Tectonic Signatures Research Articles

Provenance, Environments, and Tectonic Signatures of Tanjong-Sandakan Formations, Northeast Sabah, Malaysia: Implications for Sedimentology, Palynology, and Geochemistry

Provenance, Environments, and Tectonic Signatures of Tanjong-Sandakan Formations, Northeast Sabah, Malaysia: Implications for Sedimentology, Palynology, and Geochemistry

LOMU Type Alkali Basalts in East Asia Sourced from Subduction Recycling of Multiple Ancient Crustal Components

Abstract The origin of alkali basalts with distinctive time-integrated low U/Pb (low μ, LOMU) from East Asia is controversial due to the complex geochemical and tectonic signatures reported from this region. We report new data on the petrology and geochemistry of the Higashi-Matsuura and Kita-Matsuura alkali basalts from Southwest Japan, which confirm the presence of a LOMU-type mantle component below the Japanese Islands, similar to East Asian intraplate volcanoes. We use whole rock geochemistry and mineral chemistry to determine mantle melting, magma evolution conditions and isotopic evolution of the LOMU mantle source. Petrological studies show that the Higashi-Matsuura alkali basalts (~3 Ma) were derived from a hydrous mantle source with ~880 μg/g H2O, at a pressure of 1.8 to 2.3 GPa at a mantle potential temperature of ~1300°C. Trace element modelling concludes that these basalts may have been generated through the hydrous upwelling and partial melting of the asthenospheric mantle containing ~15% dehydrated eclogitic component. Higashi-Matsuura alkali basalts show 206Pb/204Pb values of 17.72 to 18.04 which are among the lowest values from Southwest Japan. Relatively older (6–8 Ma) alkali basalts from the Kita-Matsuura area showing similar physicochemical characteristics, do not show LOMU-type isotopic trends. Trace element and Pb–Sr–Nd isotopic data indicate that the Higashi-Matsuura mantle component is similar to the extreme LOMU components reported from the Northeast China alkali basalts, as well as the recently discovered Petit Spot volcanoes on the Pacific Plate. We model the origin of the LOMU signature from the lowest reported Pb isotope ratios in East Asia, from intraplate alkali basalts in Northeast China. Our model suggests that at least two separate subduction events of marine sediments, at 1.8 and 2.2 Ga, are required to explain the observed Pb isotopic variation in the East Asian region. Other LOMU type basalts from East Asia, including Southwest Japan and Petit Spot, define a linear trend between the Xiaogulihe basalts and lithospheric mantle xenoliths, with 40% to 60% Pb in Higashi-Matsuura basalts being derived from the lithospheric mantle during upwelling. This suggests that the LOMU array in East Asia may have been formed by mixing between multiple ancient, subducted sediment components accumulated at the mantle transition zone for about 2 billion years, and its recent upwelling due to dehydration of the stagnant Pacific slab and related melting of the metasomatised asthenospheric mantle.

Petrogenetic implications, geochemistry and tectonic framework of albitite hosted magnetite in North Delhi Fold Belt: Application of geochemistry and apatite chemistry

The iron ore bodies in the form of lensoidal and pocket type deposits occur within the Meso to Neo-Proterozoic meta-sedimentary rocks and albitites of Ajabgarh Group of Delhi Supergroup in Khetri basin, Western India. These are exposed as detached and linear ore bodies at the southern (western flank) and south-eastern (eastern flank) part of the Khetri copper deposit which follows the regional trend of the host rocks. The detailed field and petrographic studies reveal that the magnetite and hematite are associated with major silicates like quartz, actinolite, muscovite, biotite, albite and, K-feldspar. Some minor occurrences of apatite, sphene, ilmenite and monazite are also found. This work is focused on understanding the evolution of albitite-hosted magnetite with respect to tectonic signatures, the source of mineralizing fluid, and its spatial relationship with the regional albitites and meta-sediments. To interpret the above, whole rock geochemistry of albitite rocks, magnetite ore and mineral chemistry of fluorapatite, were adopted.The whole rock geochemistry of albitite suggests that the parent rocks are of calc-alkaline magma series that are originated from the subduction-related rift zone of volcanic arc environment. The apatite chemistry signifies that these are evolved from the highly fractionated granitic rocks (granitoid) at moderate to high oxidizing conditions. Some deformed apatite grains are also encountered having small distorted monazite inclusions. Thus, it has an indication of high-temperature metasomatic/hydrothermal fluids that may be responsible for the removal of REEs from apatite and re-nucleate as monazite inclusions within it. Therefore, the overall evidence and associations of albitite with apatite-magnetite is demonstrating that the source rock of the study area could have evolved from the arc system of active continental margin which was abundant with Fe, U, REEs, etc. Hence, the sources rocks were suitably metasomatized by hydrothermal fluids channelized through the regional shear zone and simultaneous precipitation of the magnetite in the study area. The dominance of magnetite, associated REEs, apatite, and non-association of sulfide minerals with the magnetite in the area of study has close similarities with the IOA type deposits.

