Tectonic Mechanisms Research Articles

Transient Cenozoic tectonic stages in the southern margin of the Caribbean plate: U-Th/He thermochronological constraints from Eocene plutonic rocks in the Santa Marta massif and Serranía de Jarara, northern Colombia

We use U-Th/(He) zircon and apatite thermochronology and Al in hornblende geobarometry from Eocene granitoids of the Sierra Nevada de Santa Marta and Guajira uplifted massifs in northern Colombia to elucidate the exhumation history of the northern South America continental margin and its bearing to Cenozoic Caribbean– South American plate interactions. Aluminium in hornblende geobarometry from the Eocene Santa Marta batholith yields pressures between 4.9±0.6kbar and 6.4±0.6kbar, which indicate that at least, 14.7-19.2km of unroofing took place since 56-50Ma in the northwestern Sierra Nevada de Santa Marta. In the Guajira Peninsula, calculated pressures for the Eocene Parashi stock are 2.3±0.6kbar and 3±0.6kbar. Stratigraphic considerations pertaining to Oligocene conglomerates from the Guajira area suggest that 6.9-9km of crust was lost between 50Ma and ca. 26Ma. U-Th/He zircon and apatite thermochronology from granitoids in the Sierra Nevada de Santa Marta shows the existence of major exhumation events in the Late Eocene (ca. 45-40Ma), Late Oligocene (ca. 25Ma) and Miocene (ca. 15Ma). The Guajira region records the Late Eocene to Early Oligocene (35-25Ma) event, but it lacks evidence for the Miocene exhumation phase. These differences reflect isolation of the Guajira region from the Sierra Nevada de Santa Marta and the Andean chain due to extensive block rotation and transtensional tectonics that affected the region during post-Eocene times. The post-Eocene events correlate in time with an increased convergence rate and the frontal approach of North and South America. It is suggested that the two major tectonic mechanisms that govern exhumation in these Caribbean massifs are: 1) subduction of the Caribbean plate, and 2) post Eocene changes in plate convergence obliquity and rates that caused the South American continental margin blocks to override the Caribbean plate. Temporal correlation with other Caribbean and Northern Andean events allows to resolve the regional Cenozoic plate tectonic reorganizations experienced by the South American, Caribbean and Pacific plates at a regional scale

Slab rupture and delamination under the Betics and Rif constrained from receiver functions

We map the lithospheric structure under the westernmost Mediterranean convergent setting interpreting P-receiver functions obtained from a dense seismic network. No orogenic root occurs under the eastern and great part of the central Betics. However, the subducted South Iberian continental lithosphere is found beneath the western Betics where the Iberian Moho reaches depths of approximately 65km, dipping gently towards the SE. Meanwhile, at the Rif, strong crustal and lithospheric thickness contrasts occur across the Nekor NW-SE sinistral fault that overlies the region of present slab tearing. East of the Nekor fault there is no orogenic root and the crust has been thinned to approximately 22km, whilst to the west the crust reaches 55km thickness and the Maghrebian continental lithosphere is attached to the lithospheric slab imaged by tomography under the Alboran basin. These data suggest that subduction rollback under the Alboran and Algerian basins, together with continental slab tearing or detachment producing edge delamination under the Betics and Rif have been the main tectonic mechanisms driving extension, magmatism and regional uplift in the westernmost Mediterranean since the Late Miocene until present. The surface expression of edge-delamination and slab tearing is marked by regional uplift, denudation of HP rocks in elongated core-complex type domes, late Miocene volcanism in the Eastern Betics and Rif, and by large NE-SW strike-slip transfer faults like the Alpujarras, Crevillente, Torcal or Nekor faults that accommodate strong gradients in crustal displacements. The Iberian slab is still attached to the oceanic slab imaged under the Alboran basin at the western Betics where intermediate depth seismicity, recent dextral strike-slip faulting and folding, could reflect slab tearing. Meanwhile, active faulting and differential GPS-measured displacements would mark slab tearing beneath the Rif coinciding with the trace of the sinistral Nekor fault.

