Tectonic Evidence Research Articles

Angular Unconformity of the Late Quaternary Strata in the Hetao Basin, North of the Ordos Block (West China): Timing and Its Tectonic Implications

Following the uplift of the Tibet Plateau and the continuous subduction of the Pacific Plate, graben faulting began to appear around the Ordos Block in the Cenozoic. The Hetao Basin is a Cenozoic rift basin between the Ordos Block and the Yinshan Mountains, and Late Quaternary sedimentary strata, which have lacustrine facies, are widely distributed inside this basin. However, the evolution of the Hetao Basin and its related fault systems has been debated for a long time due to the lack of tectonic evidence. In this study, four sections named Haolaigou, Bianqianghao, Huhesala, and Hazigai are selected along the north margin of the Hetao Basin. With the lithology and structural analysis of the Upper Pleistocene series in these sections, two new angular unconformities are found within the 10 m thick sedimentary sequence of the lacustrine sediments. Based on the dating results, we speculate that these two upper and lower angular unconformities are formed between 33 ka BP and 40 ka BP, and 60 ka BP and 80 ka BP, respectively. The angular unconformities also provide tectonic constraints for the latest and ongoing tectonic activity in the Quaternary. This tectonic movement begins at around 80 ka BP and causes two different unconformities of the lower strata with varying degrees of deformation (tilt) but also leads to the final death of the ancient lake.

The Pennsylvanian System in the Sacramento Mountains, New Mexico, USA

Pennsylvanian sedimentary rocks in the Sacramento Mountains, New Mexico, comprise an ~1 km thick stratigraphic section. The Morrowan-Desmoinesian Gobbler Formation was deposited by shallow marine processes in and near the Alamo clastic trough. In this trough, the Desmoinesian-Missourian Gray Mesa Formation (Bug Scuffle Member, Gobbler Formation) is a relatively thin unit (Space History Member) representing the glacioeustatic Amado event. The Missourian-Virgilian Beeman Formation includes the lower, siliciclastic Indian Wells Canyon Member and overlying, carbonate-rich Horse Ridge Member. The Virgilian Holder Formation consists of algal bioherms (Little Dry Canyon Member) overlain by the mixed carbonate-siliciclastic Mill Ridge Member. The Virgilian-Wolfcampian Bursum Formation is mixed siliciclastic-carbonate strata that represent shallow marine and nonmarine paleoenvironments. Animal and plant remains occur throughout the section. Unit age determinations are primarily based on conodont faunas recovered from the Gobbler, Gray Mesa, and Beeman Formations. Many conodont faunas correlate with Midcontinent cyclothems. Extensive algal and foraminiferal fossils also were identified in limestones from the section and contributed to age determinations. The Beeman Formation in particular contains an extensive Missourian macroflora. The macroflora is of “mixed” composition, containing typical wetland elements intimately intermixed with taxa indicative of seasonally dry habitats. A seasonally wet-dry background climate is indicated. It is unlikely that droughttolerant plants were transported exclusively from “uplands.” Some plant remains have arthropod-feeding evidence. Previous analyses identified late Paleozoic ice-age glacioeustasy as the primary depositional driver of Pennsylvanian sedimentation in the Sacramento Mountains. We question this because of problems with those analyses and because of ample evidence of local tectonics and microclimate changes as important drivers of sedimentation in this area. Three Pennsylvanian Ancestral Rocky Mountain orogeny tectonic pulses can be identified in the Sacramento Mountains: Morrowan-Atokan, Missourian, and late Virgilian-Wolfcampian.

Evolution and properties of young oceanic crust: constraints from Poisson's ratio

