a physical basis for the frictional melting of some rock forming minerals abstract under conditions of coseismic slip 0 1 2 0 m s 1 within the upper crust differences in the shear yield strengths fracture toughnesses melting points and to a lesser extent thermal conductivities of rock forming minerals result in a hierarchy of friction melting susceptibilities in many so called pseudotachylytes it is observed that phyllosilicates and amphiboles are preferentially consumed to form the melt phase whilst quartz and feldspar tend to survive as clasts this can be explained in terms of the disequilibrium flash melting of those mineral phases possessing the lower shear yield strengths fracture toughnesses and thermal conductivities rather than by eutectic melting that would otherwise generate equilibrium minimum melts indicative of igneous processes at high strain rates typical of coseismic faulting the mechanical behaviour of minerals is primarily controlled by their fracture toughnesses with progressive comminution during slip leading to fusion the friction melting dependence on the mechanical properties of minerals indicates that there may be an important relationship between wall rock lithology and earthquake magnitude if frictional melting results in fault lubrication

tricyclic terpane maturity parameters response to heating by an igneous intrusion the effects of thermal maturation upon the abundance and composition of the tricyclic terpanes have been investigated within a single sedimentary horizon within the dun caan shale member isle of skye scotland intruded by a 0 9 m thick tertiary dolerite dyke heating by the igneous body caused the concentrations of all 13 h 14 h and 13 h 14 h tricyclic terpanes to increase towards or within the effective oil window recording generation from abundant non hydrocarbon precursors such as kerogen and asphaltene bound components progressive changes in tricyclic terpane composition accompany these concentration changes an increase in the c23 maturity parameter towards the dyke resulted from the relatively greater rate of generation of the isomer combined with the earlier decline in concentration by degradation or isomerisation of the component the tricyclics tricyclics hopanes parameter shows a dramatic increase within the latter part of the oil window due to the greater relative thermal stability of the tricyclic terpanes the quantitative data demonstrate the importance of the processes of generation from non hydrocarbon fractions of the sedimentary organic matter and thermal degradation in the operation of tricyclic terpane maturity parameters

alkyldibenzofurans in terrestrial rocks influence of organic facies and maturation abstract dibenzofuran dbf dibenzothiophene dbt and phenanthrene phe have been determined along with their methyl derivatives mdbfs mdbts mps in coals carbonaceous shales and mudstones of lower cretaceous to upper carboniferous ages from the elmworth gas field canada and from the sakoa madagascar and ruhr coal germany basins the samples contained mainly terrestrial organic matter with thermal maturities in the 0 7 1 9 mean vitrinite reflectance rr range the pronounced predominance of mdbfs over mdbts in the sakoa samples is typical of plant debris deposited in lacustrine swamp environments onset of intense mdbf generation occurs at fairly low rank 0 77 rr as indicated by elevated concentrations of 50 120 g mdbfs g total organic carbon toc influences of lithology organic matter type and maturity on distributions of the target compounds were extracted from the molecular concentration data by multivariate statistical methods such as principal components analysis pca and fuzzy c means cluster analysis with nonlinear mapping nlm a specific source effect on 1 mdbt concentrations suggests that lichens contributed to the senescent coal forming forest in the sakoa coal basin alkyldibenzofurans hence are potential lichen biomarkers mdbfs can be used as maturity indicators only beyond 1 0 rr where ruhr coals displayed a distinct increase in the 1 4 mdbf ratio

crustal thicknesses in se brazilian shield by receiver function analysis implications for isostatic compensation hypothesis are proposed to explain the data 1 a lower density by 30 40 kg m 3 i n the lithospheric mantle under the archean block of the sao francisco craton relative to the proterozoic lithosphere is responsible for maintaining the high elevations in the plateau area relatively low density and high p wave velocity are compatible with a depleted low feo composition for the archean lithosphere 2 alternatively if the density contrasts between archean and proterozoic lithospheres are smaller than the values above then the crust beneath the paranabasin must be more dense than that of the craton higher crustal density and high poisson s ratio would be consistent with magmatic underplating in the lower crust beneath the paranabasin as inferred from other studies index terms 7205 seismology continental crust 1242 9360 information related to geographic region south america 7218 seismology lithosphere and upper mantle keywords crust upper mantle brazil parana basin san francisco craton

conductive versus convective thermal modelling of the bushveld complex time to decide a reply to the comment by cawthorn on robb and mungall 2020 earth planet sci lett 534 116084

lithospheric structure of the chaco and parana basins of south america from surface wave inversion surface wave data from a portable broadband array have been used to invert for the velocity structure of the crust and upper mantle beneath the chaco and parana basins of central south america the upper mantle velocity structure beneath the parana basin is cratonic in character whereas that beneath the chaco basin is tectonic or asthenospheric in character the surface wave analysis used broadband recordings from a subset of a 14 station array deployed in a roughly east west sawtooth arrangement along 20 s latitude with a total e w aperture of 1 400 km results from receiver function analysis as well as direct p wave regional travel time data were used in the inversions to help constrain moho depths and crust and upper mantle velocities s wave structure for the intracratonic parana basin was determined using interstation phase and group velocities for rayleigh waves fundamental and first higher mode and love waves fundamental mode only based on seven events with paths which traverse the eastern parana basin and one event with a path across the western parana basin the average moho depth in the eastern parana basin is 42 km the high velocity upper mantle lid has a maximum s wave velocity of 4 7 km s with no resolvable low velocity zone to at least 200 km depth this cratonic velocity structure indicates the presence of a lithospheric root beneath the parana basin despite emplacement of the parana plume the limited data from the western parana basin are consistent with a homogeneous upper mantle structure throughout the parana basin waveform inversion of fundamental mode and first higher mode rayleigh waves from a single subandean event was used to obtain estimates for pure path dispersion along propagation paths through the chaco basin and the western half of the parana basin the data were partitioned to isolate the partial path contribution of the phase and group velocities for the chaco basin the phase and group velocities from this somewhat sparse data set were inverted to obtain a velocity depth model for the chaco basin the distinguishing features of the chaco model consist of a rather shallow moho depth 32 km and low asthenospheric upper mantle s wave velocities about 4 2 km s with velocity increasing only slightly to about 4 3km s at 150 km depth

the proterozoic of eastern bolivia and its relationship to the andean mobile belt abstract the previously unmapped bolivian sector of the central brazil shield 220 000 km in area has been studied during an anglo bolivian technical co operation programme the metamorphic basement comprises the lomas maneches granulitic complex the chiquitania gneissic complex and the san ignacio schist supergroup which crops out as metasedimentary belts rb sr dates on some of the granulites suggest a protolith age of 2000 ma but the outcrop pattern of the metamorphic rocks as a whole is due to the 1300 ma san ignacio orogeny which was accompanied by widespread granite emplacement and metamorphism up to granulite facies the sunsas cycle began with the deposition of the supracrustal sunsas vibosi group and closed with the development of the sunsas and aguapei mobile belts at 1000 ma the upper proterozoic brasiliano cycle is represented by a bifurcating sedimentary basin in the se of the area with a closing tectonic event of 520 ma correlation is suggested with units in the central brazil shield the bolivian belts are extrapolated east into brazil and their limits indicated to the west the belts show trends parallel to the phanerozoic andean chain and they reflect the major deflection in the chain the arica elbow furthermore when the belts are traced towards the andes their ages broadly correspond to those of the sub andean precambrian basement this leads to the proposal of an enlarged andean orogenic belt system 1000 km wide comprising a 1300 ma old proto andean orogen which has controlled the trends of subsequent mobile belts

tectonic evolution of south america during the late proterozoic abstract this paper discusses the evolution and the late proterozoic assembly of the south american continent i e the west gondwana under the light of global tectonics the northern portion of the continent the amazonian craton is a pre brasiliano domain it may be a fragment derived from laurentia this domain behaved as a large continental plate which was accreted to the central and southeastern portions of the continent brasiliano domain the brasiliano domain had a more complex history and composition during the brasiliano cycle middle proterozoic to early paleozoic in the brasiliano domain several late proterozoic continental plates have been identified sao luiz west africa congo kasai sao francisco rio de la plata arequipa antofalla which acted as forelands to the brasiliano orogenic belts that surround and amalgamate these plates smaller fragments of pre brasiliano continental lithosphere are common in the interior of the brasiliano orogenic belts and played diversified roles microplates microcontinents structural highs some of them have acted as true backlands hinterlands flanked by arc magmatism two principal types of brasiliano orogenic belts are recognized between the continental plates a marginal orogenies to the late proterozoic plates with pelitic carbonatic rocks and discrete volcanism which were thrust by collision onto the margin of the plates and which seem to represent final products of previous passive margin sequences b distal orogenics internal belts within the continental masses sometimes a branching system of orogenics that contain varied clastic and minor carbonate sedimentary rocks accompanied by important bimodal volcanism and calc alkaline plutonism for these orogenies a greater variety of basinal scenarios and tectonic settings may be visualized the amalgamation of the continent during the late proterozoic i e the formation of the brasiliano structures indicates the existence of practically synchronous multilateral stresses within lithospheric plates this can be produced by complementary adjustment of the continental plates as a response to major global events elsewhere analogous to modern continental collisions two large and important tectonic zones may be candidates to such a scenario the hijaz mozambique zone site of connection of east and west gondwana and the transbrasiliano lineament which cuts across south america and extends to north africa following the zone of connection between pre brasiliano and brasiliano domains other small ocean openings and closings inside the various brasiliano areas dom feliciano gariep for instance were minor events and complementary to the other two major tectonic zones mentioned above

teleseismic p wave tomography beneath the pantanal parana and chaco parana basins se south america delimiting lithospheric blocks of the sw gondwana assemblage

