Extensional Stage Research Articles

DEFINITION OF A TEST-BED FOR EVALUATION OF HOUSE MECHANICAL VENTILATION SYSTEMS

The aim of this work is to study the Anisotropy of the Magnetic Susceptibility (AMS) in two Jurassic–Cretaceous synclines located in the northern border of the Central High Atlas (Morocco): the Aït Attab and Ouaouizaght basins. AMS is used in order to obtain the magnetic fabric and its relationship with the kinematic evolution of both basins. The tectonic evolution of the basins, still under discussion, is mostly considered as the result of inversion during Tertiary and perhaps since Bathonian, of extensional and/or strike-slip Jurassic basins. Both basins are filled with Upper Jurassic to Lower Cretaceous silts and sandstones, with less frequent marine marly limestones.The bulk magnetic susceptibility (km) generally shows higher values in the red facies (163.2 E−6 in AT and 168.6 E−6 in WZ) than in the yellowish marly limestones (97.88 E−6 in AT and 132 E−6 in WZ). Most sites show an oblate magnetic fabric. The rock magnetic analyses indicate that the main carrier of the magnetic susceptibility for the red facies is hematite, whereas in the yellowish facies there is a dominance of paramagnetic minerals. In both basins, the magnetic lineation (long axis of the ellipsoid, kmax axes) shows a predominant E–W direction. The overlapping of the stress fields during the Atlasic basins evolution, in both compressional and extensional regimes and hinder the straightforward interpretation of the magnetic fabrics. However, a coeval N–S compression during the times of sedimentation with an E–W transtension can explain the magnetic lineation found in many of the sites analyzed in the present work. There are also other less frequent directions of kmax axes (NE–SW and NW–SE) are interpreted as the result of local change of the stress field during the early extensional stage of basin formation.

Geochemistry and petrogenesis of the Huashan granites and their implications for the Mesozoic tectonic settings in the Xiaoqinling gold mineralization belt, NW China

The Huashan complex granitic batholith, occurred in the core of the Xiaoqinling orogen and gold mineralization belt along the south margin of the North China Craton, consists of amphibole monzogranite and biotite monzogranite in the Wengyu and Fangshanyu valleys, respectively. Zircon LA-ICP-MS U–Pb dating yields ages of 205±2Ma and 132±1Ma for the Wengyu and Fangshanyu granites, respectively, which represent the two major episodes of Mesozoic magmatism in the region. These rocks are characterized by metaluminous, high silica and total alkalinity, and have high FeOtotal/(FeOtotal+MgO) ratios, high large-ion lithophile elements (LILEs, especially Sr and Ba) and light rare earth elements (LREEs), and low heavy rare earth elements (HREEs) and Y concentrations and insignificant negative Eu anomalies. The Wengyu granites have higher Ba and Sr contents, which can be classified as high Ba–Sr granites, whereas the Fangshanyu granites show much lower Ba and Sr contents and display an adakitic affinity. Elemental and isotopic compositions suggest that the main sources of both the Wengyu and Fangshanyu granites are likely the ancient basement materials of Taihua Group. However, the high Ba and Sr contents of the Wengyu granites require addition of small amounts of enriched lithospheric mantle metasomatised by fluid/melt derived from pelagic sediments-bearing subducted slab in the sources. The Wengyu granites were most likely generated earlier by partial melting of the lowest part of the crust due to the subducted slab break-off under the post-collision extensional stage of the continental collision orogeny. The Fangshanyu granites might be mainly derived later from the partial melting of thickened lower crustal materials, representing by the Taihua Group basement rocks, corresponding to the tectono-magmatism of the post-orogeny to rift extensional environment during the tectonic transition from the Paleo-Tethys subduction-collision system to the Paleo-Pacific regime.

