Contractional Deformation Research Articles

Study on Strength and Microstructure of Red Clay Reinforced by F1 Ionic Soil Stabilizer

High-liquid limit red clay has poor engineering characteristics, namely poor water stability, low strength, and large expansion and contraction deformation. The clay may be reinforced with an F1 ionic soil stabilizer. The engineering characteristics of this reinforced clay were studied, specifically concerning its basic physical parameters, shear strength parameters, and micropore structure. The F1 ionic soil stabilizer significantly improved the water sensitivity, compaction characteristics, and shear strength of red clay. We determined that the optimal F1 ionic soil stabilizer mix was 0.5 L/m3, resulting in a reinforced clay with plastic limit increased by 45.74%, optimal moisture content increased by 12.12%, maximum dry density increased by 5.8%, liquid limit reduced by 8.4%, plasticity index reduced by 43.8%, infiltration coefficient reduced by 41.8%, cohesion increased 1.64-fold, and internal friction angle increased 1.30-fold. Freeze-thaw cycles reduced the shear strength parameters of the reinforced red clay, although even after 15 cycles, it still had 18.4% higher cohesion and 57.1% higher internal friction angle than undisturbed red clay. The F1 ionic soil stabilizer significantly reduced the pore size and area of red clay, the complex connected pore structure is adjusted to a more regular structure. The reinforced clay had 56.64% lower pore area ratio, 32.27% lower average Feret diameter, and 2.43% lower fractal dimension.

Sediment provenance of the Lulehe Formation in the Qaidam basin: Insight to initial Cenozoic deposition and deformation in northern Tibetan plateau

AbstractUnravelling early Cenozoic basin development in northern Tibetan Plateau remains crucial to understanding continental deformation mechanisms and to assessing models of plateau growth. We target coarse‐grained red beds from the Cenozoic basal Lulehe Formation in the Qaidam basin by combining conglomerate clast compositions, paleocurrent determinations, sandstone petrography, heavy mineral analysis and detrital zircon U–Pb geochronology to characterize sediment provenance and the relationship between deformation and deposition. The red beds are dominated by matrix‐supported, poorly sorted clastic rocks, implying low compositional and textural maturity and short transport distances. Although most sandstones have high (meta)sedimentary lithic fragment contents and abundant heavy minerals of metamorphic origin (e.g., garnet, epidote and chlorite), spatiotemporal differences in detrital compositions are evident. Detrital zircon grains mainly have Phanerozoic ages (210–280 Ma and 390–480 Ma), but Proterozoic ages (750–1000 Ma, 1700–2000 Ma and 2300–2500 Ma) are also prominent in some samples. Analysed strata display dissimilar (including south‐, north‐ and west‐directed) paleocurrent orientations. These results demonstrate that the Cenozoic basal deposits were derived from localized, spatially diverse sources with small drainage networks. We advocate that initial sedimentary filling in the northern Qaidam basin was fed by parent‐rocks from the North Qaidam‐South Qilian belts and the pre‐Cenozoic basement within the Qaidam terrane interior, rather than southern distant Eastern Kunlun regions. Seismic and drilling well stratigraphic data indicate the presence of paleohighs and syn‐sedimentary reverse faults and noteworthy diversity in sediment thickness of the Lulehe Formation, revealing that the Qaidam terrane exhibited as several isolated depocenters, rather than a coherent basin, in the early stage of the Cenozoic deposition. We suggest the Cenozoic Qaidam basin to have developed in a contractional deformation regime, which supports models with synchronous deformation throughout most of Tibet shortly after the India‐Eurasia collision.

Laramide contractional folding in the Devils River Uplift, west Texas, U.S.A.

