Brittle Normal Faults Research Articles

Morphotectonics, slope stability and paleostress studies from the Bhagirathi river section, western Himalaya (Uttarakhand, India)

We study parts of Tethyan, Higher and Lesser Himalayan rocks along the Bhagirathi river valley for morphotectonic analysis. The spatial and linear properties of the 21 watersheds (WSs) and the Bhagirathi main watershed provide strong evidence of active tectonics mainly in the WS 3 (through which the South Tibetan Detachment passes), WS 9 (no fault runs), WS 12 (Vaikrita and Munsiari Thrusts cross) and WS 17 (Basul and Tons Thrusts occur). The Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) has been used to rank the sub-watersheds as per the intensity of their recent tectonic activity. Seven morphometric parameters are used for the TOPSIS analysis. From the Lesser Himalayan section additionally, we perform landslide and paleostress studies. Eleven slopes cuts and 24 landslides were investigated to determine the mode of failure in a portion of the Rishikesh-Gangotri Highway. Landslides in soil strata is caused mainly by the low cohesion and due to the presence of coarse-grained loose materials. In the present study, most landslides (and earthquakes) have occurred in the vicinity of major thrusts. Where there is a high frequency of slickenside related to brittle normal faulting (K2 zone, near Dunda, Singuni and Dharasu Thrusts), a higher earthquake frequency of 4.0 - 6.6 magnitude is observed from the data set of around last 60 years. Paleostress analysis on data-sets of normal, reverse and strike-slip movements using the WinTensor software (ver. 5.8.8) yields NNE-SSW direction of extension for normal slip, NE-SW compression for reverse movement, and a pure strike-slip tensor with NNE-SSW shortening and WWN-SSE direction of maximum extension. The K2 zone where these deformations were most documented, is also the place of slope instability and a much higher present-day tectonic activity. (Words: 283)

Polyphase deformation-controlled giant Renli Nb–Ta deposit, South China: Constraints from systematic structural analysis

The Early Cretaceous Renli Nb–Ta deposit was recently discovered near the southern margin of the Late Jurassic–Early Cretaceous Mufushan composite granitic batholith, central Jiangnan Orogenic Belt, and represents the largest known rare metal deposit in South China. Substantial breakthroughs have been made in prospecting the pegmatite-dike-type of Nb–Ta mineralization in the deposit, especially from the perspective of geochemical exploration. However, there is still a lack of systematic structural studies in this area which has seriously hindered the in-depth understanding of the emplacement of pegmatite dikes and the distribution patterns of Nb–Ta ore bodies. In order to shed new light on this key issue, this study presents new and detailed structural data from the Mufushan region. The results indicate that the Renli and adjacent areas have experienced several deformational events, including D1 Neoproterozoic NE–SW contraction and upright-folding, D2 latest Late Jurassic to Early Cretaceous regional extension accompanied by granitic magma intrusion, and D3 Early Cretaceous brittle normal faulting and fracturing with granitic magma emplacement and Nb–Ta mineralization. The D1 event, corresponding to the Jiangnan Orogeny, formed regional-scale NW–SE-striking upright folds with ubiquitous subvertical S1 cleavages and subhorizontal L1 crenulation lineations. The D2 event represented orogenic exhumation and produced a domal structure cored primarily by gneissic/mylonitic rocks and granites and mantled by schists, which dominated the bulk structural architecture of the Mufushan area. This was followed by the D3 event which controlled the formation of numerous extension fractures in the study area. The Nb–Ta-bearing magmatic-hydrothermal fluids migrated along the pre-existing D2 shear zone and subsequently filled in the widespread D3 extension fractures to form pegmatite dikes. The D2 to D3 phases can be considered as a progressive exhumation event from plastic to brittle regimes. The D2 and D3 structures dictate the spatial distribution of Nb–Ta ore bodies.

