Regional Strike-slip Research Articles

Detachment levels of Colombian caribbean mud volcanoes

Regional analysis of mud volcanoes demonstrates the regional extension of these processes in Northern Colombia. Mud volcanoes are active systems that manifest the characteristics of the underlying sedimentary sequences on the surface, as well as the presence of hydrocarbons. These may which provide information about the oil systems and the characterization of new migration paths. New data acquired during field geology studies, along with the evaluation of acquired aerial images by dron, allowed to observe variations in terms of morphology and neotectonic process, being distinctive between mud volcanoes formed in different structural domains. Mud volcanoes formed in areas of basement without thrust faults (back stop zone) are usually circular, connected to the basement by regional faults. Other mud volcanoes formed in older and younger deformed belts tend to present ellipsoidal shapes, with drainages patterns that suggest local stress fields associated with regional strike slip movements of major faults. The analysis of U/Pb ages in detrital zircons extracted from mud volcanoes and outcropping sedimentary sequences in the Colombian Caribbean, together with the analysis of foraminiferal and palynomorph faunas, suggest different levels of detachment. Clay mineralogy and geochemistry indicate that mud volcanoes formed in the back stop and the Northern part of the San Jacinto deformed belt have sludge material originated in sedimentary sequences with contributions from continental basement rocks, while the mud volcanoes located in the central and Southern parts of the studied area tend to show sediments provided from deepest stratigraphic levels, derived from less evolved magmatic sources (dioritic basements). Gas and water analysis obtained from studied mud volcanoes suggest that the old deformed belt, Paleocene accretionary wedge and back stop areas, have evidences of thermogenic oil systems, while in the domain of the younger deformed belt the tendency is to indicate evidences of microbial process.

Rapid Exhumation Processes of the Gaoligong Mountain Range in the Southeastern Margin of the Qinghai–Tibet Plateau Since the Late Cenozoic

Three continent-scale ductile shear zones trending N-S are distributed in the southeast margin of the Qinghai–Tibet Plateau. Their tectonic deformation and exhumation histories are of great significance to understanding the orogenic processes within the continent and the growth as well as expansion mechanism of the plateau. The Gaoligong shear zone (GLGSZ) is the westernmost zone near the boundary of the Indian subduction plate and is less well studied than the Ailaoshan–Red River shear zone (ASRRSZ) in the east. In this study, low-temperature thermochronological methods including apatite (U-Th)/He (AHe), zircon (U-Th)/He (ZHe), and apatite fission track (AFT) were used to date the vertical profile samples from the Gaoligong Mountain to understand the exhumation processes that occurred in the late Cenozoic. Our results show that the GLGSZ has experienced two stages of rapid exhumation events since the late Cenozoic: in the middle Miocene (∼14.5 Ma) and early Pleistocene (∼2.9 Ma). Based on our data, we divided the late Cenozoic tectonic deformation and exhumation processes in the Gaoligong Mountain area into two stages: 1) From the middle to the late Miocene, large-scale regional dextral strike-slip movements and lateral compressions controlled the ductile shear zone and continuously denuded the mountain surface; 2) in the Pleistocene, rapid river erosion and undercutting caused by fluctuations of the monsoon system, together with the continuous activity of brittle faults, drove the latest rapid exhumations. A comparison of the thermochronological data of the different areas along the Gaoligong Mountain shows that the exhumation rate in its northern transect is significantly higher and the time of the onset of exhumation is earlier than that in the southern transect. These results indicate that the deformation processes began in the north and continued southwards and controlled the geomorphological characteristics of the Gaoligong Mountain, whose elevation is higher in the northern part than in the southern part.

Geochemical, geochronological, and isotopic constraints for the Archean metamorphic rocks of the westernmost part of the Carajás Mineral Province, Amazonian Craton, Brazil

