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1.
《Geodinamica Acta》2013,26(5):327-351
A geological study carried out in the southern part of the Larderello geothermal area (Northern Apennines) provides new information on the development mechanism and timing of the earlier extensional structures that formed during the Miocene post-collisional tectonics which affected the orogen. Staircase low-angle normal faults (LANFs) affected a multilayered thickened upper crust after the collisional stage, producing the lateral segmentation of the Tuscan Nappe, the deeper non-metamorphic tectonic unit of the Northern Apennines in the Tuscan area. The tectonic history recorded in two Tuscan Nappe discontinuous bodies revealed that the LANFs took place during the Middle–Late Miocene, displacing collisional structures developed from the Late Oligocene. These Tuscan Nappe bodies are delimited by detachment faults located at the base, within the Tuscan evaporites, and at the top within the Ligurian Units. Their western and eastern margins coincide with east-dipping ramps. These structures and the Tuscan Nappe bodies were later dissected by Pliocene–Quaternary high-angle normal faults. The reconstructed deformation history implies that the Tuscan Nappe bodies are extensional horses developed through an earlier asymmetrical east-dipping extensional duplex system, involved in block faulting during the later, Pliocene-Quaternary, stage of extension. 相似文献
2.
The role of regional extension on the rise and emplacement of granites in the crust is still debated. Pluton ascent and emplacement widely occurred in Tuscany (Italy) since late Miocene during the post-orogenic collapse of the inner Apennines, and are presently occurring in the geothermal areas of Amiata and Larderello. Tuscany offers a preferred test site to study the role of regional extension on pluton ascent and emplacement at different crustal levels. Ductile extension enhanced the segregation and ascent of granitic melts in the lower crust, controlling pluton emplacement in correspondence with the brittle–ductile transition. In the brittle crust, magma ascent occurred through subvertical faults and fractures compatible with the regional extension direction; pluton emplacement mainly occurred by means of roof lifting. The case of Tuscany suggests that the extensional structures enhance melt segregation and ascent in the ductile crust, but are not efficient alone to provide a pathway for the ascent of granitic magmas in the brittle-extending crust. The estimated magmatic strain rates due to pluton emplacement in the geothermal areas are much larger than the regional tectonic strain rates. This suggests that regional tectonics did not control magma emplacement in the brittle crust and explains why nontectonic processes (roof lifting) accommodated the space required for pluton emplacement. 相似文献
3.
《Geodinamica Acta》2000,13(6):345-366
In the Radicondoli–Travale area of the Larderello geothermal field (Italy) new structural and petrologic data on some metamorphic units, cored from geothermal wells, reveal the existence of a wide complex of hornfels rocks. The development of the complex is related to the emplacement of Neogene magmatic rocks at a shallow depth within a Permo–Triassic terrigenous metasedimentary sequence of a low metamorphic grade. The sequence was subjected to alpine tectonic-metamorphic events. This reconstruction gives new insights in the tectonic setting of metamorphic units below the Tuscan Nappe, in southern Tuscany. Particularly, the medium and high grade metamorphic rocks seem strictly related to the development of Neogene thermal aureoles and do not represent remnants of a Paleozoic basement. Furthermore, in this sector of the Larderello geothermal system, the presence of deep geothermal reservoir at a depth of 3 000 m is mainly linked to the enhanced permeability caused by fracturing in these hornfels rocks. This fracturing is a consequence of the dehydration reaction occurring in the metapelites due to Neogene thermal metamorphism. This mechanism allowed the development of a long-lived hydrothermal system, shown by the secondary mineralogical assemblages. These testify the presence of at least two hydrothermal stages which are well preserved in the less permeable units. 相似文献
4.
