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1.
Field observations and interpretations of satellite images reveal that the westernmost segment of the Altyn Tagh Fault (called Karakax Fault Zone) striking WNW located in the northwestern margin of the Tibetan Plateau has distinctive geomorphic and tectonic features indicative of right-lateral strike-slip fault in the Late Quaternary. South-flowing gullies and N–S-trending ridges are systematically deflected and offset by up to ~ 1250 m, and Late Pleistocene–Holocene alluvial fans and small gullies that incise south-sloping fans record dextral offset up to ~ 150 m along the fault zone. Fault scarps developed on alluvial fans vary in height from 1 to 24 m. Riedel composite fabrics of foliated cataclastic rocks including cataclasite and fault gouge developed in the shear zone indicate a principal right-lateral shear sense with a thrust component. Based on offset Late Quaternary alluvial fans, 14C ages and composite fabrics of cataclastic fault rocks, it is inferred that the average right-lateral strike-slip rate along the Karakax Fault Zone is ~ 9 mm/a in the Late Quaternary, with a vertical component of ~ 2 mm/a, and that a M 7.5 morphogenic earthquake occurred along this fault in 1902. We suggest that right-lateral slip in the Late Quaternary along the WNW-trending Karakax Fault Zone is caused by escape tectonics that accommodate north–south shortening of the western Tibetan Plateau due to ongoing northward penetration of the Indian plate into the Eurasian plate.  相似文献   

2.
Frictional sliding of gabbro gouge under hydrothermal conditions   总被引：12，自引：0，他引：12  
We investigated the frictional sliding behaviour of gabbro gouge under hydrothermal conditions. Experiments were performed on 1-mm-thick gabbro gouge sandwiched between country rock pieces (with gouge inclined 35° to the sample axis) in a triaxial testing system with argon gas as the confining medium. In the first series, experiments were conducted under effective normal stresses of 200 MPa and 300 MPa respectively, with pore pressure of 10 MPa. For temperature over 400 °C, pore pressure of 30 MPa was also applied to implement supercritical water conditions. At temperatures up to 615 °C, slip rate steps ranging from 0.0488 μm/s to 1.22 μm/s were applied to obtain the rate dependence of friction.At 200 MPa effective normal stress and a pore pressure of 10 MPa, the steady state rate dependence a–b shows velocity-weakening behaviour for temperatures between  200 and  310 °C. The higher temperature limit for velocity-weakening behaviour to occur extends up to  510 °C under supercritical water conditions with a pore pressure of 30 MPa. For the limited sliding distance in our experiments, only velocity-strengthening behaviour occurred at 300 MPa effective normal stress. Considering the limited displacement (< 3.5 mm), velocity-weakening behaviour may not be excluded in the high effective normal stress case for temperature below  510 °C.The coefficient of friction shows an increasing trend with increasing temperature in the low temperature range. The cut-off temperatures for the increasing trend are  250 °C and  440 °C, respectively for the 200 MPa and 300 MPa effective normal stress cases. Above the cut-off temperatures, the coefficient of friction at 1.83 mm permanent displacement varies around an average of 0.73, which is identical to the average for the oven-dried case [He, C., Yao, W., Wang, Z., Zhou, Y., 2006. Strength and stability of frictional sliding of gabbro gouge at elevated temperatures. Tectonophysics 427, 217–229, doi:10.1016/j.tecto.2006.05.023]. Together with the small value of rate dependence (a–b < 0.0073) for the whole temperature range, these results indicate the absence of fluid-assisted creep.With the result of our experiments as a constraint on strength of frictional sliding, comparison between converted strength for strike–slip faults and creep strength of gabbro-like rocks implies fracturing and faulting behaviours in the lower crust of a cool area (Zhangbei) in North China.  相似文献   