Structures in Cretaceous strata in the Devils River Uplift of west Texas include folds, thrust faults, and bedding-parallel veins indicative of contractional deformation in a thrust faulting stress regime. In this study we analyze a series of small-wavelength (meters to tens of meters) folds in roadcut exposures northwest of Del Rio, Texas. The folds have WNW-ESE dominant trends, gently plunging axes, and are gentle to tight with upright to inclined axial surfaces of variable vergence. These structures are interpreted as detachment folds, with detachments within volcanic ash or mudrock beds. Observed thickening in anticline cores was by ductile flow of volcanic ash or mudrock, small-scale folding, and thrust or reverse faulting. Bed-perpendicular calcite veins are common and in some cases are offset by flexural slip on fold limbs, indicating that folding occurred after brittle failure and extension vein formation in lithified rock. Although rare, bed-parallel calcite veins are also present in the exposure and indicative of a thrust faulting stress regime. Occasional normal and reverse or thrust faults are also present in the exposures. Fluid inclusion analysis of vein cements from extension fractures and faults yielded homogenization temperatures of 71–101 °C, and estimated trapping depths of 2.2–3.5 km prior to and coeval with folding. We interpret these folds as having formed during NNE-SSW directed Laramide shortening (∼80–40 Ma) in a thrust-faulting stress regime with >2 km of overburden, rather than as either penecontemporaneous with deposition or after exhumation by caliche formation, as previously interpreted.

Structural and geochronological constraints on a Late Mesozoic tectonic transformation in the West Qinling Orogenic Belt, China

ABSTRACT The Qinling Orogenic Belt was formed by multi-stage convergence and subsequent collision between the North China Block and South China Block during the Palaeozoic to Late Triassic. Following the collision between the South Qinling Belt and the South China Block in the Late Triassic, the whole Qinling Orogenic Belt evolved into an intracontinental orogeny. However, the exact termination time of the Late Mesozoic intracontinental contractional deformation and the tectonic transformation from compression to extension remains unclear. The extensional shear zone developed in the West Qinling Orogenic Belt provides a crucial clue to revealing the tectonic transformation and uplift and exhumation of the orogenic belt. This study focuses on kinematics, deformation temperature, and geochronology of the shear zone in the West Qinling Orogenic Belt through detailed field structural analysis and laboratory analysis. The Taibai shear zone (TBSZ), located in the West Qinling Orogenic Belt, cuts across the Qinling Orogenic Belt along the NE–NEE direction and separates the Taibai pluton in the east from the Baoji pluton in the west. The NE–NEE-striking TBSZ is an extensional shear zone showing a top-to-the-NW sinistral shear sense. The TBSZ was formed under the high greenschist–low amphibolite facies conditions (300–550°C) in the middle-upper crust level (8–15 km). Zircon U–Pb dating and biotite and muscovite 40Ar/39Ar dating indicate that the TBSZ occurred at 120–113 Ma. The extensional TBSZ and normal faults on its hanging wall indicate the tectonic transformation and the extensional collapse of the Mesozoic intracontinental orogenic belt during the late Early Cretaceous. The Cretaceous extensional collapse of the Qinling Orogenic Belt also led to the rapid uplift and exhumation of the Taibai pluton and the North Qinling Belt in the east.

Deep-water Tectono-Stratigraphy at a Plate Boundary Constrained by Large N-Detrital Zircon and Micropaleontological Approaches: Peninsular Ranges Forearc, Baja California, Mexico

