Deformation Partitioning Research Articles

A numerical study of contemporary crustal deformation partitioning across the Southwestern Tian Shan orogen

The Tian Shan region is a typical example of crustal deformation within an intracontinental environment, where the lithospheric rheological properties are marked by significant spatial heterogeneity. However, the role of lithospheric rheology in crustal strain partitioning within the interior of the Tian Shan orogenic belt remains nebulous. Here, we utilized a two-dimensional viscoplastic model with contact elements constrained by GPS velocities and fault slip rates to investigate the influence of the block's strength on crustal deformation in the southwestern Tian Shan region. Our founding suggests that a weaker lower crust beneath the northern Tian Shan region offers a potential mechanism for the contemporary deformation patterns. Contemporary crustal deformation at Tian Shan is mainly concentrated along the piedmont thrust-and-fold belts and diffuses in the orogen's interior. This pattern of crustal strain partitioning is closely linked to the interplay between fault slips and lithospheric deformation. This finding emphasizes the significant role that the lithospheric rheology and pre-existing faults played in the deformation partitioning in the interior of the Tian Shan orogeny.

Interaction of accretion mechanisms and deep fluids in continental orogenesis

Mountain-building often involves the subduction of continental margins during the convergence of tectonic plates. Rheological heterogeneities and structural inheritance in the subducting lithosphere are main factors controlling the evolution of the process and deformation partitioning in orogenic wedges. Here, we present tomographic evidence of a main along-strike change in the structure of the Apennines belt, where frontal accretion diminishes laterally, as well as plate bending, and the contribution of underplating to crustal thickening becomes evident. We hypothesize that the diverse mechanisms of mountain formation can be attributed to rheological heterogeneity in the crust and interactions with fluids liberated during the subduction process. The development of shallow decollements versus underplating is favored by differing amounts and styles of deep fluid liberation from the subducting crust.

Geomorphic signatures of neotectonic activity along the Western Andean Front in the Chilean Andes (35°S to 37°S)

AbstractCharacterizing neotectonic activity along mountain fronts in active orogens is the key to better understanding trajectories of landscape evolution, climatic‐tectonic couplings and natural hazards. One such prominent mountain front is the Western Andean Front (WAF), which defines the boundary between two main physiographic units along the Chilean Andes (15°S to 39°S): the Central Depression and Principal Cordillera. This study analyses the geomorphic imprints of neotectonic fault activity in the WAF between 35°S and 37°S. We used several topographic metrics, including the mountain‐front sinuosity index (Smf), the valley floor width‐to‐height ratio (Vf) and the stream length‐gradient (SL) index to gain information about the regional distribution of deformation patterns and its relative tectonic activity for distinctive segments. In addition, a raster map combining rock strength (RS) and mean annual precipitation (MAP) allowed discriminating lithology‐climate vs. tectonics influence on SL anomalies from catchments spanning from the main deformation zone to the regional drainage divide. We use the local relief and swath profiles to quantify the incision and transient state of mountain fronts to evaluate the geomorphic response to tectonic and/or climatic input. Our analysis highlights significant variations from north to south, delineating two distinct segments with different topographic, geomorphic and geometric fault characteristics. These segments are indented at approximately 36°S. From north to south, there is a segment with an inactive thrust front with a primarily climate‐driven fluvial response over long timescales and a second segment with a transient state adjusting to relatively high uplift rates. Morphometric data analysis, DEM‐based morpho‐structural maps, geological maps and previous research support the interpretation of two sets of neotectonic faults: (1) NNE‐striking reverse faults with a dextral slip component and (2) NW‐striking sinistral slip faults, likely a response to deformation partitioning caused by oblique subduction on a continental margin.