SUMMARYThe seismic velocity of the oceanic crust is a function of its physical properties that include its lithology, degree of alteration and porosity. Variations in these properties are particularly significant in young crust, but also occur with age as it evolves through hydrothermal circulation and is progressively covered with sediment. While such variation may be investigated through P-wave velocity alone, joint analysis with S-wave velocity allows the determination of Poisson's ratio, which provides a more robust insight into the nature of change in these properties. Here we describe the independent modelling of P- and S-wave seismic data sets, acquired along an ∼330-km-long profile traversing new to ∼8 Myr-old oceanic crust formed at the intermediate-spreading Costa Rica Rift (CRR). Despite S-wave data coverage being almost four-times lower than that of the P-wave data set, both velocity models demonstrate correlations in local variability and a long-wavelength increase in velocity with distance, and thus age, from the ridge axis of up to 0.8 and 0.6 km s−1, respectively. Using the Vp and Vs models to calculate Poisson's ratio (σ), it reveals a typical structure for young oceanic crust, with generally high values in the uppermost crust that decrease to a minimum of 0.24 by 1.0–1.5 km sub-basement, before increasing again throughout the lower crust. The observed upper crustal decrease inσ most likely results from sealing of fractures, which is supported by observations of a significant decrease in porosity with depth (from ∼15 to <2 per cent) through the dyke sequence in Ocean Drilling Program borehole 504B. High Poisson's ratio (>0.31) is observed throughout the crust of the north flank of the CRR axis and, whilst this falls within the ‘serpentinite’ classification of lithological proxies, morphological evidence of pervasive surface magmatism and limited tectonism suggests, instead, that the cause is porosity in the form of pervasive fracturing and, thus, that this is the dominant control on seismic velocity in the newly formed CRR crust. South of the CRR, the values of Poisson's ratio are representative of more typical oceanic crust, and decrease with increasing distance from the spreading centre, most likely as a result of mineralization and increased fracture infill. This is supported by borehole observations and modelled 3-D seismic anisotropy. Crustal segments formed during periods of particularly low half-spreading rate (<35 mm yr−1) demonstrate high Poisson's ratio relative to the background, indicating the likely retention of increased porosity and fracturing associated with the greater degrees of tectonism at the time of their formation. Across the south flank of the CRR, we find that the average Poisson's ratio in the upper 1 km of the crust decreases with age by ∼0.0084 Myr−1 prior to the thermal sealing of the crust, suggesting that, to at least ∼7 Myr, advective hydrothermal processes dominate early CRR-generated oceanic crustal evolution, consistent with heat flow measurements.

The Cenozoic Malaguide Basin from Sierra Espuña (Murcia, S Spain): An Example of Geological Heritage

The Cenozoic Malaguide Basin from Sierra Espuña (Internal Betic Zone, S Spain) due to the quality of outcropping, areal representation, and continuity in the sedimentation can be considered a key-basin. In the last 30 years, a large number of studies with very different methodological approaches have been done in the area. Models indicate an evolution from passive margin to wedge-top basin from Late Cretaceous to Early Miocene. Sedimentation changes from limestone platforms with scarce terrigenous inputs, during the Paleocene to Early Oligocene, to the deep basin with huge supplies of turbidite sandstones and conglomerates during the Late Oligocene to Early Miocene. The area now appears structured as an antiformal stack with evidence of synsedimentary tectonics. The Cenozoic tectono-sedimentary basin evolution is related to three phases: (1) flexural tectonics during most of the Paleogene times to create the basin; (2) fault and fold compartmentation of the basin with the creation of structural highs and subsiding areas related to blind-fault-propagation folds, deforming the basin from south to north during Late Oligocene to Early Aquitanian times; (3) thin-skin thrusting tectonics when the basin began to be eroded during the Late Aquitanian-Burdigalian. In recent times some works on the geological heritage of the area have been performed trying to diffuse different geological aspects of the sector to the general public. A review of the studies performed and the revisiting of the area allow proposing different key-outcrops to follow the tectono-sedimentary evolution of the Cenozoic basin from this area. Eight sites of geological interest have been selected (Cretaceous-Cenozoic boundary, Paleocene Mula Fm, Lower Eocene Espuña-Valdelaparra Fms, Middle Eocene Malvariche-Cánovas Fms, Lowermost Oligocene As Fm, Upper Oligocene-Lower Aquitanian Bosque Fm, Upper Oligocene-Aquitanian Río Pliego Fm, Burdigalian El Niño Fm) and an evaluation has been performed to obtain four parameters: the scientific value, the educational and touristic potential, and the degradation risk. The firsts three parameters obtained values above 50 being considered of “high” or “very high” interest (“very high” in most of the cases). The last parameter shows always values below 50 indicating a “moderate” or “low” risk of degradation. The obtained values allow us considering the tectono-sedimentary evolution of this basin worthy of being proposed as a geological heritage.

