Fault-slip Data Research Articles

High-Detail Fault Segmentation: Deep Insight into the Anatomy of the 1983 Borah Peak Earthquake Rupture Zone (Mw 6.9, Idaho, USA)

Abstract Following observations made in a survey campaign along the Lost River Fault (Idaho, USA) in 2019, we integrate both original and previously published data to obtain a detailed segmentation of the fault sections that failed in the 1983 Borah Peak earthquake (Mw 6.9). The earthquake ruptured the topographic surface with an oblique-normal faulting mechanism, activating two SW-dipping fault segments (Thousand Springs and Warm Springs) and a branching SSW-dipping fault (Arentson Gulch Fault) and producing coseismic surface ruptures with up to 3 m of vertical separation. We augment the 1983 earthquake description by interpreting high-resolution topography and fault mapping. We use quality vertical separation data, rupture zone width measurements, and fault slip data to analyze major and minor structural-geometric complexities, highlighting a partition of the deformation and a fault segmentation into four detail levels (i.e., segments, sections, subsections, and sectors). Our work provides new details of the 1983 Borah Peak earthquake, constraints for paleoseismic and seismotectonic studies, and a methodological approach applicable in other areas of the world. Our fault-slip data show variations along fault-strike that we interpret as kinematic partitioning. In 1983, the main southern segment had a large rupture zone width, while the northern segment localized the deformation. The distributed ruptures accommodate a large portion of the rupture length (~19.5 km versus 31 km for the main rupture) and displacement (~66%). 83% of the surface faulting and 80% of the displacement are located at the hanging wall of the main rupture. There is a strong correlation between vertical separation, rupture zone width, rupture position (footwall or hanging wall), and fault geometry. We highlight the control of the obliquity and kinematic partitioning in the surface expression of the earthquake propagation. We interpret the coseismic (i.e., 1983) and long-term (i.e., Quaternary) behavior, showing that the two activated segments had similar cumulated behaviors in distributing the deformation between synthetic and antithetic ruptures, despite the different geometries. Our results have implications for fault rupture behavior with application to rupture hazard.

Structural complexities and tectonic barriers controlling recent seismic activity in the Pollino area (Calabria–Lucania, southern Italy) – constraints from stress inversion and 3D fault model building

Abstract. We reconstruct the 3D fault model of the structures causative of the 2010–2014 Pollino seismic activity by integrating structural–geological and high-resolution seismological data. We constrained the model at the surface with fault-slip data, and at depth, by using the distributions of selected high-quality relocated hypocenters. Relocations were performed through the non-linear Bayloc algorithm, followed by the double-difference relative location method HypoDD applied to a 3D P-wave velocity model. Geological and seismological data highlight an asymmetric active extensional fault system characterized by an E- to NNE-dipping low-angle detachment, with high-angle synthetic splays, and SW- to WSW-dipping, high-angle antithetic faults. Hypocenter clustering and the time–space evolution of the seismicity suggest that two sub-parallel WSW-dipping seismogenic sources, the Rotonda–Campotenese and Morano–Piano di Ruggio faults, are responsible for the 2010–2014 seismicity. The area of the seismogenic patches obtained projecting the hypocenters of the early aftershocks on the 3D fault planes, are consistent with the observed magnitude of the strongest events (Mw=5.2, and Mw=4.3). Since earthquake-scaling relationships provide maximum expected magnitudes of Mw=6.4 for the Rotonda–Campotenese and Mw=6.2 for the Morano–Piano di Ruggio faults, we may suppose that, during the sequence, the two structures did not entirely release their seismic potential. The reconstructed 3D fault model also points out the relationships between the activated fault system and the western segment of the Pollino Fault. The latter was not involved in the recent seismic activity but could have acted as a barrier to the southern propagation of the seismogenic faults, limiting their dimensions and the magnitude of the generated earthquakes.

