Thrust Faults Research Articles

Relocation and Stress Analysis of the 2018 Mandali-Sumar Earthquake Sequence, Iraq-Iran Border

The 2018 Mandali-Sumar earthquake sequence started in January 2018 at the Iraq – Iran border within the Low Folded Zone of the Zagros Fold-Thrust Belt. To study the fault that is responsible for this earthquake sequence, the relocation of 32 earthquakes was conducted. In addition, moment tensor solutions of 9 large earthquakes were collected to know the fault motion and stress regime in the study area. The Computer Programs in Seismology (CPS) was used to analyze waveform data taken from 33 seismic stations located in Iraq and Iran to relocate earthquakes. The location of the earthquake sequence based on the IRSC bulletin shows a scattered spatial distribution, but the results of the relocations show that the earthquakes of the sequences are aligned in a longitudinal feature parallel to an anticline limb located in Ilam and Kermanshah provinces near the Iraqi border. The moment tensor solutions indicate that the sequence is related to a thrust fault with a 342º strike direction, 35º NE dip angle, and 76º rake angle. The TENSOR program was used to perform formal stress inversion of moment stress axes, which revealed that the azimuth of the maximum horizontal stress axis is 63º. The epicenters of the sequence are located between the Mountain Front Fault to the NE and the Zagros Foredeep Fault to the Southeast. We believe that the 2018 Mandali-Sumar earthquake sequence is related to displacement on the surface of the Zagros Foredeep Fault within the uppermost basement and the lowermost Phanerozoic cover.

Geophysical characterization of the bedrock and regolith in the Pranmati basin critical zone, Uttarakhand Himalaya

The young active Himalayan mountain is characterized by steep slope and dissected topography in overall compressive tectonic setting. The mountain belt has primarily coarse textured soil with poor water holding capacity and is highly prone to erosion. The erosion not only affects many ecosystems located at downstream but also has detrimental effects on the critical zone (CZ). In the present study, we have carried out DC electrical resistivity study in the Pranmati catchment of the Alaknanda basin, a Himalayan critical zone in the Lesser Himalaya, to understand the pattern of soil erosion, transportation and deposition by characterizing the bedrock architecture and hence regolith thickness. A total of 6 electrical resistivity tomogram (ERT) profiles were laid at two locations in the catchment, one in a plain grassland and another at a crop field located on a hill slope of >25o. The study area in the Baijnath klippe, consists of quartz-biotite gneisses with layers of quartz mica-schist enclosed by thrust faults. Electrical resistivity sections of the downslope grassland site show a sharp resistivity contrast between the southwest and northeast transects suggesting south-eastern increase in dip of the bedrock, oblique to the north-east facing surface topography and a thick regolith (> 10 m). The resistivity sections of the site located on the hillslope yield a very thin layer of regolith (< 2 m) indicating significant soil erosion and high weathering of the bedrock. We propose that the water–rock interaction within the porous regolith facilitated by subsurface water circulation might be a potential source for the thick regolith. The observations substantiate existing hypotheses for the evolution and development of deep critical zones. From the results, it has been hypothesized that the bedrock architecture and water channel paths within the CZ together control the regolith thickness.

Along-strike extent of earthquakes on multi-segment reverse faults; insights from the Nevis-Cardrona Fault, Aotearoa New Zealand

Evaluating fault segmentation is important for our understanding of seismic hazard assessment and fault growth. However, it is still unclear what controls if reverse fault earthquakes will rupture across segment boundaries. Here, we combine fault mapping and trench data from the low slip rate (0.04-0.15 mm/yr) multi-segment Nevis-Cardrona Fault (NCF) in the South Island of Aotearoa New Zealand to assess if it has ruptured in single or multi-segment earthquakes during the late Quaternary. Two new trenches on its Nevis segment provide stratigraphic evidence for two surface rupturing earthquakes, which through Optically Stimulated Luminscence dating and OxCal modelling, are constrained to have occurred at 28.9 +12.9 -9.1 ka and 12.8 ± 4.9 ka. The most recent timing is only weakly correlated to surface rupture timings from two trenches along the NCF's NW Cardrona segment. Furthermore, the 2 ± 1 m Nevis segment single event displacements we estimate would be unusually low for a ~85 km long NCF multi-segment rupture. We therefore surmise that late Quaternary NCF surface rupturing earthquakes did not rupture through ~30-50° bends that link these segments. Our trench data and fault mapping also indicate lower slip rates on the Nevis segment than previous studies (0.04-0.1 mm/yr vs 0.4 mm/yr).

