Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy最新文献

Geophysical and field-based studies indicate that granitic plutons occur as either tabular (disk) or wedge (funnel) shapes whose length (L) to thickness (T) ratio is controlled by the empirical power law, T = 0.6(±0.15)L^0.6(±0.1). The dimensions of plutons are self-similar to other natural subsidence phenomena (calderas, ice cauldrons, sinkholes, ice pits) and it is proposed that they grow in a similar fashion by withdrawal of material (melt) from an underlying source, which is then transferred to the growing pluton within the crust. Experimental studies show that growth of subsidence structures occurs by vertical inflation ⪢ horizontal elongation of an initial depression with L ≈ width of the source region. If pluton growth is modelled in the same way, the empirical power law relating T and L defines limits for pluton growth that are imposed by the width, thickness and degree of partial melting from a lower crustal source. Several growth modes that predict testable internal structural patterns are identified for plutons, depending on whether they are tabular or wedge-shaped, grow by continuous or pulsed magma delivery and whether magma is accreted from bottom to top, or vice versa. Rates of pluton growth are geologically fast (hundreds to hundreds of thousands of years) if magma supply is effectively continuous, but can also take millions of years if the time between magma delivery events is much longer than magma injection events. Plutons formed by melt extraction from an area directly beneath require large degrees of partial melting and or very thick sources. Lower degrees of partial melting and thinner sources are permitted when melt extraction occurs over a larger region, which can lead to the formation of spaced plutons. Tabular pluton growth will tend to favour widely spaced plutons, unless degrees of partial melting in the source are high. Wedge-shaped plutons can form much closer together and require lower degrees of partial melting. These results are in general agreement with current geophysical, petrological and experimental estimates of partial melting in the lower continental crust.

A new model is proposed to numerically simulate transfer of melt between microscopic pores and macroscopic veins in a deforming porous matrix. Matrix rheology is assumed to be visco-elastic. Darcy flow of porous melt through the matrix is calculated in accord with the theory of poroelasticity. Veins of melt are described separately. The model is realized using a code for a 2-D rectangle that is deformed at a constant strain rate. We reproduce in 2-D the main analytical results derived by Sleep (1988) but add calculations concerning the flow and local compaction processes around veins with different inclinations to the maximum (compressive) deviatoric stress. Inclusions perpendicular to σ₁ tend to close while those parallel to σ₁ tend to grow. Surrounding regions either compact or dilate and inclined veins propagate parallel to σ₁. The incremental porosity decreases exponentially with distance from the vein walls by a factor equal to the compaction length. Local redistribution of melt from microscopic pores to macroscopic veins strongly enhances melt segregation into the vein networks which can lead to bodies sufficiently massive to become buoyant.

The Closepet Granite, in South India, is a large, syntectonic Archaean granitic complex. Differential erosion has exposed it from the lower (25 km) to upper crust (5 km). Four main parts are recognized from bottom to top: (i) A root zone, where magmas formed, collected and rose within active shear zones, leaving schlieren behind. The surrounding crust was highly ductile, leading to diffuse deformation. (ii) A transfer zone, where the magma was progressively enriched in K-feldspar phenocrysts during its ascent. In this part, the granite rose as a mush moving as a whole within a less ductile crust. Slow cooling was responsible for a long magma residence time under conditions favoring to fabric enhancement and strain partitioning, leading to horizontal and vertical melt migration. (iii) A “gap” (dyke complex that acted as a filter zone), were the ascent of the mush was stopped, probably due to high phenocryst load and high viscosity contrast with the wall rocks. Only crystal-poor melts could continue their ascent through the dykes. (iv) A zone of shallow intrusions, where the liquids extracted from the mush filled small, elliptical plutons, cooling quickly and developing only very weak fabrics.

The Cretaceous Mt. Stuart batholith was syntectonically emplaced within amphibolite grade metasedimentary rocks of the Cascades Crystalline Core, Washington State. The northern part of the batholith defines a NW—SE trending antiformal fold. We present fabric data from that part of the batholith, collected during field mapping and anisotropy of magnetic susceptibility (AMS) measurements. The significance of the data is discussed in terms of regional tectonic deformation and plate kinematics. The data were collected from rocks with well preserved igneous textures and the fabrics therefore formed during magmatic deformation. The AMS provides measurements of the preferred orientations of Fe-rich minerals (biotite ± hornblende ± traces of pyrrhotite and magnetite) which are consistent with field measurements of the mesoscopic fabrics defined by plagioclase, biotite and hornblende crystals. The magnetic fabrics are also consistent with the orientations of folds, mineral fabrics and boudinage structures that record high-temperature subsolidus deformation in the margin of the pluton and in its host rocks. The lineations are parallel to the stretching direction associated with small increments of strain that occurred during deformation of the magmatic arc, as the batholith was crystallizing and deforming in the tectonic stress field, ca. 93 Ma. The fabrics in the Mt. Stuart batholith are used to infer emplacement in a magmatic arc during either 1) plate displacement perpendicular to a NW-SE trending plate margin, or 2) wrench dominated transpression. In the second case the analysis suggests a nearly N-S plate vector along the western North American margin during plutonism. The results demonstrate the potential usefulness of magmatic fabrics in syntectonic plutons for plate tectonic analyses of orogenic belts.

Granitic (and other) magmas with crystal contents between 50 and ca.70% are expected to show dilatant behavior during deformation. The grain size at which the magma has been crystallised is shown to be relevant to the development of excess pore pressure at continued shearing. The reigning pressure regime is compared to the stresses required for fracturing of the skeletal elements. At rates of loading in excess of average tectonic rates (≥ 10⁻¹⁴ s⁻¹), shear-induced dilation in granitic magmas with high solidosities (crystal contents >50%), can lead to fracture. The available excess skeletal pressure at a given strain rate is a function of two coupled parameters, grain size and tortuosity, with higher skeletal pressures favoured by smaller mean particle size. Our analysis suggests that the common occurrence of brittle-like features thought to have formed in the magmatic state during pluton crystallisation can only be achieved where strain rates (emplacement loading) are at least of the order 10⁻¹³ s⁻¹ or greater, consistent with similar estimates of strain rates during pluton emplacement based on field studies.

Analog experiments of melt segregation and migration in lower crustal rocks have been conducted using paraffin wax. The wax has a mechanical planar anisotropy which reproduces the pervasive foliation of high-grade metamorphic rocks. The shortening of a layer of partially molten wax (melt fraction between 15 and 20%) results in the movement of a part of the liquid from the microscopic porosity of the wax to the outside of the layer in large accumulation sites. Four stages can be identified: (1) from the beginning of the shortening, melt segregates into dilatant foliation-parallel veins; (2) the development of a fold occurs with an increasing accumulation of liquid in the limbs; (3) strain localization and vein connection allows the nucleation of shear bands; (4) melt migration is channelled by the shear band toward external pockets. The first two stages involve melt percolation from kinematically controlled high-stress areas around growing veins. The third stage is associated with local attainment of a segregated melt critical concentration estimated at 14–15%. The last point involves both horizontal and upward migration of the melt. Melt segregation and migration are highly scale- and strain-dependent mechanisms.