Structural architecture and tectonic evolution of the Shabrawet Syrian Arc inverted zone at the Northern termination of the Gulf of Suez rift, Egypt

ABSTRACT The present study offers a novel opportunity to characterize the structural style and tectonic evolution of the Mesozoic Shabrawet inverted structure zone and its interaction with the northward propagation of the Gulf of Suez rift, Egypt. Based on the integration of fieldwork, detailed surface mapping and subsurface imaging by borehole data and seismic interpretation, the geology of Shabrawet area is characterized into the northern, central, and southern structural subareas. The central subarea has exposures of Cretaceous rocks whereas the northern subarea has the Jurassic/Cretaceous subcrop capped by the Oligo/Miocene rocks with remarkable stratigraphic hiatus probably due to differential erosion or non-deposition of the Lower Tertiary-Cretaceous sediments. The southern subarea represents a SW-dipped gulf-parallel block of Middle/Late Eocene and Oligo/Miocene sediments. The structural elements of the Shabrawet area are represented by a group of plunging and double plunging folds of NE-ENE-, E-W, WNW- trends that are dissected by NE-reverse faults, ENE-trending transpressional faults, N-S sinistral faults, E-W to WNW – trending dextral faults and NW-trending normal faults. Structural analysis, fault slip data, and the constructed subsurface seismic profiles and thickness maps reveal the tectonic history of the area through a Late Cretaceous-Eocene transpressional inversion of E to ENE-extensional Jurassic border faults. This ENE-inverted Syrian arc structure zone played as structural high with non-deposition of Eocene sediments and differential erosion of Cretaceous sediments. During the Oligo-Miocene NE-extension, this high-oblique inverted zone terminates the northward propagation of gulf parallel faults and SW- block rotation of the northern structural province of the Gulf of Suez rift at the southern subarea and the central Bitter Lake. Further westward, this ENE-inverted Shabrawet-Abu Sultan intrabasinal Jurrassic half-graben of N- dipping might be structucturally correlatable with the NE- inverted structural belt of Gebel Maghara in Northern Sinai.

Post-collisional orogen-parallel extension in the Trans-North China Orogen: Evidence from syn-kinematic pegmatite dikes

Orogen-parallel extension is common in collisional settings, but its cause is debated. The Paleoproterozoic Trans-North China Orogen (TNCO) provides insights into the process of orogen-parallel extension. We present structural and geochronological data from pegmatite dikes in the Xiaoqinling region of central China to constrain the time and nature of Paleoproterozoic extension in the TNCO. The pegmatite dikes strike roughly E–W, have various dip angles, and are either deformed or undeformed. A progression from ductile to brittle deformation is commonly observed in the deformed dikes, suggesting synchronous orogenic uplift probably driven by isostatic adjustments. The deformation structures demonstrate that the intrusion of the pegmatite dikes was associated with a single phase of regional extension, thus making them syn-kinematic. Both ductile fabrics and fault-slip data from the dikes indicate that N–S (orogen-parallel) extension was responsible for dike deformation. Zircon U–Pb ages for the syn-kinematic pegmatite dikes constrain the time of extension to between 1840 and 1781 Ma, suggesting a duration of ∼ 59 Myr for the orogen-parallel extension. These data suggest that the extension postdated the syn-collisional shortening in the TNCO, which occurred between 1863 and 1840 Ma. The orogen-parallel extension, the synchronous isostatic uplift, and the intense magmatism in the TNCO are consistent with a gravitational collapse model in a post-collisional setting.

Tectonic evolution of the central part of the East Anatolian Fault Zone, Eastern Turkey

The Eastern Anatolian Fault Zone (EAFZ), having a prominent place in the tectonic evolution of the Eastern Mediterranean, is a structural element of tectonic indentor due to the convergence between the African-Arabian plates and the Eurasian Plate. This study investigates the central part of EAFZ between Doğanyol (Malatya) and Çelikhan (Adıyaman). The geometry of the fault and the morphotectonic structures were determined by the field studies. Moreover, fault-slip data are measured according to the fault planes along the deformation zone for paleostress analysis. The paleostress analysis revealed three deformation phases that developed from the Late Eocene to the present due to the convergence between the Arabian Plate and the Anatolian Block. The first deformation phase is characterized by NW-SE compressional stress between Late Eocene and Late Oligocene periods. The second deformation phase is related to N-S compressional stress from the Middle Miocene to Pliocene. The most recent deformation phase shows the strike-slip faulting under the NNE-SSW compressional stress from the Late Pliocene to the present. The EAFZ developed during the last phase of these deformation stages. In addition, elongated ridges parallel to the fault, sinistral offsets of drainage networks, linear valleys, and fault terraces observed along the segment show that the study area exhibits active tectonic morphology of the EAFZ. The distribution of seismic activity that occurred during and after the recent mainshock (24 January 2020, Sivrice-Doğanyol earthquake) is compatible with the geometry of the segment and confirms strongly the active tectonics of the segment.