Age of Alpine Corsica ophiolites revisited: Insights from in situ zircon U–Pb age and O–Hf isotopes

Knowledge of the age and timing of ophiolite sequences is essential for understanding the mechanisms of plate tectonics. The ophiolites in the Schistes Lustrés and the Upper nappes of Alpine Corsica represent remnants of the Liguria–Piemonte ocean basin that formed as a branch of the Central Atlantic basin during the opening of the Mesozoic Western Alpine Tethys. Despite numerous isotopic and paleontological studies, the age and timing of the ophiolites in the Schistes Lustrés nappe are still controversial. This study presents integrated in situ analyses of zircon U–Pb age and O–Hf isotopic data for ophiolitic gabbros and plagiogranites from three localities in the Schistes Lustrés nappe of Eastern Corsica. Our new results demonstrate that these rocks crystallized synchronously at ~159Ma, approximately 10m.y. younger than the ophiolites in the Balagne Upper nappe. Zircons from the gabbros and plagiogranites are characterized by highly positive εHf(t) (+15.0 to +15.9) and mantle-like δ18O (5.2–5.4‰) values. Thus, these ophiolitic rocks were cogenetic, and crystallized from magmas produced by partial melting of a depleted, N-MORB type mantle. By contrast, in the Balagne Upper nappe, the ~169Ma ophiolites contain numerous xenocrystic zircons inherited from a continental crust. Our current knowledge of isotopic geochronology and geochemistry supports a paleogeographic reconstruction, in which the earliest ophiolites in the Balagne nappe were emplaced close to a continental margin at ~169Ma, while the N-MORB type ophiolites in the Schistes Lustrés nappe were likely formed approximately 10m.y. later in the central part of the Liguria–Piemonte oceanic basin. The relative location of the Schistes Lustrés and Balagne Upper nappes with respect to continental margins is discussed.

Late Quaternary climatic and tectonic mechanisms driving river terrace development in an area of mountain uplift: A case study in the Langshan area, Inner Mongolia, northern China

The Langshan Range is located in the western Yin Mountain orogenic belts and the western Hetao fault-depression zone in Inner Mongolia, northern China. This area is on the northwestern margin of the East Asian monsoon region. The fluvial terraces in the transverse drainage of the Langshan Range represent a primary geomorphic response to local tectonic uplift and climatic changes. The terrace evolution was reconstructed using a combination of optically stimulated luminescence (OSL) dating and terrace tread measurements. The terraces, designated T4 through T1, were abandoned at about 58.00, 46.25, 32.19, and 15.79ka BP, respectively. Their aggradation occurred primarily during cold periods of the last glacial stage, and incision occurred primarily during shifts from cold to warm climate stages. Geomorphic analysis showed the terrace heights were controlled by the tectonic uplift in the area. Differences in river incision rates and terrace geomorphic features indicate that the uplift of the Langshan Range included a component of tilting north to south during the period of 58.00–41.28ka BP, whereas the uplift of the Langshan area tended to be equal on a regional scale after 32.19ka BP.

Earth’s youngest known ultrahigh-temperature granulites discovered on Seram, eastern Indonesia

Episodes of ultrahigh-temperature (UHT, ≥900 °C) granulite metamorphism have been recorded in mountain belts since the Neoarchean. However, evidence for the tectonic mechanisms responsible for the generation of such extreme thermal conditions is rarely preserved. Here we report the discovery of 16 Ma UHT granulites—the youngest identified at the Earth’s surface—from the Kobipoto Mountains of Seram in eastern Indonesia. UHT conditions were produced by a modern tectonic system in which slab rollback–driven lithospheric extension caused core complex–style exhumation of hot subcontinental lithospheric mantle. Overlying continental crust, heated and metamorphosed by exhumed lherzolites, developed spinel + quartz and sapphirine-bearing residual assemblages, shown by phase equilibria modeling to have required temperatures of ∼950 °C at ∼8 kbar pressure. Seram is therefore a possible modern analogue for ancient orogens that incorporate UHT granulites.

