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101.
102.
The footwall gneisses beneath the western part of the Paleoarchean (3.8 Ga) Isua Greenstone Belt, southern West Greenland, are interpreted here in terms of a 3.64 Ga stack of mylonitic crystalline thrust-nappes, the oldest example known on Earth. In present coordinates, the kinematic history of the thrust-nappe stack is couched in terms of initial longitudinal (strike-parallel) thrusting towards the southwest, followed by transverse thrusting to the northwest, and subsequent extensional collapse of the thickened crust toward the southeast.Diorite and tonalite that form the western margin of granitoids, structurally overlying the western part of the Isua Greenstone Belt and its footwall, contain 3.5 Ga mafic dykes, some of which are deformed and/or truncated at fault contacts within the granitoids. Accordingly, a component of the deformation structurally above the Isua Greenstone Belt occurred after 3.5 Ga, significantly later than the formation of the underlying mylonitic nappes, probably during the Neoarchean.The structural regime of mylonitic thrust-nappe stacking is very similar to that in modern mountain belts. It would appear that the deformational behaviour, rheological constitution and overall strength of Paleoarchean and modern continental crust were similar. 相似文献
103.
Gravity and magnetic data of the Kachchh basin and surrounding regions have delineated major E–W and NW–SE oriented lineaments and faults, which are even extending up to plate boundaries in the north Arabian Sea and western boundary of the Indian plate, respectively. The epicentral zone of Bhuj earthquake and its aftershocks is located over the junction of Rann of Kachchh and median uplifts viz. Kachchh mainland and Wagad uplifts, which are separated by thrust faults. Gravity data with constraints from the results of the seismic studies along a profile suggest that the basement is uplifted towards the north along thrust faults dipping 40–60° south. Similarly gravity and magnetic modeling along a profile across Wagad uplift suggest south dipping (50–60°) basement contacts separating rocks of high susceptibility and density towards the north. One of these contacts coincides with the fault plane of the Bhuj earthquake as inferred from seismological studies and its projection on the surface coincides with the E–W oriented north Wagad thrust fault. A circular gravity high in contact with the fault in northern part of the Wagad uplift along with high amplitude magnetic anomaly suggests plug type mafic intrusive in this region. Several such gravity anomalies are observed over the island belt in the Rann of Kachchh indicating their association with mafic intrusions. The contact of these intrusives with the country rock demarcates shallow crustal inhomogeneities, which provides excellent sites for the accumulation of regional stress. A regional gravity anomaly map based on the concept of isostasy presents two centers of gravity lows of −11 to −13 mGal (10−5 m/s2) representing mass deficiency in the epicentral region. Their best-fit model constrained from the receiver function analysis and seismic refraction studies suggest crustal root of 7–8 km (deep crustal inhomogeneity) under them for a standard density contrast of −400 kg/m3. It is, therefore, suggested that significant amount of stress get concentrated in this region due to (a) buoyant crustal root, (b) regional stress due to plate tectonic forces, and (c) mafic intrusives as stress concentrators and the same might be responsible for the frequent and large magnitude earthquakes in this region including the Bhuj earthquake of January 26, 2001. 相似文献
104.
105.
R. O. Greiling 《Journal of Earth System Science》1997,106(4):209-220
Structural geological field work, microscopic and magnetic fabric studies have been applied in order to assess the structural
origin of a gneiss dome, based on a regional example from the Neoproterozoic Pan-African Belt of NE Africa, the Wadi Hafafit
Culmination (WHC). The culmination is dominated by a number of major shear zones, which form both the boundaries between the
gneissic core and surrounding low grade successions as well as those of minor structural units within the gneisses. These
shear zones form a linked fault system, which, based on shear criteria, fault-bend fold and overall geometric interrelationships,
can be classified as an antiformal stack. The relative age sequence of the shear zones/thrusts with the highest thrust oldest
and the lowermost youngest points to a forward-propagating thrust system. This, together with the shear criteria, exclude
an origin of the WHC as a metamorphic core complex, where the highest shear zone should be youngest. The geometry of the WHC
antiformal stack is documented by maps and sections as well as section balancing and restoration. Microscopic work showed
brittle deformation in feldspar and dynamic recrystallization in quartz ribbons. The asymmetry of the fabric confirmed the
macroscopically determined shear sense. However, there is one example of an earlier, perhaps extensional shear movement. Mylonitic
foliation and transport-parallel lineation have also been determined by magnetic fabric studies. The observations suggest
that thrusts may cut across both previously folded crystalline rocks as well as homogeneous granitoid plutonic bodies. According
to the regional tectonic picture the large-scale structure of the gneiss dome originated after a phase of (late-orogenic)
extensional collapse. It is speculated that during late-orogenic cooling the upper part of the lithosphere was sufficiently
strong to allow brittle thrusting whilst the lithosphere as a whole was still weak enough to allow large-scale compressional
deformation, perhaps in a transitional stage from lateorogenic to intra-cratonic deformation. 相似文献
106.
Normal faults on mesoscopic scale are observed in the Panjal Thrust Zone in the Dalhousie area of western Htmachal. The boundary
between the southern margin of the Higher Himalaya Crystalline (HHC) of Zanskar and the Chamba syncline sequence is also described
as a normal fault, referred to as Bhadarwah Normal Fault in the Bhadarwah area of Doda district on the basis of field mapping
and shear sense criteria using S-C fabric and porphyroblast rotation. The occurrence of these normal faults suggests that
the extensional tectonic regime was not restricted only to the Zanskar shear zone area but that it also occurs south of the
Higher Himalayan range. This suggests NE-directed subhorizontal extension and exhumation of deeper level rocks of Higher Himalaya
Crystallines. 相似文献
107.
