Zones Of Lithospheric Weakness Research Articles

Asymmetric post-spreading magmatism in the South China Sea: based on the quantification of the volume and its spatiotemporal distribution of the seamounts

ABSTRACT The volume of seamounts is an essential indicator of the intensity of mantle convection during the evolution of the oceanic lithosphere. Drilling and dredging samples suggest volcanic seamounts are widely distributed in the oceanic basin of the South China Sea, and most of them were formed after cessation of seafloor spreading. By using an SRTM15_PLUS Digital Elevation Model with a 15-arc-second grid, we developed a spatial filtering method based on the Top Hat Transform to extract seamounts. With a combined analysis of basalt dating results from previous studies and gravity anomaly data, an accurate estimation of seamount volume and its spatiotemporal distribution have been obtained. In addition to an asymmetric distribution of the seamounts with a larger volume in the northern flank, clusters of seamounts can be observed at certain locations in the abyssal plain. Due to the consistency between the distribution of the seamount volume and the seafloor spreading features in the South China Sea basin, we propose the ridge jump may induce additional partial melting zones which account for the larger number of seamounts developed in the northern flank while the re-orientation of the extension during seafloor spreading dominated the distribution of post-spreading magmatism. Similar to other marginal basins, magmas formed by spontaneous partial melting would migrate through weak lithosphere where the extension direction changed, resulting in post-spreading magmatism in those lithospheric weak zones.

Understanding the Meaning of the Positive Bouguer Anomaly of Waza (Northernmost Cameroon, Central Africa)

Gravity and magnetic data were analyzed in conjunction with available geological data to determine the origin of observed positive gravity anomaly at Waza and its possible relationship to mafic intrusions. The data analysis includes the construction of a Bouguer gravity anomaly, isostatic residual gravity anomaly, total magnetic gradient and Euler solutions maps, and two and one-half dimensional gravity models. A positive gravity anomaly that occurs in Waza has amplitude of 35 mGal and can be separated into two parts. The top of the disturbing body causing the anomaly cannot be deeper than 4 km. We believe the anomaly is probably caused by a body of basaltic rocks lying at the upper surface of the Precambrian basement and originate from a self-propagating disturbance of magmatic loads on a pre-existing zone of lithospheric weakness. Melt migration might have been aided by tectonic and flexural stresses, such that the intrusion is limited in extent to the melting region of the plume and did not reach the surface. The model calculated to satisfy the observed anomaly consists of a 2.5 Km-thick slab underlain by a 10.5 km deep column. This model is interpreted to represent a sheet or saucer-like mafic intrusion that has a thick deep feeder. Detailed geophysical studies would be necessary to locate any possible economic occurrences.

Subduction and retreating of the western Pacific plate resulted in lithospheric mantle replacement and coupled basin-mountain respond in the North China Craton