Detrital zircon provenance from the Neuquén Basin (south-central Andes): Cretaceous geodynamic evolution and sedimentary response in a retroarc-foreland basin

The surface response, in terms of drainage pattern changes, to the Cretaceous geodynamic reorganization of the Andean subduction zone between 36°S and 41°S is reconstructed through the geochronology-based provenance study of alluvial detrital zircons. The age spectra obtained by 500 spot U-Pb ages record an eastward provenance of detritus coming from the foreland during the Early Cretaceous backarc extensional stage, followed by westward-sourced clastics coming from the Cordillera during the Cenomanian. This drainage pattern reversal fits the regional unconformity in the sedimentary record that is linked to the geodynamic reorganization of the continental margin from an extensional to a compressional regime, forcing the Neuquen Basin to evolve from a retroarc to a foreland stage. After this inversion, the clastic systems progressively returned to be mainly fed by the foreland, due to the uplift of the peripheral bulge as a consequence of the Late Cretaceous thrust front migration. This tectonic evolution of the Neuquen Basin and the related response of the drainage pattern are thought to be the surface expression of the dip decrease of the Benioff subduction zone.

Switch of Mesozoic extensional tectonic style in the Lusitanian basin (Portugal): Insights from magnetic fabrics

Magnetic fabrics in the Lusitanian basin show extreme variability in the orientation of strain axes during part of its Mesozoic extensional stage. A detailed study of Anisotropy of Magnetic Susceptibility (AMS) was done in Triassic and Upper Jurassic units in the onshore part of the basin. In the Triassic samples, the magnetic lineation shows a dominant NW–SE orientation, perpendicular to the main normal fault set and also to the extension directions inferred from the analysis of brittle mesostructures. For the Late Jurassic period, the AMS analysis indicates a dominant N–S to NE–SW extension. Moreover, these main directions are complicated by perpendicular solutions co-existing in the same sites. Comparison with syn-sedimentary faults reveals the difficulty in interpreting AMS data in these settings, although magnetic stability for the obtained solutions can be tracked through different mineralogical composition for the sampled rocks. Interpretation of the obtained fabrics through a Triassic radial extension regime is consistent with axes switching and local re-orientation of strain axes close to major normal faults related to the opening of the Northern Atlantic during the Jurassic.

Neotectonic Evolution of an Actively Growing Superimposed Basin in Western Anatolia: The Inner Bay of İzmir, Turkey

İzmir Bay is an actively growing shallow marine basin controlled by active faults trending NE, NW, N-S and E-W, in the West Anatolian Extensional Province. The bay of İzmir is a lazy L-shaped superimposed basin which is topographically divided into an E-W-trending inner bay and a NW-trending outer bay. The Inner Bay of İzmir is an asymmetric graben structure approximately 5-7 km wide and 25 km long containing (i) upper Cretaceous-Palaeocene basement, (ii) an older succession of lower to upper Miocene basin fill, overlain with angular unconformity by (iii) a younger Plio-Quaternary basin fill. The older succession contain a 0.5- to 1.5-km-thick, folded and coal-bearing continental volcano-sedimentary sequence. The younger succession of the Inner Bay of İzmir includes the upper Pliocene-Pleistocene Görece formation and Holocene to recent alluvial fan, fan delta to shallow marine deposits. This study reveals that the İzmir Bay region was above sea level and the site of a NE-SW-trending lacustrine environment associated with calc-alkaline to alkaline volcanism during the early to late Miocene. By the late Pliocene the modern coastal areas of İzmir were inundated beneath the waters of the Aegean Sea by the creation of the E-W-trending Inner Bay of İzmir. Detailed geological mapping, geomorphological evidence and computer-based kinematic analysis show that the tectonic evolution of the basin since the early Miocene can be divided into four stages. Two early extensional stages resulted in the formation of NE-SW-trending lacustrine volcano-sedimentary basins; the third stage, characterized by dip- to strike-slip faulting, deformed the older basin fill units, and the final extensional stage resulted in the opening of the Inner Bay of İzmir since the Late Pliocene.

Mesozoic rifting in the Basque–Cantabrian Basin (Spain): Inherited faults, transversal structures and stress perturbation

Terra Nova, 24, 70–76, 2012AbstractIn the Basque–Cantabrian Basin (Northern Iberian Margin), inherited WNW–ESE‐striking faults were reactivated during the Mesozoic extensional stage associated with the opening of the Bay of Biscay. Syn‐rift structures in the area include different sets, apparently indicating multiple extensional stages. In particular, abundant NE–SW‐striking extensional elements have been interpreted in the past as having developed during a late rifting, left‐lateral transtensional stage. However, the development of these structures also fits well with an analytical model of ridge‐perpendicular extension. Along‐strike stretching imposed by displacement variations along inherited faults locally produced an additional stress field oriented obliquely to the regional one. This caused rotation of the local minimum stress and development of anomalously oriented transversal structures. A single extensional stage is thus inferred during Mesozoic rifting, with a stretching direction oriented approximately perpendicular to the trend of the Bay of Biscay oceanic ridge.

