Synsedimentary Normal Faults Research Articles

Extensional tectonics, structural architecture modeling and geodynamic evolution in the Cretaceous Tinghir-Errachidia-Boudenib basin (Pre-African Trough, Morocco)

Structural evolution of the Cretaceous Tinghir-Errachidia-Boudenib foreland basin in the south front of the Moroccan High Atlas belt is documented here. Fieldwork, lithostratigraphic and new structural analysis coupled with borehole and geoelectrical data identify two ENE-to NE-trending sub-basins in the large Cretaceous Tinghir-Errachidia-Boudenib (TEB) basin. One named Errachidia sub-basin is situated in the west and the other Boudenib sub-basin is located in the east. They are separated by a major NE-SW trending anticlinal, inherited from a paleohigh structure, where the thickening of the Cretaceous series increases towards the center of the sub-basins and in their northern borders with the central-eastern High Atlas. The Cretaceous series clearly show a considerable thickness variation, angular unconformities and lateral facies change that was related to the syn-sedimentary normal faulting activity. The Albian-Turonian period is characterized by the development of ENE- and NW-trending fault systems. These latter are linked to an extensional tectonics event responsible for half-graben basin geometry associated with significant sedimentary rates. This considerable thickness depocenters geometry is related to normal fault coupled with the intra-basin faults. The intensity of subsidence increases from south to north and from west to east, with a high rate in the Boudenib sub-Basin. The compressive Cenozoic events caused the inversion of these extensional faulting and the folding of the High Atlas area with ENE-trending structures.

Revisiting the mechanisms of mid-Tertiary uplift of the NE Tibetan Plateau.

Contrasting views exist on timing and mechanisms of Tertiary crustal uplift in the NE Tibetan Plateau based on different approaches, with many models attributing surface uplift to crustal shortening. We carry out a comprehensive investigation of mid-Tertiary stratigraphy, sedimentology, and volcanism in the West Qinling, Hoh Xil and Qaidam basin, and the results challenge previous views. It was held that the discordance between Oligocene and Miocene strata is an angular unconformity in the West Qinling, but our field observations show that it is actually a disconformity, indicative of vertical crustal uplifting rather than crustal shortening at the Oligocene to Miocene transition. Widespread occurrence of synsedimentary normal faults in mid-Tertiary successions implicates supracrustal stretching. Miocene potassic-ultrapassic and mafic-ultramafic volcanics in the Hoh Xil and West Qinling suggest a crucial role of deep thermomechanical processes in generating crust- and mantle-sourced magmatism. Also noticeable are the continuity of mid-Tertiary successions and absence of volcanics in the Qaidam basin. Based on a holistic assessment of stratigraphic-sedimentary processes, volcanic petrogenesis, and spatial variations of lithospheric thicknesses, we speculate that small-sale mantle convection might have been operating beneath northeast Tibet in the mid-Tertiary. It is assumed that northward asthenospheric flow was impeded by thicker cratonic lithosphere of the Qaidam and Alxa blocks, thereby leading to edge convection. The edge-driven convection could bring about surface uplift, induce supracrustal stretching, and trigger vigorous volcanism in the Hoh Xil and West Qinling in the mid-Tertiary period. This mechanism satisfactorily explains many key geologic phenomena that are hardly reconciled by previous models.

Petrogenesis of late Ediacaran volcanic rocks of the Kerdous and Tagragra d’Akka inliers (Anti-Atlas Morocco): Involvement of slab-failure

Late Ediacaran rocks of the Ouarzazate Group define a thin pile of sediments and pyroclastic deposits between the Paleoproterozoic basement and the Cambrian sedimentary cover in the southern Kerdous and Tagragra d’Akka inliers. In the Tafeltast area, terrigenous sediments with volcano-sedimentary and pyroclastic rocks are located in grabens controlled by syn-sedimentary normal faults. The sedimentation is characterized by a maximum flooding outlined by the Agoujgal carbonates. In the Tagragra-n-Daouizid area, the Ouarzazate Group represents the surficial emission of rhyolitic pyroclastic flows. This pyroclastic activity was associated with continental coarse-grained sedimentary deposits. In the Tagragra Ouahallal area, the volcanic sequence is comparable to that observed in subglacial volcanoes. Massive ash layers, intrusive pillow lavas in waterlogged deposits, stacking of lava flows, alternation of emerged and submerged periods under shallow water, are characteristics described in sub-glacial mafic volcanoes.According to geochemical data, volcanic rocks of the Ouarzazate Group are: (i) mafic rocks with continental flood basalt and calc-alkaline basalt affinity; (ii) high-K calc-alkaline felsic rocks, generated by partial melting of crustal lithologies, or by an enriched mantle with involvement of crustal component; and (iii) intermediate to felsic slab failure rocks derived through mantle processes including melting of mafic upper portion of the subducted slab.

