Rheological behaviour of the crust in Southern Apennine (Italy): results from a thermal and seismological study

To add information on the brittle/ductile transition of the Southern Apennine, we investigated a rheological profile of the crust along a WSW–ENE‐oriented cross‐section running from Neapolitan Volcanic Zone to Apulia foreland. The rheological model was obtained computing a thermal modelling and constrained by a relocalization of earthquakes of the area. Results show that the area is characterized by horizontal rheological variations, with two horizons interlayered with ductile horizons, that are quite predominant with respect to vertical ones. The horizontal stratification of lithospheric rheology has important geodynamic consequence and could provide new insights to better understand the tectonic processes which have played a major role to construct the Southern Apennine belt. Results of this study suggested that well‐constrained rheological models, built integrating information from both relocated earthquakes and thermal state, could give important hints on the mechanical behaviour of the crust and its related tectonic processes.     

Late Neogene–Quaternary evolution of the intermontane Clusone Basin (Southern Alps,Italy): integration of seismic and geological data

In the Clusone Basin (a large intermontane basin filled by thick late Neogene–Quaternary sediments in the Middle Val Seriana, Southern Alps), two high‐resolution seismic profiles have been acquired in order to reconstruct the geometries of the sediments that fill the depression, with a maximum thickness of more than 200 m as documented by available well data, and to define their relationships with the bedrock, consisting of Late Triassic carbonates. In addition to standard seismic reflection processing, a seismic refraction inversion technique has been applied. The integration of geological (both surface and well data) and seismic data indicates a complex history of the drainage patterns of the Clusone Basin, documenting a shift of the Serio River from a palaeodrainage toward the southeast (Val Borlezza) to the present situation, toward the south (Val Seriana): between the older and the present‐day drainages an important depositional stage occurred, as documented by the thick sediments that fill the Clusone Basin, controlling the capture of the Serio River along the Val Seriana. Copyright © 2004 John Wiley & Sons, Ltd.     

Microtremor HVSR study for assessing site effects in the Bovec basin (NW Slovenia) related to 1998 Mw5.6 and 2004 Mw5.2 earthquakes

The Bovec basin, which is filled with glacial and fluvial sediments, has recently been struck by two strong earthquakes (1998 and 2004) which caused extensive damage (VII–VIII EMS-98). Strong site effects resulted in large variations in damage to buildings in the area, which could not be explained by the surface variations in Quaternary sediments. The microtremor horizontal-to-vertical-spectral ratio (HVRS) method was therefore applied to a 200 m dense grid of free-field measurements to assess the fundamental frequency of the sediments. Large variations in the sediment frequency (3–22 Hz) were obtained, with most of the observed values in the range 6–12 Hz. The observed frequencies cannot be related to the total thickness of Quaternary sediments (sand, gravel), but can be explained by the presence of conglomerate or lithified moraine at shallow depths. The results were compared also with the velocity structure derived from seismic refraction data. Microtremor measurements performed in several two and some three- and four-storey houses (masonry with RC floors), which prevail in the Bovec basin, have shown that the main building frequencies in the area are in the range 7–11 Hz. This indicates that damage to houses in both earthquakes in some parts of the basin was enhanced by site amplification and soil-structure resonance. Areas of possible soil-structure resonance were identified in the settlements Bovec–Brdo, Bovec–Mala vas, Čezsoča and Kal-Koritnica. Considerable changes in fundamental frequencies within short distances were established in the town of Bovec. Their values are as high as 22 Hz in the central part of the town, but diminish to 6–11 Hz in the adjacent Brdo and Mala vas districts. This is in agreement with the distribution of damage in both earthquakes, which was considerably higher in Brdo and Mala vas, although the houses in the central part of the town are older.

Microtremor investigations have proved an effective tool for assessment of site effects in cases of complex geological structure commonly encountered in young Alpine basins filled with glaciofluvial sediments which are partly cemented. Lithified layers can considerably change the fundamental frequency and, consequently, the site effects. By taking additional measurements in buildings possible soil-structure resonance can be identified.     