3.
Talc is one of the weakest minerals that is associated with fault zones. Triaxial friction experiments conducted on water-saturated talc gouge at room temperature yield values of the coefficient of friction, μ (shear stress, τ/effective normal stress, σ′N) in the range 0.16–0.23, and μ increases with increasing σ′N. Talc gouge heated to temperatures of 100°–400 °C is consistently weaker than at room temperature, and μ < 0.1 at slow strain rates in some heated experiments. Talc also is characterized by inherently stable, velocity-strengthening behavior (strength increases with increasing shear rate) at all conditions tested. The low strength of talc is a consequence of its layered crystal structure and, in particular, its very weak interlayer bond. Its hydrophobic character may be responsible for the relatively small increase in μ with increasing σ′N at room temperature compared to other sheet silicates.Talc has a temperature–pressure range of stability that extends from surficial to eclogite-facies conditions, making it of potential significance in a variety of faulting environments. Talc has been identified in exhumed subduction zone thrusts, in fault gouge collected from oceanic transform and detachment faults associated with rift systems, and recently in serpentinite from the central creeping section of the San Andreas fault. Typically, talc crystallized in the active fault zones as a result of the reaction of ultramafic rocks with silica-saturated hydrothermal fluids. This mode of formation of talc is a prime example of a fault-zone weakening process. Because of its velocity-strengthening behavior, talc may play a role in stabilizing slip at depth in subduction zones and in the creeping faults of central and northern California that are associated with ophiolitic rocks.  相似文献   

4.
A low-angle thrust fault places high-PT granulites (hangingwall) of the Internal Zone of the Neoproterozoic Brasília Belt (Tocantins Province, central Brazil) in contact with a lower-grade footwall (External Zone) comprised of nappes of distal passive margin- and back-arc basin-related supracrustals. The footwall units were emplaced at  750 Ma onto proximal sedimentary rocks (Paranoá Group) of the São Francisco paleo-continent passive margin. The high-PT belt is comprised of 645–630 Ma granulite-facies paragneiss and orthogneiss, and mafic–ultramafic complexes that include three major layered intrusions and metavolcanic rocks granulitized at  750 Ma. These complexes occur within lower-grade metasedimentary rocks in the hangingwall of the Maranhão River Thrust, which forms the Internal Zone–External Zone boundary fault to the north of the Pirineus Zone of High Strain. Detailed lithostructural studies carried out in Maranhão River Thrust hangingwall and footwall metasedimentary rocks between the Niquelândia and Barro Alto complexes, and also to the east of these, indicate the same lithotypes and Sm–Nd isotopic signatures, and the same D1–D2 progressive deformation and greenschist-facies metamorphism. Additionally, footwall metasedimentary rocks exclusively display a post-D2 deformation indicating that the Maranhão River Thrust propagated through upper crustal rocks of the Paranoá Group relatively late during the tectonic evolution of the belt. Fault propagation was a consequence of intraplate underthrusting during granulite exhumation. The results allow for a better tectonic understanding of the Brasília Belt and the Tocantins Province, as well as explaining the presence of the Pirineus Zone of High Strain.  相似文献   

5.
The integration of new and published geochronologic data with structural, magmatic/anatectic and pressure–temperature (P–T) process information allow the recognition of high-grade polymetamorphic granulites and associated high-grade shear zones in the Central Zone (CZ) of the Limpopo high-grade terrain in South Africa. Together, these two important features reflect a major high-grade D3/M3 event at ~ 2.02 Ga that overprinted the > 2.63 Ga high-grade Neoarchaean D2/M2 event, characterized by SW-plunging sheath folds. These major D2/M2 folds developed before ~ 2.63 Ga based on U–Pb zircon age data for precursors to leucocratic anatectic gneisses that cut the high-grade gneissic fabric. The D3/M3 shear event is accurately dated by U–Pb monazite (2017.1 ± 2.8 Ma) and PbSL garnet (2023 ± 11 Ma) age data obtained from syntectonic anatectic material, and from sheared metapelitic gneisses that were completely reworked during the high-grade shear event. The shear event was preceded by isobaric heating (P = ~ 6 kbar and T = ~ 670–780 °C), which resulted in the widespread formation of polymetamorphic granulites. Many efforts to date high-grade gneisses from the CZ using PbSL garnet dating resulted in a large spread of ages (~ 2.0–2.6 Ga) that reflect the polymetamorphic nature of these complexly deformed high-grade rocks.  相似文献   

