organic matter maturity in northern italy some determining agents abstract in previous agip studies concerning the origin of oil in northern italy thematurity evolution of the source rocks beneath a thrust plate and the anomalous depth trends of maturity parameters were exained this paper focuses on the possible effects of different thermal histories and of high pressure on the catagenesis of kerogen the effects of overthrusting on the degree of maturity of source rocks is investigating in two areas of the northern apennines near the cortemaggiore oil and gas field and ponte del olio no 1 well respectively the model by angevine and turcotte is used for reconstructionof the temperature evolution and that by tissot and espitalie to model the hydrocarbon generation in addition it was examined how different thermal histories affect the oil to gas ratio computed according to tissot espitalie the results for sediments beneath thrust plates which are subjected to rapid increases in temperature are compared to those typical of regularly subsiding basin in the po basin intervals of abnormally pressured sediments were drilled in the lombardy area the organic matter appears immature in early jurassic and late triassic sediments at depth even down to 6000 m as recognized by different maturity parameters possible causes for the low maturity levels of the organic matter are examined with particular emphasis on the effects of high pressures

the iberian type of volcano sedimentary massive sulphide deposits the iberian pyrite belt located in the sw iberian peninsula contains many paleozoic giant and supergiant massive sulphide deposits including the largest individual massive sulphide bodies on earth total ore reserves exceed 1500 mt distributed in eight supergiant deposits 100 mt and a number of other smaller deposits commonly with associated stockwork mineralizations and footwall alteration haloes massive sulphide bodies largely consist of pyrite with subordinated sphalerite galena and chalcopyrite and many other minor phases although substantial differences occur between individual deposits both in mineral abundance and spatial distribution these deposits are considered to be volcanogenic roughly similar to volcanic hosted massive sulphides vhms however our major conclusion is that the iberian type of massive sulphides must be considered as a vhms sub type transitional to shms

thrust faulting and hydrocarbon generation reply this paper is a rebuttal of an earlier paper trying to refute the claims of these authors they believe this reply illustrates that the simplifications used by warner and royse 1987 in their model are generally inappropriate to describe the thermal history of units in the western overthrust belt as well as other overthrust regions although their shortcuts produce results that are generally consistent with measured maturity values for some specific modeling sites the differences in maturation history and the mismatch for the general thrust case make their model a less effective predictive tool at this stage in the understanding of overthrust thermal processes you cannot predict a priori when the thermal effects of thrusting will be significant and when they will be minimal defining the exact relationship between thrusting and hydrocarbon accumulations in overthrust areas requires additional work and data collection in particular workers must consider case histories where the hanging wall contains thick sequences of resistant strata by incorporating additional constraints instead of returning to the simplified assumption of constant thermal gradients in tectonic regions the understanding of the evolution and maturation history of thrust belts is improved

composition and structure of the central aleutian island arc from arc parallel wide angle seismic data new results from wide angle seismic data collected parallel to the central aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry the 3 d ray tracing 2 5 d inversion of this sparse wide angle data set which incorporates independent phase interpretations and new constraints on shallow velocity structure produces a faster and smoother result than a previously published velocity model middle crustal velocities of 6 5 7 3 km s over depths of 10 20 km indicate an andesitic to basaltic composition high lower crustal velocities of 7 3 7 7 km s over depths of 20 35 km are interpreted as ultramafic mafic cumulates and or garnet granulites the total crustal thickness is 35 37 km this result indicates that the aleutian island arc has higher velocities and thus more mafic compositions than average continental crust implying that significant modifications would be required for this arc to be a suitable building block for continental crust lateral variations in average crustal velocity below 10 km roughly correspond to trends in major element geochemistry of primitive mg 0 6 lavas the highest lower crustal velocities and presumably most mafic material are detected in the center of an arc segment between unmak and unalaska islands implying that arc segmentation exerts control over crustal composition

evidence for preservation of crustal root beneath the proterozoic lapland kola orogen northern fennoscandian shield derived from p and s wave velocity models of polar and hukka wide angle reflection and refraction profiles and fire4 reflection transect 1 we present an analysis of the crust and upper mantle in the northern fennoscandian shield based on new p and s wave 2 d velocity models of the polar and hukka wide angle reflection and refraction profiles and results of a new seismic reflection experiment in finland finnish reflection experiment the profiles are almost collocated and crossed the proterozoic lapland kola orogen a substantial difference of the depth of the wide angle moho 40 42 km and the reflection moho 47 50 km was found in order to explain this difference we compared the velocity models to published values of vp and vp vs for the main types of lower crustal and mantle rocks we found that the main reason for disagreement is that the wide angle moho and the reflection moho correspond to different petrological boundaries in the southwest and northeast portions of the profiles the wide angle moho marks contact of either anorthositic or granulitic lower crust with a reflective layer in the upper mantle composed of peridotites and pyroxenites the reflection moho represents the bottom of this lower layer in the center of the profile the wide angle moho marks the top of a large eclogitic body in the upper mantle representing a well preserved crustal root beneath the lapland kola orogen formed because of the collision of three former archaean crustal blocks terranes or microcontinents lack of postorogenic tectonic collapse suggests another mechanism for stabilization of the lithosphere in the area upper mantle reflectors at depth of 65 75 km may mark the upper boundary of the cold and mechanically strong archaean upper mantle wedge alternatively these reflectors may represent a top of uplifted asthenosphere that can explain preservation of crustal root

sedimentology and hydrocarbon potential of yager structural complex possible paleogene source rocks in eel river basin northern california the yager structural complex of coastal northern california is composed of paleogene mudrocks turbidites and related deep marine deposits the strata are folded but lack the severe and pervasive stratal disruption that characterizes surrounding accreted terranes such as the franciscan coastal belt lithofacies associations and depositional cycles define three environmental settings slope feeder channel and restricted basin the rocks probably represent a sequence of trench slope and slope basin deposits that accumulated above a broadly coeval basement of accreted coastal belt rocks yager strata are possible source beds for hydrocarbons within the late cenozoic eel river basin type iii kerogen is dominant and organic carbon content and hydrocarbon yield are relatively low nevertheless the thermal maturity of yager shales is well within the window of peak hydrocarbon generation this level of maturation is attributed to a combination of middle tertiary thrust faulting and subsequent depositional burial beneath the neogene wildcat group