Geophysical exploration of tectonic signatures in proterozoic quartzite of the Kovilpatti region, Southern India: a study in the Kurumalai and Oodumalai hills

Geophysical exploration of tectonic signatures in proterozoic quartzite of the Kovilpatti region, Southern India: a study in the Kurumalai and Oodumalai hills

Cretaceous stratigraphic framework and sedimentary architecture, Pakistan: signatures of rift-drift tectonics and submarine volcanism along the western continental margin of the Indian Plate

Abstract Abstract The lithostratigraphy, biostratigraphy and depositional environments of the Cretaceous successions Pakistan were investigated to reveal the major geological events, including local, regional and global scale tectonics, phases of volcanism, basin condensation, emergence of structural highs and episodes of non-deposition. The succession is entirely missing at Salt Range, but were excellently exposed in Kirthar-Sulaiman Fold-Thrust belts. Such deviating nature (e.g., Hazara, Kalla Chitta, Surghar & Kohat) suggesting episodical tectonic activities were associated with inter-intra rifting and drifting of the India-Madagascar-Seychelles and Antarctica-Australian Plates during Cretaceous time. Correspondingly, several unconformities within the Cretaceous succession at numerous stratigraphic intervals consistently attest the effect of tectonics throughout the period. The recognized unconformities over Kawagarh, Lumshiwal, Parh and Fort Munro formations testify episodic uplifts on local and/or regional scales during the Coniacian-Santonian, Coniacian-Maastrichtian, Campanian and Maastrichtian in response to the separation of Madagascar from the India-Seychelles plate. In contrast, there was continuous sedimentation of carbonate, clastic and/or submarine volcanics in the Kirthar and Indian-Eurasian suture zone. Significant geological events in the restricted regions e.g., local scale submarine volcanism (Bibai Formation), ironstone deposition (Dilband Formation) and basin condensation (Khuzdar) were also associated with such rifting-drifting phases along the WNW margin of the Indian Plate.

Facies, diagenesis, and palaeo-environment significances of the Plio-Quaternary fluvio-lacustrine deposits of Ain Cheggag region, Sais foreland basin, Morocco

Fluvio-lacustrine deposits are promising archives that afford to decipher climate and tectonic signatures, typically when evidence of changes in the depositional settings therein is remarkable. As such, the present work coveys a case study whose fluvio-lacustrine deposits may serve to provide insights into climate and tectonic controls on palaeoenvironmental settings. The study area lies in the northwestern portion of Ain Cheggag, southeastern the Sais foreland basin (northern Morocco). The lithostratigraphical analysis of Plio-Quaternary fluvio-lacustrine deposits in the area has enabled the identification of nine facies. The depositional systems are organized into three deposit units, each of which reflects a typical depositional environment. The first (lower) unit consists of detrital facies implying deposition in alluvial-dominated settings. The alternation of detrital and carbonate facies giving way to the second (middle) unit suggests sedimentation in a fluvio-lacustrine environment. Whilst being carbonate-rich, the third (upper) unit reflects a shift into lacustrine-dominated settings. Accordingly, the vertical succession of the sedimentary pile delivers remarkable insights into a general palaeo-environmental shift from alluvial settings, with enhanced pedogenesis processes, towards carbonate-dominated lacustrine conditions where early- and late-stage diagenesis processes would have prevailed. This broad shift in depositional systems is likely to be the response of palaeohydrological changes that are modulated both by the neotectonic subsidence background and, specifically, the general palaeoclimate evolution towards more humid conditions. Bioturbation, desiccation, and void-filling with internal sediments are key features of early-stage diagenesis processes, whereas cementation, micritization, recrystallization, dissolution, and iron and manganese oxidation are the main features of late-stage diagenesis altering the primary structures of the studied deposits. Finally, we highlight the significance of fluviolacustrine deposits in deciphering palaeoclimatic signatures and tectonic implications.