deep seismic profile of the amazonian craton northern brazil deep seismic profiling of the amazonian craton northern brazil reveals previously unknown deep structural features buried beneath a thin relatively flat phanerozoic sedimentary cover amazon basin the seismic signature of the precambrian crust is characterized by a transparent shallow basement down to 8 9 km followed by a highly reflective middle and lower crust middle and lower crustal events are dominated by abundant dipping asymmetric arcuate reflections and diffractions the shallowest of which occur beneath a structural arch jutai arch although wide angle refraction data are unavailable a decrease in the number of coherent reflections at about 15 s two way travel time 45 km and small drops in amplitude and increases in average frequency are interpreted to mark the moho the deep amazonian seismic line is located near the westernmost boundary of the amazonian craton imaging the rondonian province one of the youngest geochronological provinces of the craton middle proterozoic to the south the rondonian basement crops out revealing schists gneisses granulites and important volumes of anorogenic granites the prominent laterally extensive arcuate reflections observed within the middle and lower crust may be related to this anorogenic magmatic activity perhaps corresponding to layered intrusive complexes on the other hand intrusion of mantle derived material during an early paleozoic rifting as well as the mesozoic episodes of mafic intrusion may also have contributed to this seismic signature similar seismic signatures found in eastern australia and on cocorp profiles of the central u s craton suggest that such cratonic terranes may have undergone similar tectonic evolution in the upper crust probable fault plane reflections associated with ene wsw reverse faults of jurassic early cretaceous age may sole into the top of the reflective zone suggesting that the lower limit of the transparent upper crust may represent a rheological boundary

experimental frictional heating of coal gouge at seismic slip rates evidence for devolatilization and thermal pressurization of gouge fluids high velocity 1 m s friction experiments on bituminous coal gouge display several earthquake related phenomena including devolatilization by frictional heating gas pressurization and slip weakening stage i is characterized by sample shortening and reduction in the coefficient of friction from 1 to 0 6 stage ii is characterized by high frequency 5 hz oscillations in stress and strain records and by gas emissions stage iii is marked by rapid weakening 0 1 to 0 35 and sample shortening together with continued gas emissions stage iv produces stable stress records and continued weakness 0 2 but without gas emission stage i shortening is due to compaction of the gouge and the weakening is attributed to mechanical or thermal effects stage ii behavior is interpreted as due to coal gasification and fluctuations in fluid pressure resulting in high frequency stick slip type behavior dramatic reduction in shear stress in stage iii is attributed to gas pressurization by pore collapse and corresponds to a frictional instability analogous to nucleation of an earthquake microstructural observations indicate the deformation was brittle during stages i and ii but ductile during stages iii and iv time dependent finite element frictional heat models indicate the center of the samples became hot 900 c during stage ii whereas the edge of samples remained relatively cold 300 c vitrinite reflectance of coal samples shows an increase in reflectance from 0 5 to 0 8 over the displacement interval 20 40 m 20 40 s indicating that the reflectance responds to frictional heating on a short time scale the energy expended per unit area in these low stress large displacement experiments is similar to that of higher stress 50 mpa short displacement 1 m earthquakes 107 j m2

the upper critical zone of the eastern bushveld complex precursor of the merensky reef the upper critical zone in the central sector of the eastern bushveld complex consists predominantly of norites although bronzitites anorthosites chromitites and other chromitic rocks appear at intervals in the sequence gabbronorites appear near the middle of the zone the rock sequence is divided into 14 principal units plus the series from the base of the merensky reef to the top of the bastard reef mineralogical compositions of successive units have been determined from modal analyses and compositions of bronzite and plagioclase in the various rock units have been determined by microprobe analysis the results indicate that although parts of the zone show normal fractionation the zone as a whole is not the product of progressive fractionation of a magma crystal settling appears to have played a relatively minor role in evolution of the zone successive pulses of parent magma into the chamber may account for some of the mineralogical and chemical variations but features of certain thick norite units suggest stratification of the magma in the chamber the study suggests that the merensky reef registers a sudden event in the bushveld magma chamber an event that altered the chemistry of the system and triggered precipitation of sulfides and platinoid minerals the alternative hypothesis that the reef is the culmination of a long period of magma fractionation is not supported by the evidence presented chromite occurs in a variety of associations with plagioclase and bronzite the associations are consistent with phase relations in the system mg 2 sio 4 iron oxide caal 2 si 2 o 8 sio 2 at elevated pressure

biomarkers heat up during earthquakes new evidence of seismic slip in the rock record during earthquakes faults heat up due to frictional work however evidence of heating from paleoearthquakes along exhumed faults remains scarce here we describe a method using thermal maturation of organic molecules in sedimentary rock to determine whether a fault has experienced differential heating compared to surrounding rocks we demonstrate the utility of this method on an ancient pseudotachylyte hosting megathrust at pasagshak point alaska measurements of the ratio of thermally stable to thermally unstable compounds diamondoids n alkanes show that the melt bearing rocks have higher thermal maturity than surrounding rocks furthermore the mineralogy of the survivor grains and the presence of any organic molecules allow us to constrain the temperature rise during the ancient earthquakes to 840 1170 c above ambient temperatures of 260 c from this temperature rise we estimate that the frictional work of the earthquake was 105 228 mj m 2 using experimental friction measurements as a constraint we estimate that the minimum slip necessary for heating was 1 8 m this paper demonstrates that biomarkers will be a useful tool to identify seismic slip along faults without frictional melt

thermal considerations in inferring frictional heating from vitrinite reflectance and implications for shallow coseismic slip within the nankai subduction zone abstract frictional properties within the upper few kilometers of subduction zones are generally thought to inhibit rupture propagation understanding whether large rapid slip propagates to the surface during megathrust earthquakes is important for characterizing tsunami hazard recent vitrinite reflectance analysis by sakaguchi et al 2011 on cores from the nantroseize drilling transect at the nankai trough japan has been interpreted to suggest that these faults reached temperatures 380 c considerably larger than background temperature values implying they hosted coseismic slip at shallow depths analysis of other temperature proxies on the megasplay by hirono et al 2009 however suggests temperatures have not exceeded 300 c and is inconclusive as to whether the fault slipped at high velocity we evaluate the effects of frictional heat generation on the spatial distribution of vitrinite reflectance its sensitivity to slip zone thickness and slip duration and the cumulative effects of numerous events we build on the analysis of sakaguchi et al 2011 by estimating frictional heating scenarios that are consistent with both the peak and spatial extent of anomalous vitrinite reflectance data our results imply coseismic slip magnitudes of several 10s of meters peak temperature estimates from the vitrinite reflectance data can be reconciled with the other geochemical constraints only by assuming the vitrinite reflectance results from the cumulative effects of multiple earthquakes however this results in unrealistically large estimates of total displacement our results imply that current understanding of how vitrinite reflectance is affected by fault slip is incomplete

organic thermal maturity as a proxy for frictional fault heating experimental constraints on methylphenanthrene kinetics at earthquake timescales abstract biomarker thermal maturity is widely used to study burial heating of sediments over millions of years heating over short timescales such as during earthquakes should also result in measurable increases in biomarker thermal maturity however the sensitivity of biomarker thermal maturity reactions to short higher temperature heating has not been established we report on hydrous pyrolysis experiments that determine the kinetic parameters of methylphenanthrene maturation at timescales and temperatures relevant to earthquake heating samples of woodford shale were heated at temperatures up to 343 c over 15 150 min the thermal maturity of the samples as measured by the methylphenanthrene index 1 mpi 1 increased with heating time and temperature we find that mpi 1 increases with time and temperature consistent with a first order kinetic model and arrhenius temperature relationship over the timescales tested here mpi 1 is strongly affected by maximum temperature and less sensitive to heating duration production of new phenanthrene isomers and expulsion of a liquid pyrolyzate also occurred differential expulsion of methylphenanthrene isomers affected the apparent maturity of the rock at lower temperatures and may need to be considered for organic rich fault rocks our results demonstrate that the overall mpi 1 reaction extent in both the rock and pyrolyzate are a useful measure of thermal maturity and reflect temperature history during rapid heating

seismic heating signatures in the japan trench subduction plate boundary fault zone evidence from a preliminary rock magnetic geothermometer frictional heating during earthquake rupture reveals important information on earthquake mechanisms and energy dissipation the amount of annealing varies widely and is as yet poorly constrained here we use magnetic susceptibility versus temperature measurements during cycling to increasingly elevated temperatures to constrain the maximum temperature a slip zone has experienced the case study comprises sheared clay cored from the japan trench subduction plate boundary fault zone decollement which accommodated the large slip of the 2011 mw 9 0 tohoku oki earthquake the decollement was cored during the integrated ocean drilling program iodp expedition 343 the japan trench fast drilling project jfast heating signatures with estimated maximum temperatures ranging from 300 to over 500 c are determined close to the multiple slip surfaces within the decollement since it is impossible to tie a specific slip surface to a certain earthquake thermal evidence for the cumulative effect of several earthquakes is unveiled this as yet preliminary rock magnetic geothermometer would be a useful tool to detect seismic heating along faults that experienced medium temperature rise a range which is difficult to assess with other approaches

deep crustal structure of the parana basin from receiver functions and rayleigh wave dispersion evidence for a fragmented cratonic root 1 the deep crustal structure of the parana basin of southern brazil is investigated by analyzing p and pp wave receiver functions at 17 brazilian lithosphere seismic project stations within the basin the study area can be described as a typical paleozoic intracratonic basin that hosts one of the largest large igneous province of the world and makes a unique setting for investigating models of basin subsidence and their interaction with mantle plumes our study consists of 1 an analysis of the moho interaction phases in the receiver functions to obtain the thickness and bulk vp vs ratio of the basin s underlying crust and 2 a joint inversion with rayleigh wave dispersion velocities from an independent tomographic study to delineate the detailed s wave velocity variation with depth the results of our analysis reveal that moho depths and bulk vp vs ratios including sediments vary between 41 and 48 km and between 1 70 and 1 76 respectively with the largest values roughly coinciding with the basin s axis and that s wave velocities in the lower crust are generally below 3 8 km s select sites within the basin however show lower crustal s wave velocities slightly above 3 9 km s suggestive of underplated mafic material we show that these observations are consistent with a fragmented cratonic root under the parana basin that defined a zone of weakness for the initial paleozoic subsidence of the basin and which allowed localized mafic underplating of the crust along the suture zones by cenozoic magmatism