AbstractIn the Radicondoli–Travale area of the Larderello geothermal field (Italy) new structural and petrologic data on some metamorphic units, cored from geothermal wells, reveal the existence of a wide complex of hornfels rocks. The development of the complex is related to the emplacement of Neogene magmatic rocks at a shallow depth within a Permo–Triassic terrigenous metasedimentary sequence of a low metamorphic grade. The sequence was subjected to alpine tectonic-metamorphic events. This reconstruction gives new insights in the tectonic setting of metamorphic units below the Tuscan Nappe, in southern Tuscany. Particularly, the medium and high grade metamorphic rocks seem strictly related to the development of Neogene thermal aureoles and do not represent remnants of a Paleozoic basement. Furthermore, in this sector of the Larderello geothermal system, the presence of deep geothermal reservoir at a depth of 3 000 m is mainly linked to the enhanced permeability caused by fracturing in these hornfels rocks. This fracturing is a consequence of the dehydration reaction occurring in the metapelites due to Neogene thermal metamorphism. This mechanism allowed the development of a long-lived hydrothermal system, shown by the secondary mineralogical assemblages. These testify the presence of at least two hydrothermal stages which are well preserved in the less permeable units. © 2000 Editions scientifiques et médicales Elsevier SAS 相似文献
5.
E. Pandeli C. Bartolini A. Dini E. Antolini 《International Journal of Earth Sciences》2010,99(6):1357-1381
In order to add new data to the knowledge of the paleogeography of Southern Tuscany and the Tuscan shelf, a multidisciplinary
study on the petrology and distribution of magmatic clasts of the widely exposed Messinian, Pliocene, and Quaternary sediments
in Southern Tuscany was carried out. The magmatic clasts consist of porphyric aplites and subordinate granite porphyries,
which derive from eroded subvolcanic acidic bodies. The porphyric aplite clasts were analyzed in detail to define their textural,
petrographic, and chemical (major and trace elements, Sr and Nd isotopic composition) features. The porphyric aplite clasts
show strong affinities with the 8.4–7.9 Ma old Capo Bianco aplite, whereas the granite porphyry clasts have affinities with
the Portoferraio porphyry (ca. 8 Ma) which intruded the Cretaceous and Paleogene Flysch Units and the Ophiolitic Unit in central-western
Elba Island (Northern Tyrrhenian Sea). The present outcrops of the Capo Bianco aplite in central Elba Island cannot be considered
as the source rock for the Late Messinian gravels, because at that time such Capo Bianco aplite (located at the lower portion
of the laccolitic stack) was there buried at several kilometers depth. However, other Capo Bianco-like aplitic bodies outcropped
during the latest Miocene in the host rocks above and also around the 6.8 Ma M. Capanne Pluton in the western Elba areas.
The exposure of such bodies was made possible by the activation of the central Elba detachment fault (=CEF), due to the uplift
of the M. Capanne pluton. This uplift displaced a significant portion of the cover (including the upper portion of the laccolithic
stack) of the granitoid body at about 6, 7 Ma, thus allowing the erosion of the lower part of the laccolith complex made up
of the Capo Bianco aplite and of the Portoferraio porphyry and leaving still buried the M. Capanne pluton. The paleogeographic
picture of Southern Tuscany arising from the collected evidences is the following: during Late Messinian, the clasts were
dispersed by a SSW/NNE-trending complex fluvial system in the Colline Metallifere area. The Early Pliocene extensional tectonics
cut off the detrital inputs from the Messinian source areas, because of the onset of the Piombino Channel and of the Campiglia–Gavorrano
Ridge. During the Pliocene, the clasts were cannibalized from the previous sedimentary units and reached the easternmost areas
due the eastward progressive uplift of the Colline Metallifere, likely connected with the coeval magmatic intrusions. Finally,
the Quaternary regional uplift allowed a drainage reversal and a backward displacement of the aplitic clasts toward the Tyrrhenian
coast. These data point to a rapidly evolving drainage pattern in Southern Tuscany during the considered time interval, which
was mostly driven by the intrusion and uplift of the Messinian to Quaternary plutons. The morpho-tectonic evolution is well
framed also within the models since long accepted for the Northern Apennine geodynamics, characterized by an overall eastward
shift of the orogenic front. 相似文献
6.
