Crustal-scale Structure Research Articles

A multi-disciplinary geophysical approach reveals a Paleoproterozoic cryptic suture in the Amazon Craton – Implications for Proterozoic Supercontinent reconstructions

The Amazon Craton evolved in the Paleo-Mesoproterozoic through successive continental magmatic arcs, which ages decrease westwards. However, when and if these magmatic arcs collided with another continent, closing an ocean basin is a subject of ongoing debate. It is speculated that the Baltica and Amazon cratons formed a purely long-lived accretionary margin for at least 900 Ma and only collided during the formation of the Rodinia Supercontinent at 1.2 – 1.0 Ga. Identifying relicts of suture zones is challenging since Proterozoic orogens tend to preserve the continental arc and the reworked components so that less than 10 % of the orogens are composed of oceanic evidence. Moreover, the Phanerozoic cover overlays more than 50 % of the proterozoic rocks in the Amazon Craton. Therefore, we propose a robust geophysical assessment to investigate the upper and lower crusts. We acquired new magnetotelluric data and integrated it to the available potential field (satellite, airborne, and ground), geochronological data (magmatic and metamorphic ages), crustal thickness and depth to Curie temperature estimates. A major crustal-scale structure (more than 1000 km in extension) separates two tectonic fragments with remarkable differences in the magnetic gradient and structure trend. We also find along this major boundary high and low Bouguer anomalies of long-wavelength paired, a step in the Mohorovic discontinuity, and a lower crust conductor. This evidence supports the hypothesis of a collision in Paleoproterozoic times. Our findings suggest that the orogen did not remain purely accretionary and evolved to a collisional type between 1.63 and 1.69 Ga. This contrasts with the purely accretionary orogens described in the Baltica Craton, thereby undermining the hypothesis that the Amazon and Baltica cratons shared the same peri-Nuna subduction system from 2.0 to 1.3 Ga.

Resistivity imaging from magnetotelluric soundings of the Rio Preto Belt (NE Brazil): The role of lateral variations in the Precambrian basement on the evolution of phanerozoic basins

We present a resistivity model for the boundary of the Paleozoic Parnaíba Basin and the Cretaceous Sanfranciscana Basin (NE Brazil). The boundary underlies the Neoproterozoic Rio Preto belt, whose basement consists of reworked Archean and Paleoproterozoic rocks. In order to characterize the subsurface geoelectrically, 17 broadband magnetotelluric (MT) soundings were acquired and distributed along a ∼90 km profile across Rio Preto belt. The transfer functions were obtained with a robust code, thus obtaining reliable results for all four components of the impedance tensor. Some apparent resistivity and phase curves were shown to be heavily contaminated by local electromagnetic noise at low frequencies, and others showed minimal radio signals associated with the deadband frequency range. In this study, the MT data set was interpreted using 2D and 3D inversion procedures. Comparing the two MT models, the crustal-scale structure is well resolved in the 2D model, but the 3D model explains significantly better the measured data related to deep/shallow structures and proved to be more sensitive to conductive anomalies. The final resistivity image derived from the 3D inversion denotes a resistive basement with vertical conductive anomalies in the SE portion and a conductive domain in the NW portion. The homogeneous conductive block corresponds to the Archean domain of the Rio Preto belt, and the heterogeneous resistive block has the morphology and physical properties of the Paleoproterozoic block. Lateral geological correlation and gravity data support that the basement of the Parnaíba Basin in the study area is formed by the Archean block of the Rio Preto Belt. The boundary between the resistive blocks is a reverse fault that is also the tectonic front of a sub-horizontal detachment fault. In the Paleoproterozoic block, the conductors are associated with graphitic and volcano-sedimentary sequences, the dome and keel structures explain the geometry of the conductors. Additionally, graben-likes features were revealed in this block, interpreted as Cambrian basins filled by Cretaceous sediments of the Urucuia and Areado formations. These structures have a good correlation with those defined in the eastern edge of the Parnaíba Basin in previous studies. Towards the NW end of the section, the resistive blocks abruptly turn into a high conductivity domain (HCD) covered by a shallow conductive layer of Parnaíba Basin sandstones that modifies the electrical properties of the basement by interaction of basal fluids.