The Archean nucleus of the Amazonian Craton, represented by the Carajás Mineral Province (northern Brazil), crops out as a broad exposition of igneous, sedimentary, and metamorphic rocks showing a complex polyphase geologic history. A succession of orthogneisses (metagranitoids), amphibolites, and metavolcano-sedimentary rocks that show greenstone belt characteristics occur in Tancredo Neves Village district, northeast of São Félix do Xingu city (state of Pará), is the focus of this study. Detailed field, petrographic, and stratigraphic data reveal that these metamorphic rocks underwent two different tectono-metamorphic events: (i) the regional metamorphism (>2.8 Ga), which occurred under greenschist-to mid-upper amphibolite-facies conditions; and (ii) the dynamic metamorphism (>2.6 Ga) which reached amphibolite-facies conditions and overprinted previous metamorphic textures. Associated regional strike-slip NW–SE to EW-trending ductile shear zones generated mylonitic, ultra-mylonitic, and migmatitic zones. Whole-rock geochemistry discriminates four groups of metagranitoid protoliths. G1 group is composed of peraluminous trondhjemitic rocks samples, calc-alkaline, weakly fractionated, and enriched in HREE. G2 group comprises metaluminous and alkaline monzogranite samples with high Zr, Y, Nd, HREE, and intermediate fractionation. U–Pb (LA-MC-ICP-MS) dating of G2 zircons yields a crystallization age of 2745.6 ± 5.0 Ma. Groups G3 and G4 encompass peraluminous, calc-alkaline to alkali-calcic, anorogenic, fractionated monzogranitic and granodioritic rocks. G3 and G4 groups are further distinguished by high and low La/Yb(N) ratios, respectively. The results obtained for Tancredo Neves Village district very much resemble the Canaã dos Carajás (Carajás Mineral Province) area characteristics, comprising intense magmatism (2.7–3.0 Ga) and reworking related to at least two distinct metamorphic events in the Archean. As Canaã dos Carajás, Tancredo Neves Village can be a promising area for mining exploration.

Prospectivity of the Triassic successions of the North West Shelf of Australia: New insights from a regional integrated geoscience study

The North West Shelf (NWS) of Australia is a prolific hydrocarbon province hosting significant volumes of hydrocarbons, primarily derived from Jurassic and Cretaceous targets. A new regional, integrated geoscience study has been undertaken to develop insights into the paleogeography and petroleum systems of Late Permian to Triassic successions, which have been underexplored historically in favor of Jurassic to Cretaceous targets. Within the NWS study area, graben and half-graben depocenters developed in response to intracratonic rifting that preceded later fragmentation and northward rifting of small continental blocks. This, coupled with contemporaneous cycles of rising sea levels, brought about the development of large embayments and shallow, epeiric seas between the Australian continental landmass and outlying continental fragments in the early stages of divergence. Key elements of the study results discussed herein include the study methodology, the paleogeographic and gross depositional environment mapping, and the reservoir and source kitchen modeling. The study results highlight the presence of depocenters that developed within oblique rift zones due to regional Permo-Triassic strike-slip tectonics that bear compelling similarities to modern-day analogues. These intracratonic rift zones are well-known and prominent tectonic features resulting from mantle upwelling and weakening of overlying lithospheric crust, initiating the development of divergent intraplate depocenters. The comprehensive analysis of these depocenters from a paleogeographic and petroleum system perspective provides a basin evaluation tool for Triassic prospectivity.

Metallogenic model of the Jinchang Au-Ni deposit in the Ailaoshan belt, SW China, determined on the basis of pyrite trace element contents, in-situ sulfur isotope composition and PGE geochemistry