Extensive, mainly acidic peraluminous magmatism affected the Tuscan Archipelago and the Tuscan mainland since late Miocene, building up the Tuscan Magmatic Province (TMP) as the Northern Apennine fold belt was progressively thinned, heated and intruded by mafic magmas. Between 3.8 and 1.3 Ma an intrusive complex was built on Larderello area (Tuscan mainland) by emplacement of multiple intrusions of isotopically and geochemically distinct granite magmas. Geochemical and isotopic investigations were carried out on granites cored during drilling exploration activity on the Larderello geothermal field. With respect to the other TMP granites the Larderello intrusives can be classified as two-mica granites due to the ubiquitous presence of small to moderate amounts of F-rich magmatic muscovite. They closely resemble the almost pure crustal TMP acidic rocks and do not show any of the typical petrographic features commonly observed in the TMP hybrid granites (enclaves, patchy zoning of plagioclase, amphibole clots). On the basis of major and trace elements, as well as REE patterns, two groups of granites were proposed: LAR-1 granites (3.8–2.3 Ma) originated by biotite-muscovite breakdown, and LAR-2 granites (2.3–1.3 Ma) generated by muscovite breakdown. At least three main crustal sources (at 14–23 km depth), characterized by distinct εNd(t) and 87Sr/86Sr values, were involved at different times, and the magmas produced were randomly emplaced at shallow levels (3–6 km depth) throughout the entire field. The partial melting of a biotite-muscovite-rich source with low εNd(t) value (about −10.5) produced the oldest intrusions (about 3.8–2.5 Ma). Afterwards (2.5–2.3 Ma), new magmas were generated by another biotite-rich source having a distinctly higher εNd(t) value (−7.9). Finally, a muscovite-rich source with high εNd(t) (about −8.9) gave origin to the younger group of granites (2.3–1.0 Ma). The significant Sr isotope disequilibrium recorded by granites belonging to the same intrusion is interpreted, as due to the short residence time of magmas in the source region followed by their rapid transfer to the emplacement level. Partial melting was probably triggered by multiple, small-sized mafic intrusions, distributed over the last 3.8 Ma that allowed temporary overstepping of biotite- and muscovite-dehydration melting reactions into an already pre-heated crust. Dilution in time of the magmatic activity probably prevented melt mingling and homogenization at depth, as well as the formation of a single, homogeneous, hybrid pluton at the emplacement level. Moreover the high concentrations of fluxing elements (B, F, Li) estimated for the LAR granites modified melt properties by reducing solidus temperatures, decreasing viscosity and increasing H2O solubility in granite melts. The consequences were a more efficient, fast, magma extraction and transfer from the source, and a prolonged time of crystallization at the emplacement level. These key factors explain the long-lived hydrothermal activity recorded in this area by both fossil (Plio-Quaternary ore deposits) and active (Larderello geothermal field) systems. 相似文献
7.
We have developed a significant body of new field-based evidence relating to the history of crustal extension in western Turkey. We establish that two of the NE–SW-trending basins in this region, the Gördes and Selendi Basins, whose sedimentary successions begin in the early Miocene, are unlikely to relate to late-stage Alpine compressional orogeny or to E–W extension of Tibetan-type grabens as previously suggested. We argue instead that these basins are the result of earlier (?) late Oligocene, low-angle normal faulting that created approximately N–S “scoop-shaped” depressions in which clastic to lacustine and later tuffaceous sediments accumulated during early–mid-Miocene time, separated by elongate structural highs. These basins were later cut by E–W-trending (?) Plio–Quaternary normal faults that post-date accumulation of the Neogene deposits. In addition, we interpret the Alaşehir (Gediz) Graben in terms of two phases of extension, an early phase lasting from the early Miocene to the (?) late Miocene and a young Plio–Quaternary phase that is still active. Taking into account our inferred earlier phase of regional extension, we thus propose a new three-phase “pulsed extension” model for western Turkey. We relate the first two phases to “roll-back” of the south Aegean subduction zone and the third phase to the westward “tectonic escape” of Anatolia. 相似文献
8.