6.
Yigui  Shihong  Franco  Yu  Yuanhou   《Gondwana Research》2009,16(2):255
The Machaoying fault zone extends along the southern margin of the North China Craton (NCC) and controlled the regional structures and hydrothermal mineral systems in this area. The fault underwent at least two major deformational phases, as revealed by macro- and micro-structural observations from a well-developed segment of the fault in the Hongzhuang–Baitu area, located south of the Xiong'er Mountains. Early ductile deformation is characterized by thrusting from north to south, which was subsequently overprinted by late brittle faulting. Syntectonic strain shadows of biotite are preserved around rotated porphyroclasts of quartz amygdales in mylonite. The biotite yields a 40Ar–39Ar plateau age of 524.9 ± 1.9 Ma, which is interpreted as the time of regional thrusting along the Machaoying fault zone. The thrusting may be temporally correlated with an Early Cambrian discontinuity in sedimentation observed in the rocks sequences of the NCC, suggesting a compressional regime in this area and a craton-wide tectonic event. Many 540–500 Ma tectonic events have been previously identified in the Qinling–Qilian–Kunlun Orogenic Belt of central China and in massifs in northeastern China, both of which surround the NCC, and some of these were interpreted to be associated with assembly of Gondwana. However, paleomagnetic data indicate that the NCC was unlikely to have been connected with Gondwana in the Early Cambrian and thus our new biotite date cannot record deformation along the Gondwanan margin. Dating of K-feldspar from a quartz–K-feldspar vein formed along one of the brittle faults of the Machaoying fault zone yields a much younger 40Ar–39Ar plateau age of 119.5 ± 0.7 Ma. This is a minimum age for the brittle deformation along the southern margin of the NCC, which also overlaps the age of widespread gold and molybdenum mineralization in the region.  相似文献   

7.
The Cuiabá Gold Deposit is located in the northern part of the Quadrilátero Ferrífero, Minas Gerais State, Brazil. The region constitutes an Archean granite–greenstone terrane composed of a basement complex (ca. 3.2 Ga), the Rio das Velhas Supergroup greenstone sequence, and related granitoids (3.0–2.7 Ga), which are overlain by the Proterozoic supracrustal sequences of the Minas (< 2.6–2.1  Ga) and Espinhaço (1.7 Ga) supergroups.The stratigraphy of the Cuiabá area is part of the Nova Lima Group, which forms the lower part of the Rio das Velhas Supergroup. The lithological succession of the mine area comprises, from bottom to top, lower mafic metavolcanics intercalated with carbonaceous metasedimentary rocks, the gold-bearing Cuiabá-Banded Iron Formation (BIF), upper mafic metavolcanics and volcanoclastics and metasedimentary rocks. The metamorphism reached the greenschist facies. Tectonic structures of the deposit area are genetically related to deformation phases D1, D2, D3, which took place under crustal compression representing one progressive deformational event (En).The bulk of the economic-grade gold mineralization is related to six main ore shoots, contained within the Cuiabá BIF horizon, which range in thickness between 1 and 6 m. The BIF-hosted gold orebodies (> 4 ppm Au) represent sulfide-rich segments of the Cuiabá BIF, which grade laterally into non-economic mineralized or barren iron formation. Transitions from sulfide-rich to sulfide-poor BIF are indicated by decreasing gold grades from over 60 ppm to values below the fire assay detection limit in sulfide-poor portions. The deposit is “gold-only”, and shows a characteristic association of Au with Ag, As, Sb and low base-metal contents. The gold is fine grained (up to 60 μm), and is generally associated with sulfide layers, occurring as inclusions, in fractures or along grain boundaries of pyrite, the predominant sulfide mineral (> 90 vol.%). Gold is characterized by an average fineness of 0.840 and a large range of fineness (0.759 to 0.941).The country rocks to the mineralized BIF show strong sericite, carbonate and chlorite alteration, typical of greenschist facies metamorphic conditions. Textures observed on microscopic to mine scales indicate that the mineralized Cuiabá BIF is the result of sulfidation involving pervasive replacement of Fe-carbonates (siderite–ankerite) by Fe-sulfides. Gold mineralization at Cuiabá shows various features reported for Archean gold–lode deposits including the: (1) association of gold mineralization with Fe-rich host rocks; (2) strong structural control of the gold orebodies, showing remarkable down-plunge continuity (> 3 km) relative to strike length and width (up to 20 m); (3) epigenetic nature of the mineralization, with sulfidation as the major wall–rock alteration and directly associated with gold deposition; (4) geochemical signature, with mineralization showing consistent metal associations (Au–Ag–As–Sb and low base metal), which is compatible with metamorphic fluids.  相似文献   

