Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere

The evolution of the European Cenozoic Rift System (ECRIS) and the Alpine orogen is discussed on the base of a set of palaeotectonic maps and two retro-deformed lithospheric transects which extend across the Western and Central Alps and the Massif Central and the Rhenish Massif, respectively.During the Paleocene, compressional stresses exerted on continental Europe by the evolving Alps and Pyrenees caused lithospheric buckling and basin inversion up to 1700 km to the north of the Alpine and Pyrenean deformation fronts. This deformation was accompanied by the injection of melilite dykes, reflecting a plume-related increase in the temperature of the asthenosphere beneath the European foreland. At the Paleocene–Eocene transition, compressional stresses relaxed in the Alpine foreland, whereas collisional interaction of the Pyrenees with their foreland persisted. In the Alps, major Eocene north-directed lithospheric shortening was followed by mid-Eocene slab- and thrust-loaded subsidence of the Dauphinois and Helvetic shelves. During the late Eocene, north-directed compressional intraplate stresses originating in the Alpine and Pyrenean collision zones built up and activated ECRIS.At the Eocene–Oligocene transition, the subducted Central Alpine slab was detached, whereas the West-Alpine slab remained attached to the lithosphere. Subsequently, the Alpine orogenic wedge converged northwestward with its foreland. The Oligocene main rifting phase of ECRIS was controlled by north-directed compressional stresses originating in the Pyrenean and Alpine collision zones.Following early Miocene termination of crustal shortening in the Pyrenees and opening of the oceanic Provençal Basin, the evolution of ECRIS was exclusively controlled by west- and northwest-directed compressional stresses emanating from the Alps during imbrication of their external massifs. Whereas the grabens of the Massif Central and the Rhône Valley became inactive during the early Miocene, the Rhine Rift System remained active until the present. Lithospheric folding controlled mid-Miocene and Pliocene uplift of the Vosges-Black Forest Arch. Progressive uplift of the Rhenish Massif and Massif Central is mainly attributed to plume-related thermal thinning of the mantle-lithosphere.ECRIS evolved by passive rifting in response to the build-up of Pyrenean and Alpine collision-related compressional intraplate stresses. Mantle-plume-type upwelling of the asthenosphere caused thermal weakening of the foreland lithosphere, rendering it prone to deformation.     

A top to bottom lithospheric study of Africa and Arabia

We study the lithospheric structure of Africa, Arabia and adjacent oceanic regions with fundamental-mode surface waves over a broad period range. Including group velocities with periods shorter than 35 s allows us to examine shallower features than previous studies of the whole continent. In the process, we have developed a crustal thickness map of Africa. Main features include crustal thickness increases under the West African, Congo, and Kalahari cratons. We find crustal thinning under Mesozoic and Cenozoic rifts, including the Benue Trough, Red Sea, and East, Central, and West African rift systems, along with less abrupt crustal thickness changes at passive continental margins. We also find crustal thickness differences in North Africa between the West African Craton and East Saharan Shield. Crustal shear wave velocities are generally faster in oceanic regions and cratons, and slower in more recent crust and in active and remnant orogenic regions. Deeper structure, related to the thickness of cratons and modern rifting, is generally consistent with previous work. Under cratons we find thick lithosphere and fast upper mantle velocities, while under rifts we find thinned lithosphere and slower upper mantle velocities. However, we also find the lack of a thick cratonic keel beneath the central portion of the Congo Craton. There are no consistent effects in areas classified as hotspots, indicating that there seem to be numerous origins for these features. Finally, it appears that the African Superswell, which is responsible for high elevation and uplift over large portions of Africa, has had a significantly different impact (as indicated by features such as temperature, time of influence, etc.) in the north and the south. This is consistent with episodic activity at shallow depths, which is well-expressed in northeastern Africa and Arabia today.     

