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1.
The Arabian-Nubian Shield (ANS) is a rapidly emerging world-class province for gold resources mainly in structurally-controlled quartz ± carbonate veins that are best classified as late Neoproterozoic orogenic gold deposits. Gold has been mined in the Eastern Desert of Egypt, in the northwestern part of the ANS, for >6000 years, that is since the times of the Pharaohs, but production prior to the 1900s was likely only about 25 t and mainly from alluvial workings. In the first half of the 1900s, about 7 t Au was recovered from the El Sid orogenic gold deposit. Today, Sukari is the single major producing mine in the Eastern Desert but many other significant gold occurrences are being actively explored.Formation of the ANS took place during closure of the Mozambique Ocean between the East and West Gondwana continental blocks. Ocean closure led to amalgamation of numerous ca. 870–625 Ma juvenile arc and back-arc igneous and sedimentary rock sequences, with many resulting terrane sutures marked by mafic-ultramafic ophiolitic assemblages and fragments. The 100 m.y. of orogeny beginning at ca. 650 Ma included crustal shortening, lithospheric reworking, escape tectonics, and eventual orogenic collapse. Peak metamorphism was reached in different parts and depths of the orogen diachronously between 620 and 585 Ma, magmatism was widespread during 650–580 Ma, and rapid exhumation of the metamorphosed rocks and mid-crustal intrusions took place from ca. 600 to 580 Ma. Regional fault sets that controlled much of the gold occurrences were related to initial transpression by oblique convergence between the arcs and associated with subsequent sinistral shearing reported as overlapping the exhumation. Because existing geological data are not adequate to fully evaluate the overall terrane history, we have subdivided the Eastern Desert into nine structural blocks, rather than arc terranes, based commonly on bounding shear zones and major faults.The greatest abundance of gold deposits is associated with the NW-trending Najd Fault System that comprises many splays throughout the blocks in the Central Eastern Desert that underwent episodes of shearing at ca. 640–570 Ma. Important deposits are also notably widespread along reactivated east-west thrust faults in the Allaqi-Sol Hamed block of the South Eastern Desert, with significant shearing at 610–580 Ma. Sulfide mineralogy of the Eastern Desert gold-bearing veins is dominated by pyrite, arsenopyrite, and (or) pyrrhotite, in addition to subordinate chalcopyrite, sphalerite, galena and tetrahedrite as well as alteration minerals that include white mica, chlorite, and carbonate, are those typical of orogenic gold deposits. Many gold occurrences are located along sheared margins to granitic intrusions or along contacts between different lithologies; sheared silica- and carbonate-altered ultramafic rocks along many fault zones are particularly widely associated with many of the gold occurrences. Ore-forming fluids were generally low-salinity aqueous-carbonic with most measured δ18O of mineralized quartz ranging from 8‰ to 15‰, δ13C for associated carbonate from −10‰ to −1‰, and δ34S for ore-related sulfides varying between −5 and +10‰, but much more consistent for individual occurrences or deposits. Gold was deposited at temperatures of generally between 250° and 370 °C, depending on location, and largely variable pressures. The few obtained absolute dates on ore formation, from the Fawakhir and Atalla deposits in the Central Eastern Desert, indicate that at least some of the mineralization was formed at ca. 600 Ma.Orogenic gold formed after the cessation of arc-terrane accretion in the ANS and during a period in which most of the shield became established with a 30–40 km-thick continental crust and underwent a transition from compressional/transpressional terrane accretion to post-amalgamation transtensional shearing. This also was marked by a petrogenetic transition from pre- to syn-accretionary, arc-related calc-alkaline I-type magmatism to late- or post-tectonic A-type magmatism within the newly formed shield. Concurrently, the Eastern Desert was affected by widespread crustal flow of aqueous-carbonic fluids, particularly on the through-going, extensive strike-slip shears of the Najd Fault System. Gold and sulfide minerals were deposited in the Eastern Desert shallow and middle crust coevally with rapid terrane exhumation, during changes in fluid chemistry associated with pressure cycling and multiple vein quartz precipitation. 相似文献
2.