The North China Craton (NCC) witnessed Mesozoic vigorous tectono-thermal activities and transition in the nature of deep lithosphere. These processes took place in three periods: (1) Late Paleozoic to Early Jurassic (~170 Ma); (2) Middle Jurassic to Early Cretaceous (160–140 Ma); (3) Early Cretaceous to Cenozoic (140 Ma to present). The last two stages saw the lithospheric mantle replacement and coupled basin-mountain response within the North China Craton due to subduction and retreating of the Paleo-Pacific plate, and is the emphasis in this paper. In the first period, the subduction and closure of the Paleo-Asian Ocean triggered the back-arc extension, syn-collisional compression and then post-collisional extension accompanied by ubiquitous magmatism along the northern margin of the NCC. Similar processes happened in the southern margin of the craton as the subduction of the Paleo-Tethys ocean and collision with the South China Block. These processes had caused the chemical modification and mechanical destruction of the cratonic margins. The margins could serve as conduits for the asthenosphere upwelling and had the priority for magmatism and deformation. The second period saw the closure of the Mongol-Okhotsk ocean and the shear deformation and magmatism induced by the drifting of the Paleo-Pacific slab. The former led to two pulse of N-S trending compression (Episodes A and B of the Yanshan Movement) and thus the pre-existing continental marginal basins were disintegrated into sporadically basin and range province by the Mesozoic magmatic plutons and NE-SW trending faults. With the anticlockwise rotation of the Paleo-Pacific moving direction, the subduction-related magmatism migrated into the inner part of the craton and the Tanlu fault became normal fault from a sinistral one. The NCC thus turned into a back-arc extension setting at the end of this period. In the third period, the refractory subcontinental lithospheric mantle (SCLM) was firstly remarkably eroded and thinned by the subduction-induced asthenospheric upwelling, especially those beneath the weak zones (i.e., cratonic margins and the lithospheric Tanlu fault zone). Then a slightly lithospheric thickening occurred when the upwelled asthenosphere got cool and transformed to be lithospheric mantle accreted (~125 Ma) beneath the thinned SCLM. Besides, the magmatism continuously moved southeastward and the extensional deformations preferentially developed in weak zones, which include the Early Cenozoic normal fault transformed from the Jurassic thrust in the Trans-North Orogenic Belt, the crustal detachment and the subsidence of Bohai basin caused by the continuous normal strike slip of the Tanlu fault, the Cenozoic graben basins originated from the fault depression in the Trans-North Orogenic Belt, the Bohai Basin and the Sulu Orogenic belt. With small block size, inner lithospheric weak zones and the surrounding subductions/collisions, the Mesozoic NCC was characterized by (1) lithospheric thinning and crustal detachment triggered by the subduction-induced asthenospheric upwelling. Local crustal contraction and orogenesis appeared in the Trans-North Orogenic Belt coupled with the crustal detachment; (2) then upwelled asthenosphere got cool to be newly-accreted lithospheric mantle and crustal grabens and basin subsidence happened, as a result of the subduction zone retreating. Therefore, the subduction and retreating of the western Pacific plate is the outside dynamics which resulted in mantle replacement and coupled basin-mountain respond within the North China Craton. We consider that the Mesozoic decratonization of the North China Craton, or the Yanshan Movement, is a comprehensive consequence of complex geological processes proceeding surrounding and within craton, involving both the deep lithospheric mantle and shallow continental crust.

Geodynamics of the East African Rift System ∼30 Ma ago: A stress field model

The East African Rift System (EARS) is thought to be an intra-continental ridge that meets the Red Sea and the Gulf of Aden at the Ethiopian Afar as the failed arm of the Afar triple junction. The geodynamics of EARS is still unclear even though several models have been proposed. One model proposes that the EARS developed in a local tensile stress field derived from far-field loads because of the pushing of oceanic ridges. Alternatively, some scientists suggest that the formation of the EARS can be explained by upwelling mantle plumes beneath the lithospheric weak zone (e.g., the Pan-African suture zone). In our study, a shell model is established to consider the Earth’s spherical curvature, the lithospheric heterogeneity of the African continent, and the coupling between the mantle plumes and the mid-ocean ridge. The results are calculated via the finite element method using ANSYS software and fit the geological evidence well.To discuss the effects of the different rock mechanical parameters and the boundary conditions, four comparative models are established with different parameters or boundary conditions. Model I ignores the heterogeneity of the African continent, Model II ignores mid-ocean spreading, Model III ignores the upwelling mantle plumes, and Model IV ignores both the heterogeneity of the African continent and the upwelling mantle plumes. Compared to these models is the original model that shows the best-fit results; this model indicates that the coupling of the upwelling mantle plumes and the mid-ocean ridge spreading causes the initial lithospheric breakup in Afar and East Africa. The extension direction and the separation of the EARS around the Tanzanian craton are attributed to the heterogeneity of the East African basement.

Topography of the Moho Undulation in Cameroon from Gravity Data: Preliminary Insights into the Origin, the Age and the Structure of the Crust and the Upper Mantle across Cameroon and Adjacent Areas

In this paper, the complete Bouguer anomaly data from Cameroon and part of the neighboring countries has been examined to compute the topography of the Moho undulations. This work is based on an improved filtering technique and an appropriate density contrast between the crust and the upper mantle. Comparison with seismic data shows that our Moho map defines better the continuity and morphology of the crust-mantle interface than the scattered seismic data in Cameroon. The present relief map, although may not give real depths at some areas provides a better surface correlation with the surface geology better than seismic techniques. Comparison between the Moho undulations and the topography maps reveals that the crust in Cameroon seems to not be in isostatic equilibrium. The Moho in the Central African Shear Zone (CASZ) must be linked with different dextral movements during the opening of the south Atlantic in the Cretaceous time. In the Chad basin, the Moho is associated to the opening of the central and south Atlantic ~130 Ma. In the case of the Congo basin, the Moho undulations are related to the post rift subsidence. The correlation between the Moho undulations map with the surface geology indicates that the actual morphology of the crust mantle interface in Cameroon can be related to the build-up of the West and Central African rift system dating back to the Early Cretaceous to Palaeogene, where the presence of intraplate tensional stresses reactivated previous shear zones of lithospheric weakness during the break up of Gondwana.

