Extensional Faults Research Articles

Subsurface structure of the Mazatlán basin, southeastern Gulf of California, Mexico, from seismic reflection and well data

The structure of the southeastern Gulf of California rifted margin is only partly known because of the thick sedimentary cover of the continental shelf and coastal areas of Sinaloa and Nayarit. Here we report the subsurface structure and probable age of the Mazatlán basin, one of the largest buried basins, based on interpretation of reprocessed by multichannel 2D reflection seismic profiles and an exploratory well drilled by Petroleos Mexicanos (PEMEX) between 1979 and 1980, as well as gravimetric data. The Mazatlán basin is located just north of the active subduction zone of the Rivera plate and west of the new oceanic crust accreted since the Pliocene at the mouth of the Gulf. The basin is bounded to the west by a high of the Late Cretaceous batholiths exposed in the Tres Marias Islands and in the normal faults bounding the Nayarit Troughs, and to the east by the late Oligocene-early Miocene extensional faults bounding the Sierra Madre Occidental. Based on the characteristics of the seismic profiles, the basin can be divided into three main sectors. The transition between the northern and central sectors is defined by an abrupt deepening of the basement that corresponds to the westward prolongation of ENE-WSW trending accommodation zones separating opposite tilted domains in the Sierra Madre Occidental. The acoustic basement is clearly defined in the central sector by a set of high-amplitude reflectors suggesting an igneous origin. The deepest part of the basin is located toward the coastal plain of Nayarit and coincides with a gravimetric minimum. We identify several high amplitude reflectors interpreted as mafic sills, which become more abundant toward the south. At least part of them can be correlated with the widespread mafic volcanism emplaced at 9–12 Ma along the eastern margin of the Gulf of California rift. In the northern and central sectors we identified three main stratigraphic boundaries (L1, L2, and L3) within the basin filling, which define seismo-stratigraphic sequences. The shallower sequences, limited by the L1 and L2 boundaries and attributed to the Pleistocene and part of the Pliocene, lack evidence of significant deformation. The sequence above L3 boundary, can be assigned to the earliest Pliocene and the Miocene since they are intruded by a ~ 10 Ma basaltic sill cut by the Huichol-1 exploratory well. The lower part of the basin is characterized by high amplitude reflectors that pinches out against the acoustic basement and correspond to volcanic tuffs and sedimentary deposits correlative with the lower member of the Comondú group exposed in Baja California, which includes ~23 to ~19 Ma distal part of ignimbrites sourced from the Sierra Madre Occidental in Nayarit. Faults with the most significant displacements have a listric geometry and dip from the E to NE. The maximum sediment thickness reported in the Huichol-1 well is 2800 m, with a two-way travel time of 2.7 s, but it reaches 3.7 TWTT in some areas. Based on sedimentation rates, the minimum age for the onset of sedimentation in the basin is ~19 Ma, which is consistent with the regional tectonic evolution of the margin.

Climate-driven stress changes and normal fault behavior in the Lake Malawi (Nyasa) Rift, East Africa

Climate-triggered fluctuations of surface masses, including ice and water, can cause transient stress in the Earth's crust, further affecting the slip behavior of faults over different temporal and length scales. In particular, lakes developed within active continental rifts may modulate the stress states and slip rates of rift border faults and intrarift faults. Here, we utilize a numerical model in a case study of the Malawi (Nyasa) Rift to understand the response of faults to mass fluctuations on the Earth's surface. The water level of Lake Malawi rose 600 m over the last 150 kyr, and significantly influenced the stress state of faults in rift valley. We find that such water load fluctuations can exert 4.6 MPa normal stress on fault planes and produce a negative Coulomb stress (down to −2.0 MPa) on fault planes as well as a pronounced reduction of slip (∼2 m) on fault planes within the rift. Moreover, along-strike differences in fault geometries and their position relative to the center of the water column load resulted in variable along-strike stress and slip changes. These results suggest that lakes that develop within continental rifts play an important role in the evolution of extensional faulting. Our case study provides a basis for evaluating the relationship between climate-driven surface mass variations and the subsurface stress state of fault planes and associated seismic potentials.

