Maria Luisa Crawford

Melt-enhanced deformation: A major tectonic process

Convergent tectonics between continental crustal blocks result in deep burial and anatexis of supracrustal rocks. Anatectic and/or mantle-derived melts combine to form a melt-weakened zone in the thickened lower crust. The accumulation of melt eventually leads to crustal failure along melt-lubricated shear zones. Rapid (>1 mm/yr vertical component) movements of large crustal blocks result. We refer to these crustal displacements as tectonic surges. The melt-lubricated shears are characterized by the close association of sheared country rocks with foliated or massive igneous sills and plutons. Rocks that formed at different crustal levels are juxtaposed across these shear zones. One result of surges with large lateral component of movement is metamorphic inversion with high-pressure and high-temperature metamorphic rocks structurally over lower pressure and temperature assemblages. Large and rapid vertical surges may displace crust containing abundant melt and may result in high T/P (including granulite facies) metamorphism and preservation of metamorphic textures caused by rapid decompression.

U-Th-Pb geochronology of the Coast Mountains batholith in north-coastal British Columbia: Constraints on age and tectonic evolution

Previously published and new U-Pb geochronologic analyses provide 313 zircon and 59 titanite ages that constrain the igneous and cooling history of the Coast Mountains batholith in north-coastal British Columbia. First-order findings are as follows: (1) This segment of the batholith consists of three portions: a western magmatic belt (emplaced into the outboard Alexander and Wrangellia terranes) that was active 177–162 Ma, 157–142 Ma, and 118–100 Ma; an eastern belt (emplaced into the inboard Stikine and Yukon-Tanana terranes) that was active ca. 180–110 Ma; and a 100–50 Ma belt that was emplaced across much of the orogen during and following mid-Cretaceous juxtaposition of outboard and inboard terranes. (2) Magmatism migrated eastward from 120 to 80 (or 60) Ma at a rate of 2.0–2.7 km/Ma, a rate similar to that recorded by the Sierra Nevada batholith. (3) Magmatic flux was quite variable through time, with high (>35–50 km 3 /Ma per km strike length) flux at 160–140 Ma, 120–78 Ma, and 55–48 Ma, and magmatic lulls at 140–120 Ma and 78–55 Ma. (4) High U/Th values record widespread growth (and/or recrystallization) of metamorphic zircon at 88–76 Ma and 62–52 Ma. (5) U-Pb ages of titanite record rapid cooling of axial portions of the batholith at ca. 55–48 Ma in response to east-side-down motion on regional extensional structures. (6) The magmatic history of this portion of the Coast Mountains batholith is consistent with a tectonic model involving formation of a Late Jurassic–earliest Cretaceous magmatic arc along the northern Cordilleran margin; duplication of this arc system in Early Cretaceous time by >800 km (perhaps 1000–1200 km) of sinistral motion (bringing the northern portion outboard of the southern portion); high-flux magmatism prior to and during orthogonal mid-Cretaceous terrane accretion; low-flux magmatism during Late Cretaceous–Paleocene dextral transpressional motion; and high-flux Eocene magmatism during rapid exhumation in a regime of regional crustal extension.

Reactive bulk assimilation: A model for crust-mantle mixing in silicic magmas

Bulk assimilation of small (millimeters to ∼1 km) fragments of crust—driven and (ultimately) masked by reactions during xenolith melting and magma crystallization—is an important mechanism for crust-mantle mixing. Xenoliths containing mica or amphibole undergo dehydration melting when incorporated into a host magma, yielding mainly plagioclase, pyroxene, Fe-Ti oxides, and hydrous melt. The xenolith is physically compromised by partial melting and begins to disintegrate; xenolithic melt and crystals are mixed into the host magma. Xenocrystic zircon is liberated at this stage. The cryptic character of assimilation is greatly enhanced in any hydrous magma by hydration crystallization reactions (the reverse of dehydration melting). All pyroxenes and oxides (phenocrysts, xenocrysts, or crystals having a hybrid signature) will be subject to these reactions, producing feldspars, amphiboles, and micas that incorporate material from several sources, a particularly effective mixing mechanism. Implicit in the model is a reduced energy penalty for bulk assimilation—much of the assimilant remains in solid form—compared to melt-assimilation models. A large role for bulk assimilation supports stoping as a credible mechanism for the ascent of magmas. While the assimilation of low-density crust and concomitant fractionation provide the isostatic impetus for ascent, the wholesale incorporation and processing of crustal rocks in the magma chamber helps create the room for ascent.