8.
Structural, petrographic and geochronologic studies of the Kampa Dome provide insights into the tectonothermal evolution of orogenic crust exposed in the North Himalayan gneiss domes of southern Tibet. U–Pb ion microprobe dating of zircons from granite gneiss exposed at the deepest levels within the dome yields concordia 206Pb/238U age populations of 506 ± 3 Ma and 527 ± 6 Ma, with no evidence of new zircon growth during Himalayan orogenesis. However, the granite contains penetrative deformation fabrics that are also preserved in the overlying Paleozoic strata, implying that the Kampa granite is a Cambrian pluton that was strongly deformed and metamorphosed during Himalayan orogenesis. Zircons from deformed leucogranite sills that cross-cut Paleozoic metasedimentary rocks yield concordant Cambrian ages from oscillatory zoned cores and discordant ages ranging from ca. 491–32 Ma in metamict grains. Since these leucogranites clearly post-date the metasedimentary rocks they intrude, the zircons are interpreted as xenocrysts that are probably derived from the Kampa granite. The Kampa Dome formed via a series of progressive orogenic events including regional ~ N–S contraction and related crustal thickening (D1), predominately top-to-N ductile shearing and crustal extension (D2), top-to-N brittle–ductile faulting and related folding on the north limb of the dome, localized top-to-S faulting on the southern limb of the dome, and crustal doming (D3), and continued N–S contraction, E–W extension and doming (D4). Structural and geochronologic variability amongst adjacent North Himalayan gneiss domes may reflect changes in the magnitude of crustal exhumation along the North Himalayan antiform, possibly relating to differences in the mid-crustal geometry of the exhuming fault systems.  相似文献   

9.
The Central African Belt in the Nkambe area, northwestern Cameroon represents a collisional zone between the Saharan metacraton and the Congo craton during the Pan-African orogeny, and exposes a variety of granitoids including foliated and massive biotite monzogranites in syn- and post-kinematic settings. Foliated and massive biotite monzogranites have almost identical high-K calc-alkaline compositions, with 73–67 wt.% SiO2, 17–13 wt.% Al2O3, 2.1–0.9 wt.% CaO, 4.4–2.7 wt.% Na2O and 6.3–4.4 wt.% K2O. High concentrations of Rb (264–96 ppm), Sr (976–117 ppm), Ba (3680–490 ppm) and Zr (494–99 ppm), with low concentrations of Y (mostly< 20 ppm with a range 54–6) and Nb (up to 24 ppm) suggest that the monzogranites intruded in collisional and post-collisional settings. The Sr/Y ratio ranges from 25 to 89. K, Rb and Ba resided in a single major phase such as K-feldspar in the source. Garnet was present in the source and remained as restite at the site of magma generation. This high K2O and Sr/Y granitic magma was generated by partial melting of a granitic protolith under high-pressure and H2O undersaturated conditions where garnet coexists with K-feldspar, albitic plagioclase. CHIME (chemical Th–U-total Pb isochron method) dating of zircon yields ages of 569 ± 12–558 ± 24 Ma for the foliated biotite monzogranite and 533 ± 12–524 ± 28 Ma for the massive biotite monzogranite indicating that the collision forming the Central African Belt continued in to Ediacaran (ca 560 Ma).  相似文献   

10.
L. Millonig  A. Zeh  A. Gerdes  R. Klemd 《Lithos》2008,103(3-4):333-351
The Bulai pluton represents a calc-alkaline magmatic complex of variable deformed charnockites, enderbites and granites, and contains xenoliths of highly deformed metamorphic country rocks. Petrological investigations show that these xenoliths underwent a high-grade metamorphic overprint at peak P–T conditions of 830–860 °C/8–9 kbar followed by a pressure–temperature decrease to 750 °C/5–6 kbar. This P–T path is inferred from the application of P–T pseudosections to six rock samples of distinct bulk composition: three metapelitic garnet–biotite–sillimanite–cordierite–plagioclase–(K-feldspar)–quartz gneisses, two charnoenderbitic garnet–orthopyroxene–biotite–K-feldspar–plagioclase–quartz gneisses and an enderbitic orthopyroxene–biotite–plagioclase–quartz gneiss. The petrological data show that the metapelitic and charnoenderbitic gneisses underwent uplift, cooling and deformation before they were intruded by the Bulai Granite. This relationship is supported by geochronological results obtained by in situ LA-ICP-MS age dating. U–Pb analyses of monazite enclosed in garnet of a charnoenderbite gneiss provide evidence for a high-grade structural-metamorphic–magmatic event at 2644 ± 8 Ma. This age is significantly older than an U–Pb zircon crystallisation age of 2612 ± 7 Ma previously obtained from the surrounding, late-tectonic Bulai Granite. The new dataset indicates that parts of the Limpopo's Central Zone were affected by a Neoarchaean high-grade metamorphic overprint, which was caused by magmatic heat transfer into the lower crust in a ‘dynamic regional contact metamorphic milieu’, which perhaps took place in a magmatic arc setting.  相似文献   