3D Interpretation of a Broadband Magnetotelluric Data Set Collected in the South of the Chinese Zhongshan Station at Prydz Bay, East Antarctica

Antarctica is covered by a thick ice sheet, and the application of geophysical methods is necessary to image the subglacial structures for studying the hydrologic systems and tectonic deformations in the Antarctic continent. The magnetotelluric (MT) method is one of the best approaches to obtain the subglacial electrical resistivities. However, only a very small volume of data has been collected so far in Antarctica using this method. In this paper, we report on a broadband MT profile collected at 10 sites in the south of the Chinese Zhongshan Station at Prydz Bay, and a 3D resistivity model was constructed by inversion of these data. This 3D model shows two low resistivity zones at a depth shallower than 6 km. They are consistent with the low-velocity zones in the previous shear-wave model and can be interpreted as the result of interconnected fluids associated with a downward migration of subglacial water. In addition, a distinct eastward dipping low resistivity zoneis present in the crust, which extends from the top surface to the lower crust. Since its location coincides with the highly positive magnetization named Amery Lineament, it is proposed that this eastward dipping low resistivity zoneextending in the entire crust probably reveals the geometry structure of the Amery Lineament at depth. Besides, it can be inferred from this new 3D resistivity model that the Amery Lineament is at least a crustal-scale structure, which probably outcrops on the land surface but was covered by the ice sheets in the study area.

Relation between intrusive and deformational processes in oblique subductive margins. The case of the zoned Flamenco pluton in northern Chile

Oblique convergent margins, like the subduction of Phoenix beneath the South American plate during Jurassic and Early Cretaceous times, are characterized by strain partitioning and a positive feedback loop between strike-slip deformation and magma ascent along the magmatic arc.Located in the Coastal batholith of northern Chile, the Flamenco pluton is one of the youngest Andean intrusives emplaced in the western active margin of South America. Besides an older SW domain of granodioritic rocks (c.a. 213 Ma), the NW and E domains of the Flamenco pluton were emplaced between 194 and 186 Ma. They present a normally zoned structure constituted by external gabbroic to Qtz-dioritic magmatic facies and tonalites and granodiorites located in inner areas of the intrusive body. These domains are separated by a central strip of stretched coarse-grained Crd-schists that presents ductile asymmetrical folding and S–C structures that point to the NW-directed displacement of the E domain of the pluton. This syn-emplacement shear zone shows kinematic compatibility and continuity to the north and south with folded and mylonitic metasediments out of the contact aureole of the pluton. Together, these segments constitute a large, steeply dipping sigmoidal structure of average N–S direction; the called here Chañaral transcurrent shear zone.Contemporary to the emplacement of the Flamenco pluton, slight variations in the trend of the crustal-scale structure generated strike-slip and transpressive sectors along the Chañaral shear zone, which favored the access of intruding magmas to the final emplacement level. As a paradigmatic example, the curviplanar Flamenco shear zone, an internal, magmatic branch of the main structure that traverses the E domain of the pluton, is defined by the sinistral and reverse shearing under magmatic conditions of the previously mingled mafic and felsic batches. Consequently, the transpressive Flamenco shear zone is interpreted as an ascent conduit where gabbroic and granodioritic liquids interacted during the building of the intrusive body. In addition, these sheared rocks were affected by late textural coarsening processes that evidence the slow and cyclical cooling of the growing magma reservoir. In contrast with the steeply dipping contacts and structures found to the east, the NW domain of the pluton shows sharp and gently dipping contacts between almost horizontal magmatic layers. We suggest that the western block of the Chañaral shear zone was a relatively passive footwall dominated by horizontal flow trajectories and lower replenishment rates according to the inverse emplacement sequence, i.e., late external mafic batches intruded along the margins of the felsic core. The variable structural arrangement of the crustal rocks that hosted the Flamenco pluton was the result of the complex interaction between far-field and local, magmatic forces during the emplacement process, besides the interference with pre-Andean structures.The presence of the Chañaral shear zone favoring the emplacement of the Flamenco pluton demonstrates that the Late Jurassic to latest Early Cretaceous Atacama Fault System had earlier precursors and both the magmatic arc axis and the transcurrent shear zones migrated landward during Jurassic times.

3D crustal-scale structure of the West Iberia margin: a novel approach to integrated structural characterization of passive margins