Kinematic analysis of fault-slip data in the Nanling Tectonic Belt and Cretaceous to Paleogene tectonic evolution of the Central South China Block

• Six tectonic events were inversed from fault-slip analysis in the Nanling Tectonic Belt. • Four Cretaceous tectonic events resulted from the Paleo-Pacific subduction . • Two Paleogene tectonic events were due to the far field effect of the India-Asia collision. Cretaceous tectonic events and Cenozoic geodynamics of the central South China Block are still under debate. A long-term extensional regime or two extension-contraction tectonic cycles were proposed for the Cretaceous tectonic evolution. While Cenozoic geodynamics of the central South China Block were ascribed to the Pacific subduction or the far field effect of the India-Asia collision. Kinematic analysis of fault-slip data were carried out for the Cretaceous to Paleogene Chaling-Yongxing Basin and the Lianshan-Yangshan Domain in the Nanling Tectonic Belt. Kinematic analysis of fault-slip data (409 groups of faults and striations) reveal four Cretaceous tectonic events and two Paleogene tectonic events. i) The early Early Cretaceous extensional regime is represented by a NW-SE horizontal minimum principal stress and a vertical maximum principal stress, which generated NE-striking boundary normal faults of Early Cretaceous basins. ii) The late Early Cretaceous strike-slip regime is characterized by a N-S horizontal maximum principal stress and a E-W horizontal minimum principal stress, which led to tectonic inversion of Early Cretaceous extensional basins and nearly N-S-striking folding of the Lower Cretaceous. iii) The early Late Cretaceous extensional regime is represented by a NW-SE horizontal minimum principal stress and a vertical maximum principal stress, which activated the NE-striking boundary normal faults of Late Cretaceous basins. And iv) the latest Late Cretaceous strike-slip regime is characterized by NW-SE horizontal maximum principal stress and a NE-SW horizontal minimum principal stress, which led to tectonic inversion of Late Cretaceous extensional basins and NNE-striking folding of the Upper Cretaceous. Orientations of maximum or minimum principal stresses imply that two extension-contraction tectonic cycles during the Cretaceous resulted from the Paleo-Pacific subduction. Two Paleogene tectonic events are v) the strike-slip regime characterized by NE-SW horizontal maximum principal stress and a NW-SE horizontal minimum principal stress, and vi) the subsequent extensional regime represented by a NE-SW minimum principal stress and a vertical maximum principal stress. The former generated NW-striking reverse faults and reactivated strike-slipping of NE- and NW-striking conjugate faults whereas the latter formed NW-striking normal faults in the Lower Paleogene clastic rocks. Orientations of maximum or minimum principal stresses indicate that two Paleogene tectonic events in the central South China Block were attributed to the far field effect of the India-Asia collision and subsequent stress relief.

Evolution of the stress field at the junction of Talesh – Alborz – Central Iran during the past 5 Ma: Implications for the tectonics of NW Iran