The distribution of sedimentary systems on Earth’s surface is intimately linked to tectonics, therefore, at plate boundaries the stratigraphic archive can unlock the timing and style of tectonism and relative plate motions. Using large-n detrital zircon and micropaleontological analyses, tied to field mapping and data collection, we unravel the timing of strike-slip motion and its influence on the development of a Cretaceous submarine canyon on a long-lived oblique-convergent margin. Structural analysis demonstrates that the canyon bedrock, composed of fluvial rocks (La Bocana Roja Fm., of maximum depositional age (MDA): 93.6±1.1 Ma), underwent both syn- and post-depositional contractional and extensional deformation during the Cenomanian-Turonian in response to dextral strike-slip movement. Relative sea-level rise associated with basin subsidence and hinterland uplift was coincident with incision and fill of a submarine canyon system (Punta Baja Fm., MDA 87.1±1.5 Ma to 84.9±2.0 Ma), which exploited structural lineaments in the bedrock. The canyon was filled by sediment derived from an uplifted magmatic arc during the Coniacian to Santonian, most likely shed from erosional topography associated with plutonic intrusions to the NE. Structural data suggest that oblique dextral strike-slip motion on the Pacific margin controlled the development and location of submarine erosion, and had ended by the earliest Santonian, significantly earlier than previously estimated. Basinward tilting led to uplift, followed by transgression and wave ravinement of the canyon fill, which was then overlain by a shallow-marine to fluvial system. Thus, the canyon was cut, filled, buried, uplifted and rotated basinward, planed off through wave ravinement, and onlapped by shallow-marine to fluvial sediments within an 8 Myr period. Our findings, in part, reconcile contrasting basin evolution models for the Late Mesozoic Pacific margin.

Interaction of inherited structures and contractional deformation in the South Dezful Embayment: Insights from the Gachsaran oilfield, SW Iran

Identifying the timing, geometry, mechanism and multiphase reactivation of the inherited structures on the structural style of the fold-and-thrust belt is important for hydrocarbon exploration and production.In this work, we present a multidisciplinary approach including field data, seismic profiles, well data analyses, isopach maps, 2D balanced cross-sections construction, and 2D sequential restoration. Such an approach is applied to decipher the multiphase deformation of the NNE-SSW trending deep-seated Kharg-Mish Fault (KMF) that controlled the tectono-sedimentary evolution and structural style of the Gachsaran Anticline (South Dezful Embayment - SDE). Kinematic analysis of the KMF reveals two different mechanisms from normal to reverse faulting (positive inversion) since the Cenomanian. From the Late Cretaceous-Late Miocene, the sedimentary cover thickness ∼2500 m decreases along-strike of the Gachsaran Anticline towards the east, over the KMF, due to regional erosion and pinch-out.The eastern sector of the Gachsaran Anticline shows an angular and smaller wavelength fold in the levels of Asmari and Sarvak formations due to the efficient local middle décollements of the Kazhdumi and Pabdeh formations. Different amounts of shortening, 19.4% for the western and 16.7% for the eastern sector, and fore-thrust propagation developed a tear fault that accommodated differential deformation. Long-term structural evolution and strain localization in the eastern sector of the Gachsaran Anticline over the KMF produced dense fracturing. Our study suggests that mechanism and timing of deformation along the pre-existing basement structures are important in structural evolution of the sedimentary cover, oil migration and accumulation in such a hydrocarbon province.

Thermal deformation and microstructure characteristics of low-heat Portland cement-based concrete in a high plateau environment

In this study, low-heat Portland cement concrete (LHPC-C) and ordinary Portland cement concrete (OPC–C) were prepared in a plateau region above 3000 m of altitude. The physical properties and thermal and deformation characteristics of the two types of concrete were studied. A JMZX-215HAT sensor was used to record the concrete strain and temperature in a plateau region. XRD, TG, SEM and MIP were employed to evaluate the product of concrete and micro-structure analysis. Results showed that Fresh LHPC-C displayed physical properties superior to fresh OPC-C in the plateau environment. And the rate of release of hydration heat and the total heat release of OPC-C were higher than those of LHPC-C. What's more, OPC-C exhibited a large contraction deformation, whereas LHPC-C exhibited a small fluctuation. Besides, LHPC-C exhibited a denser structure than OPC-C after 180 days.