Superimposition of oligo-miocene extensional and contractional basins in the northpatagonian Andean retroarc: The Alumine Basin, Neuquen, Argentina.

The Alumine Basin is a volcano-sedimentary depocentre located in the Andean retroarc at 39°S. It is the southern part of a major depositional system: the Bio Bio-Alumine Basin. Stratigraphical, geochronological, sedimentological and structural analysis allow us to conclude that the basin evolved through two stages: an Oligocene extensional stage filled with basalts and reworked volcaniclastic and pyroclastic deposits (Rancahue Formation) and a Miocene contractional stage, filled with alluvial and pyroclastic deposits covered by basaltic lavas (Chimehuin and Tipilihuque formations). The volcanic-rich (basaltic) infill of the extensional stage has a western provenance and was derived from widespread magmatic activity that took place during the Oligocene in the adjacent cordilleran axis. The contractional stage basin is an intramontane depocentre limited by uplifted blocks generated mainly by the inversion of Mesozoic extensional faults. It is strongly asymmetric and shows eastern provenance. Structural and geomorphological evidence indicate the existence of a non-depositional hiatus between the two stages.

Cuencas sedimentarias de Costa Rica: evolución geodinámica y potencial de hidrocarburos

The Upper Cretaceous/Cenozoic basins of Costa Ricaare underlain by oceanic crustand located along the western margin of the Caribeean Plate. Since Upper Cretaceous time, an intraoceanic arc was formed along the western margin of the Caribeean Plate. This arc marked the beggining of the subdution of the Farallon Plate in the region and also the creationof the first land bridge joining North and South America. Continued subdution of the Farallon Plate from Campanian until the present, created a convergent marginal tectonic setting; with, from SW to NE: a trench, an outer arc, a forearc region, a volcanic arc and a backarcr egion. Localized platforms of shallow-water carbonates, overlain by extensive cyclical turbidites system, characterized the sediments of most of the basis until the Eocene, when a major tectonic compressive event affected the area. As well, about this time, a major sinistral east-west fault (COSTA RICA TRANSCURRENT FAULT SYSTEM) divided the region into north and south segments and transtensional or pull apart basins developed along this major fault system. In the Oligocene time, the north segment was emerged and subject to erosion, whereas in the south segment the sedimentation continued. From Miocene on, shallow marine deposition was widespread in both sements. In the north segment, and extensional stage in the backarc region, created a structural depression filled with Neogene sediments. About the same time, the south segment experienced compressional tectonicis due to noth-directed movementof the Panama block. Later in the Pleistocene, northeasterly-directed shallow subdution of the thick Cocos Ridge, resulted in adiotional thrusting and the developmentof a foreland basin, both, in the back arc and fore arc regions. Organic-rich, deep water, marginally to termally mature source rock, from Cretaceous age, occur in several basins. Also, source rocks of moderate to high TOC values are present in all basins, in the Tertiary. Potential carbonate reservoirs in some of these basins include shallow water carbonates, rudistic reef buildups and grainstone accumulations. Potential clastic reservoirs include turbidite sandstones, regressive deltaic deposits and near-shore porous sandstones. The presence of potential structural and stratigrafic traps in all basins, confer an important as far as the pretoleum potential is concern.