ANALYSIS OF FAULT PATTERNS THAT CAUSE DESTRUCTIVE EARTHQUAKES IN MAINLAND WEST JAVA

West Java is one of the provinces in Indonesia with a high level of seismicity. During August 2024 alone, there have been 134 earthquakes with 23 earthquakes centered on land. The high seismicity on land is caused by the many faults on the mainland of West Java. This paper presents the results of research on fault patterns in mainland West Java from earthquake data from 1953 - 2023. The data of this research was obtained from the USGS, BMKG, GFZ, IRIS and Global CMT websites. The selected data is an earthquake with a magnitude of ≥ 5 SR and a depth of ≤ 40 km with an area boundary of 5°85 ́- 8°13 ́S and 106°74 ́- 108°76 ́ E. Using the Focal Mechanism Method, it can be found that the fault pattern that causes earthquakes in West Java in 1953-2023 is 5 reverse fault, 2 Normal fault, and 1 Strike-slip Fault. The results of this research are expected to be used as material for the preparation of earthquake disaster mitigation in West Java.

Investigation of local soil properties of Erzurum province (Eastern Türkiye) by Horizontal/Vertical Spectral ratio method

Erzurum province is a basin developed under the effect of strike-slip faults in the Eastern Anatolia region. Erzurum province is generally influenced by the left strike-slip Erzurum Fault Zone, the left- strike-slip Aşkale fault, and the Başköy-Kandilli reverse fault. It is also located approximately 80 km from the Karlıova joint, which is the intersection of the North Anatolian and East Anatolian Faults. When the earthquakes of the instrumental and historical periods are analyzed, it is seen that many damaging earthquakes of medium to large magnitude have occurred in Erzurum province. Erzurum basin is generally covered with old alluvium at the edges of the plains, while the flat areas in the central parts are covered with new alluvium. Determination of local soil properties in regions with high earthquake hazard plays an important role in reducing earthquake risks. For this purpose, single station microtremor measurements were applied at 25 sites in Palandöken and Yakutiye districts of Erzurum province. The measurements were taken for at least 30 minutes and evaluated according to the Horizontal/Vertical Spectral Ratio method. As a result of the analysis, the dominant period, H/V ratio and vulnerability index (Kg) values of the measurement points were calculated. The period values obtained vary between 0.15 s and 3.7 s, while the H/V ratios vary between 2.2 and 8.5. The Kg value obtained using these parameters is defined as the vulnerability of the soil. It is concluded that high period, high H/V and high Kg values are obtained in areas with recent alluvium and multidisciplinary analyses should be performed in soil investigations in these regions.

Interaction law between mining stress and fault activation and the effect of fault dip angle in longwall working face

Study of the interaction between fault activation and mining stress evolution in the longwall working face is helpful to provide a targeted area for fault type heavy mine pressure disaster control. Combining theoretical analysis, physical and numerical simulation, the mechanical mechanism of fault activation is analyzed, the interaction law between mining stress and fault activation is studied, and the influence of fault dip angle on the evolution of fault activation and mining stress is discussed. The minimum critical dip angles α of normal and reverse fault activation are π/4 + φ/2 and π/4-φ/2, respectively. During the mining process, the activation position of the fault surface, the peak values of stress and displacement gradually increase and transfer from the high position of the fault to the low position, and the peak value of the advance abutment pressure reaches the maximum at the fault. The advancing distance of the working face required for fault activation gradually decreases with the decrease of the fault dip angle, and the peak elevation area of the working face gradually increases with the decrease of the fault dip angle. Combined with the on-site microseismic monitoring results, it can be seen that when the working face is about 20 m away from the fault, the stress and energy increase sharply, which is the main control area of the impact disaster.