The Ailaoshan metallogenic belt in southwestern China contains numerous shear zone-related Au deposits with Au reserves greater than 200 tonnes. However, the source of ore-forming fluids and metals is controversial, hampering the construction of genetic models. In this study, we selected the Jinchang Au-Ni deposit in the northern segment of the Ailaoshan belt for detailed investigation of pyrite chemistry and in-situ sulfur isotope compositions using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The petrographic and pyrite chemistry analyses revealed four types of hydrothermal pyrite. Py1 pyrite is euhedral to subhedral and has in-situ sulfur isotope (δ34S) values of −5.30 to 6.26‰ (median: −2.93‰) and low concentrations of Au, Ni, As, Sb, Cu, Zn, Pb, and Ag. Py1 is interpreted to have formed during the early hydrothermal stage. Py2 pyrite is subhedral to anhedral and porous with variable size. Although Py2 and Py3 show different textural relationships, we conclude that both of them formed during the intermediate hydrothermal stage as evidenced by similar sulfur isotope compositions and trace element contents, and common intergrowth textures. Py2 and Py3 pyrite have in-situ δ34S values ranging from −11.6 to 1.29‰ (median: −4.45‰) and relatively elevated concentrations of Au, Ni, As, Sb, Cu, Zn, Pb, and Ag. In Py2 and Py3 pyrite, Ni concentrations correlate with Co concentration, whereas Au exhibits moderate positive correlations with Ni, Co, and As, indicating that Au, Ni and other metals were predominantly sourced from the Jinchang ultramafic rocks. Py4 pyrite is euhedral in shape and has relatively low in-situ δ34S values of −23.5 to −4.34‰ (median: −8.33‰). The metal concentrations in Py4 pyrite are lower than those in Py2 and Py3 pyrite. In general, the metal concentrations in the different pyrite populations display systematic changes, with low levels in Py1, higher levels in Py2 andPy3, and low levels again in Py4.The average total platinum-group element (PGE) concentration of the Jinchang Ni-bearing ultramafic rocks is 26.3 ppb, similar to the average value of the primitive mantle of 22.0 ppb. The Au-Ni ores from the Jinchang deposit have an average Pd/Pt ratio of 5.98, similar to that of hydrothermal Ni ores but higher than that of typical magmatic Ni ores. In contrast, the Jinchang ultramafic rocks have low Pd/Pt ratios between 0.57 and 1.13, suggesting that the PGEs in the Jinchang intrusion are of magmatic origin. Similarly, the Rh/Ir ratios of Au-Ni ores are consistent with those of hydrothermal Ni ores but differ from those of magmatic Ni ores.The pyrite chemistry, in-situ sulfur isotope compositions, and PGE geochemistry suggest that the ore-forming fluids and metals associated with the Jinchang Au-Ni deposit were dominantly derived from the Jinchang ultramafic intrusion. Mesozoic felsic magmatism or Cenozoic regional strike-slip activity may have triggered migration of the ore-forming fluids, leaching of metals from the Jinchang intrusion and precipitation of these metals at the boundary between the intrusions and carbonaceous rocks.

Regional strike-slip and initial subsidence of Korea Plateau, East Sea: tectonic implications for the opening of back-arc basins

This study focuses on the initial subsidence mechanism of Korea Plateau in the East Sea. New interpretation of digitized multichannel seismic profiles (vertically unexaggerated) in the southern part of the South Korea Plateau delineates that acoustic basement comprises a number of normal-fault blocks, forming intra-plateau basins and troughs. In the western part of the plateau, the normal faults are oblique to the continental margin, whereas in the eastern part, they are bounded by an orthogonal transfer fault. The entire faults are suggestive of a large-scale dextral strike-slip fault system. The dextral fault system in the plateau was accompanied by a regional strike-slip fault along the North Korean continental margin, which offsets the prominent NE-SW granite belts on land and under the sea. The regional dextral strike-slip fault system activated during the late Oligocene/early Miocene to middle Miocene. The regional dextral strike-slip along the western continental margin of the sea was coupled with the complex dextral fault on the eastern margin from Sakhalin to Honshu. The regional strike-slip deformation was due to the changes in plate motion, i.e., southward drift (late Oligocene/early Miocene) and clockwise rotation (middle Miocene) of the Japanese Arc away from the Eurasian Plate.

Andesite–Basaltic Dike Magmatism in the Paleoproterozoic Rift System of the Kola Craton, Baltic Shield

New structural and petrogeochemical data are obtained on poorly known dikes composed of quartz dolerites of andesite–basaltic composition and located at the northwestern termination of the Murmansk block (Kola Craton). These data allowed us to compare the studied dikes with more well-known units from the dike swarm in the area of the settlement of Liinakhamari and the volcanics of the Pechenga structure, and to discuss their joint geodynamic position. Dolerite dikes are 2.3 Ga in age and intrude granites and plagiogranites of 2.4 and 2.8 Ga in age, respectively. The specificity of the composition of the rocks of andesite–basaltic composition from the dike series, as well as that of the volcanics from the first (Akhmalakhti) formation of the Pechenga structure, is determined by their structural position in the marginal part of the “fading” Sumian plume and in the zone of dynamic influence of regional strike–slip fault zones.

Geology, geochronology, geochemical characteristics and origin of Baomai porphyry Cu (Mo) deposit, Yulong Belt, Tibet