Understanding deformation band formation across a range of length scales in the Paradox Basin, Utah

In evaporite basins, salt flow can drive complex deformation of high porosity sedimentary units adjacent to salt and this has been demonstrated by recent numerical modeling studies, however field evidence of this deformation has remained elusive. Detailed analysis of high-porosity wall and cover rocks adjacent to a large salt-cored anticline in Salt Valley, UT yields three observable deformation band gradients. A long-wavelength gradient of steeply dipping normal/shear bands is recognized along structural transects up to 5 km away from the Salt Valley anticline axis. A shorter-wavelength (5–60 m) deformation band gradient shows band density increasing toward similarly-oriented brittle normal faults. A third gradient of intense compactional deformation banding is recognized adjacent to the salt diapir. Importantly, cement phases are undamaged in these bands, indicating that they likely experienced plastic critical-state deformation prior to cementation. The steeply-dipping deformation bands and normal faults are interpreted to be broadly contemporaneous and result from outer arc extension of the Salt Valley anticline; the dense compactional band network adjacent to salt likely accommodated compression during diapiric rise contemporaneous with folding. This integration of field and petrologic studies with forward modeling makes a critical step toward developing predictive models for deformation band formation in hydrocarbon systems.

The 2022 Twin Earthquakes and Tectonic Environment in Kyaing Tong Basin, Eastern Myanmar

The first earthquake of magnitude 4.8 struck at 57 km south of Kyaing Tong, latitude 21.1141°N longitude 99.850°E, on 21st July 2022, at 16:40:08 UTC ( 11:07 a.m. local time ) at the depth of 10 km. Immediately after 30 minutes on the same day of 21st July 2022, the second earthquake with magnitude 5.9 occurred, 53 km south of Kyaing Tong at latitude 21.150°N 99.863°E, 17:07:26 UTC (11:37 a.m. local time) at 10 km depth. Focal mechanism solution for the first event was left-lateral strike-slip faulting and the second by normal faulting with a component of strike-slip component (United States Geological Survey). Many lesser aftershocks followed the main shocks and continued since July and still ongoing up to date in 2022. Twin earthquakes ruptured along the Wan Ha fault which bounds the southeast side of the Kyaing Tong basin, eastern Myanmar. The rupture propagated southward. The Kyaing Tong basin is one of the tectonic basins in the Paleo-Tethys suture zone in eastern Myanmar. These earthquakes are due to the movement of Wan Ha fault, affected the town Kyaing Tong where it caused some houses and schools collapsed. No casualty was reported. According to data from USGS, and other earthquake agencies, there is a clear increase in detected earthquakes, particularly smaller quakes of magnitude around ≥ 1.0 and ≥ 2.0 at shallower depth of 1000 meter. From the studies of satellite images, aerial photographs, shaded relief map and Digital Elevation Model map, several lineaments are visible and detailed mapping of these lineaments show the strike-slip faults in ENE-WSW direction and normal faults in NE-to NNE-striking direction. NW-SE extension is evident by the occurrence of brittle normal faults and NE-to ENE-oriented sinistral strike-slip fault. Within this zone is the trace of regional compression in a NE-SW direction. This region is presently undergoing active tectonic deformation, as indicated by current seismicity and earthquake focal mechanism solution.

The low-angle breakaway system for the Northern Snake Range décollement in the Schell Creek and Duck Creek Ranges, eastern Nevada, USA: Implications for displacement magnitude

AbstractDocumenting the kinematics of detachment faults can provide fundamental insights into the ways in which the lithosphere evolves during high-magnitude extension. Although it has been investigated for 70 yr, the displacement magnitude on the Northern Snake Range décollement in eastern Nevada remains vigorously debated, with published estimates ranging between <10 and 60 km. To provide constraints on displacement on the Northern Snake Range décollement, we present retrodeformed cross sections across the west-adjacent Schell Creek and Duck Creek Ranges, which expose a system of low-angle faults that have previously been mapped as thrust faults. We reinterpret this fault system as the extensional Schell Creek Range detachment system, which is a stacked series of top-down-to-the-ESE brittle normal faults with 5°–10° stratigraphic cutoff angles that carry 0.1–0.5-km-thick sheets that are up to 8–13 km long. The western portion of the Schell Creek Range detachment system accomplished ~5 km of structural attenuation and is folded across an antiformal culmination that progressively grew during extension. Restoration using an Eocene unconformity as a paleohorizontal marker indicates that faults of the Schell Creek Range detachment system were active at ~5°–10°E dips. The Schell Creek Range detachment system accommodated 36 km of displacement via repeated excision, which is bracketed between ca. 36.5 and 26.1 Ma by published geochronology. Based on their spatial proximity, compatible displacement sense, overlapping deformation timing, and the similar stratigraphic levels to which these faults root, we propose that the Schell Creek Range detachment system represents the western breakaway system for the Northern Snake Range décollement. Debates over the pre-extensional geometry of the Northern Snake Range décollement hinder an accurate cumulative extension estimate, but our reconstruction shows that the Schell Creek Range detachment system fed at least 36 km of displacement eastward into the Northern Snake Range décollement.