Extensional tectonic setting of the Triassic metallogenic event in the Xiaoqinling goldfield: Evidence from <sup>40</sup>Ar-<sup>39</sup>Ar chronology of the tectonite

小秦岭金矿田位于华北克拉通南缘，同时也是秦岭复合型造山带的北缘组成部分。这里是我国第二大黄金产地，大规模金的成矿作用形成于早白垩世岩石圈大规模减薄的区域伸展构造背景。此外，越来越多的同位素年代学数据显示区内还存在三叠纪的成矿事件，发育钼、铅、铀、铌等与岩浆热液活动密切相关的多金属矿床，主要分布在小秦岭的南北边缘。目前，对于三叠纪多金属成矿作用的构造背景研究尚属薄弱，还没有直接对有关的构造变形开展过专题研究。为此，我们对小秦岭金矿田东端边缘的构造带进行构造观察和测量，并采用⁴⁰Ar-³⁹Ar年代学方法厘定变形时限，揭示三叠纪成矿事件的构造背景，为深入认识区内三叠纪多金属成矿作用特征和规律提供构造证据。研究表明，小秦岭金矿田三叠纪成矿事件发生于晚三叠世的伸展构造背景，可以划分为两个阶段，早期（221~216Ma）是在碰撞向后碰撞转折阶段的重力滑脱构造环境中，发育火成碳酸岩型钼铅、铀铌铅多金属矿床；晚期（214~203Ma）是在后碰撞阶段由俯冲板片断离或岩石圈拆沉作用等深部过程导致的区域伸展构造环境中，发育石英脉型和斑岩型钼矿床。小秦岭中生代晚三叠世和早白垩世两期成矿事件都是在伸展构造背景中的构造-岩浆-流体活动的产物。;The Xiaoqinling goldfield is located in the southern margin of the North China craton, which is also a marginal part of the Qinling complex orogeny. This goldfield is the second largest gold producer in China, where the large-scale gold mineralization developed in the extensional tectonic setting due to regional lithospheric thinning in Early Cretaceous. In addition, a growing number of isotope chronological data indicates that there also developed metallogenic events in Triassic here, which resulted in the formation of polymetallic deposits, such as molybdenum, lead, uranium, and niobium, etc. The Triassic deposits are closely related to magmatic hydrothermal activities, which are mainly distributed in northern and southern margins of the Xiaoqinling goldfield. However, there is little study on the tectonic setting of the Triassic polymetallic mineralization so far, especially the work on tectonic deformation is almost absent. Therefore, we have carried out the observation and measurement on the structure developed within the tectonic belt occurred in the eastern end of Xiaoqinling goldfield, and then employed the ⁴⁰Ar-³⁹Ar chronology method to determine the time limit of deformation, to reveal the tectonic setting of Triassic metallogenic event, providing tectonic evidence for further understanding characteristics and patterns of Triassic polymetallic mineralization in the Xiaoqinling goldfield. Our study shows that the metallogenic event occurred in the extensional tectonic setting in Late Triassic, which can be divided further into two stages. The early stage (221~216Ma) is of the transition stage from collision to post-collision, developing gravitational decollement tectonics, and carbonatite-type molybdenum, lead, uranium, and niobium polymetallic deposits are well developed in this extensional tectonic setting. The late stage (214~203Ma) is of post-collision, developing regional extensional tectonics caused by the break off of subducting slab or lithospheric delamination, and quartz vein-type and porphyry molybdenum deposits are well developed. The two metallogenic events in Late Triassic and Early Cretaceous in the Xiaoqinling goldfield are the products of the tectonic deformation-magmatic activity-fluid flow in extensional tectonic setting.

Channel Profile Response to Abrupt Increases in Mountain Uplift Rates: Implications for Late Miocene to Pliocene Acceleration of Intracontinental Extension in the Northern Qinling Range-Weihe Graben, Central China

Abstract Cenozoic extension of the Qinling range-Weihe Graben system has occurred in response to the uplift and growth of the Tibetan Plateau. Rapid exhumation of the northern Qinling range since the late Miocene is also regarded as resulting from the eastward expansion of the northeast part of Tibet. Tectonic evidence of this in the landscape remains unclear, but the fluvial system can provide a sensitive proxy record of tectonic forcing through space and over time scales of 105–107 a. Here, we present a study of channel profiles in the northern Qinling range, which forms a footwall highland separated from the southern Weihe Graben by active normal faults. We identify a population of knickpoints that separate river profiles with a gentle upstream gradient from steeper downstream reaches. Above the knickpoints, steepness indices increase from the central part towards the west and east, whereas channel steepness shows its highest values in the Huaxian-Huayin section. We observed no systematic changes of channel steepness pattern as a function of rock resistance, drainage area, or channel concavity. Correlation analysis between channel steepness and basin elevation and relief documents the control of tectonic forcing on regional topography. While bearing no relation to geological outcrop boundaries, the knickpoints show a strong correlation between retreat distance, catchment area, and river length. We infer that the knickpoints formed in response to an increase in mountain uplift rates and retreated as a kinematic wave. Under linear slope exponent n, we calibrated channel erodibility K~1.00±0.44×10−6 m0.1/a and derived knickpoint ages of 5.59±1.80 Ma. Combining the ages of onset of active faulting and mountain growth in the NE Tibetan Plateau (8–10 Ma, e.g., Liupan Shan, Jishi Shan, and eastern segments of the Haiyuan and Kunlun faults) and in the southwest Qinling range (9–4 Ma), we conclude that growth of the NE Tibetan Plateau began in the mid-Miocene time and expanded eastwards to the Qinling range-Weihe Graben during the late Miocene and early Pliocene.