the aguablanca cu ni ore deposit extremadura spain a case of synorogenic orthomagmatic mineralization age and isotope composition of magmas sr nd and ore s abstract the aguablanca cu ni orthomagmatic ore deposit is hosted by mafic and ultramafic rocks of the aguablanca stock which is part of the larger high k calc alkaline santa olalla plutonic complex this intrusive complex ca 338 ma in age is located in the ossa morena zone omz of the iberian variscan belt mineralization consists mainly of pyrrhotite pentlandite and chalcopyrite resulting from the crystallization of an immiscible sulphide rich liquid isotope work on the host igneous rocks sr nd and the ore s suggests that contamination with an upper crustal component took place at some depth before final emplacement of the plutons end338 6 to 7 5 sr 338 0 7082 to 0 7100 34s sulphides near 7 4 assimilation fractional crystallization afc processes are invoked to explain early cumulates and immiscible sulphide magma formation intrusion took place at the beginning of the type a oblique subduction of the south portuguese zone under the ossa morena zone and was probably driven by transpressive structures strike slip faults the mineralization is thus synorogenic aguablanca is probably the first case referred to in the literature of a magmatic cu ni ore deposit hosted by calc alkaline igneous rocks

thrust faulting and hydrocarbon generation discussion following the discovery of large volumes of hydrocarbons in the cordilleran thrust belt of southwestern wyoming and northern utah angevine and turcotte 1983 edman and surdam 1984 and furlong and edman 1984 discussed the role of thrust faulting in hydrocarbon generation the authors concluded that overthrusting plays an important or even dominant role in the process of hydrocarbon generation in overthrust terrains these conclusions may be in error because they are based on a mathematical analysis of thermal and geologic models that have little resemblance to actual thrust belt geology in this discussion the authors discuss shortcomings of the thermal model used in the papers referred to above emphasize the differences between their model and the geologic facts as they exist in thrust belts in general and in western wyoming in particular and caution readers about applying the conclusions based on such a model in the search for hydrocarbons they contend that the temperature effects of thrust faulting were rather insignificant in the process of hydrocarbon generation in the wyoming thrust belt

cenozoic stratigraphy and basin tectonics of the andes mountains 20 28 south latitude clastic sedimentary basins have evolved during the past 40 m y in the central andes lat 20 28 s in response to shifting patterns of magmatism and deformation the distribution of these basins and their genetic relations to uplifted areas are analogous to the basins and mountain belts of the north american rocky mountains during the late cretaceous and early cenozoic petroleum exploration has focused on zones underlying the upper cenozoic strata along the eastern margin of the andes mountain belt between about 40 and 25 ma a nonmarine basin extended across the region that is now the andes mountains between about 25 and 10 ma the western part of the former basin became the site of a volcanic arc sediment accumulation continued in the east where marine intercalations demonstrate the low elevation of the basin after 10 ma the volcanic arc remained active and locally widened and crustal shortening caused regionally important thrust and reverse faulted ranges during the past 10 m y up to 4 000 m of coarse clastic debris accumulated in a foreland basin on the eastern flank of the mountains meanwhile in the interior of the mountains over 4 000 m of fine grained strata and evaporites accumulated in local depocenters

seismic imaging and modelling of the lithosphere of sw iberia abstract data from a closely spaced wide angle transect has been used to study the middle to lower crust and the moho in sw iberia a low fold wide angle stack image reveals a highly heterogeneous seismic signature at lower crustal levels changing laterally along the profile the lower crust features an irregular distribution of the reflectivity that can be explained by a heterogeneous distribution of physical properties the moho discontinuity also features a high variability in its seismic character that correlates with the different tectonic terranes in the area a 2d finite difference code was used for solving the elastic wave equation and to provide synthetic wide angle shots relatively simple layer cake model derived from conventional refraction interpretation generates the main events of the shot records however these models cannot account for the lateral variability of the seismic signature in order to obtain more realistic simulations the velocity model was modified introducing stochastic lensing at different levels within the crust the moho was modelled as a 3 km thick layered structure the resulting average velocity models include a high velocity layer at mid crustal depth a highly reflective lower crust and a relatively thin horizontal moho this heterogeneous model can be achieved by lensing within the crust a layered mafic intrusion and a strongly laminated lower crust and moho

a new scenario for related iocg and ni cu mineralization the relationship with giant mid crustal mafic sills variscan iberian massif both magmatic ni cu and hydrothermal iron oxide copper gold mineralization coexist in ancient belts but their relationship remains poorly known geochronology and field geology evidence show that in sw iberia both styles of mineralization were coeval with widespread metaluminous to peraluminous variscan magmatism 350 330 ma we propose that mineralization was probably related to a hidden large layered mafic ultramafic layered complex recently inferred from geophysics that was emplaced in a mid crustal decollement zone contamination of a primitive mantle derived magma with continental crust in the layered complex led to fractional crystallization accompanied by high t low p metamorphism and the incorporation of volatiles into the melt hot hypersaline co2 ch4 bearing brines were subsequently released and focused along major thrusts and strike slip faults to produce the iocg mineralization assimilation of continental crust also led to the formation of sulphide magmas that were tectonically injected high into the crust leading to the formation of pipe like breccia hosted ni cu ore bodies all these processes took probably place as a consequence of oblique ridge and or continent continent collision

geochemistry and habitat of the oils in italy all varieties of liquid petroleum ranging from condensates 50 degrees api to immature sulfur rich heavy oils as low as 5 degrees api have been found in italy however nonbiodegraded heavy oils account for about 70 percent of the total original oil in place geochemical analyses indicate that 11 oil groups are present in the italian basins and two main types of source rocks have been identified triassic carbonates and tertiary shales about 95 percent of the oils were originated from middle and upper triassic carbonates containing type ii kerogen about 1 percent 4total organic carbon toc and 500 mg hydrocarbon g toc only a relatively minor amount of oil was generated by tertiary shales containing type iii kerogen with toc generally less than 1 percent timing of generation and migration and bulk properties of oils were controlled by geodynamic histories of the three main italian geologic settings 1 apennine and southern alp thrust belts 2 foredeep depression bordering the thrust belts and 3 foreland nondeformed african continental margin within the apennine thrust belts deep burial during the neogene resulted in the generation of substantially lighter oils not only from deeply buried triassic but sometimes also from tertiary source rocks in the late neogene foredeep depocenters located in the central adriatic and southern sicily high subsidence up to 1000 m m y a low geothermal gradient 22 degrees c km and compressional tectonics caused the generation of immature heavy oils generally at depths below 5000 m nd temperatures greater than 100 degrees c rapid burial and higher geothermal gradients 32 degrees c km which occurred since the jurassic resulted in the generation of light oils from the late cretaceous to the oligocene in the southern sector of adriatic foreland