11.
We have analyzed the Nojima fault NIED 1800 m drill core samples by ESR (Electron Spin Resonance) to detect seismic frictional heating events, especially during the 1995 Kobe Earthquake. Dark gray fault gouge with foliation > 10 cm away from the fault plane at about 1140 m in depth, which was produced by ancient fault movements, has a FMR (ferrimagnetic resonance) signal. Heating experiments show that this FMR signal is derived from ferrimagnetic trivalent ion oxides (γ-Fe2O3: maghemite) with imperfect crystallinity, which is produced by thermal dehydration of γ-FeOOH (lepidocrocite) or Fe(OH)3 (limonite). The existence of the FMR signal means that dry heating such as frictional heating once occurred, and that the frictional heat temperature along the dark gray fault gouge may have risen to over 350 °C during ancient seismic fault slip. In order to detect frictional heating events in fault zones, the increase of the FMR signal and the color change of fault gouge into dark gray or black are important indexes. On the other hand, no FMR signal is detected from the fault gouges just on two fault planes at about 1140 m and 1300 m in depth, which are considered to be possible main fault planes in the 1995 Kobe Earthquake. These two fault planes may not have played an important role of fault slip in the Earthquake.  相似文献   

12.
Application of hornblende thermobarometry and fluid inclusion studies to the Palaeoproterozoic (1.7 Ga) basement rocks from Maddhapara, NW Bangladesh, provide information on the pressure and temperature (P–T) conditions of crystallization, the emplacement depth and composition of magmatic fluid. The basement rocks are predominantly diorite or quartz diorite with a mineral assemblage of plagioclase, hornblende, biotite, quartz, K-feldspar, titanite, and secondary epidote and chlorite. The calculated P–T conditions of the dioritic rocks are 680–725 °C and 4.9–6.4 kbar, which probably correspond to crystallization conditions. Fluid inclusion studies suggest that low- to medium-salinity (0–6.4 wt.% NaCleq) H2O-rich fluids are trapped during the crystallization of quartz and plagioclase. The isochore range calculated for primary aqueous inclusions is consistent with the P–T condition obtained by geothermobarometry. The basement rocks likely crystallized at a depth of 17–22 km, with a minimum average exhumation rate of 12–15 m/Ma during Palaeoproterozoic to Lopingian time. Such slow exhumation indicates low relief continental shield surface during this period.  相似文献   

13.
We present particle size data from 31 samples of carbonate cataclastic rocks collected across the 26 m thick fault core of the Mattinata Fault in the foreland of the Southern Apennines, Italy. Particle size distributions of incoherent samples were determined by a sieving-and-weighting technique. The number of weight-equivalent spherical particles by size is well fitted by a power-law function on a log–log space. Fractal dimensions (D) of particle size distributions are in the 2.091–2.932 range and cluster around the value of 2.5. High D-values pertain to gouge in shear bands reworking the bulk cataclastic rocks of the fault core. Low D-values characterise immature cataclastic breccias. Intermediate D-values are typical of the bulk fault core. Analysis of the ratio between corresponding equivalent particle numbers from differently evolved cataclastic rocks indicates that the development of particle size distributions with D>2.6–2.7 occurred by a preferential relative increase of fine particles rather than a selective decrement of coarse particles. This preferentially occurred in shear bands where intense comminution enhanced by slip localisation progressed by rolling of coarse particles whose consequent smoothing produced a large number of fine particles. Our data suggest that during the progression of cataclasis, the fragmentation mode changed from the Allègre et al.'s [Nature 297 (1982) 47] “pillar of strength” mechanism in the early evolutionary stages, to the Sammis et al.'s [Pure and Applied Geophysics 125 (1987) 777] “constrained comminution” mechanism in the subsequent stages of cataclasis. Eventually, localised shear bands developed mainly by abrasion of coarse particles.  相似文献   

