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1.
A cellular automaton model is presented that simulates melt extraction from migmatites. The varying parameters are the relative amount of melt, thresholds for melt connection leading to movement and melt escape, and type of deformation (pure and/or simple shear). It is shown that when the melt percentage is about half that of the melt escape threshold, the competition between production and deformation-assisted extraction results in a discontinuous rate of melt extraction. The process can be compared to stick-slip motion during frictional experiments. At higher melt fraction, it is continuously extracted. Deformation assists significantly in segregating melt, particularly when a pure shear (compaction) component is added to simple shear. 相似文献
2.
Kazuaki Nakamura Vincent Renard Jacques Angelier Jacques Azema Jacques Bourgois Christine Deplus Kantaro Fujioka Yozo Hamano Philippe Huchon Hajimu Kinoshita Pierre Labaume Yujiro Ogawa Tetsuzo Seno Akira Takeuchi Manabu Tanahashi Akinori Uchiyama Jean-Louis Vigneresse 《Earth and Planetary Science Letters》1987,83(1-4)
Leg 2 of the French-Japanese 1984 Kaiko cruise has surveyed the Suruga and the Sagami Troughs, which lie on both sides of the northwestward moving and colliding Izu-Bonin Ridge, the northernmost part of the Philippine Sea plate. The transition from the Nankai Trough to the Suruga Trough is characterized by northward decrease in width of the accretionary prism, in good agreement with the increasing obliquity between the through axis and the direction of the convergence, as the strike of the convergent boundary changes from ENE-NNE to south-north. South of the area, the southern margin of the Zenisu Ridge shows contractional deformations. This supports the interpretation made by the team of Leg 1 who studied the western extension of the area we studied, that it is an intra-oceanic thrusting of the ridge over the Shikoku Basin. In the Sagami Trough, where the relative motion is highly oblique to the plate boundary, active subduction is mostly confined in the east-west trending portions of the trough located south of the Boso Peninsula and along the lower Boso Canyon, near the TTT triple junction. In between, the present motion is mainly right-lateral along the northwest trending Boso escarpment. However, an inactive but recent (Pliocene to lower Pleistocene) accretionary prism exists south of the Boso escarpment, which suggests that the relative motion was more northerly than at present before about 1 Ma ago. 相似文献
3.
When applied to homogeneous rocks, the statistical technique of autocorrelation helps in visualizing the preferential alignment of mineral barycenters. We examine the behaviour of each element within a regular lattice deformed in either simple or pure shear. Preferential directions immediately appear depending upon the relative orientation of the shear or flattening plane with respect to the lattice. Autocorrelation amplifies these directions (either one direction with a high intensity signal or two directions with weak intensity signals). They depend on the geometrical shape of the elementary mesh of the lattice which is changing during the deformation. From the autocorrelated diagram, geometrical parameters of the elementary mesh can be estimated. Resulting from the numerical simulations, graphs are constructed which provide strain estimates in the case of simple shear. Application to natural rocks deformation is given in the case of simple-sheared peridotites. 相似文献
4.
Thermal data represent a valuable remote sensing aid in studying crustal evolution. Heat flow (Q) results from the heat loss by the cooling earth and from the heat production ( A ) of the radiogenic elements brought to the upper crust by magmatic intrusives. Heat flow is often observed to be linearly related to heat production. The slope of this relation, or thermal depth (D), has been used to infer a global upward enrichment in heat-producing elements. This thermal depth has been equated with the thickness of granites, but such an interpretation has not been confirmed everywhere. The depth to which granitic plutons are rooted can be computed from the inversion of gravity data. It averages 7±2 km and is much smaller than the thermal depth. Granulite facies rocks are assumed to be present in the lower crust on the basis of seismic and geochemical data. These rocks are generally depleted or initially poor in radiogenic elements (U, Th, K). It is suggested that the thermal depth reflects the depth to the depleted layer in continental regions and that it corresponds to the granulitic layer in most places. Worldwide thermal and seismic data support this relationship, although surface heterogeneities introduce complications. Thermal data can therefore be used to constrain the structure of the crust and its evolution through time. 相似文献
5.