Geological, fluid inclusion microthermometry and geochemical constraints on the ankeritic-sideritic iron deposits in the Delijan mining district, Malayer-Esfahan metallogenic belt, Iran

The Kahak, Raveh, and Khoreh stratabound sideritic-ankeritic iron deposits are located in the Malayer-Esfahan metallogenic belt (MEMB), Iran. Geological, geochemical, fluid inclusion and S isotope studies have been carried out on the Kahak, Raveh, and Khoreh deposits which are hosted by different types of Lower Cretaceous dolomite, sandstone, and tuff in the Delijan mining district (DMD). Ankerite and siderite ores display a strong lithological control and form stratabound stacked ore lenses hosted in different horizons by Kc2, Kcv, Kc3, and Km units. Economic orebodies are restricted to the hanging wall of the syn-sedimentary normal faults. Three ore types can be distinguished based on the petrographic studies: stringer zone ore facies, bedded ore facies, and massive-replacement ore facies. Ore minerals are predominantly ankerite, siderite, barite, chalcopyrite, sphalerite, galena, and pyrite. Ore mineralization in the Kahak, Raveh, and Khoreh deposits has emplaced in two stages. The fine-grained ore bands (stage 1) which are intricately interlayered with host rock beds, exhibit sedimentary textures such as lamina and bedding and indicate a syn-sedimentary origin. The coarser-grained stage 2 ores show massive and vein-veinlet textures that are considered to form by replacement during sub-seafloor fluid flow. Hydrothermal alteration minerals developed in the wall rock include dolomite, quartz, and calcite. A microthermometric study of fluid inclusions from the barite samples indicates homogenization temperatures between 162 and 210 °C. Salinities of ore-forming fluids range from 7.8 to 16.8 wt% NaCl eq. Fluid inclusion studies suggest that the ore-forming fluids were derived from basinal or formational brines. Fluid mixing (basinal brine and seawater) may have played an important role during Fe mineralization. Calculated δ34S values for the ore fluid vary between + 10.69 and + 31.98‰. Sulfur isotopic compositions suggest that the source of sulfur for barite was derived from seawater. These data show that the Kahak, Raveh, and Khoreh ankeritic-sideritic iron deposits are comparable with those of sub-seafloor carbonate-replacement SEDEX-type deposits.

Oblique strike-slip superimposed structure in Yingxiong range, western Qaidam basin and its response to Altyn Tagh fault and Eastern Kunlun fault

ABSTRACT The Yingxiong Range is the largest anticlinal belt within the triangular overlapping zone between the Altyn Tagh Fault and the Eastern Kunlun Fault and thus records the critical information concerning the strike-slip superimposition between the two large-scale left-slip faults. The detailed structural analysis on the basis of 3D seismic data shows that the Shizigou (SZG) and Youshashan (YSS) structures in the Yingxiong Range are controlled by NE-dipping low-angle thrust faults at shallow depth and three groups of faults including the NW-striking inverted faults, the WNW-striking XI fault and the N–S-striking Shidong fault zone at deep levels. The inverted faults were right-stepping en-échelon sinistral transtensional normal faults in the Eocene. Fracture analysis on cores of the U. Xiaganchaigou Formation shows the existence of syn-sedimentary normal faults which indicate Eocene extension deformation in the Yingxiong Range. The Eocene transtensional normal faults constituted a half-graben structural system and controlled the formation of the SZG sag. In the early Miocene, the XI fault obliquely superimposed the pre-existing Eocene transtensional normal faults, causing the reactivation of these faults in a reverse sense. Simultaneously, the Shidong fault zone started to develop as a dextral tear fault. The oblique strike-slip superimposition from transtension to transpression in the Yingxiong Range reflects the differential controlling effects of the asynchronous initiation of the Altyn Tagh Fault and the Eastern Kunlun Fault on the structural evolution and oil-gas reservoir development in the western Qaidam Basin.