Investigations into co-seismic deformation and strain in South Korea following the 2011 Tohoku-oki earthquake using GPS CORS data

Geographically located in between Japan and China, the crustal movement of the Korean Peninsula is important in understanding the mechanisms of earthquakes and plate tectonics in the northeast Asian region. The Tohoku-oki earthquake which occurred on March 11, 2011, caused crustal movements in both Korea and Japan. This study attempts to calculate the amount of crustal movement of the Korean Peninsula as a result of the Tohoku-oki earthquake and to compare the calculation with previous crustal movements of the Korean Peninsula, thereby analyzing the Tohoku-oki earthquake’s impact on the Korean Peninsula. We found that the crust of South Korea instantly moved 2.8 cm eastward on average due to the Tohoku-oki earthquake; when this figure is compared with the past crustal movements of the Korean Peninsula, the instant shift is equivalent to the peninsula’s annual shift of coordinates. In terms of the change in strain, analysis showed that the Korean Peninsula was compressed in the south-north direction while expanding in the east-west direction by the Tohoku-oki earthquake, which is opposite to the crustal deformation pattern in the past.

The Faroe–Shetland Basin: a regional perspective from the Paleocene to the present day and its relationship to the opening of the North Atlantic Ocean

Abstract The Faroe–Shetland Basin is located offshore NW Scotland on the SE margin of the Atlantic Ocean and comprises numerous sub-basins and intra-basin highs that are host to a number of significant hydrocarbon discoveries. The principal hydrocarbon discoveries are in Paleocene–Eocene strata, although earlier strata are known, and their existence is therefore intimately linked to the opening and evolution of the North Atlantic from 54 Ma. The final rifting and separation of Greenland from Eurasia is commonly attributed to the arrival of a mantle plume which impacted beneath Greenland during early Tertiary time. Moreover, the ensuing plate separation is commonly described in terms of instantaneous unzipping of the North Atlantic, whereas in reality proto-plate boundaries were more diffuse during their inception and the linked rift system which we see today, including connections with the Arctic, was not established until Late Palaeogene–Early Neogene time. From a regional analysis of ocean basin development, including the stratigraphic record on the adjacent continental margins, the significance of the Greenland–Iceland–Faroe Ridge and the age and role of Iceland, we propose a dual rift model whereby North Atlantic break-up was only partial until the Oligo-Miocene, with true final break-up only being achieved when the Reykjanes and Kolbeinsey ridges became linked. As final break-up coincides with the appearance of Iceland, this model negates the need for a plume to develop the North Atlantic with rifting reliant on purely plate tectonic mechanisms, lithospheric thinning and variable decompressive upper mantle melt along the rifts.

Origin(s) of Antarctica's Wilkes Subglacial Basin

AbstractThe Wilkes Subglacial Basin (WSB), the largest subglacial basin in East Antarctica, is a topographic depression of continental proportions that lies beneath the East Antarctic continental ice sheet. Discovered by the US Victoria Land Traverse 1959–60, the origin of the WSB and the influence of palaeoclimate on its overlying continental ice sheet have remained uncertain since the time of its discovery. Most explanations of origin favour lithospheric structural control as a function of tectonic activity. Lithospheric flexure due to thermally or isostatically induced uplift of the Transantarctic Mountains was suggested in the 1980s. Lithospheric extension and rifting was proposed in the 1990s. More recent investigations have revealed the presence of fold and thrust belts, casting doubt on flexural and extensional hypotheses as the primary mechanisms, suggesting instead a compressional scenario. While remaining inconclusive, these tectonic mechanisms in one form or another, or in combination, are now believed to have provided the structural control for the origin of the WSB. Not yet comprehensively examined, however, is the role of non-tectonic processes in the formation of the WSB, as they may have influenced the size, configuration, subglacial sedimentation and subglacial topography of the WSB. In this paper we review the tectonic hypotheses and examine post-tectonic climate change along with glacial and marine processes as potentially significant factors in the present condition and configuration of the WSB. In the process, we find that there are a number of features not included in previous investigations that may have been major factors in the modification of the subglacial basin.

Different-scale tendencies of the evolution of the composition of volcanogenic pyrite deposit ores

The changing contents and ratios of certain primary and secondary elements in the composition of volcanogenic pyrite deposit ores reveal two tendencies that have different scales of their manifestation in geological time and space. The first one is expressed in terms of the evolution of geochemical differences of deposit ores during relatively short periods of pyrite formation in some ore provinces. It is determined by the changing stages of tectonic development and corresponding alterations of magma composition within the provinces. The other tendency is global; it is expressed in terms of century-old changes of ore geochemical characteristics in the deposits that have developed in different pyrite formation epochs over the last 3.5 billion years, probably as a result of the differentiation of Earth material, a gradual temperature drop in the upper mantle, and the associated development of tectonic conditions and magma-formation mechanisms.

