Extensional Tectonic Processes Research Articles

Influence of extensional strike-slip fault systems on hydrocarbon accumulation: case studies from Huanghekoudong and Miaoxi’nan sub-sags

Extensional strike-slip basins have unique structural features that influence both sub-sags and traps. Despite previous investigations into the extension-strike-slip fault system, there remains a gap in the comprehensive analysis of its influence on sub-sag formation and trap development. The Huanghekoudong and Miaoxi’nan sub-sags within the Bohai Bay Basin, intersected by the Tan-Lu Fault Zone, offer exemplary cases for examining the interplay between strike-slip and extensional tectonic regimes. The neotectonic strike-slip movements along the Tan-Lu Fault Zone have given rise to various extensional strike-slip overlap zones in these sub-sags, rendering these locales ideal for investigating the dynamics of strike-slip and extensional tectonic processes. Employing three-dimensional seismic data, well logs, and a newly constructed sequence stratigraphic framework, our research delineates the geometric and kinematic phenomena characteristic of the extensional strike-slip fault system, including the dynamics of extensional fault deformation and interaction through time. By scrutinizing the fault system’s vital role in shaping sub-sag evolution and trap genesis, we present a comprehensive model that significantly contributes to our understanding of structural trapping dynamics. This model not only fine-tunes existing trapping models but also offers invaluable insights for future exploration strategies within the Bohai Bay Basin and other similar extensional strike-slip basins worldwide. Our findings highlight the novel and significant implications of the extensional strike-slip fault system in controlling sub-sag and trap features, thus bridging a notable gap in existing geotechnical knowledge.

Changes in the stratigraphic architecture as the response to the Upper Cretaceous tectonic and climatic interplay in the Sanfranciscana Basin, Brazil

During the Cretaceous, the Brazilian Intracratonic Sanfranciscana Basin was characterised by fluvial and aeolian deposition, whose facies, palaeosols, and stratigraphic architecture indicate significant changes in tectonics and climate. This work proposes a low-frequency sequence stratigraphic model, using the facies and palaeosols distribution as the stratigraphic proxy for identifying variations in the accommodation space, sediment supply, and their relationship with the tectonic and climatic events. The Unconformity U-0 was generated by the extensional tectonic processes during the Gondwana breakup in the Early Cretaceous and marked the initial continental sedimentation. The Lower Cretaceous Sequence SF1 overlies this unconformity, constituted by fluvial and lacustrine deposits. From south to north, the unconformity U-1 truncates the top of the Lower Cretaceous Sequence SF1 and the Precambrian basement, respectively. Overlaying Unconformity U-1 are the Upper Cretaceous volcanic rocks, fluvial, and aeolian deposits of the Sequence SF2. Because of the vertical and lateral changes, Sequence SF2 is divided into two sequences, SF2A and SF2B, separated by the regional Unconformity U-2, representing climatic and tectonic changes in the basin. During the late stage of sedimentation of Sequence SF1, the ratio between accommodation (A) and sediment supply (S) was positive in the south and negative in the basin's north. The evolution of Sequence SF2A records a negative A/S ratio, followed by an abrupt change to a positive A/S ratio in Sequence SF2B, leading to the genesis of the regional erosive surface of Unconformity U-2. Sequence SF2B also records increased humidity from arid climatic conditions, marked by the 3D regional distribution of facies and palaeosols, linked to the climatic changes detected in South America for the Late Cretaceous.

İZMİR-ANKARA-ERZİNCAN SUTUR ZONUNDAKİ (SİVAS KUZEYDOĞUSU) EOSEN YAŞLI KARATAŞ VOLKANİTLERİNİN JEOKİMYASAL ÖZELLİKLERİ

Karatas volcanics, is the product of Eocene volcanics crop out in the form of two belts along the northern and southern boundaries of the Izmir-Ankara-Erzincan Suture Zone. According to geochemical data, these volcanics have alkaline basic-intermediate character and consist of basaltic trachyandesite, trachyandesite and trachyte. This volcanic activity has been controlled by fractional crystallization and crustal contamination from basaltic trachyandesite to trachyte. Orientation of the samples towards amphibole area on the Rb/Sr - Ba/Rb ratio diagrams, dispersion of the Zr/Ba ratios (0.08 - 0.33) in the lithospheric mantle range, increase in the Ba/Rb ratio, decreases in the MgO, Ni and Cr contents point out that this volcanism originated from enriched lithospheric mantle rather than asthenospheric mantle. Geochemical data show that this enriched lithospheric mantle material is upper continental crustal material, main part of enrichment resulted by the subduction related fluids and also the contribution of the sedimentary materials. This situation may be explained that; the melts, derived from N-MORB or OIB bearing material ascended into the continental crust in a pre-collisional period and were reactivated by extensional tectonic and/or delamination processes during the post-collisional period, possibly caused the partial melting within the upper continental crust and produced the Karatas volcanics.

