an early history of pure shear in the upper plate of the raft river metamorphic core complex black pine mountains southern idaho abstract although commonly obscured by simple shear pure shear fabrics occur locally within many metamorphic core complexes the cover rocks to the raft river metamorphic core complex exposed within the black pine mountains display an early coaxial strain history which developed prior to the formation of low angle fault bounded allochthons at higher structural levels this is documented by pressure shadows with straight sutures and oppositely rotated antitaxial calcite veins these rocks were sub horizontally extended parallel to bedding by up to 160 in an e w direction the marbles of the lowest exposed levels exhibit well developed crystallographic preferred orientation fabrics calcite c and a axis pole figures are symmetric to slightly asymmetric with obliquities ranging from 0 to 8 which indicate that the percent pure shear ranges between 100 and 76 initial constraints on the timing of this layer parallel extension suggest that extension was occurring locally within the sevier belt hinterland during the late cretaceous these data coupled with previous studies suggest that pure shear might be a much more common component of strain in rocks deformed in extensional environments than is generally acknowledged

the bright moho reflection in the 1986 nevada passcal seismic experiment abstract detailed study of the reflected phases from the moho identified in the seismic reflection recordings of the 1986 nevada passcal lithospheric seismic experiment place important constraints on the fine structure and geological nature of the mohorovicic discontinuity two different spectral ratio techniques are used to estimate the reflectivity of the moho we evaluate the ratio of the reflected pmp phase to the incident source amplitude we also calculated the spectral ratio of the pmp phase with a reference reflection reflectivity estimates calculated using both spectral ratio techniques are very consistent high reflectivity within the range 0 28 0 39 was obtained the analysis of the spectral characteristics suggests a layered structure for the moho with an average thickness between 85 and 250 m these results require a layered moho structure with high impedance contrasts amplitude modeling using a reflectivity algorithm demonstrates that constructive interference from a 1 0 1 5 km thick layered structure produces amplitudes and waveforms comparable to the data to achieve such high reflectivities 20 25 of the moho consists of layers with velocities below 6 5 km s the remaining layers have velocities between 7 7 and 8 1 km s geologically the low velocities are consistent with velocities characteristic of basaltic magmas partial melts the high velocities are characteristic of mantle material the partial melts may represent an expression of present day underplating

the miocene great basin of western north america as an extending back arc region abstract a narrow continental volcanic arc has lain near the western margin of the north american continent since mid miocene time its south end began retreating northward 18 m y ago the active remnant of this andesitic arc today is the cascade range of oregon washington and british columbia a large part of the great basin and region to its north was situated throughout this period first wholly then partially in a back arc position relative to the arc this region had and still has most of the characteristics of back arc regions elsewhere in the world a thinned faulted and extended crust and lithosphere limited development of a geophysical bilateral symmetry a mid to late miocene symmetry axis is marked by narrow swarms of basaltic dikes with associated positive linear magnetic anomalies that are parallel to the contemporary trench high heat flow low bouguer gravity and seismic wave velocity values notable seismic attenuation in the mantle and a fault plane solution record dominated by normal fault mechanisms this back arc region differs somewhat from most others those composed of oceanic crust because of its continental setting and recent geologic history despite its name the great basin stands high nearly 2 km above sea level this high elevation owes in part to the fact that extension and heating of the continental lithosphere are still underway though not as a back arc phenomenon the present lithospheric stretching results from oblique extension near the transform boundary of the north american plate the san andreas fault system the latest stretching has probably maintained the physical state of the lithosphere first developed during back arc spreading