The lithospheric density structure of the Eastern Alps

The three-dimensional (3D) lithospheric density structure of the Eastern Alps was investigated by integrating results from reflection seismics, receiver function analyses and tomography. The modelling was carried out with respect to the Bouguer gravity and the geoid undulations and emphasis were laid on the investigations of the importance of deep lithospheric features. Although the influence of inhomogeneities at the lithosphere–asthenosphere boundary on the potential field is not neglectable, they are overprinted by the response of the density contrast at the crust–mantle boundary and intra-crustal density anomalies. The uncertainties in the interpretations are in the same order of magnitude as the gravity field generated by the deep lithosphere.After including the deep lithospheric geometry from the tomographic model it is shown that full isostatic equilibrium is not achieved below the Eastern Alps. However, calculation of the isostatic lithospheric thickness shows two areas of lithospheric thickening along the central axis of the Eastern Alps with a transition zone below the area of the TRANSALP profile. This is in agreement with the tomographic model, which features a change in lithospheric subduction direction.     

Discrete element modelling of the influence of cover strength on basement-involved fault-propagation folding

A discrete element model is used to investigate the influence of sedimentary cover strength on the development of basement-involved fault-propagation folds. We find that uniformly weak cover best promotes the development of classical, trishear-like fault-related folds showing marked anticlinal thinning and synclinal thickening, with cover dips increasing downwards towards the fault tip. Uniformly strong cover results in more rounded fold forms with only minor hinge thickening/thinning and significant basement fault-propagation into the sedimentary cover. Heterogeneous, layered, cover sequences with marked differences in strength promote the development of more complex and variable fold forms, with a close juxtaposition of brittle and macroscopically ductile features, which diverge from the predictions of simple kinematic models. In these structures the upper layers are often poor indicators of deeper structure. In addition, we find that in layered cover sequences fault-propagation into the cover is a complex process and is strongly buffered by the weaker cover units.     

Mantle structure and rifting processes in the Baikal–Mongolia region: geophysical data and evidence from xenoliths in volcanic rocks

The origin of the Baikal rift zone (BRZ) has been debated between the advocates of passive and active rifting since the 1970s. A re-assessment of the relevant geological and geophysical data from Russian and international literature questions the concept of broad asthenospheric upwelling beneath the rift zone that has been the cornerstone of many “active rifting” models. Results of a large number of early and recent studies favour the role of far-field forces in the opening and development of the BRZ. This study emphasises the data obtained through studies of peridotite and pyroxenite xenoliths brought to the surface by alkali basaltic magmas in southern Siberia and central Mongolia. These xenoliths are direct samples of the upper mantle in the vicinity of the BRZ. Of particular importance are suites of garnet-bearing xenoliths that have been used to construct P–T- composition lithospheric cross-sections in the region for the depth range of 35–80 km.Xenolith studies have shown fundamental differences in the composition and thermal regime between the lithospheric mantle beneath the ancient Siberian platform (sampled by kimberlites) and beneath younger mobile belts south of the platform. The uppermost mantle in southern Siberia and central Mongolia is much hotter at similar levels than the mantle in the Siberian craton and also has significantly higher contents of ‘basaltic’ major elements (Ca, Al, Na) and iron, higher Fe/Si and Fe/Mg. The combination of the moderately high geothermal gradient and the fertile compositions in the off-cratonic mantle appears to be a determining factor controlling differences in sub-Moho seismic velocities relative to the Siberian craton. Chemical and isotopic compositions of the off-cratonic xenoliths indicate small-scale and regional mantle heterogeneities attributed to various partial melting and enrichment events, consistent with long-term evolution in the lithospheric mantle. Age estimates of mantle events based on Os–Sr–Nd isotopic data can be correlated with major regional stages of crustal formation and may indicate long-term crust–mantle coupling. The ratios of ^143/144Nd in many LREE-depleted xenoliths are higher than those in MORB or OIB source regions and are not consistent with a recent origin from asthenospheric mantle.Mantle xenoliths nearest to the rift basins (30–50 km south of southern Lake Baikal) show no unequivocal evidence for strong heating, unusual stress and deformation, solid state flow, magmatic activity or partial melting that could be indicative of an asthenospheric intrusion right below the Moho. Comparisons between xenoliths from older and younger volcanic rocks east of Lake Baikal, together with observations on phase transformations and mineral zoning in individual xenoliths, have indicated recent heating in portions of the lithospheric mantle that may be related to localised magmatic activity or small-scale ascent of deep mantle material. Overall, the petrographic, P–T, chemical and isotopic constraints from mantle xenoliths appear to be consistent with recent geophysical studies, which found no evidence for a large-scale asthenospheric upwarp beneath the rift, and lend support to passive rifting mechanism for the BRZ.     

