Late Cenozoic Faults Research Articles

Large non-karst caves of the Altai-Sayan mountain region: morphology and genesis

In the Altai-Sayan mountain region, located in southern Siberia, there are a number of caves developed in rocks in which karst processes are impossible to occur. In East Sayan, these are the Big Oreshnaya and Dudinskaya caves, the total length of which is more than 50 km and 35 km, respectively. Within the Altai Mountains, these are the Altaiskaya and Kek-Tash caves with a total length of 4.7 and 3.2 km, respectively. Mapping of the listed caves revealed that the morphology of their passages and the general structure differ significantly from those of caves in carbonate rocks. The passages have a flattened shape in cross section. Their width is usually meters and tens of meters, vertical extension is tens and hundreds of meters, and lateral extension is hundreds of meters. In general, the pattern of the passages is arranged in a box-like structure, where the pasages fit into a system of disjoint extended subvertical planes with rare short subvertical planes connecting them. The structure of the passages of the studied caves resembles the structure of karst caves, rotated vertically by 90°. The main directions of the passages there are not aligned in subhorizontal planes, but in subvertical ones. The revealed structure corresponds to the geometry of the structural pattern of Late Cenozoic fault systems formed as a result of subhorizontal compression. Signs of abundant argillization (alteration of the initial substance by hydrothermal processes and its replacement by newly formed clay minerals) were found in the studied caves. The formation mechanism of the studied caves is offered: removal of the substrate argillized by hydrothermal processes along the fault zones, by groundwater without significant participation of karst processes.

Time Constraints on the Late Cenozoic Fault Evolution Along the Northern Margin of the Iranian Plateau in the Arabia‐Eurasia Collision Zone

AbstractThe collisional Iranian Plateau and its recent kinematic evolution represent a natural example to study the intraplate response to the transferred deformation from an active convergent plate margin. The late Cenozoic deformation and structural evolution of the Plateau is not well understood. Here, we integrate structural, tectonostratigraphic, and morphotectonic field observations with low‐temperature thermochronometric data along the NW‐SE trending Kushk‐e‐Nosrat (KN) Fault to unravel the exhumation history and the kinematic change at the northwestern boundary of the Iranian Plateau. We found different sets of strike‐slip related structures along the KN Fault zone, which are classified into four categories based on their cross‐cutting relations and the superimposition of kinematic indicators. These include dextral transtension, dextral, dextral transpression, and sinistral kinematics. The unreset zircon (U‐Th)/He and apatite fission track results and the reset apatite (U‐Th)/He data from the restraining area along the KN Fault suggest 80–60°C of cooling during the early Miocene (∼20–18 Ma) and late Miocene–early Pliocene (∼7–5 Ma) due to dextral and dextral transpressional kinematics along the KN Fault zone, respectively. The dextral transtentional faulting was recorded as deposition of the Qom Formation within the releasing overlap areas along the KN Fault at >20–18 Ma. The kinematics of the KN Fault changed to sinistral during Pliocene–Quaternary times presumably triggered by the simultaneous clockwise rotation of central Iran, Alborz Mountains, and the South Caspian block. Our study proposes that the morphological and tectonostratigraphic evolution of the northern margin of the Iranian Plateau has mainly been controlled through local uplift and exhumation in restraining areas and local thick deposition in releasing areas of the major strike‐slip faults during the late Cenozoic time.

New Magnetotelluric Data Reveal Deep Fault Boundaries and Contrasting Late Cenozoic Fault Kinematics Between the Qilian Shan Thrust Wedge and Beishan‐Alxa Block, Western China

AbstractThe structural connectivity and kinematic relationship between the Altyn Tagh sinistral strike‐slip fault (ATF) and Qilian Shan fold‐and‐thrust belt along the north Tibetan margin east of 96°E is an important question for tectonicists interested in the evolving active deformation field of Central Asia and associated earthquake hazards of China's Hexi Corridor region. New results from a detailed 130‐km‐long N‐S magnetotelluric (MT) survey from the Qilian Shan to Beishan elucidates the locations and down‐dip orientations of major faults. Importantly, the results indicate that the Heishan‐Jinta’Nanshan fault system roots steeply into the lower crust, is unconnected to the Qilian Shan thrust wedge, and has reactivated the margin of the North China Craton and an older, regional ductile shear belt. The structurally linked ATF‐Heishan‐Jinta’Nanshan system defines a fundamental kinematic boundary in central Asia between the NE directed Qilian Shan thrust belt to the south and the eastwardly extruding Beishan‐Alxa Block to the north.