Mesozoic structural evolution of the Northern South China Sea margin using potential field modelling

This study conducts a comprehensive geophysical inspection of the Northern South China Sea Margin (NSCSM), a complex and pivotal area for unraveling the tectonic history of Southeast China. We employ structural interpretation of the gravity and magnetic data by applying processing techniques and 2D forward modelling to delineate and subdivide the primary subsurface structures, and also to identify the sedimentary basins. Our findings reveal that the NSCSM exhibits a succession of extensional forearc basins above a heterogeneous basement, including a Mesozoic Andean-type magmatic arc and a Western Pacific-type marginal oceanic basin. It has substantial variations in crustal densities and susceptibilities across the NSCSM, reflecting its intricate geological history. The integrated interpretation of this study highlights the substantial impact of the Indosinian and Yanshanian thrusting and orogenies. These geological processes have played a significant role in deformation partitioning, leading to the development of elongated troughs and basins intersected by numerous prominent structures trending in the NE, ENE, and EW directions. The Indosinian faults provide insights into the boundary between these structures, which formed due to the Dian-Qiong Suture and the Paleo-Pacific Plate subduction. These tectonic processes have major structural trends, particularly impacting the offshore forearc region in the NE and EW directions, characterized by deep basins overlying thin crust. Our study also identifies multiple minor trends, including horse-tails and secondary fault systems of the Yanshanian origin, which inherited from the regional tectonics. These trends suggest that post-orogenic stresses and shearing have played a role in rotational and ridge jump processes during the opening and closing phases of the South China Sea (SCS). Furthermore, an uneven relationship exists between the calculated magnetic anomaly and the thickness of Cenozoic sediments, with a significant increase in sediment depth observed in the southern NSCSM compared to both crustal thickness and its heterogeneity.

Impact of Ancient Tectonics on Intracontinental Deformation Partitioning: Insights From Crustal Structures of the East Junggar‐Altai Area

AbstractCompressive stress generated at collision fronts can propagate over long distances, inducing deformation within the continent's interior. Nevertheless, the factors governing the partitioning of intracontinental deformation remain enigmatic. The Altai Mountains serve as a type‐example of ongoing intracontinental deformation. Here, we investigate the crustal architecture of the Chinese Altai Mountains, using receiver functions obtained from newly deployed dense seismic nodal arrays. The new seismic results reveal distinct crustal features, including (a) a negative polarity discontinuity beneath Chinese Altai Mountains, suggesting a low‐velocity layer; (b) a north‐dipping mid‐crustal structure beneath the suture zone between East Junggar and Chinese Altai, indicating underthrusting of East Junggar's lower crust beneath the Chinese Altai Mountains; (c) a double Moho structure beneath East Junggar, revealing a high‐velocity lower crustal layer. In conjunction with constraints from previous multi‐disciplinary regional studies, the double Moho structures are interpreted as mafic restite from Late Paleozoic magma underplating. The addition of mafic materials can significantly enhance the rheological strength of East Junggar's crust, causing it to function as an indenter that thrust beneath the Chinese Altai Mountains during the subsequent convergence process. As a consequence, significant deformation occurs in the Chinese Altai region, resulting in the emergence of decollements, as evident by the negative polarity discontinuity. The presence of pre‐existing decollements makes the Altai Mountains region more susceptible to deformation, thereby facilitating the concentration of intracontinental deformation. These findings illuminate the evolution history of the Chinese Altai Mountains and highlight the great impacts of ancient tectonics on intracontinental deformation partitioning.

Stress Patterns and Crustal Anisotropy in the Eastern Alps: Insights From Seismological and Geological Observations