Multisensory Flexible Braille Interactive Device Based on Liquid Crystal Elastomers

Traditional electronic braille devices’ rigid and complex structure limits their portability while satisfying braille output. Main-chain monodomain liquid crystal elastomers (m-LCEs) can achieve high contraction deformation under external stimulation conditions; it has broad application prospects in the human–machine interaction with the biological tissue safety and flexibility of LCEs. A multisensory flexible braille interactive device based on LCEs is proposed in this article. It utilizes the good reversible shape-memory performance of m-LCEs to solve the problems of complex structure design, poor portability, high cost, and low control stability in traditional rigid mechanical braille readers. The fixed-point stretchable convex lattice and the nonactuation performance-based film are accurately distinguished and fabricated by compression molding. Combined with the ultrathin flexible electric heating layer, the reversible contraction of the braille point is realized, and the braille point is given the dual perception of height difference and temperature difference. Moreover, matching circuit design can realize braille input by press and braille output by convex–concave conversion. Furthermore, we try to introduce thermochromic powders, referring to the principle of light-emitting diode (LED) imaging, to transform the pattern concept of visually impaired people in minds into color information that ordinary people can observe.

Notched specimen cracking test: a novel method to directly measure low-temperature thermal stress of asphalt binder

ABSTRACT A new method, the notched specimen cracking test (NSCT), is presented to directly measure the thermal stress and critical temperature of asphalt binders at low temperatures. The NSCT contains a notched specimen that contraction deformation is limited by special invar steel fixtures exposed to a continuous cooling temperature field provided by a programable environmental chamber. To simulate the actual thermal contraction state–strain level of asphalt pavement in the laboratory, the geometry of the binder specimen was carefully designed by theoretical computation based on the superposition principle. The preliminary test result shows that NSCT for evaluation of the potential cracking risk of modified or non-modified binders at low temperatures is reliable and easy to operate. The coefficient of variations of thermal stress results is at least 50% smaller than the traditional thermal cracking measurement method.

Jurassic contractional deformation in the central–western North China Craton in response to multi-plate convergence in the East Asia

The Jurassic multi-plate convergence in East Asia led to the intra-continental deformation in the North China Craton (NCC). However, much controversy is highlighted in the intra-continental deformation style and timing, and unanimous kinematics. The central-western North China Craton of the East Asia, typical of the Jurassic structural belts around the Ordos Block, is a key region to delineate the intra-continental deformation. Here, using new structural analysis, the definition of Jurassic growth strata, and paleo-stress inversion, grouping with our previous work on the NCC, we reconstructed a multi-directional shortening pattern in the central-western NCC during the Middle–Late Jurassic (170–160 Ma), which is characterized by N-S shortening along the northern margin of the NCC, by NW-SE shortening in the western margin and along the central zone of the NCC. In combination with regional structural analysis, we propose that the central-western North China Craton is roughly situated at the center of the multi-plate convergence during the Middle-Late Jurassic, and the southward compression was caused by the closing of the Mongolia-Okhotsk Ocean in the north, the northwestward compression by the subduction of paleo-Pacific plate beneath the East Asia continent in the east, and southeastward compression by the closure of the Meso-Tethys in the west.

An experimental investigation challenging the thermal collapse of sand

Currently, highly contradictory knowledge characterises the thermally induced deformation of sands and the occurrence of the thermal collapse phenomenon: a volumetric contraction of such granular materials upon heating, which has been observed in some laboratory studies as opposed to a volumetric expansion. In this context, the effects of applied stress level and relative density, which seem to account for the inconclusive findings, remain disputed. A key challenge inherent to investigations of the thermally induced deformation of sands is the requirement for tremendous measurement accuracy (i.e. trueness and precision) and the ability to minimise measurement errors. This paper addresses this challenge and the conflicts presently characterising this subject by reporting a laboratory experimental campaign that accurately and repeatedly measures the thermally induced deformation of sand for different relative densities, stress levels and particle shapes by means of temperature-controlled oedometer tests. The results of this study show a monotonic volumetric expansion of sand upon heating and contraction upon cooling, with an irreversible contractive deformation remaining after one heating–cooling cycle. Specifically, irrespective of the stress level, relative density or particle shape, no thermal collapse is observed, challenging the actual occurrence of such a phenomenon in sands.