coal maturation by frictional heat during rapid fault slip 1 the detection of frictional heating effects along faults provides key insight into the dynamics of earthquakes and faulting thermal maturity of organic matter has been considered a possible fault thermometer that records the frictional heat signature of ancient earthquakes however whether or not organic matter can mature on the order of seconds typical earthquake rise time remains uncertain here we present the results of experiments aimed at revealing coal maturation by frictional heat generated at slip velocities representative of natural earthquakes of up to 1 3 m s our results show that coal can mature coseismically in 11 seconds at temperatures induced by frictional heat ranging from 26 to 266 c even with a temperature rise to only 28 7 c over 15 m displacement in 3 2 hours coal can slightly mature within a shear localized zone the commonly used kinetic model of vitrinite maturation cannot predict the experimental results a kinetic model involving the effect of flash temperature at grain contacts and mechanochemical effects on reaction kinetics is necessary to better estimate heat generation along a fault

upper mantle structure of south america from joint inversion of waveforms and fundamental mode group velocities of rayleigh waves 1 a new tomographic s wave velocity model for the upper mantle beneath south america is presented we developed and applied a new method of simultaneously inverting regional s and rayleigh waveforms and fundamental mode rayleigh wave group velocities to better constrain upper mantle s velocity structure and moho depth we used 5700 rayleigh wave group velocity dispersion curves and 1537 regional wave trains with paths principally passing through the south american continent the joint inversion of this data set provided a new three dimensional 3 d upper mantle s velocity model and a moho depth model for south america which fits both the group velocity and regional waveform data sets well new features of the final three dimensional 3 d s velocity and moho depth model correlate well with known geotectonic units on a regional scale the moho depth ranges from 30 km in the central chaco basin to 42 km beneath the amazonian craton and 45 70 km beneath the orogenic andean belt the imaged s velocity indicates an average lithosphere thickness of around 160 km for the amazonian craton high velocities are imaged beneath the amazon and part of the parana and parnao ba basins down to about 150 km low to very low velocities are imaged

pseudotachylytes rarely generated rarely preserved or rarely reported pseudotachylyte is the only fault rock that is known to form exclusively at seismic slip rates so it is unique in preserving direct evidence of the dynamic processes in action during earthquakes it is commonly assumed that pseudotachylyte is rare and debate has centered on whether it is rarely generated or commonly generated but rarely preserved we present field and electron microscope observations of eight new pseudotachylytes from faults in the sierra nevada that have previously been the focus of many detailed studies of fault growth and mechanics these pseudotachylytes range from being abundant and easy to recognize in outcrop to being impossible to identify without microscope observations our data show that pseudotachylytes are much more common in the sierra nevada than has previously been reported we suggest that pseudotachylytes may be present within many fault zones but remain unreported primarily due to difficulty in identifying very thin or reworked pseudotachylytes in the field and therefore the use of these fault rocks to interpret dynamic earthquake processes must be revisited

neoproterozoic backarc basin sensitive high resolution ion microprobe u pb and sm nd isotopic evidence from the eastern pampean ranges argentina the eastern pampean ranges comprise high grade supracrustal sequences with linear belts of mafic ultramafic bodies representing ophiolite remnants new u pb and nd isotopic data suggest that the tectonic evolution of the pampean ranges started ca 640 ma with the deposition of supracrustal sequences in a backarc basin between a neoproterozoic magmatic arc to the east and the pampia terrane to the west ophiolite remnants of this backarc basin yielded a whole rock isochron indicating the age of 647 77 ma 2 and nd initial time t of 5 2 sensitive high resolution ion microprobe u pb data for detrital metasediments show provenance patterns with two main age populations the older 1 1 0 9 ga and the younger between ca 0 7 and 0 6 ga the neoproterozoic population is relatively more abundant in sediments of the easternmost units of the eastern pampean ranges and becomes less abundant toward the west depleted mantle ages show a similar pattern with ages generally increasing from east 1 42 ga to west 1 76 ga suggesting the presence of neo proterozoic sources to the east of the ranges the provenance data do not support previous evolution models for the eastern pampean ranges according to which the supracrustal sequence represents the passive margin of the rio de la plata craton early cambrian collision and high grade metamorphism mark the final stages of evolution of the belt and were shortly followed by calc alkaline metaluminous and peraluminous granitic magmatism ca 530 514 ma the results suggest that the geological evolution of the eastern pampean ranges took place between ca 640 and 514 ma coeval with other brasiliano orogens in brazil e g the paraguay and araguaia fold belts

reaction kinetics of alkenone and n alkane thermal alteration at seismic timescales recent experiments and field observations have indicated that biomarker molecules can react over short timescales relevant to seismic slip thereby making these compounds a useful tool in studying temperature rise in fault zones however short timescale biomarker reaction kinetics studies have previously focused on compounds that have already experienced burial heating here we present a set of hydrous pyrolysis experiments on pleistocene aged shallow marine sediment to develop the reaction kinetics of long chain alkenone destruction change in the alkenone unsaturation ratio uk 37 and change in the n alkane chain length distribution our results show that biomarker thermal maturity provides a useful method for detecting temperature rise in the shallow reaches of faults such as subduction zone trench environments through the course of our work we also noted the alteration of total alkenone concentrations and uk 37 values in crushed sediments stored dry at room temperature for durations of months to years but not in the solvent extracts of these materials this result though parenthetical for our work in fault zones has important implications for proper storage of sedimentary samples to be used for alkenone paleotemperature and productivity analysis this article is protected by copyright all rights reserved

paraguay araguaia belt conductivity anomaly a fundamental tectonic boundary in south american platform imaged by electromagnetic induction surveys 1 we discovered and mapped a 1200 km long zone of enhanced electrical conductivity through a large array of magnetometers over a narrow corridor parallel to the paraguay and araguaia belts which surround the amazon craton the anomaly has been further studied using magnetotelluric measurements made along a 300 km profile crossing the paraguay belt modeling shows the anomaly at this location as a very strong 100 km wide upper crust to mid crust conductor located under a region where surficial rocks show an increase in deformation and metamorphism within the belt from the high conductivity of the anomalous structure its observed geometry and surface rock exposures the most likely interpretation of its source is graphitized biogenic material in metasediments now deeply underthrust in a neoproterozoic or early cambrian suture zone this result strongly supports the hypothesis that the collision between the amazon plate and the western parana block has probably closed an ocean in the paraguay belt region contrary to former propositions of an ensialic evolution for this belt although the precise age of this ocean closure have been a matter of debate our finding significantly constrains the tectonic setting of west gondwana amalgamation involving the amazon plate and its surrounding blocks

extractable organic material in fault zones as a tool to investigate frictional stress abstract the detection of a frictional heating signature is an attractive but elusive way to measure frictional stress on exhumed faults with no evidence of melting we present a new paleothermometer for fault zones that utilizes the thermal maturity of extractable organic material to determine the maximum frictional heating experienced by the fault because there are no retrograde reactions in these organic systems maximum heating is preserved we investigate the punchbowl fault an ancient strand of the san andreas fault in southern california according to the methylphenanthrene index for thermal maturity there is no differential heating between the fault and off fault samples indicating that the fault did not get hotter than the surrounding rock during slip additionally the thermal maturity of the rocks falls within the range of heating expected from the bounds on burial depth and time and adds some constraint to the burial depth range simple heating models indicate that the fault must have had either 1 an extremely low shear stress during slip 2 an active slipping zone thicker than 1 mm or 3 earthquakes smaller than m6 to lack a differential heating signature after 44 km slip

the puncoviscana trough a large basin of late precambrian to early cambrian age on the pacific edge of the brazilian shield during the late precambrian to early cambrian a large basin with a homogeneous psammitic pelitic sediment fill existed in the area of the nw argentine andes it is now exposed in different tectonic levels this basin of meridional elongation was situated on a stable continental margin at the western edge of the brazilian shield it was underlain by segmented older continental crust according to the modal and chemical composition of the greywackes and subgreywackes a predominant metasedimentary source may be supposed four clastic facies types indicate a transport by gravity currents within a submarine fan system while massive red pelites represent cut off periods and rare carbonates deposition on rises the gravity currents derived from easterly directions the sedimentation age is proved by a rich ichnofauna and some medusoid impressions

proterozoic early paleozoic evolution in western south america a discussion abstract the hypothesis of exotic terranes in peru bolivia argentina and chile generated discussions on the mode of transfer and extent of accretional events that may have occurred in the southern andes during the late proterozoic early paleozoic initially a tectogenesis based on autochthonous mobile fold belts was discussed following ideas emphasised the fragmentation of the supercontinent rodinia laurentia moving along the west gondwana border and colliding with the gondwana western margin the most important effect of this laurentia gondwana relationship was attributed to the argentine precordillera or cuyania terrane splitting off from laurentia and docking to gondwana in the early paleozoic in this study the most cited arguments for this laurentia precordillera relationship are discussed emphasising paleontological considerations it is shown that these arguments do not exclude a close original vicinity of the precordillera terrane to gondwana the precordillera terrane is suggested to be part of a hypothetical platform which developed between south america africa and antarctica safran platform and which was displaced to its actual position by transcurrent faults the collisional events in the sierras pampeanas ensued from strike slip movements and were responsible for the s and i type transpressional magmatism along the pampean and famatinian terranes the final result of this continent parallel movement of terrane slices is similar to that of a terrane split off from laurentia but the first named way of formation easier explains the general continuity of plate convergence at the western border of gondwana than the laurentia precordillera connection does

cooling of the bushveld complex south africa implications for paleomagnetic reversals igneous rocks record the direction of the earth s magnetic field as they cool through their curie temperature the mafic magmas of the 8 km thick bushveld complex of south africa took 65 k y to be emplaced 180 k y to solidify to 900 c and a further 500 k y for the entire intrusion to cool below 580 c the curie temperature of magnetite once solid the cooling of this intrusion occurred mainly from the top downward with slower cooling through its floor as a result the upper rocks cooled through their curie temperature before those at the base the portion 6 km below the upper contact was the last to reach the curie temperature thus the intrusion records a mainly top down sequence of three paleomagnetic reversals starting with n normal direction the last two are also recorded from the base of the mafic sequence upward as it cooled through 580 c later than the top the lateral variations in thickness of the bushveld complex are important in this interpretation because thinner sections cooled more quickly hence reversals do not always correlate with stratigraphy specific reversals provide a cooling marker horizon that may crosscut the stratigraphic layering the interpretation of the order and number of paleomagnetic reversals presented here differs from previous interpretations that envisage the oldest paleomagnetic directions to be recorded sequentially from the base upward and has implications for the interpretation of paleomagnetic results from all thick intrusions mafic and felsic