Periclinal fold systems in thick-bedded mudstones: A case study of the Early Cretaceous Hekou Group, Lanzhou Basin, NW China

A new type of structural association comprising periclinal folds is present in thick-bedded calcareous mudstones and siltstones of the Early Cretaceous Hekou Group in the Lanzhou Basin, NW China. The periclinal fold system consists of meso-scale, open anticlines and synclines with fold axial trends in two perpendicular orientations (NE-SW and NW-SE) and two sets of conjugate thrust faults that form intersections in the same two orientations. Folded and faulted layers of the periclinal fold system are not sedimentary bedding surfaces but sub-parallel curviplanar foliations formed by compaction of the original strata soon after deposition. Many folded curviplanar foliations connect with the unfolded, conjugate thrust faults, indicating that faulting was active during and/or soon after folding. The NE-SW and NW-SE-trending fold axes and fault planes indicate bilateral, subhorizontal contraction acting simultaneously during the formation of the periclinal fold system. In some cases, converging slickenlines on curviplanar foliation planes within anticline cores point to localized isotropic contraction probably caused by scaping fluids and increased hydrostatic pressure. Similarities with the better-known polygonal fault system include development in fine-grained sediments and layer boundness, whereas main differences are a dominance of contractional faults in the periclinal fold system versus extensional faults in the polygonal fault system and a higher consolidation of sediments needed for the development of the periclinal fold system. This study suggests that collision between the Lhasa and Qiangtang Blocks combined with oblique convergence and subsequent collision along the southeastern margin of the Eurasian plate following compaction-related volume loss of fine-grained sediments led to the formation of the periclinal fold system in the Hekou Group in the Late Cretaceous.

U–Pb geochronology of Late Silurian (Wenlock to Pridoli) volcanic and sedimentary rocks, central Newfoundland Appalachians: targeting the timing of transient extension as a prelude to Devonian orogenic gold mineralization

Bimodal igneous suites and associated immature clastic sedimentary rocks are characteristic of many orogenic gold-mineralized, crustal-scale fault zones globally. In the central Newfoundland Appalachian orogen, the Rogerson Lake Conglomerate belt and Botwood basin are Late Silurian (Wenlock to Pridoli), fault-controlled sedimentary rock sequences and magmatic suites closely associated with orogenic gold mineralization; however, the spatio-temporal evolution of faulting and associated sedimentation and magmatism are not fully resolved. U–Pb zircon geochronological results were obtained by using an integrated approach employing LA-ICPMS (laser ablation-inductively coupled plasma mass spectrometry) followed by CA-ID-TIMS (chemical abrasion-isotope dilution-thermal ionization mass spectrometry) on the same detrital samples. Using this approach, a maximum depositional age for sedimentary rocks of the Rogerson Lake Conglomerate sequence is 421.9 ± 1.0 Ma (Pridoli), which confirms that they are younger than, and stratigraphically overlie, ca. 422–420 Ma igneous rocks exposed along the central Newfoundland gold belt. Towards the stratigraphic middle of the Botwood basin in north-central Newfoundland, a tuffite layer intercalated with graded siltstone produced a maximum depositional age of 427.9 ± 3.1 Ma (Wenlock; Homerian). The age of emplacement of an autobrecciated, flow-banded rhyolite dome of the Charles Lake volcanic belt along the northwestern Botwood basin is 429.3 ± 0.7 Ma (Wenlock; Homerian). The high-precision CA-ID-TIMS zircon data establish a clear link between Wenlock to Pridoli magmatism and sedimentation throughout central Newfoundland. Furthermore, these geochronological results are consistent with a structural model involving the southeastward (present-day coordinates) advancement of a transient extensional fault system across strike of the Exploits Subzone between ca. 429 and 418 Ma, with propagation along strike to the southwest (Rogerson Lake Conglomerate belt) between ca. 422 and 418 Ma. Extensional faulting may have contributed to basin formation, subsidence, and exhumation of pre-Late Silurian rocks of the Exploits Subzone. Time-transgressive, extension-related magmatism and clastic sedimentation appear to mark the transition between the Salinic and Acadian orogenic cycles along the central Newfoundland gold belt. Transient Wenlock to Pridoli lithospheric extension may have been important for increasing heat and fluid flow in the crust as a prelude to Devonian crustal thickening, fluid focussing, and orogenic gold mineralization.