Tectonic stresses governed by the Arabia – Central Iran collision are transferred throughout the Iranian Plateau. The orientation of the maximum horizontal stress (σ1) significantly changes (~90°) in the Persian block of NW Iran. However, the exact spatial extent and boundary conditions for these changes are poorly defined. This study investigates the late Cenozoic stress regime in a key region in NW Iran including the Mahneshan – Mianeh Cenozoic Basin (MMCB), the Garmachay Fault Zone (GFZ), and the southern flank of the Bozqush Mountains (SBM). We mapped folds and fault zones in detail using high-resolution satellite imageries and field surveys. We have measured fault-slip data at 49 localities distributed over the study area. The results of fault-slip inversion analysis indicate significant temporal and spatial changes in both the modern and paleostress fields. In the MMCB and GFZ, the orientation of the horizontal σ1 axis changed from N148°E to N041°E, during the Pliocene and Quaternary times. During the same period of time, it changes from NE-SW to N138°E in the SBM. Our study reveals that the southern boundary of the Persian block corresponds to the deep-seated North Tabriz fault (NTF), which is spread out over several splays at the southern flank of the Bozqush Mountains. The NTF acts as a boundary condition in both the stress and strain fields of the region. The GFZ defines the southeastern border of the Persian block, where the NW-trending maximum compression encounters the remote stress field in the hinterland of the Zagros collision. Our results indicate that independent movements of crustal blocks transfer crustal stresses beyond the collision zone and produce large-scale stress perturbations by discarding compatibility in regional strain/stress fields.

POSSIBLE INFLUENCE OF A MAJOR INFERRED FAULT ON THE OCCURRENCE OF SINISTRAL TOP-TO-THE NW DEFORMATION IN THE ZAGROS HINTERLAND FOLD-AND-THRUST BELT, IRAN

In the present study, fault slip data, the geometry of en-echelon vein arrays (tension gash), and pressure-solution seams (stylolites), in the northeastern margin of Fars Province were analyzed. The results of this study indicate that in the time of the development of these structures, the maximum principal (σ1) stress axes were generally horizontal and directed towards NE-SW and ENE-WSW. This general direction is compatible with the expected directions of σ1 stress axes responsible for the occurrence of the sinistral top-to-the NW ductile and brittle-ductile shear zones of the area. This compatibility suggests a long-lasting stable stress condition over a long period and different pressure/temperature conditions. This longstanding constant stress state can be interpreted as the result of the occurrence of a major strike-slip NW-SE trending fault in the NE of the Zagros Hinterland Fold-and-Thrust Belt. The dextral activities of the Main Zagros Thrust and this inferred fault, which are subparallel, might result in the sinistral topto-the NW deformation in the area between this fault, which we named it Abarkuh Fault and the Main Deep Fault. This inferred fault has been covered by Quaternary alluvium of the Abarkuh plain, but the great age difference of rock units of the Esteghlal Anticline and its northeastern rock exposures, and the significant change in topography between the Abarkuh plain and its southwestern mountains can be two consequences of the existence of this probable fault.

Hard classification of polyphase fault-slip data: Improvement and application

The quickest approach to the separation of polyphase fault-slip data into single-phase subsets is perhaps the recognition in stress space of the hyperplanes, consisting of their corresponding datum vectors, whose normals are parallel to the solutions of the stress vector. However the uncertainty of the solution sign makes it impossible to distinguish the subsets with respect to the opposite real stress vectors. In order to overcome this drawback the existing hard-classification method was improved by incorporating the slip senses during the separation. A series of artificial two-phase fault-slip datasets with the measurement errors generated under the imposed stresses were taken to test the improved method. There is a high accuracy of the stress solution even when the measurement errors have a range of [-12°, 12°]. The new method is further applied to data of shear bands measured on the limbs of the NNE-trending syncline at the Shihtipin resort, east of Taiwan. Two stress estimates representing the strike-slip or compressional tectonic regimes were obtained on each limb. They indicate an anticlockwise rotation of the maximum principal stress axis from ESE–WNW or SE–NW to E–W during the folding, probably in response to the oblique collision between the Luzon volcanic arc and the South China continent.