Active tectonics and volcanism in the southernmost Okinawa Trough back-arc basin derived from deep-towed sonar surveys

The southernmost Okinawa Trough back-arc basin is an active and young basin formed just after the collision of the Philippine Sea Plate against the Eurasian continental margin. The back-arc extension occurs intensively because of the southward or southeastward migration of the southernmost Ryukyu Arc, or the roll-back of the Philippine Sea Plate. To better understand the active tectonics and volcanism of the southernmost Okinawa Trough, we have conducted deep-tow sub-bottom profiler and side-scan sonar surveys across the back-arc basin. Our results show that the volcanism of the southernmost Okinawa Trough is distributed in the southern half of the back-arc basin and occurs along some linear or branched zones roughly parallel to the trough axis. Volcanic seamounts are obviously located along the central depression of the basin and their sizes show lateral variation. On the other hand, the northern half of the southernmost Okinawa Trough back-arc basin has almost no volcanic activity, but contains more brittle normal faults. It is noted that gas plumes out of seafloor are generally associated with hydrothermal mounds or activities, instead of volcanic seamounts. We suggest that the more complete rifting of the southernmost Okinawa Trough back-arc is limited to the east of ~122o30′E. To the west of ~122o30’E, the back-arc extension could be still influenced by the inherited NE-SW trending structures of the continental crust created during the former Taiwan orogeny in this area.

New Timing and Depth Constraints for the Catalina Metamorphic Core Complex, Southeast Arizona

AbstractThe Santa Catalina‐Tortolita‐Rincon Mountains of Southeast Arizona are a classic metamorphic core complex (MCC) and represent footwall exposures of crustal rocks exhumed by a detachment system. This study presents new evidence for the formation of the majority of ductile deformation during the Eocene (~46 Ma), synchronous with the emplacement of the regionally significant Wilderness Sills Suite (57–45 Ma). The evidence is provided by Eocene U‐Pb ages of syn‐ to late kinematic dikes emplaced in the principal ductile mylonitic fabric of the Catalina forerange, earlier than the brittle normal fault system and the formation of Tucson basin beginning with the latest Oligocene. Well‐documented shear sense indicators may not reflect extension at that time (Eocene), but more likely the direction of crustal flow now rotated during later extension. Muscovite‐plagioclase Rb‐Sr isochron ages of three mylonitic rocks are all clustered around 34 Ma, which is inferred to be the last age when these rocks were being deformed under ductile conditions following the emplacement of the Wilderness Sills Suite and various related dikes. Biotite‐plagioclase Rb‐Sr ages on the same rocks demonstrate that the section cooled below ~300°C at 25–26 Ma during the development of normal faulting. Normal faulting was synchronous with the emplacement of the Catalina Intrusive Suite. New U‐Pb age results for Catalina Intrusive Suite indicate a combined mean age of 24.9 Ma. Chemical compositions of hornblende‐plagioclase pairs were obtained on six Catalina Intrusive Suite samples; depth estimates for the emplacement of the Catalina Intrusive Suite average of about 6 km. These results suggest that the exposed Catalina ductile detachment system was at about 5 km beneath the surface at 25 Ma. These new data bring new light into the development of this core complex and suggest that the similarity in orientations of principal ductile and brittle fabrics at the Catalina MCC locality are coincidental. Neither was the principal ductile fabric developed during the low‐angle normal faulting of the latest Oligocene nor was this exposed section a midcrustal one at that time. Transient, pluton emplacement‐enhanced, and extension‐related ductile deformation at shallow crustal levels operated locally at ~25 Ma but that does not account for the development of the majority of the Catalina MCC mylonites.

