings, highlight the significance of core complexes for lithosphere dynamics, and propose a few possible directions for future research.

Extension rates, crustal melting, and core complex dynamics

Two-dimensional thermomechanical experiments reveal that the crystallization versus exhumation histories of migmatite cores in metamorphic core complexes give insights into the driving far-field extensional strain rates. At high strain rates, migmatite cores crystallize and cool along a hot geothermal gradient (35–65 °C km−1) after the bulk of their exhumation. At low strain rates, migmatite cores crystallize at higher pressure before the bulk of their exhumation, which is accommodated by solid-state deformation along a cooler geothermal gradient (20–35 °C km−1). In the cases of boundary-driven extension, space is provided for the domes, and therefore the buoyancy of migmatite cores contributes little to the dynamics of metamorphic core complexes. The presence of melt favors heterogeneous bulk pure shear of the dome, as opposed to bulk simple shear, which dominates in melt-absent experiments. The position of migmatite cores in their domes reveals the initial dip direction of detachment faults. The migmatitic Shuswap core complex (British Columbia, Canada) and the Ruby–East Humboldt Range (Nevada, United States) possibly exemplify metamorphic core complexes driven by faster and slower extension, respectively.

Partial melting and decompression of the Thor-Odin dome, Shuswap metamorphic core complex, Canadian Cordillera

Abstract The Thor-Odin dome region of the Shuswap metamorphic core complex, British Columbia, contains migmatitic rocks exhumed from the deep mid-crust of the Cordilleran orogen. Extensive partial melting occurred during decompression of the structurally deepest rocks, and this decompression path is particularly well recorded by mafic boudins of silica-undersaturated, aluminous rocks. These mafic boudins contain the high-temperature assemblages gedrite+cordierite+spinel+corundum+kyanite/sillimanite±sapphirine±hogbomite and gedrite+cordierite+spinel+corundum+kyanite/sillimanite+garnet±staurolite (relict)±anorthite. The boudins are interlayered with migmatitic metapelitic gneiss and orthogneiss in this region. The mineral assemblages and reaction textures in these rocks record decompression from the kyanite zone ( P >8–10 kbar) to the sillimanite–cordierite zone ( P T ∼750 °C, with maximum recorded temperatures of ∼800 °C. Evidence for high-temperature decompression includes the partial replacement of garnet by cordierite, the partial to complete replacement of kyanite by corundum+cordierite+spinel (hercynite)±sapphirine±hogbomite symplectite, and the replacement of some kyanite grains by sillimanite. Kyanite partially replaced by sillimanite, and sillimanite with coronas of cordierite±spinel are also observed in the associated metapelitic rocks. Partial melt from the surrounding migmatitic gneisses has invaded the mafic boudins. Cordierite reaction rims occur where minerals in the boudins interacted with leucocratic melt. When combined with existing structural and geochronologic data from migmatites and leucogranites in the region, these petrologic constraints suggest that high-temperature decompression was coeval with partial melting in the Thor-Odin dome. These data are used to evaluate the relationship between partial melting of the mid-crust and localized exhumation of deep, hot rocks by extensional and diapiric processes.

Microhardness, toughness, and modulus of Mohs scale minerals

Abstract We report new results of microhardness and depth-sensing indentation (DSI) experiments for the fist nine minerals in the Mohs scale: talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, and corundum. The Mohs scale is based on a relative measure of scratch resistance, but because scratching involves both loading and shearing, scratch resistance is not equivalent to hardness as measured by modern loading (indentation) methods; scratch resistance is also related to other material properties (fracture toughness, elastic modulus). To better understand the relationship of hardness to scratch resistance, we systematically determined hardness, fracture toughness, and elastic modulus for Mohs minerals. We measured hardness and toughness using microindentation, and modulus and hardness with DSI (.nanoindentation.) experiments. None of the measured properties increases consistently or linearly with Mohs number for the entire scale.

Flow of partially molten crust and origin of detachments during collapse of the Cordilleran Orogen

Abstract In metamorphic core complexes two types of detachments develop, coupled by flow of partially molten crust: a channel detachment and a rolling-hinge detachment. The channel detachment, on the hinterland side of the orogen, represents the long-lived interface that separates the partially molten crust flowing in a channel from the rigid upper crustal lid. On the foreland side of the core complex, a rolling-hinge detachment develops. This detachment dips toward the foreland, probably affects the whole crust, and its geometry is governed by strain localization at the critical interface between cold foreland and hot hinterland. Activation of the rolling-hinge detachment drives rapid decompression and melting, leading to the diapiric rise of migmatite domes in the footwall of the detachment. A kinematic hinge (switch in sense of shear) separates the two types of detachments. Structural, metamorphic and geo/thermochronological studies in the Shuswap core complex (North American Cordillera), combined with an anisotropy of magnetic susceptibility study of leucogranites concentrated in the detachments, suggest that this orogen collapsed rapidly through the development of channel and rolling-hinge detachments in the early Eocene. The kinematic hinge is currently located approximately 40 km west of the footwall in which it originated, corresponding to a mean exhumation rate of >5 km Ma−1, which explains the near-isothermal decompression recorded within the migmatite dome.