The late Silurian–Middle Devonian long-term eustatic cycle as a possible control on the global generic diversity dynamics of bivalves and gastropods

Abstract A long-term eustatic cycle (fall and subsequent rise of the global sea level) embraced the late Silurian-Middle Devonian time interval. Potentially, these sea-level changes could drive global biodiversity. The stratigraphic ranges of 204 bivalve genera and 279 gastropod genera included into the famous Sepkoski database allow reconstructing changes in the total diversity and the number of originations and extinctions of these important groups of marine benthic macro- -invertebrates during this interval. None of the recorded parameters coincided with the long-term global sea-level cycle. It cannot be not excluded, however, that the global sea-level changes did not affect the regions favourable for bivalve and gastropod radiation because of regional tectonic mechanisms; neither can it be excluded that the eustatic control persisted together with many other extrinsic and intrinsic controls. Interestingly, the generic diversity of gastropods increased together with a cooling trend, and vice versa. Additionally, the Ludlow, Eifelian, and Givetian biotic crises affected, probably, both fossil groups under study. There was also a coincidence of the relatively high bivalve generic diversity, initial radiation of gastropods and the entire biota, and the diversification of brachiopods with the Early Devonian global sea-level lowstand, and this may be interpreted as evidence of a certain eustatic control on the marine biodiversity.

Are subduction zones invading the Atlantic? Evidence from the southwest Iberia margin

Subduction initiation at passive margins plays a central role in the plate tectonics theory. However, the process by which a passive margin becomes active is not well understood. In this paper we use the southwest Iberia margin (SIM) in the Atlantic Ocean to study the process of passive margin reactivation. Currently there are two tectonic mechanisms operating in the SIM: migration of the Gibraltar Arc and Africa-Eurasia convergence. Based on a new tectonic map, we propose that a new subduction zone is forming at the SIM as a result of both propagation of compressive stresses from the Gibraltar Arc and stresses related to the large-scale Africa-Eurasia convergence. The Gibraltar Arc and the SIM appear to be connected and have the potential to develop into a new eastern Atlantic subduction system. Our work suggests that the formation of new subduction zones in Atlantic-type oceans may not require the spontaneous foundering of its passive margins. Instead, subduction can be seen as an invasive process that propagates from ocean to ocean.

Intraplate volcanism off South Greenland: caused by glacial rebound?

Plate tectonic mechanisms explain most of the dominant types of volcanism. Intraplate volcanism has been attributed to hot spots associated with mantle plumes, though alternative explanations have been proposed for settings not fed by plumes. We report young intraplate volcanism in an amagmatic margin south of Greenland in a 40 Ma old plate. An elongated structure with a length of 110 km and a height of 2500 m above basement extends at an oblique angle to the Mid Atlantic ridge and the fracture zones. We propose that this structure is related to reactivation of a rudimentary fracture zone by edge-driven convection across Minna Fracture Zone and partial melting caused by glacial rebound during the periodic melting of ice on Greenland in the last 3 Ma.

Tectonostratigraphic evolution of the Carboniferous arc-related basin in the East Junggar Basin, northwest China: Insights into its link with the subduction process