Syn-kinematic inversion in an intracontinental extensional field? A structural analysis of the Waterberg Thrust, northern Namibia

A structural analysis of a key area along the Waterberg Thrust in northern Namibia clarifies its geometric and timing relationships to the deposition of rift-related Karoo sediments. The fault brings late Precambrian crystalline rocks up against Permo-Triassic and Lower Jurassic Karoo clastic sediments of the Waterberg Basin between two of the major regional basement-rooted lineaments that cut across northern Namibia, using those subparallel lineaments as transfer faults. Detailed mapping and kinematic analysis have been used to construct and restore several sections across the thrust that put initiation of reverse offset as a thrust in Triassic time. Thin-skinned footwall deformation created several unconformities in a footwall cutoff system. The unconformities were subsequently folded during continued motion on the fault. Various dating techniques constrain the initiation of contractional deformation to the Triassic to Jurassic (Karoo time) with episodic progression during the development of the basin, followed by major further reactivation during regional uplift related to the Atlantic margin rifting and subsequent continental breakup. This early phase is linked in time so closely to the basin development that we attribute the driver of inversion to be kinematically linked to the displacement field of Neoproterozoic basement blocks themselves. Hence, we term this relationship as ‘syn-kinematic’ to distinguish it from wholly external, superimposed driving mechanisms (‘allo-kinematic’).

(U-Th)/He ages of Proterozoic-Paleozoic basement rocks from northern Chile (18-19°S) and implications on the Neogene uplift history of the Western Cordillera

In the Western Cordillera of northern Chile, the Proterozoic-Paleozoic Belén Metamorphic Complex is covered by late Oligocene-early Miocene (25-18 Ma) rocks, and both units are involved in west-vergent contractional deformation, which results in exhumation. A Miocene age (18 to 6 Ma) for deformation has been previously constrained by stratigraphy and cross-cutting relationships. To understand the youngest exhumation event and reverse faulting, we obtained 21 (U-Th)/He ages from two samples of the metamorphic rocks and the associate inverse thermal modeling. Five zircon (U-Th)/He ages from one sample are 113 to 226 Ma, very scattered, while five zircon ages from the other, are 20 to 49 Ma. The high dispersion of zircon (U-Th)/He data prevents the geological interpretation of results. Apatite grains from both samples yielded 11 (U-Th)/He ages between 10.4 and 18.7 Ma, with 9 values from 12.0 to 15.5 Ma. A slight positive correlation between apatite single-grain dates and effective uranium for 4 crystals of one sample suggests relatively slow cooling. The T-t model including these 4 apatite ages shows continuous cooling from 15 to 0 Ma with a relatively more marked cooling period at 11-7 Ma. The middle-late Miocene thermal signal agrees with the geologic evolution of the region and would permit to date the last activity of the Chapiquiña-Belén reverse fault, which uplifted and exhumed the metamorphic rocks. This signal is relatively similar to that the eastern Altiplano, but differs considerably from that the forearc.

Early Mesozoic polyphase contractional deformation in the Yanshan belt: Implications for the destruction of the north China Craton