Reativações neotectônicas na Zona de Cisalhamento do Rio Paraíba do Sul (sudeste do Brasil)

O presente trabalho tem como objetivo investigar a atuação de mecanismos neotectônicos na região da Zona de Cisalhamento do Rio Paraíba do Sul, importante feição geotectônica adjacente ao segmento central do Rift Continental do Sudeste do Brasil, e áreas adjacentes. A metodologia empregada baseia-se na integração de análises morfoestratigráficas e de dados estruturais, enfatizando a identificação de pares falha/estria afetando sedimentos cenozoicos e as rochas do embasamento alteradas, no intuito de caracterizar os campos de esforços neotectônicos. A partir das relações entre diferentes padrões de fraturamento e as unidades geológicas afetadas, e da percepção de superposição de estrias em um mesmo plano de falha, foi possível o reconhecimento de três fases neotectônicas, correlacionáveis a eventos descritos para o Sudeste do Brasil: transcorrência sinistral E-W, de idade miocênica; transcorrência dextral E-W, datada do limite Pleistoceno-Holoceno; e regime distensivo NW-SE, holocênico. As fases neotectônicas identificadas têm forte relação com a reativação das anisotropias estruturais preexistentes.

Paleostress analysis of Cenozoic faulting in the Kraishte area, SW Bulgaria

Paleostress calculations from fault planes in Paleogene sediments and in the underlying basement were used to determine the orientation and the chronology of the principal stresses during the Cenozoic tectonic evolution of the Kraishte area (southwest Bulgaria). The results show that middle Eocene–early Oligocene WSW–ENE extension led to the formation of grabens and half grabens filled with thick continental to marine deposits in the hanging wall of extensional detachments.The Paleogene sediments and their basement were then intruded by subvolcanic bodies and dykes during SW–NE extension between 32 and 29 Ma. These WSW–ENE and SW–NE extensional stages are related to the general rollback of the Hellenic slab.From the late Oligocene to the earliest Miocene, SSE–NNW transtension generated coal-bearing sedimentary basins. The anticlockwise rotation of the main tensile axis by almost 50° with respect to the previous tectonic stage was probably related to a switch from the Aegean back-arc extension to crustal stretching and extrusion of continental fragments around the Moesian platform.Since the middle Miocene extension in the Kraishte area, accommodated by faults with relatively small displacement, led to the formation of restricted basins filled with alluvial to lacustrine deposits.

The geological and metallogenic map of the Baccu Locci mine area (Sardinia, Italy)

Please click here to download the map associated with this article. The study area is in the nappe zone of the Sardinian Variscides in the southeast part of the Island of Sardinia (Italy), and extends between 39°33′14″/9°30′14″ (NW corner) and 39°30′09″/9°35′36″ (SE corner). The area shows a section of the Variscan orogen in Sardinia with three tectonic units that were stacked and folded during the Middle Carboniferous Variscan tectonics. The presented 1:10,000 scale geological map, the cross sections and the 3D models illustrate the complicated tectonic setting of the area, resulting from the polyphasic Variscan collisional evolution as well as from later extensional stages. The geometry resulting from progressive deformation is strongly noncylindrical; it is not balanceable because the polypahsic deformation with different tectonic transport directions and the loss in volume in the different formations occurred under greenschist facies metamorphism. The use of 3D modelling of geological surfaces greatly improved both the map and cross-sections. The Variscan basement of the study area hosts one of the most important mining zones of SE Sardinia (Baccu Locci mine area), which was active until 1961 for the extraction of AsPy, PbS and ZnS. Recent studies also found a noteworthy occurrence of Au. The Baccu Locci mine is assumed to be the eastern part of a mineralized corridor linked with the Variscan shear zone. A metallogenic map of the Baccu Locci mine area at 1:7,500 scale is included in this paper. The primary map associated with this paper actually represents a 4D model (spatial and time dimensions) of ore bodies hosted in a crystalline basement and highlights the overprinting of different paragenetic sequences of mineralizations and their relationships with pre-existing structures.

Neogene to Quaternary extensional reactivation of a fold and thrust belt: The Agrio belt in the Southern Central Andes and its relation to the Loncopué trough (38°–39°S)