On macroseismic attenuation parameters in the comparison of theoretical seismic hazard maps for Southern Calabria and Eastern Sicily (Southern Italy)

For two different zonations of the Calabro-Sicilian region the macroseismic intensity attenuation parameters are computed using the Grandori model.Some modifications to this relation are proposed here in order to allow its applicability also to those cases in which the data set available for each source zone does not present three successive epicentral intensity levels (I ₀) as required for its application.Maps of theoretical distribution of maximum expected intensities were plotted for both zonations adopted, in order to test the reliability, also using other attenuation models.The results of the analysis show how the proposed modifications to the Grandori relation allow the macroseismic intensity attenuation modelling even in cases in which only one intensity level is available.Finally, the comparison of theoretical maximum expected intensity distribution maps, computed for different attenuation models and seismogenic zonations, shows that the maps plotted using the Grandori model and the zonation adopted by G.N.D.T. (Gruppo Nazionale Difesa dai Terremoti - C.N.R. Italy) are more reliable.     

Seismic demands and analysis of site effects in the Marmara region during the 1999 Kocaeli earthquake

The characteristics of the strong ground motion accelerograms from the 1999 Kocaeli earthquake are investigated in detail in this study. The emphasis is on the comparison of the response spectra for the fault normal (FN) and fault parallel (FP) components of the ground motions. The results show that the near-fault records with directivity effects characterize themselves with increased base shear demands rather than increased displacement demands for both the FN and FP components and a narrower velocity sensitive region for the FN component. This study also shows that the effectiveness of base isolation may vary from site to site and for a given site, from component to component. The site effects in the Marmara region during the 1999 Kocaeli earthquake are examined. Site amplifications are predicted by the classical spectral ratio (CSR) and the receiver function (RF) methods. The CSR method gives higher estimates for the site amplifications compared to the RF method and is in better conformity with the observed damage during the Kocaeli earthquake. The districts of Istanbul that are especially susceptible to site amplification hazard are determined. It is apparent from the results that the site amplification hazard risk is the highest for Avcilar and Bakirkoy districts. This study also shows that for sites which have the risk of soil amplification for long-period structures, liquefaction may not be beneficial as a natural base isolator, and may result in shifting the eigenperiod of the low- and mid-rise structures to the critical periods with high site amplifications. This may be especially the case for Avcilar and Bakirkoy districts. In Fatih, Bakirkoy, and Cekmece districts, the predominant period of the ground motion is calculated to be very close to the eigenperiods of the typical residential buildings. Therefore, these three districts are expected to experience heavier damages in future earthquakes due to resonance effects.     

Evaluation of local amplification in the seismic microzonation: comparison between punctual multidisciplinary integrated studies and macroseismic methods in Fivizzano's area (Toscana, Italy)

In the seismic microzonation the evaluation of the local amplification issue may be solved using different methodological approaches. Taking the hint from the studies made within VEL project framework, sponsored by Tuscany Region on Fivizzano's area we are herewith comparing the results of geotechnical and geophysical integrated multidisciplinary studies with the existing data available from previous studies, mainly using a macroseismic approach, on the same area. This qualitative method does not seem to be able to make any forecasting on the scale whereas generally microzoning is performed. On the contrary, the knowledge of the geological-technical model of the investigated area, obtained by punctual multidisciplinary integrated studies, allows a direct collection of the data to insert in input step of numerical modelling, aimed to local effects evaluation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrogeological and geochemical study of the springs in San Severino Lucano territory (Southern Italy)

A hydrogeological and geochemical study is presented for the San Severino Lucano region of southern Italy. In this region, groundwater circulation occurs in rocks lithologically different from one another (metaophiolites, carbonate rocks, etc.). Many springs drain this region. The Frido springs are the most important both for their great volume of flow and for their water quality. A water balance estimated for the recharge area of the Frido springs suggests that during the period 1938–1958 the evapotranspiration represents 54.8 percent, runoff 21.2 percent, and infiltration to groundwater 34 percent of rainfall. The springs studied have a meteoric origin and their waters are mostly acid carbonate-alkaline earth type. The reservoir rocks appear to be the only discriminating factors for the chemical composition of the waters analysed.     