Sub-axial deformation in oceanic lower crust: Insights from seismic reflection profiles in the Enderby Basin and comparison with the Oman ophiolite

We analyzed high-quality seismic reflection profiles across the ocean-continent transition in the Enderby Basin between the Kerguelen Plateau and the Antarctic margin. There, we observe numerous high-amplitude dipping reflections in the lower oceanic crust which was accreted at a magmatic spreading center as testified by the almost uniform 6.4-7 km thick crust and its unfaulted, flat top basement. The deep reflections are rooting onto the Moho and are dipping both ridgeward and continentward. They occur in dense networks in mature oceanic crust as well as close to the continentward termination of oceanic crust and in the ocean-continent transition zone. The comparison with field observations in the Oman ophiolite suggests that these lower crustal dipping reflectors could correspond to syn-magmatic faults. In Oman, very high temperature (up to syn-magmatic), high temperature (sub-solidus plastic deformation) and low temperature (brittle) deformation coexist along the same fault over distances of a few hundred meters at Moho level. This very high temperature gradient may be explained by the sudden and intense interaction between crystallizing magmas and hydrothermal fluids induced by the episodic nucleation of faults in a context of continuous magmatic spreading. The igneous layering becomes extremely irregular compared to its monotonous sub-horizontal orientation away from the faults which, together with enhanced hydrothermal alteration restricted to the fault zones, might change the physical properties (velocity, density) and increase the reflectivity of syn-magmatic faults. We further speculate that these processes could explain the brightness of the lower crustal dipping reflectors observed in our seismic reflection data. Both the seismic reflection profiles of the Enderby Basin and the Oman ophiolite show evidence for syn-accretion tectonism at depth together with the systematic rotation of originally horizontal lava flows or originally vertical dikes, pre-dating cessation of magmatic activity. This indicates ubiquitous deformation processes within the axial zone of magmatic spreading centers.

An extensional collapse model for the Lhasa–Qiangtang orogen in Central Tibet

Large-scale geodynamic processes not only result in magmatism, but also generate corresponding tectonic and sedimentary evidence. Therefore, magmatic, sedimentary, and tectonic evidences can be used to investigate lithospheric geodynamics. This study reports Late Cretaceous magmatic–sedimentary–tectonic evidence from the Tari Co area in central Tibet of post-collisional extensional collapse of the lithosphere. Zircon UPb dating shows that the Tari Co Late Cretaceous igneous rocks formed during ca. 90–75 Ma, including (1) thickened lower crust-derived adakitic granodioritic porphyry (90.4 ± 1.0 Ma and 90.7 ± 1.0 Ma) with high Sr/Y (34.4–53.8) ratios but low Y (5.48–9.14 ppm), Cr (10.2–14.4 ppm), and Ni (4.33–6.63 ppm) contents; (2) delaminated lower crust-derived dioritic porphyry dike (80.5 ± 1.6 Ma) with high Mg# (48–53) and moderate SiO2 (57.9–63.6 wt%), Cr (40.3–54.1 ppm) and Ni (43.5–56.4 ppm) contents; and (3) mafic dikes (74.8 ± 1.0 Ma) derived from different magma sources. According to sedimentary facies, gravel composition, and depositional age, the Upper Cretaceous Jingzhushan Formation in study area is divided into the lower subunit and upper subunit. Different from the typical lower subunit of the Jingzhushan Formation with sub-rounded limestone gravels as the main components, the upper subunit of the Jingzhushan Formation comprises slump breccias and coarse conglomerates. The gravel clasts in the upper subunit are mostly adakitic granodioritic porphyry (90.1 ± 1.4 Ma), indicating a sedimentary response to the crustal uplift associated with lithospheric delamination. In terms of structural deformation, the study area gradually entered the post-collisional setting in the late Late Cretaceous time (ca. 79-75 Ma), forming an east-west trending extensional tectonic system comprising extensional and step faults, grabens and horsts, and mafic dikes. Based on the Late Cretaceous geological records and previous studies, we suggest a three-stage post-collisional model including melting of thickened crust, lithospheric delamination, and gravitationally driven ductile collapse. Our results provide new insights into the evolution of continental orogens.