Hyperextended margins are very heterogeneous along the entire length of the margin, so the definition of tectonic domains made exclusively from 2D seismic sections presents serious limitations. In this work we present an approach of the 3D crustal-scale structure of the West Iberia margin (WIM) by modelling eight lithospheric sections, using seismic, wells and gravity data. The continuous nature of gravity data allowed us to propose a new map of tectonic domains within the WIM. Maps of total horizontal (THD) and vertical gradients (dZ) of Bouguer anomaly have been calculated and compared with other criteria such as the crustal structure and thinning factor. This comparative analysis has been carried out on a section proposed as a model for the Western Iberian Margin (Tugend et al. in Tectonics, 2014; Cadenas et al. in Tectonics 37:758–785, 2018), and on four 2 + 1/2D gravimetric models transversal to the margin. The results point out a significant variation in the absolute values of Bouguer anomaly, thinning factor and crustal structure along the margin and, therefore, of the position of the different domain boundaries. Clear patterns that correlating the Bouguer anomaly signal and its derivatives to the tectonic domain are evidenced. Most significantly, the necking-zone and its transition to the hyperextended domain are characterized by high values of the THD of the Bouguer anomaly. The observed patterns in Bouguer anomaly and its derivatives provide a solid constraint for mapping the boundaries between different tectonic domains along the margin, even in those areas where limited deep seismic information could lead to uncertain interpretations. The results of this work can also inform on the general kinematics of the WIM.

The Cantabrian Fault at Sea. Low Magnitude Seismicity and Its Significance Within a Stable Setting

The Cantabrian fault (CF) is a crustal-scale structure that cuts obliquely the western North Iberian Margin (NIM) for 160 km and continues onshore transecting the Cantabrian Mountains (CM) for another 150 km as the Ventaniella fault (VF). For most of its length inland, the fault system is aseismic, except for a 70 km long segment at its southern end. Within this segment, a gently north-dipping linear arrangement of earthquakes was interpreted as related to the intersection of a slightly oblique fault to VF with the basal thrust of the CM. In addition to earthquake nucleation along parts of its length, the CF–VF also stands out regionally as a major seismotectonic boundary, separating a seismically active area to the West from an essentially aseismic region to the East. Contrasting tectonothermal evolution in the crust on either side during the Mesozoic rifting may underlie the observed differences. On the other hand, the seismicity within the subsea segment is low magnitude, persistent, and understudied. The scarcity of the permanent seismic stations distribution in the area did not allow to establish more than a generalized consensus relating the offshore events to the submarine structure. A recent local seismic network monitored the area providing the highest accuracy information on the offshore events to date. Although the location of foci is partially challenged by the lack of recording stations from northern azimuths at sea, the observed pattern shows indeed a broad linear trend in the submarine domain in relation to the crustal-scale structure. Specifically, this study shows that the distribution of foci offshore display two preferential areas along the CF–VF within its southern crustal block. Considering the basement rock types and the deep architectural disposition of the margin crust, two possible explanations for the origin of the clusters are put forward in this contribution.

Interpretation of buried basement in the southwestern Athabasca Basin, Canada, from integrated geophysical and geological datasets

Recent discoveries of basement-hosted uranium deposits in the Patterson Lake corridor in the southwestern Athabasca Basin of Canada have brought vigorous exploration interest to the region. New lithostratigraphic constraints, geochronology and airborne geophysical surveys have dramatically improved the understanding of the host basement geology, warranting a re-examination of the remote predictive mapping and geophysical responses of the buried basement rocks. This study took a two-step approach to examine the regional basement geology and architecture. First, a mosaic of the long-wavelength response of potential field (gravity and magnetic) datasets was examined to divide the basement into regional domains based on bulk physical property variations. The interpretive geological model was then refined using textural and lineament analysis of new airborne gravity and magnetic datasets, geological drill hole logs and magnetic susceptibility measurements. The new basement map identifies and updates major features including a crustal-scale structure that separates the southern Tantato Domain from the newly defined eastern Taltson Domain. This structure may have played a role in localizing fluid flow in the Patterson Lake corridor, defining the spatial extents of structurally controlled buried felsic intrusions, and redefines the boundaries of the Taltson, Clearwater and Tantato Domains. In addition, the potential field enhancements delineated significant regional faults that controlled the geometry of Paleoproterozoic cover sequences and have implications for understanding the crustal architecture of the southern Rae Province. These new interpretations shed light on the tectonic history of the region to support on-going exploration activities and delineate regionally prospective areas in this understudied area of the Canadian Shield. Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

Neotectonic evidences associated with Achankovil shear zone using morphometric analysis and field investigations