• The northern North China Craton was in a contractional setting in early Mesozoic. • Post-collisional contractional deformation was due to the lithosphere delamination . • A decoupling process dominated the early stage of the destruction of the NCC. After the collision of the Mongolian arc terranes with the amalgamated Bainaimiao arc and North China Craton in the latest Permian to earliest Triassic, the northern North China Craton ended the Andean-type accretionary orogenesis. The nature of the subsequent post-collision tectonic setting in early Mesozoic in the northern North China Craton remains controversial. Some workers proposed that the northern North China Craton evolved into a post-orogenic extensional stage. However, this view was mainly built on geochemistry studies of Triassic plutonic rocks in northern North China Craton while it had never received support from studies of tectonic deformation. In contrast, our recent field mapping combining with geochronological studies in the Yanshan belt suggest that the northern North China Craton was in an overall polyphase multi-directional contractional settings in early Mesozoic. We have identified four stages of contractional deformation (D 1 -D 4 ) in this region. D 1 (250–231 Ma) is the intracratonic response to the collision of the Mongolia arc terranes with the NCC. D 2 (214 ∼ 207 Ma) and D 3 (204 ∼ 195 Ma) are post-collisional intracratonic contractional deformation which occurred synchronously with the lithosphere thinning. D 2 and D 3 and the intervened plutonism might be a crustal response to the delamination and sinking of the over-thickened lithosphere of the northern North China Craton. D 4 is the result of the west-ward subduction of the Izanagi plate beneath the eastern Asian continent plate in the Early Jurassic. These findings together with previous studies indicate that the northern North China Craton experienced a long period of polyphase contractional deformation before its final destruction in the Early Cretaceous. The early stage of the destruction of the North China Craton is characterized by coeval lithosphere thinning and crustal thickening.

Nonlinear Solid–Fluid Coupled Seismic Response Analysis of Layered Liquefiable Deposit

A seismic response analysis of layered, liquefiable sites plays an important role in the seismic design of both aboveground and underground structures. This study presents a detailed dynamic site response analysis procedure with advanced nonlinear soil constitutive models for non-liquefiable and liquefiable soils in the OpenSees computational platform. The stress ratio controlled, bounding surface plasticity constitutive model, PM4Sand, is used to simulate the nonlinear response of the liquefiable soil layers subjected to two seismic ground motions with different characteristics. The nonlinear hysteretic behavior of the non-liquefiable soil under earthquake excitations is captured by the Pressure Independent Multi Yield kinematic plasticity model with a von Mises multi-yield surface. The soil elements are modelled utilizing the solid–fluid fully coupled plane-strain u-p elements. The seismic response of the layered liquefiable site in terms of the development of excess pore water pressure, acceleration, ground surface settlement, and stress–strain and effective stress path time histories under two representative earthquake excitations are investigated in this study. The numerical results indicate that both the characteristics of ground motions and the site profile have a significant influence on the dynamic response of the layered liquefiable site. Under the same intensity of ground motion, the loose sand layer with a 35% relative density is more prone to liquefaction and contractive deformation, which causes irreversible residual deformation and vertical settlement. The saturated soil layer can effectively filter the high-frequency components and amplify the low-frequency components of ground motions. Moreover, the liquified site produces a 40% post-earthquake consolidation settlement after the excess pore pressure dissipation.

Mountain Growth under the Combined Effects of Paleostress and Paleoclimate: Implications from Apatite (U-Th)/He Thermochronology of Taibai Mountain, Central China

Abstract Tectonics and climate are the two competitive factors sculpturing landforms. Observations on the Earth surface are affected by signals from both tectonic and climatic agents. How to clarify these signals is a key issue. We categorize factors affecting mountain growth as horizontal (extension, compression, and transpression) and vertical (mantle upwelling and climate change) forces to evaluate the driving forces of accelerated exhumation in Taibai Mountain. Based on apatite (U-Th)/He thermochronology, we document two stages of accelerated exhumation at ca. 52-46 Ma and ca. 24-19 Ma from the age-elevation relationship, confirmed by 1D half-space modeling and QTQt inverse modeling. In the framework of paleostress, the two accelerated exhumation events occurred during transpressional periods in the early Eocene and late Oligocene-early Miocene in East China. These two events were triggered by the localized contractional deformation at the intersection of the North Qinling and Fengxian-Taibai faults: The opposite-direction shearing of these two faults was responsible for the former event with an unroofing magnitude of ~1 km; the same-direction shearing of the two faults resulted in the latter event with an unroofing magnitude of ~0.6 km. The far-field effects of both India-Eurasia collision and Pacific subduction drove the accelerated exhumation at ca. 52-46 Ma. The lateral extrusion of the Tibetan Plateau acted as the main driving force for the accelerated exhumation at ca. 24-19 Ma, which may have been slightly influenced by the intensified Asian summer monsoon in the early Miocene.