《Precambrian Research》2005,136(1):27-50
The Wadi Mubarak belt in Egypt strikes west–east (and even northeast–southwest) and crosscuts the principal northwest–southeast trend of the Najd Fault System in the Central Eastern Desert of Egypt. The belt therefore appears to be a structural feature that formed postdate to the Najd Fault System. In contrast, it is shown here that the deformation in the Wadi Mubarak belt can be correlated with the accepted scheme of deformation events in the Eastern Desert of Egypt and that its geometry and apparently cross-cutting orientation is controlled by a large granite complex that intruded prior to the structural evolution. Structural correlation is facilitated by a series of intrusions that intrude the Wadi Mubarak belt and resemble other intrusions in the Eastern Desert. These intrusions include: (1) an older gabbro generation, (2) an older granite, (3) a younger gabbro and (4) a younger granite. The structural evolution is interpreted to be characterized by early northwest directed transport that formed several major thrusts in the belt. This event is correlated with the main deformation event in the Eastern Desert, elsewhere known as D2. During this event the regional fabric of the Wadi Mubarak belt was wrapped around the El Umra granite complex in a west–east orientation. The Wadi Mubarak belt was subsequently affected during D3 by west–east and northwest–southeast trending sinistral conjugate strike–slip shear zones. This event is related to the formation of the Najd Fault System. Detailed resolution of superimposed shear sense indicators suggest that D3 consisted of an older and a younger phase that reflect the change of transpression direction from east-southeast–west-northwest to eastnortheast–westouthwest. The El Umra granite complex is dated here with single zircon ages to consist of intrusion pulses at 654 and 690 my. These ages conform with the interpretation that it intruded prior to D2 and that the structural pattern of the Wadi Mubarak belt was initiated early during D2. 相似文献
3.
The northerly dipping Sha’it–Nugrus shear zone (SNSZ) is the boundary separating the Central Eastern Desert from the South Eastern Desert of Egypt. The hangingwall of this shear zone is composed of low-grade metavolcanics and ophiolitic nappes of the Central Eastern Desert, while the footwall consists of South Eastern Desert high-grade metapsammitic gneisses (Migif-Hafafit gneissic complex). The SNSZ is about 700 m thick and represents the shear foliated lower parts of the hangingwall and upper parts of the footwall. A significant part of the SNSZ has been truncated by a later normal fault along Wadi Sha’it, however the SNSZ is well-preserved along Wadi Nugrus. Features of the SNSZ include shear-related schistosity (termed Ss), mylonite zones, sheared syn-kinematic granitoid intrusions, diverse metasomatism and metamorphic effects (higher T overprinting of hangingwall lithologies and retrogression of footwall lithologies). Shear-sense indicators clearly show top-to-N or NW displacement sense. SNSZ structures overprint arc collision related nappe structures (～680 Ma) and are therefore post-arc collision. SNSZ syn-kinematic intrusives have been dated at ～600 Ma. The SNSZ is deformed (regionally and locally folded and thrust dissected) during later NE–SW compressive tectonism. The SNSZ had an originally approximately E–W strike, low-angle N-dip and a normal shear sense, making this an example of a low-angle normal ductile shear (LANF) or detachment fault. The steep NE dip of Ss foliations and low-pitching slip lineations along Wadi Nugrus are due to NW–SE folding of the SNSZ, and do not indicate a sinistral strike-slip shear zone. The normal shear sense activity is responsible for juxtaposing the low-grade Central Eastern Desert lithologies against South Eastern Desert gneisses. A displacement of 15–30 km is estimated on the SNSZ, which is comparable to LANF displacements in the Basin and Range province of the western USA. Frictional resistance along this shear was probably reduced by high magmatic fluid pressure and hydrothermal fluid pressure. The vastness and diversity of the hydrothermal activity along this shear zone is a characteristic of other LANFs in the Eastern Desert, e.g. at Gabal El-Sibai, and may be Gabal Meatiq. The SNSZ formed during the Neoproterozoic extensional tectonic phase of Eastern Desert that began ～600 Ma, and followed arc collision and NW-ward ejection of nappes. 相似文献
4.