Active faulting and continental slope instability in the Gulf of Patti (Tyrrhenian side of NE Sicily,Italy): a field,marine and seismological joint analysis

The Gulf of Patti and its onshore sector represent one of the most seismically active regions of the Italian Peninsula. Over the period 1984–2014, about 1800 earthquakes with small-to-moderate magnitude and a maximum hypocentral depth of 40 km occurred in this area. Historical catalogues reveal that the same area was affected by several strong earthquakes such as the Mw = 6.1 event in April 1978 and the Mw = 6.2 one in March 1786 which have caused severe damages in the surrounding localities. The main seismotectonic feature affecting this area is represented by a NNW–SSE trending right-lateral strike-slip fault system called “Aeolian–Tindari–Letojanni” (ATLFS) which has been interpreted as a lithospheric transfer zone extending from the Aeolian Islands to the Ionian coast of Sicily. Although the large-scale role of the ATLFS is widely accepted, several issues about its structural architecture (i.e. distribution, attitude and slip of fault segments) and the active deformation pattern are poorly constrained, particularly in the offshore. An integrated analysis of field structural geology with marine geophysical and seismological data has allowed to better understand the structural fabric of the ATLFS which, in the study area, is expressed by two major NW–SE trending, en-echelon arranged fault segments. Minor NNE–SSW oriented extensional structures mainly occur in the overlap region between major faults, forming a dilatational stepover. Most faults display evidence of active deformation and appear to control the main morphobathymetric features. This aspect, together with diffused continental slope instability, must be considered for the revaluation of the seismic and geomorphological hazard of this sector of southern Tyrrhenian Sea.     

Active faulting and continental slope instability in the Gulf of Patti (Tyrrhenian side of NE Sicily,Italy): a field,marine and seismological joint analysis

The Gulf of Patti and its onshore sector represent one of the most seismically active regions of the Italian Peninsula. Over the period 1984–2014, about 1800 earthquakes with small-to-moderate magnitude and a maximum hypocentral depth of 40 km occurred in this area. Historical catalogues reveal that the same area was affected by several strong earthquakes such as the Mw = 6.1 event in April 1978 and the Mw = 6.2 one in March 1786 which have caused severe damages in the surrounding localities. The main seismotectonic feature affecting this area is represented by a NNW–SSE trending right-lateral strike-slip fault system called “Aeolian–Tindari–Letojanni” (ATLFS) which has been interpreted as a lithospheric transfer zone extending from the Aeolian Islands to the Ionian coast of Sicily. Although the large-scale role of the ATLFS is widely accepted, several issues about its structural architecture (i.e. distribution, attitude and slip of fault segments) and the active deformation pattern are poorly constrained, particularly in the offshore. An integrated analysis of field structural geology with marine geophysical and seismological data has allowed to better understand the structural fabric of the ATLFS which, in the study area, is expressed by two major NW–SE trending, en-echelon arranged fault segments. Minor NNE–SSW oriented extensional structures mainly occur in the overlap region between major faults, forming a dilatational stepover. Most faults display evidence of active deformation and appear to control the main morphobathymetric features. This aspect, together with diffused continental slope instability, must be considered for the revaluation of the seismic and geomorphological hazard of this sector of southern Tyrrhenian Sea.