Cadomian orogenic collapse in the Ibor and Alcudia anticlines of the Central Iberian Zone, Spain

AbstractIn the Central Iberian Zone, the Cadomian orogenic collapse is represented by chaotic megabreccias, olistostromes and mélange deposits reflecting a drastic change from slope-related deposits, fed by denudation of the Cadomian arc, to offshore-dominant settings episodically punctuated by phosphogenetic processes. In the Ibor and Alcudia anticlines, the pre-rift unconformity is marked by paraconformable to angular discordant contacts separating variable tilted strata of the Ediacaran Lower Alcudian – Domo Extremeño Supergroup and the upper Ediacaran – lower Terreneuvian Ibor Group from the overlying Terreneuvian San Lorenzo and Fuentepizarra formations. The sedimentation of the San Lorenzo Formation reflects two palaeogeographic scenarios: (i) a low-angle stable basement recording shoaling-upward siliciclastic cycles, and (ii) perturbations of basement fault scarps feeding slope-related conglomeratic channels, with NE-directed palaeocurrents, and sourced from topographic palaeohighs controlled by the movement along synsedimentary normal fault systems, such as the so-called El Guijo Fault. The intra-Fortunian age of the pre-rift unconformity is constrained by the ichno- and microfossil content of the succession, and is bracketed between the first occurrence of Treptichnus pedum in the Arrocampo Formation (Ibor Group) and of Anabarella plana in the Fuentepizarra Formation.

Reconstruction of Mesozoic paleostress associated with the tectono-sedimentary evolution of the basins bordering the Aouli inlier (Upper Moulouya, Morocco)

During the Mesozoic, the basins bordering the Aouli inlier (BBAI) experienced a geodynamic evolution marked by five successive stages: (i) during the Upper Triassic-Middle Liassic (Middle Carixian?), several sedimentary basins occurred with detrital, volcanic, and carbonate filling. Sedimentation and volcanism are controlled by NE-SW to E-W normal faults developed under a NW-SE to NNW-SSE extensional tectonic regime related to the Atlas rifting stage; (ii) during the Middle Liassic (Upper Carixian?-Domerian)-Toarcian, a pure extensive tectonic regime with NE-SW trending σ3 axis is responsible for the appearance NW-SE synsedimentary normal faults. Lateral variations of facies and thicknesses of layers and basin subsidence are controlled by these faults; (iii) during the Aalenian-Bajocian, the tectonic regime is extensive with NNW-SSE σ3 axis responsible for southward deepening of the sedimentary basins bounded by ENE-WSW normal faults. Sedimentary series are represented by bioclastic limestones and a thick succession of marls; (iv) during the Infracenomanian, the tectonic regime becomes transpressive with a sub-horizontal NNW-SSE trending σ1 axis. This is responsible for the development of ENE-WSW reverse faults, NE-SW and NW-SE strike-slip faults, the folding of Jurassic series, and the deposit of ante-Cenomanian continental succession unconformably overlying the underlying series; (v) during the Cenomano-Turonian, the setting up of carbonate platform marks a marine transgression on the entire Upper Moulouya domain. This is related to an extensive tectonic regime with sub-horizontal NW-SE σ3 axis, responsible for the development of NE-SW trending normal faults that control marine transgression and the subsidence of Cretaceous basins.

Mimicking Alpine thrusts by passive deformation of synsedimentary normal faults: a record of the Jurassic extension of the European margin (Mont Fort nappe, Pennine Alps)

The Mont Fort nappe, former uppermost subunit of the Grand St-Bernard nappe system, is an independent tectonic unit with specific structural and stratigraphic characteristics (Middle Penninic, NW Italy and SW Switzerland). It consists in a Paleozoic basement, overlain by a thin, discontinuous cover of Triassic-Jurassic metasediments, mainly breccias, called the Evolène Series. The contact of this Series over the Mont Fort basement is debated: stratigraphic or tectonic? We present new observations that support the stratigraphic interpretation and consequently imply that the Evolène Series belongs to the Mont Fort nappe. We moreover show that the Mont Fort nappe was strongly affected by normal faulting during Jurassic. These faults went long unnoticed because Alpine orogenic deformation blurred the record. Alpine strain erased their original obliquity, causing confusion with an Alpine low-angle thrust. These Jurassic faults have been passively deformed during Alpine tectonics, without inversion or any other kind of reactivation. They behaved like passive markers of the Alpine strain. Detailed field observations reveal the link between observed faults and specific breccia accumulations. Areas where the Evolène Series is missing correspond to sectors where the fault scarps were exposed on the bottom of the sea but were too steep to keep the syn- to post-faulting sediments. The Mont Fort nappe thus represents an example of a distal rifted margin. The succession of synsedimentary extensional movements followed by orogenic shortening generated a situation where passively deformed normal faults mimic an orogenic thrust.