Jean-Louis Vigneresse 《地学前缘(英文版)》2019,10(4):1613-1622
Common base and noble metals represent an important economic factor in the actual industrial development.For instance the world resources for copper are actually estimated for about the next 30 years only.The situation requires rethinking the way major ore deposits form,leading to new guides for exploration.The present paper briefly examines the processes leading to ore formation,in relation with granitic or granodioritic intrusions.It identifies the importance of metal enrichment during the magmatic stage.Within the magma chamber that forms porphyry intrusions,metals may incorporate to first formed crystals,becoming inert;concentrate into the residual melt of a mush;or segregate by diffusion into the exsolved magmatic volatile phase(MVP)into which they are transported and precipitated.A competition results between elements partitioning and diffusivity.Hence,a specific Peclet number for each metal(Cu,Au,Ag,Mo,W,Sn,and REE)controls the ratio between the diffusive and the advective flux.Metals diffusivity in the melt shows differential behavior relative to a threshold of about10 13 m2/s.Metals with slower diffusivity values(e.g.As)will not concentrate.Conversely,fast diffusive metals(Au,Ag,Cu)may rapidly incorporate the MVP,provided an adequate component(halogens or S)is attractive for metals.The chemistry of the MVP escaping the magma induces different alteration patterns.Their relative content in F,Cl or S,attested by the composition of biotites and apatites,links with the preferential content of metals in o re deposits,representing a valuable tool for exploration.Finally the model is replaced in a set of coupled mechanical-chemical instabilities,within a three phase material. 相似文献
6.
From structural studies in seventeen ophiolite massifs, information has been drawn about the activity of the asthenosphere beneath oceanic spreading centers. This information, together with geophysical data pertaining to oceanic ridges, has been integrated into a numerical model. It is inferred that for a fast-spreading ridge (< 5 cm/yr) a local diapiric uprise of melted peridodite is superimposed to the main circulation driven by the lithosphere drifting. As a result, the upward flow splits, at a depth of 35 km, into a 10–20 km wide axial jet and a horizontal flow diverging away from the ridge. The small diapir channels upward most of the partially molten peridotites. Magmatic activity in the crust is thus constrained to an area of similar horizontal extension, in agreement with ophiolitic and oceanic data. Also in agreement with findings in ophiolites, the meeting at a few tens of kilometers away from the ridge of the two diverging mantle circulations, creates a change in shear sense of the flow. From there on, the horizontal diverging flow is compatible with lithosphere drift. 相似文献
7.
For a long time, granites have been considered as passive bodies ascending under intrinsic negative density and viscosity contrasts with their host rocks. Chemical variations within a granitic body resulted from in situ differentiation and crystal fractionation. Since the mid 1980s, this global view has been significantly modified by (i) shifting melting from water-saturated conditions to fluid-absent reactions, (ii) increasing the role played by the mantle during granite generation, (iii) reassessing the rheology of partially molten rocks, (iv) demonstrating stepwise segregation and ascent of magmas by analogue and numerical models, (v) combining structural, geophysical and geochemical studies to reveal the internal structures in granitic plutons. It results that a granitic body is built up by a discontinuous accumulation of successive magma intrusions. The discontinuous nature of magma emplacement has also significant consequences for its ability to generate ore. The processes that lead to ore deposits are examined, with a brief review of the magmatic and fluid phases that concentrate ore forming elements. Examples are taken from crustal-derived granites and porphyry-type deposits. Those are considered as the two end-members of magmatic and hydrothermal ore deposits. The source characteristics of the magma, the emplacement mechanisms and magma mixing processes are the frame that controls the potential to carry base metals with the magma. The distribution of elements is controlled by diffusion, partition between minerals and melt, solubility and redox conditions. Variations of those parameters are examined by considering their activation energy which controls the exponential dependence with temperature. A characteristic length depending on the activation energy, temperature variation and time is estimated for a characteristic time lag of 30 ka. The intrusion of a magma into a magma chamber of similar composition, hence temperature, has few effects on diffusion, partition coefficient and redox conditions, because of a too low temperature contrast. The intrusion of a mafic magma into a felsic one induces a variation of 300 °C in both magmas. The characteristic length of diffusion may vary by up to two orders of magnitude, whereas the variation of partition coefficients is only one order of magnitude. The redox conditions are about 2.5 log unit in the mafic magma, but they can vary by 7 log units in the felsic magma. Hence, a strong decrease in δD values is observed in porphyry-type deposits. The effect is a removal of the elements with higher activation energy (W, Sn, Zr) from the mafic to the felsic magma. Deformation during the late stages of emplacement also controls ore formation. 相似文献
8.