The Influence of Basement Terranes on Tectonic Deformation: Joint Earthquake Travel‐Time and Ambient Noise Tomography of the Southern South Island, New Zealand

AbstractDetermining how the crustal structure developed in previous tectonic regimes influences current deformation is important for understanding seismogenic behavior. Paleozoic‐Mesozoic convergent‐margin tectonics produced the southeastern South Island crust, which is currently involved in late Cenozoic transpressional collision. We examine crustal velocity structure through joint inversion of earthquake travel times and ambient noise derived group velocities to increase the resolution of Vp (P‐wave velocity) and Vp/Vs across the region. This method improves the details of the shallow velocity structure and highlights lithological properties, uplifted regions, and distributed fracture zones. The Otago schist shows thick lower crust while contrasting higher Vp below 15 km depth characterizes the Matai terrane and Median batholith, both backstops during accretion and deformation. The complex velocity structure, integrated with late Cenozoic faults, structural domains, seismicity patterns, and geodynamic modeling, shows how Cenozoic structures relate to deeper properties. Seismicity within the schist is distributed within transition regions rather than along specific faults, and these zones contain low Vp and low Qp (1/attenuation). The most prominent seismicity zone is along the transition from northern Otago ranges to the Southern Alps, where faulting and brittle deformation corresponds to the eastern edge of ductile crustal thickening above the bending of the strong oceanic lower crust. The thick weak crust rheology of Otago, relative to Canterbury, enables broad deformation, with numerous late Cenozoic faults that may not persist to great depth. Along the southern stronger plutonic terranes, there is a band of sparse deeper (15–35 km) seismicity, consistent with strong elastic brittle crust.

Late Cenozoic faulting deformation of the Fanshi Basin (northern Shanxi rift, China), inferred from palaeostress analysis of mesoscale fault-slip data

AbstractThe Fanshi Basin is one of the NE–SW-striking depocentres formed along the northern segment of the fault-bounded Shanxi rift. In order to understand the crustal driving stresses that led to the basin formation and development, a palaeostress analysis of a large quantity of fault-slip data collected mainly at the boundaries of the basin was accomplished. The stress inversion of these data revealed three stress regimes. The oldest SR1 was a Neogene stress regime giving rise to a strike-slip deformation with NE–SW contraction and NW–SE extension. SR1 activated the large faults trending NNE–NE, i.e. (sub)parallel to the main strike of the Shanxi rift, as right-lateral strike-slip faults. It was subjected to the Shanxi rift before the activation of the Fanshi Basin boundary fault, i.e. the Fanshi (or Wutai) fault, as a normal fault. The next is a short-lived NE–SW extensional stress regime SR2 in early Pleistocene time, which shows the inception of the basin’s extension. A strong NW–SE to NNW–SSE extensional stress regime SR3 has governed the northern segment of the Shanxi rift since late Pleistocene time and is the present-day extension. It gave rise to the current half-graben geometry of the Fanshi Basin by activating the Fanshi (or Wutai) fault as a normal fault in the southern part of the graben. Because of the dominance of the NW–SE to NNW–SSE extension, which is perpendicular to the NE–SW extension, mutual permutations between σ3and σ2due to inherited fault patterns might have occurred while the crustal stresses in the Fanshi Basin changed from the SR1 to SR3 stress regimes.