Interpretation of a ca. 1600–1580 Ma metamorphic core complex in the northern Gawler Craton, Australia

The Mount Woods Domain in the Gawler Craton, South Australia records a complex tectonic evolution spanning the Palaeoproterozoic and Mesoproterozoic. The regional structural architecture is interpreted to represent a partially preserved metamorphic core complex that developed during the ~1600–1580 Ma Hiltaba Event, making this one of the oldest known core complexes on Earth. The lower plate is preserved in the central Mount Woods Domain, which comprises the Mount Woods Metamorphics. These rocks yield a detrital zircon maximum depositional age of ~1860 Ma and were polydeformed and metamorphosed to upper amphibolite to granulite facies during the ~1740–1690 Ma Kimban Orogeny. The upper plate comprises a younger succession (the Skylark Metasediments) deposited at ~1750 Ma. Within the upper plate, sedimentary and volcanic successions of the Gawler Range Volcanics were deposited into half graben that evolved during brittle normal faulting. The Skylark Shear Zone represents the basal detachment fault separating the upper and lower plate of the core complex. The geometry of normal faults in the upper plate is consistent with NE-SW extension.Both the upper and lower plates are intruded by ~1795–1575 Ma Hiltaba Suite granitic and mafic plutons. The core complex was extensively modified during the ~1570–1540 Ma Kararan Orogeny. Exhumation of the western and eastern Mount Woods Domain is indicated by new 40Ar/39Ar biotite cooling ages that show that rock packages in the central Mount Woods Domain cooled past ~300 °C ± 50 °C at ~1560 Ma, which was ~20 million years before equivalent cooling in the western and eastern Mount Woods Domain. Exhumation was associated with activity along major syn-Kararan Orogeny faults.

The timing of brittle deformation: An example from the Cenozoic faults in the Youjiang fold‐thrust belt in southwestern China

The Cenozoic evolution of SE Asia is marked by thousands of kilometres extension on the continental margin, including normal faults, half‐grabens, and volcanic rocks, but dating of brittle faults in the shallow crust is difficult because of a lack of synkinematic minerals. The Cenozoic extension in the Youjiang fold‐thrust belt in southwest China is marked by brittle normal faults and half‐grabens filled with Paleogene sediments in the region. Here, we identified a strongly weathered mafic sill that was truncated by normal faults in the fold‐thrust belt and present the age and chemistry of zircons from a weathered sill. The zircons from the sill yielded a mean age of ~35 Ma. Integrating cross‐cutting relationship and zircon U–Pb dating suggest that the Cenozoic extension was initiated later than ~35 Ma in Youjiang fold‐thrust belt in southwest China. Combined with structural analysis and previous fission track data in the region, we suggest that the normal faults in this study provide evidence of surface fault activity of Cenozoic extension in SE Asia caused by retreat of a trench system. Furthermore, this study provides a case study to discuss how to decipher brittle normal faulting using zircon geochronology and structural and textural analysis from the mesoscale to the microscale.

Insights into post-orogenic extension and opening of the Palaeo-Tethys Ocean recorded by an Early Devonian core complex in South China