New insights into active tectonics and seismogenic potential of the Italian Southern Alps from vertical geodetic velocities

Abstract. This study presents and discusses horizontal and vertical geodetic velocities for a low strain rate region of the south Alpine thrust front in northeastern Italy obtained by integrating GPS, interferometric synthetic aperture radar (InSAR) and leveling data. The area is characterized by the presence of subparallel, south-verging thrusts whose seismogenic potential is still poorly known. Horizontal GPS velocities show that this sector of the eastern Southern Alps is undergoing ∼1 mm a−1 of NW–SE shortening associated with the Adria–Eurasia plate convergence, but the horizontal GPS velocity gradient across the mountain front provides limited constraints on the geometry and slip rate of the several subparallel thrusts. In terms of vertical velocities, the three geodetic methods provide consistent results showing a positive velocity gradient, of ∼ 1.5 mm a−1, across the mountain front, which can hardly be explained solely by isostatic processes. We developed an interseismic dislocation model whose geometry is constrained by available subsurface geological reconstructions and instrumental seismicity. While a fraction of the measured uplift can be attributed to glacial and erosional isostatic processes, our results suggest that interseismic strain accumulation at the Montello and the Bassano–Valdobbiadene thrusts it significantly contributing to the measured uplift. The seismogenic potential of the Montello thrust turns out to be smaller than that of the Bassano–Valdobbiadene fault, whose estimated parameters (locking depth equals 9.1 km and slip rate equals 2.1 mm a−1) indicate a structure capable of potentially generating a Mw>6.5 earthquake. These results demonstrate the importance of precise vertical ground velocity data for modeling interseismic strain accumulation in slowly deforming regions where seismological and geomorphological evidence of active tectonics is often scarce or not conclusive.

Seismological evidence for the earliest global subduction network at 2 Ga ago.

The earliest evidence for subduction, which could have been localized, does not signify when plate tectonics became a global phenomenon. To test the antiquity of global subduction, we investigated Paleoproterozoic time, for which seismic evidence is available from multiple continents. We used a new high-density seismic array in North China to image the crustal structure that exhibits a dipping Moho bearing close resemblance to that of the modern Himalaya. The relict collisional zone is Paleoproterozoic in age and implies subduction operating at least as early as ~2 billion years (Ga) ago. Seismic evidence of subduction from six continents at this age is interpreted as the oldest evidence of global plate tectonics. The sutures identified can be linked in a plate network that resulted in the assembly of Nuna, likely Earth's first supercontinent. Global subduction by ~2 Ga ago can explain why secular planetary cooling was not appreciable until Proterozoic time.

Pangea Rifting Shaped the East Antarctic Landscape

AbstractEast Antarctica remains one of the few continental regions on Earth where an understanding of the origin and causal processes responsible for topographic relief is largely missing. Low‐temperature thermochronology studies of exposed Precambrian basement revealed discrete episodes of cooling and denudation during the Paleozoic–Mesozoic; however, the significance of these thermal events and their relationship to topography across the continental interior remains unclear. Here we use zircon and apatite (U‐Th)/He thermochronology to resolve the low‐temperature thermal evolution of a poorly exposed section of East Antarctic basement in the Bunger Hills region and gain insights into the chronology and style of landscape evolution across East Antarctica. Thermal history modeling results indicate that Precambrian basement in the Bunger Hills region experienced a distinct cooling episode during the Late Paleozoic–Triassic, which we relate to ~2–4 km of regional exhumation associated with intracontinental rifting, followed by a second episode of localized cooling and ≤1 km exhumation during the Late Jurassic–Cretaceous separation of India from East Gondwana. These findings, combined with existing thermochronological and tectonic evidence, support a continent‐scale denudation event associated with uplift and exhumation of large sections of Precambrian basement during Late Paleozoic–Triassic Pangea‐wide intracontinental extension. By contrast, continental extension associated with the Jurassic–Cretaceous breakup of East Gondwana resulted in significant denudation only locally in regions west of the Bunger Hills. We propose that the combined effects of these Paleozoic–Mesozoic tectonic events had a profound impact on the topography across the East Antarctic interior and influenced the long‐term landscape evolution of East Antarctica.