Geologic map of Slate Range Crossing area, California, USA

This detailed geologic map and supplemental digital data set1 examine and demonstrate the complex Neogene–Quaternary deformation in the Slate Range Crossing area (California, USA) of the active dextral transtension of the Death Valley region and Walker Lane belt. This map integrates the late Cenozoic structures and geologic units with the Mesozoic geologic units and deformation as a data set to examine the controls on reactivation of older structures. These geologic data were collected to study pre-, syn-, and post-kinematic rocks to examine the deformation history of the area and to find palinspastic markers to examine the late Cenozoic fault displacement and displacement history across Panamint Valley to the east, as reported in Andrew and Walker (2009). The study focused on defining the Miocene and Pliocene rocks and deposits and examining lateral changes and depositional sources of clasts. There are two different volcanic-sedimentary sequences in this area. A Miocene section contains mafic to felsic volcanic units, numerous debris-flow to laharic deposits, and several associated conglomerates and breccias containing exotic clasts. The exotic clasts are matched to rocks in the Panamint Range on the east side of Panamint Valley as reported in Andrew and Walker (2009) as displacement vectors for palinspastic reconstructions. These Miocene strata ubiquitously dip eastward 20–40º. A younger volcanic-sedimentary sequence contains relatively thin mafic lava flows and associated locally derived, coarse-grained mass wasting deposits. These younger basaltic lavas generally have gentle dipping lava flow features and foliation. Numerous faults cut the different age deposits allowing a chronology of Neogene to Quaternary faulting; additionally, there are numerous fabrics associated with Jurassic contraction and Cretaceous(?) dextral shear. The area near Slate Range Crossing has a conspicuous zone of earthquake foci; this study found that some of this seismic activity coincides with a zone of southwest-striking, moderately dipping to the north, sinistral-oblique normal faults, which cut across the northernmost Slate Range. These faults form a structural boundary between the Argus and Slate Ranges and link the fault networks in Panamint Valley with those in Searles Valley. This mapping and structural data demonstrate the two-stage Neogene fault history of the Walker Lane belt deformation in this area and show that regional tilting of rocks occurred after ca. 13 Ma and before ca. 4 Ma; this eastward down-tilting appears to be a discrete event and may mark the change from extension to transtension. This detailed geologic mapping and collection of structural data for the rocks in the eastern Argus and northern Slate Ranges and Panamint Valley were created using digital in-the-field geographic information systems software running on a field-hardened laptop computer. This map is a simplification of detailed geologic mapping data collected at 1:6000 scale and reduced to 1:20000 scale. Structural data includes kinematic and relative timing of deformation information.

Straight to Low-Sinuosity Drainage Systems in a Variscan-Type Orogen—Constraints from Tectonics, Lithology and Climate

A holistic-modular approach has been taken to study the evolution of three straight to low-sinuosity drainage systems (=SSS) in an uplifted basement block of the Central European Variscides. The development of the SSS is described by means of a quadripartite model. (1) The geological framework of the SSS: Forming the lithological and structural features in the bedrock as a result of different temperature, pressure and dynamic-metamorphic processes. (2) Prestage of SSS: Forming the paleo-landscape with a stable fluvial regime as a starting point for the SSS. (3) Proto-SSS: Transition into the metastable fluvial regime of the SSS. (4) Modern SSS: Operation of the metastable fluvial regime Tectonics plays a dual role. Late Paleozoic fold tectonic creates the basis for the studied SSS and has a guiding effect on the development of morphotectonic units during the Neogene and Quaternary. Late Cenozoic fault tectonics triggered the SSS to incise into the Paleozoic basement. The change in the bedrock lithology has an impact on the fluvial and colluvial sediments as well as their landforms. The latter reflects a conspicuous modification: straight drainage system ⇒ higher sinuosity and paired terraces ⇒ hillwash plains. Climate change has an indirect effect controlling via the bedrock the intensity of mechanical and chemical weathering. The impact on the development of the SSS can be assessed as follows: Tectonics >> climate ≅ bedrock lithology. The three parameters cause a facies zonation: (1) wide-and-shallow valley (Miocene), (2) wide-angle V-shaped valley (Plio-Pleistocene), (3) acute-angle V-shaped valley (Pleistocene), (4) V-shaped to U-shaped valleys (Pleistocene-Holocene). Numerical data relevant for the hydrographic studies of the SSS are determined in each reference area: (1) Quantification of fluvial and colluvial deposits along the drainage system, (2) slope angles, (3) degree of sinuosity as a function of river facies, (4) grain size distribution, (5) grain morphological categorization, (6) grain orientation (“situmetry”), (7) channel density, (8) channel/floodplain ratios. Thermodynamic computations (Eh, pH, concentration of solubles) are made to constrain the paleoclimatic regime during formation of the SSS. The current model of the SSS is restricted in its application to the basement of the Variscan-Type orogens, to an intermediate crustal maturity state.