Several studies have considered Achankovil shear zone as NW–SE trending Precambrian crustal scale structure. Two major faults, namely Thenmala and Thenmala south faults, are identified associated with this shear zone. The present study identified NW–SE trending segmented lineaments in these zones. Major drainages and rock units surrounding these faults show NW–SE trend. The present study applied both conventional and recent geomorphic parameters to identify anomalies related to these structures. Morphometric results suggest that the area between Thenmala fault and Thenmala south fault, especially the central part, exhibits anomalies supporting active tectonism. Field investigation carried out in this zone identified continuities of NW–SE trending faults as brittle deformation in the southeastern part of the study area. Faults observed in the well sections along these lineaments show multiple slip planes and gouge formations. Morphometric results and field evidences suggest reactivation of these NW–SE structures associated with the present stress regime of peninsular India. Moreover, several instances of historic and recent earthquakes reported from this area. Considering the general trend of the seismic source zone reported in the peninsular India the NW–SE trending faults can generate a magnitude > 5.0. In view of this peak ground acceleration is calculated for a magnitude of 5.5 as the maximum credible earthquake that can be generated by these two faults. Frequent earthquakes within short duration around these faults indicate that the above-mentioned structures may be tectonically active and may generate a ground acceleration between 2.6 and 2.8 in the nearby cities.

Zircon oxygen and hafnium isotope decoupling during regional metamorphism: implications for the generation of low δ18O magmas

Measurements of U–Th–Pb, Lu–Hf and oxygen isotopes as well as selected trace and rare earth elements were carried out on zircon grains from high-grade metasedimentary rocks from the Albany–Fraser Orogen in southwest Australia. Oxygen isotopes from detrital zircon domains yield δ18O (VSMOW) values ranging from 5.8 to 8.0‰ and exhibit coupled Hf- and O-isotope compositions. In contrast, metamorphic zircon domains show considerably less variability in O-isotope compositions with a median δ18O of 5.6 ± 0.5‰ and exhibit decoupled Hf- and O-isotope compositions. 176Hf/177Hf isotope ratios of metamorphic zircon grains are within the evolutionary trend defined by the detrital grains implying negligible input from external sources during their growth. In addition, the relatively low δ18O value of the metamorphic zircons implies crystallisation from a relatively 18O depleted crustal melt. Such δ18O values are lower than those typically observed in most metamorphic zircon, including those preserved in partial melts of metasediments (10–12‰) and suggest that the sedimentary protolith was hydrothermally altered by meteoric fluids prior to high-temperature metamorphism. We suggest that alteration of the sedimentary protolith is related to the proximity to the Fraser Shear Zone, a major crustal scale structure within the orogen, which may represent an old reactivated structure. Occurrences of low δ18O metamorphic zircon, and potentially also low δ18O igneous rocks, in ancient collisional settings may, therefore, delineate long-lived fluid pathways within the crust.

Diagenetic to incipient metamorphic zones of the Benton Uplift, Ouachita Orogen, Arkansas, USA

AbstractVitrinite reflectance analysis and the mean diameter of metamorphosed quartz, suggest that the eastern Ouachita region, Arkansas, USA reached the lower greenschist facies, regardless of burial depth. An increase in “crystallinity” may explain that the maturation is from the exhumation of the rocks from a greater depth. Shale and sandstone samples collected from outcrops along the crest of the Benton Uplift exhibit illite and chlorite crystallinities that yield higher indices compared to younger rocks adjacent to this crustal-scale anticlinal structure. Illite crystallinity d(001) of air-dried, fine (<0.2 μm) authigenic clays (Δ°2θ (CuKα) = 0.417° to 0.875°) yield a range of conditions from diagenetic temperature to lower anchizone temperatures up to ∼200 °C. Air-dried, fine authigenic chlorite clays (Δ°2θ (CuKα) = 0.259° to 1.570°) yield a wide range of diagenetic to metamorphic conditions that span the diagenetic zone through epizone, which indicate a maximum temperature slightly above ∼300 °C. These results are in contrast to prior thermal maturation data. No additional heat sources, such as from pluton emplacement, were identified by major areas of anomalous high maturation based on crystallinity. Regionally, the illite and chlorite crystallinity increases toward the central axis of the Benton Uplift. Exhumation of the rocks from greater depth is all that is required to explain the illite and chlorite crystallinity data.

Origin of the Piché Structural Complex and implications for the early evolution of the Archean crustal-scale Cadillac – Larder Lake Fault Zone, Canada