The Baomai porphyry Cu-Mo deposit is a newly discovered deposit located in the northern section of the Yulong porphyry Cu belt, eastern Tibet. It is associated with Eocene biotite monzogranite intrusions that were emplaced into Paleoproterozoic Ningduo Group and Middle-Upper Triassic sedimentary rocks. A biotite monzogranite porphyry with a zircon U-Pb age of 42.7 Ma was intruded by several post-ore diorite porphyry dikes (ca. 37.8 Ma). Alteration mineral assemblages consistent with argillic, phyllic, and hornfels were identified in ore-bearing porphyries and host rocks at Baomai. Argillic and phyllic alteration assemblages are spatially related to high-grade Cu and Mo mineralization, whereas hornfels alteration is accompanied by weak Cu mineralization. Molybdenite from the mineralized porphyry returned a Re-Os isochron age of 42.6 Ma, and is considered to represent the age of porphyry-style mineralization at the deposit. Ore-bearing porphyries in Baomai are high-K (K2O/Na2O >1), alkaline-rich intrusions rich in LREE, LILE (e.g., Rb, U, K), depleted in HFSE (e.g., Nb, Ta, P and Ti) and contain absent or slightly negative Eu anomalies and no significant Ce anomalies. Slightly high 87Sr/86Sr ratios (0.7064–0.7076), low εNd(t) values (−2.5 to −4.6), uniform (206Pb/204Pb)i ratios (18.70–18.75) and positive εHf(t) values (0–5.3) of ore-bearing porphyries at Baomai indicate that they share the same or similar petrogenesis to other mineralized porphyries within the Yulong porphyry Cu belt. The porphyries were likely generated from partial melting of phlogopite-bearing amphibolite or amphibolite eclogite with a low degree of crystal fractionation, and related to enrichment mantle (EMII) transformed or mixed by older subducted slab-derived fluid. Actually, there are no significant three stages of ore-bearing magmatic activities in Yulong porphyry Copper belt (Hou et al., 2003b), Eocene intrusions (37–43 Ma) and regional strike slip faulting were all important factors in controlling the formation of these Cu-polymetallic deposits during the Eocene India-Tibetan collision.

Fault and fracture patterns around a strike-slip influenced salt wall

The trends of faults and fractures in overburden next to a salt diapir are generally considered to be either parallel to the salt margin to form concentric patterns, or at right angles to the salt contact to create an overall radial distribution around the diapir. However, these simple diapir-related patterns may become more complex if regional tectonics influences the siting and growth of a diapir. Using the Sedom salt wall in the Dead Sea Fault system as our case study, we examine the influence of regional strike-slip faulting on fracture patterns around a salt diapir. This type of influence is important in general as the distribution and orientation of fractures on all scales may influence permeability and hence control fluid and hydrocarbon flow. Fractures adjacent to the N-S trending salt wall contain fibrous gypsum veins and injected clastic dykes, attesting to high fluid pressures adjacent to the diapir. Next to the western flank of the salt wall, broad (∼1000 m) zones of upturn or ‘drape folds’ are associated with NW-SE striking conjugate extensional fractures within the overburden. Within 300 m of the salt contact, fracture patterns in map view display a progressive ∼30°–35° clockwise rotation with more NNW-SSE strikes immediately adjacent to the salt wall. While some extensional faults display growth geometries, indicating that they were syn-depositional and initiated prior to tilting of beds associated with drape folding, other fractures display increasing dips towards the salt, suggesting that they have formed during upturn of bedding near the diapir. These observations collectively suggest that many fractures developed to accommodate rotation of beds during drape folding. Extensional fractures in the overburden define a mean strike that is ∼45° anticlockwise (counter-clockwise) of the N-S trending salt wall, and are therefore consistent with sinistral transtension along the N-S trending Sedom Fault that underlies the salt wall. Our outcrop analysis reveals fracture geometries that are related to both tilting of beds during drape folding, and regional strike-slip tectonics. The presence of faults and fractures that interact with drape folds suggests that deformation in overburden next to salt cannot be simply pigeon-holed into ‘end-member’ scenarios of purely brittle faulting or viscous flow.

2½-D inversion constraints on the palinspastic retro-deformation of Siluro-Devonian structures in the Black Range region, western Victoria - the “Crab Nebula” untangled

The Stavely Arc developed during the Cambrian above a continent (west) dipping subduction zone within the Grampians-Stavely Zone in present-day western Victoria. Siluro-Devonian regional dextral trans-tensional strike-slip faults recently mapped by the Geological Survey of Victoria in Cambrian basement, previously only recognised in Grampians Group cover, have segmented what was a relatively simple Cambrian configuration of Stavely Arc volcanic fault slices into a complex array of segmented and variably rotated arc fragments, including the until now enigmatic “Crab Nebula” in the Black Range. Inversion of magnetic data has provided an understanding of the 3D geometries of the three volcanic belts in the Black Range. Applying this understanding, together with over-printing criteria evident in filtered potential field data, within the context of a trans-tensional strike-slip stress regime, has enabled a retro-deformation of the Black Range belts into a single, west dipping belt of arc volcanics — an untangled, pre-Silurian “Crab Nebula”. The proven prospectivity of the Black Range Belt (by the presence of the McRaes/Eclipse prospect) can now be extended to all three of the Black Range belts.