Tectono‐Stratigraphic and Thermal Evolution of the Western Betic Flysch: Implications for the Geodynamics of South Iberian Margin and Alboran Domain

AbstractThe Betic‐Rif orogen is a key region to understand the evolution of the plate boundary between Africa and Iberia/Europe. This study focuses on the Flysch Complex, which is considered the sedimentary cover of a domain originally positioned between the Iberian and Alboran margins. Based on stratigraphic and depositional evolution constraints, evidence for salt tectonics, combined with new apatite fission‐tracks (AFT) and (U‐Th‐Sm)/He ages from the Flysch Complex and the Subbetic Zone, we propose a geodynamic interpretation for the formation of the Betic Cordillera, accounting for moderate N‐directed transport of the Flysch Complex and synchronous exhumation between External and Internal Zones of the Betic. Early contraction between Africa and Iberia/Europe is reflected in the Cretaceous Flysch basin by a prolonged period of residence in the partial annealing zone for AFT and onset of foreland subsidence at 50 Ma. This stage lasted until the Early to Middle Miocene (20–15 Ma), marked by the rapid succession, in less than 5 Ma, of the deposition of Cenozoic flysch and their rapid exhumation. This event is interpreted to reflect the W‐directed retreating mantle delamination between Africa and Iberia margins at the origin of the collapse of the proto‐Betic orogenic domain and formation of the Alboran domain.

Geology of Grimaldi Basin on the Moon: Evidence for volcanism and tectonism during the Copernican period

The Grimaldi Basin, centered at (5.2° S, 68.6° W), is a ~400 km diameter highly degraded, Pre-Nectarian double-ring impact structure on the Moon located near the western edge of the Oceanus Procellarum. Hyperspectral data from Moon Mineralogy Mapper (M3), FeO estimates from Kaguya Lunar Multiband Imager (MI) data and TiO2 estimates from WAC 321/415 nm ratio have been used to study the compositional make-up of the Grimaldi mare basalt. Additionally, morphological studies and crater chronology have been carried out using moderate to very high-resolution images from Lunar Reconnaissance Orbiter (LRO) to decipher the geological evolution of the Grimaldi Basin. Mare Grimaldi are dominantly composed of clino-pyroxenes with olivine and/or of feldspathic mixing with lateral as well as depth-wise variations in composition. In the south central part, the basin experienced Copernican aged volcanism ~700 Ma ago resulting in the formation of olivine bearing basalts with high FeO and TiO2 content. Cross-cutting of small Copernican craters by fresh wrinkle ridges and lobate scarps has been observed at several places in the basin suggesting that tectonic activities occurred in the basin within the past ~50 Ma-1 Ga. Thus, similar to the Oceanus Procellarum region, Grimaldi Basin was also geologically active during the Copernican period.

Geomorphology, morphometry and evidence of tectonics in the Araçaji Chart, Eastern Edge of Paraíba

The present article has as objective to analyse the interrelations between the relief, the lithology, the hydrography and the conspicuous evidence of tectonics on the area covered by the Araçaji 1: 25.000 chart, State of Paraíba, Northeast Brazil. The results showed the existence of anomalies in the study area, such as altimetry with disproportionate dimensions to the regional average, strong channel inflections, rectilinear channels and modified patterns in the drainage network influenced mainly by the local geological framework and possibly the Cenozoic tectonics. Allied to this qualitative analysis, morphometric indices were applied, for which the values obtained corroborate the tectonic influence in the area. The methodology used in this article requires a greater comparison to results obtained in other areas of the Atlantic continental border. Geomorfologia, morfometria e evidências de tectônica na Folha Araçaji, Borda Oriental da ParaíbaR E S U M OO presente artigo tem como objetivo analisar as interrelações existentes entre o relevo, a litologia, a hidrografia e a conspícua evidência tectônica da área da folha Araçaji na escala de 1:25.000, Estado da Paraíba, Nordeste do Brasil. Os resultados mostraram a existência de anomalias na área em questão, como altimetria com cotas desproporcionais à média da região, fortes inflexões na direção de canais, canais retilíneos e padrões modificados na rede de drenagem influenciados, principalmente, pelo arcabouço geológico local e, possivelmente, pela tectônica cenozoica. Aliados a essa análise qualitativa, foram aplicados índices morfométricos, nos quais os valores obtidos corroboram com a influência tectônica na área. A metodologia empregada neste artigo carece de maior comparação com resultados obtidos em outras áreas da borda continental atlântica.Palavras-chave: Geomorfologia Estrutural; Bacia Hidrográfica; Rio Mamanguape; Rede de drenagem.