The Ab-Bagh Late Jurassic-Early Cretaceous sediment-hosted Zn-Pb deposit, Sanandaj-Sirjan zone of Iran: Ore geology, fluid inclusions and (S–Sr) isotopes

The Ab-Bagh clastic- and carbonate-hosted stratabound and stratiform Zn-Pb deposit in the Sanandaj-Sirjan zone, in the southeastern corner of the Malayer-Esfahan metallogenic belt of Iran, is typical of several sedimentary–exhalative (SEDEX) deposits in this metallogenic belt. The deposit is hosted in a late Jurassic-early Cretaceous sedimentary sequence. Based on position, there are two Zn-Pb-bearing stratigraphic ore horizons. Ore horizon 1 is hosted by late Jurassic-early Cretaceous black shale and siltstone. The wedge-shaped ore body is located close to a synsedimentary normal fault. Petrographic studies indicates that mineralization comprises three sulfide ore facies: stockwork, bedded and massive ore facies. Ore horizon 2 occurs in early Cretaceous carbonates and comprises a massive-replacement ore facies that is concordant with host rock layering; it is also underlain by a stockwork facies. Textures include framboidal, laminated, breccia, replacement, massive and vein-veinlet. Hypogene sulfide minerals are predominantly sphalerite, galena, pyrite and chalcopyrite. Calcite and quartz are major gangue minerals. Silicification and carbonatization are the main wallrock alteration styles; alteration intensity increases towards the feeder zone. Based on relationships between ore minerals and rock-forming minerals, sulfide mineralization in the Ab-Bagh deposit formed during two main stages: fine-grained sulfide bands (stage I) are intricately interlayered with organic matter-rich beds of black shale and siltstone. Sulfide bands exhibit classic sedimentary textures, such as laminations and bedding, indicative of a synsedimentary origin. Coarser-grained stage II base metal sulfides show breccia and vein-veinlet textures, and are considered to have formed by replacement during sub-seafloor fluid flow. Fluid inclusion microthermometry was carried out on calcite associated with sulfides of the stockwork ore facies. Homogenization temperatures are in the range of 180–301 °C, and the average salinity is 17.93 wt% NaCl eq. The size distribution of pyrite framboids of the bedded ore, geochemistry of host rock and presence of organic matter, collectively suggest anoxic to locally suboxic ambient conditions for the host basin. The δ34S composition of pyrite, sphalerite, and galena ranges from −5.08 to +4.51‰. The highest δ34S values are from massive and stockwork ore facies (+2.66 to + 4.51‰) and the lowest from bedded ore (−5.08 to −4.72‰). In the bedded ore facies, the main sulfur source was bacterially reduced seawater sulfate. Whereas in massive and stockwork ore facies, thermochemically reduced seawater sulfate was the main source of sulfur. The 87Sr/86Sr ratios of calcite indicate that origin of ore forming fluids is Paleozoic radiogenic basement. Collectively, the isotopic, sedimentological, textural, mineralogical, fluid inclusion, and geochemical data for the Ab-Bagh deposit support a vent-proximal sub-seafloor replacement SEDEX ore deposit.

Sulfide ore facies, fluid inclusion and sulfur isotope characteristics of the Tappehsorkh Zn-Pb (± Ag-Ba) deposit, South Esfahan, Iran

The stratiform, stratabound Tappehsorkh Zn-Pb (±Ag-Ba) deposit, located in the southeastern part of the Malayer-Esfahan Metallogenic Belt of Iran, formed during Lower Cretaceous back-arc extension. Sulfide mineralization occurs within dolostone, black siltstone, and crystal lithic tuff and andesite associated with the Gushfil-Baghabrisham synsedimentary normal fault. Three sulfide ore facies (massive, bedded, and stockwork) occur in the deposit. Sulfide minerals are sphalerite, galena, tetrahedrite and pyrite with minor chalcopyrite and bornite, and gangue minerals are barite, dolomite and quartz. Sulfide mineralization textures are massive, replacement, vein-veinlet, laminated, disseminated, and brecciaThree mineralization stages are distinguished: early, main and post-ore. An early fine-grained sulfide ore (early ore, stage I) occurs in the bedded facies which has synsedimentary textures such as laminated, disseminated and framboidal pyrite, galena and sphalerite. Coarse-grained sulfides in the brecciated, massive replacement, and vein-veinlet textures comprise main ore stage of mineralization (main ore, stage II) and formed during diagenesis via sub-seafloor replacement of protore by mixing of hydrothermal brines with cold seawater. Late, coarse-grained sulfides (post-ore, stage III) were formed during deformation and orogenic events and occur as strain fringes, foliation-like, and spindle textures. Pervasive hydrothermal alteration styles are predominantly dolomitization and silicification and minor sericitization occurs in all host rocks in the hanging-wall of the GBF.Fluid inclusions within massive sphalerite of the main ore stage have trapping temperatures ranging from 118–199 °C, and salinities ranging from 6.2 to >23.32 wt. % NaCl equivalent. The δ34S values of the sulfide minerals from main ore stage ranges from -2.4 to -34.1‰, whereas barite ranges from +10.3 to +16.9‰. The negative values of the δ34S as well as the narrow range of δ34S in galena and sphalerite suggest that sulfur was derived by bacterial reduction of seawater sulfate.The back-arc extensional tectonic setting, presence of a normal fault controlling sulfide mineralization and hydrothermal alteration, the siliciclastic-carbonate-volcanic lithology of the host rocks, magmatism in the region (pyroclastic and volcanic rocks, that also host the sulfide mineralization), the distribution of the three ore facies, and the negative sulfur isotope values all support a sub-seafloor diagenetic replacement SEDEX-type classification for the Tappehsorkh deposit, which was then affected by Laramide (?) orogenic events which imparted the post-ore stage mineral textures.