the new esa satellite only gravity field model via the direct approach 1 reprocessed gravity field and steady state ocean circulation explorer goce gravity gradient data were combined with data from laser geodynamics satellite lageos 1 2 and gravity recovery and climate experiment grace to generate a satellite only gravity field model to degree 260 using the direct approach named dir r4 when compared to earth gravitational model 2008 egm2008 it is more accurate at low to medium resolution thanks to goce and grace data when compared to earlier releases of esa goce models it is more accurate at high degrees owing to the larger amount of data ingested it is also slightly more accurate than esa s fourth release of the time wise model tim r4 as demonstrated by gps leveling orbit determination tests and an oceanographic evaluation according to the formal probably too optimistic by a factor of 2 2 5 cumulated geoid 1 3 cm and gravity anomaly 0 4 mgal errors at 100 km resolution the goce mission objectives have been reached

application of aromatic compounds as maturity indicators in source rocks and crude oils concentration ratios of aromatic compounds such as naphthalenes phenanthrenes and dibenzothiophenes are at present attracting increasing attention as maturity indicators in sedimentary rocks and crude oils generally these indicators rely either on an increase with maturity in the degree of alkylation of a given parent compound or a shift in the isomer distribution of alkyl aromatic homologues towards thermally more stable isomers a combination of both concepts in the methylphenanthrene index mpi resulted in an excellent maturity parameter as demonstrated for rock extracts by an improved correlation with mean vitrinite reflectance rm between 0 65 and 1 4 i e in the zone of oil formation in cases that rm measurements were ambiguous this empirical relationship has proved useful in the calculation of theoretical vitrinite reflectance rc values from the mpi on the other hand for potential petroleum source rocks reaching maturity at relatively low temperatures the influence of the organic matter type on the aromatic distributions cannot be neglected consequently organic facies effects should be taken into account in the interpretation of aromatic distributions of such source rocks and related crude oils since the impact of the organic facies is strongly reduced with increasing maturity aromatic maturity parameters are particularly useful in the maturity evaluation of post mature crude oils and condensates in contrast to biological markers substantially higher concentrations of the key alkyl aromatics persist at elevated maturation levels thus aromatic maturity parameters should be considered as versatile tools in organic maturation studies and can complement established maturity indicators based on steranes and terpanes

disappearing ink how pseudotachylytes are lost from the rock record abstract melt origin pseudotachylytes are the most widely accepted feature recording earthquake slip in the fault rock record however reports of pseudotachylytes are rare compared to the frequency and distribution of earthquakes in active faults suggesting melting occurs only under exceptional circumstances and therefore that pseudotachylytes are rarely formed in this paper we document the processes whereby pseudotachylytes are overprinted destroyed and otherwise removed from the rock record we present examples of recrystallized altered and cataclastically and crystal plastically deformed pseudotachylytes from a variety of ancient faults based on these observations we identify characteristics of pseudotachylytes that are resistant to change over geologic time and develop criteria to allow recognition of relict pseudotachylytes our results imply that pseudotachylytes are vastly under reported due to their vulnerability to destruction and the resulting difficulty in identification as a consequence the significance of frictional melting is underestimated the criteria we propose to distinguish relict pseudotachylytes can help to reconcile the observed frequency of earthquakes with the difficulty of demonstrating ancient seismic slip in the rock record

aromatic hydrocarbon biomarkers in terrestrial organic matter of devonian to permian age abstract this paper presents the results of a detailed study on the occurrence and distribution of alkylnaphthalenes and alkylphenanthrenes in 39 coal and sediment samples from the late palaeozoic containing predominantly terrestrial organic matter most of the samples represent the euramerian flora realm and originate from locations beyond the variscan front in eastern northern and central europe i e especially england and the moscow basin the samples are of relatively low maturity 0 32 to 1 80 rr and consist mainly of mixtures of type iii and ii kerogen quantification of the c0 to c4 naphthalenes and the c0 to c4 phenanthrenes showed that these compound classes were highly abundant in all samples aromatic hydrocarbon maturity parameters in general are in good agreement with measured vitrinite reflectance values 1 2 7 trimethylnaphthalene an assumed diagenetic product of oleanane type biomarkers was present in all investigated samples this observation provides evidence that oleanane type lipids were biosynthesised by organisms predating the evolution of angiosperms or that assumed formation pathways for 1 2 7 trimethylnaphthalene are insecure 1 2 8 trimethylphenanthrene was significantly enriched in samples from the permian and the one sample from the devonian investigated in this study the precursor s of 1 2 8 trimethylphenanthrene therefore may have been produced by organisms which were least abundant in the carboniferous depositional environments represented by our samples

seismic stratigraphy and subsidence analysis of the southern brazilian margin campos santos and pelotas basins abstract the campos santos and pelotas basins have been investigated in terms of 2d seismo stratigraphy and subsidence the processes controlling accommodation space e g eustacy subsidence sediment input and the evolution of the three basins are discussed depositional seismic sequences in the syn rift barremian to the drift holocene basin fill have been identified in addition the subsidence uplift history has been numerically modeled including i sediment flux ii sedimentary basin framework iii relation to plate tectonic reconfigurations and iv mechanism of crustal extension although the initial rift development of the three basins is very similar basin architecture sedimentary infill and distribution differ considerably during the syn rift sag to the drift basin stages after widespread late aptian early albian salt and carbonate deposition shelf retrogradation dominated in the campos basin whereas shelf progradation occurred in the santos basin in the tertiary these basin fill styles were reversed since the paleogene shelf progradation in the campos basin contrasts with overall retrogradation in the santos basin in contrast long term cretaceous paleogene shelf retrogradation and intense neogene progradation characterize the pelotas basin its specific basin fill and architecture mainly resulted from the absence of salt deposition and deformation these temporally and spatially varying successions were controlled by specific long term subsidence uplift trends onshore and offshore tectonism in the campos and santos basins affected the sediment flux history distribution of the main depocenters and occurrence of hydrocarbon stratigraphic structural traps this is highlighted by the exhumation and erosion of the serra do mar serra da mantiqueira and ponta grossa arch in the hinterland as well as salt tectonics in the offshore domain the pelotas basin was less affected by changes in structural regimes until the eocene when the andean orogeny caused uplift of the source areas flexural loading largely controlled its development and potential hydrocarbon traps are mainly stratigraphic

seismic slip propagation to the updip end of plate boundary subduction interface faults vitrinite reflectance geothermometry on integrated ocean drilling program nantro seize cores seismic faulting along subduction type plate boundaries plays a fundamental role in tsunami genesis during the integrated ocean drilling program iodp nankai trough seismogenic zone experiment nantro seize stage 1 the updip ends of plate boundary subduction faults were drilled and cored in the nankai trough offshore japan where repeated large earthquakes and tsunamis have occurred including the a d 1944 tonankai mw 8 1 earthquake samples were obtained from the frontal thrust which connects the deep plate boundary to the seafloor at the toe of the accretionary wedge and from a megasplay fault that branches from the plate boundary decollement the toe of the accretionary wedge has classically been considered aseismic but vitrinite reflectance geothermometry reveals that the two examined fault zones underwent localized temperatures of more than 380 c this suggests that frictional heating occurred along these two fault zones and implies that coseismic slip must have propagated at least one time to the updip end of the megasplay fault and to the toe of the accretionary wedge

andean trending mobile belts in the brazilian shield much of the phanerozoic andean mountain chain is floored by precambrian basement1 3 in places as old as 2 000 myr4 5 but relations between these rocks and those of the brazilian shield have long been obscure mainly because of the intervening cenozoic sedimentary cover we describe here recent work over the bolivian sector of the brazilian shield fig 1 which shows a pattern of proterozoic mobile belts approximately parallel with the andean chain to the west when the trends of these bolivian belts are projected towards the andes under the cenozoic cover their ages broadly accord with those of the andean basement our interpretation is that the two domains are linked in the form of a chain of five andean trending mobile belts with a combined width of 1 000 km becoming progressively younger to the west with ages ranging from 1 300 myr to the present

magnetotelluric evidence of the tectonic boundary between the rio de la plata craton and the pampean terrane chaco pampean plain argentina the extension of the transbrasiliano lineament abstract the transbrasiliano lineament represents a continental shear belt that transversely intersects the south american platform from nne to ssw much evidence of this lineament exists to the north but it remains uncharacterized in the distal andean foreland to the south where it is associated with the tectonic boundary between the rio de la plata craton and the pampean terrane this tectonic boundary is mostly unexposed in the chaco pampean plain argentina debate continues about the existence of an east or west dipping subduction zone and a later collisional event between the terranes here we report the results of a magnetotelluric survey along a w e profile at 27 s between 63 45 and 60 30 w to characterize the geoelectric structure the magnetotelluric data were processed the dimensionality and distortion of the geoelectrical medium were analyzed and a 2 d inversion model was developed the distribution of the resistivities at the lithospheric scale indicates a highly resistive crust 20 000 m on the east side of the profile that is correlated with the rio de la plata craton on the west side of the profile a less resistive but still highly resistive crust 12 000 m is correlated with the pampean terrane the highly resistive blocks are separated by an east dipping conductive anomaly 150 250 m that is correlated with the transbrasiliano lineament this conductive feature is consistent with the east dipping subduction model and can be explained by the presence of graphite in paleosutures in long stable geological terranes oblique convergence between the terranes may have developed the transpressional shear belt the results provide new geophysical evidence of the tectonic boundary between the rio de la plata craton and the pampean terrane beneath the chaco pampean plain that extends southward into the transbrasiliano lineament the study also improves the knowledge of the amalgamation of western gondwana