Morphostructural evidence of Late Quaternary tectonics at the Po Plain-Northern Apennines border (Lombardy, Italy)

The landscape of foreland basins adjacent to active mountain ranges evolves under the control of basinward tectonic propagation of the structural fronts, in competition with climate dynamics. The resulting palimpsest landscapes record the sequence of geomorphic evolutionary steps, and the spatial-temporal relations between the active geological processes. However, a precise evolutionary sequence is hard to decipher from low-relief settings, like the wide alluvial plains worldwide. About these topics we investigate the sequence of tectonic- and climate-driven processes which shaped the palimpsest landscape of a large Quaternary foreland basin, the Po Plain in Northern Italy. Above the average glacio-fluvial and alluvial plain of the basin, Late Quaternary intra-basin reliefs emerge, owing to syndepositional ramp folding and uplift driven by N-wards propagation of the outermost thrust fronts of the active N-Apennine chain.The incremental tectono-morphological and depositional history of the region permits to describe the propagation of the structural front as a polyphase process, involving uplift, wrenching and late collapse of the reliefs. This history is documented by: relicts of uplifted planation surfaces covered by latest Pleistocene weathered loess units; polygonal facets, landslides and slope wedges along faults delineating the slopes of the largest among the intra-basin reliefs (San Colombano hill); hydrographic anomalies on the relief network (river diversions, piracy and perching of valleys, paleovalley fills), river diversions and cross-cut relations on the adjacent alluvial plain. These features testify the thrust-fold-related, pre-LGM uplift of the Late Pleistocene alluvial stratigraphy, the subsequent segmentation of the relief in differently uplifting blocks along Riedel faults, owing to activation of a transfer fault zone, and the LGM extensional collapse of the relief, along inherited fault systems. A late, post-glacial outwards propagation of the Apennine thrusts was buttressed by the opponent Alpine thrust-belt front and induced the entrenchment of the river network. River diversions on the plain were controlled by the orientation of the San Colombano hill fault systems. The distribution of historical earthquakes and the present-day geodetic data are coherent with this evolution.The study documents how, in foreland basins, the syndepositional propagation of “blind” orogenic fronts shapes the palimpsest landscape with superimposed morphological, tectonic, and stratigraphic features of paleoseismic significance, and permits comparisons with the other low-relief basin settings worldwide.

Mysterious tsunami in the Caribbean Sea following the 2010 Haiti earthquake possibly generated by dynamically triggered early aftershocks

Dynamically triggered offshore aftershocks, caused by passing seismic waves from main shocks located on land, are currently not considered in tsunami warnings. The M7.0 2010 Haiti earthquake epicenter was located on land 27 km north of the Caribbean Sea and its focal mechanism was oblique strike-slip. Nevertheless, a tsunami recorded on a Caribbean Deep-Ocean Assessment and Reporting of Tsunami (DART) buoy and a tide gauge produced runup heights of 1–3 m along Haiti southeast coast. Earthquake finite-fault model inversions of the DART waveform suggest that a reverse fault doublet with magnitudes of M6.8 and M6.5 located 85 km southwest of the epicenter may have excited the tsunami. This doublet collocates with dynamically triggered aftershocks, derived from back-projection analysis, that occurred 20-60 s after the main shock of the Haiti earthquake. The aftershocks are within a region of maximum dynamic strain predicted by the main shock, on a possibly tectonically active submarine ridge southwest of Haiti's Southern Peninsula. The agreement between the tsunami finite-fault source models and the seismic and tectonic evidence suggests that earthquakes on land, even strike-slip faults, can generate tsunamis by dynamically triggering offshore aftershocks.

Intensifying aeolian activity following the end‐Permian mass extinction: Evidence from the Late Permian–Early Triassic terrestrial sedimentary record of the Ordos Basin, North China

AbstractSedimentary successions provide direct evidence of climate and tectonics, and these give clues about the causes of the mass extinction around the Permian–Triassic boundary. Terrestrial Permian–Triassic boundary strata in the eastern Ordos Basin, North China, include the Late Permian Sunjiagou, Early Triassic Liujiagou and late Early Triassic Heshanggou formations in ascending order. The Sunjiagou Formation comprises cross‐bedded sandstones overlaid by mudstones, indicating meandering rivers with channel, point bar and floodplain deposits. The Liujiagou Formation was formed in braided rivers of arid sand bars interacting with some aeolian dune deposits, distinguished by abundant sandstones where diverse trough and planar cross‐bedding and aeolian structures (for example, inverse climbing‐ripple, translatent‐ripple lamination, grainfall and grainflow laminations) interchange vertically and laterally. The Heshanggou Formation is a rhythmic succession of mudstones interbedded with thin medium‐grained sandstones mainly deposited in a shallow lacustrine environment. Overall, the sharp meandering to braided to shallow lake sedimentary transition documents palaeoenvironmental changes from semi‐arid to arid and then to semi‐humid conditions across the Permian–Triassic boundary. The die‐off of tetrapods and plants, decreased bioturbation levels in the uppermost Sunjiagou Formation, and the bloom of microbially‐induced sedimentary structures in the Liujiagou Formation marks the mass extinction around the Permian–Triassic boundary. The disappearance of microbially‐induced sedimentary structures, increasingly intense bioturbation from bottom to top and the reoccurrence of reptile footprints in the Heshanggou Formation reveal gradual recovery of the ecosystem after the Permian–Triassic boundary extinction. This study is the first to identify the intensification of aeolian activity following the end‐Permian mass extinction in North China. Moreover, while northern North China continued to be uplifted tectonically from the Late Palaeozoic to Late Mesozoic, the switch of sedimentary patterns across the Permian–Triassic boundary in Shanxi is largely linked to the development of an arid and subsequently semi‐humid climate condition, which probably directly affected the collapse and delayed recovery in palaeoecosystems.