migration of orogenic fluids through the siluro devonian helderberg group during late paleozoic deformation constraints on fluid sources and implications for thermal histories of sedimentary basins abstract fluid inclusion data and fracture filling cement fabrics in deformed but non overthrusted carbonate and siliciclastic rocks of the siluro devonian helderberg group central appalachians indicate post cementation late paleozoic migration of high pressure high temperature fluids void filling quartz and calcite cements contain secondary two phase fluid inclusions liquid vapor which occur in annealed microfractures most melting temperatures last occurrence of ice are 20 to 25 c which correspond to salinities of greater than 22 equivalent wt nacl pressure corrected liquid vapor homogenization temperatures are 200 to 300 c and greatly exceed maximum paleotemperatures 150 c given by conodont cai values vitrinite reflectance illite crystallinity or temperatures calculated from known sedimentary overburden completely cemented sandstone and limestone also have multiple crosscutting trains of secondary hydrocarbon inclusions hydrocarbon inclusions occur along deformation microstructures in cements some hydrocarbon inclusion trains crosscut cement filled fractures the nature of occurrence of the secondary inclusions suggests that fluids moved along intracrystalline deformation microstructures either during or after late paleozoic deformation rare fractures contain transported skeletal grains exotic clasts lithologies different from wall rock recemented clasts of fracture filling cement and mud cement clasts contain solid inclusions of mud and skeletal fragments and indicate several episodes of particle transport cementation and refracturing prior to final fracture filling and cementation these fracture fills locally crosscut stylolites coarse grained poorly sorted clastic fracture fills suggest migration of rapidly moving fluids which were capable of transporting clasts through fracture conduits under deep burial conditions conduction calculations place upper limits on size and duration of thermal anomalies which were caused by extrabasinal fluids these calculations indicate that hot fluid migration events most likely were short lived heat transfer by extraformational fluids moving along faults and fractures may have been the mechanism by which thermal anomalies developed in the helderberg group stratigraphic constraints suggest that likely sources for the hot brines are overthrusted terranes to the east of the study area possible stratigraphic horizons which were sources for the overpressured hot fluids may have been thick cambrian shales which are important detachment horizons for major late paleozoic thrust sheets in the eastern valley and ridge province east of the study area fluids may have migrated during thrusting along thrust faults which ramp upwards from east to west into stratigraphically higher units thrust faults ultimately die out in devonian units some of the dissolved ions in the secondary two phase inclusions may have been incorporated when shale derived fluids moved through silurian evaporites or cambro ordovician carbonates which contain evidence of cryptic evaporites potential source rocks for secondary hydrocarbon inclusions may have been lower silurian through lower devonian strata based on lopatin modeling and assuming that migration occurred during late paleozoic deformation this study may help to explain many apparently anomalous geologic phenomena in deformed foreland basin sediments it also has implications for tracking migration pathways for fluids which facilitate regional scale deformation in fold and thrust belts and how tectonic fluids may affect the rocks through which they move

geochemistry and habitat of natural gases in italy abstract systematic chemical and isotopic analyses were carried out on natural gases from 92 fields which represent more than 90 of the original gas reserves of italy a quantitative assessment of the various genetic gas types indicates that most of the gaseous hydrocarbons 80 were generated through bacterial activity and or diagenetic processes while only a minor amount 20 was produced by thermal degradation of organic matter the formation of the gas accumulations was geologically controlled by the neogene tectonic phases and by the sedimentary events associated with the growth of the apennines and southern alps in fact about 80 of italian natural gas reserves have been found in the apennine foredeep while the remaining part is equally distributed in the folded belt and in the foreland furthermore the influence exerted by tectono sedimentary characteristics is emphasized by the fact that thermal degradation of organic matter was the only process responsible for most of the gas generated in the apennine folded belt 97 thermogenic gases on the other hand the high rates of subsidence turbiditic sedimentation and very early compressional tectonics in the foredeep favored processes leading to the accumulation of very large quantities of biogenic gas 82 moreover geochemical and geological data as well as mathematical modelling demonstrate that migration and accumulation of italian natural gases mainly occurred later than 5 my ago and in some cases these phenomena are still active in fact in the multipay zone gas fields of the northern adriatic a kind of steady equilibrium is reached between loss of gas through the thin cap rocks often less than 1 m and continous replenishment from the bacterial fermentation and or low temperature thermochemical reactions

hydrocarbon possibilities of concealed mesozoic paleogene sediments below himalayan nappes reappraisal hydrocarbon exploration of the ganga ganges and sub himalayan basins indicates the presence of upper neogene quaternary continental molasse and the absence of paleogene sediments regarded as hydrocarbon source beds marine to brackish water paleogene lower neogene sediments closely fringing the sub himalayan foothills increase in thickness northward their presence and facies in the frontal schuppen zone and in the lesser himalayan windows commonly in close association with upper mesozoic shelf sediments indicate a formerly greater width for the late mesozoic early neogene shelf miogeosynclinal basin the pre tertiary rocks of the lesser himalayas are in nappes floored by the flat lying sole thrust redesignated the main boundary thrust mbt schuppen structure and ateral shortening complicate the geometry of the mbt downfaulted klippen and decken also are present in grabens of pre tertiary rocks in the sub himalayan schuppen zone pre tertiary tectonic grain and metamorphism ubiquitously developed within the lesser himalayan cover rocks are typically absent in the subjacent paleogene lower neogene window zone sediments deformations within the latter are much less than in pre tertiary cover rocks with which they are commonly intersliced owing to the development of schuppen structure tertiary deformations are unlikely to affect adversely the possible hydrocarbon resource within the thick mesozoic lower neogene autochthonous sediments or those which have migrated to the lesser himalayan cover rocks this opinion is corroborated by a few oil and gas shows from the eocene limestone outcrops close to the mbt and also from the lesser himalayan pre tertiary metamorphic cover rocks

geophysical evidence of a mantle derived intrusion in sw iberia 1 deep seismic reflection data acquired as part of the sw iberia europrobe project across the transpressional variscan orogen sample three tectonic terranes the south portuguese zone the ossa morena zone and the central iberian zone the seismic data reveal the existence of a mid crustal reflective body 140 km long and of variable thickness up to 5 km the iberian reflective body the conductivity image provided by coincident mt soundings the amplitude characteristics of the seismics mineralization studies related to magmatic ore deposits and the surface geology suggest that the irb is a mantle derived mafic intrusion the geophysical geological and petrological data suggest that the irb is most probably an early carboniferous approximately at 350 340 ma mantle derived intrusion possibly linked to plume activity that took place in europe in the carboniferous and permian

graphic approach to determination of hydrocarbon maturation in overthrust terrains with the current exploration efforts in areas of overthrust tectonics it is important to evaluate efficiently and effectively the effects of thrust related thermal perturbations on hydrocarbon maturation a simple graphic technique which combines lopatin diagrams with theoretical models of temperatures in overthrust regimes allows one to develop relatively detailed models for the timing of hydrocarbon maturation in overthrust terrains these models provide the critical combination between rigorous consideration of thermal perturbations and simplification necessary for routine application the approach described here is applicable not only in cases of simple overthrusting at the surface but can also be modified to include the effects of multiple thrusting events she ts thrust in the subsurface and post thrusting erosion all of these models provide critical constraints on the timing of maturation and migration as well as information on the maturity of potential source rocks