Geological and geochemical constraints on the Farahabad vent-proximal sub-seafloor replacement SEDEX-type deposit, Southern Yazd basin, Iran

Farahabad is a Lower Cretaceous-age sandy dolomite, sandstone and silty limestone hosted zinc-lead deposit in the southern Yazd basin, Iran. This deposit occurs in the lower part of Taft Formation. The Farahabad deposit is a stratiform and stratabound accumulation of hydrothermal sulfides. The mineralized zone at Farahabad is lens shaped and has a relatively flat top. It also ranges greatly in thickness, from a few meters to >30 m. Three styles of ore facies have been differentiated at the deposit. Feeder zone, bedded ore facies, whereas the bulk of ore is contained within sphalerite and galena breccias that record a prolonged and texturally complex history of ore replacement (massive-replacement ore facies). The shape and size of the mineralized body suggest that dolomites replaced with sulfide minerals in a submarine channel. Dolomitization and sulfide mineralization's are the major hydrothermal products in the Farahabad deposit, and they occur in all host rocks, but only in the immediate vicinity of the syn-sedimentary normal fault that served as the main conduit for the mineralizing fluids. Based on microscopic studies, sulfide mineralization in the Farahabad deposit emplaced during two stages: fine-grained sulfide bands (stage I) are intricately interlayered with organic matter-rich beds and thin turbidite beds. They exhibit classic sedimentary textures, such as laminae, cross bedding, graded bedding and bedding, indicative of a synsedimentary origin. Main-stage (stage II) mineralization involved the progressive replacement of preexisting sulfides and the dolomite breccias, initially by replacement of the breccia matrix and ultimately by replacement of clasts.Dolomite minerals from the feeder zone and massive-replacement ore facies have 115–234 °C trapping temperature with salinities ranging from 0.99 to 16.70 eq. wt% NaCl eq. δ34S values of sulfide minerals range from −19.47 to +3.9‰, suggesting that sulfur in the hydrothermal fluids was derived predominantly from reduction of seawater sulfate by bacteriogenic sulfate reduction. The δ13CPDB and δ18OSMOW values of host limestone's and hydrothermal dolomites plot the near marine carbonate rocks field in a plot of δ13CPDB vs. δ18OSMOW diagram. It suggests that CO2 in the hydrothermal fluids was mainly originated from marine carbonate rocks. Abrupt lateral changes in facies and thickness, along with the existence of synsedimentary breccia's and debris flows within the ore sequence, suggest the proximity of synsedimentary faults and tectonic activity contemporaneous with the sedimentation in the Lower Cretaceous, favorable to the formation of deposit. So this deposit formed by combination of sub-seafloor replacement and on the seafloor processes. Characteristics of the Farahabad deposit are compatible with a vent-proximal sub-seafloor replacement SEDEX-type classification.

Seismic imaging of the Olduvai Basin, Tanzania

A 5.6-km-long line of refraction and reflection seismic data spanning the Pliocene-Pleistocene fill of the Olduvai Basin, Tanzania is presented. The line is oriented along a northwest-southeast profile through the position of Olduvai Gorge Coring Project (OGCP) Borehole 2A. Our aims are to (1) delineate the geometry of the basin floor by tracing bedrock topography of the metaquartzitic and gneissic basement, (2) map synsedimentary normal faults and trace individual strata at depth, and (3) provide context for the sequence observed in OGCP cores. Results with refraction tomography and poststack migration show that the maximum basin depth is around 405 m (±25 m) in the deepest portion, which quadruples the thickness of the basin-fill previously known from outcrops. Variations in seismic velocities show the positions of lower density lake claystones and higher density well-cemented sedimentary sequences. The Bed I Basalt lava is a prominent marker in the refraction seismic results. Bottom-most sediments are dated to >2.2 Ma near where Borehole 2A bottoms out at the depth of 245 m. However, the seismic line shows that the basin-fill reaches a maximum stratigraphic thickness of around 380 m deep at Borehole 2A, in the western basin where the subsidence was greatest. This further suggests that potential hominin palaeoenvironments were available and preserved within the basin-fill possibly as far back as around 4 Ma, applying a temporal extrapolation using the average sediment accretion rate.