Three-dimensional seismic modelling of crustal structure in the TESZ region based on POLONAISE''97 data

This paper reports the results of 3-D tomographic modelling of crustal structure in the Trans European Suture Zone region (TESZ) of Poland, eastern Germany and Lithuania. The data are the product of a large-scale seismic experiment POLONAISE'97, which was carried out in 1997. This experiment was designed to provide some 3-D coverage. The TESZ forms the boundary between the Precambrian crustal terranes of the East European Craton (EEC) and the younger Phanerozoic terranes to the southwest. The 3-D results generally confirm the earth models derived by earlier 2-D analyses, but also add some important details as well as a 3-D perspective on the structure. The velocity model obtained shows substantial horizontal variations of crustal structure across the study area. Seismic modelling shows low (<6.1 km/s) velocities suggesting the presence of sedimentary rocks down to a depth of about 20 km in the Polish basin. The shape of the basin in the vicinity of the profile P4 shows significant asymmetry. Three-dimensional modelling also allowed tracing of horizontal irregularities of the basin shape as well as variations of the Moho depth not only along profiles, but also between them. The slice between P2 and P4 profiles shows about 10-km variations of the Moho over a 100-km interval. The crustal thickness varies from about 30 km in SW, beneath the Palaeozoic platform, to about 42 km beneath East European Craton in NE. High seismic velocities of about 6.6 km/s were found in the depth range 2–10 km, which coincides with K trzyn anorthosite massif. The results of this 3-D seismic modelling of the POLONAISE'97 data will ultimately be supplemented by inversion of seismic data from previous experiments.     

3D numerical modeling of forward folding and reverse unfolding of a viscous single-layer: Implications for the formation of folds and fold patterns

The main aims of this study are to show (i) that non-cylindrical three-dimensional (3D) fold shapes and patterns can form during a single, unidirectional shortening event and (ii) that numerical reverse modeling of 3D folding is a feasible method to reconstruct the formation of 3D buckle-folds. 3D viscous (Newtonian) single-layer folding is numerically simulated with the finite element method to investigate the formation of fold shapes during one shortening event. An initially flat layer rests on a matrix with smaller viscosity and is shortened in one direction parallel to the layering. Forward modeling with different initial geometrical perturbations on the flat layer and different lateral boundary conditions generates non-cylindrical 3D fold shapes and patterns. The simulations show that, in reality, the initial layer geometry and the boundary conditions strongly control the final fold geometry. Fold geometries produced from the forward folding models are used as initial setting in numerical reverse folding models with parameters identical to those of forward models. These reverse models accurately reconstruct the initial geometry of forward models with also only one extension event parallel to the previous shortening direction. The starting geometry of the forward models is inaccurately reconstructed by the reverse models if a significantly different viscosity ratio than in the forward models is used. This work demonstrates that reverse modeling has a high potential for reconstructing the deformation history of folded regions and rheological constraints such as viscosity ratio. Reverse models may be applied to natural 3D fold shapes and patterns in order to determine if they formed (i) during a single or multiple deformation events and (ii) as active buckle-folds with a viscosity ratio 1 or as passive, kinematic folds without buckling. This approach may find much application to fold interference patterns, in particular.     