integrated lithospheric modeling combining thermal gravity and local isostasy analysis application to the ne spanish geotransect a two dimensional algorithm to determine the steady state thermal structure of the lithosphere that integrates thermal gravity and local isostasy analyses is presented gravity analyses together with seismic data are used to constrain spatial variations in density and crustal structure while absolute elevation is used to determine the lithospheric mantle thickness the calculation is performed using a finite element technique that links the different physical equations the program optionally calculates the temperature at any material boundary and with given rheological parameters the strength distribution and the total lithospheric strength in selected columns we apply the algorithm to the northeastern spanish geotransect which extends from the pyrenees to the balearic promontory and along which a strong variation in crustal and lithospheric thickness is evident we assess the use of two different inferred density models for the lithospheric mantle the first assumes a linear decrease in density with increasing temperature using the asthenospheric density as a reference the second model assumes a constant density for the whole lithospheric mantle although conceptually the two hypotheses differ substantially the results obtained do not show significant differences lithospheric thicknesses of 120 130 km below the pyrenees 60 65 km in the valencia trough and 65 75 km below the balearic promontory are deduced in all cases the mean lithospheric mantle density has to be 40 60 kg m 3 higher than the asthenospheric density the algorithm is shown to be a powerful tool in lithospheric thermal modeling especially in areas where surface heat flow is poorly constrained because of the temperature density elevation relationship

tectonic evolution of the boundary between the central iberian and ossa morena zones variscan belt southwest spain three tectonometamorphic units can be differentiated in the boundary between the central iberian and ossa morena zones of the iberian massif variscan belt the three units have been named the northern unit central unit and southern unit the northern unit corresponds to the border of the central iberian zone it evolved at low temperature and intermediate or low pressure metamorphic conditions and was affected by top to the se ductile shearing the central unit placed under the northern unit preserves high pressure silurian eclogitic assemblages retrograded by high to medium temperature and finally low temperature ductile shearing with top to the nw sense of movement oblique left lateral the central unit is superposed on the southern unit the latter corresponds to the border of the ossa morena zone and underwent low pressure intermediate temperature metamorphism synchronous with right lateral top to the se ductile shearing the envisaged tectonic evolution is as follows after a stage of lower paleozoic rifting subduction of the central unit under the northern unit took place in silurian times as a result of the crustal thickening a gravitational instability developed giving way to left lateral extensional shearing that affected the entire central unit the combined action of oblique thrusting at the front of the wedge and oblique extensional shearing at the rear caused the exhumation of the high pressure rocks of the central unit this tectonic evolution reveals that the boundary between the central iberian and ossa morena zones is a suture of the variscan belt

thermal subsidence and compaction in sedimentary basins application to baltimore canyon trough cooling with the subsidence of continental crust is likely to be an important mechanism for the formation of sedimentary basins if the lithosphere was initially very thin and sedimentation continued to fill the basin to the original depth of basement a similarity solution is possible in this paper the similarity solution is extended to include sediment compaction and a depth dependent thermal conductivity the similarity solution is also used to determine the thermal evolution and petroleum potential of sedimentary basins experience with similarity solutions in many branches of applied physics indicates that they commonly give good results even when the required assumptions are rather poor approximations an empirical subsidence relation valid for older basins is al o extended to include sediment compaction these results are applied to the evolution of the baltimore canyon trough on the outer continental margin of the eastern united states as given by the cost b 2 well good agreement is obtained with both the subsidence record and the present thermal gradient it is predicted that optimal thermal conditions for the conversion of kerogens to petroleum occur between depths of 4 and 6 km

processing and inversion of refraction and wide angle reflection data from the 1986 nevada passcal experiment we derive averaged one dimensional velocity models for the crust beneath the basin and range in western nevada by inverting p composites extracted from slant stacks of refraction and wide angle reflection data a combination of high cut filtering in the frequency domain with coherency filtering in the p domain has proved effective in smoothing severe static shifts due to lateral heterogeneities and suppressing artifacts due to spatial aliasing we supplement the low frequency information provided by the refraction records with higher frequency arrivals picked from the normal incidence and wide angle reflection sections generalized least squares inversion of data from eight shots for p and s wave velocity structure beneath two perpendicular profiles yields consistent estimates for total crustal thickness p wave 30 31 km s wave 30 32 km and average crustal velocity 6 2 km s 3 4 3 5 km s average poisson s ratio beneath the n s profile is 0 27 0 28 p wave velocities near the base of the crust 7 4 0 3 km s are estimated by inverting precritical reflections from the moho standard errors in depth for these models range from 1 to 3 km extremal depth bounds in general are about twice as large uncertainties in velocity estimated from resolving kernels range from 0 1 to 0 3 km s estimates of poisson1s ratio as a function of depth are characterized by large standard errors reflecting the poor resolution of shear wave velocities the combination of low average p and s wave velocities with a high average poisson s ratio in the upper half of the crust however is consistent with the presence of fluid filled cracks these results highlight the need for three component recording to better resolve shear wave velocity structure

generation and migration of oil in the maturin subbasin eastern venezuelan basin abstract regional geochemical evaluation of the possible cretaceous and tertiary source rocks and oil oil and oil source rock correlation studies have identified the calcareous shales and limestones of the upper cretaceous querecual and san antonio formations of the guayuta group with amorphous marine type ii organic matter as the source rocks for oil in the maturin subbasin the deposition of the source rocks took place on a passive continental margin whilst the generation and migration of oil occurred in a foreland basin during miocene recent times the oils found in the northern part of the subbasin in different reservoirs cretaceous eocene oligocene miocene and pliocene formations are of mature marine type with or without bacterial alteration geochemical data indicate that some of these oils were mixed with the highly altered residue of a previously accumulated oil based on maturity and migration modelling it appears that the oils essentially migrated from active upper miocene recent kitchens in the cretaceous source rocks below the main thrusts the highly altered residue was formed during the upper miocene by the biodegradation of oil migrating during the middle and upper miocene large areas of mature to overmature cretaceous source rocks now exposed in the interior mountain front were in active oil kitchens during the lower and middle miocene before being involved in thrusting around 12 myr bp long distance 150 300 km southward lateral migration of oils during the middle miocene gave rise to the altered moderately mature marine oils in the shallow miocene stratigraphic reservoirs of cerro negro orinoco oil belt in the southern fringe of the subbasin