     

Dating synmagmatic folds: a case study of Schlingen structures in the Strona-Ceneri Zone (Southern Alps, northern Italy)

The Strona-Ceneri Zone (Southern Alps) contains folds with moderately to steeply inclined axial planes and fold axes, and amplitudes of up to several kilometres (so-called 'Schlingen'). These amphibolite facies folds deform the main schistosity of Late Ordovician metagranitoids and are discordantly overlain by unmetamorphic Permian sedimentary rocks. Mutually cross-cutting relationships between these folds and garnet-bearing leucotonalitic dykes indicate that these dykes were emplaced during folding. Sm–Nd systematics and the strongly peraluminous composition of these dykes point to an anatectic origin. Pb step leaching of magmatic garnet from a leucotonalitic dyke yielded a 321.3±2.3  Ma intrusive age. Rb–Sr ages on muscovites from leucotonalitic dykes range from 307 to 298  Ma, interpreted as cooling ages during retrograde amphibolite facies metamorphism. Conventional U–Pb data of zircons from an older granodioritic dyke that pre-dates the Schlingen folds yielded discordant U–Pb ages ranging from 371 to 294  Ma. These ages reflect a more complicated multi-episodic growth history which is consistent with the observed polyphase structural overprint of this dyke. Schlingen folding was accompanied by prograde amphibolite facies metamorphism, during the thermal peak of which the leucotonalitic dyke material was generated by partial melting in a deeper source region from where these S-type magmas intruded the presently exposed level. Because partial melting may occur in a relatively late stage of a clockwise P–T–t path, or even during decompression on the retrograde path, we do not exclude the possibility that Schlingen folding had already started in Early Carboniferous time. Schlingen folds also occur in Penninic and Austroalpine basement units with a very similar pre-Alpine history, indicating that Variscan folding affected large segments of the future Alpine realm.     

Sensitivity analysis of seismic hazard for Western Liguria (North Western Italy): A first attempt towards the understanding and quantification of hazard uncertainty

The use of logic trees in probabilistic seismic hazard analyses often involves a large number of branches that reflect the uncertainty in the selection of different models and in the selection of the parameter values of each model. The sensitivity analysis, as proposed by Rabinowitz and Steinberg [Rabinowitz, N., Steinberg, D.M., 1991. Seismic hazard sensitivity analysis: a multi-parameter approach. Bull. Seismol. Soc. Am. 81, 796–817], is an efficient tool that allows the construction of logic trees focusing attention on the parameters that have greater impact on the hazard.In this paper the sensitivity analysis is performed in order to identify the parameters that have the largest influence on the Western Liguria (North Western Italy) seismic hazard. The analysis is conducted for six strategic sites following the multi-parameter approach developed by Rabinowitz and Steinberg [Rabinowitz, N., Steinberg, D.M., 1991. Seismic hazard sensitivity analysis: a multi-parameter approach. Bull. Seismol. Soc. Am. 81, 796–817] and accounts for both mean hazard values and hazard values corresponding to different percentiles (e.g., 16%-ile and 84%-ile). The results are assessed in terms of the expected PGA with a 10% probability of exceedance in 50 years for rock conditions and account for both the contribution from specific source zones using the Cornell approach [Cornell, C.A., 1968. Engineering seismic risk analysis. Bull. Seismol. Soc. Am. 58, 1583–1606] and the spatially smoothed seismicity [Frankel, A., 1995. Mapping seismic hazard in the Central and Eastern United States. Seismol. Res. Lett. 66, 8–21]. The influence of different procedures for calculating seismic hazard, seismic catalogues (epicentral parameters), source zone models, frequency–magnitude parameters, maximum earthquake magnitude values and attenuation relationships is considered. As a result, the sensitivity analysis allows us to identify the parameters with higher influence on the hazard. Only these parameters should be subjected to careful discussion or further research in order to reduce the uncertainty in the hazard while those with little or no effect can be excluded from subsequent logic-tree-based seismic hazard analyses.     