Kinematic variability of late Cenozoic fault systems and contrasting mountain building processes in the Alxa block, western China

Abstract In this study, new structural field data and geological observations are coupled with remote sensing analysis and compilation of previous published work to establish a new kinematic framework for the neotectonic reactivation of the Alxa block, China. The Alxa block occupies a key position in Central Asia north and east of the contractional and sinistrally transpressive deforming zones of the Qilianshan-Hexi Corridor-easternmost Altyn Tagh Fault and Beishan deforming belts, south of the sinistrally transpressive southernmost Gobi Altai, and west of – or overlapping with - Ordos block extensional and transtensional graben systems. In addition, the Alxa block is internally transected by important fault systems (Bayan Nuru, Bayanwula, Chahanbulage) that record previously unrecognized Late Cenozoic activity. Active mountain building processes in the Alxa block show remarkable variation from large extensional horst blocks (Helanshan, SW Langshan, SW Yabrai) to localized uplifts along sinistral wrench-belts (Bigesitai) to dominantly contractional (Longshoushan) and sinistral transpressional ranges (Helishan). The triangular wedge-shape of the Alxa block likely pre-disposed it to eastward extrusion in response to NE-directed India-Eurasia collision-derived compressional stresses from the south, and E-W tensional forces driven by Pacific margin rollback to the east. A combination of inherited regional lithospheric structure and the modern east-directed geodetically derived crustal velocity field led to: 1) crustal thinning and subsidence of the Ejina basin to the west in the wake of the eastward extruding Alxa block, and 2) distributed sinistral transtension and crustal extension within the Alxa block interior, and in the Hetao and Yinchuan grabens to the east.

New insights on Franciscan Complex geology, architecture, depositional age, and provenance for the western Mt. Tamalpais area, Marin County, California

ABSTRACT New geologic mapping of the Franciscan Complex in the western Mt. Tamalpais area northwest of San Francisco, California, reveals a tectonostratigraphic stack of accretionary units that young westward and structurally downward. Serpentinite-matrix mélange at the top of the section, beneath a possible basal fragment of Coast Range Ophiolite, and a structurally low black-rock-bearing, mudrock-matrix Ocean Plate Stratigraphy (OPS) mélange located near the base of the section, bracket lithic and feldspathic metasandstone-dominated accretionary units. The ages of the metasedimentary accretionary units are constrained by U-Pb age dating of detrital zircons. Maximum depositional ages fall between 124 Ma and 88 Ma. Sediment provenance is dominantly in the Sierra Nevada arc to the east, but minor numbers of detrital zircons were derived from Paleozoic and Precambrian sources. The provenance age profiles (cumulative distribution plots) reveal changing age components of zircon populations over time, reflecting the history of unroofing in the Sierra Nevada arc. An unusual post-accretion thrust fault and atypical greenschist facies metamorphism uncharacteristic of the Franciscan Complex, plus probable Late Cenozoic faults, complicate the character of the geology of the Mt. Tamalpais area.