Sierra Negra, Galápagos: A resurgent-block basaltic caldera

Caldera resurgence is rare at mafic volcanoes. Here we consider the well-exposed resurgence at Sierra Negra caldera (Galápagos) to investigate how resurgence develops at a mafic system. Based on topographic and field analysis, the structure of the resurgence consists of an eastward-tilted block bounded by a fault-propagation fold activated by a steep inward-dipping reverse fault with dip decreasing toward the surface. Extension of the uplifted part of the reverse fault results from gravitational instability and is accommodated by horst-and-graben structures over a zone several hundred meters wide. Extension culminates in the main normal fault responsible for the inward tilt of the lava pile, forming a distinctive ridge. The resurgence results from spatially and temporally distinct unrest episodes, promoted by the shallow accumulation of large volumes of magma. Sierra Negra is the first documented example of piecemeal resurgence, as shown by the recent uplift episodes associated with eruptions in 2005 and 2018. The example of Sierra Negra suggests that the formation of resurgent blocks depends on the initial location of the feeding system, with non-centered feeding systems developing asymmetric (trapdoor) blocks. Finally, Sierra Negra demonstrates that mafic volcanoes without well-developed rift zones may promote resurgence when reaching a mature stage with significant amounts of cumulates, favoring shallow magma accumulation.

Research on the surface subsidence characteristics and prediction models caused by coal mining under the reverse fault

Predicting and understanding the phenomenon of surface subsidence caused by coal mining in working faces with faults are important issues for safe coal mining and efficient production. In numerical simulation experiments, it was found that the phenomenon of surface subsidence manifests when faults exist, and the degree of influence of faults with different dip angles on surface subsidence varies. This phenomenon is attributed to fault activation. According to the experimental results, the impact of faults with different dip angles on surface subsidence falls into three levels: level I for 35° faults, level II for 45° and 55° faults, and level III for 65° and 75° faults. Similarly, the relationship between the difficulty of fault activation and the dip angle of faults can be categorized as 35° faults prone to activation, 45° and 55° faults difficult to activate, and 65° and 75° faults not prone to activation. The probability integral correction model for fault mining, which integrates the surface subsidence values caused by fault-induced attenuation and the subsidence arising from separation spaces, was introduced, thereby constructing a surface subsidence prediction model. This proposed prediction model can accurately predict surface subsidence, with a root mean square error of 10.74 mm between the predicted and measured values, as validated using DInSAR results from the III 6301 working face in the Jincheng mining area.

On the origin of the Azzel-Matti circular structure (southern Algeria): Insights from remote sensing, geological and geophysical data

The Azzel-Matti circular structure (25°51′ N, 0°35’ E) is located on the Tanezrouft plateau at the boundary between the West African Craton and the Bled El Mass compartment. Its morphology exhibits a diameter of about 6.5 km, with a raised rim that stands above the surrounding terrain. This study is based on a combination of remote sensing data (Landsat-8-OLI images and Shuttle Radar Topography Mission (SRTM) images), aeromagnetic and seismic data, and field investigations in and around the Azzel-Matti circular structure to analyze its morphology and ascertain its origin. The investigation of the area surrounding the structure did not reveal any evidence of magmatism, diapirism or impact. However, it did confirm the presence of new faults of a deeper character, as highlighted by remote sensing, aeromagnetic, and seismic analysis. This suggests that tectonic deformation is a probable factor contributing to the formation of this circular structure. The morphology of the structure is explained by the counter-clockwise rotation of the strata, likely resulting from the interplay between movements along the sub-meridian faults marking the eastern and western boundaries and the ESE-WNW oblique reverse fault marking the northern boundary. These faults are coherently arranged to facilitate the counter-clockwise rotation of the strata. The movement of the faults and their depth indicate an NE-SW compressional event corresponding to the Late Paleozoic Variscan (Hercynian) orogeny. This is supported by the presence of horizontal lacustrine carbonate formations, presumably of Jurassic age, which are discordant with the Carboniferous strata, thus providing evidence for the age of the Azzel-Matti circular structure between the Moscovian and the Jurassic.