Initial gold enrichment within a Neoarchean granite-greenstone belt: Evidence from ore-bearing and ore-barren samples in the Jiapigou deposits, NE China

The Jiapigou gold mineralization hosted in a Neoarchean granite-greenstone belt in the southern Jilin Province, at the easternmost part of the northern margin of the North China Craton (NCC), is one of the important gold deposits in China. However, its evolution, in particular the initial Au source and enrichment, have not been well constrained. The initial gold enrichment defined by Au concentrations of Neoarchean unaltered gneisses of this region, range from <0.005ppm to 0.001ppm, increasing to 0.006–0.031ppm Au for the ore-hosting Neoarchean altered mylonite gneisses. A combined study of zircon trace element, UPb dating, Hf isotope compositions, as well as whole rock chemical compositions was performed for dioritic gneisses from both ore-bearing (close to the ore body) and ore-barren zones (within the nearby rock body) with a view to constrain the gold initial enrichment. The dioritic gneisses have varied SiO2 contents (54.19–61.34wt.%), Al2O3 (13.06–16.75wt.%) with moderate to high K2O/Na2O ratios (0.7–1.3) and MgO (2.20–3.77%). Based on their chemical compositions, positive zircon εHf(t) values, and their formation ages of 2512±9Ma, we infer that these dioritic rocks were generated by partial melting of a juvenile source, most likely thickened mafic lower continental crust (LCC) with residual plagioclase associated with minor amphibole. The ore-bearing sample (H5) from a ductile shear zone has been extensively altered by fluids and is the sample containing zircons with blurred zoning and reverse discordance. Zircons from H5 have much higher radiogenic Pb but lower U and Pb contents than those in the ore-barren sample (H3), which contains zircons with oscillatory zoning and represents the basement rock. Based on the available petrological, geochronological and geochemical constraints, we propose the following model for Au initial enrichment within the Neoarchean granite-greenstone belt. At ca. 2.54Ga, associated with the accretionary processes in the proto-NCC, subduction-related fluids infiltrated the metasomatised subcontinental lithospheric mantle (SCLM), scavenging Au and Pb (mainly radiogenic Pb, henceforth Pb*) and triggering partial melting. The resulting basaltic magmas underplated the LCC and were re-melted at 2512±9Ma, when the onset of an extensional regime permitted the advection of heat and melts/fluids into the LCC. This generated voluminous dioritic magmas with juvenile Hf isotope signatures with little evidence of interaction with the mantle. Subsequent ca. 2505Ma re-melting of some granodioritic rocks led to the formation of potassic granite and residual fluids further enriched in Au and Pb*, both focused into ductile shear zones and other lithospheric weak zones. The fluids with anomalous UPb fractionation resulted in the rarely reported zircon reverse discordance, alteration and also enrichment in Au, as observed in sample H5 and H4, representing the initial stage of Au enrichment within the Neoarchean granite-greenstone belt.

Plume‐orogenic lithosphere interaction recorded in the Haladala layered intrusion in the Southwest Tianshan Orogen, NW China