Active deformation to the north of the Altyn Tagh Fault: Constraints on the northward growth of the northern Tibetan Plateau

Sinistral transpressive deformation occurs in the region north of the Altyn Tagh fault due to the Indo-Eurasian collision. In this region, several late Cenozoic faults have developed, and the Sanweishan fault is the key feature with clear landforms. In this study, slip rate measurements combined with several dating methods imply that the Sanweishan fault has been active with sinistral strike-slip rates of 0.33 ± 0.04 mm/a since 14 ka BP and 0.28 ± 0.03 mm/a since 20 ka BP, which are more than an order of magnitude smaller than that of the ATF to the south. The horizontal displacements are 1.7–5.5 m in the eastern segment and 25.0–76.0 m in the middle and western segments of the Sanweishan fault. The increment of horizontal displacement along the main Sanweishan fault is 3.0–4.0 m between paleoearthquakes, and the magnitude of the latest paleoearthquake on the middle and eastern segments of the fault is up to M = 7.2–7.5. The whole length of the Sanweishan fault is ~360 km or longer, which include active faults in the Yangguan and Danghe Reservoirs in the west. More components of thrusting occur on the western and eastern segments of the Sanweishan fault. The growth of the Sanweishan fault is similar to that of the ATF, but the former fault is younger, and its slip rate is much smaller. The differential slip rates on the Sanweishan fault and ATF may result in the clockwise rotation and eastward movement of the intervening block, which compresses regions to the northeast.

Dynamics of the Zones of Strong Earthquake Epicenters in the Arctic–Asian Seismic Belt

Our comprehensive study of the Russian Arctic region aims to clarify the features and types of seismotectonic deformation of the crust in the Arctic–Asian Seismic Belt, specifically in the zones of strong earthquakes in the Laptev Sea Segment, the Kharaulakh Segment, and the Chersky Seismotectonic Zone. We have analyzed modern tectonic structures and active fault systems, as well as tectonic stress fields reconstructed by tectonophysical analysis of the Late Cenozoic faults and folds. The investigated neotectonic structures are ranked with respect to the regional classification principles. Changes in the crustal stress–strain state in the lithospheric plate boundaries between the Eurasian, North American, and Okhotsk Sea Plates are analyzed, and regularities of such changes are discovered. A set of models has been constructed for the studied segments of plate boundaries with account of the dynamics of the regional geological structures. The models can give a framework for the assessment of potential seismic risks of seismogenerating structures in the Russian Arctic region.

Neotectonics of the Kharaulakh sector of the Laptev Shelf

Seismotectonic deformation and crustal stress pattern have been studied comprehensively in major seismogenic structures of the Kharaulakh sector of the Verkhoyansk fold system and adjacent parts of the Chersky seismotectonic zone. The study focuses on neotectonic structures, deep structure, and systems of active faults, as well as tectonic stress fields inferred by tectonophysical analysis of Late Cenozoic faults and folds. The results, along with geological and geophysical data, reveal main strain directions and structural patterns of crustal stress and strain in the Arctic segment of the Eurasia-North America plate boundary. The area is a junction of mid-ocean and continental structures evolving in a mixed setting of extension, compression, and their various combinations. The rotation pole of the two plates is presumably located near Buor-Khaya Bay. In this case, extension is expected to act currently upon the neotectonic structures north of the bay and compression to control those in the south and southeast. This inference is consistent with the identified zoning of stress and strain in the Kharaulakh sector.

Neotectonic activity and parameters of seismotectonic deformations of seismic belts in Northeast Asia

The Arctic-Asian and Okhotsk-Chukotka seismic belts bordering the Kolyma-Chukotka crustal plate are the subject of our study aimed at reconstructing the stress–strain state of the crust and defining the types of seismotectonic deformation (STD) in the region. Based on the degrees of activity of geodynamic processes, the regional principles for ranking neotectonic structures were constrained, and the corresponding classes of the discussed neotectonic structures were substantiated. We analyzed the structural tectonic positions of the modern structures, their deep structure parameters, and the systems of active faults in the Laptev, Kharaulakh, Koryak, and Chukotka segments and Chersky seismotectonic zone, as well as the tectonic stress fields revealed by tectonophysical analysis of the Late Cenozoic faults and folds. From the earthquake focal mechanisms, the average seismotectonic strain tensors were estimated. Using the geological, geostructural, geophysical and GPS data, and corresponding average tensors, the directions of the principal stress axes were determined. A regularity in the changes of tectonic settings in the Northeast Arctic was revealed.