origin of the bolivian andean orocline a geologic study utilizing landsat and shuttle imaging radar abstract the origin of the major andean orocline in bolivia where the north trending southern andes abruptly change direction by 45 and continue northwestward into peru has been investigated by photogeologic techniques utilizing landsat imagery and shuttle imaging radar sir a in conjunction with published literature and maps the evidence suggests that this orocline might have been formed beginning in the miocene by the pacific coastal region being bent around the resistant brazilian shield as a result of the relative motions between the eastward moving coastal region and the westward moving south american continent the eastward motion of the coast was further increased about 10 m y b p when the opening of the east pacific rise increased the rate of eastward motion of the subducting nazca plate the brazilian shield with its deep roots provided greater resistance to this motion resulting in the decoupling of the andes along a wide left lateral strike slip fault zone extending east west from the altiplano through santa cruz and into the interior indicated left lateral motion across this fault decreases from 180 km on the east side of the andes to 50 or 60 km in the altiplano the difference being taken up in compressing the eastern cordillera and sub andean foredeep from an average width of 350 km in southern bolivia and argentina to an average width of 220 km in northern bolivia and peru the resistance of the shield may also have resulted in counter clockwise rotation of and lateral angular shear in the andes north of the break zone

geochemical study on a well in the western canada basin relation of the aromatic distribution pattern to maturity of organic matter abstract thirty one rock samples from a western canada basin well have been analysed for aromatics using glass capillary gas chromatography with simultaneous flame ionization and sulfur selective flame photometric detection a uniform aromatic distribution pattern with a marked predominance of di and tricyclic aromatic hydrocarbons was observed throughout a depth interval of 3000 meters comprising cretaceous and jurassic sediments the very high relative abundance of sulfur aromatics at two narrow intervals in the triassic and mississippian is attributed to a major change in facies gradual changes with increasing depth have been observed for a series of compound ratios which had been calculated from the isomers of methyl and dimethylphenanthrene and their parent compounds these changes reflect the increasing thermal maturation of the sedimentary organic matter a methylphenanthrene index mpi has been introduced which exhibited a very good correlation with vitrinite reflectance data the mpi is the first maturity parameter which is based on a series of aromatic hydrocarbons this hydrocarbon internal maturity parameter permits the recognition and definition of maturity levels in extracts which can be compared to the maturity e g vitrinite reflectance of source rocks

brasiliano crustal structure and the tectonic setting of the parnaiba basin of ne brazil results of a deep seismic reflection profile a 1430 km deep crustal seismic reflection profile of the parnaiba basin shows an asymmetric structured western margin and a gently dipping eastern margin the 3 km thick phanerozoic sedimentary section overlies a pronounced planar regional unconformity that crosses three precambrian blocks with differing seismic facies the amazonian araguaia block the parnaiba block and the borborema block the blocks are separated by steep crustal scale boundaries across which seismic facies change abruptly in the west the ophiolitic metasedimentary rocks of the araguaia group overlie the amazonian craton both craton and metasediments terminate eastward against a steep east dipping fault zone defining the amazonian araguaia block and parnaiba block boundary reactivation of this neoproterozoic margin in the late triassic and late jurassic early cretaceous folded and elevated basement and basin over 2 km a second crustal boundary defines the eastern margin of the parnaiba block with the neoproterozoic borborema block this boundary is interpreted as the extension of the transbrasiliano shear zone these data demonstrate that the basement of the parnaiba basin was formed during brasiliano orogenesis by west directed collision related thrusting succeeded by lateral accretion along steep crustal scale boundaries after formation of a post brasiliano peneplain the parnaiba basin developed seamlessly across three very different crustal blocks and appears to have been significantly larger than its present outline no extensive underlying rift system is evident suggesting that basement structure had little to do with basin formation but that episodic reactivation of the boundary zones and basement fabric has controlled the structuring and preservation of the basin

integrated correlation of the vendian to cambrian arroyo del soldado and corumba groups uruguay and brazil palaeogeographic palaeoclimatic and palaeobiologic implications abstract the corumba group of sw brazil and the arroyo del soldado group asg of uruguay are correlated on the basis of litho bio and chemostratigraphy both units represent marine sedimentation with alternating siliciclastics and carbonates developed on a stable continental shelf in the corumba basin sedimentation began in the varangerian represented by the glaciomarine puga formation a series of sea level fluctuations coupled with climatic changes are recorded up section while uppermost deposits of the asg are of lowermost cambrian age sedimentation ceased in the latest vendian in the corumba basin an assemblage of six species of organic walled microfossils dominated by bavlinella faveolata and soldadophycus bossii three species of vendotaenids and two species of skeletal fossils cloudina and titanotheca is described from the corumba group the vendotaenid eoholynia corumbensis sp nov is described from siltstones of the guaicurus formation an important diversity of skeletal fossils in the corumba arroyo del soldado and nama groups points to favourable vendian palaeoclimatic conditions in sw gondwana preliminary carbon isotopic data show a series of alternating positive and negative excursions corroborating the upper vendian age indicated by fossils for both units previously reported strontium isotopic data are also consistent with this age it is postulated that the corumba and asgs were deposited onto the same shelf which opened to the east the rio de la plata superterrane craton extends farther to the north than previously expected or it was already amalgamated with the amazonian craton by vendian times collision of the platform with the parana block caused closure of the basin during the cambrian early ordovician finally models of neoproterozoic glaciations based on enhanced bioproductivity driven by high nutrient availability are discussed

paleozoic crustal blocks of onshore and offshore central argentina new pieces of the southwestern gondwana collage and their role in the accretion of patagonia and the evolution of mesozoic south atlantic sedimentary basins abstract the southwestern gondwana basement block configuration in the central argentinean offshore area was analyzed using gravimetric magnetic and seismic data and existing onshore tectonic models the resultant maps the distribution of the mesozoic and cenozoic basins and paleozoic structural features were used to validate the interpretations and to produce a new regional tectonic model pre carboniferous southwestern gondwana of south america was interpreted as an open margin formed from east to west by the dom feliciano belt the rio de la plata craton the pampean belt and the pampia and cuyania terranes the collision of the patagonia allochthonous terrane during the late paleozoic resulted in the development of the ventania cape fold belt which was mapped for the first time off the argentinean coast out to 600 km from the shore a strong change in the orientation of the fold belt is referred to as the colorado syntaxis a mirror image of the cape syntaxis in south africa this change reflects the buttressing effect of the cratonic blocks that hamper the northward propagation of syncollisional deformation and resulted in a 180 km shift of the orogenic front the mesozoic basins and the basement block distribution were analyzed the pampean belt a deformed area produced by the pampia accretion to the cratonic area is the locus to two episutural basins the general levalle and macachin basins the salado basin was interpreted as an episutural basin controlled by a 2 1 2 0 ga suture within the rio de la plata craton the colorado basin is composed of four segmented depocenters that reflect different emplacement controls the location of the western colorado basin was controlled by the upper paleozoic orogen the distributions of the central and eastern colorado depocenters orthogonal to the continental boundary were also strongly influenced by the upper paleozoic structures and were offset by lineaments that reflect dom feliciano fabric the colorado basin external depocenter that parallels the continental margin was also controlled by these lineaments we interpret a time gap of some 50 ma between the beginning of the evolution of the margin orthogonal depocenters and the atlantic breakup

the proterozoic history of eastern bolivia abstract about 100 000 km2 of the previously unmapped bolivian sector of the central brazil shield has been studied by proyecto precambrico an anglo bolivian technical cooperation programme the lower proterozoic is represented by the lomas maneches granulite group and the bulk of the chiquitania paragneiss complex which were formed during the trans amazonic orogenic cycle 2000 ma the middle proterozoic spans the orogenic cycles of san ignacio 2000 1300 ma and sunsas unmetamorphosed upper proterozoic and possibly cambrian sediments on the southern and eastern flanks of the shield represent marine transgressions related to the intra continental braziliano orogenic cycle east trending dolerite dykes were probably intruded during this period within the shield

testing emplacement models for the rustenburg layered suite of the bushveld complex with numerical heat flow models and plagioclase geospeedometry abstract conventional models of the genesis of the rustenburg layered suite rls of the bushveld complex postulate that it is the product of a single multiply replenished magma chamber that cooled as a single massive unit i e a monolithic magma chamber this concept has been challenged by alternative hypotheses involving out of sequence emplacement of some of its major layers as sills both scenarios have partial support from high precision u pb geochronological data to test these contradictory hypotheses new zircon geothermometric data and compositional data for plagioclase from the upper critical zone are presented and the available geochronological data are tested against the predictions of the thermal evolution of the entire rls for both emplacement and cooling scenarios crystallization temperatures of zircon within units of the upper critical zone were estimated using ti in zircon thermometry and range between 799 45 c and 865 64 c the data are used in conjunction with a detailed numerical model of heat flow in the bushveld complex that is used to approximate the duration and extent of thermal perturbations felt throughout the system and delimit the time intervals during which zircon crystals could have grown at the measured crystallization temperatures the results show that prolonged cooling histories 1 myr that would be experienced near the centre of a large slowly cooled plutonic magma chamber would erase signs of plagioclase zoning at 4 kbar and moderate f h 2 o however plagioclase crystals with zoning profiles spanning at least 10 mole an are common in the upper critical zone in contrast emplacement of the major units of the rls at widely separated intervals as recorded by the existing high precision u pb dataset would impose transient thermal effects on the system against a backdrop of generally much lower ambient temperatures throughout its evolution in this scenario apparent ages of zircon crystals would record the time of cooling through temperatures near the water saturated biotite granite solidus corresponding to the final intercumulus mineral assemblage in both mafic and ultramafic macrolayers subsequent reheating of these rocks would be unable to reset primary zircon crystallization dates but might in some examples result in some amount of remelting or regrowth of zircon comparison of the evolving temperature fields of the two types of emplacement and cooling histories shows that the existing geochronological data and preservation of plagioclase zoning are consistent with out of sequence emplacement and rapid cooling of individual ultramafic sills into previously solidified lithologies of the rls but not with the conventional view of a single monolithic magma body

tearing up rodinia the neoproterozoic palaeogeography of south american cratonic fragments rodinia was initially defined as a long lived supercontinent that assembled all the continental fragments around laurentia and remained stable from 1000 up to 750 ma nonetheless recent work has cast doubt on the rodinia palaeogeography and even on the timing of its assembly and break up the geochronological and palaeomagnetic databases accumulated for south america and africa in the last decade show that most of these continental fragments were not part of rodinia a wide brasiliano ocean separated most of the south american and african cratons from the laurentia amazonia rio apa west africa margin this ocean was closed between 940 and 630 ma along the pampean paraguay araguaia pharusian mobile belts moreover accretion along the south american and african platforms was a diachronous and long lived process that involved several intra oceanic and continental magmatic arcs and microcontinents this evolution started at around 1000 ma and ended at around 520 ma with the final assembly of gondwana