Late Palaeozoic mineralization and tectonic evolution of the West Junggar metallogenic belt, Central Asia: constraints from Re–Os and 40Ar/39Ar geochronology

ABSTRACTThe West Junggar Metallogenic Belt (WJMB) is located between the Tianshan fault system and the Ertix fault system in the western part of the Central Asian Metallogenic Domain (CAMD). The belt features widespread late Palaeozoic granitic plutons, strike-slip faults, and porphyry copper and orogenic gold deposits. We collected nine molybdenite samples from the Baogutu III–IV Cu–Mo deposit and the Suyunhe Mo–W deposit, and 12 granitoid samples from the Jiaman, Kangde, Kulumusu, Bieluagaxi, Hatu, Akbastau, Miaoergou, Baogutu, Karamay, and Hongshan plutons in the WJMB. Molybdenite Re–Os dating gives metallogenesis ages of 312.7 and 299.7 Ma for the Baogutu III–IV and Suyunhe deposits, respectively. 40Ar/39Ar thermochronology yields biotite ages ranging from 326 to 302 Ma and K-feldspar ages from 297 to 264 Ma, indicating a regional medium-temperature cooling history in the WJMB during the late Carboniferous to middle Permian. By integrating these data with previous zircon U–Pb, amphibole 40Ar/39Ar, and zircon and apatite fission-track ages, we reconstruct the whole thermal history of the WJMB, which includes late Palaeozoic intrusive magmatism, porphyry Cu and W–Mo mineralization, and late Mesozoic tectonic uplift and exhumation of the WJMB. The regional 40Ar/39Ar cooling ages are consistent with the timing of regional sinistral strike-slip faulting, thereby indicating the tectonic significance of the cooling ages. We suggest that the biotite 40Ar/39Ar ages represent the static cooling of the granitic plutons after emplacement, since the ages are consistent with the U–Pb ages of the plutons. Thereafter, the oldest K-feldspar 40Ar/39Ar age may record the initiation of sinistral strike-slip movement on the Darabut, Mayile, and Baerluke faults. The regional faulting resulted in significant uplift of the WJMB during the early and middle Permian.

New insights into the emplacement mechanism of the Late Triassic granite plutons in the Qinling orogen: A structural study of the Mishuling pluton

Numerous geochronological and geochemical studies of the Late Triassic granite plutons in the Qinling orogen have been conducted over the past few decades. These studies have extensively discussed the genesis and correlations of granite plutons with the collisional processes between the North and South China blocks. However, several contradictory conclusions on the tectonic settings of these plutons (subduction, syncollision, postcollision, or intraplate) have been reached. Moreover, in all these studies, compressional or extensional structures have been always considered to control the magma emplacement, but no direct evidence has been presented so far. In order to clarify the emplacement mechanism of these Late Triassic plutons and avoid the ambiguities from purely geochemical studies, we conducted a multidisciplinary structural study on the Mishuling pluton in West Qinling, one of the biggest Late Triassic plutons in the orogen. New single-grain zircon dating confirmed that the Mishuling pluton was emplaced at 212 ± 2 Ma. Gravity investigations show an overall wedge shape to the pluton: A deep root is located in its western part, the thickness gradually decreases to the east, and it finally pinches out at its eastern extremity. We studied internal structures of the Mishuling pluton through field, microscopic, and AMS (anisotropy of magnetic susceptibility) observations, which show that the pluton did not undergo penetrative secondary deformation, and preserved cryptic internal fabrics were mainly acquired during the final stage of emplacement. In the western part of the pluton, internal fabrics display a concentric pattern. In the eastern part, the lineations mainly plunge west, while the foliations form an analogous W-plunging synform, mainly dipping SW in the northern part and dipping NW in the southern part. The three-dimensional shape and internal structures of the Mishuling pluton are concordant with the wrench tectonics of country rocks that developed during the late stage of the Late Triassic collision between the North and South China blocks. We propose an integrated emplacement model involving both the three-dimensional shape and internal fabrics of the Mishuling pluton, and regional structures of the country rocks. In this model, the magma rose in the western part and then spread eastward parallel to the regional strike-slip movements.