Luminescence dating of Late Pleistocene faults as evidence of uplift and active tectonics in Sardinia, W Mediterranean

AbstractPrecise dating of the activity of Late Pleistocene to Holocene neo‐tectonic structures is crucial to quantify the rate of deformation in low‐seismicity regions. Sardinia is a relatively stable continental fragment set in the middle of the tectonically active Western Mediterranean belt. This paper provides evidences of significant uplift of northwest Sardinia that support an ongoing tectonic activity since the Marine Isotopic Stage 7 (MIS 7; ca. 220 ka). In particular, it documents for the first time Late Pleistocene to Holocene tectonics based on luminescence dating of travertine sealing a major NNE‐SSW fault.

Fold-Axis Parallel Extension along the Southern Ending of the Quito (Ecuadorian Andes) Fault System: Implications in River Network and Aquifer Geometry

Extension parallel to fold axis represents a rarely documented tectonic process that occur in different tectonic settings such as large-scale orogenic belts, mountain ranges, and thrust and fold belts. Along the Ecuadorian Andes, extensional systems have only been reported as tectonic boundaries of local intermontane basins, pull-apart depressions and outer-arc stretching faults along hinge surfaces in compressional antiforms. The Quito fault system shows in planform view a right-stepping en-echelon arrangement which southernmost anticlinal ridge terminates as an arcuate structure locally dissected by the Saguanchi Gorge. Valley hillslopes of this transverse drainage display evidence of tectonic extension featured by several swarms of normal faults commonly dipping towards the valley bottom. Magnetotellurics sounding obtained across a N‒S line parallel to the crestline axis reveals three contiguous and distinguishable domains of which the central one shows a wedge-shaped geometry pointing down. This tectonic evidence for ongoing extensional deformation suggests that the process of fold-axis-parallel extension controlled the tectonic development and dissection of the Saguanchi Gorge across the anticlinal ridge. This episode marks a significant change in the groundwater recharge and aquifer storage conditions between the Quito basin and the inter-Andean Depression aquifer systems, currently in production to provide the potable water supply for the Quito city.

Evidence for olivine deformation in kimberlites and other mantle-derived magmas during crustal emplacement

This paper highlights published and new field and petrographic observations for late-stage (crustal level) deformation associated with the emplacement of kimberlites and other mantle-derived magmas. Thus, radial and tangential joint sets in the competent 183 Ma Karoo basalt wall rocks to the 5 ha. Lemphane kimberlite blow in northern Lesotho have been ascribed to stresses linked to eruption of the kimberlite magma. Further examples of emplacement-related stresses in kimberlites are brittle fractures and close-spaced parallel shears which disrupt olivine macrocrysts. In each of these examples, there is no evidence of post-kimberlite regional tectonism which might explain these features, indicating that they reflect auto-deformation in the kimberlite during or immediately post-emplacement. On a microscopic scale, these inferred late-stage stresses are reflected by fractures and domains of undulose extinction which traverse core and margins of some euhedral and anhedral olivines in kimberlites and olivine melilitites. Undulose extinction and kink bands have also been documented in olivines in cumulates from layered igneous intrusions. Our observations thus indicate that these deformation features can form at shallow levels (crustal pressures), which is supported by experimental evidence. Undulose extinction and kink bands have previously been presented as conclusive evidence for a mantle provenance of the olivines—i.e. that they are xenocrysts. The observation that these deformation textures can form in both mantle and crustal environments implies that they do not provide reliable constraints on the provenance of the olivines. An understanding of the processes responsible for crustal deformation of kimberlites could potentially refine our understanding of kimberlite emplacement processes.