paleo stress estimates on ancient seismogenic faults based on frictional heating of coal 1 faulted coals from eastern kentucky and montana usa and the south wales coalfield uk show fault related vitrinite reflectance anomalies interpreted as due to frictional heating during seismic slip the vitrinite reflectance anomalies are converted to temperature using kinetic based software easy r for post seismic cooling rates shear stresses are calculated using lachenbruch s 1986 piezometer which depends only on the area under the thermal anomaly and fault displacement for maximum displacements of 1 3 meters the minimum shear stresses are 13 64 mpa and overlap with seismological estimates of stress drops during modern earthquakes the calculated local mean normal stress is 100 mpa resulting in coefficients of friction of 0 1 0 6 thrust and 0 2 0 4 normal faults which are lower than laboratory based values

proterozoic crustal evolution at the southern margin of the amazonian craton in the state of mato grosso brazil evidence from rbsr and k ar data abstract geologic mapping on a 1 100 000 scale in the sao jose dos quatro marcos sjqm area southwest of the state of mato grosso central western brazil showed a remarkable variety of precambrian lithologic units predominant are grey tonalitic gneisses with a rb sr isochron age of 1971 770 ma an initial 87 sr 86 sr ratio of 0 7071 and a k ar biotite cooling age of 1500 ma the gneisses gneisses show interlayered first generation amphibolite lenses and bands k ar amphibole age of 1500 ma and some rare relicts of grey gneissic granulites together with associated pink augen gneisses rare outcrops of deformed banded calc silicate rocks are recrystallized by contact metamorphism these units are intruded by a granitoid suite constituted by early usually somewhat deformed tonalites and granodiorites and late massive pink granodiorites and monzo and syegogranites rb sr isochron age 1472 19 ma i r 0 7037 they show enclaves of grey gneisses and second generation amphibolites a large amphibolite body to the west is also considered part of this second generation basic sequence scattered rhyolite outcrops to the east represent the southernmost remnants of the volcanic rio branco group the grey gneisses here formally identified as the sao jose dos quatro marcos gneiss and probably derived from lower proterozoic tonalites were complexly deformed and recrystallized at 2 0 ga into predominantly amphibolite facies rocks the first and second generation amphibolites document two distinct commingling episodes separated by at least 0 5 ga each directly associated with the corresponding host rock granitoid magmatism the porphyritic granites were converted into pink augen gneisses before 1500 ma and are thus not contemporaneous to the 1380 ma syn kinematic san ignacio granitoids from eastern bolivia calc silicate rocks are interpreted as remnants of supracrustal sequences perhaps related to the quatro meninas group the 1500 ma granitoids formally named the jaboti intrusive suite invaded under a syn to late kinematic regime determining a unique thermal magmatic event in this region

neoproterozoic crustal accretion in central brazil recent geochronological studies have demonstrated the existence of several neoproterozoic orthogneiss and metavolcanic are suites ca 900 to 600 ma in the southern part of the tocantins province a major neoproterozoic orogenic area in central brazil exposed between the amazon and sao francisco cratons nd and sr isotopic characteristics of these metamorphic suites are primitive initial 87 sr 87 sr ratios vary from 0 7024 to 0 7042 and initial nd values are positive varying between 0 2 and 6 9 the most primitive initial sr and nd isotopic compositions are very similar to model depleted mantle compositions at the time of formation of the original magmas the mantle like characteristics of these are suites in western goias contradict previous ensialic evolution models for the tocantins province and reveal that crustal accretion processes were important in large areas of central brazil during neoproterozoic time

u pb age data from the sunsas region of eastern bolivia evidence for the allochthonous origin of the paragua block abstract ion microprobe u pb dating of detrital zircons from the chiquitania and san ignacio groups of eastern bolivia show that these rocks were derived mostly from a 1765 ma source and were deposited after 1690 ma the emplacement of the lomas maneches granitoid suit dated at 1690 1660 ma accompanied the deposition of these metasediments these rocks which collectively define the basement of eastern bolivia were subsequently deformed and metamorphosed during two discrete periods of orogenesis the oldest of these events the san ignacio orogeny deformed the terrane through three successive phases of deformation and was accompanied by voluminous syn tectonic granite intrusion and migmatisation of the 1690 ma sediments and intrusions u pb age data from igneous and metamorphic zircon constrain the san ignacio orogeny to between 1340 and 1320 ma while the orientation of structures suggests deformation was the result of w nw directed shortening a 300 million years hiatus during which the overlying sunsas group was deposited separated the san ignacio orogeny from the sunsas orogeny shortening during sunsas orogenesis resulted in an intense nw trending structural overprint in the southern and western extremes of the study area the intensity of deformation decreased toward the northeast such that in the san ignacio region sunsas tectonism resulted in distinct dome and basin fold interference patterns late syn to post tectonic granites were emplaced in the region of highest sunsas strain one of these intrusions the taperas granite yielded an age of ca 1075 ma when taken collectively these new data highlight a number of important points 1 the deposition of the chiquitania and san ignacio groups and the synchronous emplacement of the lomas maneches granitoid suite 1690 1660 ma have no temporal equivalents in the either the rondonia and mato grosso regions of brazil to the north or east or the proterozoic pre andean inliers to the west 2 the trend of the structures attributed to the san ignacio orogeny are oriented at a high angle to the nw trending pre sunsas margin of amazonia and 3 the sunsas orogeny represents the only geological event with a common age and direction of crustal shortening recorded in both the amazon craton and the rocks of eastern bolivia these data are interpreted to suggest that the rocks of eastern bolivia represent a geologically distinct microcontinent the paragua craton that was accreted to the southern margin of amazon craton during the sunsas orogeny

40ar 39ar constraints on the age and thermal history of the urucum neoproterozoic banded iron formation brazil urucum is one of the youngest banded iron formations bifs yet its exact age remains uncertain 40 ar 39 ar geochronology on late diagenetic to early metamorphic cryptomelane from the urucum sequence reveals a minimum depositional age of 587 7 ma metamorphic braunite age spectra yield flat segments defining apparent ages of 547 3 ma to 513 4 ma interpreted as recrystallization or cooling ages metamorphic muscovite grains from a meta arkose interbedded with the bif yield reproducible plateau ages of 513 3 ma cryptomelane ages are interpreted to record the minimum timing of the extensional tectonic events that generated the graben systems at the southeastern border of the amazon craton including the chiquitos tucavaca aulacogen and the jacadigo basin which hosts the urucum banded iron formation structurally controlled hydrothermal alteration resulted in crystallization of braunite and muscovite during the interval between 547 and 513 ma coeval with post collisional decompression that resulted in granite emplacement e g sao vicente granite in the paraguay belt subsequent uplift and erosion of the precambrian sequence possibly succeeded metamorphism a chemically distinct cryptomelane generation identified in surface samples yield ca 60 ma results these ages provide evidence for supergene recrystallization after exhumation in the mesozoic cenozoic 40 ar 39 ar systematics implies that the oldest cryptomelane samples experienced ar loss probably via thermally induced diffusion this suggests that the urucum bif was buried and heated to significant temperatures in the late proterozoic consistent with present mineralogy and isotopic composition that do not reflect the original characteristics of the depositional environment

biomarker thermal maturity experiments at earthquake slip rates abstract evidence of temperature rise is useful for identifying where within a fault zone earthquake slip has occurred as well as some attributes of the earthquake such as energy output biomarker thermal maturity can elucidate structures within a fault zone that were hotter than surrounding rock and therefore likely to have hosted earthquakes we performed a series of friction experiments on thin gouge layers of woodford shale at slip rates of 1 m s our results show that biomarker thermal maturity can clearly delineate zones of slip that are as thin as 150 m we establish that the kinetics of methylphenanthrene reaction previously determined at longer durations and lower temperatures are applicable at seismic slip rates and timescales temperature estimates show that once the slipping zone reached 500 c biomarker reaction was significant as predicted by the reaction kinetics our results demonstrate that temperature rise in fault zones is a function of both the work density and power density during the earthquake biomarker reactions along with other arrhenius style reactions such as those that lead to dynamic shear weakening are both a function of slip speed and frictional work

geological evolution of the basement rocks in the east central part of the rondonia tin province sw amazonian craton brazil u pb and sm nd isotopic constraints on the basis of geologic petrologic and u pb geochronologic data the basement rocks in the east central part of the rondonia tin province rtp southwestern amazonian craton are grouped into five lithologic associations 1 tonalitic gneiss 1 75 ga 2 enderbitic granulite 1 73 ga 3 paragneiss 4 granitic and charnockitic augen gneisses 1 57 1 53 ga and 5 fine grained granitic gneiss and charnockitic granulite 1 43 1 42 ga the first three are related to development of the paleoproterozoic rio negro juruena province and represent the oldest crust in the region the tonalitic gneisses and enderbitic granulites show calc alkaline affinities and nd isotopic compositions initial end 0 1 to 1 5 tdm of 2 2 2 1 ga that suggest a continental arc margin setting for the original magmas the paragneisses yield tdm values of 2 2 2 1 ga suggesting that source material was primarily derived from the ventuari tapajos and rio negro juruena crusts but detrital zircon ages and an intrusive granitoid bracket deposition between 1 67 and 1 57 ga the granitic and charnockitic augen gneisses show predominantly a type and within plate granite affinities but also some volcanic arc granite characteristics the initial end values 0 6 to 2 0 indicate mixing of magmas derived from depleted mantle and older crustal sources these rocks are correlated to the 1 60 1 53 ga serra da providencia intrusive suite that reflects inboard magmatism coeval with the cachoeirinha orogen located to the southeast the fine grained granitic gneiss and charnockitic granulites represent the first record of widespread magmatism at 1 43 1 42 ga in northern rondonia their geochemical signatures and the slightly positive initial end values 0 7 to 1 2 are very similar to those of the most evolved granites of the calc alkaline santa helena batholith farther southeast u pb monazite and sm nd whole rock garnet ages demonstrate that a high grade tectonometamorphic episode occurred in this region at 1 33 1 30 ga this episode attained upper amphibolite conditions and is interpreted as the peak of the rondonian san ignacio orogeny the u pb and sm nd data presented here and data published on rapakivi granites elsewhere indicate that the east central part of the rtp is a poly orogenic region characterized by successive episodes of magmatism metamorphism and deformation between 1 75 and 0 97 ga