Tectono‐stratigraphic evolution of Shillong Plateau, North East India through the Permian‐Eocene window

The Shillong Plateau in the north‐eastern Indian Plate signifies a cut‐off patch of the Indian Peninsular Shield, bordered by fault and thrust sheets, with thick alluvium cover along its periphery. The basement rock in the plateau accomplished about 2 km relief difference with the adjoining Sylhet Trough of Bangladesh in the south. The plateau witnessed Permo‐Carboniferous Gondwana sedimentation at its western margin, with Early Cretaceous basaltic traps and continuous fluvio‐marine sedimentation along its southern edge since the late Cretaceous. Although it is largely believed that strike‐slip movement along the Dauki Fault shifted the plateau about 250 km eastward, however, the driving mechanism for Shillong Plateau detachment and conversion of rift‐intercratonic basin to platformal configuration has drawn more attraction. This article focuses on the tectono‐stratigraphic evolution of Shillong Plateau and its adjoining regions on the basis of previous works, to try to understand the distinct tectonic and environmental factors. Early Permian continental rifting to the south‐west of the plateau and south‐east and south‐west of the Malda High presumably separated the Shillong Plateau from Chotanagpur Granite Gneiss Complex (CGGC). Extrusion of Rajmahal Traps west of the Malda High and Bengal‐Sylhet‐Mikir volcanism along an Early Cretaceous geofracture probably accelerated the east‐northeast migration of Shillong Plateau with respect to the CGGC. Since the Late Cretaceous, sedimentation at the southern fringe of Shillong Plateau commenced with an intracratonic basin which gradually transformed into a passive margin configuration during the Late Palaeocene‐Eocene Epoch during growth of the Indian Ocean. Differential subsidence in southern Shillong Plateau caused clastic sedimentation without considerable transportation. At the beginning, the Late Cretaceous intracratonic basin had received volcanoclastic, fluvio‐lacustrine sediments in a series of graben/half‐grabens associated with extensional faulting and volcanism, as represented by the Lower Mahadek and Jadukata formations in the plateau. The upper Mahadek and later Palaeogene sequences along the southern margin of the plateau record marine sedimentation, initiated with an estuarine‐marshy environment during the Late Cretaceous to Palaeocene, followed by a platformal marine succession since the Mid‐Eocene. The Calcutta‐Mymensingh Eocene Hinge Zone that separates a stable shelf from the deep basinal part might be traced through the south‐eastern fringe of Shillong Plateau and presumably continues further north‐east between the Naga and Disang thrusts in the eastern part of Dhansiri Valley.

How do tectonics influence the initiation and evolution of submarine canyons? A case study from the Otway Basin, SE Australia

The architecture of canyon-fills can provide a valuable record of the link between tectonics, sedimentation and depositional processes in submarine settings. We investigated the role of plate tectonics in the initiation and evolution of submarine canyons. We demonstrate that events at the plate tectonic scale (i.e. continental break-up and shortening) have a first-order influence on the initiation and development of submarine canyons. The Late Cretaceous ( c. 65 Ma) separation of Australia and Antarctica initially resulted in extensional fault systems, which then formed a steep stair-shaped palaeoseabed. Subsequently, the Late Miocene ( c. 5 Ma) collision of Australia and Eurasia resulted in substantial uplift and exhumation on the SE Australian continental margin. These tectonic events led to elevated seismicity, which ultimately gave rise to gravity-driven processes (i.e. turbidity currents and mass wasting processes) and formed the base of the canyon. The inherited stair-shaped topography then facilitated further gravity-driven processes that established a mature sediment conduit extending from the shallow marine shelf to the abyssal plain. We suggest that the stratigraphic architecture of canyons can be used as an archive to record tectonic movements. Moreover, the factors that precondition and trigger gravity-driven processes can induce canyon initiation and facilitate canyon development. Supplementary material: A PDF file containing all the clean, uninterpreted seismic sections is available at https://doi.org/10.6084/m9.figshare.c.5937760

Outer trench slope extension to frontal wedge compression in a subducting plate

The occurrence of faulting in subducting plates is a major process that changes the mechanical properties of the subducting lithosphere and carries surface materials into mantle wedges. Two ocean-bottom seismometer networks deployed on the frontal accretionary wedge of the northern Manila trench in 2005 and on the outer slope of the trench in 2006 were used to detect earthquakes in the subducting plate. All available P and S manually picked phases and the waveforms of 16 short-period, three-component stations were used. Relocation was performed using the double-difference method with differential times derived from the phase-picked data. Two intraplate earthquake sequences of small-to-moderate magnitudes in the northern Manila subduction system were investigated in this study. The results revealed distinct fault planes, but a contrasting seismogeny over the northern Manila Trench. The seismicity in the frontal wedge (as measured in 2005) was mainly contributed by a fluid overpressure sequence, whereas that in the incoming plate (as measured in 2006) was contributed by the aftershocks of an extensional faulting sequence. The obtained seismic velocity models and Vp/Vs ratios revealed that the overpressure was likely caused by high pore-fluid pressure within the shallow subduction zone. By using the near-field waveform inversion algorithm, we determined focal mechanism solutions for a few relatively large earthquakes. Through the use of data obtained from global seismic observations, we determined that stress transfer may be responsible for the seismic activity in the study area during the period of 2005–2006. In late 2005, the plate interface in the frontal wedge area was unlocked by the overpressure effect due to a thrusting-dominant sequence. This event changed the stress regime across the Manila Trench and triggered a normal fault extension at the outer trench slope in mid-2006. However, in the present study, a hybrid focal mechanism solution indicating reverse and strike–slip mechanisms was implemented, and it revealed that the plate interface locked again in late 2006.Graphical