The Junggar Basin in northwest China is located in the southern part of Central Asian Orogenic Belt (CAOB). The tectonic mechanisms controlling sedimentary fill through space and geologic time within dynamic subduction environments remains poorly understood in the Junggar Basin. This paper presents an integrated tectonostratigraphic and tectonic evolution of Carboniferous volcanic arcs and associated basin in the East Junggar Basin using multichannel seismic reflection data, well-line data, geochronological and geochemical data. We propose a new tectonic-sedimentary model for the East Junggar Basin during the Carboniferous time based on geochemistry of volcanic rocks and four seismic tectonostratigraphic sequences. The Carboniferous succession of volcanogenic sediments represents progradational sequences mainly supplied from the northeast. The negative Ta, Nb and Ti anomalies, low Rb/Ba ratios (0.01–0.15) and high incompatible trace element (such as Th, U) abundances of the basalts suggest that metasomatism may have occurred during the Late Carboniferous subduction. The northeastward increase of the Ba/La ration, K, Ca and decrease of Na, Ti and Fe indicate the northeast-dipping subduction polarity in the East Junggar Basin. During the Early Carboniferous, tectonism in the East Junggar Basin took place in two stages. Deep marine sediments widely covered the Junggar region, and then the depocenter slightly shifted towards southwest, which probably resulted from the northeast-dipping subduction that led to the southwestward tilting of the Dinan area. During the early stage of the Late Carboniferous, mixed marine and nonmarine deposits initially onlapped the volcanic arc towards the northeast, and then gradually downlapped to the southwest. At the end of the Late Carboniferous, subduction of the Junggar Ocean continued towards northeast and depositional environment became nonmarine. The main axis of the depocenter rapidly migrated towards southwest that resulted from the southwestward tilting of the Luliang region.

Toward a dynamic concept of the subduction channel at erosive convergent margins with implications for interplate material transfer

Convergent plate boundaries accommodate intraplate displacement within a ∼100–1000 m thick shear zone. Marine geophysicists typically define this zone, the subduction channel (SC), as the sedimentary layer between the downgoing oceanic crust and the base of the upper plate. Geologists and modelers, instead, perceive the SC as a specific type of shear zone. The original theory of SCs was developed when the net accretion of marine sediments to the forearc was thought to typify a convergent margin. While erosive margins were briefly mentioned, their mechanics were not discussed in any detail. We now realize that subduction erosion is taking place at roughly half of the modern subduction margins. Here we review and revise the theory of erosive SCs (1) to unify this concept across disciplines, focusing on the meaning of the channel's boundaries; (2) to redefine the portions of the forearc included in the SC concept; and (3) to better idealize this dynamic system where material supply to the channel, fluid content, and the heterogeneity of deformation all influence the SC's upper and lower boundaries. Migration of the channel boundaries controls the downdip variation of tectonic mechanisms that shape the margin. Within the shallow, <15 km deep part of the SC, a gradual change of physical properties defines three zones; zone 1 of rapid fluid dewatering, zone 2 of overpressure, and zone 3 with metamorphic fluid release. A SC is a dynamic feature with along‐strike and downdip variations caused by changes in channel material, in trapped fluid contents, and in interplate boundary geometry.

Global pattern of earthquakes and seismic energy distributions: Insights for the mechanisms of plate tectonics

In this paper, we analyse the distributions of number of events (N) and seismic energy (E) on the Earth's surface and along its radius as obtained from the global declustered catalogue of large independent events (M≥7.0), dissipating about 95% of the Earth's elastic budget. The latitude distribution of the seismic event density is almost symmetric with respect to the equator and the seismic energy flux distribution is bimodal; both have their medians near the equator so that they are equally distributed in the two hemispheres. This symmetry with respect to the equator suggests that the Earth's rotational dynamics contributes to modulate the long-term tectonic processes.The distributions of number and energy of earthquakes versus depth are not uniform as well: 76% of the total earthquakes dissipates about 60% of the total energy in the first ~50km; only 6% of events dissipates about 20% of the total amount of energy in a narrow depth interval, at the lower boundary of the upper mantle (550–680km). Therefore, only the remaining 20% of energy is released along most of the depth extent of subduction zones (50–550km). Since the energetic release along slabs is a minor fraction of the total seismic budget, the role of the slab pull appears as ancillary, if any, in driving plate tectonics. Moreover the concentration of seismic release in the not yet subducted lithosphere suggests that the force moving the plates acts on the uppermost lithosphere and contemporaneously all over the Earth's outer shell, again supporting a rotational/tidal modulation.