the tectonic evolution of the transbrasiliano lineament in northern parana basin brazil as inferred from aeromagnetic data data from six airborne magnetic surveys were compiled and analyzed to develop a structural interpretation for the transbrasiliano lineament in northern parana basin brazil magnetic lineaments interpreted to reflect geologic contacts and structures at different depths were illuminated using the matched filter technique applied to aeromagnetic anomalies field based structural measurements generally support our magnetic analysis we estimated three primary magnetic zones i a zone of deep magnetic sources at 20 km depth ii an intermediate basement zone at 6 km depth and iii a shallow zone of near surface geological features at 1 5 km depth the deepest zone exhibits three major ne trending crustal discontinuities related to the transbrasiliano lineament dividing the region into four geotectonic blocks anomalies associated with the intermediate zone indicate directional divergence of subsidiary structures away from the main transbrasiliano fault which strikes n30 e the shallow magnetic zone includes near surface post brasiliano orogenic granites our analysis identified ne trending sigmoidal lineaments around these intrusions indicating intense zones of deformation associated with probable shear structures at the shallow depth zone magnetic anomalies caused by cretaceous alkaline intrusive bodies and basalts of the serra geral formation are enhanced by the matched filter method these igneous bodies are related to extensional nw striking lineaments and seismicity aligned along these lineaments suggests that they currently are reactivated prior to flexural subsidence of the parana basin reactivation processes along transcurrent elements of the transbrasiliano lineament promoted extensional processes and formed initial parana basin depocenters cretaceous and more recent sedimentation also correlate with reactivations of ne striking structures

the preservation of fossil biomarkers during meteorite impact events experimental evidence from biomarker rich projectiles and target rocks a devonian siltstone from orkney scotland shows survival of biomarkers in high velocity impact experiments the biomarkers were detected in ejecta fragments from experiments involving normal incidence of steel projectiles at 5 6 km s 1 and in projectile fragments from impact experiments into sand and water at 2 5 km s 1 the associated peak shock pressures were calculated to be in the range of 110 147 gpa for impacts of the steel projectiles into the siltstone target and hydrocode simulations are used to show the variation of peak pressure with depth in the target and throughout the finite volume projectiles thermally sensitive biomarker ratios including ratios of hopanoids and steranes and the methylphenanthrene ratio showed an increase in thermal maturity in the ejecta and especially the projectile fragments measurement of absolute concentrations of selected biomarkers indicates that changes in biomarker ratios reflect synthesis of new material rather than selective destruction their presence in ejecta and projectile fragments suggests that fossil biomarkers may survive hypervelocity impacts and that experiments using biomarker rich rock have high potential for testing survival of organic matter in a range of impact scenarios

thermal maturation by vitrinite reflectance of woodford shale anadarko basin oklahoma the woodford shale upper devonian lower mississippian a black shale widely regarded as an important hydrocarbon source rock has attained thermal maturity to postmaturity with respect to the generation of liquid hydrocarbons in most of the anadarko basin in western oklahoma untreated whole rock well cuttings and core material from selected 10 ft intervals of the woodford shale in 28 wells were examined under reflected white light thermal maturation was determined by measuring the reflectance in oil immersion ro of first generation vitrinite particles a minimum of 45 reflectance measurements were recorded for each well representing one or more sample intervals the woodford shale was sampled at depths from 5 060 ft 1 542 m in the northeastern shelf to 25 115 ft 7 655 in the deepest part of the basin a systematic increase in mean vitrinite reflectance meanro with depth was observed over much of the anadarko basin from northeast to southwest across the basin mean ro increases from 0 48 to 2 61 high temperatures have raised the woodford shale to anthracite rank in two wells 4 29 and 4 89 ro in the deep anadarko basin an isoreflectance map for the woodford shale indicates those areas of the basin where the thermal history of the shale is optimum for preserving liquid hydrocarbons mean ro of 0 5 to 2 0 computer generated plots of vitrinite reflectance versus depth provide a regression equation that predicts the vitrinite reflectance of the woodford shale at any depth in the anadarko basin and determines the amount of erosion

crustal deformation of the andean foreland at 31 30 s argentina constrained by magnetotelluric survey abstract twenty five new long period magnetotelluric sites near 31 5 s were collected in a west east profile this profile and the previous one aligned with and adjacent to the eastern end have been merged to form a single profile of more than 700 km long extending from the precordillera to the chaco pampean plain the geotectonic scenario is characterized by a modern flat subduction zone of the nazca plate located at a depth of around 120 km and clearly defined by the distribution of earthquake hypocenters recorded by local and regional networks a bulge shape at 68 5 w with an anomalous dip to the west is observed within this segment the smooth slab deformation might result from the restriction on eastward motion due to the presence of an electrically resistive zone the magnetotelluric model shows that this thick zone of increased resistivity is found from shallow crustal levels to upper mantle depths the bulge geometry allows hot fluids and volatiles to rise from the deeper asthenospheric wedge and reach the lower crust reducing its viscosity and letting it flow the zones of low resistivity in the lower crust show spatial correlation with the areas of foreland deformation from precordillera to the sierras pampeanas and may also suggest a ductile regime shear zones reactivated by cenozoic faulting must necessarily have their roots in the levels of the ductile lower crust associated to conductive channels the zone where the lower crust is closer to the surface coincides with the areas of greatest structural relief and erosion the interface between the folded ductile lower crust and the brittle upper crust might act as the main level of decollement of the bordering structures between the precordillera sierra de pie de palo and the sierras pampeanas in addition the geometry of the interface might be conditioning the vergence of those structures

comment on was there an ediacaran clymene ocean in central south america by u g cordani and others the existence of gondwana is a subject of historical interest for geologists since it forms the basis for early scientific theories on the mobility of continents textbook examples of the common gondwanan origin of the southern continents includes the paleozoic ubiquity of glossopteris flora and

comment on testing emplacement models for the rustenburg layered suite of the bushveld complex with numerical heat flow models and plagioclase geospeedometry by s j robb and j e mungal earth planet sci lett 534 2020 116084

organic geochemistry of the woodford shale southeastern oklahoma how variable can shales be woodford shale samples obtained from a cored outcrop in southeastern oklahoma were geochemically analyzed to determine vertical variations of organic facies thermal maturity and an evaluation of their depositional environments total organic carbon values ranged from 5 01 to 14 81 indicating a good source rock potential in this area the woodford shale is marginally mature as indicated by vitrinite reflectance values rock eval data revealed that the samples are dominated by type ii kerogen biomarker ratios based on pristane phytane steranes and hopanes show a mix of marine and terrigenous organic matter high salinity conditions and water density stratification also prevailed during deposition of this formation as indicated by the presence of gammacerane the woodford shale was subdivided into lower middle and upper members based on the integration of geochemical and geologic data moreover the presence and extent of photic zone anoxia pza were determined by the presence of aryl isoprenoids the lower and upper woodford shale members were deposited under dysoxic to suboxic conditions and episodic periods of pza the middle member was deposited under anoxic conditions and persistent pza in addition aryl isoprenoids helped infer the position of the chemocline during deposition of the different members the relative hydrocarbon potential parameter was used in determining transgressive and regressive cycles within the woodford shale this study undoubtedly demonstrates the significant lithologic and chemical variability that occurs within shales the application of this workflow to regional studies can have a direct influence on exploration and production activities in shale gas systems

vitrinite reflectance and molecular geochemistry of jurassic sediments the influence of heating by tertiary dykes northwest scotland vitrinite reflectance measurements from sedimentary rocks adjacent to six dykes from sites across the inner hebrides indicate that the lateral extent of thermal alteration is reasonably similar and proportional to dyke thickness d t with alteration starting to take effect at 70 d t detailed sampling of a jurassic siltstone adjacent to a 0 9 m dyke reveals that the zone of maximum extractable organic matter eom wt g rock equivalent to the oil window is relatively narrow ca 25 of the d t the position of this oil window relative o the vitrinite reflectance profile is comparable with that from burial maturation gas chromatograms of the aliphatic hydrocarbon fractions reveal i a shift from branched and cyclic dominated hydrocarbons to an increasing predominance of straight chain components and ii the carbon number distribution of the n alkanes shifts towards the shorter chain homologues with decreasing distance from the dyke contact apart from the methylphenanthrene index mpi1 changes in each of the molecular maturity parameters are initiated by the the thermal effects of the dyke between 80 and 70 d t from the contact the mpi1 parameter was the most sensitive maturity indicator in that it responded to the dyke emplacement even in the distal samples i e distance from dyke contact 140 d t a significant reduction in toc at the dyke contact suggests that the synthesis gas reaction may have occurred i e the reaction of organic matter with steam resulting in the formation of carbon monoxide this reaction may be associated with the vitrinite reflectance reversal this study suggests that these particular dykes have had only a local effect on the maturity of the intruded sediments

western parana suture shear zone and the limits of rio apa rio tebicuary and rio de la plata cratons from gravity data abstract we present a new gravity map between 45 70 w and 5 40 s integrating open source terrestrial gravity data of argentina with the south american gravity model 2004 sagm04 a 5 min arc resolution gravity model the bouguer anomaly map reveals a 2000 km long linear gravity feature from 15 s to 30 s at longitude 55 w with a steep horizontal gradient separating two gravity domains the eastern domain is the parana basin with ne sw trending bouguer anomalies of 80 mgal in average the western domain comprises the chaco parana chaco tarija and pantanal basins with circular positive anomalies of up to 20 mgal in amplitude previous seismic studies mapped a thinner crust of less than 35 km in the western domain and the present gravity models indicate a 10 20 kg m 3 denser crust on the other hand the eastern domain has a thicker crust of more than 40 km seismic tomography models also show p and s wave velocity reduction in the western domain whereas high velocity characterises the parana basin these geophysical data indicate that the gravity gradient marks a transition between two distinct lithospheres the gravity gradient is associated with a tectonic feature referred to as the western parana suture shear zone granites of 530 570 ma ages located parallel or over the gravity gradient suggest a neoproterozoic to early cambrian age suture shear zone thus approximately synchronous and parallel to the pampean belt sediment corrected residual gravity map and its vertical derivative allow us to define the limits of the rio apa rio de la plata and rio tebicuary cratons their eastern and western limits are the western parana suture and the pampean belt respectively this study unravels precambrian tectonic elements concealed by the phanerozoic sedimentary basins adding new constraints for the amalgamation history of sw gondwana