14.
Timpanogos Cave, located near the Wasatch fault, is about 357 m above the American Fork River. Fluvial cave sediments and an interbedded carbonate flowstone yield a paleomagnetic and U–Th depositional age of 350 to 780 ka. Fault vertical slip rates, inferred from calculated river downcutting rates, range between 1.02 and 0.46 mm yr− 1. These slip rates are in the range of the 0–12 Ma Wasatch Range exhumation rate ( 0.5–0.7 mm yr− 1), suggesting that the long-term vertical slip rate remained stable through mid-Pleistocene time. However, the late Pleistocene (0–250 ka) decelerated slip rate ( 0.2–0.3 mm yr− 1) and the accelerated Holocene slip rate ( 1.2 mm yr− 1) are consistent with episodic fault activity. Assuming that the late Pleistocene vertical slip rate represents an episodic slowing of fault movement and the long-term (0–12 Ma) average vertical slip rate, including the late Pleistocene and Holocene, should be  0.6 mm yr− 1, there is a net late Pleistocene vertical slip deficit of  50–75 m. The Holocene and late Pleistocene slip rates may be typical for episodes of accelerated and slowed fault movement, respectively. The calculated late Pleistocene slip deficit may mean that the current accelerated Wasatch fault slip rate will extend well into the future.  相似文献   

15.
Zircon fission-track (FT) and U–Pb analyses were performed on zircon extracted from a pseudotachylyte zone and surrounding rocks of the Asuke Shear Zone (ASZ), Aichi Prefecture, Japan. The U–Pb ages of all four samples are  67–76 Ma, which is interpreted as the formation age of Ryoke granitic rocks along the ASZ. The mean zircon FT age of host rock is 73 ± 7 (2σ) Ma, suggesting a time of initial cooling through the zircon closure temperature. The pseudotachylyte zone however, yielded a zircon FT age of 53 ± 9 (2σ) Ma, statistically different from the age of the host rock. Zircon FTs showed reduced mean lengths and intermediate ages for samples adjacent to the pseudotachylyte zone. Coupled with the new zircon U–Pb ages and previous heat conduction modeling, the present FT data are best interpreted as reflecting paleothermal effects of the frictional heating of the fault. The age for the pseudotachylyte coincides with the change in direction of rotation of the Pacific plate from NW to N which can be considered to initialize the NNE–SSW trending sinistral–extensional ASZ before the Miocene clockwise rotation of SW Japan. The present study demonstrates that a history of fault motions in seismically active regions can be reconstructed by dating pseudotachylytes using zircon FT thermochronology.  相似文献   

16.
To investigate the strength of frictional sliding and stability of mafic lower crust, we conducted experiments on oven-dried gabbro gouge of 1 mm thick sandwiched between country rock pieces (with gouge inclined 35° to the sample axis) at slip rates of 1.22 × 10− 3 mm/s and 1.22 × 10− 4 mm/s and elevated temperatures up to 615 °C. Special attention has been paid to whether transition from velocity weakening to velocity strengthening occurs due to the elevation of temperature.Two series of experiments were conducted with normal stresses of 200 MPa and 300 MPa, respectively. For both normal stresses, the friction strengths are comparable at least up to 510 °C, with no significant weakening effect of increasing temperature. Comparison of our results with Byerlee's rule on a strike slip fault with a specific temperature profile in the Zhangbei region of North China shows that the strength given by experiments are around that given by Byerlee's rule and a little greater in the high temperature range.At 200 MPa normal stress, the steady-state rate dependence a − b shows only positive values, probably still in the “run-in” process where velocity strengthening is a common feature. With a normal stress of 300 MPa, the values of steady-state rate dependence decreases systematically with increasing temperature, and stick-slip occurred at 615 °C. Considering the limited displacement, limited normal stress applied and the effect of normal stress for the temperatures above 420 °C, it is inferred here that velocity weakening may be the typical behaviour at higher normal stress for temperature above 420 °C and at least up to 615 °C, which covers most of the temperature range in the lower crust of geologically stable continental interior. For a dry mafic lower crust in cool continental interiors where frictional sliding prevails over plastic flow, unstable slip nucleation may occur to generate earthquakes.  相似文献   