resistivity saturation of hcp fe si alloys in an internally heated diamond anvil cell a key to assessing the earth s core conductivity abstract electrical resistivity and thermal conductivity of iron fe light element alloys at high pressure and temperature are key parameters to constrain the dynamics and thermal evolution of the earth s core we determined the electrical resistivity of hcp fe 2 4 and 6 5 wt silicon si alloys up to 117 gpa and 3120 k using a four terminal method in an internally heated diamond anvil cell the temperature dependence of electrical resistivity of hcp fe si alloys was suppressed as both si concentration and temperature increased which indicates the resistivity saturation phenomenon the electrical resistivity of metal asymptotically approaches the saturation resistivity our results are fully reproduced by a highly resistive saturation model and the obtained saturation resistivities for hcp fe si alloys are comparable to those for hcp pure fe at around 100 gpa if si is a major light element in the earth s core the pure fe like saturation resistivity would keep the core conductivity high enough to induce active dynamics there and rapid growth of the inner core

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

the snake range decollement an exhumed mid tertiary ductile brittle transition the snake range decollement srd in east central nevada separates supracrustal rocks extended by normal faulting from ductilely deformed igneous and metamorphic rocks a well known stratigraphy unaffected by earlier faulting permits analysis of both upper and lower plate strain leading to a better understanding of how vastly different rock types and deformational styles are juxtaposed along low angle faults in metamorphic core complexes middle cambrian to permian upper plate rocks are cut by two generations of ne trending east directed normal faults both generations were initiated as high angle 60 planar faults that flattened abruptly into the srd and rotated domino style to low angles yielding a total rotation of bedding of about 80 90 faulting is tertiary in age as 35 m y old volcanic rocks are involved and resulted in about 450 500 extension in a n55w s55e direction the srd developed as a subhorizontal surface 6 7 km deep at the top of the cambrian pioche shale lower plate granitic rocks and their late precambrian cambrian metamorphic country rocks were involved in progressive ductile to brittle extension at low greenschist grade forming a penetrative subhorizontal foliation and n55 70w lineation that increases in intensity eastward and upward toward the srd stretching and thinning in the lower plate is coaxial and comparable in magnitude to upper plate extension and is interpreted as synchronous k ar ages ranging from 20 to 40 m y in the lower plate suggests the n snake range represents a tertiary thermal anomaly we conclude that the srd developed as a ductile brittle transition zone at 6 7 km depth gravity data suggests that the gently domed srd is cut by younger basin and range faults but the geology of adjacent ranges suggests that the srd does not continue more than 60 km in any given direction the lack of stratigraphic omission across the srd rules out large amounts of movement on a surface that originally cut downsection and we suggest that extensional detachment faults such as the srd can be developed locally as boundaries between brittlely extended rocks and underlying ductile extension and intrusion

emplacement of a fold nappe betic orogen southern spain the aguilon nappe sierra alhamilla southeast spain is a north closing recumbent isoclinal fold with a greatly thinned lower limb this large scale geometry can be demonstrated using small scale structures the fold nappe is detached along the lower limb and lies on a carpet of mylonitic schist mylonite and cataclasite formation of the nappe must have involved northward relative transport a point of current controversy in the betic zone during emplacement the nappe underwent extensive ductile deformation accomplished partly by pressure solution of silica in the highly deformed footwall rocks however quartz was deformed by crystal plastic processes the change in deformational mechanism suggests a downward increase in deviatoric stress which is compatible with a gravitational mechanism of nappe emplacement the core of the low grade nappe is occupied by medium grade schist the abrupt change in grade suggests that the schist contact is an extensional fault that excised part of the original metamorphic zonation and was subsequently refolded in the nappe such relations are common in the betic zone and suggest extension by listric normal faulting a characteristic feature of nappes emplaced by gravity spreading