The Andean orogenic front between 38 and 39°S is formed by extensional structures delimiting the Loncopué trough. These structures are superimposed to Late Cretaceous–Late Miocene compressive structures that formed the Agrio fold and thrust belt. Gravity data, and limited borehole and seismic data complimented with previous field studies were used to construct two structural cross sections across the area of Neogene extensional deformation and remnants of the previous compressive structure. One of them was restituted for Late Cretaceous times previously to extensional relaxation of the fold and thrust belt and Late Oligocene–Early Miocene and Pliocene to Quaternary stages. Different modeling techniques and filters for gravity anomalies show that i) a series of deeper and larger depocenters, where Late Oligocene to Early Miocene sequences are buried, were located particularly at the Present trough boundaries; while ii) smaller and probably shallower depocenters were concentrated at the axial -eastern trough following structures at surface with young morphological expression. The former are interpreted as representative of the initial extensional stage when Late Cretaceous compressional basement structures were relaxated, while the latter could be the product of incipient-young extensional reactivation of the Agrio fold and thrust belt. General coincidence between both areas of extensional relaxation at the western Agrio fold and thrust belt implies that a common basement discontinuity is recurrently being reactivated through time, probably in coincidence with the eastern main Loncopué fault boundary.

Facies architecture of a Triassic rift-related Silicic Volcano-Sedimentary succession in the Tethyan realm, Peonias subzone, Vardar (Axios) Zone, northern Greece; Regional implications

In northern Greece, along the western edge of the Paleozoic Vertiscos terrane (Serbomacedonian massif) and within the Peonias subzone – the eastern part of the Vardar (Axios) Zone – a Silicic Volcano-Sedimentary (SVS) succession of Permo(?)–Skythian to Mid Triassic age records the development of a faulted continental margin and the formation of rhyolitic volcanoes along a continental shelf fringed by neritic carbonate accumulations. It represents the early rifting extensional stages that eventually led to the opening of the main oceanic basin in the western part of the Vardar (Axios) Zone (the Almopias Oceanic Basin). Even though the SVS succession is deformed, altered, extensively silicified and metamorphosed in the low greenschist facies, primary textures, original contacts and facies relationships are recognized in some places allowing clues for the facies architecture and the depositional environment. Volcanic and sedimentary facies analysis has been carried out at Nea Santa and Kolchida rhyolitic volcanic centres. Pyroclastic facies, mostly composed of gas-supported lapilli tuffs and locally intercalated accretionary lapilli tuffs, built the early cones which were then overridden by rhyolitic aphyric and minor K-feldspar–phyric lava flows. The characteristics of facies, especially the presence of accretionary lapilli, imply subaerial to coastal emplacement at this early stage. The mature and final stages of volcanism are mostly represented by quartz–feldspar porphyry intrusions that probably occupied the vents. At Nea Santa area, the presence of resedimented hyaloclastite facies indicates subaqueous emplacement of rhyolitic lavas and/or lobes. Moreover, quartz–feldspar–phyric sills and a partly extrusive dome featuring peperites at their margins are inferred to have intruded unconsolidated, wet carbonate sediments of the overlying Triassic Neritic Carbonate Formation, in a shallow submarine environment. The dome had probably reached above wave-base as is indicated by the presence of reworked rhyolitic clasts in the younger mixed rhyolite–carbonate epiclastic sedimentary facies. This facies is interpreted as mass- and debris-flow of mixed provenance, deposited below wave-base. The facies architecture of the SVS succession records a change in volcanic activity from explosive to effusive and then to intrusive. The depositional environment changed from subaerial-coastal to shallow submarine as the silicic volcanism evolved and carbonate sedimentation was progressively taking over, probably compensating for the gradual subsidence of the corresponding basin. Silicic magmatism and carbonate sedimentation were contemporaneous and spatially related. The timing of the rifting, the continental crustal elements involved and the accompanying tectonic, magmatic and sedimentary processes are features of the spatially and temporally evolving western peri-Tethyan region.

The Hellenic Subduction System: High-Pressure Metamorphism, Exhumation, Normal Faulting, and Large-Scale Extension