Criteria to identify sedimentary sills intruded during deformation of lacustrine sequences

Although sedimentary dykes have been widely reported across a range of settings, sedimentary sills have received somewhat less attention, perhaps due to the potential difficulties in identifying largely conformable intrusions within bedded sequences. Most outcrop descriptions of clastic intrusions are based on deep-water marine sequences, with few descriptions of sills in lacustrine settings. The recognition of sills in such settings is, however, important because lacustrine sequences are increasingly used as a record of palaeoseismic activity. The misidentification of sills that contain fragments and clasts of host stratigraphy with seismically-generated turbidites and debris flows, may lead to incorrect interpretations of palaeoseismicity. We use the Late Pleistocene Lisan Formation of the Dead Sea Basin as a case study, where laminated lake sediments preserve intricate relationships with sills. This permits us to not only establish a range of criteria used in the identification of sedimentary sills, but also examine relationships with adjacent seismically-triggered slumps and slides. Key criteria we use to recognise sills include marked changes in their thickness together with bifurcation and bridging geometries. Sills may be internally layered, contain lenses of breccia, together with aligned and folded clasts that may be truncated across upper sill contacts. Critical evidence for the interpretation of sills is also preserved along sharp but irregular upper contacts that erode and truncate bedding in the overlying host sequence. Minor apophyses and ‘wedges’ intrude both upwards and downwards from sills, while isoclinal recumbent ‘peel-back’ folds are created in host sediments by shear generated along the lower contacts of sills. We have undertaken anisotropy of magnetic susceptibility (AMS) analysis and find an oblate fabric that suggests flow and intrusion of sills along the strike of the slope, that may also help with their identification in bedded sequences. Sills form along detachments to both extensional and contractional deformation associated with seismically-generated slumps and mass transport deposits, together with sub-surface fold and thrust systems. High fluid pressures associated with injection of sedimentary sills may facilitate near-surface failure and downslope movement of the sedimentary pile.

Complex deformation history of the Keszthely Hills, Transdanubian Range, Hungary

Abstract We have investigated the deformation history of the Keszthely Hills (Transdanubian Range, W Hungary), which belongs to the uppermost slice of the Austroalpine nappe system. This Upper Triassic to Upper Miocene sedimentary rock sequence documented the deformation of the upper crust during repeated rifting and inversion events. We investigated the structural pattern and stress field evolution of this multistage deformation history by structural data collection and evaluation from surface outcrops. Regarding the Mesozoic deformations, we present additional arguments for pre-orogenic (Triassic and Jurassic) extension (D1 and D2 phases), which is mainly characterized by NE–SW extensional structures, such as syn-sedimentary faults, slump-folds, and pre-tilt conjugate normal fault pairs. NW–SE-striking map-scale normal faults were also connected to these phases. The inversion of these pre-orogenic structures took place during the middle part of the Cretaceous; however, minor contractional deformation possibly reoccurred until the Early Miocene (D3 to D5 phases). The related meso- and map-scale structures are gentle to open folds, thrusts and strike-slip faults. We measured various orientations, which were classified into three stress states or fields on the basis of structural criteria, such as tilt-test, and/or superimposed striae on the same fault planes. For this multi-directional shortening we presented three different scenarios. Our preferred suggestion would be the oblique inversion of pre-orogenic faults, which highly influenced the orientation of compressional structures, and resulted in an inhomogeneous stress field with local stress states in the vicinity of inherited older structures. The measured post-orogenic extensional structures are related to a new extensional event, the opening of the Pannonian Basin during the Miocene. We classified these structures into the following groups: immediate pre-rift phase with NE–SW extension (D6), syn-rift phase with E–W extension (D7a) and N–S transpression (D7b), and post-rift phase with NNW–SSE extension (D8).