AbstractIn the Eastern Alps, the indentation of the Adriatic promontory since the Cenozoic affected the kinematics of separate crustal domains bounded by faults that accommodate lateral extrusion processes and differential shortening. Deciphering the pattern of crustal stresses in the orogen interior is challenging, due to the lack of in situ stress measurements at crustal depths. We define stress regimes and the orientations of the most‐compressive horizontal stress (SHmax) by integrating published results with new data, including stress analysis from fault plane solutions, estimation of crustal anisotropy through the shear wave splitting analysis, paleostress determination from fault slip data, and computation of cumulative seismic displacements. The retrieved regional SHmax are generally consistent with N‐S convergence. In the northern part of the study area, current stress orientations are almost parallel to paleo‐SHmax, suggesting a rather uniform compressional regime since the late Cenozoic. Conversely, sharp deflections and divergence with paleo‐SHmax appear at the western border of the Adriatic promontory across major transpressive and extensional shear zones and in the Southalpine domain, indicating a change in tectonically induced second‐order stresses. A current strike‐slip regime with subordinate orogen‐parallel seismic displacements affects a belt to north of the Periadriatic Lineament and NE‐extension characterizes the Ortles‐Engandine region. Seismic anisotropy locally exhibits fault‐parallel fast axes (Brenner‐Giudicarie fault‐systems, Dinaric and Southalpine thrusts), whereas stress‐induced anisotropy parallel to SHmax characterizes the southern part of the orogen. Cumulative seismic displacements are small compared to geodetic ones, and unravel partitioning of deformation into second‐order transpressive and extensional belts in response to indentation.

Late Devonian syntaxis in the Northern Appalachian orogen

Abstract The pre-accretionary shapes of cratonic margins form successions of promontories and re-entrants inherited from the rifting of supercontinents. In accretionary orogens, the extent of deformation related to a collision with a continent characterized by an irregular margin is obfuscated through the partitioning of deformation along pre-existing structures. In the Northern Appalachians, the extent of the deformation related to the oblique collision of the Meguma terrane with the composite Laurentian margin is disputed. Herein, we use a framework based on modern collisional settings to investigate the Late Devonian to Mississippian deformation inboard of the Avalonia–Meguma boundary and evaluate the regional tectonic setting. We combine published shear zone kinematic interpretations, deformation ages and regional 40 Ar/ 39 Ar cooling ages with structural interpretation of aeromagnetic and gravimetric depth slices covering the Northern Appalachians. We find that the deformation related to the collision of the Meguma terrane, attributed to the Neoacadian orogeny, has a larger structural footprint than previously documented. While this deformation is partitioned in multiple structures in the Canadian Appalachians, northern New England is characterized by rapid crustal deformation, high palaeoelevation and fast erosional exhumation, similar to modern syntaxis structures.

Slip partitioning and seismogenic stress buildup adjacent to the junction of the Lenglongling fault and the Tuolaishan fault: Insights from kinematically constrained numerical simulations

Understanding fault kinematics plays a crucial role in estimating seismic hazards and linking the relationship between seismogenic stress buildup and tectonic deformation. The northwestern segment of the Qilian-Haiyuan fault zone (QHFZ) serves as a natural laboratory for investigating how the oblique block motion decomposes into a complex fault system. In this study, we constructed a 3D FEM model incorporating the active faults as contact discontinuities to obtain a continuous numerical image of fault slip rates. Our findings revealed that deformation partitioning within the context of oblique block convergence generates a spatially heterogeneous distribution of fault slip rates. The inferred strike-slip rates of the Tuolaishan fault and the Lenglongling fault (LLLF) range from approximately 2 to 4 mm/yr and 4 to 7 mm/yr, respectively. The Lenglongling fault's slip behavior exhibits a spatial transition, with a dominant strike-slip component dominating in the western part and gradually changing to a combination of reverse faulting and strike-slip in the eastern segment. The Northern Lenglongling fault (NLLLF), characterized by a listric shape, merges with the LLLF in the deep crust, resulting in the formation of a positive flower structure that enhances the accumulation of compressional seismogenic stress. The region with high shear strain rates within this area has experienced significant earthquakes, including the 2022 Menyuan MS 6.9 earthquake, the 2016 Menyuan MS 6.4 earthquake, and the 1986 Menyuan MS 6.4 earthquake. Our numerical study gained valuable insights into the relationship between slip partitioning, fault structures, and regional stress.