The Palaeo-Tethys Ocean formed during the Devonian as a consequence of blocks separating from northern Gondwana. The record of the Palaeo-Tethys Ocean stretches across Eurasia from the European Alps eastward to the southwestern South China block (SCB). Tectonic domes of the SCB record the contraction, extension, decompression, and partial melting of deep crust that occurred as a result of detachment faulting and tectonic evolution of the Palaeo-Tethys Ocean. This study investigates post-orogenic (Kwangsian) extension and opening of the Palaeo-Tethys Ocean as recorded by the Motianling core complex in the southwestern SCB, based on structural analysis and 40Ar/39Ar geochronology. The Motianling core complex in the southernmost part of the Jiangnan fold belt comprises the Sanfang gneissic granite surrounded by low-grade metasedimentary rocks. The core complex records at least three deformation (D) events: pre-D1, D1, and D2. The pre-D1 event produced large-scale NE-striking folds with axial surface cleavages and was coeval with emplacement of the Sanfang batholith. D1 deformation is represented by a top-to-the-NW ductile–brittle normal faulting system with mylonitic foliation and was coincident with greenschist-facies metamorphism, indicating that the core complex formed during NW–SE extension. D2 deformation is dominated by Cenozoic NW–SE-trending normal faults. The timing of D1 deformation and subsequent cooling was constrained by 40Ar/39Ar geochronology. Syn-kinematic muscovite yields an 40Ar/39Ar age of 417 ± 2.3 Ma (1σ) and suggests that the dome was initially developed in the Early Devonian through ductile shear zones. Magmatic biotite from the granitic mylonite yields a weighted mean 40Ar/39Ar age of 363.0 ± 3.1 Ma. Therefore, the Motianling core complex records at least two major stages of tectonic evolution: (1) Kwangsian (Caledonian) orogenic collapse and subsequent first-stage cooling at ∼417 to 363 Ma; and (2) Cenozoic regional extension and final cooling at ca. 45 Ma, as inferred from published apatite fission track data. The present results, combined with previous studies, indicate that the D1 deformation event resulted in the formation and exhumation of the Motianling core complex in a continental rifted margin during the opening of the Palaeo-Tethys Ocean.

The Xiaoqinling metamorphic core complex: A record of Early Cretaceous backarc extension along the southern part of the North China Craton

AbstractMany metamorphic core complexes (MCCs) of Early Cretaceous age are documented in the northern part of the North China Craton (NCC), which formed in a backarc extensional setting. However, whether or not the MCCs are also present in the southern part of the NCC, and where the western boundary of backarc extension lies, remain unclear. We present new structural and geochronological data to show that Early Cretaceous structures in the Xiaoqinling region (China) lying in the southern part of the central NCC represent a Cordilleran-type MCC. The NW-dipping detachment zone on the northwestern edge of the Xiaoqinling MCC is a ductile extensional shear zone that is overprinted by a later brittle detachment fault. The footwall (lower plate) consists of Archean metamorphic rocks and Mesozoic plutonic rocks, and was cut by a series of ductile normal sense shear belts and later brittle normal faults that strike predominantly NE-SW. Both the ductile and brittle structures indicate that NW-SE extension was responsible for the development of the MCC. Geochronological data suggest that the MCC initiated at 138 Ma and lasted until 100 Ma, recording a protracted extensional history. The MCC experienced an early phase of crustal-scale normal faulting (138–126 Ma) and later isostatic doming (125–100 Ma), consistent with the “rolling-hinge” model. The Xiaoqinling MCC shows similar features and a similar evolution to other intraplate MCCs in the northern and southeastern parts of the NCC, and shows that the southern part of the NCC was also involved in intense backarc extension and magmatism. Distribution of these intraplate MCCs indicates synchronous backarc extension over a length of around 1800 km. Delamination of a flat oceanic slab during roll-back is consistent with such large-scale, synchronous extension in the overriding plate.

Mesozoic tectonic evolution of eastern China: Insights from the Changshan Islands, NE China