Protracted (>60 Myrs) thermal evolution of a Neoproterozoic metasedimentary sequence from eastern Borborema Province (NE Brazil): Thermal and rheological implications for orogenic development

The thermal evolution of orogenic belts is quite varied. While some show rapid cooling, others record high temperatures for tens of millions of years. Here, we report the latter situation based on U-Pb zircon and monazite dating of paragneiss and micaschist from the Surubim Complex (eastern Borborema Province, NE Brazil). The Borborema Province and its African counterparts (Cameroon belt and Nigerian Shield) formed during amalgamation of West Gondwana in the Late Neoproterozoic (Brasiliano/Pan-African Orogeny). The location of the Surubim Complex in the central core of this large orogenic system offers the opportunity to study tectonic processes at work in the inner regions of ancient orogens. Basement-involved thick-skinned tectonics, regional low-angle foliation related to NW-directed thrusting, transcurrent shear zones and intrusion of granitic plutons characterize the Brasiliano Orogeny in the study area. Zircon data indicate provenance of the Surubim Complex mainly from proximal sources, predominantly 2.2–2.0 Ga-old basement rocks (including an orthogneiss here dated at 2132 ± 11 Ma), and from Neoproterozoic extension-related granitoids. The ages of the youngest detrital zircon grains range from 690 to 650 Ma. The weighted mean age (647 ± 4 Ma) of low Th/U zircon overgrowths in one sample is tentatively interpreted as dating syn-extensional metamorphism given the small age difference to the youngest detrital zircon population. A time lag of c. 17 Myrs, between 647 Ma and 630 Ma, suggests a temperature drop due to contractional deformation. The ages of the oldest magmatic grains from a leucosome sample and of monazite grains from a paragneiss sample, together with data from the literature, show that temperatures high enough to promote local anatexis during development of the regional low-angle foliation were reached around 630 Ma. Afterwards, as indicated by zircon and monazite growth and/or recrystallization, temperatures remained relatively elevated until the transition to the transcurrent regime, at c. 590 Ma, and persisted so up to c. 563 Ma. The results outline a protracted thermal history spanning more than 60 Myrs. This evolution is comparable to the tectonothermal evolution of long-lived hot orogens, where persistence of high temperature conditions for a long time has been related to the formation of orogenic plateaus.

Contractional salt deformation in a recently inverted basin: Miocene to current salt deformation within the central Algerian basin

AbstractField analogues illustrating the early stage of deformation of shortening structures in salt‐bearing orogenic fold‐and‐thrust belts are not yet well illustrated in literature. The relatively young Messinian salt of the Algerian basin could represent a good case example of such systems. The Algerian Basin is a deep‐water Miocene back‐arc basin including a layer of mobile Messinian evaporites up to 2 km thick. The Messinian salt was deposited in an already inverted basin, after its extensive stage. Its inversion initiated in the Tortonian, with a N‐NW shortening induced by the north‐westward motion of the African plate. In this study, we use new 2D multichannel seismic data to identify, classify and map salt structures throughout the central Algerian Basin and re‐assess its structural style. We interpret contractional salt tectonic structures, such as buckle folds, squeezed diapirs and related salt sheets as evidence of regional thick‐skinned shortening episodes. We conclude that horizontal displacement loading has driven salt deformation within the basin since its deposition. We also show spatial variations in the structural style of the central Algerian Basin, both along‐ and down‐dip. We demonstrate that the initial shortening‐related salt deformation in the late Messinian was focussed along the Algerian margin and later shifted outward toward the Balearic margin in the Plio‐Quaternary. The shifting of the deformation front is interpreted to be a result of the thickening and strengthening of the overburden. The second peak of deformation may have reactivated faults along the Emile‐Baudot escarpment with thick‐skinned deformation. We also observe a variation in the intensity of the salt deformation along the margin from SW to NE, which may be due to variable tectonic loading applied along the Algerian margin or the pre‐shortening distribution of salt.