Structural and paleostress analysis within a fossil slow-spreading ridge: Tectonic processes involved during ocean expansion

This study aims to decipher the spatio-temporal chronology of tectonic processes leading to ocean widening at a slow-spreading oceanic ridge axis, involving mantle and gabbro exhumation and volcanism. Structural and petrographic studies of deformed lithologies were performed on peridotites, gabbros and basalts from the Chenaillet ophiolite (Alps, France). Inversion of fault-slip data reveals N030° and N060° σ3 paleostress trends. The dominant N030° extension, is consistent with the NNW-SSE direction of volcano feeder-dykes and ENE dipping low-angle normal faults crosscutting the mantle. The N060° extension accords with the mantle dome structure formation to the east. Low-angle faults intersect roots of volcanoes that thus belong to their hanging walls. Hydrothermalism is contemporaneous with or postdates low-angle faulting. A feeder-dyke major virgation northwards, synchronous with eruptions, suggests a dextral transform fault consistent with a N120° σ2. Oceanic expansion mechanisms are clarified. At the surface, magma eruption occurs along on-axis active high-angle normal faults, their footwalls enabling mantle exhumation. At depth, off-axis high-angle faults become low-angle faults as they spread at shallow level. With westward drift of the lithosphere, the uppermost levels of the ridge shift westward faster, such that volcanoes move to an off-axis position while their roots are cut by low-angle faults.

Abrupt shift from trench–parallel to trench–perpendicular backarc extension: Evidence from the Kalaqin metamorphic core complex in the eastern North China Craton

The driving force of backarc extension remains a subject of debate. In this study, we investigate the Kalaqin metamorphic core complex (MCC) in the eastern North China Craton, to determine the driving mechanism of backarc extension related to subduction of the Paleo–Pacific Plate. The MCC records two phases of extensional structures. D1 structures include the NE–SW–striking Louzidian–Dachengzi and Jinshan–Meilin detachment shear zones on both sides of the core complex. These zones originally constituted a single NE–dipping, low–angle, normal–sense detachment shear zone that records deformation temperatures of 500–600 °C. The shear zone seprarted rifted upper crust from locally deformed middle–lower crust. Together, the D1 structures constitute a Cordillera–type MCC associated with magmatism. The MCC was then overprinted by a second phase of extension (D2) that produced a NE–SW–trending horst between rift basins. The interior of the horst was deformed by early normal shear belts and later normal faults, all striking NE–SW. Our U–Pb zricon dating results from deformed and undeformed igneous rocks, together with previous geochronolical data, constrain D1 at 156–150 Ma (Late Jurassic) and D2 at 135–100 Ma (Early Cretaceous). Kinematic and fault–slip data show an evolution from D1 trench–parallel to D2 trench–perpendicular extension with respect to subduction of the Paleo–Pacific Plate. The D1 trench–parallel, NE–SW extension is considered to have resulted from both west–northwestward subduction of the Paleo–Pacific Plate and southward subduction of the Mongol–Okhotsk Oceanic lithosphere, whereas the D2 trench–perpendicular, NW–SE extension took place after closure of the Mongol–Okhotsk Ocean and is ascribed to the subduction of the Paleo–Pacific Plate only.

Paleostress reconstruction of faults recorded in the Nied\u017awiedzia Cave (Sudetes): insights into Alpine intraplate tectonic of NE Bohemian Massif

Brittle structures identified within the largest karstic cave of the Sudetes (the Niedźwiedzia Cave) were studied to reconstruct the paleostress driving post-Variscan tectonic activity in the NE Bohemian Massif. Individual fault population datasets, including local strike and dip of fault planes, striations, and Riedel shear, enabled us to discuss the orientation of the principal stresses tensor. The (meso) fault-slip data analysis performed both with Dihedra and an inverse method revealed two possible main opposing compressional regimes: (1) NE–SW compression with the formation of strike-slip (transpressional) faults and (2) WNW–ESE horizontal compression related to fault-block tectonics. The (older) NE-SW compression was most probably associated with the Late Cretaceous–Paleogene pan-regional basin inversion throughout Central Europe, as a reaction to ongoing African-Iberian-European convergence. Second WNW–ESE compression was active as of the Middle Miocene, at the latest, and might represent the Neogene–Quaternary tectonic regime of the NE Bohemian Massif. Exposed fault plane surfaces in a dissolution-collapse marble cave system provided insights into the Meso-Cenozoic tectonic history of the Earth’s uppermost crust in Central Europe, and were also identified as important guiding structures controlling the origin of the Niedźwiedzia Cave and the evolution of subsequent karstic conduits during the Late Cenozoic.