The Southern Atlas Front in Tunisia and its foreland basin: Structural style and regional-scale deformation

Surface and subsurface data together with balanced cross-sections are used to emphasize the structural style and regional-scale deformation of the Southern Atlas Front in Tunisia and its foreland basin. This E–trending shaped belt reflects successive tectonic events. Mixed thick- and thin-skinned structural style characterizing this belt is defined by shallow thrusts rooted down into deep E–trending basement thrusts with major decollement level located within Triassic salt. A NW–trending system is also recognized in the front and particularly responsible for the belt's curved shape, which probably acts as transfer areas between the wide-ranging E–trending major structures. The intermediate areas are characterized by salt tectonics probably active during Jurassic and Lower Cretaceous, in response to active synsedimentary normal faults and differential sedimentation loading. The shortening amount varies and decreases from West to East and from North to South: ~7.45% in the West, ~3.3% in west-central part, ~2% in the central area to insignificant (~1.4%) in the east. From north to south, the Northern Chotts Range narrow zone accommodates overall the Cenozoic shortening with ~6.95% and less than 1% in the Chotts region. This variation coincides with the pre-Cenozoic basin's architecture acquired during Mesozoic Tethyan evolution, which generates the main Jurassic E–trending Chotts basin. Inside this basin, distributary sub-basins are formed in relation with NW–trending faults active during the Cretaceous period. The Chotts basin appears, therefore, as Mesozoic rift rollover structure well-preserved in the front of the belt. The compilation of data suggests that the Front of the southeasternmost end of the Atlassic domain is distinguished by the reactivation of inherited fault systems acting during alpine orogeny as major southward thrusts. The foreland shows a structural singularity to be formed by a preserved almost unmodified and well exposed large Mesozoic rollover structure.

Paleogene crustal extension in the eastern segment of the NE Tibetan plateau

The Tibetan plateau experienced lateral growth in response to the India–Eurasia collision, but uncertainties remain about when and how its present-day margins were formed. It was commonly viewed that crustal shortening in the NE Tibetan plateau had commenced since the Eocene and led to reverse faulting and compressional basins. Our recent investigations of some Cenozoic intermontane basins reveal that synsedimentary normal faults occur widely in the eastern segment of the NE Tibetan plateau. Normal faulting at basin bases obviously controlled basin sedimentation and presumably resulted in footwall uplift and basement exhumation. Small-scale synsedimentary normal faults are very common in basin fill sequences although they decrease in intensity upwards and eventually vanish in the uppermost Paleogene sequences. Measurement of the attitudes of synsedimentary normal faults indicates that crustal stretching was in NW–SE direction. The NW–SE extension was consistent with Paleogene stress field of the whole North China craton, which were thought responsible for the NE–SW-trending rift basins in the interior of the North China craton, such as the Yinchuan, Weihe, and Bohai Bay rift basin systems. A broad unconformity separates Paleogene and Neogene sequences in the eastern segment of the NE Tibetan plateau, contrasting strikingly with conformable Oligocene to Middle Miocene sequences in the neighboring Ningnan and Yinchuan rift basins. It thus follows that the eastern segment of the NE Tibetan plateau had not been formed in Paleogene when it was basically dictated by crustal extension, possibly as a result of far-field effect of subduction of the western Pacific plate. End-Oligocene termination of normal faulting and the presence of a regional unconformity indicate that the eastern segment of the NE Tibetan plateau began rising at the beginning of the Miocene.

Lithostratigraphic and structural controls on sulphide mineralisation at the Kamoa copper deposit, Democratic Republic of Congo

The Kamoa copper deposit, 25 km west of the Kolwezi CuCo district in Democratic Republic of Congo, is one of the largest high-grade copper deposits in the world. Kamoa is a recent discovery on the western edge of the Congolese part of the Central African Copperbelt (CACB). Two main rock units are present at Kamoa, sandstone and siltstone of the Mwashya Subgroup (Roan Group), and overlying diamictite and interbedded siltstone-sandstone of the Grand Conglomérat unit (Nguba Group). The deposit is at a redox boundary at the base of the Grand Conglomérat.Drill-core data indicate that the Kamoa deposit includes areas of higher and lower grade, some of which appear at the deposit-scale to be related to known or inferred faults. In the southern part of the Kamoa deposit, a NNW-trending zone of abrupt change in stratigraphic thickness named the “Mupaka fault” correlates with elevated copper grades. Thickness variations, facies changes, steep bedding, and rotated mesoscopic faults demonstrate that the Mupaka fault is a syn-sedimentary normal fault that was active during deposition of Nguba Group rocks. Similar to syn-depositional and subsequently inverted extensional faults known to localise ore at other CACB deposits, particularly in the Zambian Copperbelt, the Mupaka fault was an important control on mineralisation at Kamoa.