Eocene contractional deformation in the NW corner of the Arabian plate and its relation to Arabia-Eurasia collision in SE Türkiye

ABSTRACT Field evidence indicates c. 40 Ma (Early-Middle Eocene) contractional deformation in the S Amanos Mountains. Following latest Cretaceous southward ophiolite emplacement, an intra-platform basin infilled with shallow to deeper marine, dominantly carbonate sediments. Localized thrusting, reverse faulting, and folding were followed by subaerial erosion and non-marine clastic deposition, sealed by an Early Miocene marine transgression. Eocene emergence elsewhere in SE Türkiye probably also resulted from regional crustal shortening. Alternative hypotheses for collision in SE Türkiye propose latest Cretaceous, Mid-Late Eocene, Early Miocene, or Late Miocene suturing of S Neotethys. Much geological evidence in SE Türkiye and NW Iran supports Mid-Late Eocene initial continent-continent collision. Plate reconstructions suggest S Neotethys survived in this region until the Mid-Late Eocene. Biogeographical evidence points to minor Eocene mammal migration across S Neotethys, followed by Early Miocene large-scale terrestrial migrations. Seawater isotopic data suggest a major decrease in inter-Indian Ocean-Mediterranean Sea current flow during the Early Miocene. Instrumental geophysics and both numerical and analogue modelling suggest that the northern Arabian platform underthrust Eurasia (>200 km in S central Iran). In SE Türkiye, Eocene initial ‘soft’ collision was followed by Early Miocene ‘hard collision’, with crustal thickening and exhumation continuing until the Late Miocene, followed by westward ‘tectonic escape’.

High‐speed main protection for multiterminal LCC‐MMC‐UHVDC based on initial wave process comparison

AbstractThe multiterminal LCC‐MMC‐UHVDC, which employs an LCC on the rectifier side and multiple FHMMCs on the inverter side, has emerged as a cutting‐edge technology. Nevertheless, distinct disparities in their protection requirements compared to those of conventional DC grids pose notable challenges in attaining the desired attributes. This paper first derives variation patterns in the time domain of initial waves under different fault conditions. By utilizing the theoretically calculated rate‐of‐change waveform for the backward current traveling wave in an external fault scenario as a reference, a main protection relay grounded in initial wave process comparison is proposed. This approach capitalizes on the disparity observed in internal faults with the theoretical waveform. To mitigate maloperations stemming from the employment of a non‐directional start‐up criterion in reverse fault scenarios, subtle noise patterns, mimicking the theoretical waveforms, are infused into the actual waveforms. This approach averts maloperations in reverse faults and obviates the need for added delays associated with directional start‐up criteria, thereby enhancing both speed and security. Case studies demonstrate that the proposed protection offers sufficient selectivity and a resistive tolerance of 600 ohms and boasts a speed of 0.2 ms, satisfying the requirements of 800 kV UHVDC systems.

Geological inference of channel and polka-dot seismic anomalies in Yeongil bay, Pohang

The Engineering Ocean Seismic 3D system, which enables ultra-high-resolution seismic surveys on a smaller survey scale, was deployed in Yeongil Bay, Pohang, South Korea. The region is renowned for abundant shallow gas deposits and faults. Based on acoustic impedance contrast and abnormal behavior observations, two seismic anomalies termed channel and polka-dot anomalies have been identified in the seismic volume. Seismic attribute analysis based on the signal amplitude and structural characteristics reveals that these anomalies correspond to shallow biogenic gas deposits. Structural interpretation of the seismic volume revealed that the contractional deformation resulting from post-Miocene neotectonics has resulted in uplift and reverse faulting in the Pohang region, contributing to the formation of anomalies. The channel anomalies correspond to gas-saturated tidal channels that formed during eustatic sea-level changes in the post-Last Glacial Maximum period. The polka-dot anomalies are located in topographic lows and are overlain by neotectonic sediment. The different behaviors of these anomalies in a seismic volume can be attributed to the different thicknesses of overburden overlying each of the anomalies.