Active tectonics and kinematics of Fethiye-Göcek Bay, SW Turkey: Insight about the eastern edge of Pliny-Strabo Trenches

Pliny-Strabo Trench (PST)is a Subduction Transform Edge Propagator (STEP) fault developed at the northern edge of the African oceanic lithosphere connecting the Aegean and Cyprian trenches in the eastern Mediterranean. Recent studies have demonstrated that the PST terminates close to Turkish Border and links to the Cyprian trench east of Rhodes and south of Fethiye-Göcek Bay. However, it is also claimed that the PST extends into SW Anatolia along a sinistral transtensional shear zone, so-called Fethiye-Burdur Fault Zone (FBFZ) implying that a lithospheric tear in the downgoing plate extends into the over-riding plate, although, this is kinematically almost impossible since full mechanical coupling between down going and the over-riding plate is necessary. To test this hypothesis and understand the kinematics of the transition zone between PST and Cyprian trench we have conducted a rigorous paleostress inversion study combined with interpretation of 2D seismic data of which 228 km total length obtained from Fethiye-Göcek Bay.The seismic reflection data are used for the interpretation and delineation of off-shore faults and to determine their recent activity. The geometry and kinematics of the exposed on-land faults are determined by analyzing 13969 fault slip data obtained from 211 sites distributed evenly throughout in an area encompassing the bay in an area extending 70 km in E-W and 50 km in N–S that cover the whole area where the Pliny-Strabo STEP fault would emerge on-land.Results of analyses indicated that most of the faults in the study area are developed under multi-directional extension, except for some NE-SW-striking faults, which have dextral strike-slip components contrary to proposed sinistral nature of Fethiye-Burdur Fault Zone. Although there are numerous normal faults some which have been active until recently, however, there is almost no NE-SW striking sinistral strike-slip fault in the region to justify the presence of Fethiye-Burdur Shear Zone. Additionally, there are a number of earthquakes with dominantly strike-slip moment tensor solutions in the deeper part of the bay while they are in normal character in the on-shore areas suggesting that the Pliny-Strabo Trench stops very close to the shoreline and it does not propagate into SW Anatolia. Therefore, existence and alleged characteristics of the Fethiye-Burdur Fault Zone could not be verified.

Changing tectonic regimes in the central Costa Rica forearc between the Paleogene and the present: Insights from structural analysis and focal mechanisms

We discuss the Cenozoic history of the Central Costa Rica forearc, between 10° and 9°30′N, based on geological mapping, fault slip data and seismological records.The temporal variability in the regional stress-field suggests a variable and complex deformation pattern. The first stage of deformation, from the Paleogene to early Miocene, was controlled by an extensional regime in the forearc. Afterwards, a change to orthogonal convergence triggered a contractional deformation and subsequent inversion of the sedimentary extensional basins in the middle-upper Miocene. Finally, a transpressive regime has been developed since the Pliocene.

The Rosebel gold mining district (Trans-Amazonian belt, Suriname), a new structural framework

The Rosebel gold district belongs to the Paleoproterozoic Trans-Amazonian belt associated with sub-meridian crustal shortening. Here, we present new structural observations (cleavage, stretching lineations, veins, fault slip data, aeromagnetic map). The regional cleavage is steeply dipping and bears a steeply plunging stretching lineation. Finite strains are of flattening type. Fault slip data reveal a complex deformation history. The overall strain pattern in the area reflects vertical motions, a feature consistent with pop-down tectonics involving vertical stretch and burial of supracrustal deposits during horizontal shortening of a hot and weak continental lithosphere.

Paleostress variation during the same regional deformation in the Eastern Dharwar Craton (southern India)