New Facies Model and Carbon Isotope Stratigraphy for an Ediacaran Carbonate Platform From South America (Tamengo Formation—Corumbá Group, SW Brazil)

The Ediacaran is a period characterized by the diversification of early animals and extensive neritic carbonate deposits. These deposits are still not well understood in terms of facies and carbon isotope composition (δ13C). In this study we focus on the Tamengo Formation, in southwestern Brazil, which constitutes one of the most continuous and well-preserved sedimentary record of the late Ediacaran in South America. We present new detailed lithofacies and stable isotopes data from two representative sections (Corcal and Laginha) and revise the paleoenvironmental and stratigraphic interpretation of the Tamengo Formation. The Corcal section consists of neritic deposits including shallow-water limestone beds, alternated with shale and subordinate marl beds. These facies yield specimens of the Ediacaran fossils Cloudina lucianoi and Corumbella werneri. On the other hand, the Laginha section shows more heterogeneous facies, such as impure carbonates, breccias, marls, and subordinate mudstone beds, as well as no evidence of Corumbella werneri. The stable carbon isotope record is also different between the two sections, despite belonging to the same unit. The Corcal section displays higher and more homogeneous δ13C values, consistent with those of Ediacaran successions worldwide. The Laginha section, instead, displays more variable δ13C values, which suggest the influence of local and post depositional processes. The difference between the two sections was attributed to the different distance from the shore. We propose that the difference is due to topographic variations of the continental platform, which, at the Laginha site, was steeper and controlled by extensional faults. Therefore, the Corcal section is a better reference for the Tamengo Formation, whereas the Laginha is more particular and influenced by local factors. Besides, the lithofacies associations of the Tamengo Formation are like those of the Doushantuo and Dengying formatios, in South China, with no significant biogenic carbonate buildups, and different from those of other important Ediacaran units, such as the Nama Group in Nmibia and the Buah Formation in Oman. Our work highlights the complexity and heterogeneity of Ediacaran carbonate platforms and of their carbon isotopic composition. In addition, we characterize the Corcal section as a possible reference for the Ediacaran in South America.

Metallogeny of the Hunjiang basin, northeastern North China block

The Hunjiang basin in the northeastern North China block is host to a wide range of ore deposit types that are currently exploited by more than 100 past and producing mines. Extensive field and laboratory investigations were conducted to summarize the regional metallogeny, classify the different deposit types, and to place the deposits within a regional geologic and tectonic context. The Archean basement of the basin is host to magnetite deposits that have been affected by high-grade metamorphism. In addition, Proterozoic magmatic Cu-Ni deposit and orogenic Au deposits that are likely of Mesozoic age are located in the metamorphic basement rocks. Polymetallic Co-Cu and Ni-Co-Cu deposits are located in Proterozoic schists within the basin and are of pre- to syn-metamorphic origin. However, most deposits in Hunjiang basin can be related to Mesozoic igneous activity. This includes porphyry Cu and Cu-Mo deposits as well as associated skarns and carbonate replacement deposits. Economically most significant are Au deposits hosted in Proterozoic limestone and dolomite that are located distal to the Mesozoic igneous intrusive centers. This includes the White Mountain deposit near Baishan City, which contains an endowment of more than 50 tonnes Au. There are no known high-sulfidation epithermal deposits within the Hunjiang basin, which is consistent with fluid inclusion evidence demonstrating that emplacement of the porphyry intrusions occurred at significant depth below the paleosurface. Geochronological constraints indicate that Mesozoic igneous activity and associated deposit formation in the Hunjiang basin occurred as a result of Jurassic subduction processes and subsequent extension after slab rollback and lithospheric delamination under the Eastern North China Block in the Cretaceous. Future mineral exploration efforts in the Hunjiang basin should focus on the distal sedimentary rock-hosted Au deposits associated with Mesozoic intrusions, with special emphasis on the northern part of the basin where these deposits occur preferentially at the intersection of extensional faults with a vanished evaporite unit that is distinctly enriched in Fe oxides providing a chemical trap.