Lower Crustal Rheology Controls Strain Partitioning and Mode of Intracontinental Deformation

AbstractThe factors that control strain partitioning along plate boundaries and within continental interiors remains poorly resolved. Plate convergence may be accommodated via distributed crustal shortening or discrete crustal‐scale strike‐slip faulting, but what controls these differing modes of deformation is debated. Here we address this question by examining the actively deforming regions that surround the Tarim Basin in central Asia, where deformation is uniquely partitioned into predominately strike‐slip faults in the east and distributed fold‐thrust belts in the west to accommodate Cenozoic India‐Asia plate convergence. We present integrated geological and geophysical observations to elucidate patterns in crustal deformation and compositional structure in and around the Tarim Basin. The thrust‐dominated western Tarim Basin correlates with a strongly‐magnetic lower crust, whereas strike‐slip faulting along the eastern margins of the Tarim Basin lack such magnetic signals. We suggest that the lower crust of the western Tarim is more mafic and stronger than in the east, which impacts intra‐plate strain partitioning. A stronger lower crust results in vertical decoupling to drive mid‐crust horizontal detachments and facilitate thrust faulting, whereas a more homogenized crust favored vertical transcrustal strike‐slip faulting. These rheological differences likely originated from the impingement of the Permian Tarim plume focused in the west. A comparison with the Longmen Shan of eastern Tibetan Plateau reveals remarkably similar strain partitioning that correlates with variations in foreland rheology. Our results highlight how variations in lower‐crust viscosity impact strain partitioning in an intra‐plate setting and how plume processes exert a strong control on later continental tectonic processes.

Ransiki earthquakes, northeastern Bird’s Head Peninsula, northwestern New Guinea, Indonesia: Deformation partitioning in oblique plate convergence

The plate boundary between the Pacific-Caroline and Australian plates in northwestern New Guinea is associated with a geographic concentration of earthquakes developed in the Ransiki region of the northeastern Bird’s Head Peninsula (West Papua, northwestern New Guinea) at the intersection of the Ransiki and Yapen faults. We examine these earthquakes based on regional geomorphological and lithostratigraphical frameworks, field observations of surface ruptures and liquefaction phenomena, and focal mechanisms of historical earthquakes. The Ransiki earthquakes are a set of 29 earthquakes from the Global Centroid Moment Tensor catalogue in the period 1977–2019 (magnitudes of Mw4.9 to Mw7.5). In the east, focal mechanisms show sinistral movement along the east-west trending Yapen Fault including the Mw6.7 earthquake on 21 April 2012. The largest earthquake was on 10 October 2002 (Mw7.5) and along with other earthquakes mainly in the southwest were associated with dextral movement indicated by focal mechanism solutions on the northwest trending Ransiki Fault south of its intersection with the Yapen Fault. The southern part of the Ransiki Fault therefore indicates local north-northeast compression that is also evident in the newly recognised Wainoei Fault south of Yapen Island. The two largest earthquakes (10 October 2002, 21 April 2012) show ground effects associated with liquefaction, indicated by surface offsets, open fissures, and sand blows, that all occurred in saturated sediments of the Ransiki delta. Earthquakes in the Ransiki region show that west-southwest oblique plate convergence between the Australian and Pacific-Caroline plates is partitioned into east-west sinistral strike-slip motion along the Yapen Fault and north-northeast compression associated with the Ransiki Fault.

Tonian Continental Arc Magmatism of the Porongos Complex, Dom Feliciano Belt, Southern Brazil

This study provides new information on the deformation, metamorphism, and tectonic setting of the metavolcanic rocks of the southern portion of the Porongos Complex, southern Brazil. The structural analysis enabled the identification of three deformational phases, formed under ductile to semi-ductile conditions. D1 progressed through deformation partitioning, comprising closed to isoclinal folds and an axial plane foliation. Then, simple shear prevailed, resulting in isoclinal folds, sigma-type porphyroclasts, foliation transposition, and mylonitic rocks. D2 comprises open to gentle folds and an axial plane cleavage. The formation of shear bands is ascribed to the semi-ductile D3. The mineral assemblage represented by phengite + chlorite + clinozoisite-epidote + actinolite + albite + quartz associated with the temperatures obtained through the chlorite geothermometer (316°C) indicates greenschist facies conditions. The protoliths of these metavolcanic rocks are geochemically discriminated as calc-alkaline, dacite-to-rhyodacite, with peraluminous compositions. The bulk trace element compositions show enrichment in large-ion lithophile elements and light rare-earth elements and depletion in heavy rare-earth elements. Also, negative Nb, P, Ti, and Ta anomalies are observed in the multielement diagram. All these geochemical features are typical of rocks formed in continental magmatic arcs. Geochemical comparison with other pre-collisional Tonian orthometamorphic rocks from the Dom Feliciano Belt demonstrates strong similarities, which corroborates the prior interpretation of a continental arc setting for the origin of these Tonian rocks.