The Cenozoic history of the retreating Hellenic subduction system in the eastern Mediterranean involves subduction, accretion, arc magmatism, exhumation, normal faulting, and large-scale continental extension from ∼60 Mya until the Recent. Ages for high-pressure metamorphism in the central Aegean Sea region range from ∼53 Ma in the north (the Cyclades islands) to ∼25−20 Ma in the south (Crete). Younging of high-pressure metamorphism reflects the southward retreat of the Hellenic subduction zone. The shape of pressure-temperature-time paths of high-pressure rocks is remarkably similar across all tectonic units, suggesting a steady-state thermal profile of the subduction system and persistence of deformation and exhumation styles. The high-pressure metamorphic events were caused by the underthrusting of fragments of continental crust that were superimposed on slab retreat. Most of the exhumation of high-pressure units occurred in extrusion wedges during ongoing lithospheric convergence. At 23–19 Mya large-scale lithospheric extension commenced, causing metamorphic core complexes and the opening of the Aegean Sea basin. This extensional stage caused limited exhumation at the margins of the Aegean Sea but accomplished the major part of the exhumation of high-grade rocks that formed between 21 and 16 Mya in the central Aegean. The age pattern of extensional faults and contoured maps of fission-track cooling ages do not show a simple southward progression. Our review of lithologic, structural, metamorphic, and geochronologic data is consistent with a temporal link between the draping of the subducted slab over the 660-km discontinuity and the large-scale extension causing the opening of the Aegean Sea basin.

Relative contributions of crust and mantle to generation of Campanian high-K calc-alkaline I-type granitoids in a subduction setting, with special reference to the Harşit Pluton, Eastern Turkey

We present elemental and Sr–Nd–Pb isotopic data for the magmatic suite (~79 Ma) of the Harşit pluton, from the Eastern Pontides (NE Turkey), with the aim of determining its magma source and geodynamic evolution. The pluton comprises granite, granodiorite, tonalite and minor diorite (SiO2 = 59.43–76.95 wt%), with only minor gabbroic diorite mafic microgranular enclaves in composition (SiO2 = 54.95–56.32 wt%), and exhibits low Mg# (<46). All samples show a high-K calc-alkaline differentiation trend and I-type features. The chondrite-normalized REE patterns are fractionated [(La/Yb) n = 2.40–12.44] and display weak Eu anomalies (Eu/Eu* = 0.30–0.76). The rocks are characterized by enrichment of LILE and depletion of HFSE. The Harşit host rocks have weak concave-upward REE patterns, suggesting that amphibole and garnet played a significant role in their generation during magma segregation. The host rocks and their enclaves are isotopically indistinguishable. Sr–Nd isotopic data for all of the samples display I Sr = 0.70676–0.70708, ε Nd(79 Ma) = −4.4 to −3.3, with T DM = 1.09–1.36 Ga. The lead isotopic ratios are (206Pb/204Pb) = 18.79–18.87, (207Pb/204Pb) = 15.59–15.61 and (208Pb/204Pb) = 38.71–38.83. These geochemical data rule out pure crustal-derived magma genesis in a post-collision extensional stage and suggest mixed-origin magma generation in a subduction setting. The melting that generated these high-K granitoidic rocks may have resulted from the upper Cretaceous subduction of the Izmir–Ankara–Erzincan oceanic slab beneath the Eurasian block in the region. The back-arc extensional events would have caused melting of the enriched subcontinental lithospheric mantle and formed mafic magma. The underplating of the lower crust by mafic magmas would have played a significant role in the generation of high-K magma. Thus, a thermal anomaly induced by underplated basic magma into a hot crust would have caused partial melting in the lower part of the crust. In this scenario, the lithospheric mantle-derived basaltic melt first mixed with granitic magma of crustal origin at depth. Then, the melts, which subsequently underwent a fractional crystallization and crustal assimilation processes, could ascend to shallower crustal levels to generate a variety of rock types ranging from diorite to granite. Sr–Nd isotope modeling shows that the generation of these magmas involved ~65–75% of the lower crustal-derived melt and ~25–35% of subcontinental lithospheric mantle. Further, geochemical data and the Ar–Ar plateau age on hornblende, combined with regional studies, imply that the Harşit pluton formed in a subduction setting and that the back-arc extensional period started by least ~79 Ma in the Eastern Pontides.