Hydrochemical baseline condition of groundwater at the Mizunami underground research laboratory (MIU) 总被引：2，自引：0，他引：2
Hydrochemical conditions up to depths of 1000 m below ground level around the Mizunami Underground Research Laboratory were investigated to construct a “baseline condition model” describing the undisturbed hydrochemical environment prior to excavation of the underground facilities at Mizunami, Gifu, Japan. Groundwater chemistry in this area was classified into a Na–Ca–HCO3 type of groundwater in the upper part of sedimentary rock sequence and a Na–(Ca)–Cl type of groundwater in the deeper part of the sedimentary rock sequence and basement granite. The residence time of the groundwaters was estimated from their 14C contents to be approximately 9.3 ka in the middle part of the sedimentary rock and older than 50 ka in the deep part of the granite. The evolution processes of these groundwaters were inferred to be water–rock interactions such as weathering of plagioclase, dissolution of marine sulphate/sulphide minerals and carbonate minerals in the Na–Ca–HCO3 type of groundwater, and mixing between “low-salinity water” in the shallow part and “higher-salinity water” in the deeper part of the granite in the Na–(Ca)–Cl type of groundwater. The source of salinity in the deeper part of the granite was possibly a palaeo-hydrothermal water or a fossil seawater that recharged in the Miocene, subsequently being modified by long-term water–rock interaction. The Cl-depth trend in granitic groundwater changes at a depth of −400 m below sea level. The hydrogeological properties controlling the groundwater flow and/or mixing processes such as advection and diffusion were inferred to be different at this depth in the granite. This hydrochemical conceptual model is indispensable not only when constructing the numerical model for evaluating the hydrochemical disturbance during construction and operation of the MIU facility, but also when confirming a hydrogeological model. 相似文献
9.
We re-processed the seismic reflection survey data of the Kakkonda geothermal field. The pre-stack migration delineates a strong and continuous reflector between 1800- and 2800-m depth, below which formations are not reflective. Earthquake data exhibit seismicity in the upper crust. The lower boundary of seismogenic layer is interpreted as the brittle–ductile transition. The thermal structure is thought to be the major factor controlling its depth. We compared the strong reflector with the thermal and rheological structure from drillholes. The depth of the reflector corresponds to the top of the highly–very highly fractured zone observed from formation microscanner imagery (FMI) logging in the Miocene formations. The density of fracture in the Kakkonda granite is very low, suggesting that granite corresponds to the nonreflective zone. The temperature–depth profile of well WD-1a shows that the temperature at the highly–very highly fractured zone is about 350 °C. This corresponds to a hydrothermal convection zone filled with two-phase geothermal fluid. The cut-off depth of seismicity that indicates the brittle–ductile transition lies at the isotherm of 300–350 °C near the reflector. We conclude that the strong seismic reflector is a strong contrast in acoustic impedance at the top of the fractured layer. The fractured layer could be a decoupling plane caused by different tectonic behaviors between the upper brittle and the lower ductile layers or a dehydration front by thermal diffusion. The similarity between the strong reflector and K-horizon, the strong reflector, found in southern Tuscany, Italy suggests that the P-wave reflector at the top of highly fractured zone at the brittle–ductile transition be common in areas with magmatic activity. 相似文献
10.