Slope deformation partitioning and monitoring points optimization based on cluster analysis

Slope deformation partitioning and monitoring points optimization based on cluster analysis

Impact response of metastable dual-phase high-entropy alloy Cr10Mn30Fe50Co10

Plate impact experiments are conducted on a metastable dual-phase high-entropy alloy (HEA) Cr10Mn30Fe50Co10 with different matrix grain sizes and phase fractions. Free surface velocity histories are obtained along with microstructure characterizations. Postmortem samples are characterized with transmission electron microscopy and electron backscatter diffraction. Multiple deformation mechanisms are observed, including dislocation slip and stacking fault in the face-center cubic (FCC) phase, dislocation slip and deformation twinning in the hexagonal close-packed (HCP) phase, and the FCC to HCP transformation. For spallation, damage is ductile in nature. Despite different microstructures, micro-voids prefer to nucleate at high-angle grain boundaries within the FCC phase, leading to similar spall strength. Upon shock loading, a dynamic plastic deformation partitioning behavior among the two phases is observed; at high shock stress, denser dislocations in the HCP phase result in increased fractions of voids around the FCC/HCP phase interface and within the HCP phase during spallation damage.

Modelling of till microstructure development during subglacial deformation using ring shear experiments

Micromorphology has become a principal tool in glacial sedimentology providing a wealth of data on the geometry and kinematics of deformation histories and aiding our understanding of progressive and polyphase deformation. This approach has been applied to the detailed microstructural analysis of fabric development in nine samples of experimentally deformed till subjected to simple shear in a ring shear apparatus in order to model the progressive microfabric development during simulated subglacial deformation. Thin section analysis revealed that the relative strength and complexity of the clast microfabrics increases with increasing cumulative shear distance (0–1152 cm) and cumulative shear strain (0–549), whereas microfabric strength increases rapidly during the initial stages of shearing (cumulative strain up to ∼20) and remains relatively stable at higher cumulative strain (>200). In addition microfabric development is heterogeneous reflecting the partitioning of deformation at a microscale. A conjugate pattern of clast long axis alignment identified in the un-sheared sample taken at the start of the experiment formed during the initial consolidation and draining of the till. This emphasises that water-rich unconsolidated tills have the potential to be highly responsive to any applied stress even at very low strains. This un-sheared fabric was overprinted at a very early stage of the experiment when cumulative shear strains were below 5. The complex microfabrics formed during simple shear record the progressive development of up-shear dipping P-shears, down-shear dipping R-shears, and subhorizontal Y-shears. Sigmoidal fabric geometries, comparable to S–C and ECC-type fabrics, associated with the Riedel shears record a consistent sinistral sense of shear, i.e. the direction of shear imposed by the rotating plate of the ring shear. Observed changes in the relationships between these microfabrics record the switching between the localised imposition of a compressional fabric and P-shear formation, to subhorizontal shear and the formation of Y-shears accompanied by extension and the growth of low-angle R-shears, and back again. This process may potentially lead to alternating phases of “stick” (compression) and “slip” (subhorizontal shear) recognised as a key feature of glacier motion occurring in response to soft-sediment bed deformation. Multiple cycles of microstructure overprinting and rejuvenation pose a major challenge on determining the cumulative strain of subglacially deformed tills of unknown history from micromorphological signatures alone.