Identification of Detrital Carbonate in East Cepu High

East Cepu High is a part of horst – graben series which formed by extensional tectonic processes during Paleogene in North East Java Basin. Due to excellent paleogeography position, the carbonate build-up was growth very well and as the main reservoir in East Cepu High. Sea level change have important factor to provide variation of facies in each carbonate buildup, one of emerging facies is detrital carbonate. Detrital carbonate indicated by onlap horizon featured with carbonate build up body. Based on paleogeography, fluctuation of sea level change and sediment source, detrital carbonate formed in leeward area in lowstand or highstand phases. Distinguish between detrital carbonate facies with other facies, advanced seismic processing performed by using continuous wavelet transform (CWT) and seismic inversion. CWT is one method of spectral decomposition used to find the frequency that represent a facies. The result from seismic inversion will support the interpretation for facies distribution. As the result, seismic data which have interval frequency 10 – 45 Hz and Acoustic Impedance (AI) value above 35000 (from cross plot between acoustic impedance and gamma ray) can be interpreted as detrital carbonate. Based on seismic interpretation, detrital carbonate facies distributed along leeward area with geometrical spreading. The lateral facies change from detrital carbonate to shale was identified which causing this facies become potential as hydrocarbon reservoir with stratigraphic trap. Based on the earlier studies, North East Java Basin have a strong hydrocarbon migration to fill the reservoir, therefore the detrital carbonate have high chance to be a new hydrocarbon prospect in this area.

The Rieti Land Reclamation Authority relevance in the management of surface waters for the irrigation purposes of the Rieti Plain (Central Italy)

The Rieti Plain is crowned by calcareous-marly reliefs (Rieti and Sabini Mountains) and represents an intra-Apennine Plio- Quaternary alluvial and fluvial-lacustrine basin formed after multistage extensional tectonic processes. This territory presents huge amounts of water resources (Velino and Turano rivers; several springs; Lungo and Ripasottile lakes, relics of ancient Lacus Velinus). The aquifers occurring in the reliefs often have hydraulic continuity with the Rieti plain groundwater (detected at about 1-4 m below ground surface), which has general flow directions converging from the reliefs to the lake sector. Hydraulic exchanges between groundwater and surface waters are variable in space and time and play a relevant role for groundwater resource distribution. The Rieti Land Reclamation Authority was instituted in 1929 by Royal Decree N. 34171-3835, and integrates eight former authorities, dating the end of 1800s. It contributes to maintain the reclamation actions in the Rieti Plain, which started with the realization of the Salto and Turano artificial reservoirs, along two left tributaries of Velino River. The hydroelectric energy production purposes struggle with the reclamation and flood mitigation activities in the plain. The Land Reclamation Authority actuated the Integrated Reclamation General Project through the realization of pumping stations, connection and drainage canals, forestry-hydraulic works, rural roads, movable dams along Velino River and irrigation ditches. The irrigation activities, granted by the derivation of 5 m3/s from the Velino River, are carried out through 194,000 hectares within the territory of 42 municipalities of the Rieti Province. The Rieti Land Reclamation Authority contributes to the irrigation needs and to the environmental and hydrogeological protection and monitoring.

Pre- and post-Marinoan carbonate facies of the Democratic Republic of the Congo: Glacially- or tectonically-influenced deep-water sediments?

The upper carbonate-rich parts of the West Congo Supergroup (~1000–560Ma) from the Democratic Republic of the Congo have hitherto been considered as a record of abrupt eustatic and climatic events accompanying glaciation and deglaciation of a Snowball Earth-type Marinoan ice age that was of global extent. These strata have however never been investigated in detail. Results of new sedimentological work at key outcrops over a 1300km outcrop belt show that pre- and post-Marinoan carbonates are respectively, storm-influenced sediments deposited principally in a mid/outer-ramp setting, and deep-water slope carbonates (calicturbidites) representing a lobe-fringe or levee-overbank setting. The Upper Diamictite Formation held previously by some to be a subglacial tillite, comprises gravity flows (debrites) deposited in deep water below wave base along the unstable margins of a carbonate ramp. A direct glacial influence on sedimentation for diamictites or any accompanying facies cannot be readily identified. Sedimentary facies reported here primarily record the presence of deep-water submarine to alluvial fan systems related to extensional tectonic processes of the central-southern Macaúbas Basin (now located in Brazil) between 700Ma and 660Ma followed by the 630-Ma onset of the pre-collisional magmatic arc in the Araçuaí-West Congo Orogen. No extreme short-lived climatic or eustatic events of a Snowball Earth-type ice age are recorded in the studied succession, which primarily reflects long-term overriding regional tectonic controls resulting in diachronous sedimentation along the western margin of the Congo Craton.