AbstractThe plume‐orogenic lithosphere interaction may be common and important for the generation of large igneous provinces. The information regarding such a process is recorded by the Haladala gabbroic intrusion (~300 Ma), the largest layered ultramafic‐mafic intrusion hosting V‐Ti magnetite deposits in the Southwest Tianshan Orogen, NW China. The Haladala gabbros exhibit unfractionated chondrite‐normalized rare earth element patterns with negative Nb and Ta anomalies and positive Pb anomaly on the primitive mantle‐normalized multielement variation diagram. They are characterized by low initial Sr isotopes, slightly decoupled but high positive bulk rock εNd(t) and εHf(t), and high 207Pb/204Pb and 208Pb/204Pb relative to 206Pb/204Pb, delineating a DUPAL signature in the sources. The Haladala gabbros cannot be arc or postcollisional magmatism, given the lack of hydrous minerals and low K contents, respectively. This is further supported by the relatively low oxygen fugacity required for the gradual enrichment of V‐Ti magnetite during the magma fractionation and by an overall anhydrous mantle source suggested by troctolite mineral assemblage (olivine + plagioclase). The emplacement age of the Haladala gabbros is identical to that of the Wajilitag kimberlites in the Tarim's interior, which have been interpreted as the first magmatic expression of the Tarim mantle plume. We thus propose that the Haladala gabbroic intrusion was generated in a hybrid geodynamic setting in which the Southwest Tianshan Orogen was impacted by an upwelling mantle plume. In this sense, the Haladala layered gabbroic intrusion records the early phase of magmatism of the Tarim plume, which was preferentially emplaced in a lithospheric weak zone.

Evidence for a long‐lived accommodation/transfer zone beneath the Snake River Plain: A possible influence on Neogene magmatism?

AbstractGeochronologic data compiled from 12 metamorphic core complexes and their flanking regions outline important differences in tectonic and magmatic histories north and south of the Snake River Plain‐Yellowstone Province (SRP‐Y). Magmatism, crustal flow, metamorphism, and extensional exhumation of core complexes north of the SRP occurred mostly between 55 and 42 Ma as compared to 42–25 Ma south of the SRP, with final exhumation of the southern complexes occurring only during younger Miocene (20–0 Ma) Basin and Range faulting. These significant differences in the timing of events suggest that the now lava‐covered SRP, which is at a high angle to Cordilleran trends, may have at times operated as a steep shear or transfer zone accommodating difference in strain to the north and south. Following previous suggestions, we infer that this proposed accommodation or transfer zone developed above an important lithospheric boundary localized above a tear in the subducting slab (shallower slab angle to the south) used to explain both the locus of Late Cretaceous‐Paleocene magmatism and the different ages and mechanisms of slab reconfiguration and removal north and south of the SRP during the Cenozoic. The details of these different histories help outline the complex evolution of this zone and also suggest that this zone of lithospheric weakness may have subsequently focused Miocene SRP‐Y hot spot magmatism.

Plate tectonics on the Earth triggered by plume-induced subduction initiation.

Scientific theories of how subduction and plate tectonics began on Earth--and what the tectonic structure of Earth was before this--remain enigmatic and contentious. Understanding viable scenarios for the onset of subduction and plate tectonics is hampered by the fact that subduction initiation processes must have been markedly different before the onset of global plate tectonics because most present-day subduction initiation mechanisms require acting plate forces and existing zones of lithospheric weakness, which are both consequences of plate tectonics. However, plume-induced subduction initiation could have started the first subduction zone without the help of plate tectonics. Here, we test this mechanism using high-resolution three-dimensional numerical thermomechanical modelling. We demonstrate that three key physical factors combine to trigger self-sustained subduction: (1) a strong, negatively buoyant oceanic lithosphere; (2) focused magmatic weakening and thinning of lithosphere above the plume; and (3) lubrication of the slab interface by hydrated crust. We also show that plume-induced subduction could only have been feasible in the hotter early Earth for old oceanic plates. In contrast, younger plates favoured episodic lithospheric drips rather than self-sustained subduction and global plate tectonics.

Geon 12 crustal extension in the central Grenville Province, implications for the orogenic architecture, and potential influence on the emplacement of anorthosites

Remnants of a ca. 1.24 Ga old volcanic belt formed in a within-plate setting (Manicouagan Crustal Extension Belt) are exposed over several tens of kilometres in the hinterland of the central Grenville Province, and can be recognized despite granulite-facies Grenvillian metamorphism and deformation. This belt mainly consists of layered felsic–mafic rock units, some of which are documented here for the first time. In the vicinity of the 1.24 Ga belt, a Geon 14 arc-related mafic suite and a Geon 15 metasedimentary package were pervasively injected by felsic (and mafic) material, show evidence of a Geon 12 thermal event, and therefore may represent remnants of rifted crust. Following Geon 12 crustal extension, lithospheric-scale magmatic activity in the immediate region continued intermittently, producing mafic dykes at 1.17 Ga and anorthosite at 1.16 and 1.05 Ga. A close association to 1.16 Ga or younger anorthosite is conspicuous to several Geon 12 rock units known in the central and eastern Grenville Province. We therefore suggest that subsequent anorthositic magmatism was focused on zones of lithospheric weakness inherited from Geon 12 crustal extension and remained active intermittently until the end of the Grenvillian orogeny.