Seismic evidence of divergent rifting and subsequent deformation in the southern Japan Sea, and a Cenozoic tectonic synthesis of the eastern Eurasian margin

Neogene rift system configuration for the back-arc of southwest Japan, southern rim of the Japan Sea, is argued on the basis of reflection seismic interpretation. Divergent rifting and subsequent contraction provoked by an arc–arc collisional event are manifested by the formation of faulted grabens and their inverted deformation, respectively. We identified the following four Cenozoic tectonic epochs as a decomposition process of the eastern Eurasian margin based on reliable paleomagnetic data: (1) Plate margin rearrangement on a regional left-lateral fault through southwest Japan and Sikhote Alin, which constituted a continuous geologic province before the early Tertiary differential motion; (2) Early Tertiary clockwise rotation (>20°) of the east Tan-Lu block relative to the North China block; (3) Oligocene to early Miocene divergent rifting and spreading of the Japan Sea, which divided southwest Japan from the east Tan-Lu block; (4) Middle Miocene bending and back-arc inversion of southwest Japan caused by collision with the Izu-Bonin arc. According to the estimation of relative motions during these events, a paleogeographic reconstruction is presented through Cenozoic time.     

Evolution of the lithosphere in the area of the Rhine Rift System

The Rhine Rift System (RRS) forms part of the European Cenozoic Rift System (ECRIS) and transects the Variscan Orogen, Permo-Carboniferous troughs and Late Permian to Mesozoic thermal sag basins. Crustal and lithospheric thicknesses range in the RRS area between 24–36 km and 50–120 km, respectively. We discuss processes controlling the transformation of the orogenically destabilised Variscan lithosphere into an end-Mesozoic stabilised cratonic lithosphere, as well as its renewed destabilisation during the Cenozoic development of ECRIS. By end-Westphalian times, the major sutures of the Variscan Orogen were associated with 45–60 km deep crustal roots. During the Stephanian-Early Permian, regional exhumation of the Variscides was controlled by their wrench deformation, detachment of subducted lithospheric slabs, asthenospheric upwelling and thermal thinning of the mantle-lithosphere. By late Early Permian times, when asthenospheric temperatures returned to ambient levels, lithospheric thicknesses ranged between 40 km and 80 km, whilst the thickness of the crust was reduced to 28–35 km in response to its regional erosional and local tectonic unroofing and the interaction of mantle-derived melts with its basal parts. Re-equilibration of the lithosphere-asthenosphere system governed the subsidence of Late Permian-Mesozoic thermal sag basins that covered much of the RRS area. By end-Cretaceous times, lithospheric thicknesses had increased to 100–120 km. Paleocene mantle plumes caused renewed thermal weakening of the lithosphere. Starting in the late Eocene, ECRIS evolved in the Pyrenean and Alpine foreland by passive rifting under a collision-related north-directed compressional stress field. Following end-Oligocene consolidation of the Pyrenees, west- and northwest-directed stresses originating in the Alps controlled further development of ECRIS. The RRS remained active until the Present, whilst the southern branch of ECRIS aborted in the early Miocene. Extensional strain across ECRIS amounts to some 7 km. Plume-related thermal thinning of the lithosphere underlies uplift of the Rhenish Massif and Massif Central. Lithospheric folding controlled uplift of the Vosges-Black Forest Arch.     

Oblique strain partitioning and transpression on an inverted rift: The Castilian Branch of the Iberian Chain

The Iberian Chain is a wide intraplate deformation zone formed by the tectonic inversion during the Pyrenean orogeny of a Permian–Mesozoic basin developed in the eastern part of the Iberian Massif. The N–S convergence between Iberia and Eurasia from the Late Cretaceous to the Lower Miocene times produced significant intraplate deformation. The NW–SE oriented Castilian Branch of the Iberian Chain can be considered as a “key zone” where the proposed models for the Cenozoic tectonic evolution of the Iberian Chain can be tested. Structural style of basin inversion suggests mainly strike–slip displacements along previous NW–SE normal faults, developed mostly during the Mesozoic. To confirm this hypothesis, structural and basin evolution analysis, macrostructural Bouguer gravity anomaly analysis, detailed mapping and paleostress inversions have been used to prove the important role of strike slip deformation. In addition, we demonstrate that two main folding trends almost perpendicular (NE–SW to E–W and NW–SE) were simultaneously active in a wide transpressive zone. The two fold trends were generated by different mechanical behaviour, including buckling and bending under constrictive strain conditions. We propose that strain partitioning occurred with oblique compression and transpression during the Cenozoic.     