Great Basin mammal diversity in relation to landscape history

The alternating mountain ranges and desert basins of the Great Basin in the western United States support higher species diversity of mammals than any other region of comparable area in temperate North America. Topographically complex regions have strong environmental gradients and heterogeneous habitats that result in fragmented geographic ranges over long periods and may promote speciation. In order to evaluate the influence of topography and climate on mammal diversification, we traced the history of mammal diversity during the formation of basin-and-range topography. We compiled species-occurrence data from the NeoMap database of fossil mammals and calculated standing diversity, origination rate, extinction rate, diversification rate (originations − extinctions), and turnover rate (originations + extinctions) for million-year intervals. We evaluated changes in faunal composition (species in mammalian orders and species in rodent families) over time and assessed whether significant changes in diversity and faunal composition tracked major changes in landscape history. Neogene geologic evolution generated by tectonic activity and changing climate created the topographic complexity and habitat heterogeneity of the Great Basin. Over the last 30 million years, extensional tectonic processes caused an older high plateau to collapse and stretch from east to west, resulting in longitudinal expansion by ~235 km to form the Great Basin. Average elevation decreased by 1–3 km. Global warming and associated increases in regional precipitation from 17 to 14 million years ago (mya; the Miocene Climatic Optimum [MCO]) interrupted the long-term trend of Cenozoic cooling and aridification. From 30 to 2 mya, Great Basin mammal diversity peaked during the MCO, then declined over the later Miocene and Pliocene. The major changes in diversity over time were robust to sampling effects. Faunal composition changed episodically, with increasing proportions of rodents, lagomorphs, and carnivores and decreasing proportions of ungulates and proboscideans. The highest diversification rate occurred during the MCO with smaller but significant diversification rates later in the Neogene. The highest turnover rates occurred during and immediately following the MCO. Comparison of rodent faunas of the Great Basin and the Great Plains showed substantial differences in the timing and magnitude of diversification and changes in taxonomic composition. These patterns support the hypothesis that climate change over complex topography stimulated diversification in the Great Basin.

Mesozoic tectono-magmatic activities in South China: Retrospect and prospect

The South China Block was formed through the collisional orogeny between the Cathaysia Block and the Yangtze Block in the Early Neoproterozoic. The northern, western and southern sides of the South China Block were affected by disappearance of the Paleo-Tethyan Ocean during the Paleozoic. The southern and northern sides of the South China Block were respectively collided with the Indo-China Block and North China Block in the latest Paleozoic to form the basic framework of the Eastern China. The Eastern China has been affected by the westward subduction of the Pacific Plate since the Mesozoic. Therefore, the South China Block was influenced by the three major tectonic systems, leading to a superposed compound tectonics. The comparative study of the Mesozoic geology between the South China Block and its surrounding areas suggests that although the Mesozoic South China Block was adjacent to the subduction zone of the western Pacific, no juvenile arc-type crust has been found in the eastern margin. The main Mesozoic geology in South China is characterized by reworking of ancient continental margins to intracontinental tectonics, lacking oceanic arc basalts and continental arc andesites. Therefore, a key to understanding of the Mesozoic geology in South China is to determine the temporal-spatial distribution and tectonic evolution of Mesozoic magmatic rocks in this region. This paper presents a review on the tectonic evolution of the South China Block through summarizing the magmatic rock records from the compressional to extensional tectonic process with the transition at the three juncture zones and using the deformation and geophysic data from the deep part of the South China continental lithosphere. Our attempt is to promote the study of South China’s geology and to make it as a typical target for development of plate tectonic theory.

Ongoing lithospheric removal in the western Mediterranean: Evidence from Ps receiver functions and thermobarometry of Neogene basalts (PICASSO project)

The western Mediterranean tectonic system consists of the Betic Mountains in southern Spain and the Rif Mountains in northern Morocco curved around the back-arc extensional Alboran basin. Multiple tectonic models have been developed to explain the coeval compressional and extensional tectonic processes that have affected the western Mediterranean since the Oligocene. In order to provide constraints on these evolutionary models, we use Ps teleseismic receiver functions (RF), thermobarometric analyses of post-Oligocene basalts, and previous teleseismic tomography images to investigate the lithospheric structure of the region. Ps RFs were calculated using seismic data from 239 broadband seismic stations in southern Iberia and northern Morocco and thermobarometric analysis was performed on 19 volcanic samples distributed throughout the region. The RF images reveal a highly variable Moho depth (∼25 to ∼55 km), as well as a strong positive, sub-Moho horizon between ∼45 and ∼80 km depth beneath the central Betic and Rif Mountains, which we interpret to be the top of the previously imaged Alboran Sea slab. Thermobarometric constraints from magmas in the eastern Betics and Rif indicate mantle melting depths between 40 and 60 km, typical of melting depths beneath mid-oceanic ridges where little to no lithosphere exists. Together, the RF and thermobarometric data suggest ongoing and recent slab detachment resulting from delamination of the continental lithosphere.