Extensional tectonics during Late Cretaceous evolution of the Southern Central Andes: Evidence from the Chilean main range at ~35°S

The study of Late Cretaceous-earliest Paleogene series in the Chilean Andes at ~35°S provides new constraints on the coeval evolution of the area and the Southern Central Andes. These correspond to the continental series of BRCU and the overlying Plan de los Yeuques Formation (PYF). Zircon age determinations reveal that these represent an almost continuous record of deposition, respectively from the Cenomanian-early Campanian to the Campanian-Danian. According to paleogeographic reconstructions, the BRCU series corresponds to the coeval foreland basin deposits, in a near arc position, which was westerly bounded by the earliest Late Cretaceous orogen. Detrital zircon age spectra for this reveal only a Cretaceous component, thus indicating a depocenter disconnected to that represented by coeval synorogenic deposits in Argentina. The volcanic rocks of the PYF record an eastern migration of the magmatic loci into the area at least since ~80 Ma. Its outcrops display a series of syn-sedimentary normal faults indicating extensional conditions throughout its deposition. Its volcanic rocks conform an apparent bimodal suite characterized by arc-like affinities, low pressure conditions for the corresponding magmas, and juvenile isotopic signatures for all the compositional spectra. Comparison and modeling of such compositional characteristics indicate a derivation from parental magmas similar to those currently sourcing the southern segment of the Andean Southern Volcanic Zone (~37–42°S). Altogether these characteristics suggest an extensional intra-arc basin setting for the deposition of this unit probably framed by a normal to thin crust of ~30–35 km. The extensional conditions observed for this unit are also recorded in numerous Late Cretaceous arc deposits along the Andean margin, thus indicating that this corresponds to a regional scale feature. This evidence records a change from Late Cretaceous from compressional to extensional conditions in the arc area which agrees well with major plate tectonic configuration proposed for this time span in the Andean margin.

Structural evolution and tectonic style of the Tunisian central Atlas; role of inherited faults in compressive tectonics (Ghoualguia anticline)

Geological mapping, field cross sections, structural analyses and new subsurface data were used to characterize the geometry and tectonic setting of the Ghoualguia structure, which is an E-W-trending anticline located between the Kalaa Khasba and Rouhia troughs of the central Tunisian Atlas.The results show an important NE-SW extensional phase during the Mesozoic, as demonstrated by synsedimentary normal faults (NW-SE and E-W) and thickness variations. In the Aouled Mdoua area, the absence of Paleocene-Eocene rocks indicates that the eastern and western parts of the Ghoualguia structure were separated by high topography. In addition, the angular unconformity observed between the Upper Cretaceous unit (Abiod Fm.) and the upper Eocene series (Souar Fm.) provide evidence of a tilted-block structure delineated by North-South faults. A major compressional phase during the middle to late Miocene created various detachment levels that originated mainly in the Triassic and Cretaceous deposits. Faults were reactivated as thrust and strike-slip faults, creating fault-related fold structures. In the core of the Ghoualguia fold, an original S-dipping normal fault underwent reverse movement as a back thrust. Fault-slip data indicate that the area records a major NE-SW extensional phase that took place during the late Miocene and Pliocene. A balanced cross section provides insight into the existence of two main detachment levels rooted in the Triassic (depth ± 6 km) and the lower Cretaceous (depth ± 2.5 km). The balanced cross section highlights a shortening of about 2.5 km along cross section and 1.5 km in the central part of the Ghoualguia anticline. This work underlines the predominant role of the inherited Mesozoic structures during the evolution of the Atlassic range and their influence on the geometry of the central Tunisian atlas.

The early Cretaceous Mansourabad shale-carbonate hosted Zn–Pb (-Ag) deposit, central Iran: An example of vent-proximal sub-seafloor replacement SEDEX mineralization