The Changshan islands are scattered among the Yellow Sea east of the Liaodong Peninsula, west of North Korea and north of the Sulu Orogenic Belt. They consist of three lithotectonic units: i) Archean gneiss, ii) Paleoproterozoic marble and micaschists, and iii) Neoproterozoic weakly metamorphic sedimentary rocks. Three stages of deformation (D1-D3) documented in this study indicate that Changshan islands are a key region to decipher the Mesozoic tectonic evolution of eastern China. The earliest deformation D1 corresponds to a ductile deformation with NE-SW trending mineral and stretching lineations and top-to-the-NE sense of shear. The deformation D2, observed in the micaschists and sedimentary rocks, is characterized by the SW-verging folds with NE-dipping axial planar cleavage S2 and SW-directed thrusting. D2 could be considered as the back-folding and back-thrusting of the D1. D3 is expressed by the NE-SW trending low-angle brittle and ductile detachment faults representing a NW-SE extensional event.To the east of Guanglu and Guapi islands, two samples from a granite pluton that only record D3 fabrics yield SIMS zircon ages of 165.4 ± 1.6 Ma and 164.7 ± 1.5 Ma, which indicate that D1 and D2 occurred earlier than 165 Ma, while D3 is younger than this age. Similar geometric and kinematic features with those of the South Liaodong Peninsula suggest that D1 and D2 occurred in the Middle-Late Triassic as the structural response of overriding plate to the continental subduction of the South China Block (SCB) beneath the North China Craton (NCC), while the D3 occurred in the Early Cretaceous during the development of the Liaonan Metamorphic Core Complex (MCC). The ductile shear zone and brittle normal faults of the Changshan islands represent the early extensional structures of the Liaonan MCC. They may develop further southeastward and controlled the early development of the North Yellow Sea Basin.

Tectono-metamorphic evolution of shallow crustal levels within active volcanic arcs. Insights from the exhumed Basal Complex of Basse-Terre (Guadeloupe, French West Indies)

In order to decipher the tectono-metamorphic evolution of shallow crustal levels of the active volcanic arc of the Guadeloupe archipelago (Lesser Antilles) we present new geochemical, geochronological, mineralogical and structural investigations of the so-called Basal Complex, the oldest and most eroded volcanic complex of Basse-Terre in Guadeloupe. Based on geochemical and mineralogical criteria we propose an updated geological map of this northern area of Basse-Terre. Using40Ar–39Ar geochronology we demonstrate first that the eroded “Gros Morne” of Deshaies belong to the Basal Complex, and second that this complex is characterized by 4.3 to 2 Ma old volcanism. Structural analysis reveals a long-lived deformation history with the development through time of N80-N100 schistose zones; N110-N140 and N160-N10 oriented hydrothermal breccias and N140-N150 brittle normal faults. The boundary between the Basal Complex and the southernmost Septentrional Chain corresponds to a series of faults with N 150° and N 50° main directions. Detailed mineralogical and petrological investigations, including thermodynamic modeling, allow the identification of three phases of post-magmatic mineralogical transformations with first a high-temperature stage under Greenschist to sub-Greenschist facies conditions (0.6–2 kbar for 250–300 °C), a re-equilibration under Zeolite facies conditions and finally a sub-surface alteration. The consistency between P–T conditions of metamorphism and the present day measured geothermal gradient demonstrates that the metamorphic pattern is the record of hydrothermal fluids circulation during building and cooling of the Lesser Antilles magmatic arc. The tectono-metamorphic evolution recognized in the Basal Complex enables us to propose a conceptual model for heat and fluid transport within shallow crustal levels of the Guadeloupe active volcanic arc.

Tectonics of the Laptev Shelf, Siberian Arctic

Abstract The Laptev Sea in the Siberian Arctic represents a unique tectonic junction of an active spreading ridge, the Gakkel Ridge in the Eurasian oceanic basin, with the Siberian Arctic continental margin. New long-offset seismic profiles acquired in recent years provide a reliable basis for deciphering the structural and seismic stratigraphic characteristics of the Laptev Rift System. The tectonic development of the Laptev Shelf represents a sequence of four phases controlled by relative plate movements: (1) intense brittle normal faulting (an initial rifting or stretching phase) affected the entire shelf in the Late Cretaceous(?)–Paleocene(?); (2) a thinning/exhumation phase resulted in exhumation of the lower continental crust and probably upper mantle in the western part of the rift system – this phase is inferred to have occurred during the Late Paleocene to Early Eocene, preceding and accompanying continental break-up in the Eurasia Basin; (3) a stalled rift phase characterized by either a dramatically reduced rate of extension, or a non-extension/compression regime controlled by major reorganization of the plate movements – the onset of this fourth phase is inferred to coincide with the initiation of seafloor spreading in the southern Eurasia Basin at around 53–50 Ma; and (4) reactivation of the rifting in the mid-Miocene (a second rift phase).

Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

Abstract. High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range Province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada), suggest the presence of two distinct rheological transitions in the middle crust: (1) the brittle–ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear, and (2) the localized–distributed transition, or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths.In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. The LDT is not always exhumed, or it may be obscured by later deformation, but in the Whipple Mountains, it can be directly observed where high-strain mylonites captured from the middle crust depart from the brittle detachment along a mylonitic front.

Seismically induced soft-sediment deformation structures revealed by X-ray computed tomography of boring cores

X-ray computed tomography (CT) allows us to visualize three-dimensional structures hidden in boring cores nondestructively. We applied medical X-ray CT to cores containing seismically induced soft-sediment deformation structures (SSDSs) obtained from the Kanto region of Japan, where the 2011 off the Pacific coast of Tohoku Earthquake occurred. The CT images obtained clearly revealed various types of the seismically induced SSDSs embedded in the cores: a propagating sand dyke bent complexly by the preexisting geological structure, deformed laminations of fluidized sandy layers, and two types of downward mass movement (ductile downward folding and brittle normal faulting) as compensation for upward sand transport through sand dykes. Two advanced image analysis techniques were applied to the sand dyke CT images for the first time. The GrowCut algorithm, a specific digital image segmentation technique that uses cellular automata, was used successfully to extract the three-dimensional complex sand dyke structures embedded in the sandy sediments, which would have been difficult to achieve using a conventional image processing technique. Local autocorrelation image analysis was performed to detect the flow pattern aligned along the sand dykes objectively. The results demonstrate that X-ray CT coupled with advanced digital image analysis techniques is a promising approach to studying the seismically induced SSDSs in boring cores.

Rapid change from compression to extension in the North China Craton during the Early Cretaceous: Evidence from the Yunmengshan metamorphic core complex

The Yunmengshan metamorphic core complex (YMMCC) is situated in the northern part of the eastern North China Craton (NCC). The main structures in the YMMCC area include the arcuate Sihetang shear zone (SHTSZ) and the NE-striking Shuiyu shear zone (SYSZ). We present structural and geochronological evidence to show that the SHTSZ was a top-to-the-SSW thrusting zone formed between 140 and 137Ma. The SYSZ is an extensional, top-to-the-SE shear zone developed between 131 and 114Ma, in association with intense magmatism and the development of the Huairou rifted basin in the hanging wall. Ductile shearing on the SYSZ was followed by isostatic rebound from 114Ma to the end of the Early Cretaceous and then was replaced by brittle normal faulting. The normal displacement on the SYSZ and subsequent isostatic rebound led to the formation of the YMMCC. The mechanism of formation is consistent with a rolling-hinge model of detachment faulting and core complex formation. The structural evolution of the Yunmengshan area, in common with the whole eastern NCC, thus records a rapid switch from a NNE–SSW shortening regime to a NW–SE extension one in the earliest Cretaceous. The intense NW–SE extension of the Early Cretaceous, as recorded by the YMMCC, was associated with peak lithospheric thinning in the eastern NCC. We attribute the shortening to the final closure of the Mongol–Okhotsk Ocean between the Siberian Craton and the North China–South Mongol block, and the extension to the back-arc deformation related to the subduction of the Izanagi Plate in the Pacific Ocean. We suggest that the Yunmengshan area illustrates how back-arc compression can be driven by the overriding plate motion, while connection of the overriding plate to a larger continent favors subduction-related back-arc extension.