Interdigitated fluvial clastic deposits and calcareous tufa testifying an uplift of the catchment area: An example from the Pianizzoli area (southern Tuscany, Italy)

Calcareous tufa, as well as other types of continental carbonates, are largely sensitive to climate variations. As a consequence, the relationship between climate variations and tufa deposition is relatively well known and calcareous tufa deposits are often used as a reliable climatic proxy in many geological settings. Conversely, the response of tufa to tectonics and uplifting processes is relatively poorly known, due to their genetic independence from geothermally heated groundwater springs and associated tectonic-controlled upwelling conduits in tectonically active settings.The aim of this paper is to describe a peculiar stratigraphic succession where calcareous tufa deposits are closely associated and interbedded with coarse-grained clastic deposits. The study area is located in southern Tuscany (Massa Marittima), where extensional tectonic processes, responsible for the emplacement of magmatic bodies at shallow crustal levels, have been active from Pliocene to Quaternary age, and caused a diffuse uplift as well as local volcanic eruptions and hydrothermal fluid circulation.The succession described in the paper recorded phases of calcareous tufa-dominated deposition (i.e. overall, medium/low hydrodynamic energetic settings) repeatedly interbedded with coarse-grained clastic fluvial sediments which testify an abrupt change in both the energy of the depositional environment and the sediment supply. Nevertheless, during the phases dominated by clastic sedimentation, tufa deposition was not inhibited as it would be expected for climate-influenced settings, suggesting that the observed stratigraphic settings were a consequence of a local uplift of the catchment area.These findings indicate that calcareous tufa deposits could be a sensitive proxy to tectonics when they are associated with coarse-grained clastic deposits, in a geological setting where the origin of the clastic sediments cannot be explained by autocyclic or climate-induced factors.

From mush to eruption in centuries: assembly of the super-sized Oruanui magma body

The magmatic systems that give rise to voluminous crystal-poor rhyolite magma bodies can be considered to operate on two contrasting timescales: Those governed by longer-term processes by which a magma acquires its chemical and isotopic characteristics (e.g., fractional crystallisation and assimilation), and those operating at shorter timescales during the physical accumulation of the melt-dominant magma body that finally erupts. We explore the compositional and textural relationships between amphibole and orthopyroxene crystals from the 25.4 ka, 530 km3 (magma) Oruanui eruption products (Taupo volcano, New Zealand) to investigate how processes related to the physical assembly of the pre-eruptive magma body are represented in the crystal record. Over 90 % of orthopyroxenes from the volumetrically dominant high-SiO2 (>74 wt%) rhyolite pumices record textural evidence for a significant disequilibrium event (partial dissolution ± resorption of cores and interiors) prior to the growth of 40–500 μm thick rim zones. This dissolution/regrowth history of orthopyroxene is recorded in the chemistry of co-crystallising amphiboles as a prominent inflection in the concentrations of Mn and Zn, two elements notably enriched in orthopyroxene relative to amphibole. Textural and chemical features, linked with in situ thermobarometric estimates, indicate that a major decompression event preceded the formation of the melt-dominant body. The decompression event is inferred to represent the extraction of large volumes of melt plus crystals from the Oruanui crystal mush/source zone at pressures of 140–300 MPa (~6–12 km depth). Orthopyroxene underwent partial dissolution during ascent before reestablishing in the melt-dominant magma body at pressures of 90–140 MPa (~3.5–6 km). We model Fe–Mg diffusion across the core-rim boundaries along the crystallographic a or b-axes to constrain the timing of this decompression event, which marked establishment of the melt-dominant magma body. Maximum modelled ages indicate that this event did not begin until ~1,600 years before eruption, consistent with constraints from zircon model-age spectra. Once extraction began, it underwent runaway acceleration with a peak extraction age of ~230 years, followed by an apparent period of stasis of ~60 years prior to eruption. The rapidity of the extraction and accumulation processes implies the involvement of a dynamic driving force which, in the rifted continental arc setting of the Taupo Volcanic Zone, seems likely to be represented by magma-assisted extensional tectonic processes.