tectonics of the beja acebuches ophiolite a major suture in the iberian variscan foldbelt the beja acebuches ophiolitic complex baoc south portugal spain corresponds to a high grade metamorphic belt along the boundary between ossa morena and the south portuguese zones and comprises a lithostratigraphic sequence including from top to bottom metabasalts metamorphosed multiple dyke intrusions in gabbro flasergabbros and metaserpentinites it is affected by three deformation phases d1 affects the ophiolite lower stratigraphic units and is represented by a mylonitic cleavage with a stretching lineation where shear criteria indicates the sense of shear to be towards the north north east this deformation event can be related to the ophiolite emplacement above the crystalline footwall of the serpa antiform affecting precambrian basement and cambrian cover rocks the obduction polarity ist north eastwards similar to the subduction polarity that generates the beja gabbroic complex bgc implying a flake geometry the second deformation phase d2 is represented by an intense wnw ese sinistral shear event which is responsible for the shattered appearance of the suture d2 is reactivated later by a more brittle d3 event involving thrusting to the south west again with a sinistral component 40ar 39ar isotopic ages were obtained for metamorphosed multiple dyke intrusions in the baoc s gabbro 342 6 1 4 ma for metagabbroic cumulates 340 7 1 9 ma and for the undeformed unmetamorphosed bgc 341 1 1 3 ma occurring to the north of the ophiolitic suture these ages reflect a last regional cooling event in the area which post dates the ophiolite emplacement and the intrusion of the bgc through this oceanic sequence

synfolding magnetization in the jurassic preuss sandstone wyoming idaho utah thrust belt the jurassic preuss sandstone exposed in five thrust plates of the wyoming idaho utah thrust belt carries directions of remanent magnetization that group most tightly after only partial unfolding field petrographic and rock magnetic evidence indicates that the carrier of this magnetization is detrital low ti titanomagnetite the detrital titanomagnetite was remagnetized at low temperatures 75 150 c probably completely during folding anisotropy of magnetic susceptibility and petrographic observations indicate that the detrital titanomagnetite has been affected by tectonic strain we suggest that low temperature remagnetization of the detrital titanomagnetite was either a viscous partial thermoremanent magnetization the acquisition of which was enhanced by stress or a piezoremanent magnetization that involved stress induced movement of domain walls during intracrystalline strain or was a combination of the two mechanisms stress may promote remagnetization at temperatures much lower than predicted by current theoretical models other mechanisms such as acquisition of chemical remanent magnetization during folding deflection of a prefolding magnetization by internal strain or combination of components of magnetization with different direction cannot account for the geometry of magnetization in the preuss the locus of acquisition of synfolding magnetization in the preuss migrated in conjunction with deformation in the thrust belt a model is presented in which synfolding magnetization was acquired during cooling and folding as strata moved up thrust ramps a lack of reverse polarity directions remains a puzzling feature of the remanence the remanent direction is tentatively interpreted to reflect the predominant polarity state during its acquisition over an extended rather than a discrete time period during folding in late cretaceous and early tertiary periods of predominantly normal polarity

an interpretation of wide angle compressional and shear wave data in southwest germany poisson s ratio and petrological implications shear wave velocity especially when used with compressional wave velocity is an important constraint on estimates of the composition and physical state of the continental lithosphere we present shear velocity s models derived from high quality s wave data along three profiles in southwest germany which combine with existing p wave velocity p models to give poisson s ratio in the crust and uppermost mantle we then construct a laterally varying petrological model of the crust by comparing our p and s models with published laboratory measured rock velocities in general poisson s ratio is 0 25 in the uppermost crust 0 22 0 23 in the mid crust but reaching a low of 0 15 in the black forest low p zone and varies laterally from 0 24 to 0 30 in the lower crust the average poisson s ratio of the whole crust is everywhere about 0 25 the laminated signature of the lower crust prominent on reflection and p wave refraction data is absent on the s wave refraction data implying a strong high low alternation of poisson s ratio in the lower crust this suggests that the lower crustal laminae represent mafic silicic compositional layering for example basic sills in an originally more quartz rich lower crust rather than alternating zones of high and low pore pressure there is no evidence for a shear wave low velocity zone lvz in the black forest mid crust despite the presence of a strong p wave lvz this suggests high quartz content and the presence of pore fluids at low not high pore pressure a clear p to s converted moho reflection pm s is visible on several shots indicating that the moho is at least locally a first order discontinuity no shear wave refractions from the upper mantle are present even where strong p wave refractions are observed possibly indicating an increase of poisson s ratio with depth in the upper mantle the petrological modeling indicates that the upper and middle crust are probably of granitic composition that the mid crustal low p zones have high quartz content and that the lower crust probably consists of granulites of laterally varying bulk composition from granodioritic to gabbroic the lower crust is more mafic in areas of high reflection density on seismic reflection profiles suggesting that the laminations are caused by mafic intrusions into the lower crust either prior to the variscan orogeny or during tertiary volcanic activity

variscan intracontinental deformation the coimbra cordoba shear zone sw iberian peninsula the coimbra cordoba shear zone is a major lineament roughly parallel to the nw se variscan structures of the southern iberian peninsula inside the shear zone evidence for a non coaxial strain path exists from structural microstructural and lattice preferred orientation data on quartz plane strain is suggested by the study of microstructures in the xy and yz sections in all formations occurring in the shear zone with increasing strain a mylonitic series develops the deformation contemporaneous with epizonal to mesozonal metamorphism is responsible for 1 1 the regional subvertical foliation oblique to the shear zone 2 2 the well defined subhorizontal lineation striking nw se 3 3 the discrete planes of shear roughly parallel to the shear zone the data are interpreted as resulting from a left lateral slip motion with a displacement of at least 72 km in the light of plate tectonics it is inferred that the coimbra cordoba lineament could have been a suture zone which evolved into an intracontinental left lateral shear zone during the variscan orogeny of the ibero armorican arc

erratum to a new style of ni cu mineralization related to magmatic breccia pipes in a transpressional magmatic arc aguablanca spain the aguablanca deposit represents a new style of magmatic ni cu mineralization in discordant sulfide rich pyroxenitic breccia pipes the orebody is hosted by variscan calc alkaline diorites and gabbros which intruded during an oblique subduction collision event transpressional transtensional left lateral structures facilitated the intrusion of primitive magmas to shallow depths in the crust a two stage genetic model is proposed in the first stage a transitional deep magma chamber formed the primitive magma interacted at depth with wall rocks resulting in extensive crustal contamination concomitant sulfide magma immiscibility and settling of orthopyroxene clinopyroxene and sulfide rich cumulates to form a layered magmatic complex geochemical and mineralogical evidence including the virtual disappearance of olivine heavy 34s values 7 4 distinctive nd sr and pb signatures high au contents and the presence of spinel and graphite indicate a major interaction with the upper crust probably with pyrite rich carbonaceous slates of late proterozoic age the second stage was related with the emplacement of residual calc alkaline gabbroic to noritic melts and the development of an intrusive breccia containing fragments of the consolidated layered complex rocks and associated disseminated to massive sulfides