Composition of plagioclase and associated minerals in some schists from Vermont, U.S.A., and South Westland, New Zealand, with inferences about the peristerite solvus

Two suites of regionally metamorphosed semi-pelitic schists were studied in order to investigate the paragenesis of low temperature plagioclase, from which something may be inferred as to the nature of the peristerite solvus at the temperatures and pressures of formation of these rocks: one from the Gile Mountain Formation in the Hanover and Mt. Cube quadrangles, eastern Vermont, U.S.A.; the other from the Alpine schists along the Haast River, South Westland, New Zealand. Plagioclase, muscovite, biotite, chlorite, carbonate, and garnet compositions were determined with an ARL EMX electron probe microanalyzer. The variation in plagioclase composition with increasing grade in the Vermont schists suggests that the peristerite solvus is asymmetrical with a near vertical albite-rich side and a sloping oligoclase-rich side. The top of the solvus appears to lie slightly above the temperature expressed by the almandine isograd in these schists. The compositions of the coexisting albite and oligoclase in the New Zealand rocks suggest a lower geothermal gradient than in Vermont, creating a different pattern of variation in plagioclase composition. Distribution diagrams of Mg, Ti, and AlIV for muscovite-biotite and chlorite-biotite pairs in both suites of rocks support the hypothesis that the plagioclase relations observed represent equilibrium.

Structural history of the crustal-scale Coast shear zone north of Portland Canal, southeast Alaska and British Columbia

Abstract Structural, metamorphic and U–Pb geochronologic data reveal how a steep, crustal-scale shear zone influenced the evolution of the Paleogene Coast Mountains batholith during and since its emplacement. We document two distinct stages of deformation ( D CSZ 3 and D CSZ 4 ) that produced the Coast shear zone north of Portland Inlet. Between 65 Ma and 57 Ma, deformation now preserved within the eastern side of the Coast shear zone ( D CSZ 3 ) produced a moderately to gently, north-northeast-dipping foliation and north-east-plunging mineral lineations. D CSZ 3 involved dominantly east-side-up, top-to-the-southwest displacements during and after the intrusion of tabular tonalite and granodiorite plutons. Widespread crustal thickening followed by rapid exhumation, east-side-up tilting of the batholith, and decompression of rocks equilibrating at 5.6±0.4 kbars, 710±30°C occurred at this time. Prior to D CSZ 3 , deformation ( D WTB 1–2 ) now preserved west of the Coast shear zone resulted in tectonic imbrication of lithologically distinctive crustal fragments at 8–9 kbars, and west- to southwest-vergent ductile thrust faults before ∼92 Ma. From ∼57 Ma to 55 Ma, deformation in the western Coast shear zone ( D CSZ 4 ) produced a narrow, 1–2 km wide, zone comprised of a steeply-dipping to subvertical foliation that overprints and transposes all D WTB 1–2 and D CSZ 3 structures. D CSZ 4 involved bulk east-side-down displacements parallel to a steeply-plunging, down-dip sillimanite lineation and regional tilting of the batholith. This east-side-down displacement may reflect a final period of crustal readjustment and collapse following an earlier period of crustal thickening during batholith construction. The variable history of motion within the Coast shear zone appears to reflect a response to different periods of batholith development within a convergent to obliquely-convergent continental margin.

Batholith emplacement at mid-crustal levels and its exhumation within an obliquely convergent margin

Abstract Emplacement of the central part of the Coast Mountains batholith of northern coastal British Columbia occurred within a regime characterized by oblique convergence between the Farallon/Kula and North American plates. We use new structural, kinematic and U–Pb isotopic data to show that the locations, geometry, and mechanisms of pluton emplacement within this batholith were controlled by displacements within a network of normal faults and transtensional shear zones. These data also show that the most active period of pluton emplacement, from ∼67 to ∼51 Ma, coincided with a change in style of deformation within the batholith. Prior to ∼67 Ma plutons were emplaced within an arc dominated by regional-scale contractional shear zones. In contrast, emplacement of 67–51 Ma plutons occurred in an arc increasingly dominated by normal faults with arc-parallel to oblique displacement and by sinistral transtensional shear zones. We have identified and mapped the structure of three plutonic complexes composed of 67 to 51 Ma plutons: the Khyex sill complex, Arden Lake plutonic complex and Quottoon plutonic complex. Shear-zone-controlled emplacement of plutons within the batholith accounts for the widely different orientations and structural features that characterize plutons within these three complexes. During and after this latest Cretaceous–Paleogene period of intense plutonic activity and accompanying deformation, the deep roots of the batholith were rapidly unroofed by ductile normal faulting prior to 50 Ma.