Gravity data inversion for the lithospheric density structure beneath North China Craton from EGM 2008 model

The density structures of the crust and upper mantle beneath the North China Craton (NCC) are reconstructed in this paper by using the Bouguer gravity anomaly derived from EGM08 model. The Occam’s inversion method and preconditioned conjugate gradient (PCG) are adopted in our paper for solving the 3D density inversion problems. Then the depth weighted function is incorporated into model parameters to avoid the skin effects in inversion. In our study, the Bouguer gravity anomaly derived from EGM08 model is used to study the crust and upper mantle structures beneath the NCC. For reducing the non-uniqueness of our inversion problem, we use the inverted density distribution from 0.5° × 0.5° P-wave tomographic results as our initial model. Finally, the high-resolution 3D density model is established down to 250 km depth for the first time in this region. The density images at eight representative layers (10, 25, 40, 60, 80, 120, 160, 200 km) and eight vertical cross-sections (linear profiles along latitude 35°, 36°, 37°, 38°, 39°, 40°, longitude 108°, 119°) are displayed and compared with the P-wave velocity tomographic images. The 3D-dimensional density model indicates that lateral heterogeneities are widely distributed in different units within the NCC, while their density patterns and depth extensions are significantly different for three main units of NCC, suggesting different tectonic mechanisms that have dominated the evolution of these regions in the Phanerozoic. In particular, our inverted density images demonstrate that the Phanerozoic lithospheric reactivation and thinning may not have influenced the central and western NCC to the same extent as the eastern NCC, most of which preserves relatively thick cratonic lithosphere today. However, localized lithospheric thinning may exist in the circular-Ordos rift systems, whose ancient tectonic belts around the Ordos plateau may have been affected by multi-phase tectonic events in its long evolution history.

Ultra Low Frequency (ULF) European multi station magnetic field analysis before and during the 2009 earthquake at L'Aquila regarding regional geotechnical information

Abstract. This work presents ground based Ultra Low Frequency (ULF) magnetic field measurements in the frequency range from 10–15 mHz from 1 January 2008 to 14 April 2009. In this time period a strong earthquake series hit the Italian Abruzzo region around L'Aquila with the main stroke of magnitude M = 6.3 on 6 April 2009. In the frame of the South European Geomagnetic Array (SEGMA), a European collaboration runs ULF fluxgate instruments providing continuously magnetic field data recorded in mid- and south Europe. The main scientific objective is the investigation of signal variations due to seismic activity and the discrimination between other natural and human influences. The SEGMA station closest to the L'Aquila earthquake epicenter is L'Aquila observatory located in the epicenter region. For the scientific analysis we extract the nighttime period from 22:00–02:00 UT and determine the power spectral density (PSD) of the horizontal (H) and vertical (Z) magnetic field components and the standardized polarization ratio (Z) over (H). To discriminate local emissions from global geomagnetic effects, data from three SEGMA stations in distances up to 630 km from the epicenter region are analyzed and further compared to the independent global geomagnetic ∑ Kp index. Apart from indirect ionospheric effects, electromagnetic noise could be originated in the lithosphere due to tectonic mechanisms in the earthquake focus. To estimate the amplitude of assumed lithospheric electromagnetic noise emissions causing anomalies in the PSD of the (Z) component, we consider magnetotelluric calculations of the electric crust conductivity in the L'Aquila region. Results found at L'Aquila observatory are interpreted with respect to the lithosphere electrical conductivity in the local observatory region, the ∑ Kp index, and further in a multi station analysis. Possible seismic related ULF anomalies occur ~2 weeks before the main stroke.

Geological and Palaeoseismological Evidence for Late Pleistocene−Holocene Activity on the Manisa Fault Zone, Western Anatolia

In West Anatolia near the cities of İzmir and Manisa, the historical occurrence of large earthquakes suggests the presence of important seismogenic faults. However, these faults have yet to be investigated in detail. The Manisa Fault Zone (MFZ) is an active large-scale normal fault system in this area, and thus field observations and palaeoseismological studies of this zone are important for predicting future earthquakes. Hence we sought to document geological and palaeoseismological evidence for Holocene activity on the MFZ. We performed trenching to determine the magnitude and timing of past surface-faulting events using detailed fault-trace mapping, measurements of Upper Pleistocene-Lower Holocene sediments, and radiocarbon dating. By comparing the trench data with palaeoearthquake records, we find evidence for three palaeoearthquakes which correspond to 926 AD, 1595 or 1664 AD, with the most recent event in 1845 AD. We also find this in the central and western sectors of the MFZ, which together with the eastern sector comprise the three major seismogenic zones. The Pliocene-Quaternary vertical offset at fault scarps is far less than that in the western sector, suggesting that activities of these sectors are highly independent. Evaluation of field observations suggests that the MFZ has been the source of multiple Late Pleistocene and Holocene surface-rupturing earthquakes. Our results constitute the first palaeoseismic evidence on the causative faults of historical earthquakes that affected Manisa, and point to their underlying tectonic mechanisms.