Waleed S. IBRAHIM 《《地质学报》英文版》2021,95(6):2045-2062
The Wadi Ibib area is situated in the northern part of the Neoproterozoic Hamisana Shear Zone (HSZ), which is a high strain zone evolved during the late stages of the Pan-African orogeny, likely as a tectonic escape structure. Amphibolite facies pelitic metasedimentary windows crop out in the axial parts of the HSZ and are noticeably associated with numerous N-trending pegmatite dikes. Whole-rock geochemistry of the pegmatites reveals a peraluminous (S-type) affinity, with low K/Rb ratios and elevated concentrations of U, Th, REE, Rb, Li, Cs, Y, Nb and Ta. Structurally, the pegmatite sets intrude along the shear plane of the HSZ, corresponding to the regional N-trending tectonic fabrics, such as axial planar foliation and dextral-shearing in the metasedimentary host rock. Field relationships, including structural context, coupled with geochemical characteristics of the Wadi Ibib pegmatites, do not support their formation as a complementary part of evolved granitic magmas. Space-localized decompression-induced partial melting of peraluminous garnet-bearing metapelites was alternatively the underlying process for formation of these pegmatites. Such decompression was associated with regional escape tectonics and stress axes permutations during the late deformation stage (D3) in the evolution of the south Eastern Desert terrane, due to end-orogeny system pressure-release. 相似文献
5.
Geotectonics - Poly-deformed late-Proterozoic metamorphic domains covering Um Ashira area at the western segment of Wadi Allaqi shear zone, South Eastern Desert of Egypt, show evidences of inverted... 相似文献
6.
Multistage Extension and Age Dating of the Xiaoqinling Metamorphic Core Complex,Central China 总被引：2，自引：0，他引：2
ZHANG Jinjiang 《《地质学报》英文版》1999,73(2):139-147
There are two extensional systems in the Xiaoqinling metamorphic core complex (XMCC). One is the detachment fault system developed along the peripheries of the XMCC, which extended in an ESE-WNW direction and whose upper plate moved towards the WNW. The other extensional system includes the retrograde shear zones and normal faults developed within the XMCC, which represent the collapse of the XMCC. Ar-Ar and K-Ar dating shows that the extension of the detachment fault system continued from 135 to 123 Ma, i.e. in the late stage of its evolution at about 127 Ma. The collapse represented by the extensional system within the XMCC was operative during 120(106 Ma, and its main activity occurred about 116 Ma ago. These suggest that the XMCC experienced two extensional stages in its evolution, i.e., the syn-orogenic regional extension and post-orogenic collapse extension. 相似文献
7.
Tertiary basalt is widespread in the area south of Wadi Hodein, south Eastern Desert, Egypt. It is the youngest unit in the basement rocks of the Central Eastern Desert classification of El Shazly (Proc 22nd Intl Geol Congr, New Delhi 10:88–101, 1964) and El Ramly (Ann Geol Surv Egypt II:1–17, 1972), traversed all the previous succession of the basement rocks as well as the Nubia Sandstone of Cretaceous age, forming sheets, small hills, ridges, and dikes. This Tertiary basalt is strongly associated with the opening of the Red Sea. Geologic, petrographic, and petrochemical studies as well as microprobe and X-ray analyses were performed on samples from Wadi Hodein Tertiary basalt. Field and petrographic studies classified the Tertiary basalt in south Wadi Hodein into porphyritic olivine basalt, plagiophyric basalt, and doleritic basalt. Opaque minerals (magnetite and ilmenite) constitute 6–7.5% of this basalt. Petrochemical studies and microprobe analyses reveal that they are low-TiO2 basalt with low uranium and thorium contents, classified as being basaltic andesite to andesite, originated from calc-alkaline magma, and developed in within-plate tectonic environment. Scanning electron microscopy shows that magnetite and ilmenite are the prevalent opaque minerals in this Tertiary basalt. Field radiometric measurements of the Tertiary basalt in south Wadi Hodein reveals low uranium and thorium contents. Uranium contents range from 0.5 to 0.9 ppm, while thorium contents range from 1.2 to 3.2 ppm. Fractional crystallization and mass balance modeling indicate that the most-silica low-TiO2 Tertiary basalt in south Wadi Hodein can be derived from the relatively less-silica low-TiO2 Tertiary basalt of south Quseir and Gabal Qatrani through fractional crystallization of plagioclase, olivine, augite, and titanomagnetite oxides. Tertiary basalts in south Wadi Hodein and south Quseir have nearly the same age, 25 Ma (Sherif, The Fifth International Conference on the Geology of Africa, 2007), 24 Ma (Meneisy and Abdel Aal, Ain Shams Sci Bull 25(24B): 163–176, 1984), and 27 Ma (El Shazly et al., Egypt J Geol 1975), respectively. Finally, the fractionation modeling and geochemical characteristics of these basalts suggested their origination from one basaltic magma emplaced in late Oligocene. 相似文献
8.