lithospheric transition from the variscan iberian massif to the jurassic oceanic crust of the central atlantic abstract a 1000 km long lithospheric transect running from the variscan iberian massif vim to the oceanic domain of the northwest african margin is investigated the main goal of the study is to image the lateral changes in crustal and lithospheric structure from a complete section of an old and stable orogenic belt the variscan iberian massif to the adjacent jurassic passive margin of sw iberia and across the transpressive and seismically active africa eurasia plate boundary the modelling approach incorporates available seismic data and integrates elevation gravity geoid and heat flow data under the assumptions of thermal steady state and local isostasy the results show that the variscan iberian crust has a roughly constant thickness of 30 km in opposition to previous works that propose a prominent thickening beneath the south portuguese zone spz the three layers forming the variscan crust show noticeable thickness variations along the profile the upper crust thins from central iberia about 20 km thick to the ossa morena zone omz and the ne region of the south portuguese zone where locally the thickness of the upper crust is

thermal histories and overthrusting application of numerical simulation technique abstract overthrust belts frequently contain potential exploration targets and an assessment of the effects of overthrusting on temperature histories and hydrocarbon hc generation can reduce exploration costs however many models oversimplify the problem for example by neglecting the physical and thermal characteristics of lithologies or by using instantaneous thrust sheet emplacement rates the adaptation of numerical simulation techniques to handle overthrusting in a two dimensional simulation enables the emplacement of a previously deposited and therefore physically and thermally well defined sedimentary sequence over a sequence or partial sequence of previously deposited units the combined effects of factors such as complex lateral and vertical lithological variations thrust sheet thicknesses movement rates compaction and forced fluid flow can be taken into account the method is demonstrated by its application to a schematic model the following variables are tested thrust sheet emplacement rates thermal conductivities and heat flow a thickness of 5 km is used in order to limit the total amount of variables the results show that simple one dimensional models can considerably overestimate temperature differences and effects across thrust sheet planes through time this can lead to incorrect conclusions on the establishment of a temperature equilibrium in the thrusted sequence and on hc generation timing

thrust loads and foreland basin evolution cretaceous western united states two dimensional modeling of loading during the formation of the idaho wyoming thrust belt shows that regional isostatic compensation by flexure of an elastic lithosphere is sufficient to control the formation of a foreland basin the flexural rigidity of the lithosphere is inferred to have been approximately 1023 nm 1030 dyne cm on the basis of palinspastic comparison of predicted downwarping due to the thrust plate loads to the shape of the sedimentary wedge on the west side of the cretaceous western interior seaway erosion of part of the uplifted thrust plates redistributed the load depositing it farther to the east thereby causing subsidence over a much wider area than could have been accomplished only by the loading by thrust plates paleotopography after major cretaceous thrust events was calculated the resulting mountainous terrain gentle alluvial plain and flat sea floor correspond well with the topography of the modern foreland thrust belt and basin system in the andes of south america and to paleogeographic reconstructions in the western united states thrust belt topography is controlled by the subsurface geometry of the thrust faults particularly the positions of ramp zones and by isostatic subsidence

composition of the uralide crust from seismic velocity v p v s heat flow gravity and magnetic data abstract p wave velocity vp s wave velocity vs poisson s ratio heat flow potential field and surface geological data are integrated to constrain a model for the composition of the uralide crust along the urseis transect the model is constructed using published laboratory measurements of vp vs and density for a variety of crustal rock types these laboratory data have been corrected for depth pressure and the uralides temperature depth function the model shows clear differences between the composition of the old continental crustal nucleus of the east european craton and the newly added crust of the accreted arc terranes to the east the crust of the east european craton is more felsic than that of the magnitogorsk and east uralian zones and the latter two have a lowermost crust whose characteristics indicate a high garnet content mafic garnet granulite and or the presence of hornblendite the overall composition of the arc terranes is basaltic physical properties suggest that eclogite is not present in the arc terranes or if present it exists in small amounts that are below the resolution of the data set the lack of eclogite in the lower crust favours an intracrustal differentiation model for the evolution of the bulk composition of the continental crust nevertheless the absence of surface uplift the lack of metamorphism and late orogenic mantle melts and the current crustal thickness indicate that crustal thinning did not affect the bulk composition of the uralide crust

timing of organic maturation coalification relative to thrust faulting in the southeastern canadian cordillera abstract in order to access the effects if any of overthrusting tectonic burial on levels of organic maturation coalification detailed vitrinite reflectance profiles were constructed for 11 deep boreholes that penetrated stacked thrust sheets of mesozoic and paleozoic strata in the southeastern canadian cordillera recognition of the timing and degree of maturation resulting from emplacement of thrust sheets is largely dependent on the accuracy with which the preorogenic level of maturation of the strata is known the vitrinite reflectance profiles of several boreholes suggest that a significant component of organic maturation coalification postdates emplacement of overthrust sheets and have resulted from tectonic burial for other boreholes however the maturation profiles are ambiguous and suggest that postorogenic maturation if any was insufficient to overprint preorogenic levels of maturity maturity levels and reflectance gradients of mesozoic strata in general are lower in the southern part of the southeastern cordillera than in the northern part which imply higher paleoheat flow in the north

electromagnetic imaging of variscan crustal structures in sw iberia the role of interconnected graphite abstract the western part of the iberian peninsula iberian massif is the best exposed fragment of the variscan orogen in europe its southern half was generated by an oblique collision between three continental terranes belonging to the margins of laurassia avalonia the south portuguese zone spz and gondwana the ossa morena zone omz and the central iberian zone ciz the boundaries between them are considered to be suture zones a 200 km long magnetotelluric profile across the three variscan terranes was done in a nne direction approximately perpendicular to the main tectonic features the results of two dimensional inversion of the mt dataset reveal high conductivity zones coinciding with the transitions spz omz and omz ciz these conductive bodies related to the sutures at depth were interpreted as graphite enrichments along shear planes formed due to the overall transpressive regime a high conductivity layer extending along the whole omz was found at a depth of 15 25 km the top of which spatially correlates with a broad reflector detected by a recently acquired deep seismic reflection profile the high conductivity was interpreted as caused by the precambrian serie negra graphite rich rocks carbon and oxygen x ray mapping with electron microprobe on polished sections of serie negra samples from omz revealed the presence of interconnected graphite which supports the hypothesis that graphite is determinant for the high conductivity two graphite types which help to record the geological evolution were identified graphite accumulations in the schistosity surfaces produced by folding and metamorphism and metallic films of graphite developed along late faults the conductive layer shows blobs of higher conductivity suggesting macro anisotropy additional mylonitisation and shearing produced by thrusting at depth can be the origin of these zones of enhanced conductivity given that the detachment level is located within the serie negra several high resistivity features were found in the upper crust related to devonian and carboniferous successions and probably to some unexposed plutons in the spz and the palaeozoic series of omz plus some granitic intrusions in the ciz a high resistivity zone extending to the whole crust is correlated with extensive late variscan granite intrusions