A cross section of the eastern Betic Cordillera (SE Spain) according field data and a seismic reflection profile

The BT3 multichannel seismic profile was acquired by the C.G.G. (Compagnie General de Géophysique) in 1977 for hydrocarbon exploration in the eastern Betic Cordillera. REXIMseis Ltd scanned and vectorized a paper copy and then performed post-stack processing, including coherence filtering and deconvolution. The receiver functions of a broad-band seismic station located near the village of Vélez Rubio, at the SE end of the profile, were analysed by Julia et al. [Julia, J., Mancilla, F., Morales, J., 2005. Seismic signature of intracrustal magmatic intrusions in the Eastern Betics (Internal Zone), SE Iberia, Geophysical Research Letters 32, L16304, doi:10.1029/2005GL023274.] to determine the structure of the underlying crust. We have used these Vp data to convert the profile to depth. The profile has a mean SE–NW trend, with a SE-Section 44 km in length followed by a NW-Section 20 km in length. The record includes the first 4 s (twtt), which corresponds to 11 km.Two main areas can be seen in the profile. At the SE-end, a band of high-amplitude discontinuous reflectors dips towards the north. The band is 100 to 200 ms thick, increasing even more northwards. This band reaches the surface at the top of the Maláguide Complex (the upper complex of the Internal Zones). Above these reflectors, an area with chaotic seismic facies and no reflectors corresponds to the outcrops of the olistostromes and turbidites of the Solana Formation, and it is in turn overlain by discontinuous reflectors of the Subbetic rocks.At the NW-end of the profile, a set of high-amplitude continuous reflectors with SE dips point to the location of the Prebetic. Below this section, oblique reflectors of intermediate amplitude indicate the Variscan basement. Over the Prebetic, we have marked the basal thrusts of the Intermediate Units and the Subbetic. Using this seismic data, as well as field observations, we propose a geological cross-section of the upper crust of the eastern Betic Cordillera and a model of the most recent evolution of the orogen. In this model, the Internal Zones and the Subbetic have been welded together from the Middle Burdigalian to the present day and acted as an orogenic wedge that deformed the Intermediate Units and the Prebetic.     

GIS-based hydrostratigraphic modeling of the city of Rome (Italy): analysis of the geometric relationships between a buried aquifer in the Tiber Valley and the confining hydrostratigraphic complexes

A preliminary hydrostratigraphic model of the city of Rome (Italy) was completed through the analysis of geological and hydrogeological data, and by using geographic information system (GIS)-based operations. The complex lithostratigraphic setting of a local Quaternary volcano-sedimentary multilayer was simplified into hydrostratigraphic complexes considering textural properties of terrains and hydraulic conductivity values. The Tiber River Valley, in the middle of the urban area, was analyzed separately from the surrounding areas. GIS tools were used in a multiple-step procedure to reconstruct the geometry of bounding surfaces of the main hydrostratigraphic complexes. Particular care was given to the characterization of a confined gravelly aquifer lying at the base of the upper Pleistocene?CHolocene alluvium that fills the Tiber Valley. Updated isobaths and thickness maps of this layer were produced. Three-dimensional reconstruction identifies areas of potential recharge between the alluvium aquifer and the surrounding hydrostratigraphic complexes. In the central sector of the city, the gravel deposits are laterally confined by a thick aquitard corresponding to the Pliocene clayey bedrock of the city; in contrast, a potential recharge area can be hypothesized in the northern areas, where the upper Pleistocene?CHolocene valley cross cuts the Paleotiber Graben, filled with older and highly permeable gravels and clays.     

Magnetic and ground probing radar measurements for soil pollution mapping in the industrial area of Val Basento (Basilicata Region, Southern Italy): a case study

The joint application of electromagnetic techniques for near-surface exploration is a useful tool for soil pollution monitoring and can also contribute towards describing the spatial distribution of pollutants. The results of a geophysical field survey that was carried out for characterizing the heavy metal and waste disposal soil pollution phenomena in the industrial area of Val Basento (Basilicata region, Southern Italy) are presented here. First, topsoil magnetic susceptibility measurements have been carried out for defining the spatial distribution of superficial pollution phenomena in the investigated area. Second, detailed and integrated measurements based on a high-resolution magnetic mapping and ground probing radar (GPR) profiling have been applied to investigate the subsurface in two industrial areas located in more polluted sites that were identified during the first phase. Our monitoring strategy discloses the way to rapidly define the zone characterized by high pollution levels deriving from chemical industries and traffic emissions and to obtain the way information about the presence of local buried sources of contamination.     