Late Cenozoic faulting and the stress state in the south-eastern segment of the Siberian platform

We have studied the structural geology and geomorphology of the fault zones in the junction area of the Angara-Lena uplift and the Predbaikalsky trough. We have analyzed faults and folds and reconstructed paleostresses for this junction area named the Irkutsk amphitheatre. Our study shows that syn-fold (Middle Paleozoic) faults include thrusts, reverse faults and strike-slip faults with reverse components, that occurred due to compression from the neighbouring folded region. Recently, contrary to compression, faulting took place under the conditions of extension of the sedimentary cover: most of these recent faults have been classified as normal faults. In the Late Cenozoic, the platform cover was subjected to brittle and partly plicative deformation due to the NW–SE-trending extension that is most clearly observed in the adjacent Baikal rift. Thus, the divergent boundary between the Siberian block of the North Eurasian plate and the Transbaikalia block of the Amur plate is a zone of dynamic influence, which occupies the area considerably exceeding the mountainous region on the Siberian platform. Important factors of faulting are differentiated vertical movements of the blocks comprising the platform. Such vertical movements might have been related to displacements of brine volumes. In the Late Cenozoic basins, movements along separate faults took place in the Late Pleistocene – Holocene.

Recent structures and tectonic regimes of the stress–strain state of the Earth’s crust in the northeastern sector of the Russian Arctic region

A comprehensive investigation aimed at determining seismotectonic types of destruction and the stress–strain state of the Earth’s crust in the main seismogenerating structures of the Arctic–Asian seismic belt is conducted for the territory of the northeastern sector of the Russian Arctic region. Based on the degree of activity of geodynamical processes, the regional principles for ranking neotectonic structures are elaborated, and neotectonic zoning is carried out based on the substantiated differentiation of the corresponding classes. Within the limits of the Laptev Sea, Kharaulakh, and Lena-Anabar segments, we analyzed I the structural–tectonic position of the most recent structures, II the deep structure parameters, III the parameters of the active fault system, and IV the parameters of the tectonic stress field, as revealed from tectonophysical analysis of Late Cenozoic fault and fold deformations. Based on the seismological data, the mean seismotectonic deformation tensors are calculated to determine, in combination with geological and geophysical data, the orientations of the principal stress axes and to reveal the structural–tectonic regularity for tectonic regimes of the stress–strain state of the Earth’s crust in the Arctic sector of the boundary between the Eurasian and North American lithospheric plates.

Cenozoic faults and faulting phases in the western Tarim Basin (NW China): Effects of the collisions on the southern margin of the Eurasian Plate

The Bachu Rise in the western Tarim Basin is the fore-bulge of the Kunlun late Cenozoic intra-continental foreland basin system formed under the far-field effect of India-Asia collision. Cenozoic faults and faulting are abnormally developed in the Bachu Rise and its adjacent area. Taking the Niaoshan-Gudongshan area on the southern boundary of the Bachu Rise as the key study area, 5 Cenozoic faulting phases were identified in the Bachu Rise and its adjacent area after careful seismic interpretation. They are end Cretaceous∼beginning Paleogene (ca. 65Ma) décollement-thrusting, end Paleogene∼beginning Neogene (ca. 23Ma) décollement-thrusting, end Miocene∼beginning Pliocene (ca. 5Ma) basement-involved thrusting, late Pliocene∼early Pleistocene (ca. 3–2Ma) normal faulting, middle Pleistocene∼Holocene (ca. <1.5Ma) décollement-thrusting and strike-slip faulting. The Middle Cambrian and Paleogene gypsum-salt layers serve as the two main décollement layers in the study area. Thrusting of ca. 65Mawas under the far-field effect of the collision between Lhasa (part of the Cimmerian Continent) and Asia; and the other 4 Cenozoic faulting phases were all under the far-field effect of the India-Asia collision. The late Cenozoic faulting is characterized by pulse thrust. There is one tectonic pause between each two successive thrust pulses. The compressive tectonic stress is weaker and even evolved into a slight tensional tectonic stress and forms normal fault in the tectonic pauses.