大型走滑断裂对青藏高原地体构架的改造 总被引：15，自引：5，他引：10
青藏高原的大型走滑断裂有13条,已确定的大型韧性走滑断裂主要形成于3个时期:早古生代、印支期和新生代以来.印度/亚洲碰撞(60～50Ma)以来形成的大型韧性走滑构造位于青藏高原的南部,而且主要在喜马拉雅山链的东、西两侧,如西侧的喀喇昆仑和恰曼韧性右行走滑断裂,东侧的鲜水河-小江和哀牢山-红河韧性左行走滑断裂、崇山-澜沧江、嘉黎-高黎贡山和萨盖韧性右行走滑断裂等.主要的变形特征表现为早期具有地壳深部的韧性走滑剪切带性质,在后期抬升过程中,由韧性→韧脆性→脆性应变转化;而在青藏高原北部,表现为古韧性走滑剪切带的再活动,如阿尔金-康西瓦、东昆仑左行走滑断裂,以及新生的脆性断裂,如海源左行走滑断裂等.本文在青藏高原13条大型走滑断裂研究及综合研究的基础上,阐述不同时期的大型走滑断裂,以及它们在青藏高原地体拼合及碰撞造山中的作用,包括走滑断裂与走滑型褶皱造山、走滑断裂与挤压/转换型造山、走滑断裂与挤压盆-山体系、走滑断裂与地体相对位移和走滑断裂与地体的侧向挤出,以及走滑断裂与构造结的形成. 相似文献
9.
小秦岭金矿田中生代构造演化与矿床形成 总被引：23，自引：4，他引：23
作为中国金矿主产地之一,小秦岭变质核杂岩经历两期不同性质的伸展。第一期为沿周缘拆离断层发育、方向与造山带平行的同造山伸展,上盘向WNW运动,活动时代为距今135～123Ma,属燕山期陆内造山形成的地壳增厚和岩浆活动共同作用的结果。第二期为退化变质糜棱岩带和正断层组成的变质核杂岩内部伸展构造,代表造山后进一步隆升导致的垮塌,时代为距今120～106Ma。小秦岭变质核杂岩内部发育与垮塌伸展同期的挤压性逆冲断层,由造山后残余挤压作用和构造剥蚀导致的伸展驱动力降低所致。小秦岭中蚀变千糜岩型金矿受退化变质糜棱岩带控制,成因为典型的伸展控矿机制,石英脉型金矿产于内部逆冲断层,成矿机制与小秦岭变质核杂岩垮塌伸展过程中的构造反转相关。 相似文献
10.
《地学前缘(英文版)》2022,13(2):101049
The Aegean Sea area is thought to be an actively extending back-arc region, north of the present day Hellenic volcanic arc and north-dipping subduction zone in the Eastern Mediterranean. The area shows extensive normal faulting, ductile ‘extensional’ shear zones and extensional S-C fabrics throughout the islands that have previously been related to regional Aegean extension associated with slab rollback on the Hellenic Subduction Zone. In this paper, we question this interpretation, and suggest the Cenozoic geodynamic evolution of the Aegean region is associated with a Late Cretaceous–Eocene NE-dipping subduction zone that was responsible for continent-continent collision between Eurasia and Adria-Apulia/Cyclades. Exhumation of eclogite and blueschist facies rocks in the Cyclades and kyanite-sillimanite grade gneisses in the Naxos core complex have pressures that are far greater than could be accounted for purely by lithospheric extension and isostatic uplift. We identify four stages of crustal shortening that affected the region prior to regional lithospheric extension, herein called the Aegean Orogeny. This orogeny followed a classic Wilson cycle from early ophiolite obduction (ca. 74 Ma) onto a previously passive continental margin, to attempted crustal subduction with HP eclogite and blueschist facies metamorphism (ca. 54–45 ?Ma), through crustal thickening and regional kyanite – sillimanite grade Barrovian-type metamorphism (ca. 22–14 ?Ma), to orogenic collapse (<14 ?Ma). At least three periods of ‘extensional’ fabrics relate to: (1) Exhumation of blueschists and eclogite facies rocks showing tight-isoclinal folds and top-NE, base-SW fabrics, recording return flow along a subduction channel in a compressional tectonic setting (ca. 50–35 ?Ma). (2) Extensional fabrics within the core complexes formed by exhumation of kyanite- and sillimanite gneisses showing thrust-related fabrics at the base and ‘extensional’ fabrics along the top (ca. 18.5–14 ?Ma). (3) Regional ductile-brittle ‘extensional’ fabrics and low-angle normal faulting related to the North Cycladic Detachment (NCD) and the South(West) Cycladic Detachment (WCD) during regional extension along the flanks of a major NW–SE anticlinal fold along the middle of the Cyclades. Major low-angle normal faults and ductile shear zones show symmetry about the area, with the NE chain of islands (Andros, Tinos, Mykonos, Ikaria) exposing the NE-dipping NCD with consistent top-NE ductile fabrics