Genesis and evolution of a curved mountain front: paleomagnetic and geological evidence from the Gran Sasso range (central Apennines, Italy)

The Gran Sasso range is a striking salient formed by two roughly rectilinear E–W and N–S limbs. In the past 90° counterclockwise (CCW) rotations from the eastern Gran Sasso were reported [Tectonophysics 215 (1992) 335], suggesting west–east increase of rotation-related northward shortening along the E–W limb. In this paper, we report on paleomagnetic data from Meso-Cenozoic sedimentary dykes and strata cropping out at Corno Grande (central part of the E–W Gran Sasso limb), the highest summit of the Apennine belt. Predominant northwestward paleomagnetic declinations (in the normal polarity state) from both sedimentary dykes and strata are observed. When compared to the expected declination values for the Adriatic foreland, our data document no thrusting-related rotation at Corno Grande. The overall paleomagnetic data set coupled with the available geological information shows that the Gran Sasso arc is in fact a composite structure, formed by an unrotated-low shortening western (E–W trending) limb and a strongly CCW rotated eastern salient. Late Messinian and post-early Pliocene shortening episodes documented along the Gran Sasso front indicate that belt building and arc formation occurred during two distinct episodes. We suggest that the southern part of a late Messinian N–S front was reactivated during early–middle Pliocene time, forming a tight range salient due to CCW rotations and differential along-front shortening rates. The formation of a northward displacing bulge in an overall NW–SE chain is likely a consequence of the collision between the Latium-Abruzzi and Apulian carbonate platforms during northeastward propagation of the Apennine wedge, inducing lateral northward extrusion of Latium-Abruzzi carbonates towards ductile basinal sediment areas.     

A Source Study of the 9 September 1998 (Mw 5.6) Castelluccio Earthquake in Southern Italy using Teleseismic and Strong Motion Data

Teleseismic and strong motion data are used to derive the source parameters of the September 9, 1998 Castelluccio earthquake (M5.6). Teleseismic body-wave modeling indicates normal faulting (Plane 1: strike = 328°, dip = 50°, rake = −75°; Plane 2: strike = 126°, dip = 42°, rake = −107°) along NW–SE striking planes. Both nodal planes of the computed focal mechanism are tested for their capability of reproducing the mainshock acceleration time histories at three strong motion stations. Synthetic accelerograms are estimated using the stochastic method for finite sources in combination with the H/V ratios technique for the incorporation of the site effect. Our preferred model, which provided the best fit between synthetic and observed waveforms and corresponding Fourier amplitude spectra, consists of a NE-dipping normal fault with dimensions 8 × 7 km. The rupture nucleation point, which is assumed to coincide with the hypocenter location, was confined to the southeasternmost, deepest part of the fault. Our results are in good agreement with the so far released information regarding the aftershock sequence of the examined event, as well as the general seismotectonic knowledge on the broader epicentral area.     

Effects of detrital carbonate on stable oxygen and carbon isotope data from varved sediments of the interglacial Piànico palaeolake (Southern Alps,Italy)