The objective of this study is to evaluate progressive variation in the state of stress at ca 2.5 Ga in Hutti region of Eastern Dharwar Craton (EDC, southern India). Paleostress analysis is performed using quartz vein orientations in metavolcanic and granitoid rocks (ca 2.5 Ga) of the region. It is established that veins in the metavolcanic rocks formed during regional D3 deformation under N–S compression that is related to N–S directed far-field stress, which is also responsible for D3 folds with E-W striking axial planes in parts of the EDC. Anisotropy of magnetic susceptibility data from the granitoids yields NNW-SSE striking fabric anisotropy that developed during regional D1/D2 deformation. Paleostress analysis using veins in the granitoids of the region yields NE-SW compression. However, the kinematics of formation of these veins is shown to be compatible with dextral simple shear along pre-existing NNW-SSE oriented fabric under N–S directed D3 compression. Therefore, it is established that veins in the metavolcanic and granitiod rocks of the study area formed under the same N–S directed D3 far-field stress, an interpretation that is further supported by the similar values of stress ratio obtained from 3D Mohr circle analysis of vein data from the two lithologies. Thus, the study demonstrates that there are pitfalls in direct evaluation of paleostress using only vein orientations and that understanding kinematics of vein emplacement is critical in paleostress analysis. Our data further reveal that the granitoids were subjected to faulting during the final stages of D3 due to which the veins are displaced. Paleostress analysis using fault-slip data from the granitoids yields NNE-SSW compression. This local variation of paleostress in granitoids is due to late-D3 brittle deformation during cratonization of the Dharwar Craton at ca 2.5 Ga.

Structural styles of the Aras fault zone with implications for a transpressive fault system in NW Iran

• NE-striking Aras Fault Zone (AFZ) bounds the Talesh and Lesser Caucasus blocks. • The AFZ is characterized by well-developed geological and geomorphological features. • Paleostress results along the AFZ indicate a similar tectonic regime since Pliocene. • The transpressive tectonic regime controls the structural evolution of the AFZ. • Transpressive tectonic regime leads to strain partitioning along the AFZ. Aras Fault Zone (AFZ) is one of the most important intra-plate fault zones within the Turkish-Iranian Plateau. The AFZ is divided into five fault branches based on geometrical and structural properties called the Aras Main, Horadiz, Khodafarin, Asadkandi, and Aslanduz fault branches. Our geometric and kinematic analysis indicates that these faults exhibit compressional left-lateral strike-slip fault characteristics with general directions of NE-SW and dipping to SE. Structural mapping has revealed that the NE section of the AFZ shows typical horsetail fault geometry. Few meters to approximately 17 km of left-lateral displacements were observed along the AFZ, indicating the occurrence of numerous historical earthquake events. Paleostress analysis of fault slip data reveals that the AFZ has developed in the dominant NNE-SSW and NE-SW oriented compressional stress regime. Evaluating the Ratio of Principal Stress Differences (R), Stress Regime Index (R'), and principal stress directions show that the zone is under the transpressional tectonic regime. Paleostress orientations are consistent with orientations of the GPS velocity vectors, which indicates that the region, including the AFZ, is under the effect of the same tectonic regime, at least since Pliocene. The amounts of strike-slip partitioning on discrete strike-slip faults are determined as 33–41%, which means at least 41% of total wrench components in the region have been accommodated by discrete strike-slip faulting along the AFZ. Results indicate that the simple shear component of total stress applied to the area is consistent with the GPS slip velocities.

Spatio-temporal variation of the strain field in the southern Central Andes broken-foreland (27°30′S) during the Late Cenozoic

We present an integrated structural and Anisotropy of Magnetic Susceptibility (AMS) study focused on the characterization of the spatio-temporal variation of the strain field during the main Neogene deformation, of the Argentine Precordillera and the northern Sierras Pampeanas, immediately to the south of the Puna plateau (27°30′S). The AMS from 43 sites and fault-slip data analysis from mesoscale faults (n = 540) were carried out in Lower Miocene to Middle Pliocene sedimentary and volcaniclastic rocks. The AMS results show moderate values of anisotropy degree near to deformation zones, and low and high values of bulk susceptibility. The most predominant magnetic fabric is sedimentary type I and II, with a weak magnetic lineation. In areas where the rocks are deformed, the incipient magnetic lineation are subparallel to the regional structural trend and present a main N–S orientation in the Precordillera and Fiambalá basin, while in the Sierras Pampeanas they are mostly NE-oriented. Combining these results with kinematic analysis from fault-slip data, we identify, in the Precordillera and Fiambalá basin areas, a contractional event with E-W main direction, from ~23 Ma to 5 Ma, mostly active from the onset of deposition of Neogene sequences. During this period, in the NW Sierras Pampeanas, the contraction directions follow a NW orientation. The onset of N–S to NNW contraction, mostly recognized in rocks younger than 5 Ma, exposed in the northern areas of Precordillera and Sierra Pampeanas, documents a change of the strain field during the Early-to-Middle Pliocene, associated with a shift in the contractional direction from E-W to N–S. We relate the change to the rapid uplift of the southern Puna, generating a juxtaposition of an area under N–S extension, affecting the topographically-higher Puna, with the other area under N–S contraction in the adjacent Precordillera and Sierras Pampeanas. We associate these two events as part of the evolution of this segment of the Andes, where the broken foreland setting is active during this time. By combining the AMS and kinematic analysis results, we obtain a timing for the spatio-temporal change of contraction directions, which allows us to compare both the broken-foreland and foreland basins to understand spatio-temporal strain variations.