The Cenozoic Multiple-Stage Uplift of the Qiangtang Terrane, Tibetan Plateau

Cenozoic collision between the Indian and Asian continents is generally considered as the main driver forming the high Tibetan Plateau (TP). However, it remains hotly debated when and how the relatively flat and highly elevated TP was formed. Here, we present combined analyses of the apatite fission track (AFT) and apatite (U-Th)/He (AHe) of 18 granite samples along three steep topographic transects in the central part of the Qiangtang Terrane (QT), TP. The results indicate that the AFT ages of all samples are mostly between 130 Ma and 80 Ma, while the AHe ages range from 80 Ma to 40 Ma. Further thermal history modeling indicates that no significant cooling occurred after 40 Ma for most samples, except those lying close to the Reganpei Co extensional fault in the QT. The results are generally consistent with other low-temperature thermochronological data, as well as structural and sedimentologic data from the QT, suggesting that low relief and the relatively flat topography of the QT were almost completely formed before ∼40 Ma. As both megafossils and pollen had undergone a sharp change from subtropical- to psychro-species, indicating a relatively low elevation (∼2 km) at ∼40 Ma and >2 km uplift during the Oligocene. We use simple one-dimensional isostatic modeling to assess the contribution of convective removal of the lithospheric mantle to the present elevation of the QT. The results suggest that a combined effect of isostatic rebound (≥2 km) and thermal expansion related to asthenosphere upwelling and subsequent crustal base heating (∼0.4 km) led to the final uplift of the QT. Therefore, the QT experienced multiple-stage uplift processes which were controlled by crustal thickening before ∼40 Ma and lithospheric mantle delamination during the Oligocene, respectively.

Seismic interpretation and hydrocarbon assessment of the post-rift Cenomanian Bahariya reservoir, Beni Suef Basin, Egypt

The geophysical interpretations for a collection of high-quality 2D seismic data plus well logging data for six drilled wells exposed the structural controls and the hydrocarbon potentiality of the Beni Suef Basin, north Western Desert, Egypt. The created structural map on the top of the Early Cenomanian Bahariya Formation (the significant oil reserve in north Western Desert), exhibits a series of NW–SE normal faults as well as a master ENE strike-slip major fault. The extensional faults formed horst structures while the strike-slip fault created a negative flower structure in the central part of the study area. These structural features represent the main petroleum traps within Beni Suef Basin. The reservoir quality was assessed through the petrophysical analysis of the Bahariya Formation in wells; Azhar A2, Azhar E-2X, Azhar E3, Yusif-4X, Fayoum-2X and Beni Suef W-1X. This analysis proved that the Lower Bahariya unit has better reservoir quality than the Upper Bahariya reservoir, with shale volume less than 10%; effective porosity between 10 and 20%, water saturation between 25 and 75% and total pay thickness between 11 and 88 ft. The obtained results encourage drilling more exploratory wells in Beni Suef Basin to enhance the productivity from the sandstones of the Lower Bahariya reservoir.

Structural Characteristics and Tectonic Evolution of the Wunansha Uplift in the South Yellow Sea Basin, China

By selecting typical seismic sections to carry out detailed structural interpretation, the structural style features of the Wunansha Uplift in the South Yellow Sea basin were systematically combined, and the compressional structures (imbricate, opposite/back thrust, and Y-shaped structures), strike-slip faults (positive flower-shaped faults), and extensional normal faults (listric-shaped normal faults) were identified. On this basis, combined with the characteristics of the regional stress field and the background of deep geodynamics, the genetic mechanism and structural evolution of the structural style in the Wunansha Uplift were defined. The stress mechanism of the compressional structures originated from the initial high-speed and low-angle NW subduction of the paleo-Pacific plate during the early Yanshanian movement in the Early Jurassic. The regional strike-slip fault was mainly a positive flower structure with compression and torsion characteristics, and its stress mechanism originated from sinistral shear caused by the Early Cretaceous low-angle NNW subduction of the paleo-Pacific plate. The Tan-Lu fault in eastern China also had sinistral shear characteristics in this period. The extensional normal fault was characterized by a listric shape, which developed along the northern boundary of the Wunansha Uplift, that is, the connection between the Wunansha Uplift and the Southern Depression of the South Yellow Sea basin. The stress mechanism was derived from the transition of the paleo-Pacific plate from low-angle to high-angle subduction during the late Yanshanian movement in the Late Cretaceous. Simultaneously, the tectonic stress system in eastern China also changed from compressional to tensional.