Seamounts off the West Antarctic margin: A case for non-hotspot driven intraplate volcanism

New radiometric age and geochemical data of volcanic rocks from the guyot-type Marie Byrd Seamounts (MBS) and the De Gerlache Seamounts and Peter I Island (Amundsen Sea) are presented. 40Ar/39Ar ages of the shield phase of three MBS are Early Cenozoic (65 to 56Ma) and indicate formation well after creation of the Pacific–Antarctic Ridge. A Pliocene age (3.0Ma) documents a younger phase of volcanism at one MBS and a Pleistocene age (1.8Ma) for the submarine base of Peter I Island. Together with published data, the new age data imply that Cenozoic intraplate magmatism occurred at distinct time intervals in spatially confined areas of the Amundsen Sea, excluding an origin through a fixed mantle plume. Peter I Island appears strongly influenced by an EMII type mantle component that may reflect shallow mantle recycling of a continental raft during the final breakup of Gondwana. By contrast the Sr–Nd–Pb–Hf isotopic compositions of the MBS display a strong affinity to a HIMU-type mantle source. On a regional scale the isotopic signatures overlap with those from volcanics related to the West Antarctic Rift System, and Cretaceous intraplate volcanics in and off New Zealand. We propose reactivation of the HIMU material, initially accreted to the base of continental lithosphere during the pre-rifting stage of Marie Byrd Land/Zealandia to explain intraplate volcanism in the Amundsen Sea in the absence of a long-lived hotspot. We propose continental insulation flow as the most plausible mechanism to transfer the sub-continental accreted plume material into the shallow oceanic mantle. Crustal extension at the southern boundary of the Bellingshausen Plate from about 74 to 62Ma may have triggered adiabatic rise of the HIMU material from the base of Marie Byrd Land to form the MBS. The De Gerlache Seamounts are most likely related to a preserved zone of lithospheric weakness underneath the De Gerlache Gravity Anomaly.

State of stress and age offsets at oceanic fracture zones and implications for the initiation of subduction

The recycling of oceanic lithosphere back into the Earth's interior through subduction is a central component of plate tectonics. The process by which new subduction zones initiate, however, remains poorly understood. Several different mechanisms for subduction initiation have been proposed, including passive margin collapse (aided by sediment loading and/or rheological weakening due to the presence of volatiles) and forced convergence across a zone of preexisting lithospheric weakness. In this paper we focus on the latter type of model, which identifies three conditions necessary for subduction initiation: a zone of weakness such as a fracture zone, an age (and therefore density) offset along the fracture zone, and significant normal compressive stress which leads to shortening. We identify regions on the present-day Earth which meet these conditions and which may correspond to regions of relatively likely subduction initiation in the near future. Using a digital seafloor age model, we have created a database of oceanic fractures and quantified the associated age offsets. We have evaluated the state of stress on these lithospheric weak zones using two different global stress models. We find that the conditions needed to initiate subduction via the forced convergence model are relatively rare on the present-day Earth, and that there is little indication of incipient subduction at regions identified as relatively likely for subduction initiation. Using the same technique, we have evaluated the state of stress and seafloor age offset at regions of inferred present-day incipient subduction, and find that most of these regions are not associated with both high far-field compressive stresses and large age (and thus density) offsets. Subduction has likely initiated via forced convergence across preexisting zones of lithospheric weakness in the past, but our results indicate that the conditions needed for this type of subduction initiation are rare on the present-day Earth.

Deep resistivity cross section of the intraplate Atlas Mountains (NW Africa): New evidence of anomalous mantle and related Quaternary volcanism

The Atlas Mountains are characterized by high elevations and Quaternary volcanism. Long period magnetotelluric data acquired along a NNW‐SSE transect reveal the presence of a conductive anomalous mantle below the High Atlas. Data dimensionality analyses show a preferent N80°E strike of the deep resistivity structure in agreement with the induction vector alignment at long periods. Accordingly, a 2D inversion of the data set was carried out. Large resistive bodies at the crustal basement most likely correspond to batholiths emplaced in more conductive metapelites. They are covered by outcropping conductive sedimentary detritic and carbonate rocks. Lithospheric thinning producing anomalous mantle and basin development in the Atlas probably started during Triassic‐Jurassic rifting. Inversion tectonics since the Oligocene produced low shortening on previous lithospheric weak zones, with thrusting of the Atlas above the stable African plate. Melting at the top of the anomalous mantle is connected with Quaternary basaltic volcanism in the Middle Atlas.