The Loncopué Trough: A Cenozoic basin produced by extension in the southern Central Andes

Abstract The Loncopue Trough is located in the hinterland Andean zone between 36°30′ and 39°S. It constitutes a topographic low bounded by normal faults and filled by lavas and sediments less than 5 Ma old. Reprocessed seismic lines show wedge-like depocenters up to 1700 m deep associated with high-angle faults, correlated with the 27–17 Ma Cura Mallin basin deposits, and buried beneath Pliocene to Quaternary successions and Late Miocene foreland sequences. The southern Central Andes seem to have been under extension in the hinterland zone some 27 Ma ago and again at approximately 5 Ma ago. This last extensional period could have been the product of slab steepening after a shallow subduction cycle in the area, although other alternatives are discussed. Orogenic wedge topography, altered by the first extensional stage in the area, was recovered through Late Miocene inversion, and was associated with foreland sequences. However, since the last extension (

Volcanic evolution of the back-arc Pleistocene Payun Matru volcanic field (Argentina)

Abstract For the first time, about 30 volcanic formations of the back-arc Payun Matru volcanic field (Payun Matru volcanic field, Argentina, 36°S, 69°W) have been sampled for K–Ar geochronology and geochemistry in order to reconstruct the eruptive history of this key province in the Andean back-arc. The Payun Matru volcanic field has been built since final Pleistocene until present with ages ranging from 280 ± 5 to 7 ± 1 ka. Erupted lavas belong to calc-alkaline series, with characteristics of both arc and intraplate magmas. From previous studies, three main units are distinguished: (1) a basaltic field (Los Volcanes), which covers a large surface of the Payun Matru volcanic field, composed of strombolian cones and associated lava flows emitted from 300 ka to Holocene times, (2) the stratovolcano Payun, with intermediate compositions, built around 265 ka, and (3) the shield volcano Payun Matru s.s. characterized by trachytic compositions and a large summit caldera. The earlier stages of the Payun Matru volcano are not dated, but we constrain the major explosive event, related to the eruption of a widespread ignimbrite and to the formation of the caldera, between 168 ± 4 ka (internal wall of caldera) and 82 ± 1 ka (flow within the caldera). Based on the geochemical similarities of the ignimbrite and the upper lava flow of the pre-caldera cone, we suggest that the age of this event is most probably at the older end of this interval. Numerical modeling using a GIS program has been used to reconstruct the morphological evolution for Payun Matru volcano before and after the caldera collapse. The ancient edifice could be modeled as a flattened cone, 2300 m high, with a volume of about 240 km3. The ignimbrite eruption associated with the Payun Matru caldera formation could be related to the regional tectonic environment, which is characterized by multiple Plio-Pleistocene extensional stages during the last 5 Myr. The evolution of the Nazca plate subduction from a flat slab to a normal dip induced an input of fluid mobile elements and asthenosphere plume-like mantle source beneath the Patagonian lithosphere, which yields the observed intraplate signature. We also interpret this geodynamic evolution as the influence of extensive processes in the upper crust leading to caldera-forming eruptions as observed throughout this province.

Subsidence history from a backstripping analysis of the Permo‐Mesozoic succession of the Central Southern Alps (Northern Italy)

ABSTRACTSeven tectonic subsidence curves, based on outcrop data, have been calculated in order to constrain the geodynamic evolution of the Permian–Mesozoic sedimentary succession (up to 10 km thick) of the Central Southern Alps basin (Italy). The analysis of the tectonic subsidence curves, covering a time span of about 200 Ma, allowed us to quantify the subsidence rates, to document the activity of syndepositional fault systems and calculate their slip rates. Different stages, in terms of duration and magnitude of subsidence‐uplift trends, have been identified in the evolution of the basin. The fault activity, reconstructed by comparing subsidence curves from adjacent sectors, resulted as highly variable both temporally and spatially. Strike‐slip tectonics was coeval to Permian sedimentation, as suggested by the strong differences in the subsidence rates in the sections. The evolution and subsidence rates suggest a continental shelf deposition from Early Triassic to Carnian, when subsidence came to a stop. A rapid resumption of subsidence is observed from the Norian, with a subsidence pulse in the Late Norian, followed by the regional uplift, in the Late Rhaetian. The following Early Jurassic subsidence is characterized by tectonic subsidence similar to that of the Norian. The Norian and Early Jurassic pulses were characterized by the highest slip rates along growth faults and are identified as two distinct tectonic events. The Norian–Rhaetian event is tentatively related to transtensional tectonics whereas the Early Jurassic event is related to crustal extension. The Early Jurassic subsidence records a shift in space an time of the beginning of the extensional stage, from Late Hettangian to the east to Late Pliensbachian–Toarcian to the west. From the Toarcian to the Aptian, the curves are compatible with regional thermal subsidence, later followed (Albian–Cenomanian) by uplift pulses in a retrobelt foreland basin (from Cenomanian onward).