along 200 ?km of strike. In contrast, from the Greek mainland (Attica) along the SE chain of islands (Kea, Kythnos, Serifos) a SW-dipping low-angle normal fault and ductile shear zone, the WCD is inferred for at least 100 ?km along strike. Islands in the middle of the Cyclades show deeper structural levels including kyanite- and sillimanite-grade metamorphic core complexes (Naxos, Paros) as well as Variscan basement rocks (Naxos, Ios). The overall structure is an ~100 ?km wavelength NW–SE trending dome with low-angle extensional faults along each flank, dipping away from the anticline axis to the NE and SW. Many individual islands show post-extensional large-scale folding of the low-angle normal faults around the domes (Naxos, Paros, Ios, Sifnos) indicating a post-Miocene late phase of E–W shortening. 相似文献
11.
Transpressional deformation has played an important role in the late Neoproterozoic evolution of the ArabianNubian Shield including the Central Eastern Desert of Egypt. The Ghadir Shear Belt is a 35 km-long, NW-oriented brittleductile shear zone that underwent overall sinistral transpression during the Late Neoproterozoic. Within this shear belt, strain is highly partitioned into shortening, oblique, extensional and strike-slip structures at multiple scales. Moreover, strain partitioning is heterogeneous along-strike giving rise to three distinct structural domains. In the East Ghadir and Ambaut shear belts, the strain is pure-shear dominated whereas the narrow sectors parallel to the shear walls in the West Ghadir Shear Zone are simple-shear dominated. These domains are comparable to splay-dominated and thrust-dominated strike-slip shear zones. The kinematic transition along the Ghadir shear belt is consistent with separate strike-slip and thrustsense shear zones. The earlier fabric(S1), is locally recognized in low strain areas and SW-ward thrusts. S2 is associated with a shallowly plunging stretching lineation(L2), and defines ～NW-SE major upright macroscopic folds in the East Ghadir shear belt. F2 folds are superimposed by ～NNW–SSE tight-minor and major F3 folds that are kinematically compatible with sinistral transpressional deformation along the West Ghadir Shear Zone and may represent strain partitioning during deformation. F2 and F3 folds are superimposed by ENE–WSW gentle F4 folds in the Ambaut shear belt. The sub-parallelism of F3 and F4 fold axes with the shear zones may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation in fold zones. Dextral ENEstriking shear zones were subsequently active at ca. 595 Ma, coeval with sinistral shearing along NW-to NNW-striking shear zones. The occurrence of upright folds and folds with vertical axes suggests that transpression plays a significant role in the tectonic evolution of the Ghadir shear belt. Oblique convergence may have been provoked by the buckling of the Hafafit gneiss-cored domes and relative rotations between its segments. Upright folds, fold with vertical axes and sinistral strike-slip shear zones developed in response to strain partitioning. The West Ghadir Shear Zone contains thrusts and strikeslip shear zones that resulted from lateral escape tectonics associated with lateral imbrication and transpression in response to oblique squeezing of the Arabian-Nubian Shield during agglutination of East and West Gondwana. 相似文献
12.