Pressure-temperature-deformation-time (P-T-d-t) exhumation history of the Voltri Massif HP complex, Ligurian Alps, Italy

[1] An integrated structural, petrological, and geochronological/thermochronological study was undertaken to constrain the tectonic controls on the exhumation of the Voltri Massif high-pressure (HP) complex, located in the Ligurian portion of the Western Alps (Italy). Petrotextural analyses were performed to identify the pressure-temperature interval of (1) peak metamorphism (the D1-M1 stage) to eclogitic/blueschist facies conditions and (2) the main retrogressive event (the D2-M2 stage) to greenschist facies conditions. U-Pb SHRIMP dating on a zircon rim (33.8 ± 0.8 Ma) and titanite grains (29 ± 5 Ma), coupled with 40Ar-39Ar analyses on phengite (∼64 Ma to ∼34 Ma) placed temporal constraints on the exhumation path from the D1-M1 to the D2-M2 stages at the Eocene-Oligocene boundary. Apatite fission track thermochronology, with older ages of 23.9 ± 4.9 Ma, confirms the existence of a regional, Late Oligocene to Miocene cooling/exhumation event for the Voltri Massif (the D3-M3 stage). The compilation of this pressure-temperature-deformation-time path supports a change in the exhumation history: (1) an initial stage, from the D1-M1 to the D2-M2, which was nearly isothermal with highly variable exhumation rates, and (2) a second stage, from D2-M2 to upper crustal levels, which was accomplished by cooling and moderate exhumation rates of ∼1–2 mm yr−1. This two-stage path can be reconciled with the dominant tectonic mechanisms responsible for exhumation of HP rocks in the Voltri Massif area. At the regional scale, this path is consistent with major geodynamic reorganization in the Mediterranean region at the Eocene-Oligocene time boundary, which involved a switch from synorogenic events during transpressive kinematics at the Alpine-Apennine plate boundaries, to postorogenic processes related to crustal thinning and opening of back-arc basins.

Role of the shale tectonics on the evolution of the Eastern Venezuelan Cenozoic thrust and fold belt

This paper presents a structural and stratigraphic analysis of the foreland-fold-belt of the Eastern Venezuelan Basin and the main conclusions about shale tectonic mechanisms in the area. The deformation of the foreland-fold-belt has been investigated analyzing the growth strata architecture preserved on the structure fold limbs. Three contractional episodes are proposed for the Eastern Venezuelan Basin: 1) Oligocene to middle Miocene, 2) late Miocene to Pliocene and 3) Pleistocene. The first episode produced contractional listric faults inside the shale and long displacement blind thrusts in the underlying Cretaceous units. The second episode produced the deformation of the Cenozoic strata into overlapping east-west-trending, convex northward anticlines that covers more than 200 kilometers in length and 40 kilometers wide, break-through normal faults product of a high sedimentary load that overcomes contraction and the formation of short-displacement blind thrusts in the underlying Cretaceous units. The last episode is related to an oblique compression and the formation of high angle extensional faults with dextral movement and NW-SE strike. The role of the shale tectonics in the evolution implies that shale deforms in two stages: 1) folding and 2) normal faulting of the crest of the anticline (Break through normal faulting). Folding controlled the sediment distribution during most of the Neogene strata, while the normal faulting of the anticlines represent basin potential for hydrocarbon. The best potential hydrocarbon plays in the basin are related to oblique-collision restricted basins and controlled by break-through normal faults and the presence of NW-SE strike faults that connect the HC source with the reservoirs. Results from this research imply that the role of sedimentation is fundamental for the overburden sand distribution and tectonic constrain of the folds.