Understanding heat transfer along extensional faults: The case of the Ambilobe and Ambanja geothermal systems of Madagascar

Understanding the role of faults where forced convective heat transfer is the dominant mechanism giving rise to hot springs is critical in geothermal exploration in extensional environments. This study uses two-dimensional models of coupled fluid flow and heat transfer along cross-sections perpendicular to faults and the regional topography to identify favorable fault conditions for geothermal system development in northern Madagascar. Structural data collected at surface were used to define fault scenarios and simulate the ascension of hot fluids to reproduce hot spring temperatures in the Ambilobe normal fault zone area and the Ambanja graben structure. Fault dips facing topography‐driven groundwater flow was shown to be favorable, and hot spring temperatures could be reproduced when the fault permeability was > 10−14 m2. Faults located in a discharge zone near a river were the most favorable for fluid ascension, regardless of their dip. Constraining the model with a basal heat flow between 90 and 148 mWm−2 at a depth of 10 km allowed the reservoir temperature to reach 150–200 °C at depths of 2 km or shallower along favorable faults.

The brittle evolution of Western Norway – A space-time model based on fault mineralizations, K–Ar fault gouge dating and paleostress analysis

Basement fracture and fault patterns on passive continental margins control the onshore landscape and offshore distribution of sediment packages and fluid pathways. In this study, we decipher the spatial-temporal evolution of brittle faults and fractures in the northern section of the passive margin of Western Norway by combining field observations of fault mineralizations and K–Ar fault gouge dating with different paleostress approaches, resulting in the following model: (1) High-T fault mineralizations indicate Silurian NW-SE compression followed by NW-SE extension in the Early to Mid-Devonian. (2) Epidote, chlorite and quartz fault mineralizations indicate a dominant strike-slip stress field in the Late Devonian to early Carboniferous. (3) E-W extensional stress fields which could be related to Permo-Triassic or Late Jurassic rifting are not prominent in our data set. (4) K–Ar fault gouge ages indicate two extensive faulting events under a WNW-ESE transtensional stress regime with related precipitation of zeolite and calcite in the mid (123-115 Ma) and late (86-77 Ma) Cretaceous. Our results show that the brittle architecture of the study area is dominated by reactivation of ductile precursors and newly formed strike-slip faults, which is different from the dip-slip dominated brittle architecture of the southern section of the West Norway margin.

QUaternary fault strain INdicators database - QUIN 1.0 - first release from the Apennines of central Italy

We present QUIN, a “QUaternary fault strain INdicators database”, designed to integrate and unify published and unpublished local-scale geological information and derive strain parameters for structural and seismotectonic analyses. It provides data on 3339 Fault Striation Pairs (FSP; fault plane and slickenline), distributed within 455 survey sites. These are exposed along the intra-Apennine Quaternary extensional faults of Central Italy. The area covers an extent of ~550 km in a NW-SE direction. We give information on FSP location, attitude and kinematics, and deformation axes. We also provide an original shapefile of the faults hosting the FSP. A large amount of homogeneously distributed Quaternary fault/slip data help to clarify and implement the contemporary geometric and kinematic deformation pattern of Central Italy that appears scattered and incomplete whenever exclusively derived from earthquake data. The high-density of structural data can help investigate stress pattern heterogeneities at local scales, with relevance for new generations of hazard assessment evaluation and a better understanding of rupture propagation and related barriers.

Imaging extensional fault systems using deep electrical resistivity tomography: A case study of the Baza fault, Betic Cordillera, Spain

Seismic hazard assessment of present-day tectonic faults may be improved by studying their structure and kinematics. These features have been mainly determined by surface geological studies of exposed fault traces, structural lateral segmentation or paleoseismic trenches. All these approaches rely mainly on two-dimensional analyses of surface outcrops, while knowledge of the faults at depth remain largely inaccessible. To improve on such limitations, geophysical methods can be applied to establish detailed information on fault morphology and segmentation at depth. This work analyzes new results of a deep electrical resistivity tomography survey acquired across the Baza Fault, a present-day tectonic fault that controls the geometry of the Neogene intra-mountainous Baza Basin (Betic Cordillera, Spain). Interpretation of our preferred resistivity model reveals its detailed structure down to approximately 1000 m depth. The survey shows a minimum 2 km wide complex normal fault system, with rotational tilting blocks bounded by potentially listric normal faults. This study presents subsurface evidence of the Baza fault (F3), coincident the main topographic scarps. However, the geophysical model and geomorphic evidence also support a fault branch (F1) that might be an additional active seismogenic source. The geophysical survey technique presented in this study provides essential data to improve assessment of the seismogenic potential of the Baza Fault.