Identification of geomorphic signatures of active tectonics in the Wadi Hagul Basin, Northwest Gulf of Suez, Egypt: insights from SRTM derived geomorphic indices and watershed analysis

The satellite data, include Landsat 7 Enhanced Thematic Mapper (ETM+), Landsat-8 (OLI), Shuttle Radar Topography Mission (SRTM), were processed and interpreted for creating an integrated geospatial map of the geomorphic indices (GI) of Wadi Hagul Basin (WHB). The present study is considered new, useful, and valuable in northwest of Gulf of Suez region to evaluate active tectonics using SRTM resulting drainage network and GI. Where fairly slight studies on active tectonics recognized on GI were prepared in the investigated area. The GI includes; Ratio of valley floor width to valley height (Vf), Transverse topographic symmetry factor (T), Stream length-gradient index (SL), Drainage basin shape (Bs), hypsometric integral (Hi), geomorphological landform and watershed analysis. Also, GI were integrated and combined with the seismic and structural lineaments intensities maps to create an integrated single index map. The morphotectonic geo-spatial distribution hazards map reveals that the high risky area is 82.75% of the total area, the moderate-risk area is 16.51%, and the low risky morph-tectonic area is 0.81%. Based on this map, it is necessary for planners and decision makers to extremely study attractive applicable action concerning the recommended mitigation measurements from this study to avoid any serious future problems in WHB and its surroundings areas.

Eruption, transport mechanisms and deposition of Paleogene ignimbrites, southwestern North Patagonian Massif, Argentina

The Paleocene-Eocene Catán Lil Ignimbrite (CLI) is one of the few occurrences within the explosive acid volcanism of this age in the extra-Andean region of the North Patagonian Massif. It represents a small-scale explosive eruption covering an area of 214 km2, with an estimated volume of 1.27 km3 and consisting of three weakly-to densely welded cooling units. The maximum thickness of this sequence is 90 m. Its basal unit is the best preserved, and has allowed a lithofacial analysis that reveals different aspects of the eruption and the emplacement mode of the Catán Lil pyroclastic flows. The CLI ignimbrites are generally massive, poorly sorted, matrix-supported deposits (tuff and lapilli tuff) resulting from dense pyroclastic currents (PDCs). They are composed mainly of juvenile fragments, scarce lithoclasts and crystalloclasts, being a product of a purely magmatic fragmentation occurred at a shallow level. A pyroclastic fountaining eruptive model is interpreted, resulting from the collapse of a low eruptive column, and a rapid and high-temperature emplacement. The CLI PDCs generally respond to slow moving pyroclastic flows with low erosive energy. The morphology and stratigraphy indicate radial emissions with thicknesses decreasing towards the periphery that become evident in Central-South ignimbrites. These PDCs flowed over a planar terrain, giving rise to most of the CLI deposits (South-Central area and North- East). The Northwestern ignimbrites are deposits in the interior (south and middle part) of the Catán Lil creek, forming thick deposits within the latter valley and thin, veneer-like deposits over the flanking Lipetrén granitoids. In the North- East region, the ignimbrites show stratigraphic relationships that suggest that the PDCs would slightly postdate those of the South-Central area. The Catán Lil Ignimbrites represent the most eastern exposures of the Paleocene-Eocene Pilcaniyeu Belt, differing from the latter in that it lacks a conspicuous lava facies, and by the unimodal character and the tectonic signature of the Catán Lil sequence.