Deformation, metamorphism and imbrication of the Indian plate, south of the Main Mantle Thrust, north Pakistan 总被引:2,自引:0,他引:2
P. J. TRELOAR R. D. BROUGHTON M. P. WILLIAMS M. P. COWARD B. F. WINDLEY 《Journal of Metamorphic Geology》1989,7(1):111-125
ABSTRACT South of the Main Mantle Thrust in north Pakistan, rocks of the northern edge of the Indian plate were deformed and metamorphosed during the main southward thrusting phase of the Himalayan orogeny. In the Hazara region, between the Indus and Kaghan Valleys, metamorphic grade increases northwards from chlorite zone to sillimanite zone rocks in a typically Barrovian sequence. Metamorphism was largely synchronous with early phases of the deformation. The metamorphic rocks were subsequently imbricated by late north-dipping thrusts, each with higher grade rocks in the hanging wall than in the footwall, such that the metamorphic profile shows an overall tectonic inversion. The rocks of the Hazara region form one of a number of internally imbricated metamorphic blocks stacked, after the metamorphic peak, on top of each other during the late thrusting. This imbrication and stacking represents an early period of post-Himalayan uplift. 相似文献
108.
The sea-floor morphology of a Mediterranean shelf fed by small rivers, northern Alboran Sea margin 总被引:1,自引:0,他引:1
F.J. Lobo L.M. Fernndez-Salas I. Moreno J.L. Sanz A. Maldonado 《Continental Shelf Research》2006,26(20):2607-2628
Depositional geometries and distribution patterns of shelf sediment wedges mainly derived from small rivers located in the northern margin of the Alboran Sea, Western Mediterranean Basin, are reported in this study, in order to understand: (1) their generation under particular physiographic and climatic conditions of river basins; (2) the interaction of shallow-water wedges with submarine valleys. A high amount of data has been used in this study, including river discharge and wave climate data, multibeam bathymetry, high-resolution seismic profiles and surficial sediment samples.The eastern shelf of the study area comprises the prodeltaic wedge off the Guadalfeo River and its eastward continuation, interrupted by the deeply indented Carchuna Canyon head. In contrast, the western shelf receives the contributions of a smaller river, the Verde River, whose associated prodeltaic wedge is limited to the inner shelf. Morphological features of both prodeltas are similar and differ from other Mediterranean prodeltaic bodies, resembling fan deltas. Those similarities include very steep foresets and bottomsets, very shallow and close to the coast offlap break, coarse sediment composition, lobate shape and common occurrence of crenulated sea floor. All these features point out to an origin linked to the activity of high-density sediment flows, rapid sedimentation and limited lateral redistribution. Those processes are favoured by the existence of an abrupt onshore physiography, a regional climate with a marked seasonality that conditioned torrential fluvial regimes and high availability of loose sand and gravel.Shelf sediment by-pass is a likely process during the Holocene in the eastern shelf, as suggested by the identification of two types of submarine valleys: (1) numerous gullies occur from the distal toe of the Guadalfeo River prodelta to the slope; (2) submarine canyon heads affect to the Holocene sedimentary wedge in the eastern sector of the study area. In the western shelf, however, the influence of shelf sedimentation processes on deeper domains is minimal, due to shelf widening, prevalence of relict features and absence of submarine valleys. 相似文献
109.
The present study investigates the factors associated with the occurrence of landslides along with the National Highway (NH
39) connecting Assam-Manipur (India) to Myanmar. Kinematics and slope stability analyses were used to gain an understanding
of the causes of slope failure despite the terrain comprising material with a high safety factor. The study area falls within
a high seismic zone along the regional Churachandpur-Mao Thrust (CMT) situated west of the Indo-Myanmar subduction zone. Based
on these studies of seismicity, slip rates, creeping, among others, it was inferred that CMT is a creeping regional fault
running parallel to the subduction zone boundary creeping segment of CMT and that the CMT requires further monitoring to assess
the landslide hazard in the region. 相似文献
110.
利用天然地震环境噪声成像研究柴达木盆地及邻区的岩石圈结构,利用工业地震剖面研究新生代构造变形的几何学与运动学特征,在此基础上讨论柴达木盆地新生代的成盆动力学过程与演化。柴达木盆地及邻区的岩石圈表现出向南和向北挠曲的特征。其中,东昆仑-可可西里地区地壳深度30~40 km 的低速层向北抬升,可与柴达木盆地内部深度15 km 左右的低速区相连接,反映了东昆仑-祁漫塔格山向柴达木盆地的逆冲推覆作用,因此在岩石圈尺度上,柴达木新生代成盆动力学过程与前陆盆地是相似的,表现为构造负荷引起的挠曲沉降。柴达木盆地新生代构造变形受控于柴西南和柴北缘两期冲断系统,柴北缘冲断系统形成于古新世-始新世路乐河-下干柴沟期,主要记录于祁连山山前、阿尔金山山前北段及冷湖和鄂博梁深层;柴西南冲断系统形成于早中新世下油砂山期以来,现今盆地南部的北西向构造带和盆地北部的冷湖和鄂博梁浅层构造都属于这期冲断系统。由于柴西南冲断系统的前锋构造已扩展至柴达木盆地北缘,并受到阿尔金山和祁连山的阻挡,缺少稳定的台盆区,因而使得柴达木盆地新生界不发育前陆盆地特有的楔状沉积结构。柴西南和柴北缘两期冲断系统的叠加,不仅使得柴达木新生代构造变形在时间和空间上呈现有次序的分布,也使得新生代盆地呈现出开启到封闭的演化格局,从而对新生界油气生成和聚集产生了重要影响。 相似文献