Pre-Andean deformation of the Precordillera southern sector, southern Central Andes

This paper presents a detailed investigation of the structure and evolution of the Precordillera southern sector (Argentina). We document the development and successive reactivation of regional and discrete structural grain through time, and discuss the existence of a large-scale mechanical anisotropy present in the lithosphere. Our kinematic studies indicate that the Permian orogeny generated a doubly vergent fold-and-thrust belt of transpressive deformation, where strain was partitioned into two different types of deformation domains. The west-vergent western domain was characterized by partitioned transpression with shortening dominating, and a strike-slip–dominated subdomain. The east-vergent eastern domain was characterized by pure contractional deformation. Our model for the Late Permian to Early Triassic evolution of the Precordillera involves a north-northwest–trending weakness zone affected by north-northeast–directed extension, generating an area with transtensional deformation during the Choiyoi volcanism development. Later, during the Triassic generation of the Cuyana rift basin, the northeast stretching direction was orthogonal to the rift trend, indicating pure extensional deformation. We propose a model where the clear parallelism between the distribution of an inferred early Paleozoic suture zone, a north-northwest–trending late Paleozoic belt, and Permian–Triassic rift-related magmatism indicates the reactivation of a north-northwest–trending long-lived lithospheric weakness zone.

Kinematic analysis of a transtensional fault system: The Atuel depocenter of the Neuquén basin, southern Central Andes, Argentina

Abstract The Atuel depocenter of the Neuquen basin originated as an Upper Triassic to Lower Jurassic rift system, later inverted during the Andean contractional deformation. In order to study the extensional architecture and the kinematic evolution of this depocenter, we collected a large amount of field and sub-surface data, consisting of slip data from outcrop-scale normal faults, thickness and facies distribution within the synrift deposits, and structural data from angular and progressive unconformities. The Atuel depocenter has a NNW trend, showing a bimodal distribution of NNW and WNW major faults (first and second order faults). On the other hand, from kinematic indicators measured on outcrop-scale faults (third and fourth order faults), we found a mean NE internal extension direction, which is oblique to the general trend of the sub-basin. Taking these particular characteristics into account, we interpreted the Atuel depocenter as an oblique rift system. We evaluated two mechanisms in order to explain the development of this transtensional system: 1) reactivation of upper-crustal NNW-oriented Paleozoic shear zones, and 2) oblique stretching of a previous NNW-oriented lithospheric weakness zone.

Martian rifts: Structural geology and geophysics

Large extensional tectonic surface structures on Mars are investigated on the basis of image and topographic data. These features have lengths of up to 1400 km, typical widths of tens of kilometers, and depths of up to 3000 m. Their structural architecture is characterized by segmentation with changing style, the development of asymmetric basins with changing polarity, and accommodation zones between such basins. Most structures are associated with varying degrees of volcanism. Based on these observations and a comparison to terrestrial continental rifts, we interpret these series of basins or grabens as rift systems. Crater counts of rift-related geologic surface units indicate that rifts on Mars are ∼ 4.0 to 3.5 Ga old. Modelling of rift flank uplift yields heat flows ranging between 28 and 66 mW m − 2 , and a corresponding thickness of the elastic lithosphere in the order of ∼ 10 km at the time of rifting. The radial orientation of some rifts with respect to Tharsis is consistent with a rifting mechanism related to regional extensional stresses in the lithosphere. Rifts with other orientations might have formed in response to stress fields induced by horizontal gradients of the gravitational potential energy, modulated by magmatic zones of lithospheric weakness.