Fault Geometry and Active Stress from Earthquakes and Field Geology Data Analysis: The Colfiorito 1997 and L’Aquila 2009 Cases (Central Italy)

The fault segmentation pattern and the regional stress tensor acting since the Early Quaternary in the intra-Apennine area of central Italy was constrained by integrating two large geological and seismological fault-slip data sets collected for the areas struck by the two most energetic seismic sequences of the last 15 years (Colfiorito 1997, M w 6.0 and L’Aquila 2009, M w 6.1). The integrated analysis of the earthquake fault association and the reconstruction of the 3D shape of the seismogenic sources were exploited to identify homogeneous seismogenic volumes associated with subsets of geological and focal mechanism data. The independent analysis of geological and seismological data allowed us to observe and highlight similarities between the attitude of the long-term (e.g., Quaternary) and the instantaneous present-day (seismogenic) extensional deformations and to reveal their substantial coaxiality. Coherently, with the results from the kinematic analysis, the stress field inversion also noted a prevailing tensional seismotectonic regime associated with a subhorizontal, NE–SW, minimum stress axis. A minor, very local, and shallow (<5 km) strike-slip component of the stress field was observed in the Colfiorito sector, where an inherited N–S oriented right-lateral fault was reactivated with sinistral kinematics. Instead, an almost total absence of strike-slip solutions was observed in the L’Aquila area. These results do not agree with those indicating Quaternary regional strike-slip regimes or wide areas characterized by strike-slip deformation during the Colfiorito and L’Aquila seismic sequences.

Seismicity and seismotectonics of the Gulf of Aqaba region

The historical seismicity of the last ten centuries and the instrumental data that occurred in the Gulf of Aqaba region during the period 1982–2008 are evaluated. It is found that 12 historical earthquakes have occurred with average recurrence periods 70–90 and 333–500 years for M ≥ 6.0 and 7.0, respectively. Those with M ≤ 6.5 appear to be incomplete and require further investigation. More than 98 % of the instrumental data has occurred in the form of swarms and sequences. The first have released about 32 % of the total energy and are most likely related to subsurface volcanic activities. Their epicentral distribution indicates that all regional faults of the gulf area are active in the present, but with clear concentration within the area bound by latitudes 28.2°–29.8° and longitudes 34.4°–35.2°. Regional strike-slip faults of the northern two basins appear to be as twice active as the normal, or more. An appreciable level of seismic hazard is envisaged as the “a” value is 6.0–6.2 while the “b” value shows a temporal variation, mostly in the range 0.8–1.05. More than 95 % of the seismic energy was released from earthquakes shallower than 22 km. This indicates a brittle upper crust and a ductile lower crust and upper mantle. Tectonic movements at depths > 22 km appear to be aseismic. The epicentral distribution of the five swarms indicates that the lengths of the causative faults varied in the range 45–70 km. The maximum expected magnitude is Mw = 6.8–7.2. This implies a seismic slip rate of about 0.54–0.8 Cm/year and some 20–30 % of aseismic tectonic movements. This and the sequence nature of the seismicity of this region result in a noticeable hazard reduction. Combining the seismicity data of the Gulf of Aqaba region with other geophysical, geological, tectonic, and environmental data, clearly indicate that the seismicity of this region is as old as the initiation of the gulf itself. No apparent southward or northward migration of activity is observed.

3D deformation in strike-slip systems: Analogue modelling and numerical restoration

Regional and local strike-slip systems in Chile are complex and pose interesting questions, such as the interaction between strike-slip and reverse faults, how they evolve, and the relationship between shortening, rotation and uplift. Within this context, we developed a new analytical method based on analogue and numerical modelling applied to 3D, pure and transtensional-transpressional strike-slip systems. Analogue modelling results indicate that in restraining stepovers of strike-slip fault systems, where antiformal pop-up structures are usually formed, pre-existent basement structures with a high angle to the main strike-slip fault will generate a higher rotation of blocks. However, when these structures are oriented at a high angle with respect to the main stress convergence vector, the rotation will be less and therefore a higher tendency to uplift will be produced. These results were applied to NW- and SE-striking basement faults oblique to N-S mega-thrust faults in central Chile (32o-35oS), for which we propose a simultaneous development based on the analogue model results. Moreover, we propose that strike-slip movement occurred on thrust faults in central Chile. Furthermore, we performed a numerical restoration of an analogue experiment which modeled a pure strike-slip system, and concluded that the restoration is very sensitive to shortening data as well as to rotational data. These results are extremely important for future numerical and regional analysis of strike-slip systems.