Application of TOPSIS model in active tectonic prioritization: Madeira watershed, South America

The Andes orogenic belt is one of the most tectonically active zones located between the convergent margins of the South American plate and the Pacific plate. From the dissected western belt to the expanded alluvial plain of Amazon basin, the Madeira watershed bears recent tectonic imprints. The tectonic signature is well documented by the spatial and linear characteristics of the five sub watersheds along with the Madeira trunk river and channels. The level of tectonic activeness has been assessed on the sub watersheds of the Madeira River, which is one of the main right-hand tributaries of the Amazon River. On the five defined sub-watersheds and the main channel watershed of Madeira, several morphometric indices, viz., asymmetry factor (AF), tilt angle (β), sinuosity index (SI), channel concavity (ɵ), elongation ratio (Re), circularity ratio (Rc), transverse topographic symmetry factor (T) and basin shape index (Bs), have been used. To rank the watersheds based on tectonic activity, we applied the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) The active tectonics was also tested by a comparative study of linear indicators as Hack profiles, longitudinal profiles and segment-wise stream gradient index (SL) of watersheds 2 (rank 1) and 3 (rank 6). The results reveal that the most tectonically active watershed 2 is located in a high seismic magnitude zone with several deep earthquake epicenters. The least tectonically active watershed 6 is located in a low seismic zone.

Geomorphic mapping and analysis of neotectonic structures in the piedmont alluvial zone of Haryana state, NW-India: a remote-sensing and GPR based approach

The Himalayan Frontal Thrust (HFT) and the surrounding piedmont alluvial region represent a zone of active deformation in the Indo-Gangetic plains. We investigated the Piedmont zone between the Ghaggar and Yamuna River basins in Haryana, India, for geomorphic signatures of active tectonics using remotely sensed data and validated by Geophysical Ground Penetrating Radar (GPR) surveys. The possible locations and the types of active tectonic features such as sub-surface fault, ridges, lineaments and warps were identified based on the presence of geomorphic signatures such as drainage gradient anomalies, abrupt change in flow direction, river offset, compressed meanders, paleochannels and topographic breaks. We used various optical satellite imageries to detect and map the temporal changes in the flow pattern of rivers in the study area. GPR investigations were done at selected test sites to locate and verify the continuity of subsurface fault. The GPR profiles were taken in the North-South direction using the common midpoint technique with 40 and 100 MHz antennae. Low frequency bi-static GPR scanning confirmed a number of dipping reflectors due to fault planes and warp surfaces in the study area. It is concluded that the piedmont zone of Haryana is actively deforming and could become a future seismic hazard zone.

Geomorphic signature of active tectonics in Simav Graben: An evaluation of indicator relative tectonic activity of Simav Fault and its seismotectonic implications for Western Anatolia

Geomorphic signature of active tectonics in Simav Graben: An evaluation of indicator relative tectonic activity of Simav Fault and its seismotectonic implications for Western Anatolia

Disentangling orbital, sub-orbital, and tectonic signatures from lacustrine sediments developed upon a half-graben (Lake Ifrah Basin, Northwest Africa): Insights into lowest-rank T-R sequences in low accommodation basins

Lacustrine settings constitute a unique environment that preserves detailed expressions of allocyclic signals such as those of climate and tectonics. Possible decryption of these signatures stems from careful scrutiny of the sedimentation dynamics (temporary base-level variations), lake-level fluctuations (accommodation), and resulting strata bounding surfaces that are used to build a conventional sequence stratigraphic framework. However, due to discrepancies between marine, to which this approach has been initially dedicated, and lacustrine settings (especially regarding the physical scale), deciphering climate from tectonic forcing becomes unwieldy in such interior basins. The present work deals with this challenge and provides insights from a case study where lacustrine sedimentation occurs on a tectonically active half-graben within a key climate region (Lake Ifrah, Northwest Africa). We conducted conventional sedimentological and high-resolution sequence-stratigraphic analyses, integrated with palaeolimnological proxies (geochemical elements and ostracod species). Up to five facies models (accounting for lithological domination, wind-driven energy, and lake-level state) and three lowest rank T-R sequences, deposited since the Marine Isotope Stage-3 (MIS-3), have been identified. Periods with sustained high lake levels appear to be mainly precession-paced (as during MIS-3 and the Early Holocene), although the role of obliquity is shown to influence the hydrological budget as well. Furthermore, sedimentation dynamics are shown to respond to millennial timescale climate variability associated with North Atlantic cooling events (Dansgaard-Oeschger stadials, Heinrich Events) and, interestingly, to enhanced Saharan winds during the deglacial period. On the other hand, tectonism had a rather instantaneous effect on lake level and sedimentation. Two tectonic pulses marking instantaneous differential hanging-wall subsidence have triggered a sharp drop in relative lake level, hence conditioning a forced regression.We highlight the importance of the conventional high-resolution sequence stratigraphy in shaping our understanding of the cyclic interplay between orbital/sub-orbital and tectonic forcings, so as the resulting sedimentation dynamics and lake-level cycles in lacustrine settings. We stress the role of the forced regression concept and associated systems tract and bounding surfaces, as well as the importance of using ostracods and geochemical proxies to trace transgressions and cryptic surfaces with sequence-stratigraphic significance, such as the maximum flooding surface, in lacustrine settings.