paleozoic and grenvillian structures in the southern appalachians extended interpretation of seismic reflection data interpretive reprocessing of seismic reflection data has elucidated paleozoic and grenvillian structures in the southern appalachians the seismic data include a 7500 km grid of adcoh seisdata and cocorp reflection profiles that traverse the blue ridge and inner picdmont geologic provinces of north carolina south carolina and georgia surface geology and potential field data were used to constrain the interpretation the reprocessed seismic reflection data have delineated the internal and external geometry of the crystalline blue ridge inner piedmont allochthon including the locations of the blue ridge master decollement haycsville fault and brevard fault zone on the basis of the reprocessed data all of the major faults within the allochthonous upper crust sole in the blue ridge master decollement reflections extending to the southeast from beneath the surface location of the hayesville fault to the blue ridge thrust might be the seismic signature of a high strain zone this implies that internal deformation of the blue ridge allochthon associated with the alleghanian orogeny might have occurred farther to the west than has been previously documented from field studies relative amplitude seismic data enabled the discrimination between blue ridge inner piedmont crystalline rocks and underlying lower paleozoic shelf strata thereby delineating the blue ridge thrust the interpreted geometry constrains the top of the shelf sequence beneath the blue ridge to depths of less than 3 km this relatively shallow depth of the shelf strata together with the presence of duplex structures and bright spots that are imaged within the sequence might imply favorable conditions for hydrocarbon exploration beneath the blue ridge midcrustal reflections from within the upper to iower crust are interpreted to originate from preserved grenvillian structures that were reactivated at the basement surface during late proterozoic early cambrian extension reflection continuity is occasionally disrupted by interpreted post grenvillian pre early cambrian low density intrusions topography at the basement surface possibly caused by the intrusions is interpreted to have controlled the formation of some of the structures within the overlying allochthon including blue ridge and brevard fault zone ramps correlation of seismic time structure contour maps with available gravity data and two dimensional gravity modeling suggest that anomalies in the gravity field can be attributed to low density sources within the autochthonous crust discontinuous reflection packages from depths of 36 42 km are interpreted to originate from the mohorovi i discontinuity the reflectors trend about n15 e with a true dip of approximately 15 nw

the structure of a major suture zone in the sw iberian massif the ossa morena central iberian contact abstract we have investigated the stratigraphy structure and metamorphism of the boundary between the ossa morena zone omz and the central iberian zone ciz two significant continental portions of the variscan iberian massif the omz ciz contact is marked by a strongly deformed and metamorphosed nw se trending narrow band namely the central unit in which partially retrogressed eclogites are included during the middle late devonian the ciz overthrust the omz and in the footwall km scale recumbent folds and thrusts developed with decoupling and underplating of the lower crust at the same time in the hanging wall there took place intense though localized back folding and back shearing in the early carboniferous a transtensional tectonic regime sank the overthrust block resulting in the exhumation of eclogites these eclogites probably came from the underthrust omz lower crust and they are at present included in the suture zone central unit of this continental collision the extension is responsible for the origin of a basin and bimodal magmatism on the southern border of the ciz a late episode of folding and fracturing significantly contributed to the final complex picture of this suture boundary

seismic reflection imaging over the south portuguese zone fold and thrust belt sw iberia 1 we reprocessed an 35 km long part of the ibesreis seismic reflection profile which runs over the iberian pyrite belt section of the south portuguese zone sw iberia with the goal to image the upper crust 15 km depth the applied processing sequence enhanced numerous prominent reflections and diffraction patterns within the uppermost 5 s travel time relative to high amplitude source generated noise a complex subsurface characterized by conflicting dips and a survey following winding roads require a crooked line prestack migration scheme for coherent imaging to interpret sources of diffracted energy we additionally employed a diffraction imaging scheme which enhances diffractions at the expense of reflections the final seismic images show south vergent imbricate fold and thrust tectonics documenting the contractive deformation that the south portuguese zone experienced during the variscan orogeny based on surface geological information we correlate a low reflective unit with the shallow upper carboniferous flysch group a highly reflective unit ranging in depth from 2 to 4 km with the middle carboniferous volcano sedimentary complex group which hosts massive sulfide deposits and a moderately reflective unit with the upper devonian phyllite quartzite group below these units another low reflective facies is present which may represent older paleozoic metasediments in addition the seismic and diffraction images reveal bands of high reflectivity and distinct diffraction patterns that were interpreted as extensive layered mafic intrusions these proposed mafic bodies may be related to the same event that triggered a huge hydrothermal activity assumed in early carbonifereous times

geopressuring mechanism of organic matter cracking numerical modeling since the 1950s hydrocarbon generation has been considered as one of the most important abnormal pressuring mechanisms hydrocarbon generation effects on pore pressure evolution 1 it increases the pore fluid volume through the conversion of solid kerogen into liquid or gaseous hydrocarbon and 2 it decreases the sediment permeability because hydrocarbons remain an immiscible phase in pore water in this paper the two products of organic matter decomposition liquid hydrocarbon oil and gaseous hydrocarbon methane are assumed to occur in sequence as a supplementary phase in pores as a basis for evaluating the effect of this pressuring mechanism a simplified three stage first order kinetic reaction model is applied to simulate the generation of oil and gas we u e a numerical basin model with a two phase hydrodynamic formulation to calculate the effect of organic maturation on geopressure evolution oil generation does not play an important role except when the organic matter content of rocks is relatively large 5 however the generation of gas has a large influence the results show that beneath a certain depth oil to gas cracking strongly influences overpressuring the effect of this mechanism becomes more important with increasing organic matter content of rocks several environmental conditions such as kerogen type temperature gradient and lithological characteristics influence to various degrees the effect of organic matter maturation on overpressure development

rheic ocean ophiolitic remnants in southern iberia questioned by shrimp u pb zircon ages on the beja acebuches amphibolites 1 the rheic ocean was a major oceanic domain between avalonia and gondwana in ordovician silurian times most of the paleozoic plate reconstructions assume that the rheic ocean suture lies within southern iberia coinciding with the contact between the south portuguese zone and the ossa morena zone this paper reports four sensitive high resolution ion micro probe shrimp u pb zircon ages from mid ocean ridge basalt morb featured rocks of the beja acebuches amphibolite unit which crops out along the boundary between the ossa morena and the south portuguese zone and is considered its most conspicuous suture unit the obtained ages range from 332 3 to 340 4 ma corresponding to the crystallization of the mafic protoliths these early carboniferous ages for the beja acebuches amphibolites imply that this unit can no longer be viewed as an ophiolite belonging to the rheic ocean suture since this oceanic domain was presumably closed in devonian times tectonic reconstructions joining in a single suture line the beja acebuches amphibolite unit in southern iberia to either the devonian lizard ophiolite in southern england or the root zone of the devonian ordovician ophiolitic units in northwest iberia must be therefore reconsidered because of the age difference we interpret the beja acebuches amphibolite unit to represent a narrow and very ephemeral realm of oceanic like crust that opened in early carboniferous times after total consumption of the rheic ocean we suggest that a mantle plume underneath southern iberia in early carboniferous times is the most plausible large scale geodynamic scenario for the formation of these morb featured rocks