The Piché Structural Complex (PSC) extends over 150 km within the Cadillac – Larder Lake Fault Zone (CLLFZ), a gold-endowed, east-trending, and high-strain corridor located along the southern edge of the Archean Abitibi Subprovince. The PSC consists of discontinuous units of volcanic rocks (<1 km thick) that host multiple gold deposits. It is spatially associated with molasse-type Timiskaming sedimentary basins. This study describes and interprets the origin of structures and lithologies within the poorly understood PSC to unravel the tectonic evolution of the CLLFZ. Field mapping, chemical analyses, as well as interpretations of cross-sections from drill-hole data, were used to interpret the geometry and structure of the PSC. The PSC is subdivided into six homogeneous fault-bounded segments or slivers. These slivers consist mostly of ultramafic to intermediate volcanic rocks and include some felsic volcanic flows and intrusions. Volcanic facies, chemical compositions, and isotopic ages confirm that these slivers are derived from the early volcanic units of the southern Abitibi greenstone belt, which are located north of the CLLFZ. Cross-cutting relationships between volcanic rocks of the PSC and the Timiskaming-aged intrusions suggest that the slivers were inserted into the CLLFZ during the early stages of the accretion-related deformation (<2686 Ma) and prior to Timiskaming sedimentation and ductile deformation (>2676 Ma). The abundant ultramafic rocks located within the CLLFZ may have focused strain, thereby facilitating the nucleation of the fault as well as the displacements along this crustal-scale structure.

Evidence for Paleoproterozoic anatexis and crustal reworking of Archean crust in the São Francisco Craton, Brazil: A dating and isotopic study of the Kinawa migmatite

New SHRIMP U-Pb zircon dating indicates that, contrary to previous thought, part of the Archean crust in the southern São Francisco Craton (SFC), NW of the Jeceaba Bom Sucesso Lineament was affected by the Paleoproterozoic Transamazonian orogeny. The Kinawa migmatite is a reworked part of the Archean crust in the southern SFC, and occurs within the Cláudio Shear Zone, a NE-NW crustal-scale structure. The protolith of the migmatite is a metagranodiorite that has an U-Pb zircon crystallization age of 2704±3Ma coeval with the voluminous potassic magmatism in the region. Moreover, the εHf(t=2704) data suggests a slightly evolved crust (−1.5 to −6.1) and thus a relatively long crustal residence (∼700Ma). Surprisingly, the migmatite itself yielded much younger ages of 2034±32Ma and 2048±25Ma, which coincide with the ages of the Mineiro Belt and Mantiqueira Complex. The distribution of ages using data from the literature and this study show that within the Archean domain the occurrence of Paleoproterozoic ages is restricted to the Cláudio Shear Zone. Inherited zircon crystals from all samples yield Neoarchean ages (∼2.7Ga), and similar Hf signatures to the metagranodiorite protolith (εHf(t=2704)=−5.5 to −0.5). The εHf(t) signatures of the Kinawa migmatite are heterogeneous and record two events, one is interpreted to be the formation of the protolith with lower 176Hf/177Hf (at ∼2.7Ga) and the other with higher 176Hf/177Hf (at ∼2.05Ga) as the anatexis that produced the migmatite. An important result is that the migmatite had two sources. Two samples are consistent with derivation from an evolved crust comparable with the metagranodiorite protolith (e.g. 176Lu/177Hf ∼0.01 and εHf(t=2034) −13.4 to −17), but the other sample has much less negative values (εHf(t=2048)=−4.6 to −8.7), which requires a different source with much lower 176Lu/177Hf, and therefore younger, possibly Post-Archean material.

Planned Products of the Mars Structure Service for the InSight Mission to Mars

The InSight lander will deliver geophysical instruments to Mars in 2018, including seismometers installed directly on the surface (Seismic Experiment for Interior Structure, SEIS). Routine operations will be split into two services, the Mars Structure Service (MSS) and Marsquake Service (MQS), which will be responsible, respectively, for defining the structure models and seismicity catalogs from the mission. The MSS will deliver a series of products before the landing, during the operations, and finally to the Planetary Data System (PDS) archive. Prior to the mission, we assembled a suite of a priori models of Mars, based on estimates of bulk composition and thermal profiles. Initial models during the mission will rely on modeling surface waves and impact-generated body waves independent of prior knowledge of structure. Later modeling will include simultaneous inversion of seismic observations for source and structural parameters. We use Bayesian inversion techniques to obtain robust probability distribution functions of interior structure parameters. Shallow structure will be characterized using the hammering of the heatflow probe mole, as well as measurements of surface wave ellipticity. Crustal scale structure will be constrained by measurements of receiver function and broadband Rayleigh wave ellipticity measurements. Core interacting body wave phases should be observable above modeled martian noise levels, allowing us to constrain deep structure. Normal modes of Mars should also be observable and can be used to estimate the globally averaged 1D structure, while combination with results from the InSight radio science mission and orbital observations will allow for constraint of deeper structure.