Cenozoic evolution and deformation in the Eastern and Western Depression of the Liaohe Subbasin, Bohai Bay Basin: Insights from seismic data

AbstractIn rift basins with multiphase extension, the reactivation of pre‐existing faults plays an important role in the evolution and partitioning of deformation. The Liaohe Subbasin is located in the northeastern Bohai Bay Basin, and the Tanlu Fault Zone (TLFZ) runs through it and experienced extension and right‐lateral strike‐slip during the Cenozoic. Therefore, it is an ideal region for studying the deformation and evolution of rift basins. Based on 3D seismic data, we document the structure and fault characteristics of the Eastern and Western Depressions of the Liaohe Subbasin, identify periods of activity on normal and strike‐slip faults, and analyse the evolution and mechanisms of the Cenozoic evolution of this subbasin. Our main results show that (1) the main faults in the Western Depression exhibited extensional motion throughout the Palaeocene, while the Eastern Depression exhibited significant right‐lateral motion during the Oligocene following the early extension stage. (2) The Cenozoic evolution of the Liaohe Subbasin can be divided into four stages: (i) an initial extension stage (the Palaeocene to the Eocene), (ii) an extension stage with initial strike‐slip (Early Oligocene), (iii) a right‐lateral strike‐slip stage (Late Oligocene) and (iv) a postrift stage (after the Neogene). (3) The upwelling of mantle material caused by the subduction and rollback of the Pacific Plate led to the extension of the Liaohe Subbasin from the Palaeocene to the Eocene. The TLFZ began right‐lateral motion at 40 Ma and experienced strong right‐lateral deformation when rifting was weak in the Oligocene and enhanced deformation after 12 Ma. (4) In this multiphase rifting process, under moderate obliquity, strike‐slip deformation is obvious in the areas close to pre‐existing faults, while extensional deformation is obvious in areas farther away. (5) Cenozoic extension and strike‐slip deformation due to partitioning of strain are caused by obliquely reactivation of TLFZ. This study suggests that the reactivation of pre‐existing strike‐slip faults plays a major role in evolution and strain partitioning. Moreover, the results will contribute to future hydrocarbon exploration in multiphase rift basins.

Crustal‐Scale Disharmonic Structural Pattern of West Junggar: Unveiling a Permian Indentation of Junggar Block Into Northern Kazakhstan Orocline

AbstractAs a crustal‐scale orocline in the Central Asian Orogenic Belt (CAOB), the Kazakhstan Orocline in particular its northern limb intervenes the Junggar Block and Chinese Altai. The West Junggar exhibits a structural pattern that is disharmonic from the major structure of the orocline. How and when such a structural complexity was formed remains poorly understood. To address this issue, we carried out detailed mapping, structural analysis and revision of geophysical data in three key areas of the West Junggar. It is shown that different arc chains experienced independent geological histories before they accreted with one another. Such an accretionary process generated the regional D1 deformation in late Carboniferous. Subsequent D2 shortening event produced variable superimposed structures, including crescent‐mushroom‐like folding in the Boshchekul‐Chingiz Arc and partitioned sinistral shearing of the West Karamay Unit. The synformal folding in the Zharma‐Saur Arc and crustal‐scale Erqis‐Zaysan sinistral strike‐slip shearing in late Permian—Triassic probably resulted from changing configuration of regional stress. Combined with previous data, we propose that the orogenic fabrics of arc chains and accretionary wedges were primarily related to shaping the basic structural framework of the Kazakhstan Orocline until late Carboniferous. The anticlockwise rotation of the northern limb of the orocline was associated with the indentation of the Junggar Block into the arc chains. Progressive indentation resulted in the first passive bending of arc chains followed by deformation partitioning into simple shear dominated transpression along the western margin of the Junggar Block and pure shear dominated transpression away from the indenter.