Quaternary extensional and compressional tectonics revealed from Quaternary landforms along Kosi River valley, outer Kumaun Lesser Himalaya, Uttarakhand

A portion of the Kosi River in the outer Kumaun Lesser Himalaya is characterized by wide river course situated south of the Ramgarh Thrust, where huge thickness (~200 m) of the landslide deposits and two to three levels of unpaired fan terraces are present. Brittle normal faults, suggesting extensional tectonics, are recognized in the Quaternary deposits and bedrocks as further supported by surface morphology. Trending E–W, these faults measure from 3 to 5 km in length and are traced as discontinuous linear mini-horst and fault scarps (sackungen) exposed due to cutting across by streams. Active normal faults have displaced the coarsely laminated debris fan deposits at two sites located 550 m apart. At one of the sites, the faults look like bookshelf faulting with the maximum displacement of ~2 m and rotation of the Quaternary boulders along the fault plane is observed. At another site, the maximum displacement measures about 0.60 cm. Thick mud units deposited due to blocking of the streams by landslides are observed within and above the fan deposit. Landslide debris fans and terrace landforms are widely developed; the highest level of fan is observed ~1240 m above mean sea level. At some places, the reworking of the debris fans by streams is characterized by thick laminated sand body. Along the South Almora Thrust and Ramgarh Thrust zones, the valleys are narrow and V-shaped where Quaternary deposits are sparse due to relatively rapid uplift across these thrusts. Along the South Almora Thrust zone, three to four levels of fluvial terraces are observed and have been incised by river exposing the bedrocks due to recent movement along the RT and SAT. Abandoned channel, tilted mud deposits, incised meandering, deep-cut V-shaped valleys and strath terraces indicate rapid uplift of the area. Thick mud sequences in the Quaternary columns indicate damming of streams. A ~10-km-long north–south trending transverse Garampani Fault has offset the Ramgarh Thrust producing tectonic landforms.

Low-temperature constraints on the Cenozoic thermal evolution of the Southern Rhodope Core Complex (Northern Greece)

The South Rhodope Core Complex (SRCC) of Northern Greece is probably the most studied metamorphic core complex of the Rhodope Massif, and yet its geological evolution has not yet been fully unravelled, especially the later stages of its thermotectonic evolution. We applied the fission-track method on apatite and zircon and the [U–Th–(Sm)]/He method on apatite in order to reconstruct the low-temperature thermal history. The main detachments responsible for the unroofing of the core complex are the Kerdilion and the Strymon Valley detachments. The Kerdilion detachment initiated the exhumation of the SRCC at the latest at 42 Ma and controlled it until about 24 Ma. Between 24 and 12 Ma, the Strymon Valley detachment accommodated the exhumation. Since 12 Ma brittle normal faults, some of them cutting the Strymon Valley detachment were responsible for the final cooling of the basement rocks in the studied area and the formation of syn-tectonic sedimentary basins. Activity along these brittle normal faults lasted until 6 Ma or probably even until today, as indicated by recent seismic activity in the area.

Structural characteristics and formation mechanism of the Kalaqin metamorphic core complex in the Yanshan area, China

Based on new detailed field observations and magmatic zircon dating, we reanalyzed deformation characteristics and tectonic evolution of the Kalaqin metamorphic core complex in the northern Yanshan tectonic belt. We propose that two stages of extension occurred in formation of the core complex in Late Jurassic and Early Cretaceous, rather than only one in Early Cretaceous as previously suggested. During Late Jurassic (~156-150 Ma), the core complex originated from a shallow detachment ductile shear zone with a top-to-the-NE shear sense under NE-SW extensional stress. The footwall was ductilely deformed, and shear zone development was associated with magmatic activity and formation of rift basins. Ductile shearing was followed by limited isostatic uplift ~149-145 Ma. During Early Cretaceous (~141-100 Ma), the extensional direction switched to NW-SE, leading to development of two NE- striking major normal faults with opposite dip directions along the eastern and western edges of the core complex. Early Cretaceous half-graben basins developed on the hanging walls of both major faults, as their shared footwall rose in an extensional dome. This doming resulted in exhumation of the Late Jurassic ductile extensional structures to shallow levels. The Kalaqin metamorphic core complex was overprinted by earlier ductile deformation belts and later brittle normal faults during the extensional doming of Early Cretaceous, the deformation shows characteristics of strain localization. Our new structural evidence allows determination of the evolution and formation mechanism of the Kalaqin metamorphic core complex, and confirms extensional deformation in both Late Jurassic and Early Cretaceous in the Yanshan tectonic belt. The Early Cretaceous extension is intense and widespread, and represents the peak destruction of the eastern North China Craton.