Pull-apart formation and strike-slip partitioning in an obliquely divergent setting, Leka Ophiolite, Norway 总被引：1，自引：0，他引：1
The Leka Ophiolite Complex (LOC) is located on the island of Leka, Norway, and belongs to the Uppermost Allochthon of the Scandinavian Caledonides. The rocks of the adjacent mainland and most of the surrounding islands are basement gneisses and supracrustal rocks not related to the ophiolite complex. Paleostress analysis, gravity inversion, and regional geology support a fault-bounded rhombochasm geometry for the LOC. The paleostress inversions revealed two types of tensors, interpreted as small strains: (1) horizontal extension, generally E–W to NE–SW, and (2) horizontal extension in the same direction with an added component of perpendicular horizontal contraction. A strong positive gravity anomaly (25 mGal) is centered on Leka, and gravity inversion indicates that the LOC lies directly below its surface exposures with steep-sided walls and a flat bottom located at 7 km depth. The faults bounding the LOC probably initiated during postorogenic extension in the Scandinavian Caledonides. The faults are regional in scale and are parallel to other NE–SW trending en echelon faults along the Norwegian coastline and on the adjacent mainland.A pull-apart structure explains the down-dropping and subsequent preservation of the LOC, as it is surrounded by rocks from lower structural positions within the nappe stack. The paleostress directions from Leka support a sinistral component of shear along these faults. The gravity inversion is consistent with a fault-bounded geometry. This pull-apart structure, as uniquely recorded by the dense ophiolitic rocks, suggests that strike-slip partitioning was active in an obliquely divergent setting. 相似文献
9.
Vincent Renard Kazuaki Nakamura Jacques Angelier Jacques Azema Jacques Bourgois Christine Deplus Kantaro Fujioka Yozo Hamano Philippe Huchon Hajimu Kinoshita Pierre Labaume Yujiro Ogawa Tetsuzo Seno Akira Takeuchi Manabu Tanahashi Akinori Uchiyama Jean-Louis Vigneresse 《Earth and Planetary Science Letters》1987,83(1-4)
Leg 2 of the French-Japanese 1984 Kaiko cruise has surveyed the trench triple junction off central Japan, where the Japan, Izu-Bonin and Sagami Trenches intersect. The Izu-Bonin Trench is deeper than the Japan Trench and filled by a thick turbiditic series. Its anomalous depth is explained by the westward retreat of the edge of the northwestward moving Philippine Sea plate. On the contrary to what happens in the Japan Trench, horst and graben structures of the Pacific plate obliquely enters the Izu-Bonin Trench, suggesting that the actual boundary between these two trenches is located to the north of the triple junction. The inner wall of the Izu-Bonin Trench is characterized in the triple junction area by a series of slope basins whose occurrence is related to the dynamics of this area. The northernmost basin is overthrust by the edge of the fore-arc area of the Northeast Japan plate. The plate boundary is hardly discernible further east, which makes it impossible to locate precisely the triple junction itself. These features suggest that large intra-plate deformation occurs there due to the interaction of the plates involved in the triple junction and the weak mechanical strength of the wedge-shaped margin of the overriding plates. 相似文献
10.
L. Amir L. Martinez J.R. Disnar J.-L. Vigneresse R. Michels F. Guillocheau C. Robin 《Tectonophysics》2005,400(1-4):227-240
Many studies investigated the thermal modeling of the Paris basin for petroleum interests during the 1970s. Most of the softwares developed by oil companies or research centers were based on the assumption of a constant thermal gradient. In order to take into consideration the variation of the thermal gradient during basin evolution, we developed the TherMO's Visual Basic 1D program. We applied our model to twenty boreholes located along a cross-section roughly running EW over 150 km in the center of the Paris basin. The numerical results were calibrated with organic matter maturity data. TherMO's simulates the amount of heat provided to the sedimentary organic matter. The heat parameter simulated shows lateral variation along the cross-section. It decreases from Rambouillet to Trou Aux Loups boreholes (87–66 mW/m2) at about 100 km more to the east whereas the heat flux value simulated is 73 mW/m2 in St. Loup borehole. The mean thermal gradient calculated for liassic horizons at 87 My for the Rambouillet well is 50.4 °C/km. This value is similar to previously published results. By integrating the calculation of the thermal gradients and conductivities related to the burial of each stratigraphic sequence, our approach points out variations in the thermal regimes the sedimentary organic matter (SOM) has been subjected to through geological time. 相似文献