Holocene Evolution of Minor Mountain Lacustrine Basins in the Northern Apennines, Italy: The Lake Moo Case Study

Sedimentary systems developed in small (<1 km2) mountain lacustrine basins represent high-resolution geological archives, able to record subtle climatic and tectonic signatures over historical times. The studied example from the Mt. Ragola ophiolitic massif in the Northern Apennines (Italy) allowed us to better understand the role of the different (neo)tectonic and climatic events on the development and distribution of large landslides and lakes/peat bogs during the last 10 kyrs. Implementing a multidisciplinary approach that includes detailed acquisition of bedrock, geomorphological, topographic, and geophysical data, we detected and mapped ridge splitting, trenches, closed depressions, double ridges, and counterscarps. These morphostructures are interpreted as relevant factors influencing the distribution of sediments in historical times by shifting the position of the local equilibrium point (i.e., erosion vs. deposition) and have been correlated to a combination of climatic (i.e., increased flood events) and tectonic (i.e., spatial–temporal clustering of seismic shocks) forcing, starting from the demise of the Little Ice Age in the mid-19th century to the present-day situation. This approach allowed us to better describe the current changes in the hydrologic cycle, reaching beyond the limits of historical instrumental data. Furthermore, allowing the recognition and dating of recent tectonic vs. gravitational deformations, it also constitutes an integrative method for assessing the local geological hazard.

Signatures of provenance, tectonics and chemical weathering in the Tawi River sediments of the western Himalayan Foreland, India

Signatures of provenance, tectonics and chemical weathering in the Tawi River sediments of the western Himalayan Foreland, India

Along-strike sedimentological variability and architectural patterns of the transgression of a “mid”-Cretaceous braidplain system (Iberian Basin, eastern Spain): A tool for depicting eustatic and tectonic signatures within the framework of a global transgression

The mid-Albian to lower Cenomanian Utrillas Group is interpreted as an arid braidplain system in the Serranía de Cuenca region (Iberian Basin, Spain). In its proximal locations, it consists of an interaction of alluvial facies and subordinate aeolian deposits, with the occurrence of tidally-influenced deposits that reflect a general transgressive trend to the top of the succession. However, the distal areas of this braidplain system, where there was a more intense interaction with the Tethys Ocean, have remained understudied to date.We present a sedimentological study of two outcrops (Pajaroncillo and Reíllo) located at the distal sector of the braidplain system and 12 km apart, defining an along-strike relative position. Both outcrops differ notably regarding the arrangement and distribution of the facies associations identified despite their relative closeness. Based on the sedimentological analysis, the main transgressive and regressive trends of the succession have been identified, along with several correlatable stratigraphic surfaces. This has permitted us to distinguish five stages (T1–T5), which reflect the overall transgressive evolution of the succession: a continental aggradational stage (T1), dominated by alluvial deposition; an initial transgression stage (T2) dominated by an open coast setting in Reíllo and a restricted mudflat system in Pajaroncillo; a regressive stage (T3), during which continental alluvial to supratidal environments deposited; a resumption in transgression (T4), characterised by an array of inter- to subtidal environments; and a deepening stage (T5), when the transgression advanced landward, giving rise to the most distal deposits of the studied succession.The studied outcrops, located at the basin edge, have been integrated within the regional sequence stratigraphy framework, which was originally devised for more central areas of the Iberian Basin (Valencia Domain) and the Prebetic. This regional approach has allowed us to interpret the studied deposits as third-order depositional sequences. Besides, the supra-regional implications of this research are discussed within the broader context of Iberia and the subtleties of its tectono-sedimentary evolution during the Albian to Cenomanian times.