闽台地区Pn速度结构反演问题

本文简要介绍了利用Pn波走时研究地壳内部结构的基本原理。以及如何运用福建数字地震台网观测资料研究闽台地区莫霍面结构的基本思路和方法。为今后开展这项工作提供参考     

Acritarchs of Las Ventanas Formation (Ediacaran, Uruguay): Implications for the timing of coeval rifting and glacial events in western Gondwana

Acritarchs and other organic-walled microfossils occurring in siltstones of the Las Ventanas Formation (Quebrada de Viera and El Perdido members) are systematically described and illustrated. The assemblage includes the following species: Leiosphaeridia tenuissima, Leiosphaeridia minutissima, Lophosphaeridium sp., Soldadophycus bossii, Soldadophycus major, Soldadophycus sp. and Vendotaenia antiqua. The microflora is characterized by low diversity (six species), dominance of L. tenuissima, absence of acanthomorphic acritarchs, and relatively large size of sphaeromorphs, reaching 400 μm in diameter. A number of species are shared with acritarch assemblages preserved in the overlying Arroyo del Soldado Group. Differences between assemblages include the occurrence of abundant Bavlinella faveolata and small size of spheroids in the Arroyo del Soldado Group. The assemblage occurring in Las Ventanas Formation is assigned to the Ediacaran Leiosphere Palynoflora, which spans the interval between the base of the Ediacaran (end of the Marinoan Glaciation, 635 Ma) and the termination of the Gaskiers Glaciation (582 Ma). An early Ediacaran age between 615 and 579 Ma is also supported by available radiometric ages. An extensional setting for Las Ventanas basin is suggested on the basis of the bimodal, synsedimentary volcanism, strong palaeorelief, great thickness of alluvial fan conglomerates and the evolution from continental to open marine environments. Diamictites occurring in the Quebrada de Viera Member are described for the first time, including associated dropstones which suggest a glacial origin. If confirmed, this would be one more example of the association between rifting and glaciation in the Neoproterozoic, coeval with a low-diversity, high-abundance acritarch microflora. A causal relationship between these tectonic, climatic and biologic events is discussed.     

酒东盆地营尔凹陷早白垩世断陷形成机制探讨

位于青藏高原和华北板块两大构造带交汇处的河西走廊带中西段酒东盆地,早白垩世为典型断陷盆地,但对其断陷形成机制存在明显争议.因早白垩世两大构造带的主体构造作用明显差异:北—南向挤压和北西—南东向伸展,为深入了解哪一种作用是控制早白垩世酒东盆地形成的主要因素,本研究特选取远离新生代强烈活动的阿尔金断裂带,在河西走廊带中西段...     

The latest geodynamics in Asia: Synthesis of data on volcanic evolution,lithosphere motion,and mantle velocities in the Baikal-Mongolian region

From a synthesis of data on volcanic evolution,movement of the lithosphere,and mantle velocities in the Baikal-Mongolian region,we propose a comprehensive model for deep dynamics of Asia that assumes an important role of the Gobi,Baikal,and North Transbaikal transition-layer melting anomalies.This layer was distorted by lower-mantle fluxes at the beginning of the latest geodynamic stage(i.e.in the early late Cretaceous) due to avalanches of slab material that were stagnated beneath the closed fragments of the Solonker,Ural-Mongolian paleoceans and Mongol-Okhotsk Gulf of Paleo-Pacific.At the latest geodynamic stage,Asia was involved in east-southeast movement,and the Pacific plate moved in the opposite direction with subduction under Asia.The weakened upper mantle region of the Gobi melting anomaly provided a counterflow connected with rollback in the Japan Sea area.These dynamics resulted in the formation of the Honshu-Korea flexure of the Pacific slab.A similar weakened upper mantle region of the North Transbaikal melting anomaly was associated with the formation of the Hokkaido-Amur flexure of the Pacific slab,formed due to progressive pull-down of the slab material into the transition layer in the direction of the Pacific plate and Asia convergence.The early—middle Miocene structural reorganization of the mantle processes in Asia resulted in the development of upper mantle low-velocity domains associated with the development of rifts and orogens.We propose that extension at the Baikal Rift was caused by deviator flowing mantle material,initiated under the moving lithosphere in the Baikal melting anomaly.Contraction at the Hangay orogen was created by facilitation of the tectonic stress transfer from the Indo-Asian interaction zone due to the low-viscosity mantle in the Gobi melting anomaly.     