Studying active fault-related folding on tectonically inverted orogens: A case study at the Yariguíes Range in the Colombian Northern Andes

Morphometric analysis is widely used in tectonic geomorphology as a useful method for identifying fold activity in terms of its lateral and vertical growth. This study evaluates the recent tectonic activity and lateral propagation of the Los Cobardes anticline (LCA), which built up the Yariguíes Range, from morphometric characterization, a structural kinematic analysis, and fieldwork. The study area is located west of the Colombian Eastern Cordillera in a tectonically active region bounded by longitudinal tectonically inverted reverse structures (e.g., the Suárez Fault) with minor transverse faults and the LCA as the central fold of the Yariguíes Anticlinorium.A comparison among the precipitation data, lithology, and the morphometric indexes leads us to conclude that for the LCA, recent tectonics drive fold activity. Based on the LCA's geomorphology and the kinematic analysis derived from fault-slip data, we define three independent fold segments bounded by transverse zones: LCA-A, LCA-B, and LCA-C. Of these, the first two show evidence of lateral propagation toward the southwest, which is closely related to the activity of the Suárez Fault and inherited transverse structures. We found qualitative and quantitative geomorphologic evidence of southwestward propagation: wind and water gaps decreasing in elevation, drainages partially flowing in the direction opposite to the regional trend, and low concavity values for normalized profiles (from −40.49 to 11.76) and Vf (from 0.07 to 0.13). Assisted by published low-temperature thermochronology and seismotectonic data, we demonstrate that the LCA has been growing since the Pliocene and remains active, accommodating deformation in a transpressive stress state. Our findings could serve as a reference to study active fold growth and its structural drivers in known tectonically inverted basins, such as the foreland thrust belt in Argentina, the High Atlas Mountains in Morocco, and the Central Mountain Range of Taiwan, among others.

Late Cenozoic paleostress reconstruction and stress regimes in Taiyuan basin of the Shanxi rift, North China

The Taiyuan basin, initiated during the late Cenozoic, is part of the left-stepping en-echelon graben systems of Shanxi. The state of stress in this area and its evolution over time is a significant concern given the existence of high shear strain and seismic risk along the basin boundary faults. In the present study, the inversion of fault-slip data through the right dihedron and the rotational optimization methods led us to identify 21 significant paleostress tensors. The maximum principal stress orientation shows two modes trending NE–SW and ENE–WSW with most of stress tensors having stress ratio values within a wrench regime. The extensional sites reveal NW–SE and NE–SW directions. Three tectonic stages with different kinematics were successfully identified since the late Cenozoic. The oldest event has been identified in our analysis since the late Miocene to late Pliocene and belongs to NW–SE extension/strike-slip stress regime. The next was generated in the early Pleistocene under the NE–SW extension stress regime, and the youngest activity recorded belongs to the NNW–SSE extension/strike-slip regime developed since the late Pleistocene. These events were dynamically driven by the northeastward extrusion of the Tibetan Plateau in response to the plate boundary motion involving the Indian and Eurasian plates and lesser by the NW subduction of the Pacific plate. The present-day stress state is characterized by ENE–WSW compression and NNW–SSE extension derived from earthquake focal mechanisms.