The geochemical and isotopic signature of Quaternary alluvial sediments filling a post-orogenic basin along the Tyrrhenian coasts of Italy (Cornia Plain, Tuscany) was investigated to unravel possible interactions with geothermal fluids from the Larderello geothermal field. Two cores located in the upper (UCP) and lower (LCP) sector of the plain were sampled to depths of up to 80 m. A third core in a neighbouring area not affected by geothermal activity was also sampled (Arno plain at Pisa), and its sediment composition was used as reference. The Cornia sediments (fraction < 65 μm) show high B, Cs and Sb concentrations related to a peculiar chemical enrichment of the clay fraction. They also show remarkable enrichments in As (up to 1000 μg g− 1) reflecting a contribution from local ore deposits.87Sr/86Sr ratios, ranging from 0.71022 to 0.71698, reveal the nature of the weathered mother rocks of the alluvial sediments, whereas the boron isotopic composition, varying from − 20‰ to − 10‰, suggests an interaction between the clay fraction and boron-rich fluids at temperatures greater than 50 °C. This implies that hydrothermal fluids widely circulated within the Cornia basin in the past, ultimately leading to the geochemical anomalies currently recorded in local sediments.Although natural (geogenic) in origin, these anomalies cause severe problems to the regional water management (groundwater exploitation) through leaching of trace elements into circulating groundwater, a phenomenon which has to be carefully studied and monitored. 相似文献
11.
《Geodinamica Acta》2013,26(6):375-387
Information from surface and subsurface geology (boreholes and seismic reflection lines) are used to depict the geometry of the extensional structures (low-angle normal faults and related Tuscan Nappe megaboudins) affecting the Mt. Amiata geothermal area and developed during the early stage of the extensional tectonics which affected the inner Northern Apennines and Tyrrhenian Sea from the Early-Middle Miocene. Normal faulting involved the thickened middle-upper crust after the collisional stage and, in the Mt. Amiata region, took place over relatively short periods (5-7 Ma) characterised by rapid extensional strain rates. Normal faults showing articulated geometry (flat-ramp-flat) characterised by subhorizontal detachments (flats) and synthetic ramps, caused widespread megaboudinage mainly in the sedimentary tectonic units and particularly in the Tuscan Nappe. Evaporites occurring at the base of the Tuscan Nappe, the deepest sedimentary tectonic unit of the Northern Apennines, controlled the geometry of the faults, and rift-raft tectonics may be the style of this first extensional phase. Three Tuscan Nappe extensional horses (megaboudins) have been recognised in the subsurface of the Mt. Amiata area. They are characterised, in map view, by elliptical shapes and show a mean NNW-SSE lengthening. They are delimited at the base and at the top by east-dipping flats, while their western and eastern margins coincide with east-dipping ramps. On the whole, considering their geometrical features, these megaboudins correspond to extensional horses belonging to an asymmetrical east-dipping extensional duplex system. Rollover anticlines deformed the western ramp of the megaboudins and rotated the uppermost flat as well as all the structures previously developed, which became steeply-dipping to the west. 相似文献
12.
In eastern Elba Island (Tuscany, Italy), a shallow crustal level felsic, tourmaline‐bearing, dyke‐sill swarm of Late Miocene age is associated with abundant tourmaline‐quartz hydrothermal veins and metasomatic masses. Development of these veins and masses in the host rocks demonstrates multiple hydro‐fracturing by magmatic, boron‐rich saline fluid. Tourmalines in felsic dykes are schorl, whereas in veins and metasomatic masses, tourmaline composition ranges from schorl‐dravite through dravite to uvite. This compositional shift is evidence for an increasing contribution to the magmatic boron‐rich fluids by a Mg‐Ca‐Ti‐rich external component represented by biotite‐rich and amphibolite host rocks. This system can be envisaged as an exposed proxy of the high temperature hydrothermal system presently active in the deepest part of the Larderello‐Travale geothermal field (Tuscany). 相似文献
13.
The Quaternary System is under threat of extinction as a chronostratigraphic unit in the international Geological Time Scale. In its place, the Neogene System (previously comprising Miocene and Pliocene Series), is to be extended to include the Pleistocene and Holocene Series. It may be possible to “save” the Quaternary by defining it as a Subsystem of the Neogene. In doing so, it would be an opportune time to extend the base of the Quaternary to 2.6 Ma to encompass the time during which (1) Earth's climate has been strongly influenced by bi-polar glaciation, and (2) the genus Homo first appeared and evolved. 相似文献
14.