Numerical simulation of rifting in the northern Viking Graben: the mutual effect of modelling parameters

Numerous basin modelling studies have been performed on the Viking Graben in the northern North Sea during the past decades in order to understand the driving mechanisms for basin evolution and palaeo temperature estimations. In such modelling, it is important to include lithospheric flexure. The values derived for the lithospheric strength by these studies vary considerably (i.e. up to a factor of 30). In this study, which is based on new interpretation of a regional transect, we show that both the estimated value of the effective elastic thickness and the derived β-profile are dependent on the assumed value of the depth of necking. The use of models that implicitly set the level of necking at a depth of 0 km generally leads to an underestimation of the lithospheric strength, and an overestimation of the thinning factors. In the northern Viking Graben, a necking depth at intermediate crustal levels gives results comparable to the observations. Extension by faulting is modelled to be a significant factor. In conclusion, rifting in the northern Viking Graben can be explained with various models of effective elastic thicknesses (EET) varying from 1 km for a zero necking depth to the depth of the 450 °C isotherm for an intermediate level of necking.It is also shown that the development of the basin during the post-rift phase cannot be explained by pure shear/simple shear extension. Two mechanisms are proposed here to explain the post-rift subsidence pattern, namely intra-plate stress and phase boundary migration. The two extreme models for EET mentioned above (1 km for a zero necking depth to the depth of the 450 °C isotherm for an intermediate level of necking) give very different responses to compressional stress, the latter gives basically no response for realistic intra-plate stress.     

Sedimentation processes and new age constraints on rifting stages in Lake Baikal: results of deep-water drilling

 With this paper we present a first attempt to combine the direct results on lithology, composition and age dating in the boreholes BDP-93, BDP-96 and BDP-97 with geological and seismic data from the areas where those sections were drilled. The sedimentary environments represented by the BDP boreholes are markedly different and possess characteristic lithological features. The results of the deep drilling provide the essential means for testing numerous age models used in geological reconstructions of the Lake Baikal rifting dynamics. Neither the basin-wide unconformity interpreted from seismic data, nor the interpreted change from shallow-water to deep-water facies at the boundary of the seismic stratigraphic complexes were found in the BDP-96 boreholes on Academician Ridge. Also, lithology does not support the proposed reconstructions of intense lake level fluctuations and transgressions during the Pliocene at Academician Ridge. The continuous deep-water hemipelagic sedimentation at Academician Ridge has existed for the past 5 Ma. The beginning of an intense rifting phase of the Neobaikalian sub-stage and related drastic changes in sedimentation processes were interpreted on seismic sections as the basin-wide unconformity B10. Different age estimates for this boundary ranged from Late Pliocene (3.5 Ma) to Plio-Pleistocene boundary. As shown by BDP-96 borehole, B10 is associated with a lithological change from diatomaceous ooze to dense silty clay and not with an erosional contact. The new age for this boundary in BDP-96 is approximately 2.5 Ma. This new age constraint suggests that the upper sedimentary strata of Northern Baikal (1.5–1.7 km thick) have formed during the past 2.5 Ma with average sedimentation rates of 60–70 cm/ka. The BDP-93 boreholes at Buguldeika suggest that uplift in Primorsky Range took place prior to 1.07–1.31 Ma, a date which exceeds the age of previous geological models. Received: 12 March 1999 / Accepted: 10 February 2000     

Metamorphic and deformational imprint of Cambrian–Lower Ordovician rifting in the Ossa-Morena Zone (Iberian Massif, Spain)