17.
The South Indian Craton is composed of low-grade and high-grade metamorphic rocks across different tectonic blocks between the Moyar–Bhavani and Palghat–Cauvery shear zones and an elongated belt of eastern margin of the peninsular shield. The Madras Block north of the Moyar–Bhavani shear zone, which evolved throughout the Precambrian period, mainly consists of high-grade metamorphic rocks. In order to constrain the evolution of the charnockitic region of the Pallavaram area in the Madras Block we have undertaken palaeomagnetic investigation at 12 sites. ChRM directions in 61 oriented block samples were investigated by Alternating Field (AF) and Thermal demagnetization. Titanomagnetite in Cation Deficient (CD) and Multi Domain (MD) states is the remanence carrier. The samples exhibit a ChRM with reverse magnetization of Dm = 148.1, Im = + 48.6 (K = 22.2, α95 = 9.0) and a palaeomagnetic pole at 37.5 °N, 295.6 °E (dp/dm = 7.8°/11.8°). This pole plots at a late Archaean location on the Indian Apparent Polar Wander Path (APWP) suggesting an age of magnetization in the Pallavaram charnockites as 2600 Ma. The nearby St. Thomas Mount charnockites indicate a period of emplacement at 1650 Ma (Mesoproterozoic). Thus the results of Madras Block granulites also reveal crustal evolution similar to those in the Eastern Ghats Belt with identical palaeopoles from both the areas.  相似文献   

18.
J.D.A. Piper   《Tectonophysics》2007,432(1-4):133-157
The Southern Uplands terrane is an Ordovician–Silurian back-arc/foreland basin emplaced at the northern margin of the Iapetus Ocean and intruded by granite complexes including Loch Doon (408.3 ± 1.5 Ma) during Early Devonian times. Protracted cooling of this 130 km3 intrusion recorded magnetic remanence comprising a predominant (‘A’) magnetisation linked to initial cooling with dual polarity and mean direction D / I = 237 / 64° (α95 = 4°, palaeopole at 316°E, 21°N). Subsidiary magnetisations include Mesozoic remanence correlating with extensional tectonism in the adjoining Irish Sea Basin (‘B’, D / I = 234/− 59°) and minority populations (‘C’, D / I = 106/− 2° and ‘D’, D / I = 199/1°) recording emplacement of younger ( 395 Ma) granites in adjoining terranes and the Variscan orogenic event. The ‘A’ directions have an arcuate distribution identifying anticlockwise rotation during cooling. A comparable rotation is identified in the Orthotectonic Caledonides to the north and the Paratectonic Caledonides to the south following closure of Iapetus. Continental motion from midsoutherly latitudes ( 40°S) at 408 Ma to equatorial palaeolatitudes by  395 Ma is identified and implies minimum rates of continental movement between 430 and 390 Ma of 30–70 cm/year, more than double maximum rates induced by plate forces and interpreted as a signature of true polar wander. Silurian–Devonian palaeomagnetic data from the British–Scandinavian Caledonides define a 430–385 Ma closed loop comparable to the distributed contemporaneous palaeomagnetic poles from Gondwana. They reconcile pre-430 Ma and post-380 Ma APW from this supercontinent and show that Laurentia–Baltica–Avalonia lay to the west of South America with a relict Rheic Ocean opening to the north which closed to produce Variscan orogeny by a combination of pivotal closure and right lateral transpression.  相似文献   

19.
Neoformed minerals in shallow fault rocks are increasingly recognized as key to the behavior of faults in the elasto-frictional regime, but neither the conditions nor the processes which wall-rock is transformed into clay minerals are well understood. Yet, understanding of these mineral transformations is required to predict the mechanical and seismogenic behavior of faults. We therefore present a systematic study of clay gouge mineralogy from 30 outcrops of 17 low-angle normal faults (LANF's) in the American Cordillera to demonstrate the range and type of clay transformations in natural fault gouges. The sampled faults juxtapose a wide and representative range of wall rock types, including sedimentary, metamorphic and igneous rocks under shallow-crustal conditions. Clay mineral transformations were observed in all but one of 28 faults; one fault contains only mechanically derived clay-rich gouge, which formed entirely by cataclasis.Clay mineral transformations observed in gouges show four general patterns: 1) growth of authigenic 1Md illite, either by transformation of fragmental 2M1 illite or muscovite, or growth after the dissolution of K-feldspar. Illitization of fragmental illite–smectite is observed in LANF gouges, but is less common than reported from faults with sedimentary wall rocks; 2) ‘retrograde diagenesis’ of an early mechanically derived chlorite-rich gouge to authigenic chlorite–smectite and saponite (Mg-rich tri-octahedral smectite); 3) reaction of mechanically derived chlorite-rich gouges with Mg-rich fluids at low temperatures (50–150 °C) to produce localized lenses of one of two assemblages: sepiolite + saponite + talc + lizardite or palygorskite +/− chlorite +/− quartz; and 4) growth of authigenic di-octahedral smectite from alteration of acidic volcanic wall rocks. These transformation groups are consistent with patterns observed in fault rocks elsewhere. The main controls for the type of neoformed clay in gouge appear to be wall-rock chemistry and fluid chemistry, and temperatures in the range of 60–180 °C.  相似文献   