Caledonian reactivation and reworking of Timanian thrust systems and implications for latest Mesoproterozoic to mid-Paleozoic tectonics and magmatism in northern Baltica

Background The Trollfjorden–Komagelva Fault Zone is the southernmost thrust fault of the Timanian Orogen and extends for thousands of kilometers from northwestern Russia to northern Norway. Though there is little about its location onshore northeastern Norway, where it is mapped as a major fault system dominantly comprised of NNE-dipping thrust faults, its continuation to the west below Caledonian nappes and offshore post-Caledonian sedimentary basins remains a matter of debate. Methods The present study provides a more definitive answer about the continuation of Trollfjorden–Komagelva Fault Zone west of the Varanger Peninsula by using seismic reflection, bathymetric, topographic, and magnetic data onshore Finnmark and offshore on the Finnmark Platform. Results The present study demonstrates that the Sørøya–Ingøya shear zone represents a portion of the Trollfjorden–Komagelva Fault Zone that was folded into a NE–SW orientation and reactivated as a top-southeast thrust during the Caledonian Orogeny, while other portions of the Trollfjorden–Komagelva Fault Zone (e.g., on the Varanger Peninsula) were reactivated as strike-slip faults. The study also documents the presence of another major, NNE-dipping Timanian shear zone with a similar geometry to the Trollfjorden–Komagelva Fault Zone north of the Varanger Peninsula. Conclusions The Trollfjorden–Komagelva Fault Zone may continue offshore as a NE–SW-striking folded structure. This has the following implications: (1) the Seiland Igneous Province likely formed in a backarc setting, (2) metasedimentary rocks of the Kalak Nappe Complex deposited along the Baltican margin of the Iapetus Ocean, possibly in a late–post-Grenvillian collapse basin, (3) the Iapetus Ocean was much narrower than the several thousands of kilometers width commonly proposed, and (4) early Neoproterozoic magmatism in northern Norway is possibly related to the initial breakup of Rodinia.

Crustal seismicity and updated stress mapping in the Bolivian Orocline, Central Andes

The Central Andes region, with significant seismic activity, has distinct faulting mechanisms across its varied topography. Notably, the sub-Andean province predominantly features reverse faulting, whereas the high Andean plateau shows a predominance of normal and strike-slip faults. Despite the importance of this extensive orogenic plateau, Bolivia remains under-documented in seismic studies. We used data from recently installed seismic stations in Bolivia and regional stations in Brazil to determine 13 new focal mechanisms of shallow crustal earthquakes in Bolivia, some of them felt in major cities. Employing probabilistic full waveform moment tensor inversion and P-wave polarities, we mapped the stress distribution across the Central Andes. The main patterns of faulting mechanisms were: reverse faulting along the NW-SE trending sub-Andean belt north of the Orocline (north of Cochabamba) with NE-SW compression; reverse faulting along the N-S trending sub-Andean belt south of the Orocline (south of Santa Cruz) with ∼ E-W oriented compression; strike-slip faulting within the Eastern Cordillera with NE-SW P axes. The results indicate that the sub-Andean belt experiences compressional forces perpendicular to the front of the Andean plateau, arising from both the spreading stresses of the plateau and the broader plate-wide compression. On the other hand, the high plateau (Altiplano) is characterized by normal and strike-slip mechanisms, suggesting a dynamic equilibrium between local extensional gravitational stresses and regional compressional forces due to the Nazca plate convergence.

Nilsen Plateau, Transantarctic Mountains, Antarctica: an update on the Gondwana stratigraphy and structure

ABSTRACT A revised and updated geological map of the Nilsen Plateau and a new geological map of the nearby Mt. Hassel-Mt. Bjaaland region are presented. The Gondwana succession, overlying pre-Devonian basement rocks, consists of Permian glacial beds, post-glacial shales and siltstones, deltaic sandstones, and coal measures succeeded by Triassic flood plain deposits. Jurassic dolerite sills intrude basement rocks and the Permo-Triassic beds. High-angle reverse faults displace Permian strata at the southern end of the range, and post-Jurassic normal faulting affected the whole range.