Tectonic shortening and strain magnitudes across the Chapada Diamantina Fold and Thrust Belt: New insights into the tectonic evolution of the Paramirim Aulacogen

In this study, strain magnitudes and tectonic shortening across the Chapada Diamantina fold-and-thrust belt are calculated by finite strain analysis, cross-section restoration and kinematic forward models. A total of 9 complete 3D strain analyses on oriented samples were conducted by the Inertia Tensor method and best fit ellipsoids were computed, resulting in strain ratios (X/Z) varying from 1.190 to 2.504. Predominant oblate shaped ellipsoids related to down-dipping stretching directions are associated with regional transport direction from SW to NE, with reverse faults, upright axial-plane fold and sub-horizontal axis. The restoration of the cross-section yielded a shortening of 19.37%, understood as the minimum tectonic shortening. The adopted model assumes the inversion of former extensional faults (steep thrusts), where the structural architecture of the area is mainly controlled by the inherited rift basin architecture, with basement involvement in a combination of thick and thin skin deformation. Similarly, the assessment of kinematic forward models resulted in a minimum shortening of 18.5%. Comparably, finite strain data integration techniques yielded a total of 21.65% of regional tectonic shortening, representing the horizontal component of ductile flattening strain across the section due to penetrative strain. Therefore, we present, by three different and independent approaches, an estimate range of tectonic shortening between 18%–22% at the western Chapada Diamantina region, but the total amount of shortening across the Paramirim aulacogen might have been greater. These results suggest that the western and eastern parts of the São Francisco craton could not be considered as rigidly linked during the Neoproterozoic–Cambrian Brasiliano orogeny, with the Paramirim aulacogen exerting an import role.

Structural and petrological characteristics of a Jurassic detachment fault from the Mont-Blanc massif (Col du Bonhomme area, France)

Necking zones, located between the proximal and distal domains of passive margins, are characterized by important crustal thinning and the occurrence of extensional detachment faults. Here we provide a petrographic, microstructural and geochemical study of an extensional detachment fault discovered in the Mont-Blanc massif, which was located in the necking zone of the European margin during Early Jurassic rifting. The footwall of the fault consists of basement gneisses, while its hanging wall presents pre-to syn-rift deposits. A careful observation of the basement/cover contact also reveals the presence of a strain gradient affecting the Variscan gneisses, which are progressively deformed to cataclasite and to black gouges at the top basement, and sealed by undeformed Jurassic syn-rift sandstones, the Grès Singuliers, which rework fault rock pebbles. Microscale observations also indicate fluid-assisted deformation, characterized by the total breakdown of feldspars and the coeval formation of white mica, leading to strain softening along the fault plane. The overall observations point toward a rift-related detachment fault responsible for the exhumation of the internal Mont-Blanc basement, an interpretation which challenges previous models of crustal exhumation in the External massifs of the Alps. Finally, the preservation of an undeformed syn-rift sedimentary cover sealing a tectonized top basement makes this area a fossil analogue for detachment faults in necking domains. It demonstrates that preservation of pre-Alpine, rift related structures is possible even in highly deformed areas, and emphasizes the role played by pre-Alpine inheritance on the building of the Alpine Mountain belt.

Multiple crustal and mantle inputs in post-collisional magmatism: Evidence from late-Variscan Sàrrabus pluton (SE Sardinia, Italy)