Age, tectonic evolution and origin of the Aswa Shear Zone in Uganda: Activation of an oblique ramp during convergence in the East African Orogen

The Aswa Shear Zone (ASZ) is a major NW–SE trending structure of over 1000 km length in East Africa. In Uganda, the ASZ is a steeply NE-dipping, up to 11 km wide mylonitic shear zone that shows multiple stage brittle reactivation. On outcrop-scale, the fabric in the ASZ is characterized by a well-developed NW–SE striking and subvertical or steeply NE or SW dipping mylonitic foliation and a subhorizontal to moderately NW- or SE-plunging stretching lineation. Sinistral kinematics and fabric are very consistent along strike. The strain is heterogeneously distributed and partitioned into lens-shaped lower strain zones dominated by folding and characterized by pure shear, which are surrounded by high strain zones, some of them thick ultramylonites, with intense simple shear combined with flattening and strong transposition of pre-existing fabrics. Ductile shearing occurred during bulk E–W shortening, commenced at amphibolite facies conditions and continued with similar kinematics at greenschist and even lower grade conditions. A number of (sub-)parallel shear zones occur to the NE and SW of the main zone at a distance of up to 20–45 km. They show similar fabrics and kinematics and are thus related to activity along ASZ reflecting strain partitioning into simple shear and pure shear domains on a regional scale.Samples of mylonitic gneisses from the shear zone have been analyzed with U–Pb LA-MC-ICPMS and show Neoarchaean crystallisation ages between 2.66 and 2.61 Ga. Timing of ductile sinistral shearing is poorly constrained by lower intercept ages of 686 ± 62 and 640 ± 44 Ma. The fabric and structural relationship of the ca. 660 Ma Adjumani Granite exposed in the northern segment of ASZ suggest that the age of shear activity can be further limited to ca. 685 and 655 Ma.The Aswa Shear Zone is interpreted as an intra-cratonic, crustal-scale structure close to the northeastern margin of the Congo Craton, possibly inherited from previous continental extension. Early Aswa Shear Zone activation is linked to underthrusting of the Congo Craton and coeval high-grade metamorphism and intense deformation in the orogen interior. During E–W convergence between ca. 690 and 650 Ma, the NE-dipping ASZ was activated as an oblique ramp leading to deflection of the transport direction and concentration of non-coaxial strain and sinistral shear along the shear zone system. During progressive convergence, between ca. 645 and 620 Ma, sinistral shearing along ASZ changed to ductile–brittle deformation mechanisms, while thrusting took place in Pan-African belts in eastern and western Uganda. Late-orogenic brittle sinistral reactivation of the ASZ can be regarded as the result of continent collision and closure of the Mozambique ocean further to the east, that potentially caused lateral escape manifested in NW–SE striking sinistral shear zones in Kenya and the southern Arabina-Nubian Shield between 620 and 570 Ma.

Crustal-scale tectonic wedging in the central Longmen Shan: Constraints on the uplift mechanism in the southeastern margin of the Tibetan Plateau

This study focuses on the upper–middle crust (UMC) deformation in the central Longmen Shan (LMS). The results of this study are constrained by the surface geology, typical seismic reflection profiles, previously available thermochronology, and deep geophysical data. Regional seismic profiles demonstrate a strata dip of about 2°, northwest trending, from the Sichuan Basin (SB) to LMS. The interpretation of shallow artificial seismic reflection data indicates that an unknown basement structure lead to the uplifting of the sedimentary cover by 3–4km. A long and wide-angle reflection seismic profile presents evidence that the middle crustal-scale structure is involved in the deformation. Geophysical data support that there is an upper detachment (D1) at the depth of ∼20km. The other lower detachment (D2) could be generated at approximately 30–40km depth in the low velocity zone. The ductile middle crust between the D1 and D2, has shortening and forming a wedge tip beneath the transition zone of the LMS and the SB. The deformation of the LMS frontal monocline belt is related to this crustal-scale wedging. Two different tectonic stages are distinguished in the Cenozoic through the axial surface analysis and chronological data. During the first stage, the crustal-scale tectonic wedge was developed between the upper and lower detachment, resulting in the uplift of the UMC. During the second stage, the middle crust could hardly be extruding and uplifting. The brittle upper crust was rapidly uplifted and shortened by the shallow major thrusts, which were developed on the D1. The D1 and D2 controlled the uplifting and shortening in the southeastern margin of the Tibetan Plateau. The lower crust (LC) may be decoupled from the D2 and subducted due to the resistance by the stable craton underlying the SB. The structural model manifests the importance of multi-detachments and the superimposed deformation in the LMS thrust belt. However, we emphasize that this crustal-scale tectonic wedging only discovered in the central LMS. The tectonic wedge is an important uplifting pattern that should be not neglected.