20.
This study provides evidence for the existence of halite and sylvite solid inclusions in igneous quartz and feldspars, the first to be reported in intrusive rocks, and to partially constrain the physicochemical environment that lets halides crystallize under magmatic conditions.Halite and sylvite solid inclusions were found included in quartz and feldspars from a micrographic–granophyric assemblage in a miarolitic aplite and, rarer, in alkali-feldspar from a miarolitic monzogranite. Monzogranite and aplite represent I-type, K-enriched postcollisional rocks of the Late Cambrian–Early Ordovician Sierra Norte–Ambargasta batholith in the Eastern Sierras Pampeanas. Both granitoids fall among the most evolved felsic rocks of the batholith, with aplite approaching haplogranitic compositions. Halite is far more common than sylvite and the presence and distribution of one or both halides are erratic within the felsic intrusive bodies. Halides occur as small skeletal grains, commonly in cross-shaped aggregates of less than 50 μm. No K or Na was found at the detection limits of EDS in either halite or sylvite respectively. Textural relationships suggest that the alkali-chlorides separated from the melt near the minima along the quartz–feldspar cotectics of PH2O > 160 < 200 MPa in a silica-, and potassium-rich magmatic system at approximately 750–700 °C, prior to the H2O-vapor saturated miarole-forming stage.Computed ratios for the magmatic volatile phase (MVP) coexisting with melt at the early stage of aplite crystallization are: NaCl/HCl = 0.11–0.97 and KCl/HCl = 0.24–1.62, being the highest range of values (0.79–0.97 and 1.45–1.62, respectively) found in those alkali-chloride-bearing samples. Maximum HCl/ΣCl(MVP) (0.28 to 0.31) indicates higher total Cl concentration in the MVP of alkali-chloride-bearing aplites, which is much higher in the halite-free aplite samples (HCl/ΣCl(MVP) = 0.59 to 0.74). One miarolitic monzogranite sample, where halite solid inclusions are present, also yielded the highest ratios for NaCl/HCl(MVP) (0.91) and KCl/HCl(MVP) (1.46), and the HCl/ΣCl(MVP) is 0.30. A high HCl concentration in the fluid phase is suggested by the log f(HF)/f(H2O) = − 4.75 to − 4.95, log f(HCl)/f(H2O) = − 3.73 to − 3.86, and log f(HF)/f(HCl) = − 0.88 to − 1.22, computed at 750 °C after biotite composition. The Cl concentrations at 800 °C, computed with a Dv/lCl = 0.84 + 26.6P (P at 200 MPa), yielded values within the range of  70 to 700 ppm Cl in the melt and  4000 to 40 000 ppm Cl in the coexisting MVP. The preferential partitioning of Cl in the vapor phase is controlled by the Dv/lCl; however, the low concentration of Cl in the melt suggests that high concentrations of Cl are not necessary to saturate the melt in NaCl or KCl.Cl-saturation of the melt and coexisting MVP might have been produced by a drop in Cl solubility due to the near-haplogranitic composition of the granitoids after extreme fractionation, probably enhanced by fluctuating reductions of the emplacement pressure in the brittle monzogranite host. Liquid immiscibility, based in the differential viscosity and density among alkali-chloride saturated hydrosaline melt, aluminosilicate felsic melt, and H2O-rich volatiles is likely to have crystallized halite and sylvite from exsolved hydrosaline melt. High degrees of undercooling might have been important at the time of alkali-chloride exsolution. The effectiveness of alkali-chloride separation from the melt at magmatic temperatures is in line with the interpretation of “halite subtraction” as a necessary process to understand the origin of the “halite trend” in highly saline fluid inclusions from porphyry copper and other hydrothermal mineralizations, despite the absence of the latter in the Cerro Baritina aplites, where this process preceded the exsolution of halite-undersaturated fluids.Pervasive alteration of the monzogranite country rock as alkali-metasomatic mineral assemblages, the mineral chemistry of some species, and the association of weak molybdenite mineralization are compatible with the activity of alkaline hypersaline fluids, most likely exsolved during the earliest stages of aplite consolidation.  相似文献   

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