The control of deformation partitioning on gold mineralization in the Qingchengzi district, Liaodong Peninsula, northeastern China

The control of deformation partitioning on gold mineralization in the Qingchengzi district, Liaodong Peninsula, northeastern China

Strain Localization: Analog Modeling and Anisotropy of Magnetic Susceptibility

AbstractThe majority of the strain in Earth crust and upper mantle is localized to the high strain zones developed at ductile‐to‐brittle condition at kilometer‐to‐micrometer scale. Therefore, they represent the key to understanding the deformation evolution of the lithosphere. The finite strain pattern recorded within these zones has been therefore a subject of research in geology. The methods studying rock magnetism such as the anisotropy of magnetic susceptibility (AMS) are frequently used techniques to characterize and quantify deformation and flow record in rocks. Numerous sedimentary, subsolidus and submagmatic deformation zones exhibit typical evolution of the AMS ellipsoid across the strain gradient suggesting indirect not straightforward correlation between AMS and strain ellipsoids. To document spatiotemporal and internal fabric evolution during strain localization, pure shear, simple shear, and shear zone (SZ) analog experiments were performed using shear‐thinning thixotropic material of plaster of Paris. The experimental results closely resemble the record from natural SZs in sedimentary rock systems but also in subsolidus SZs and submagmatic mushy systems. The magnetic fabric evolution across deformation zones is interpreted to be associated with the intersection and transposition of preexisting primary fabric with shear fabrics and evolution of synkinematic subfabrics. Their development is attributed to localization of deformation at microscale due to the self‐organized slip of anisometric particles forming microshear planes reflecting the symmetry of deformation. The experimental results when confronted with the natural examples implies that the localization and partitioning of deformation is one of the most important factors for the interpretation of AMS in deformation zones.

Fault slip of the 2022 Mw6.7 Menyuan, China earthquake observed by InSAR, and its tectonic implications

In this article the ascending and descending Sentinel-1A satellite data are used to investigate the coseismic slip model of the 2022 Mw6.7 Menyuan earthquake in Qinghai, China. The optimal slip model indicates that this event ruptured two fault segments. The main rupture concentrated on the western Lenglongling fault (LLLF) with a purely left-lateral striking-slip motion. A small part of the eastern Tuolaishan fault (TLSF) section was also ruptured, and the motion on it is mainly oblique slip at depth, with an obvious thrust component. Combined with the rupture characteristics of historical events, GPS velocity map, and slip rate studies, we suggest that the TLSF–LLLF junction is a demarcation point where the deformation partitioning pattern has changed. Along the whole LLLF segment, the oblique convergence has completely partitioned into slip on the purely strike-slipping LLLF and thrusting faults in the north. The TLSF segment accommodates a fraction of compressional shortening, which compensates for the discrepancy in the left-lateral slip rate between the LLLF and TLSF. Such transformation in the strain partitioning pattern is likely to be determined by the geometric relationship between the fault strike and the direction of regional block movement.

Stress transmission along mid-crustal faults highlighted by the 2021 Mw 6.5 San Juan (Argentina) earthquake

Understanding the mechanisms of crustal deformation along convergent margins is critical to identifying seismogenic structures and assessing earthquake hazards for nearby urban centers. In the southern central Andes (28–33^{circ }S), differences in the style of middle to upper-crustal deformation and associated seismicity are highlighted by the January 19th, 2021 (Mw 6.5) San Juan earthquake. We integrate waveforms recorded at regional and teleseismic distances with co-seismic displacements calculated from local Global Navigation Satellite System time series, to re-estimate the source parameters of the 2021 San Juan earthquake, confirming a mid-crustal nucleation depth (21 ± 2 km) and right-lateral transpressional mechanism. Considered alongside decades of seismic observations and geological data, this event provides evidence for retroarc deformation partitioning among inherited basement faults and upper-crustal structures in response to oblique convergence of the Nazca and South American plates. As they may transfer shortening to active upper-crustal faults associated with historically devastating shallower earthquakes, a better understanding of seismogenic basement faults such as the mid-crustal structure activated during the 2021 San Juan earthquake earthquake could help future re-assessment of the seismic risk in western Argentina.