Fault-controlled stratigraphy of the Late Cretaceous Abiod Formation at Ain Medheker (Northeast Tunisia)

The palaeogeographic setting of the studied Ain Medheker section represents an Early Campanian to Early Maastrichtian moderately deep carbonate shelf to distal ramp position with high rates of hemipelagic carbonate production, periodically triggered by mass-flow processes. Syndepositional extensional tectonic processes are confirmed to the Early Campanian. Planktonic foraminifera identified in thin sections and calcareous nannofossils allow the identification of the following biozones: Globotruncanita elevata, Contusotruncana plummerae (replacing former Globotruncana ventricosa Zone), Radotruncana calcarata, Globotruncana falsostuarti, and Gansserina gansseri. The following stable C-isotope events were identified: the Santonian/Campanian boundary Event, the Mid-Campanian Event, and the Late Campanian Event. Together with further four minor isotopic events, they allow for correlation between the western and eastern realms of Tunisia. Frequently occurring turbidites were studied in detail and discussed in comparison with contourites.

Slip distribution model of two small-sized inland earthquakes and its tectonic implication in north-eastern desert of Egypt

Two slip distribution models of the 1999 and 2006 Beni-Suef earthquakes ( M w 4.5 and M w 4.2, respectively) were revealed using waveform inversion with an empirical Green’s function method. Waveform data used were recorded by the Egyptian Seismic Network’s short-period stations. To identify the active fault plane associated with each event and to estimate the best fitting rupture velocity, a set of slip distribution models was recovered on both nodal planes of the focal mechanism with a range of fixed rupture velocities. The result for the 1999 event shows that the northwest trending plane consistently provided better fitting solutions than the southwest trending plane. This implies that the slip was left-lateral on a northwest trending plane dipping toward the southwest. The 2006 event caused a slip movement on a dip-slip fault dipping towards the north. The rupture velocities were founded to be 3.5 km/s and 3.0 km/s which gave a comparatively small residual for the 1999 and 2006 earthquakes, respectively. The corresponding slip distribution models for the 1999 and 2006 events provide their seismic moments of 7.6E + 15 N m and 2.5E + 15 N m and moment magnitudes of 4.5 and 4.2, respectively. From the viewpoint of tectonics in the region, the present results imply an extensional tectonic process along the pre-existing WNW–ESE/NW–SE faults that may be transferred from the Gulf of Suez–Red Sea rift towards the northeastern desert in Egypt. This finding implies the diving of the northeastern of African plate beneath the Eurasian plate.

Petrogenetic evolution of the Early Miocene Alaçamdağ volcano-plutonic complex, northwestern Turkey: implications for the geodynamic framework of the Aegean region

Extensional-tectonic processes have generated extensive magmatic activity that produced volcanic/plutonic rocks along an E-W-trending belt across north-western Turkey; this belt includes granites and coeval volcanic rocks of the Alacamdag volcano-plutonic complex. The petrogenesis of the Early Miocene Alacamdag granitic and volcanic rocks are here investigated by means of whole-rock Sr–Nd isotopic data along with field, petrographic and whole-rock geochemical studies. Geological and geochemical data indicate two distinct granite facies having similar mineral assemblages, their major distinguishing characteristic being the presence or absence of porphyritic texture as defined by K-feldspar megacrysts. I-type Alacamdag granitic stocks have monzogranitic-granodioritic compositions and contain a number of mafic microgranular enclaves of monzonitic, monzodioritic/monzogabbroic composition. Volcanic rocks occur as intrusions, domes, lava flows, dykes and volcanogenic sedimentary rocks having (first episode) andesitic and dacitic-trachyandesitic, and (second episode) dacitic, rhyolitic and trachytic-trachydacitic compositions. These granitic and volcanic rocks are metaluminous, high-K, and calc-alkaline in character. Chondrite-normalised rare earth element patterns vary only slightly such that all of the igneous rocks of the Alacamdag have similar REE patterns. Primitive-mantle-normalised multi-element diagrams show that these granitic and volcanic rocks are strongly enriched in LILE and LREE pattern, high (87Sr/86Sr)i and low e Nd(t) ratios suggesting Alacamdag volcano-plutonic rocks to have been derived from hybrid magma that originated mixing of co-eval lower crustal-derived more felsic magma and enriched subcontinental lithospheric mantle-derived more mafic magmas during extensional processes, and the crustal material was more dominant than the mantle contribution. The Alacamdag volcano-plutonic complex rocks may form by retreat of the Hellenic/Aegean subduction zone, coinciding with the early stages of back-arc extension that led to extensive metamorphic core-complex formation.

Comment on “Geodynamic evolution of the SW Europe Variscides” by António Ribeiro et al.