Lithospheric velocity structure of the New Madrid Seismic Zone: A joint teleseismic and local P tomographic study

The enigmatic seismicity in the New Madrid Seismic Zone (NMSZ) has been attributed to some abnormal lithospheric structure, including the presence of dense mafic intrusions and a low‐viscosity lower crust. However, the area's detailed lithospheric structure remains unclear. Here we invert 2,056 teleseismic P and 12,226 local P first arrival times from a recent nine‐year dataset to infer the lithospheric velocity structure beneath the NMSZ. Our results show that the seismically active zone is associated with a local, NE–SW trending low‐velocity anomaly in the lower crust and upper mantle, instead of high‐velocity intrusive bodies proposed in previous studies. The low‐velocity anomaly is on the edge of a high‐velocity lithospheric block, consistent with the notion of stress concentration near rheological boundaries. This lithospheric weak zone may shift stress to the upper crust when loaded, thus leading to repeated shallow earthquakes.

Permo-Triassic oblique extension in the Potrerillos-Uspallata area, western Argentina

The Permo-Triassic evolution of southwestern South America was characterized by the development of a great amount of volcanism under extensional conditions. Structural analyses of faults developed contemporaneously with this volcanism in the key area of Potrerillos-Uspallata suggest the existence of an oblique extensional setting controlled by the presence of a pre-existing lithospheric anisotropy. A clear parallelism between the trace of an inferred Devonian suture zone, the Late Paleozoic San Rafael orogenic belt and the Permo-Triassic rifting suggests that Early and Late Paleozoic tectonic inheritance permitted the reactivation of a NNW-trending zone of lithospheric weakness. The reactivation of this pre-existing weak zone during Late Permian to Early Triassic times has resulted in the generation of a new complex fault system, which concentrated the oblique-slip normal displacement related to a NNE–SSW stretching (N23°E).

Magmatism in the Huarina belt, Bolivia, and its geotectonic implications

The Huarina belt is a tectono-stratigraphic unit extending along the western side of the Eastern Cordillera and southern Altiplano. It includes the well-known Bolivian tin belt. Both geophysical and isotopic evidence suggest that the Huarina belt is located between three cratonic blocks in a major lithospheric weakness zone that has probably been inherited from the Proterozoic. Most of the Phanerozoic backarc magmatism in the Bolivian Andes occurred along this belt. Local Ordovician basalts and andesites, and clasts of Devonian granitoids occurring in Late Devonian–Early Carboniferous sedimentary units, are testimonies of a scarce Palaeozoic igneous activity. During the Mesozoic, rift-related magmatism resulted in peraluminous to metaluminous, high-K calcalkaline Triassic granitoids, as well as discrete Cretaceous centers of alkaline and carbonatitic rocks. In the Cenozoic, huge high-K, generally peraluminous ignimbrite sheets, along with numerous volcanic and intrusive centers, were generated along the belt. Shoshonitic, alkaline and ultrapotassic rocks have been locally described. On the basis of both geochemical and geophysical evidence, a general evolution model is proposed for the Huarina belt. During Paleozoic and Mesozoic times, the predominant extensional regime of the continental crust led to lithospheric thinning, basin generation, and production of within plate magmas. During the Cenozoic, prevailing compressional conditions led to a gradual thickening of the lithosphere, inducing delamination of its denser and less viscous root. The mantle-generated magmas interplayed widely with crustal melts, giving rise to the dominantly peraluminous magmas.

Constraints from finite element modeling on the active tectonics of northern Central America and the Middle America Trench

We have developed an elastic finite element model in order to study the role of the different forces acting on the northwestern part of the Central American Volcanic Arc and the Chortis Block. We present synthetic focal mechanisms, maps of tectonic regime, and strain crosses to analyze the results. The models show that to achieve the observed state of stress on the volcanic arc, the arc must be modeled as a lithospheric weak zone. Also, the forces related to the eastward drift of the Caribbean plate must be higher than those related to the subduction of the Cocos plate. The coupling on the subduction interface must be low, with or without slip‐partitioning due to the obliquity of the subduction at the trench. At Guatemala the western edge of the Chortis block is pinned against North America, even with low trench‐normal forces, making the triple junction between the Cocos, North American, and Caribbean plates a zone of diffuse deformation. The extension in the western part of the Chortis block, from Guatemala to the Honduras depression, is explained by the geometry of the North American‐Caribbean plate boundary and the direction of motion of the Caribbean plate with respect to North America. The direction of extension in the Chortis block is always E‐W regardless of the magnitude of the applied forces, and the main part of the deformation is absorbed between the Ipala graben and the Honduras depression, both features being consistent with our models.