Antonietta Grande Gianfranco Di Vincenzo Giacomo Prosser Alfredo Caggianelli 《地学学报》2009,21(4):293-303
The Variscan crystalline basement of the Calabria–Peloritani terrane (CPT) in southern Italy was partly reworked by ductile and brittle shear zones throughout the Alpine tectonic evolution (from thickening to exhumation). Although evidence of extensional tectonics in the CPT has already been found and roughly constrained to the Oligocene onward, no attempt has ever been made to directly date brittle fault movements. Structural (meso- and micro-scale), kinematic and petrographic analyses and 40 Ar–39 Ar laser experiments reveal that the pseudotachylyte-bearing shear zones of the Palmi area in southern Calabria formed in response to extensional shearing ∼33.5 Ma ago and overprinted compressional tectonic structures. Results provide the first direct evidence of Middle Oligocene co-seismic faulting in the area and confirm the role of extensional tectonics in promoting the Oligocene exhumation of the Calabria basement. 相似文献
13.
A Quantitative structural study of late pan-african compressional deformation in the central eastern desert (Egypt) during Gondwana assembly 总被引：2，自引：0，他引：2
The Arabian-Nubian-Shield (ANS) is composed of a number of island arcs together with fragments of oceanic lithospere and minor continental terranes. The terranes collided with each other until c. 600 Ma ago. Subsequently, they were accreted onto West Gondwana, west of the present River Nile. Apart from widespread ophiolite nappe emplacement, collisional deformation and related lithospheric thickening appear to be relatively weak. Early post-collisional structures comprise not only extensional features such as fault-bounded (molasse) basins and metamorphic core complexes, but also major wrench fault systems, and thrusts and folds. The Hammamat Group was deposited in fault-bounded basins, which formed due to N-S to NW-SE directed extension. Hammamat Group sediments were intruded by late orogenic granites, dated as c. 595 Ma old. A NNW-SSE-oriented compression prevailed after the deposition of the Hammamat Sediments. This is documented by the presence of NW-verging folds and SE-dipping thrusts that were refolded and thrusted in the same direction. Restoration of a NNW-SSE- oriented balanced section across Wadi Queih indicates more than 25% of shortening. Transpressional wrenching related to the Najd Fault System followed this stage. The wrenching produced NW-SE sinistral faults associated with positive flower structures that comprise NE-verging folds and SW-dipping thrusts. Section restoration across these late structures indicates 15 17% shortening in the NE-SW direction. At a regional scale, the two post-Hammamat compressional phases produced an interference pattern with domes and basins. It can be shown that the Najd Fault System splays into a horsetail structure in the Wadi Queih area and loses displacement towards N and NW. The present study shows a distinct space and time relationship between deposition of Hammamat Group/late-Pan-African clastic sediments and late stages of Najd Fault wrench faulting: Hammamat deposition is followed by two episodes of compression, with the second episode being related to Najd Fault transpression. Therefore, the Hammamat sediments do not represent the latest tectonic feature of the Pan-African orogeny in the ANS. The latest orogenic episodes were the two successive phases of compression and transpression, respectively. It is speculated that extension during (Hammamat) basin formation was sufficiently effective to reduce the thickness of the orogenic lithosphere until it became gravitationally stable, and incapable of further gravitational deformation. 相似文献
14.
东昆仑山南缘大型转换挤压构造带和斜向俯冲作用 总被引：28，自引：5，他引：23
东昆仑地体和巴颜喀拉－－松潘甘孜地体之间的会聚边界是一条位于东昆仑南缘的大型转换挤压构造带。研究表明该带的东段（阿尼玛卿段）和西段（东－西大滩段）构造特征不同,阿尼玛卿段的构造以印支期具往南西造山极性的逆冲叠覆岩片和新生代脆性左行走滑构造为特征,东－西大滩段是由220Ma形成的EW向韧性左行走滑剪切带及两则伴生的挤压褶皱断裂带组成,韧性变形持续至20Ma,之后表现为脆性左行走滑构造再活动。因此,东昆仑南缘大型会聚带是一条由东段的“收缩挤压”为主向西段的“转换挤压”逐渐过度的特殊复杂的构造带,它的形成与巴颜喀拉－－松潘甘孜地体往NE方向斜向俯冲于东昆仑地体之下有关。 相似文献
15.