Crustal-scale structure of South Tien Shan: implications for subduction polarity and Cenozoic reactivation

Abstract Based on new structural and petrological investigations, we present two crustal-scale cross-sections of the Kyrgyz South Tien Shan, and correlations of main faults and units between Kyrgyzstan and China. The overall structure corresponds to a doubly-vergent mountain belt. The Kyrgyz and Chinese areas exhibit identical structural and metamorphic histories. To the west, the Atbashi Range comprises high-pressure oceanic and continental units stacked by north-verging thrusts above a low metamorphic accretionary prism. High-pressure (HP) gneisses are bound to their south by a south-dipping detachment exhibiting mantle relicts. The high-pressure oceanic and continental units underwent similar pressure–temperature ( P – T ) paths with peak conditions of around 500 °C–20 kbar, followed by rapid exhumation. The overall south-dipping structure and kinematics indicate a south-dipping subduction of the Central Tien Shan Ocean at 320–310 Ma, ending with the docking of the Tarim block to the Kazakh continent. To the east, the Pobeda Massif shows a narrow push-up structure. A major north-vergent thrust exhumes deep-crustal-level granulites, constituting the highest summits, which were thrust towards the north onto low-grade Devonian–Carboniferous schists. The southern part of South Tien Shan is made up of a south-verging thrust stack that formed later during ongoing convergence, reactivated throughout post-30 Ma phases.

Crustal-scale structure and deformation of Lu-Zong ore district: Joint interpretation from integrated geophysical data

We have studied five intersecting integrated geophysical profiles for the 3D crustal structure of the Lu (Lujiang)-Zong (Zongyang) ore district to obtain a better understanding of the metallogenesis and provide in-depth information for deep mineral targeting. The profiles, totaling more than 300 km, have reflection seismics and magnetotelluric sounding. Regional gravity surveys were also integrated into this study. New discoveries were obtained regarding the upper crustal structure and deformation based on the integrated analysis of these data. The Lu-Zong ore district consists of four major crustal blocks. They are the Shaxi uplift, Qianshan-Kongcheng Depression (QKD) in the west, Lu-Zong volcanic basin, and the Along-River uplift in the east. The north–south crustal elements show the northward “step-type” uplift, juxtaposed by two steplike faults, the west–northwest/east–southeast-trending Tangjiayuan-Zhuanqiao Fault and the Lujiang-Huangguzha-Tongling Detachment (LHTD) Fault. The Lu-Zong volcanic basin presents a nonsymmetrical shape with four inward-dipping boundary faults. The northern and eastern boundary faults (BF2 and LHTD) are deep faults, which control the development and evolution of the Lu-Zong volcanic basin. There are three west–northwest/east–southeast-trending faults and six northeast–southwest-trending faults cutting over the ore district. From north to south, they are the LHTD Fault, Tangjiayuan-Zhuanqiao Fault, and Yijing-Taojiaxiang Fault (BF3); from west to east, the six faults are the Tan-Lu, Chuhe, Luohe-Quekou, Zongyang-Huangtun, Taojiawan-Shijiawan, and Changjiang Thrust Fault (CTF). The formation and evolution of the ore-district are mainly affected by the Yanshanian intracontinental orogeny, and they experience the Middle to Late Jurassic compression and subsequent Cretaceous extension, possibly due to the paleo-Pacific northwest-trending subduction. Our studies determined that the CTF is a thrust system in nature and the LHTD is a southwest-dipping detachment. Two Jurassic basins were found, surrounding the northeast and southeast of Lu-Zong volcanics, which may be due to the product of postcollision extension of Indosinian orogeny during the middle and early Late Triassic.

Geodynamic significance of preserved Carboniferous subduction complex in Atbashi range (South Tianshan, Kyrgyzstan) and inferences for crustal-scale structure of north Tarim-Tibet orogenic system