closing the clymene ocean and bending a brasiliano belt evidence for the cambrian formation of gondwana southeast amazon craton we report new paleomagnetic and geochronological data from ediacaran rift drift carbonates in the paraguai belt at the southern end of the suture zone between the amazon craton and the sao francisco and rio de plata cratons south america early thrusting resulted in remagnetization ca 528 36 ma or later the mean age is established by 40 ar 39 ar encapsulation dating of mixed authigenic and detrital illite from remagnetized carbonates from the unmetamorphosed fold thrust belt this remagnetization overlaps with a 525 ma gondwana reference pole metamorphic illite from the slate belt yields 40 ar 39 ar ages of 496 484 ma the timing of peak regional metamorphism oroclinal bending of the paraguai belt was caused by a 90 clockwise rotation of the east west limb after ca 528 ma probably refl ecting the irregular margin of the southeast amazon craton the age of the paraguai belt overlaps with that of the pampean orogeny farther south along the western margin of the rio de plata craton suggesting a coeval closure for the clymene ocean separating the amazon craton from the sao francisco and rio de plata cratons

origin of rectangular and aligned lakes in the beni basin of bolivia abstract the origin of aligned rectangular shaped lakes in the beni basin of bolivia has been investigated utilizing landsat imagery shuttle imaging radar and the available scientific literature the lakes tend to line up in two preferred orientations northwest and northeast they occur in cenozoic sediments of the beni basin situated between the brazilian shield and the andean foredeep available evidence suggests that the shapes and orientations of the lakes are primarily controlled by a nearly orthogonal fracture pattern propagated upward from the underlying granitic brazilian shield basement these orientations and shapes may reflect a reactivated ancient global fracture pattern or may be the result of an e w greatest principal compressive stress 1 and subsequent downwarping and uplifting of the beni basin during the cenozoic andean orogeny the lakes themselves may occupy down dropped basement blocks induced in the gently upwarped basement while unusual these lakes are not unique this suggests that transmission of basement structure through overlying sediments may occur in other regions but that special conditions such as little or no lateral compression of the sediments the absence of high trees and vegetation stability of surface material from drifting and a high water table are necessary to preserve and enhance such surface features

emplacement and crystallization time for the bushveld complex the bushveld complex formed by the crystallization of successive basaltic magmatism may be of the order of several million years for example the columbia river basalts hooper injections of magma which were sufficiently closely spaced in time that each previous magma had not cooled and differentiated 1988 were erupted in the period 17 12 ma with minor eruptions for a further 5 my although most outpouring significantly before the addition of the next one to constrain the emplacement and crystallization times a thermal model is presented occurred within the first 2 my it is now recognized that large intrusions were not emplaced in a single pulse but which permits the investigation of the rate of cooling of magma in an intrusion repeatedly subjected to magma addition and subresult from multiple magma injection the question is how rapidly were magma chambers such as the bc traction such modelling indicates that magmas injected into the bushveld complex were emplaced within 75 000 years at that filled and how much magma was involved answers are relevant to the dynamics of melt production storage and time injection into the complex ceased the volume of rock in the eastern and western limbs is 370 000 600 000 km however transport in the mantle and crust this paper describes a thermal modelling technique not previously applied a quantitative evaluation of the cr budget in the formation of chromitite layers indicates that large volumes of magma cannot be to magma chambers which can be used to analyse this process and to obtain an estimate of the emplacement accounted for in the preserved rock sequence similarly an evaluation of the incompatible trace element abundances such as those for zr time to present this model it is necessary to discuss the stratigraphy size and connectivity of the different limbs and k suggests that the chamber was open and that large volumes of differentiated magma escaped the volume of magma therefore of the bushveld complex and to consider the extent of tapping of the magma chamber as well as its filling greatly exceeded the preserved volume of cumulate rocks giving an estimated magma volume of over 1 10 km an average the term bushveld complex has been given several meanings in the literature and according to the south emplacement rate of 13 km year is indicated by these calculations african commission on stratigraphy 1980 includes not only the ultramafic mafic layered rocks but also the sills beneath the intrusion volcanic rocks which pre date the

hot on the trail coseismic heating on a localized structure along the muddy mountain fault nevada abstract recent advances in the use of thermal proxies to identify past earthquake slip in exhumed faults have created an extraordinary opportunity to map the geometry of past ruptures in unprecedented detail this approach can reveal along strike differences in the structure and speed of earthquake slip here we present organic thermal maturity data collected along the muddy mountain thrust in nevada to investigate where it was within the fault that earthquake slip occurred and whether this is consistent along strike we observe large changes in thermal maturity which represent temperature rise variations along the fault modeling of thermal maturity measurements yield peak temperatures of 760 1090 c where the principal slip zone psz is narrowest 0 8 mm nearby where the psz is thicker 1 5 mm these temperatures did not exceed 500 c based upon estimates of temperature rise within a submillimeter psz mean frictional work during earthquake slip was 6 3 7 1 mjm 2 these results show that thickness of the active slipping zone can vary at small spatial scales and development of a continuously narrow

seismic characteristics of central brazil crust and upper mantle a deep seismic refraction study 1 a two dimensional model of the brazilian central crust and upper mantle was obtained from the traveltime interpretation of deep seismic refraction data from the porangatu and cavalcante lines each approximately 300 km long when the lines were deployed they overlapped by 50 km forming an e w transect approximately 530 km long across the tocantins province and western sao francisco craton the tocantins province formed during the neoproterozoic when the sao francisco the paranapanema and the amazon cratons collided following the subduction of the former goias ocean basin average crustal vp and vp vs ratios moho topography and lateral discontinuities within crustal layers suggest that the crust beneath central brazil can be associated with major geological domains recognized at the surface the moho is an irregular interface between 36 and 44 km deep that shows evidences of first order tectonic structures the 8 05 and 8 23 km s 1 p wave velocities identify the upper mantle beneath the porangatu and cavalcante lines respectively the observed seismic features allow for the identification of 1 the crust has largely felsic composition in the studied region 2 the absence of the mafic ultramafic root beneath the goias magmatic arc and 3 block tectonics in the foreland fold and thrust belt of the northern brasilia belt during the neoproterozoic seismic data also suggested that the bouguer gravimetric discontinuities are mainly compensated by differences in mass distribution within the lithospheric mantle finally the goias tocantins seismic belt can be interpreted as a natural seismic alignment related to the neoproterozoic mantle domain

hematite fault rock thermochronometry and textures inform fault zone processes abstract minerals on fault surfaces record fluid rock interaction strain and or heat during fault slip fault hosted hematite is amenable to u th he he thermochronometry hematite he dates reflect fault related processes or exhumation depending on hematite growth above or below the grain size controlled closure temperature and processes that promote open system behavior deconvolving these phenomena requires multi scale hematite textural characterization and constraints on the ambient thermal history from conventional thermochronometry two examples of this approach include investigations of hematite coated fault surfaces from exhumed damage zones in the seismically active wasatch fault zone wfz ut and painted canyon fault pcf paralleling the southern san andreas fault mecca hills ca wfz fault mirrors preserve sub m scale polygonal grain textures and hematite he data patterns that together with host rock apatite he data are consistent with localized friction generated heat hematite he loss and hematite recrystallization during damage zone seismicity

closure of the clymene ocean and formation of west gondwana in the cambrian evidence from the sierras australes of the southernmost rio de la plata craton argentina abstract the formation of gondwana took place across a series of brasiliano pan african suture zones that record late neoproterozoic to earliest paleozoic collisions between precambrian cratons in south america an internal suture zone marks the disappearance of the clymene ocean that separated the amazon craton from the sao francisco and rio de la plata cratons new geochronological data from the southern end of this massive collision zone in the sierras australes of central eastern argentina document paleoproterozoic crust and suggest an ediacaran age for the oldest sedimentary rocks these two observations extend the known limit of the rio de la plata craton at least 200 km sw of previous estimates new data also confirm the occurrence of late ediacaran to late cambrian magmatism in the sierras australes the age of these hypabyssal to volcanic rocks corresponds to igneous events in the pampean belt along the western margin of the rio de la plata craton although the shallow level magma emplacement in the sierra da ventana study area contrasts with the deeply exhumed rocks of the pampean orogeny type locality these new age data are compared with a broad compilation of geochronological age clymene collision belts to the north the paraguai and araguaia belts the close overlap of the timing of orogenesis indicates the age of clymene ocean closure in its northern reaches in the south the pampean belt was unconfined allowing continued tectonic activity and crustal accretion throughout the paleozoic

the southern border of the guapore shield in western brazil and bolivia an interpretation of its geologic evolution abstract the geology of the southeastern and southwestern margins of the guapore shield is described the shield rocks comprise high grade granulite and low grade greenschist sequences these rocks are poorly known oldest recorded ages include transamazonian datings 2 ma the margins of the shield show the development of a mobile belt formed by rocks of upper proterozoic to cambrian age within this mobile belt the cuiaba group of uncertain but possibly upper proterozoic age outcrops to the east and shows tectonic contacts with younger rocks that span the vendian upper proterozoic to cambrian age the rocks of vendian to cambrian age form three isolated elongated basins which show evidence of fault control the axes of these basins are equally separated by angles of 120 and meet at a triple junction near corumba the stratigraphy of the vendian to cambrian basins is correlated from bolivia to brazil based on tectonic continuity sedimentological similarity and fossil content the central location at corumba shows instability since the middle proterozoic when important igneous activity is recorded at rincon del tigre bolivia and in brazil this igneous activity is interpreted as being related to hot spot activity in an intercratonic setting from which the younger upper proterozoic basins originated