《Gondwana Research》2006,9(4):457-471
The Arabian-Nubian-Shield (ANS) is composed of a number of island arcs together with fragments of oceanic lithospere and minor continental terranes. The terranes collided with each other until c. 600 Ma ago. Subsequently, they were accreted onto West Gondwana, west of the present River Nile. Apart from widespread ophiolite nappe emplacement, collisional deformation and related lithospheric thickening appear to be relatively weak. Early post-collisional structures comprise not only extensional features such as fault-bounded (molasse) basins and metamorphic core complexes, but also major wrench fault systems, and thrusts and folds. The Hammamat Group was deposited in fault-bounded basins, which formed due to N-S to NW-SE directed extension. Hammamat Group sediments were intruded by late orogenic granites, dated as c. 595 Ma old. A NNW-SSE-oriented compression prevailed after the deposition of the Hammamat Sediments. This is documented by the presence of NW-verging folds and SE-dipping thrusts that were refolded and thrusted in the same direction. Restoration of a NNW-SSE- oriented balanced section across Wadi Queih indicates more than 25% of shortening. Transpressional wrenching related to the Najd Fault System followed this stage. The wrenching produced NW-SE sinistral faults associated with positive flower structures that comprise NE-verging folds and SW-dipping thrusts. Section restoration across these late structures indicates 15 17% shortening in the NE-SW direction. At a regional scale, the two post-Hammamat compressional phases produced an interference pattern with domes and basins. It can be shown that the Najd Fault System splays into a horsetail structure in the Wadi Queih area and loses displacement towards N and NW. The present study shows a distinct space and time relationship between deposition of Hammamat Group/late-Pan-African clastic sediments and late stages of Najd Fault wrench faulting: Hammamat deposition is followed by two episodes of compression, with the second episode being related to Najd Fault transpression. Therefore, the Hammamat sediments do not represent the latest tectonic feature of the Pan-African orogeny in the ANS. The latest orogenic episodes were the two successive phases of compression and transpression, respectively. It is speculated that extension during (Hammamat) basin formation was sufficiently effective to reduce the thickness of the orogenic lithosphere until it became gravitationally stable, and incapable of further gravitational deformation. 相似文献
16.
In the westernmost Superior Province of Canada, the east–west alignment of granite–greenstone belts and the adjacent, highly deformed gneiss belts led to the first proposals that plate tectonics existed before 2.5 Ga ago, with the belts thrust against one another by east–west-oriented subduction zones. Here, we present seismic reflection data, which demonstrate that in this region the present juxtaposition of the Uchi granite–greenstone belt and the North Caribou gneiss terrane occurred along a late southeast-dipping extensional shear zone that extends from the surface into the lower crust. The preservation of the Uchi belt and probably the English River metasedimentary belt is directly related to their dropping along extensional shear zones, which limited subsequent erosion. The relative lateral transport of these greenstone rocks implies that they were neither derived from the immediately underlying crust, nor preserved by vertical crustal movements as might occur in the absence of plate tectonics. Extension may have been associated with the emplacement of mantle-derived magmas at 2700 Ma, which has been linked to slab break-off or lithospheric delamination, making the extension approximately coeval with local gold mineralisation. Since crustal-scale faults can facilitate the circulation of gold-bearing fluids, we suggest that greenstone rocks preserved in the hanging walls of syn- to post-accretion extensional shear zones may preferentially host Archean lode-gold deposits. In the westernmost Superior Province, our seismic observations imply that some of the late structures in the well-developed belts defined by surface mapping arose through the collapse of a collage of laterally accreted terranes. 相似文献
17.
Hot collisional orogens are characterized by abundant syn-kinematic granitic magmatism that profoundly affects their tectono-thermal evolutions. Voluminous granitic magmas, emplaced between 360 and 270 Ma, played a visibly important role in the evolution of the Variscan Orogen. In the Limousin region (western Massif Central, France), syntectonic granite plutons are spatially associated with major strike–slip shear zones that merge to the northwest with the South Armorican Shear Zone. This region allowed us to assess the role of magmatism in a hot transpressional orogen. Microstructural data and U/Pb zircon and monazite ages from a mylonitic leucogranite indicate synkinematic emplacement in a dextral transpressional shear zone at 313 ± 4 Ma. Leucogranites are coeval with cordierite-bearing migmatitic gneisses and vertical lenses of leucosome in strike–slip shear zones. We interpret U/Pb monazite ages of 315 ± 4 Ma for the gneisses and 316 ± 2 Ma for the leucosomes as the minimum age of high-grade metamorphism and migmatization respectively. These data suggest a spatial and temporal relationship between transpression, crustal melting, rapid exhumation and magma ascent, and cooling of high-grade metamorphic rocks.Some granites emplaced in the strike–slip shear zone are bounded at their roof by low dip normal faults that strike N–S, perpendicular to the E–W trend of the belt. The abundant crustal magmatism provided a low-viscosity zone that enhanced Variscan orogenic collapse during continued transpression, inducing the development of normal faults in the transpression zone and thrust faults at the front of the collapsed orogen. 相似文献
18.