[1] Ribeiro et al. [2007] have presented a geodynamic view of the SW Iberia Variscides based on data from Portugal. Their treatment of already published data is commendable, and the knowledge gained will surely encourage the discussion of the SW Europe Variscides. However, in our opinion, Ribeiro et al.’s modeling and interpretation of the Ediacaran–Lower Ordovician ( 560–470 Ma) geodynamic evolution are of limited value. In this regard, they based their analysis of the Ossa-Morena Zone (OMZ) on assumptions which are contradicted by recent published data. Ribeiro et al. ignored recent progress in the OMZ Ediacaran-Ordovician stratigraphy and, as a consequence they misunderstood the structure of domains overprinted by strong Variscan (Carboniferous) deformation and metamorphism. [2] The first arguable point of Ribeiro et al.’s [2007] work is the possibility of occurrence of a likely Grenville inlier in the OMZ. Until now, no rocks older than the Ediacaran have ever been proven to exist in the OMZ (Portugal and Spain). The oldest dated rocks are Ediacaran felsic dykes with 623 ± 3Ma (U-Pb TIMS/zircon, Loma del Aire rhyolites [SanchezGarcia et al., 2007]). The most representative Ediacaran rocks constitute the Serie Negra succession [Eguiluz et al., 2000; Pereira et al., 2006b]. Regardless of the Cambrian and Carboniferous high-grade metamorphism with anatexis, zircons from the Ediacaran Serie Negra metasediments preserve older cores and metamorphic overgrowths [OrdonezCasado, 1998; Fernandez-Suarez et al., 2002; Chichorro, 2006;Chichorro et al., 2006;Pereira et al., 2008; Linnemann et al., 2008]. The obtained ages for the metamorphic overgrowths are mainly Carboniferous, but also Ediacaran, Cambrian and Ordovician. U-Pb SHRIMP and LA-ICP-MS dating on detrital zircons from the Serie Negra metasediments indicate maximum age of deposition ranging from 560 to 540Ma [Ordonez-Casado, 1998; Fernandez-Suarez et al., 2002; Pereira et al., 2006a, 2008; Linnemann et al., 2008]. A common feature of all investigated samples is an ‘‘age gap’’ between 1.7 and 1.0 Ga. This age gap with no sign of Grenvillian zircon forming events is a characteristic of a Cadomian/West African provenance and distinguishes Cadomia/West Africa from Amazonia and Baltica. These Ediacaran sediments represent the infill of basins related to dismantling of a Cadomian continental arc [Pereira and Chichorro, 2004; Pereira et al., 2006b, 2007; Linnemann et al., 2008]. Ribeiro et al.’s [2007] statement that the Ediacaran calc-alkaline plutons are spatially limited to the northern margin of the Coimbra-Cordoba shear zone is not correct. It was recognized as plutonism of Ediacaran age in the northern margin of this major Variscan shear zone with 580 Ma (Pb-Pb Kober/zircon, Aljucen granodiorite [Talavera et al., 2008]), 573 ± 14 Ma (U-Pb SHRIMP/ zircon, Valle de la Serena porphyritic granitoid [OrdonezCasado, 1998]) and 555 Ma (Sm-Nd/garnet, MeridaMontoro gabbro-diorite [Bandres et al., 2002]) but also toward the south with 552 ± 10 Ma (U-Pb SHRIMP/zircon; Ahillones granite [Ordonez-Casado, 1998]). The Cadomian back-arc basin was active longer, at least until 545Ma. The final magmatic pulse of the Cadomian magmatic arc at 550 Ma (coeval with the crystallization of the Ahillones granite) is documented by newU-Pb LA-ICP-MS zircon data [Pereira et al., 2006a; Linnemann et al., 2008]. Closure of the Cadomian back-arc basin and final events of arc-continent collision in the OMZ occurred probably between 545 Ma and the overall onset of Cambrian intracontinental riftrelated plutonism at 530 Ma [Sanchez-Garcia et al., 2003, 2008]. [3] The second debatable point is the assertion of Ribeiro et al. [2007] that continental rifting on the Cambrian platform of northern Gondwana started around the Middle Cambrian. New zircon U-Pb zircon dating indicates an early igneous event with calc-alkaline signature and peraluminous tendency in the OMZ during the Lower Cambrian ( 530–515 Ma) associated with rift-related carbonate and siliciclastic deposition: 532 ± 4 Ma (U-Pb TIMS/zircon, Mina Afortunata leucogranite [Sanchez-Garcia et al., 2008]), 530 ± 3 Ma (U-Pb SHRIMP/zircon, Bodonal porphyroid [Romeo et al., 2006]) and 529–527Ma (U-Pb SHRIMP/zircon, Alcacovas and Santiago do Escoural orthogneisses [Chichorro, 2006; Chichorro et al., 2008]). This igneous event that seems to represent last residual melts of high-temperature, zirconundersaturated mafic magmas later affected by crustal contamination, while others indicate partial melting of crustal metasediments variably contaminated by basaltic liquids, is interpreted as the result of an extensional tectonic process accompanied by strong thermal rise [Sanchez-Garcia et al., 2003, 2008; Chichorro et al., 2008]. This situation was probably connected to an underlying mantle plume or simply TECTONICS, VOL. 28, TC4009, doi:10.1029/2008TC002430, 2009 Click Here for Full Article