The high-temperature metamorphism recorded in the Valuengo and Monesterio areas constitutes a rare occurrence in the Ossa-Morena Zone of Southwest Iberia, where low-grade metamorphism dominates. The metamorphism of the Valuengo area has been previously considered either Cadomian or Variscan in age, whereas that of Monesterio has been interpreted as a Cadomian imprint. However, these areas share important metamorphic and structural features that point towards a common tectonometamorphic evolution. The metamorphism of the Valuengo and Monesterio areas affects Late Proterozoic and Early Cambrian rocks, and is syn-kinematic with a top-to-the-north mylonitic foliation, which was subsequently deformed by early Variscan folds and thrusts. The U–Pb zircon age (480±7 Ma) we have obtained for an undeformed granite of the Valuengo area is consistent with our geological observations constraining the age of the metamorphism. We propose that this high-temperature metamorphic imprint along a NW–SE ductile extensional shear zone is related to the crustal extension that occurred in the Ossa-Morena Zone during the Cambro-Ordovician rifting. In the same way, the tectonothermal effect of the preorogenic rifting stage may have been wrongly attributed to orogenic processes in other regions as well as in this one.     

Chemical,isotopic and mineralogical characteristics of volcanogenic epithermal fluorite deposits on the Permo-Mesozoic foreland of the Andean volcanic arc in Patagonia (Argentina)

Epithermal deposits mined for fluorite in Patagonia, Argentina, are closely related to late Triassic through Jurassic magmatic activity which brought about felsic to intermediate magmatic rocks. The fluorite mineralization in the Patagonian epithermal system resulted from gaseous F-and CO₂-enriched magmas which lead to an explosive phreatomagmatic volcanism, when getting in contact with groundwater near the surface. As a result of these hydrothermal processes, rapid cooling took place in the epithermal mineralization. Changes in the viscosity along with the cooling down of mineralizing fluids caused mottled mineral colors blurring the boundaries between the stages and ore textures.The fluids accountable for the main constituents fluorite, quartz, barite and silica were operative over a vertical extension of roughly 600 m. Their temperature of formation dropped from 379 °C through 64 °C, while the pH decreased from the heat center towards the paleosurface under oxidizing conditions. This steep temperature gradient conducive to the telescoping of mineral associations into each other was accompanied by a rapid loss in CO₂, and a mixing of meteoric and magmatic fluids. Even the boundary between the hypogene and supergene alteration cannot be drawn precisely within the assemblage of Mn oxides, which bridge the gap between hypogene and supergen mineralization. The physical-chemical parameters of the fluids, particularly, the redox conditions did not allow sulfides to be preserved. A classification of the epithermal system as to its degree of sulfidation is based on K-feldspar and kaolinite which are present in significant amounts, whereas APS (aluminum-phosphate-sulfate) minerals are absent. Therefore a categorization as an epithermal fluorite deposit of low- to intermediate sulfidation is justified, because the only mineral of economic interest in the system is fluorite.The data obtained during this joint study render the Patagonian fluorite district a reference type of fluorite in an epithermal system of low- to intermediate sulfidation which are widespread in Argentina, e.g., Sierras Pampeanas, and evolved on part of the stable craton, called Gondwana and which grade into epithermal Au, Ag, In, Pb and Zn deposits.     

冀中坳陷中部现今热岩石圈厚度及地热学意义探讨

热岩石圈厚度是研究盆地的构造演化和板块动力学的重要参数,本文通过实测数据构建地壳分层模型,根据热传导的基本原理,计算了冀中坳陷中部的Moho面温度以及热岩石圈厚度,并探讨其地热学意义。结果表明:冀中坳陷中部的Moho面温度分布在500～600℃,西南侧整体温度较东北部高,热岩石圈厚度介于102～122km,其平面展布特征与华北克拉通热岩石圈厚度西厚东薄的特征相吻合,为华北克拉通受太平洋板块西向俯冲导致东部遭受破坏提供了依据,并且较薄的岩石圈使热流更易传导到地壳浅部,成为了该地区热异常的成因背景。