Bruno Della Vedova Claudio Vecellio Stefano Bellani Umberta Tinivella 《International Journal of Earth Sciences》2008,97(2):317-332
We present a 3-D thermal model of the Larderello geothermal field (Tuscany) to evaluate (1) the extent and contribution of
the heat transfer mechanisms (conduction vs. convection) at the intermediate-upper crust levels, (2) the variability of the
heat and mass fluxes entering from below and (3) the crucial role of the formation permeability. The model, composed by three
main layers, considers the upper 10 km of the crust to better constrain the simulations with experimental data from borehole,
fluid inclusion studies and hypocentral distributions. Several sets of simulations were carried out with different bottom
boundary temperatures and different formation permeabilities for the two deeper layers. The results indicate that the present
temperature (T) and pressure distributions in the Larderello field require deep reservoir rocks with higher permeability than the overlying
capping units and underlying intermediate crust. Permeability values of 1 mDarcy for the reservoir rocks are enough to allow
fluid convection, if the temperature at 10 km depth is as high as 500 ± 50°C. The presence of localized zones with formation
permeability 50–100 times higher than the surrounding rocks strongly favours the migration of over-pressurized fluids, which
episodically break through the overburden, feeding the presently exploited geothermal fields. 相似文献
15.
Forebulge dynamics and environmental control in Western Amazonia: The case study of the Arch of Iquitos (Peru) 总被引：4，自引：2，他引：4
Martin Roddaz Patrice Baby Stphane Brusset Wilber Hermoza Jos Maria Darrozes 《Tectonophysics》2005,399(1-4):87
The Iquitos Arch corresponds to a broad topographic high in the Western Amazonia. Morphostructural and geophysical data and flexural modeling show that the Iquitos Arch is the present-day forebulge of the Northwestern Amazonian foreland basin. A detailed tectono-sedimentary study of the Neogene and Quaternary deposits of the Iquitos area has been carried out in order to circumscribe the timing of the forebulge uplift and its environmental consequences. The Neogene and Quaternary sedimentary succession of the Iquitos Arch consists of six formations that evolved from tidal to fluvial environments. The first three formations exhibit Late Miocene gliding features and synsedimentary normal faults. Such soft-sediment deformations bear witness to tectonic activity ascribed to the growth of the forebulge. Regional erosive surfaces that separate the Neogene and Quaternary formations recorded the progressive forebulge emersion and the evolution of Amazonian drainage system. This uplift is related to an increase in tectonic activity within the Andes, which has provoked the eastern propagation of the orogenic wedge and caused an orogenic loading stage in the Amazonian foreland basin system. The emersion of the forebulge induced the retreat of the Pebas “marine megalake” nearby the Iquitos area and consequently caused important environmental changes in the Amazonian basin. From the end of the Late Miocene to the Pliocene, the forebulge acted as a barrier inducing the deposition of fluvial deposits in the forebulge depozone and the deposition of the “White Sand” deposits in the backbulge depozone. Since about 6 Ma, the forebulge is incised and crossed over by the modern Amazon River. The Iquitos forebulge is still growing as shown by the faulted Holocene terrace deposits. 相似文献
16.
Andrea Brogi Antonio Lazzarotto Domenico Liotta Giorgio Ranalli 《Tectonophysics》2003,363(1-2):127-139
The Larderello geothermal field is located in the Inner Northern Apennines, in an area which has been subject to extension since the Early Miocene. The latest extensional episode (Pliocene–Present) has resulted in the formation of NW-trending, NE-dipping listric normal faults, whose geometry is controlled down to 3 km by borehole data. In this paper, we integrate a new interpretation of seismic reflection lines with existing seismic, field, and borehole data to analyse the relations among listric normal faults, the top of the brittle–ductile transition, and the migration of geothermal fluids.In accordance with previous interpretations, we consider the strong reflector (K-horizon) marking the top of the reflective mid-lower crust, and located at a depth of 3–5 km in the geothermal area, to represent the top of the brittle–ductile transition. Its reflectivity most probably derives from the presence of overpressured fluids. We identify three main NW-trending, NE-dipping extensional brittle shear zones, showing listric geometry and soling out in the vicinity of the K-horizon. The latter appears to be dislocated in correspondence of the soling out of the shear zones. These shear zones, because of the associated intense fracturing, represent the most natural channels of upward migration of geothermal fluids from the magmatic sources located below the K-horizon.We suggest that these two conclusions—that listric normal faults root at or near the brittle–ductile transition, and that they act as preferential upward migration paths for magmatic fluids—may be of general validity for geothermal fields located in extensional settings. 相似文献
17.