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
19.
青藏高原南部拉萨地体的变质作用与动力学 总被引：3，自引：0，他引：3
拉萨地体位于欧亚板块的最南缘,它在新生代与印度大陆的碰撞形成了青藏高原和喜马拉雅造山带。因此,拉萨地体是揭示青藏高原形成与演化历史的关键之一。拉萨地体中的中、高级变质岩以前被认为是拉萨地体的前寒武纪变质基底。但新近的研究表明,拉萨地体经历了多期和不同类型的变质作用,包括在洋壳俯冲构造体制下发生的新元古代和晚古生代高压变质作用,在陆-陆碰撞环境下发生的早古生代和早中生代中压型变质作用,在洋中脊俯冲过程中发生的晚白垩纪高温/中压变质作用,以及在大陆俯冲带上盘加厚大陆地壳深部发生的两期新生代中压型变质作用。这些变质作用和伴生的岩浆作用表明,拉萨地体经历了从新元古代至新生代的复杂演化过程。(1)北拉萨地体的结晶基底包括新元古代的洋壳岩石,它们很可能是在Rodinia超大陆裂解过程中形成的莫桑比克洋的残余。(2)随着莫桑比克洋的俯冲和东、西冈瓦纳大陆的汇聚,拉萨地体洋壳基底经历了晚新元古代的(～650Ma)的高压变质作用和早古代的(～485Ma)中压型变质作用。这很可能表明北拉萨地体起源于东非造山带的北端。(3)在古特提斯洋向冈瓦纳大陆北缘的俯冲过程中,拉萨地体和羌塘地体经历了中古生代的(～360Ma)岩浆作用。(4)古特提斯洋盆的闭合和南、北拉萨地体的碰撞,导致了晚二叠纪(～260Ma)高压变质带和三叠纪(～220Ma)中压变质带的形成。(5)在新特提斯洋中脊向北的俯冲过程中,拉萨地体经历了晚白垩纪(～90Ma)安第斯型造山作用,形成了高温/中压型变质带和高温的紫苏花岗岩。(6)在早新生代(55～45Ma),印度与欧亚板块的碰撞,导致拉萨地体地壳加厚,形成了中压角闪岩相变质作用和同碰撞岩浆作用。(7)在晚始新世(40～30Ma),随着大陆的继续汇聚,南拉萨地体经历了另一期角闪岩相至麻粒岩相变质作用和深熔作用。拉萨地体的构造演化过程是研究汇聚板块边缘变质作用与动力学的最佳实例。 相似文献
20.
贺兰山北段结晶基底中保留有不同程度的韧性变形剪切带.通过详细的野外考察和室内显微构造研究,明确贺兰山北段的古元古代基底经历了4期韧性剪切变形:(1)早期顺层剪切带表现出中下部地壳层次的变形样式,运动学特征一致反映了近南北向的伸展;(2)麻粒岩相变质的糜棱片麻岩剪切带为南北向挤压的产物,导致经历高温高压变质的孔兹岩系从下地壳向中部地壳抬升;(3)高级糜棱岩(低角闪岩相-高绿片岩相)剪切带涉及的2次伸展运动(北西-南东向伸展和北东-南西向伸展)使得基底进一步向中部地壳抬升,可能发生在形成孔兹岩系的同一造山运动的晚期伸展垮塌过程中;(4)北东-近东西向左行逆冲绿片岩相糜棱岩剪切带则将结晶基底抬升到中上部地壳层次,其运动学特征与高级糜棱岩剪切带明显不同,可能是另一造山运动的产物.贺兰山北段与大青山-乌拉山地区有相似的韧性剪切带和构造变形,表明华北克拉通西部北缘存在一致的近东西走向的古元古代碰撞造山运动以及随后另一造山运动的改造. 相似文献