The Sàrrabus pluton is formed by multiple short-lived intrusions emplaced at about 286 Ma at shallow crustal levels within the external part of the South Variscan Orogenic Belt. A chemical and Sr and Nd isotopic study on the Variscan post-collisional magmatism from the Sàrrabus pluton reveals the repeated bimodal character of the intrusions, in which heterogeneous crustal sources and mantle-derived calcalkaline magmas are involved. Products of this magmatic activity occur as intrusive units and mafic/felsic dykes intruded in post-collisional regime along extensional faults during tectonic exhumation. Pluton growth started with an early stage of emplacement of broadly granodioritic magma with subordinate mafic magma batches (stage 1) followed by large intrusions of metaluminous to subaluminous and subordinately peraluminous granites (stage 2). In stage 1, the occurrence of remnants of stratified olivine-bearing gabbroic rocks indicates the intrusion of mafic magmas which experienced low-pressure crystal/liquid fractionation. Mafic magmas may represent an external heat supply for melting of different crustal materials belonging to an inferred Precambrian crystalline basement underlying the Paleozoic rocks of the Variscan nappe pile. Strong evidence for heterogeneous crustal sources is constrained by isotope data. Peraluminous granites and felsic dikes display initial 87Sr/86Sr in the range of 0.7140÷0.7215 and a roughly constant εNd286 (−7.4 to −7.5). Conversely, a peculiar less radiogenic character, in the range of 0.7030÷0.7067/−5.5÷ − 6.2, is observed for metaluminous to subaluminous varieties. Calculated Neodymium Crustal Index (NCI) confirmed a progressive increase in crustal magmas generation during the pluton growth, from stage 1, recording minor mixing processes between mantle- and crustal-derived peraluminous melts, to stage 2, where extensive crustal melting occurred, originating metaluminous to subaluminous granites. Possible crustal sources for metaluminous/subaluminous and peraluminous granites are (Pan-African) amphibolites and metasedimentary rocks, respectively. Two-stage depleted-mantle Nd model ages cluster at 1.4 and 1.6 Ga for metaluminous/subaluminous and peraluminous granites, respectively, well comparable with other segments of the European Variscan belt. Remarkably, last magmatic pulses resulted in widespread subalkaline bimodal mafic/felsic dykes that overlapped the SrNd signature recorded by major intrusions. This similar geochemical affinity between late dykes and the first intrusives may be related to decreasing temperature in the crust, which inhibited extensive mixing processes with the upwelling melts. At the same time, the presence of felsic intrusions in the shallow crust would have prevented the rise of more primitive basic magmas, which would have consolidated at depth. Finally, the high radiogenic character of Sàrrabus mafic products, compared to coeval Variscan mafic rocks of Corsica and northern Sardinia, may be indicative of previous fractionation and mixing processes, possibly related to magmatic underplating of the lower crust.

Genesis of orogenic gold systems in the Daduhe belt: Evidence of long-lived fertile mantle lithosphere as a source of diverse metallogeny on the western margin of the Yangtze Craton, China

The poorly documented Daduhe gold belt on the western margin of the Yangtze Craton, an important metallogenic belt in southwest China, is characterized by typical orogenic gold systems against a highly anomalous geodynamic setting. A synthesis of regional and deposit geology, mineralization ages, and S-O isotopic compositions of 33 gold deposits from the Danba-Kangding-Shimian-Mianning area of the belt reveals regional metallogenic patterns, establishes a consistent genetic model, and demonstrates a source equivalent to other diverse mineral systems on the western margin of the Yangtze Craton. The Daduhe orogenic gold deposits are controlled by the lithosphere-scale Xianshuihe and Anninghe strike-slip faults in the northern and southern segments of the belt, respectively. The alteration and ore mineral assemblages of the Daduhe orogenic gold deposits define a wide range of hypozonal, mesozonal, and epizonal sub-types. They formed during two episodes: 1) Early Jurassic gold mineralization related to closure of the Paleo-Tethys Ocean, with a large hypozonal gold deposit on the margin of the Danba dome in the northern segment; 2) Cenozoic gold mineralization associated with continental collision on the Tibetan Plateau, with small mesozonal and epizonal gold deposits in shear zones in migmatite basement and at contacts with cover sequences in both the northern and southern segments. At the belt and deposit scales, the ore-controlling structures for both mineralization episodes are NNE-trending second-order extensional faults or their intersections with the dominant NW-trending compressional faults. The three deposit sub-types have overlapping δ34S of 0 ∼ 10‰, δ18Ofluid of 6 ∼ 11‰, and low He-Ar isotope ratios, all consistent with derivation of ore fluids from metasomatized mantle lithosphere. Importantly, the Daduhe orogenic gold deposits have similar S-O-He-Ar isotope compositions to the giant IOCG, Cu-Ni-PGE, Fe-V-Ti, anomalous porphyry Cu-Mo-Au, and carbonatite-related REE deposits in the Panxi region on the shared margin of the western Yangtze Craton. This suggests that these diverse mineral systems have a common source of ore fluids and metals derived from fertile mantle lithosphere during different geodynamic and metallogenic events. Such a widespread hydrous metal-rich deep source with lithosphere-scale faults as ore fluid pathways, and a depth continuum of orogenic gold deposits in the Daduhe belt suggest a high prospectivity and a higher future gold endowment for the belt. This study adds to an increasing awareness of the diversity of mineral systems on specific craton margins globally.