Ivan S. Zagor
ev 《Geological Journal》1994,29(3):241-268
The Pirin-Pangaion Structural Zone occupies the south-western part of the Rhodope Massif. It consists of Proterozoic amphibolite facies metamorphic rocks of the Rhodopian Supergroup, and granitoids of Hercynian, Late Cretaceous and Palaeogene age. The pre-Hercynian structure of the zone is dominated by an interference pattern of three superimposed fold generations of NE-SW and NW-SE trends. These structures are cut by Hercynian granitoids, and the entire complex is affected by late Hercynian or early Alpine conical folds. The zone was overthrusted by the Ogražden and Kroussia Units (Serbo-Macedonian ‘Massif’) along the north-east vergent Mid-Cretaceous Strimon overthrust, and by the Central Rhodope Zone of the Rhodope Massif, along the south-west vergent Meso-Rhodopean Overthrust. With this thrusting event, the Pirin-Pangaion Structural Zone was brought together with the Serbo-Macedonian ‘Massif’ and the Central Rhodope Zone to form the Late Cretaceous Morava-Rhodope Zone, which acted as a ‘plateau’ along the southern edge of the Eurasian plate. Late Cretaceous granitoid magma of crustal origin intruded this zone, whereas north of it the Srednogorie volcanic island arc was the site of igneous activity with magmas originating in the upper mantle. The West Thrace Zone developed as a Palaeocene to Oligocene depression superimposed over the older basement obliquely to the southern periphery of the Rhodope Massif. In the Late Eocene and Early Oligocene, this depression represented a volcanic island arc with mantle-derived basic to intermediate magmas; contemporaneous granitoid magmas formed through crustal melting in the thickened crust of the Rhodope Massif (Pirin and Pangaion Units included). Early Miocene thrusting was most intense in the Pangaion Unit, and was followed by Late Miocene to Quaternary extension. 相似文献
18.
In the Pyrenees, the development of mylonites zones is one of the most striking structural features. Two sets of mylonites of regional extent have been recognized: large longitudinal E-W to N110°E trending zones (e.g. Mérens fault and North Pyrenean fault) and oblique NW-SE trending zones cross-cutting both the Hercynian and the post-Hercynian terrains. The longitudinal zones limit the major structural zones of the Pyrenees and are associated with NW-SE “en échelons” folds in the Mesozoic terrains and rotations of rootless plutonic or gneissic massifs, acting as competent inclusions in a more ductile matrix, in the Hercynian basement. The oblique mylonite zones limit map-scale fold-bands and appear as the sheared limbs of these folds.The age of the oblique zones and of the major movements along the longitudinal zones is clearly Alpine and the “en échelons” folds seem to have controlled the sedimentation during the Upper Albian and possibly during the Upper Cretaceous. Early movements along the longitudinal zones may have been Hercynian.The analysis of the structures at all scales leads us to interpret these mylonite zones and associated structures as the ultimate result of a transcurrent simple shear acting during the whole Mesozoic period. This strike-slip shearing was probably associated with an extension perpendicular to it from the Permian to the Upper Cretaceous and then to a shortening component also perpendicular to it from the Late Cretaceous to the Eocene.The development of the mylonite zones appears to have predated the major Alpine thrusting but to have been reactivated during this thrusting, acting as initiation sites for the thrusts or as oblique ramps in the case of the oblique mylonite zones. 相似文献
19.