On the Billefjorden fault zone in Garmdalen, central Spitsbergen: implications for the mapping of major fault zones during geological fieldwork and for the tectonic history of Svalbard

Background The present contribution reexamines the geometry of a segment of a presumably long-lived fault in Svalbard, the Balliolbreen Fault segment of the Billefjorden Fault Zone, along which presumably two basement terranes of Svalbard accreted in the early–mid Paleozoic after thousands of kilometers strike-slip displacement. Methods We performed structural fieldwork to Billefjorden in central Spitsbergen and interpreted satellite images. Results Field observations demonstrate that the Balliolbreen Fault formed as a top-west thrust fault in the early Cenozoic and that weak sedimentary units such as shales of the Lower Devonian Wood Bay Formation and coals of the uppermost Devonian–Mississippian Billefjorden Group partitioned deformation, resulting in significant contrast in deformation intensity between stratigraphic units. For example, tight early Cenozoic folds are localized in shales of the Wood Bay Formation and contemporaneous top-west brittle–ductile thrusts within coals of the Billefjorden Group, whereas Pennsylvanian deposits of the Hultberget (and/or Ebbadalen?) Formation are simply folded into gentle open folds. Rheological contrasts also resulted in the development of décollements locally, e.g., between tightly folded strata of the Wood Bay Formation and Billefjorden Group and flat-lying, brecciated limestone-dominated strata of the Wordiekammen Formation. Despite the limited quality and continuity of outcrops in the area, the eastward-thickening character (i.e., away from the fault) of Pennsylvanian deposits of the Hultberget, Ebbadalen, and Minkinfjellet formations suggests that the fault did not act as a normal fault in Pennsylvanian times. Conclusions The study suggests that strain partitioning of early Cenozoic Eurekan contraction alone may explain the deformation patterns in Paleozoic rock units in central Spitsbergen, i.e., that Late Devonian Svalbardian contraction is not required, and that a major segment of the Billefjorden Fault Zone formed in the early Cenozoic. The present work illustrates the crucial need for interdisciplinary approaches and composite educational backgrounds in science.

Investigating the influence of climate on the evolution of fold-and-thrust belts in Chinese Tianshan: A 2D doubly vergent numerical model approach

In most analogue and numerical modelling studies for investigating the evolutionary mechanisms of fold-and-thrust belts, the initial models are typically designed as “sandboxes” with one side fixed in the horizontal direction. However, such model setup has neglected the potential effects of drainage divide migration. To address this issue, in this study we developed a 2D doubly vergent numerical model that accounts for drainage divide migration to investigate the structural evolution in Tianshan fold-and-thrust belts since the late Cenozoic, with an emphasis on the influences of climatic forcing on rock deformation. The deformation styles of northern and southern Tianshan fold-and-thrust belts exhibit significant differences, with the northern Tianshan fold-and-thrust belt characterized by deep isoclinal anticlines, whereas the southern Tianshan fold-and-thrust belts are dominated by thrust faults and minor shallow-level tight anticlines. Our modelling results suggest that such differences in deformation style may be attributed to the north–south precipitation gradient across Tianshan mountain. The results also show that unilateral climatic perturbations can lead to migration of the drainage divides towards the side with lower precipitation, thereby impacting the structural evolution on the opposite side of the mountain range. Such impact mainly manifests as an increased tendency to develop more thrust faults in the direction toward which the drainage divide is shifting. This provides a new perspective for reevaluating the patterns of tectonic evolution in the global orogenic belts since the late Cenozoic.

Mechanism and controlling factors of mass transport complexes migration: A case study of the mass transport complexes in the taranaki deep water basin, New Zealand

Mass transport Complexes (MTCs) form significant sediment accumulations in continental slopes, hold key insights for natural hazard prediction and offshore oil exploration. This paper uses high-definition 3D seismic data to reconstruct the seismic geomorphology and sedimentary dynamics of MTCs, meticulously exploring the depositional systems of the Tanaraki Basin, New Zealand. It deciphers by kinematic notation, seismic faciess, quantifies megaclast morphological characteristics, in conjunction with the basal slope and channel structure development as the migration or kinematics of MTCs. Five seismic facies categories and dynamic traits—compression ridges, thrust faults, slides, grooves and slope terraces are distinguished in MTCs. Based on attributes maps and geomorphological interpretations, MTCs is segmented into four zones, showing combined effects of levée, basal slopes, and megaclast clusters on its migration. Lithological and topographical variations along these features modulate erosion properties and MTCs mobility, with base height shifts guiding local migration trajectories. The results of megaclast parameters in Zones 1 and 3 tune our understanding of stress patterns and directionality shifts, highlighting the complex dynamics at play. Notably, the differential motion triggered by levees instigates longitudinal shear zones. At critical migration disparities, MTCs fracture at these weak points, discharging pore pressure and filling fractures with fines, birthing “promontory” formations marked by low-amplitude fills. This work, therefore, establishes a groundbreaking migratory model that synthesizes the impacts of levees height, rock type variability, and megaclasts accumulation intensity, depicting a fragmented migration pattern. This study not only enriches our grasp of MTCs behavior in deep-water contexts but also furnishes a robust scientific foundation and predictive tool for gauging the hazards that MTCs may pose to underwater structures, thus carrying substantial theoretical and applied significance.