CO2-Brine immiscibility at high temperatures, evidence from calcareous metasedimentary rocks

Study of fluid inclusions in quartz segregations and in the rock matrix of a calcareous psammite and a carbonate schist suggests that brines containing 23–24 weight percent salt (NaCl equivalent) are immiscible with CO2 at the metamorphic conditions of approximately 600° and 6.5 Kb. The presence of a high temperature solvus between saline brine and CO2 is supported by other fluid inclusion studies as well as experimental measurements from the literature. As saline brines are common in metamorphic and hydrothermal systems, CO2-brine immiscibility should play an important role in petrogenesis. The fluid inclusions preserved in the quartz segregations probably represent the fluids generated by prograde metamorphic reactions, whereas the compositions of the fluids trapped in the rock matrix quartz suggest they have reequilibrated with the matrix minerals during incipient retrograde reactions. The isochores from the densest inclusions observed in this study pass close to the inferred peak metamorphic conditions; other isochores suggest an episode of deformation and recrystallization at 275° C and 1.4 Kb. Using the density information preserved in all the inclusions, a convex-downward uplift path on a P-T diagram is inferred for these rocks.

High-temperature arc-parallel normal faulting and transtension at the roots of an obliquely convergent orogen

Structural and kinematic data from northern coastal British Columbia (∼54.5°N) document intra-arc deformation patterns at mid-crustal levels during and after emplacement of the Coast Mountains batholith. Major arc-parallel displacements occurred along high-temperature (700 °C) ductile normal faults and steep sinistral transtensional shear zones within an obliquely convergent margin. Strain patterns and pluton emplacement were controlled by (1) structural anisotropies in host rocks that predate batholith emplacement, and (2) kinematic compatibility requirements created by simultaneous motion on curved, pluton-bounding shear zones. These controls superseded a partitioning of the arc-parallel and arc-normal components of oblique-plate convergence onto strike-slip and thrust faults, respectively.

Exhumation and tilting of the western metamorphic belt of the Coast orogen in southern southeastern Alaska

The uplift history of the western metamorphic belt in southern southeastern Alaska can be traced using emplacement depths and cooling ages of mid-Cretaceous plutons which intruded the metamorphic belt during the period 101 Ma to 89 Ma. During the period 101 Ma to 94 Ma the rocks of the metamorphic belt now at the surface were buried to a depth of approximately 30 km. Pressure data indicate that by 89 Ma the western portion of the metamorphic belt was at a shallower crustal level (∼23 km within the study area) while the eastern side remained at a depth of approximately 30 km. Cooling curves constructed from minerals in mid-Cretaceous plutonic rocks indicate that the western side of the metamorphic belt cooled earlier than the eastern side; an observation consistent with the inferred uplift history. Southwestward transport of metamorphic belt rocks within the hanging walls of thrust and high-angle faults may have provided one mechanism for uplift during mid- to late-Cretaceous time. The eastern portion of the western metamorphic belt cooled rapidly at ∼55 Ma, probably in response to a period of rapid uplift. The Coast shear zone, an Early Tertiary high-angle fault with west-side-up displacement, bounds the east side of the metamorphic belt. We suggest Tertiary uplift occurred by tilting of the metamorphic belt due to movement along this fault.

Pressure-temperature and evolution of fluid compositions of Al2SiO5-bearing rocks, Mica Creek, B.C., in light of fluid inclusion data and mineral equilibria

AbstractMetamorphosed pelitic rocks from Mica Creek, British Columbia contain sillimanite, kyanite with minor fibrolite and andalusite-bearing quartz pods. Mineral equilibria were used to infer peak P-T conditions and fluid compositions in equilibrium with the solid phases. Fluid inclusions in three schist samples appear to be good indicators of conditions affecting those rocks during and after peak metamorphic conditions. In samples from two localities, fluid inclusions from schist and quartz-rich segregations have densities appropriate to the peak metamorphic conditions. The observed compositions for these fluids (low salinity with ≅12 mole % dissolved CO2) agree with calculated