Andrea Brogi 《International Journal of Earth Sciences》2008,97(4):677-703
The tectonic evolution of the Mt Amiata volcano-geothermal area is under discussion. Some authors state that this region,
as well as the hinterland of the Northern Apennines, were affected by compression from the Cretaceous to the Quaternary. In
contrast, most authors believe that extension drove the tectonic evolution of the Northern Apennines from the Early Miocene
to the Quaternary. Field data, seismic analyses and borehole logs have been integrated in order to better define the structural
features of the continental crust in the Mt Amiata geothermal area. In this paper I propose the hypothesis that the structure
of the crust in the Mt Amiata volcano-geothermal area derives from two main geological processes: (1) contractional tectonics
related to the stacking of the Northern Apennines (Cretaceous–Early Miocene), (2) subsequent extensional collapse of the hinterland
of the mountain chain, and related opening of the Northern Tyrrhenian Sea (Early Miocene–Quaternary). Compressional and extensional
structures characterise the Mt Amiata region, although extensional structures dominate its geological framework. In particular
the extension produced: (a) Middle-Late Miocene boudinage of the previously stacked tectonic units; (b) Pliocene–Quaternary
normal faulting which favoured the emplacement of a magmatic body in the middle-upper crust; and (c) the eruption of the Mt
Amiata volcano, which gave rise to an acid and intermediate volcanic complex (0.3–0.19 Ma). The extension produced the space
necessary to accommodate the Middle-Late Miocene marine and continental sediments. Pliocene and Quaternary normal and transtensional
faults dissected the previous structures and influenced the Early Middle Pliocene marine sedimentation within the structural
depressions neighbouring the Mt Amiata volcano. The magmatic body was emplaced at depth (about 6–7 km) during the Pliocene
extension, and produced the eruption of the Mt Amiata volcano during the Late Pleistocene. This gave rise to local uplift,
presently reaching about 3,000 m, as well as a negative Bouguer anomaly (−16 mgal), both centred on the Mt Amiata area. The
crustal dome shows a good correspondence with the convex shape of the regional seismic marker known as the K-horizon, which
corresponds to the 450°C isotherm, and the areas with greatest heat flow. This is probably a consequence of the above-cited
magmatic body presently in the process of solidification. A Late Pleistocene eruption occurred along a crustal fissure striking
N50° (Mt Amiata Fault), which crosscuts the crustal dome. Hydrothermal circulation, proven by the occurrence of thermal springs
and gas vents (mainly CO2 and H2S), mainly occurs along the Mt Amiata Fault both in the northeastern ans southwestern sides of the volcano. 相似文献
20.
A. Del Moro M. Puxeddu F. Radicati di Brozolo I. M. Villa 《Contributions to Mineralogy and Petrology》1982,81(4):340-349
Rb-Sr and K-Ar ages have been obtained on six biotites, two muscovites and one hornblende from samples of micaschist, gneiss and amphibolite of Lower Paleozoic to Precambrian age at a depth exceeding 2,000 m in basement rocks of the Larderello-Travale geothermal region. Most of the data cluster in the range 2.5–3.7 Ma, revealing the existence of a Pliocene thermal event to which the origin of the field may be attributed. The resulting duration of the Larderello geothermal field is unexpectedly long. In the basement levels of the two wells examined, unstabilized minimum temperatures of 290° and 380° C were measured. All the biotites show almost complete 40Ar and 87Sr retention at the measured well temperatures. Petrologic evidence (stilpnomelane stability) and experimental data (activation energies and diffusion coefficients) also favour a closure temperature above 400° C for Rb-Sr and K-Ar in biotites, in agreement with recent direct experimental determinations.For the last 3 Ma mean geothermal gradients of 120°–150° C/km have been evaluated in the first 2–3 km, and 60°–65° C/km in the underlying 2 km. A rough estimate of total cooling in the last 3 Ma gives a value of 120° C at 2,500 m depth and 50° C at 4,000 m depth in Sasso 22 well. A mean uplift rate of about 0.2 mm/year is calculated independently.Research conducted under a collaboration agreement between the Italian National Research Council (CNR) and the Italian National Electricity Board (ENEL) 相似文献