Transition From Reverse to Left‐Slip on the Eastern Haiyuan Fault, NE Tibetan Plateau, From the Structure and Age of the Ganyanchi Pull‐Apart Basin

AbstractBecause the active, left‐slip Haiyuan fault is a first‐order structure along the NE margin of the Tibetan Plateau, its tectonic evolution provides insight into deformation processes along this margin over time. Late Cenozoic deformation in the area of the eastern Haiyuan fault initiated as thrust faulting, followed by left‐lateral strike slip displacement. However, the time of this kinematic change has been long debated. Here we use the structural evolution and the age of the Ganyanchi basin to date the onset of this kinematic transition. Seismic reflection data, integrated with published structural mapping, indicate that the basin is a pull‐apart controlled by strands of the eastern Haiyuan fault. Previously reported magnetostratigraphy of a core from the basin interior indicates deposition started at ∼2.8 Ma. However, this age likely postdates initiation of the bounding strike‐slip faults. We estimate that an additional 0.2–1.0 Myr of lateral slip occurred before the dated section formed. Thus, we find that the age of the eastern Haiyuan fault is at least ∼3.5 Ma, which is significantly older than early estimates of less than 2 Ma. Integrating our new data with prior work reveals that the kinematic change from NE‐SW shortening to left‐slip along the Haiyuan fault youngs to the east, which we interpret to result from growth of the Haiyuan strike‐slip fault via progressive lateral propagation from west to east. Miocene‐Pliocene onset of the Haiyuan strike‐slip fault coincides with initiation ages of other major strike‐slip faults in NE Tibet, likely implying that the NE margin of the Tibetan Plateau had switched to a strike‐slip dominated mode of deformation by this time.

Crustal Thermal Architecture, Structural Reconstructions, Field Relationships, and Geophysical Data Rule Out Deep Structural Burial of the Footwall of the Northern Snake Range Metamorphic Core Complex (Nevada, USA)

AbstractThermobarometry in the Northern Snake Range metamorphic core complex (Nevada, USA) implies pre‐extensional burial of footwall rocks to 21–30 km depths, while geologic field relationships support 7–13 km pre‐extensional depths. This has fueled a 40‐year‐long debate, which has far‐reaching implications for how pressure data are interpreted in orogenic settings. Here, we test published models for deep burial by integrating regional cross‐section reconstructions with new (n = 95) and published (n = 132) peak temperature measurements, field relationships and published geophysical data. Burial of Neoproterozoic‐Cambrian metasedimentary footwall rocks to 21–30 km depths is incompatible with a regional seismic reflection cross‐section that interprets the top of Precambrian crystalline basement at 17–20 km depths. Two reconstructed cross‐sections define 42 km and 50–65 km of displacement on the master detachment fault and demonstrate that the higher displacement ranges (&gt;66–94 km and &gt;76–102 km, respectively) necessary to exhume rocks from 21 to 30 km depths are not possible without spatially overlapping Cambrian rocks preserved in its footwall and hanging wall. The 22°C/km average Late Cretaceous thermal gradient predicted by thermobarometry is incompatible with the 46 ± 10°C/km Late Cretaceous peak thermal gradient that we calculate down to 15–20 km pre‐extensional depths. Field relationships that rule out large‐magnitude shortening invalidate models for deep footwall burial via thrust or reverse faulting. We conclude that there is no scenario for deep burial that is compatible with structural/geophysical constraints, crustal thermal architecture, and field relationships. This necessitates a non‐lithostatic interpretation for pressures from the Northern Snake Range, similar to recent interpretations for other Cordilleran metamorphic core complexes.