Metalogenia de oro y cobre en América central

Los depósitos de oro y cobre en América Central se encuentran dentro de rocas del Cretácico Tardío y más jóvenes del arco insular volcánico. Basaltos toleíticos bajos en potasio forman la base de la agrupa-ción volcánica y fueron depositados en un arco insular intraoceánico. El vulcanismo se tornó mas potásico conel tiempo, y ya para el Mioceno tardío, las andesitas calco-alcalinas dominan la secuencia volcánica. Rocas volcánicas alcalinas aparecieron hace unos cinco millones de años pero han sido subreportadas en areas más anti-guas del arco.Depósitos porfídicos de cobre se centran sobre plutones calco-alcalinos de composición intermedia, diorita cuar-cífera a diorita monzonítica y que típicamente son anómalas en oro. Aunque algunos de los depósitos porfídi-cos contienen más del billón de toneladas, ninguno está en producción. Vetas auríferas tipo bonanza se agrupanalrededor de intrusiones subvolcánicas en distritos que asemejan la distribución espacial de volcanes activos.Bonanzas de metales preciosos, algunos de los cuales produjeron por encima del millón de onzas de oro, sonresponsables por la mayor parte de la producción histórica en América Central. Se han venido descubriendo depósitos auríferos a lo largo de una inconformidad regional que marca el nacimiento del arco insular actual delocéano. A pesar de su reciente descubrimiento, algunos de estos depósitos ya exceden el millón de onzas. Lasrocas caja incluyen domos exógenos y los mantos piroclásticos que los encierran.Los esfuerzos exploratórios de hoy se concentran sobre depósitos de alto volumen: stockworks relacionados conlos pórfidos, depósitos de skarn, stockworks epitermales y depósitos diseminados. Fallas longitudinales regionales paralelas a la Depresión de Nicaragua controlan la distribución, tanto del oro como del cobre a través deCentroamérica. Dataciones proveen documentación para mineralización a lo largo de la historia del arco, un período de cerca de 100 millones de años.

Modelling the pore fluid diffusion process in aftershock initiation for 2004 Sumatra earthquake: implications for marine geohazard estimation in the Andaman region

The role of fluid injection on the occurrence and migration path for the aftershocks of 2004 Sumatra earthquake (Mw 9.3) and January 2005 Andaman earthquake swarm within the aftershock sequence is investigated here from the viewpoint of pore fluid diffusion process. The Sumatra earthquake created a regionally extensive crustal rupture plane exceeding 1,200 km length below the Andaman Sea. The r–t plots (Shapiro et al. 1997) are constructed for these aftershocks in order to examine the role of poroelastic effects as rupturing progressed with time. Their main results are as follows: the r–t plot corresponding to first 3 h of aftershock activity (when only 44 events of mb ≥ 4.5 originated) reveals that 95% of the data points occurred below the modelled parabola with relatively high D value of 20 m2/s, whereas a significantly low D value of 3.5 m2/s characterises the aftershock activity for the first 24 h (when 420 events of mb ≥ 4.0 occurred). Here, the Coulomb stress was transferred from the main shock with a rapid imposition of normal stress, thus inducing the pore-pressure change that started diminishing almost immediately by fluid diffusion, at a rate, defined by the diminishing D value. The modelling results for fault seismicity at far off distances from the main epicentre are interpreted here as potential indicators for large-scale sub-seabed rupturing—consequent to stress changes induced by bending of the Indian Ocean plate. Bathymetric slopes under the Andaman subduction zone are particularly amenable to sub-marine slides where crustal E–W hinge faults inferred seismically cut across the N–S trending regional thrust and strike-slip faults. Seabed rupturing appears to allow deep-slab hydration in these areas, producing pressure gradients along the normal faults. These features are important since they can herald marine geohazards in the Andaman region.