Dynamics of seismogenerating structures in the frontal zone of the Kolyma–Omolon superterrane

To develop a model for the dynamics of seismogenerating structures in the frontal zone of the Kolyma–Omolon superterrane (Chersky seismotectonic zone), the following aspects are analyzed: structural–tectonic position, deep structure parameters, active faults, and fields of tectonic stresses as revealed from solutions of focal mechanisms of strong earthquakes and kinematic types of Late Cenozoic fold deformations and faults. It is found that a certain dynamic setting under transpressional conditions takes place and it was caused by the interaction between structures of the Eurasian, North American, and Okhotsk lithospheric plates within regional segments of the Chersky zone (Yana–Indigirka and Indigirka–Kolyma). These conditions are possible if the Kolyma–Omolon block located in the frontal zone of the North American Plate was an indenter. Due to this, some terranes of different geodynamic origin underwent horizontal shortening, under which particular blocks of segments were pushed out laterally along the orogenic belt, on a system of conjugated strike-slip faults of different directions and hierarchical series, in the northwest and southeast directions, respectively, to form the main seismogenerating reverse-fault and thrust structures with the maximum seismic potential (M ≥ 6.5).

Morphotectonics and paleoseismology of the eastern end of the Bolnay fault (Mongolia)

The Bolnay (Hangayn) fault is an active shear system which generated the M = 8.2-8.5 Bolnay earthquake of 23 July 1905, one of world’s largest recorded intracontinental event. The fault follows the Mesozoic suture formed during the closure of the Mongolia-Okhotsk ocean. The Late Cenozoic faulting in the region was induced by propagation of strain from the India-Eurasia collision that had reached Mongolia at about 5 ± 3 Ma. The left-lateral strike slip almost all over the fault length is compensated in its western end by Late Quaternary reverse motion. We estimated coseismic slip associated with the event of 1905 and the previous earthquakes in the eastern fault end and checked whether vertical offset compensates the strike slip in this part as well. The 1905 coseismic slip measured from a displaced dry stream bed and pebble bars in the Hasany-Gol river valley was 6.5-7.5 m. The 13 ± 1 m left-lateral displacement of pebble bars in the same valley represents a cumulative slip of two events. Paleoseismological studies across the strike of surface ruptures reveal at least two generations of rupture in two events that postdated the deposition of sediments with a 14C age of 4689 ± 94 yr. Hypsometry of the alluvial surface in the zone of deformation shows gradual elevation increase toward the mountains, but without abrupt change across the fault. This means the absence of vertical offset and reactivation of the fault as a left-lateral strike slip. The horizontal slip in the eastern extension of the Bolnay fault is compensated rather by parallel fault-bounded pull-apart basins trending northeastward oblique to the principal fault strike. The age of their sedimentary fill suggests no older than middle Pleistocene normal faulting that compensated the Bolnay strike slip.

Late Cenozoic faulting in SW Bulgaria: Fault geometry, kinematics and driving stress regimes. Implications for late orogenic processes in the Hellenic hinterland