A subduction complex of carboniferous age is preserved in the South Tianshan belt of Kyrgyzstan. It is made of a LP-LT accretionnary prism comprising an obducted ophiolite thrusted by a HP complex made of a sedimentary channel including eclogites boudins and a continental unit. Its structure and metamorphic history are investigated to reconstruct the geodynamic evolution of the northern Tarim in the Upper Palaeozoic. This study gives insights into the crustal-scale structure of this mountain belt, currently intensely reactivated by the India-Asia collision. Eclogites boudins have a N-MORB type composition similar to the unmetamorphosed obducted ophiolite sequence. Evidence for eclogite facies in both acidic and mafic lithologiesand geological structure are in agreement with a deep subduction channel mainly comprised of metasediments. Eclogites boudins are investigated for a preliminary PT study. Prograde stage (I) begins in blue-schist/eclogite facies transition at 520 ± 30°C - 17 ± 1 kbar. Condit ions of peak metamorphism (II) in eclogite facies range from 550 ± 30°C - 18.5 ± 1 kbar to 540- 595°C - 21 kbar. Retrograde stage (III) is also in the eclogite facies conditions at 515 ± 30°C - 16.7 ± 1 kbar. The geological structure and metamorphic conditions of this suture zone implies the subduction of a narrow oceanic basin in a south-dipping subduction zone, while another north-dipping subduction was active below Middle Tianshan. Final stacking of Middle and South Tianshan occurred at 320-310 Ma. These antithetic subduction zones are still reflected in the main structures of Tianshan. Reactivation of the south­ dipping structures since 30 Ma is ascribed to explain the current localisation of uplift and deformation.

Improving seismotectonics and seismic hazard assessment along the San Ramón Fault at the eastern border of Santiago city, Chile

The San Ramón Fault is an active west-vergent thrust fault system located along the eastern border of the city of Santiago, at the foot of the main Andes Cordillera. This is a kilometric crustal-scale structure recently recognized that represents a potential source for geological hazards. In this work, we provide new seismological evidences and strong ground-motion modeling from hypothetic kinematic rupture scenarios, to improve seismic hazard assessment in the Metropolitan area of Central Chile. Firstly, we focused on the study of crustal seismicity that we relate to brittle deformation associated with different seismogenic fringes in the main Andes in front of Santiago. We used a classical hypocentral location technique with an improved 1D crustal velocity model, to relocate crustal seismicity recorded between 2000 and 2011 by the National Seismological Service, University of Chile. This analysis includes waveform modeling of seismic events from local broadband stations deployed in the main Andean range, such as San José de Maipo, El Yeso, Las Melosas and Farellones. We selected events located near the stations, whose hypocenters were localized under the recording sites, with angles of incidence at the receiver <5° and S–P travel times <2 s. Our results evidence that seismic activity clustered around 10 km depth under San José de Maipo and Farellones stations. Because of their identical waveforms, such events are interpreted like repeating earthquakes or multiplets and therefore providing first evidence for seismic tectonic activity consistent with the crustal-scale structural model proposed for the San Ramón Fault system in the area (Armijo et al. in Tectonics 29(2):TC2007, 2010). We also analyzed the ground-motion variability generated by an M w 6.9 earthquake rupture scenario by using a kinematic fractal k −2 composite source model. The main goal was to model broadband strong ground motion in the near-fault region and to analyze the variability of ground-motion parameters computed at various receivers. Several kinematic rupture scenarios were computed by changing physical source parameters. The study focused on statistical analysis of horizontal peak ground acceleration (PGAH) and ground velocity (PGVH). We compared the numerically predicted ground-motion parameters with empirical ground-motion predictive relationships from Kanno et al. (Bull Seismol Soc Am 96:879–897, 2006). In general, the synthetic PGAH and PGVH are in good agreement with the ones empirically predicted at various source distances. However, the mean PGAH at intermediate and large distances attenuates faster than the empirical mean curve. The largest mean values for both, PGAH and PGVH, were observed near the SW corner within the area of the fault plane projected to the surface, which coincides rather well with published hanging-wall effects suggesting that ground motions are amplified there.

Reply to comment on the article “Propagation of a lithospheric tear fault (STEP) through the western boundary of the Calabrian accretionary wedge offshore eastern Sicily (Southern Italy)” by Gallais et al., 2013 Tectonophysics

In his Comment, A. Argnani argues that our interpretation of the STEP fault (named STEP-1 fault, in the Comment) is poorly constrained by the seismic data used in Gallais et al. (2013), and he particularly disagrees with our identification north of the Alfeo Seamount. A. Argnani is convinced that the tear fault is expressed by a belt of surface deformation, close to the Malta Escarpment. However, none of the seismic images published by A. Argnani show a crustal scale structure that could be associated with the presence at depth of a lithospheric tear. In that paper we documented the presence of a sub-vertical crustal scale fault, located 50 km east of the Malta Escarpment, with a northward increasing vertical offset (Gallais et al., 2013). The track of this fault towards the north could be extended following the position the fault “F6”, mapped 20 km east of the Malta Escarpment by Nicolich et al. (2000). In fact these authors reported an offset of the crust associated with activity the fault “F6”, suggesting the lithospheric tear at depth at the edge of the Ionian slab is distinct from the Malta Escarpment.