Tectonic signatures in the Triassic sediments of the Betic External Zone (southern Spain) as possible evidence of rifting related to the Pangaea breakup

This study was supported by Projects PID2019-104625RB-I00 and PGC2018-098272-B-I00 (Secretaria de Estado de I+D+I, Spain), B-RNM-072-UGR18 (FEDER Andalucia), P18-RT-4074, and RNM-208 (Junta de Andalucia).

Constraints of lithostratigraphy on the landscape evolution in response of erosion, climate and tectonics in the Marginal Ganga Plain, India

• Continental Sequence stratigraphy of the Marginal Ganga Plain is discussed. • 3-D model reveals the regional stratigraphic framework of the MGP. • The MGP comprises three depositional sequences and four sequence boundaries. • The MGP defines variable accommodation to sediment supply ratio (A/S). • Surface morphology of the MGP is controlled by the forebulge tectonics. The Marginal Ganga Plain (MGP) exhibits up to 160 m thick fluvial sedimentary record of the Late Quaternary forebulge dynamics. The mineralogical composition of sediments from forty boreholes reveals three major depositional sequences (DS1, DS2, and DS3) and four sequence boundaries Sb1, Sb2, Sb3 and Sb4 representing where different tectonic and climatic conditions exercised parental control. These sequence boundaries are demarcated by the shift in facies architecture. The Sb1 sequence boundary is locally developed over the basement rocks. The lowermost DS1 represents a thick floodplain and low sinuosity channel association at > 114 ka under stable landscape. This DS1 indicates a low rate of sedimentation and High Accommodation Systems Tract (HAST), characterized by positive > 1 A/S (Accommodation to Sediment) ratio. The sequence boundary Sb2 lies between DS1 and overlying DS2 implies a regional unconformity in the MGP and negative A/S ratio. Further, laterally extensive asymmetrical DS2, developed between ~ 80–54 ka with an erosional base overlies the DS1 and represents Low Accommodation Systems Tract (LAST) with A/S < 1. The Sb3, present between DS2 and DS3 denotes a sharp transition in facies architecture. The DS3 made up of the interfluves sequence shows an upward fining sequence deposited by vertically aggrading minor channels developed after 54 ka and is characterized as HAST where A/S > 1. The Sb4 represents the top surface of the DS3 and is currently getting degraded by ravinement processes. The varying pattern of alluvial architecture in the MGP defines variable A/S ratio and infer forebulge tectonic signature shaping the fluvial landscape.

DELINEATION OF LINEAMENTS IN THE REGION OF OUADDAI (CHAD) USING GRAVITY DATA

The gravity map of Ouaddai in the eastern region of Chad exhibits important anomalies that are often delimitated by the high gravity gradients, resulting from the density contrasts between various anomaly sources. This presumes an important tectonic activity in zone. Because of its arid and desert nature, the region is water-poor or even lacking water resources. The main source of water in the region is made of very deep aquifers. The gravity analysis from this study helps to better understand the network of faults in the area. Many complementary approaches of the gravity data processing have been applied, namely the horizontal gradient coupled to the upward and downward continuation and the Euler method. Results from the data filtring have allowed highlighting faults network in various directions (SSW-NNE S-N E-W, NE-SW) and the main direction. In total, 37 major faults were detected with various lengths including 11 (F6, F9, F10, F12, F17, F29, F31, F34, F35, F37) for a cumulative total length of 353 km oriented towards Bongor basin and 15 others (F1, F4, F7, F8, F11, F13, F18, F20, F23, F25, F26, F27, F28, F30, F33), for a cumulative total length of 621 km, oriented towards Doba basin. These faults are of paramount importance with great potential impacts on the regions hydrocarbon reservoirs. Thise results in one hand confirmed some known faults from the previous investigations. In the other hand, the study helps to identify other new and unknown tectonic signatures.