Why is lawsonite eclogite so rare? Metamorphism and preservation of lawsonite eclogite, Sivrihisar, Turkey

A rare occurrence of lawsonite eclogite crops out in a belt of high-pressure rocks in the Sivrihisar Massif, Turkey. Although lawsonite eclogite is predicted to be common at depths of ∼45–300 km in subduction zones, lawsonite seldom survives exhumation. In the Sivrihisar Massif, lawsonite eclogite occurs as 5-cm-long to 3.5-m-long pods in lawsonite blueschist and blueschist facies quartzite and marble. We have identified >70 eclogite pods within an ∼14 km 2 area; most of them are lawsonite bearing. Phase diagrams calculated for Sivrihisar eclogite bulk compositions indicate metamorphic conditions of 21– 24 kbar, ∼422–580 °C. High-pressure minerals (omphacite, lawsonite) are synkinematic with respect to the main fabric, and therefore preserve chemical and structural features of high-pressure subduction metamorphism. The presence of both pristine and retrogressed lawsonite eclogite within meters of each other suggests that lawsonite eclogite preservation is related to rapid exhumation and other factors. For example, the effects of exhumation-related deformation ± fluid infiltration may locally overprint some rocks, producing clinozoisite and/or epidote, whereas rocks that escape these effects retain primary lawsonite.

Metamorphic history of the southern Menderes massif, western Turkey

Metamorphic mineral compositions and textures are integrated with microstructures to test tectonic models for the construction and exhumation of a mid-crustal terrane in western Turkey. The southern Menderes massif, part of the Alpine orogen, is composed of a tilted sequence of metasedimentary rocks that structurally overlies orthogneiss. Garnet-biotite equilibria for schists collected along north-south traverses consistently indicate temperatures of 430 °C for the southernmost, structurally highest garnet-bearing rocks, ∼500 °C for structurally intermediate rocks, and ∼550 °C for the structurally deepest rocks near the contact with the orthogneiss. There are no detectable discontinuities in metamorphic grade from north to south within the schist unit, or between the schist and overlying lower-grade and underlying higher-grade rocks. Pressure estimates are less certain; geobarometric results for the schists yield P ≈ 8 kbar, but mineral assemblages and structural data are consistent with lower pressures (≤6 kbar). Metamorphic textures and garnet zoning in the Menderes schists indicate metamorphism during a single thermal event accompanying development of the main penetrative foliation. Top-to-north fabrics formed during folding/thrusting and were synkinematic with garnet growth. Top-to-south chloritic shear bands truncate the earlier foliation and overprint peak-metamorphic assemblages and textures. We propose that these metamorphic features and structures indicate synmetamorphic shortening followed by extension during Alpine orogenesis.

Core complex development in central Anatolia, Turkey

Tectonic models for the central Anatolian segment of the Alpine-Himalayan orogen have not considered this region as part of the Aegean extensional province. We have found evidence, however, for Oligocene-Miocene extension and exhumation of midcrustal rocks in a metamorphic dome (Nigde massif) that is structurally and petrologically similar to Miocene core complexes >500 km to the west. Strong correspondence in the timing and kinematic development of the Menderes (western Turkey) and Nigde core complexes suggests that extensional tectonics and exhumation of mid- to lower-crustal rocks affected much of the eastern Mediterranean region. In the Nigde massif, supracrustal rocks were buried to depths of 16‐20 km at high temperatures (>700 °C) during contraction associated with closure of Neo-Tethyan seaways in late Mesozoic‐early Cenozoic time. Following cooling and decompression to <600 °C and <10 km, metasedimentary rocks underwent a second heating event at low pressures during Miocene magmatism that postdated much of the unroofing of the massif. Development of the Nigde core complex was related to exhumation of thickened and thermally weakened continental crust in the upper plate of a north-dipping subduction zone. The partial subduction of the Tauride carbonate platform in the Eocene resulted in choking of the subduction zone, followed by isostatic rebound and exhumation of the buoyant platform. This in turn caused a rapid emergence of the upper plate, resulting in erosion of upper-crustal rocks and exhumation of midcrustal rocks along the northern edge of the Inner Tauride suture zone. Although northern regions of central Anatolia contain the same protoliths as the Nigde massif and underwent extensive Tertiary magmatism, they did not undergo similar extension, suggesting that core complex development in south-central Turkey was controlled by the location of thickened and thermally weakened crust adjacent to a suture zone.