The Mansourabad stratiform and stratabound Zn-Pb (-Ag) deposit is hosted by an Early Cretaceous siliciclastic-carbonate sequence deposited in southern Yazd basin, Iran. Mineralization is mainly hosted in silty limestone and dolomite. Based on petrographic studies three sulfide ore facies can be distinguished: feeder zone, bedded and massive-replacement ore facies. Textures include replacement, breccia, massive, framboidal, laminated and vein-veinlet. Sulfide minerals are predominantly sphalerite, galena, and pyrite. Dolomite and quartz are the major non-sulfide gangue minerals. Based on microscopic studies, sulfide mineralization in the Mansourabad deposit emplaced during two stages: fine-grained sulfide bands (stage I) are intricately interlayered with organic-rich beds and thin turbidite beds. They exhibit classic sedimentary textures, such as lamina and bedding, indicative of a synsedimentary origin. Coarser-grained stage II base metal sulfides show breccia and vein-veinlet’s textures, and are considered to form by replacement during sub-seafloor fluid flow. Dolomitization and silicification are the major hydrothermal alteration styles in the Mansourabad deposit, and they occur in all host rocks, but only in the immediate vicinity of the syn-sedimentary normal fault that served as the main conduit for the mineralizing fluids.Calcite minerals from the feeder zone and massive-replacement ore facies have 115–250 °C trapping temperature with salinities ranging from 3.55 to 19.82 eq. wt.% NaCl eq. δ34S values of pyrite, sphalerite and galena range from −18.8 to +3.2‰. The highest δ34S values correspond to the feeder zone (+3.2 to +2‰), and the lowest to the bedded ore facies (−18.8 to −9.9‰) and the massive ore (+2.6 to − 4.7‰). The overall range of δ34S is remarkably higher than typical magmatic values, suggesting that sulfides formed from the reduction of seawater sulfate by bacteriogenic sulfate reduction in a closed or semi-closed system in the bedded ore, whereas thermochemical sulfate reduction likely played an important role in the feeder zone. The δ13CPDB and δ18OSMOW values of host limestone’s and hydrothermal dolomites range from +2.7‰ to +2.0‰, +8.6‰ to +9.2‰, +1.5‰ to +2.8‰ and +10.6‰ to +13.2‰, respectively, and plot the marine carbonate rocks field in a plot of δ13CPDB vs. δ18OSMOW. It suggests that CO2 in the hydrothermal fluids was mainly originated from marine carbonate rocks.Abrupt lateral changes in facies and thickness, along with the existence of synsedimentary breccia’s and debris flows within the ore sequence, suggest the proximity of synsedimentary faults and tectonic activity contemporaneous with the sedimentation in the Lower Cretaceous, favorable to the formation of deposit. So this deposit formed by combination of sub-seafloor and on the seafloor processes. S, C and O isotopes, along with sedimentological, textural, mineralogical, and geochemical evidences, suggest that this deposit should be classified as a vent-proximal sub-seafloor replacement SEDEX ore deposit.

МОРФОСТРУКТУРА ЗОНЫ СОЧЛЕНЕНИЯ СЕВЕРО-ЗАПАДНОГО КАВКАЗА И КЕРЧЕНСКО-ТАМАНСКОЙ ОБЛАСТИ, "Геоморфология"

Морфоструктурный анализ, основанный на интерпретации цифровых моделей рельефа и полевых исследованиях, а также новейшие геолого-геофизические данные подтвердили, что складчато-разрывные морфоструктуры Северо-Западного Кавказа продолжаются структурами Керченско-Таманской области и развиваются в единой обстановке сжатия. Направление сжатия при этом меняет вектор с северо-восточного на меридиональное, что выражается в переориентировке осей складчатых морфоструктур и геофизических аномалий с северо-западного на широтное. Уточнены положение и кинематические характеристики разрывных структур, играющих роль западного ограничения орогена Северо-Западного Кавказа. Широко описанная в литературе поперечная Анапско-Джигинская зона сбросового типа имеет диффузный характер и слабо выражена в рельефе. В свете новых данных роль границы горного сооружения приобретает расположенная к востоку от нее Абрауская поперечная зона сбросо-раздвиговой кинематики с вертикальной амплитудой деформаций минимум в 600 м за четвертичный этап. Она выделяется нами в качестве разрывного нарушения регионального значения с функцией ограничения орогена.

El sistema turbidítico conglomerático de Andatza (Pirineos occidentales): Estratigrafía, sedimentología y control estructural

En este trabajo se describe la estratigrafía y sedimentología de la Formación Conglomerados de Andatza. Mediante el análisis de facies se han interpretado tres asociaciones de facies principales correspondientes a un sistema turbidítico (de grano grueso) y al talud: un sistema turbidítico interno (o cañón), un talud fangoso de alto gradiente y otro de bajo gradiente. La distribución cartográfica de las asociaciones de facies junto con el análisis de paleocorrientes describen un modelo sedimentario de sistema turbidítico de grano grueso en forma de “L” para los Conglomerados de Andatza, cuya morfología fue controlada por fallas normales sinsedimentarias de basamento. El carácter grosero del tamaño de grano de este sistema turbidítico indica la proximidad del área fuente de los conglomerados, que alimentaría el cañón turbidítico desde el norte a través de una estrecha plataforma marina.