Diagenetic Evolution and Porosity Destruction of Turbiditic Hybrid Arenites and Siliciclastic Sandstones of Foreland Basins: Evidence from the Eocene Hecho Group, Pyrenees, Spain

Abstract This study aims to unravel the impact of diagenetic alterations on porosity loss of foreland-basin turbiditic hybrid arenites and associated siliciclastic sandstones of the Eocene Hecho Group (south-central Pyrenees, Spain). In this succession, hybrid arenites and calclithites are extensively cemented by mesogenetic calcite cement (δ18OVPDB = −10.0‰ to −5.8‰; Th, mode = 80° C; salinity mode = 18.8 wt% eq. NaCl), Fe-dolomite (δ18OVPDB = −8.5‰ to −6.3‰) and trace amounts of siderite. The extent of carbonate cementation is interpreted to be related to the amounts of extrabasinal and intrabasinal carbonate grains, which provided nuclei and sources for the precipitation and growth of carbonate cements. Other diagenetic alterations, such as pyrite and albitization, had no impact on reservoir quality. Scarce early diagenetic cements, coupled with abundant ductile carbonate and siliciclastic framework grains, have led to rapid porosity loss owing to compaction. Conversely, abundant quartz in the sandstones prevented rapid loss of porosity by mechanical compaction. Reservoir quality was affected by mesogenetic cementation by quartz overgrowths, calcite and dolomite intergranular pressure dissolution of quartz grains, and formation of fracture-filling calcite cement (δ18OV-PDB values from −10.4‰ to −7.8‰; Th temperatures of 150° C), which are attributed to deep circulation of hot meteoric waters during extensional stages of tectonism. The results of this study illustrate that diagenetic evolution pathways of the arenites and sandstones are closely linked to the variation in detrital composition, particularly the proportion and types of extrabasinal noncarbonates, extrabasinal carbonates, and intrabasinal carbonate grains. These insights suggest that marine turbiditic hybrid arenites and calclithites of foreland basins are subjected to more rapid and extensive porosity loss owing to compaction and cementation than associated siliciclastic sandstones. Degradation of reservoir quality makes these hybrid arenites, calclithites, and sandstones suitable as tight gas reservoirs, but only if fracture porosity and permeability develop during tectonic deformation.

Geochemistry of migmatite-granite connection: a case study from the Central Rhodope, Bulgaria

Extensive crustal melting and intrusive granite magmatism accompanied the extensional stage of the Rhodope massif Alpine evolution. Granites of different structural position and time of crystallization could reveal compositional and temporal relations between anatectic migmatization and granite magma generation. We have studied the post-kinematic Smilyan pluton and smaller syn-kinematic granite bodies hosted by the Madan unit metatexitic gneisses in the southwestern periphery of a metamorphic core complex known as Central Rhodopian Dome (CRD). The dominant geochemical features of the Madan unit granites display remarkable similarities with in situ formed anatectic melts from the CRD diatexitic core (Arda unit): felsic peraluminous compositions, low HFSE and REE, high LILE contents and LREE/HREE ratios, and negligible to positive Eu anomaly. Some deviations of the syn-kinematic granites geochemistry (metaluminous compositions, REE and HFSE enrichment, LREE/HREE ratios and Eu/Eu* variation) support an idea of separate partial melt batches extracted from different precursor compositions. Age data available (Smilyan pluton 43 Ma, and CRD core anatectic melts 37–38 Ma) preclude direct feedback relations between Madan unit intrusive granites and Arda unit migmatites. We infer that the migmatite-granite connection should be considered a common process of protracted Tertiary crustal melting that operated during CRD evolution and affected different crustal sources to produce discrete portions of granite melts. Major and trace elements geochemistry reveals potential lines of descent amongst groups of spatially related granitic rocks due to the same mechanism of melt generation.