Unusual Braarudosphaera bigelowii and Micrantholithus vesper enrichment in the Early Miocene sediments from the Slovenian Corridor, a seaway linking the Central Paratethys and the Mediterranean

During a stratigraphic survey of an Early Miocene sedimentary sequence outcropping in SE Slovenia an unusual enrichment of the calcareous nannoplankton assemblage with pentaliths – pentagonal nannoliths consisting of five crystal units and showing radial symmetry – was detected. Pentaliths were only abundant in a short interval, which was sampled in detail. The lithological composition of this interval was indistinguishable from that of the underlying and the overlying strata. The composition of nannoplankton assemblages and distribution patterns of different taxa were studied. Mineral composition, total calcite content and δ 13C values of chosen sediment samples were analysed as well. The pentalith-enriched strata were deposited in the lower Early Miocene, corresponding to the Egerian stage of the Central Paratethys and the Aquitanian of the Mediterranean, in the Slovenian Corridor — a seaway linking the Central Paratethys and the Mediterranean. The presence of rare specimens of Discoaster druggii and Helicosphaera carteri allows stratigraphic correlation with the transition of NN1 — Triquetrorhabdulus carinatus Zone and NN2 — Discoaster druggii Zone of Martini [Martini, E., 1971. Standard Tertiary and Quarternary calcareous nannoplankton zonation. In: Farinacci, A. (Ed.), Proceedings of the II Planctonic Conference, Roma, 1970, Ed. Technoscienza, pp. 739–785]. Pentaliths belong to two species: Braarudosphaera bigelowii and Micrantholithus vesper. Both species display a wide range of morphological variation and most likely represent a group of sibling species. The pattern of distribution of both species displays obvious similarities throughout most of the studied interval. In the upper part of the studied interval B. bigelowii reaches maximum abundance, while M. vesper disappears. Since the ability of B. bigelowii to thrive in hyposaline water is well known, it appears that M. vesper tolerates reduced water salinity as well, but to a lesser extent than B. bigelowii. No significant paleoclimatic and paleogeographical changes are known in this particular region during the Early Miocene, however, the onset of far reaching extensional tectonic processes took place during this time. The enrichment of nannoplankton assemblage with pentaliths may represent a reflection of a short-lived episode of hyposaline conditions. Enhanced influx of freshwater and terrigenous material is indicated by a drop in 13C values, a rise in nannoplankton productivity and the presence of sand in some samples from the pentalith-enriched strata. The most likely causes of this episode are regional tectonic changes, associated with the onset of intensive tectonic activity in the western part of the Central Paratethys.

Metamorphic evolution of anthophyllite/cummingtonite-cordierite rocks from the upper unit of the Ordenes Complex (Galicia, NW Spain)

Two types of low-Ca amphibolites are described for the first time in the Ordenes Complex (Galicia, NW Spain). The first type exhibits an initial mineral assemblage containing sillimanite, staurolite and anthophyllite, which are found as microinclusions in cordierite crystals. This association indicates medium pressure and temperatures above 600oC, and is replaced in the matrix by a medium to low-pressure assemblage of anthophyllite, cordierite and garnet. The second type is represented by lowpressure amphibolitic gneisses formed mainly of cummingtonite, cordierite, andalusite and garnet. The first type of amphibolite would appear to give rise to the second type as a result of metamorphic evolution to lower pressures . A fragment of the decompressive P-T path was obtained by a detailed study of the mineral assemblage succession and the textural relationships of these rocks. The P-T path begins with an almost isothermal decompression from ca. 6-7 kbar and 650oC. This kind of P-T path (isothermal) is usually interpreted, according to thermal models, as originating during exhumation favoured by some tectonic process. Hence, the exhumation of the uppermost unit of the Ordenes Complex, in which the anthophyllite-cummingtonite rocks are located, appears to have been facilitated by some kind of extensional tectonic process.

Application of Low-Temperature Thermochronometry to Extensional Tectonic Settings