Contamination with detrital matter is a well‐known bias in δ¹⁸O records from lake carbonates but quantitative information of this effect is yet lacking. Therefore, we developed a new methodological approach combining isotope analyses with microfacies, X‐ray diffraction and micro‐X‐ray fluorescence data and applied this in a case study for the Piànico interglacial lake record in order to provide a quantitative estimate of the effect of detrital carbonate on stable δ¹⁸O and δ¹³C data. The Piànico record contains a long series of distinct and well‐preserved calcite varves and is correlated to Marine Isotope Stage 11. Intercalated in the varve sequence are detrital layers triggered by surface erosion events. These detrital layers are mainly composed of dolomite, thus reflecting the mineralogical signature of the catchment. Microfacies analyses of a 9350 varve year interval allows the identification of detrital layers down to sub‐millimetre scale and a precise selection of three different types of samples for isotope analyses: (1) pure endogenic calcite varves (five varves per sample) without detrital contamination; (2) individual detrital layers; and (3) ‘mixed’ samples including five calcite varves and up to four thin detrital layers. Detrital samples show the isotopic signature of the catchment dolomite and are up to 5.7‰ enriched in δ¹⁸O values with respect to endogenic calcite samples. In mixed samples, the degree of isotopic enrichment is directly related to the amount of detrital contamination; δ¹⁸O of bulk carbonates is significantly biased when the detrital component amounts to more than 5% of the sample. It is also shown that samples containing detrital material have an influence on the calculation of the covariance between δ¹³C and δ¹⁸O. Covariance is high (r = 0.76) when the correlation coefficient is calculated on the base of all samples, but absent (r = ?0.43) when samples containing detrital dolomite are excluded. It has been demonstrated that microfacies analysis is a quick tool to avoid or reduce detrital contamination in bulk carbonate samples during sample selection. Copyright © 2009 John Wiley & Sons, Ltd.     

A critical revision of the seismicity of Northern Apulia (Adriatic microplate — Southern Italy) and implicationsfor the identification of seismogenic structures

Northern Apulia is an emerged portion of the Adriatic microplate, representing the foreland–foredeep area of a stretch of the Apennine chain in southern Italy. The interaction between the relatively rigid microplate and the contiguous more deformable domains is responsible for the intense seismicity affecting the chain area. However strong, sometimes even disastrous, earthquakes have also hit northern Apulia on several occasions. The identification of the causative faults of such events is still unclear and different hypotheses have been reported in literature. In order to provide guidelines and constraints in the search for these structures, a comprehensive re-examination and reprocessing of all the available seismic data has been carried out taking into consideration 1) the characteristics of historical events, 2) the accurate relocation of events instrumentally recorded in the last 20 years, 3) the determination of focal mechanisms and of the regional stress tensor.The results obtained bring to light a distinction between the foreland and foredeep areas. In the first region there is evidence of a regional stress combining NW compression and NE extension, thus structures responsible for major earthquakes should be searched for among strike–slip faults, possibly with a slight transpressive character. These structures could be either approximately N–S oriented sinistral or E–W dextral faults. In the foredeep region there is a transition toward transtensive mechanisms, with strikes similar to those of the previous zone, or maybe also towards NW oriented normal faults, more similar to those prevailing in the southern Apennine chain in relation to a dominant NE extension; this appears to be the effect of a reduction of the NW compression, probably due to a decrease in efficiency of stress transmission along the more tectonised border of the Adriatic microplate.     

钦-杭结合带硅质岩的分布特征及其地质意义

钦州—杭州(钦-杭)结合带位于中国东南部地区,跨越浙江、江西、湖南、广东和广西5省(区)。大致以南岭为界,该带可划分为北、中、南三段。中段与南岭带重合,大致分布在北纬24°~27°范围,以北为北段,包括江西、浙江及安徽南部,以南为南段,包括粤西桂东南地区。钦-杭结合带跨带内沉积硅质岩广泛发育,地质和地球化学证据展示它们主要为热水成因。硅质岩相关的热水活动分布偏向于靠近结合带的两侧。钦-杭结合带"北、中、南"三段的硅质岩时空分布存在明显差异:南段及两侧邻区大规模热水活动集中于晚古生代,中段及两侧邻区的集中于早古生代,北段及邻区的热水活动集中于元古宙。这种大规模热水活动自北向南逐渐变新的特点与钦-杭结合带分段演化具有较好的对应关系。作为热水活动地质遗迹的硅质岩,其形成与大地构造与地球动力学背景的演化有密切的关系,特别是与钦-杭结合带的几个拉张阶段存在较好的对应关系。富含硅质的热水活动伴随了丰富的成矿作用,热水喷流沉积型块状硫化物矿床和热水沉积型金矿在该构造带内均较为典型。