Tectonic settings of porphyry Cu–Mo–Au deposits in the Himalayan–Tibetan orogen, East Tethys

Porphyry Cu (Mo–Au) deposits in the Himalayan–Tibetan orogen formed during the Late Triassic, Early Cretaceous, Eocene, Oligocene, and Miocene and can be classified into different metallogenic belts according to their petrologic features, mineralization ages, and tectonic settings. A close spatial relationship to regional strike–slip faults is evident in all five belts. Porphyry Cu (Mo–Au) deposits exist in a wide range of tectonic environments, including island arc, syn-collision, post-collisional convergence, and continental-transform plate boundaries. Porphyry Cu deposits cluster in the southernmost part of the Yidun–Zhongdian Belt, along the N–S-trending Gaze River dextral strike–slip fault. Porphyry Cu deposits in the Lijiang–Jinping Belt lie along the Ailaoshan–Red River continental–transform shear zone and the associated strike–slip faults. The Yulong–Malasongduo porphyry belt is controlled by the Cesuo Fault, a NNW-trending regional dextral transcurrent fault that is associated with Palaeogene westward continental oblique subduction along the Jinsha suture. In the Gangdis Belt, Miocene porphyry Cu deposits are localized along N–S-trending normal faults, which were produced by transpression within the regional NW–SE-trending Karakoram–Jiali fault zone (KJFZ). A close spatial relationship between porphyry Cu deposits and strike–slip faults also exists for the Bangong–Nujiang Belt.

Tectonic evolution of the Northern Sicanian-Southern Palermo Mountains range in Western Sicily: insight on the exhumation of the thrust involved foreland domains

Geological mapping and structural analysis, coupled with extensive samplings on terrigenous Tertiary covers, enable us to reconstruct the structural setting as well as the tectonic evolution of the northern Sicanian-southern Palermo Mountains range in western Sicily. The analyzed region is a segment of the Apenninic-Maghrebian Orogen characterized by the occurrence of two regional superimposed tectonic edifices; the Apenninic-Maghrebian Chain (AMC) and the Pelagian Sicilian Thrust Belt (PSTB) which constitute the uppermost and lowermost structural level respectively. The different structural associations generally suggest a multiphase tectonic history in which we distinguished three main deformational events. The oldest one is characterized by the development of low-angle foreland-verging large thrust contacts. These usually propagated along Lower-Middle Miocene terrigenous levels and leads to the staking of the AMC. As a whole the AMC structured during Middle Miocene and tectonically overrode the Pelagian foreland since Tortonian time. The subsequent tectonic phase developed as effect of continental collision with the propagation of south verging high-angle thrust faults. These involve mainly the underlying Pelagian foreland giving rise to the imbrications at depth of the PSTB. The more recent tectonic phase starts since Middle Pliocene with the activation of NW-SE oriented right-lateral traspressive shear zones and associated double verging E-W striking thrust contacts. These latter occurred mainly in the overlapping area between major shear zones and are responsible for the final uplift of the previously imbricate PSTB. During this tectonic stage the previously E-W oriented structural features suffered local clockwise rotations in the neighbours of the main right-lateral shear zones. In this newly proposed structural framework, we interpreted the Mt. Kumeta and Rocca Busambra carbonate ridges as a Pliocene push-up type antiformal structures grown and exhumed at restraining stepovers in a contest of regional strike-slip kinematics

تمددي في جنوب شرق الدرع العربي قبه نايس جبل كرش: معقد تحولي

A number of gneiss-cored domes and antiforms are exposed along the regional strike-slip Najd fault system in the Arabian Shield and the eastern desert of Egypt. The mode of origin is still controversial, although plausible comparisons with modern metamorphic core complexes were made in some well-studied areas. The Kirsh dome is located within the major Ar Rika shear zone and consists of a core of orthogneiss/migmatite and an envelope of paragneisses with locally abundant kyanite-bearing quartzites. The dome is surrounded by the low-grade metasediments of the Murdama Group and is bound from the south by a low-angle dip-slip fault. Beyond the southern strand of the Ar Rika Fault is the Kibdi Basin which hosts unmetamorphosed sediments belonging to the Jibalah Group; this group occupies scattered pull-apart basins closely associated with releasing bends along the Najd fault system. Little dating has been done on the gneiss domes of the Arabian Shield; however, recent dates from similar structures in the eastern desert and Sinai range from 580 to 620 Ma. A similar, albeit younger 40Ar/39Ar age of 557 ± 15 Ma was obtained from a biotite paragneiss south of Jabal Kirsh; this age difference probably represent the time interval it took the Kirsh rocks to cool below the biotite closure temperature and would place a lower age limit for the dome. The Kirsh dome occupies an extensional zone between left-stepping faults; movement within this zone might have caused enough decompression to trigger fluid-absent melting in the middle crust especially as the rocks cross the biotite dehydration solidus. Diapiric ascent aided by strike-slip dilatancy pumping led to the emplacement of the Kirsh rocks in their present position within the Murdama Group metasediments.