Extension and associated tectonic exhumation have been described across a diverse spectrum of geodynamic environments, ranging from lithospheric-scale extension along divergent plate margins to extensional faulting within compressional orogenic belts. Normal faulting structurally dominates extensional tectonic environments such as passive continental margins, intra-continental extensional provinces and rifts, extensional back-arc basins, and mid-oceanic ridge inside corner extensional systems. In addition, structural domains characterized by normal faulting or extensional crustal attenuation commonly occur within collisional orogens and within transcurrent deformation belts. Numerous studies over the past decades have investigated extensional faulting in these different extensional tectonic environments to elucidate the architecture of extensional fault systems, the driving forces associated with crustal or lithospheric extension, and the timing and rates of extensional tectonic processes. Many important aspects of extensional tectonics remain controversial, such as (1) the forces driving initiation of extension and rifting and their variation through time (e.g., Jones et al. 1996; Atwater and Stock 1998), (2) the parameters controlling the temporal and spatial distribution of extensional strain in the lithosphere (e.g., Buck 1991; Kusznir and Park 2002), (3) the factors influencing the geometry, kinematics, and mechanics of extensional fault systems (e.g., Wernicke and Burchfiel 1982; Jackson and White 1989; Buck 1991), (4) the role of magmatism during extensional faulting (e.g., Sengor and Burke 1978; Gans et al. 1989; Armstrong and Ward 1991; Axen et al. 1993), and (5) the interplay between extensional tectonism, erosion, and sedimentation in evolving extensional provinces (e.g., Ehlers et al. 2001, 2003). One of the primary hurdles in resolving many of these questions has been the lack of constraints on the timing and rates of geological processes accommodating extension. Traditionally, the most direct way to assess the timing of normal faulting has been to date stratigraphic or intrusive/extrusive …

Early Palaeozoic magmatism in the English Lake District

SUMMARY The Lower Palaeozoic rocks of the Lake District record the rifting of the microcontinental terrane of Avalonia from Gondwana early in the Ordovician, its drift northwards across the Iapetus Ocean and collision with Laurentia and Baltica in the Silurian, and its Acadian deformation in Early Devonian times. Three discrete episodes of magmatism occurred during these events. In late Ordovician (Caradoc) times large volumes of magma were generated in response to subduction of Iapetus oceanic crust beneath Avalonia. Large-scale eruptions formed the basalt–andesite–rhyolite volcanic fields that are now preserved as the Eycott and Borrowdale volcanic groups. Large bodies of mainly granitic magma were emplaced beneath the Borrowdale Volcanic Group as part of the largely Ordovician Lake District batholith; these include, for example, the Eskdale and Ennerdale intrusions. Less than 10 Ma later, a brief episode of silicic volcanism is recorded within the marine sedimentary strata of the lowest part of the overlying Windermere Supergroup. A period of about 40 million years of apparent quiescence then ensued before further granitic masses, the Shap and Skiddaw plutons were emplaced around the margins of the batholith in Early Devonian times. All of these magmatic episodes were accompanied by minor intrusions. The Eycott and Borrowdale volcanic groups are rare examples in the geological record of the products of subaerial volcanism. Preservation is entirely due to subsidence keeping pace with the emplacement of new material. Subsidence resulted both from extensional tectonic processes and, locally, during the movement

Preliminary results of the geological structure and kinematics of deformation in Mt. Tzena (Paikon subzone, Central Macedonia, Greece)

The Tzena Mt. belongs to the Paikon subzone (Axios zone, Central Macedonia, Greece) and consists of 5 distinguishable lithological units, which are from the lower to the upper the following: the lower unit of gneisses, the marbles unit, the black phyllites, the meta-volcanosedimentary unit and the meta-flysch. These units form a metamorphic sedimentary sequence of Upper Paleozoic to Lower Cretaceous age, while the metarhyolites of the volcanosedimentary unit are of Jurassic age. Kinematic analysis of the deformation in Tzena Mt. suggests that the whole metamorphic sequence has been affected by a strong shearing and mylonitisation in ductile to semi-ductile conditions, trending SW-NE, contemporaneous with a retrograde metamorphism in greenschist facies. Kinematic indicators show sense of movement both top to the SW and NE while a streching lineation, trending SW-NE, defined by preferred orientation of white mica and chlorite remains stable all over the metamorphic sequence. The Upper Cretaceous carbonates of the adjacent to Tzena, Pinovon Mt., have been affected by this deformation as well, and hence the deformation took place in Tertiary times. However, the contact between the lower gneiss unit and the marbles unit constitute a shear zone, in ductile to semi-ductile conditions, trending SW-NE with a sense of shear top to the SW. Thus, the marbles have been moved upon the gneisses along a large shear zone towards SW, during the Tertiary deformation. Correlation between this tertiary SW-NE deformation event in Tzena Mt. with the similar tertiary shear deformation towards SW in Paikon Mt., suggests that Tzena and Paikon Mts. have the same tectonic history in Tertiary times. Analogous Tertiary tectonic events with similar geometry and kinematics of the deformation have already been established for several other areas in Greece (Olympos - Ossa Mts., Rhodope Mt., Cyclades islands etc.) where they have caused the exhumation of metamorphic core complex (Kilias & Mountrakis, 1990, Sokoutis et al., 1993, Kilias, 1995, Dinter, 1998, Kilias et al., 1999). Thus, in relation to these, an exhumation of the Tzena orogen, consisting of the metamorphic sequence, during the Tertiary extensional tectonic process could be very possible.