contrasts in thermal maturity within terranes and across terrane boundaries of the franciscan complex northern california the franciscan subduction complex of northern california includes several tectonostratigraphic units we have used the technique of vitrinite reflectance to document levels of thermal maturity for the central belt and the yager coastal and king range terranes spatial variations in thermal maturity help define the thermal structure of each franciscan terrane and each terrane displays a different geometry of thermal structure these differences are controlled by the style of structural deformation the overprinting effects of deformation following metamorphism and thermal overprinting caused by hydrothermal circulation anomalies along and contrasts across terrane boundaries help establish the relative timing and the structural mechanisms of terrane amalgamation for example localized anomalies are evident along the eel river fault central yager boundary and these increases in thermal maturity may be due to shear heating hydrothermal discharge or conductive heat transfer across the fault cross c

influence of overthrusting on maturation of hydrocarbons in phosphoria formation wyoming idaho utah overthrust belt geochemical analyses were performed on 11 phosphoria formation samples collected throughout the wyoming idaho utah overthrust belt maturation models incorporating the thermal effects of thrust faulting with lopatin s techniques were then used to interpret these geochemical data in particular four of the sample sites were selected for detailed thermal modeling these four sites include one sample site in an area unaffected by thrusting two sites located in the hanging walls of major thrusts and one site in the footwall of the darby thrust the detailed thermal modeling shows overthrusting did affect the thermal history of the phosphoria formation the effect of thrusting was to cool the overriding sheet and warm the sediments being overridden thus the hanging wall samples had a different post thrusting thermal history than the footwall sample cooling of the hanging wall slowed post thrusting maturation in the two thrust sheet samples in contrast the footwall sample was exposed to higher temperatures after thrusting and the maturation process was accelerated results of this study indicate that tectonism influences the thermal history of the rocks and should be included in any reliable source rock evaluation program involving overthrust strata a simple evaluation based only on the thickness of the overburden in general will not be sufficient to explain fully the maturation data in the absence of actual geochemical information such as ro tai or cai the models described in this paper appear to be sufficiently accurate to give a reasonable estimate of thermal maturity of a potential source bed

a 1 5d model of the steady state thermal structure of detachment folds abstract we present a 1 5d steady state model of thermal perturbations in the core of detachment anticlines the model incorporates the effects of heat conduction advection by folding radiogenic sources and conversion of work into heat by viscous dissipation we work out analytical solutions of model s equations for heat flow and temperature field for the case of low amplitude detachment folds growing in a thick homogeneous stratigraphy our analytical solutions for the simplest case of no heat sources show that detachment folds develop a thermal boundary layer conduction of heat dominates inside the layer whereas advection of heat by folding prevails outside the boundary layer we also find that heat flow in a growing detachment fold decays exponentially with the square of the thickness of the detached stratigraphic section when heat sources are included into the problem we find that radiogenic heat generates thermal anomalies at the crest of the anticlines whereas viscous dissipation produces thermal anomalies at the core of the detachment fold we analyze three cases of geological interest detachment folds growing in evaporitic sequences marine carbonates and shaley sediments for a typical uplift rate of 2 mm yr conduction dominates in the first class of rocks viscous dissipation dominates in folds growing in limestone due to its high viscosity advection and the generation of heat by radiogenic isotopes dominate the growth of detachment folds in shale additionally the cooling histories of lagrangian particles embedded in the fold for the last two cases are analyzed the core of the fold is characterized by rapid cooling whereas the limbs cool at slower rates than the core thus the development of detachment anticlines may either accelerate maturation of source rocks and expand the maturation window or halt it by removing source rocks from the generating window

oil generation in overthrust belts the burial of immature sediments beneath a thrust sheet may result in sufficient heating to generate hydrocarbons we present a model for the thermally activated generation of oil from kerogen and the subsequent destruction of the oil through cracking using this oil generation model in conjunction with a model applicable to the thermal evolution of overthrust belts we have studied the evolution of oil in sediments beneath a thrust sheet composed of sedimentary rocks oil generation may begin soon after emplacement of the thrust sheet beneath thick thrust sheets 8 km all oil in the sedimentary section may be destroyed less than 5 m y after thrusting we apply our results to the timing of oil generation in the sedimentary section beneath the absaroka thrust plate in the fossil syncline of western wyoming our calculations indicate that the paleozoic and a part of the mesozoic section were thermally mature prior to emplacement of the absaroka plate the remaining part of mesozoic sediments matured only after thrusting our results are in agreement with warner s 1980 observations that oil being produced from reservoirs in the absaroka plate was generated in the underthrust mesozoic section

shear wave modeling and poisson s ratio in the variscan belt of sw iberia in 2003 two wide angle reflection refraction seismic transects were acquired in the variscan belt of sw iberia the approximately 250 and 300 km long dense trace spacing transects revealed clear s wave arrivals in the shot gathers recorded by vertical component sensors in both transects first s wave arrivals sg and moho reflections sms are the most prominent phases that can be correlated from shot to shot sg is observed up to relatively large offsets and constrains the upper and middle crust s wave velocities the sms is seen from offset 0 18 s twtt to 150 km offset where it intercepts first s wave arrivals sg the upper mantle refracted phase sn is difficult to recognize although pms smp converted phases can be identified using a 2 d ray tracing approach two s wave velocity models for the crust of sw iberia were obtained these s wave velocity models complement the previous p wave velocity models and provide us with relatively well resolved poisson s ratio crustal sections for sw iberia the resulting poisson s ratio models present differences between tectonic zones at upper and middle crustal depths thus supporting the existence of different tectonic zones prior to the variscan collision the most noteworthy feature is the high poisson s ratio value over 0 28 coincident with high p wave velocity areas over 6 8 km s at midcrustal depths in order to constrain the possible crustal composition p wave velocities and poisson s ratios have been compared with published laboratory measurements on different crustal rock types this comparison indicates that the high p wave velocity and poisson s ratios are compatible with a mixture of mafic to ultramafic rock types alternating with felsic ones this result is consistent with the existence of mafic layered bodies in the middle crust in the same way that has been suggested by previous works in this area