We investigate the geometry and kinematics of the faults exposed in basement rocks along the Strouma River in SW Bulgaria as well as the sequence of faulting events in order to place constraints on the Cenozoic kinematic evolution of this structurally complex domain. In order to decipher the successive stress fields that prevailed during the tectonic history, we additionally carried out an analysis of mesoscale striated faults in terms of paleostress with a novel approach. This approach is based on the P–T axes distribution of the fault-slip data, and separates the fault-slip data into different groups which are characterized by kinematic compatibility, i.e., their P and T axes have similar orientations. From these fault groups, stress tensors are resolved and in case these stress tensors define similar stress regimes (i.e., the orientations of the stress axes and the stress shape ratios are similar) then the fault groups are further unified. The merged fault groups after being filled out with those fault-slip data that have not been incorporated into the above described grouping, but which present similar geometric and kinematic features are used for defining the final stress regimes. In addition, the sequence of faulting events was constrained by available tectonostratigraphic data.Five faulting events named D1, D2, D3, D4 and D5 are distinguished since the Late Oligocene. D1 is a pure compression stress regime with σ1 stress axis trending NNE-SSW that mainly activated the WNW-ESE to ENE-WSW faults as reverse to oblique reverse and the NNW-SSE striking as right-lateral oblique contractional faults during the Latest Oligocene-Earliest Miocene. D2 is a strike-slip − transpression stress regime with σ1 stress axis trending NNE-SSW that mainly activated the NNW-SSE to N-S striking as right-lateral strike-slip faults and the ENE-WSW striking faults as left-lateral strike-slip ones during the Early-Middle Miocene. D3 extensional event is associated with a NW-SE to WNW-ESE extension causing the activation of mainly low-angle normal faults of NE-SW strike and NNE-SSW to NNW-SSE striking high-angle normal faults. D4 is an extensional event dated from Late Miocene to Late Pliocene. It activated NNW-SSE to NW-SE faults as normal faults and E-W to WNW-ESE faults as right-lateral oblique extensional faults. The latest D5 event is an N-S extensional stress regime that dominates the wider area of SW Bulgaria in Quaternary times. It mainly activated faults that generally strike E-W (ENE-WSW and WNW-ESE) normal faults, along which fault-bounded basins developed. The D1 and D2 events are interpreted as two progressive stages of transpressional tectonics related to the late stages of collision between Apulia and Eurasia plates. These processes gave rise to the lateral extrusion of the Rhodope and Balkan regions toward the SE along the Strouma Lineament. The D3 event is attributed to the latest stage of this collision, and represents the relaxation of the overthickened crust along the direction of the lateral extrusion. The D4 and D5 events are interpreted as post-orogenic extensional events related to the retreat of the Hellenic subduction zone since the Late Miocene and to the widespread back-arc Aegean extension still prevailing today.

Andean earthquakes triggered by the 2010 Maule, Chile (Mw 8.8) earthquake: Comparisons of geodetic, seismic and geologic constraints

The Maule, Chile, (Mw 8.8) earthquake on 27 February 2010 triggered deformation events over a broad area, allowing investigation of stress redistribution within the upper crust following a mega-thrust subduction event. We explore the role that the Maule earthquake may have played in triggering shallow earthquakes in northwestern Argentina and Chile. We investigate observed ground deformation associated with the Mw 6.2 (GCMT) Salta (1450 km from the Maule hypocenter, 9 h after the Maule earthquake), Mw 5.8 Catamarca (1400 km; nine days), Mw 5.1 Mendoza (350 km; between one to five days) earthquakes, as well as eight additional earthquakes without an observed geodetic signal. We use seismic and Interferometric Synthetic Aperture Radar (InSAR) observations to characterize earthquake location, magnitude and focal mechanism, and characterize how the non-stationary, spatially correlated noise present in the geodetic imagery affects the accuracy of our parameter estimates. The focal mechanisms for the far-field Salta and Catamarca earthquakes are broadly consistent with regional late Cenozoic fault kinematics. We infer that dynamic stresses due to the passage of seismic waves associated with the Maule earthquake likely brought the Salta and Catamarca regions closer to failure but that the involved faults may have already been at a relatively advanced stage of their seismic cycle. The near-field Mendoza earthquake geometry is consistent with triggering related to positive static Coulomb stress changes due to the Maule earthquake but is also aligned with the South America-Nazca shortening direction. None of the earthquakes considered in this study require that the Maule earthquake reactivated faults in a sense that is inconsistent with their long-term behavior.