Origin and Precambrian paleogeography of the North Wulan terrane, northwestern China: A coherent model of the Tarim–Qilian–Quanji continent during the Columbia–Rodinia supercontinent cycle

The occurrence of pre-existing crust in orogenic belts has important implications for constraining and testing models of orogenic processes and for making paleogeographic reconstructions. Asynchronous magmatic–metamorphic rocks provide an opportunity to reconstruct the positions and constituent blocks of certain Precambrian fragments. Here we report Neoarchean to Neoproterozoic crustal rocks preserved in the North Wulan Terrane (NWT), northwestern China. Our new zircon geochronological data reveal the ca. 2.57 Ga magmatic and 2.0–1.72 Ga metamorphic events in the NWT, implying its involvement in the formation of Columbia supercontinent. Meanwhile, the NWT also recorded the 1.5–1.3, 1.1, 0.9–0.8 and 0.51–0.48 Ga magmatic-metamorphic activities, corresponding to the involvement of regional Precambrian fragments in cyclic supercontinent-related processes from Columbia to Greater Gondwana. Most of the ca. 2.57 and 0.9–0.8 Ga granites are characterized by enriched zircon εHf(t) values of –5.7 to +0.9 and –5.2 to +7.1. Moreover, some of these granites show fractionated characteristics, with high silica contents (80.3–82.3 wt% and 73.9–75.2 wt%), indicating that these rocks were derived mainly from remelting of 3.6–2.7 Ga mature crustal materials and 1.6–2.0 Ga crustal materials without or with minor contribution of juvenile materials, respectively. On the basis of these new results, regional stratigraphy, and previously determined geochemical data and tectonic signatures, it is concluded that the Quanji Massif and the Central Qilian, Hualong and North Wulan terranes probably formed a coherent Qilian–Quanji block as the southeastern extension of the Tarim Craton initiated from the Neoarchean. We propose that from the Neoarchean to early Paleoproterozoic, the primal Qilian–Quanji block was formed by long-term arc-related magma emplacement and reworking of ancient mature crustal materials in southeastern Tarim Craton. Subsequently, the juvenile Tarim–Qilian–Quanji continent was involved in tectonic cycles during its involvement in the supercontinents of Columbia and Rodinia.

Ongoing Inversion of a Passive Margin: Spatial Variability of Strain Markers Along the Algerian Margin and Basin (Mediterranean Sea) and Seismotectonic Implications

Subduction initiation is an important but still poorly documented process on Earth. Here, we document one of a few cases of ongoing transition between passive and active continental margins by identifying the geometrical and structural signatures that witness the tectonic inversion of the Algerian continental margin and the deep oceanic domain, located at the northern edge of the slow-rate, diffuse plate boundary between Africa and Eurasia. We have analyzed and tied 7900 km of deep seismic reflection post-stacked data over an area of ∼1200 km long and ∼120 km wide. The two-way traveltime lines were converted into depth sections in order to reconstruct and map realistic geometries of seismic horizons and faults from the seafloor down to the acoustic basement. Along the whole length of this young transitional domain, we identify a clear margin segmentation and significant changes in the tectonic signature at the margin toe and in the deep basement. While the central margin depicts a typical thick- and thin-skinned tectonic style with frontal propagation of crustal thrust ramps, the central-eastern margin (Jijel segment) reveals a higher strain focusing at the margin toe together with the largest flexural response of the oceanic lithosphere. Conversely, strain at the margin toe is limited in the western margin but displays a clear buckling of the oceanic crust up to the Spanish margin. We interpret these contrasting, segmented behavior as resulting from inherited heterogeneities in (1) the geometry of the Algerian continental margin from West to East (wrench faulting in the west, stretched margin elsewhere) and (2) the Miocene thermal state related to the diachronous opening of the Algerian basin and to the magmatic imprint of the Tethyan slab tearing at deep crustal levels. The narrow oceanic lithosphere of the Western Algerian basin is assumed to favor buckling against flexure. From the dimension and continuity of the main south-dipping blind thrusts identified at the margin toe, we reassess seismic hazards by defining potential lengths for ruptures zones leading to potential magnitudes up to 8.0 off the central and eastern Algerian margins.