structural discordance between neogene detachments and frontal sevier thrusts central mormon mountains southern nevada detailed geologic mapping in the mormon mountains of southern nevada provides significant insight into processes of extensional tectonics developed within older compressional orogens a newly discovered wsw directed low angle normal fault the mormon peak detachment juxtaposes the highest levels of the frontal most part of the east vergent mesozoic sevier thrust belt with autochthonous crystalline basement palinspastic analysis suggests that the detachment initially dipped 20 25 to the west and cut discordantly across thrust faults nearly complete lateral removal of the hanging wall from the area has exposed a 5 km thick longitudinal cross section through the thrust belt in the footwall while highly attenuated remnants of the hanging wall nowhere more than a few hundred meters thick structurally veneer the range the present arched configuration of the detachment resulted in part from progressive domino style rotation of a few degrees while it was active but is largely due to rotation on younger structurally lower basement penetrating normal faults that initiated at high angle the geometry and kinematics of normal faulting in the mormon mountains suggest that pre existing thrust planes are not required for the initiation of low angle normal faults and even where closely overlapped by extensional tectonism need not function as a primary control of detachment geometry caution must thus be exercised in interpreting low angle normal faults of uncertain tectonic heritage such as those seen in the cocorp west central utah and birp s moist deep reflection profiles although thrust fault reactivation has reasonably been shown to be the origin of a very few low angle normal faults our results indicate that it may not be as fundamental a component of orogenic architecture as it is now widely perceived to be we conclude that while in many instances thrust fault reactivation may be both a plausible and attractive hypothesis it may never be assumed

the mohorovicic discontinuity beneath the continental crust an overview of seismic constraints abstract the seismic signature of the moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image seismic data of different types passive source earthquake broad band recordings and controlled source seismic refraction densely recorded wide angle deep seismic reflection and normal incidence reflection using vibroseistm explosives or airguns have contributed to the description of the moho as a relatively complex transition zone of critical importance for the quality and resolution of the seismic image are the acquisition parameters used in the imaging experiments a variety of signatures have been obtained for the moho at different scales generally dependent upon bandwidth of the seismic source this variety prevents the development of a single universally applicable interpretation in this way source frequency content and source and sensor spacing determine the vertical and lateral resolution of the images respectively in most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the moho and its relationship to a petrologic crust mantle transition in regional seismic studies carried out using passive source recordings the moho is a relatively well defined structure with marked lateral continuity the characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area refraction and wide angle studies suggest the moho to be often a relatively sharp velocity contrast whereas the moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity the origin of the moho and its relation to the crust mantle boundary is probably better constrained by careful analysis of its internal details which can be complex and geographically varied unlike the oceanic moho which is formed in a relatively simple well understood process the continental moho can be subject to an extensive variety of tectonic processes making overarching conclusions about the continental moho difficult speaking very broadly 1 in orogenic belts still undergoing compression and active continental volcanic arcs the moho evolves with the mountain belt 2 in collapsed phanerozoic orogenic belts the moho under the collapse structure was formed during the collapse often by a combination of processes 3 in regions having experienced widespread basaltic volcanism the moho can result from underplated basalt and basaltic residuum in precambrian terranes the moho may be as ancient as the formation of the crust in others precambrian tectonic and magmatic processes have reset it we note that seismic reflection data in phanerosoic orogens as well as from precambrian cratonic terranes often show thrust type structures extending as deep as the moho and suggest that even where crust and mantle xenoliths provide similar age of formation dates the crust may be semi allochothonous

uplift and extension at the gulf of suez indications of induced mantle convection reconstruction of tectonic movements since the initiation of the gulf of suez rift indicates that there has been 25 27 km of extension the uplift bordering rift indicates that lithosphere heating greatly exceeds that producible by uniform lithosphere extension i propose that small scale convection induced by the rifting augments the heating introduced by extensional rifting and produces the broad uplifts flanking the rift

shear zone model for the origin of metamorphic core complexes core complex tectonite results from simple shear rotational strain within regional ductile shear zones that have accommodated a tertiary crustal stretching the emplacement of younger and or upper level nontectonite rocks on older and or deeper level tectonites is the ordinary and predictable result of normal slip translation along shear zones that cut upward through the local geologic column progressive deformation produces a structural stratigraphy marked from bottom to top by normal country rock protolith mylonitic gneiss tectonite carapace microbrecciated tectonite microbreccia decollement and upper plate detachment rocks as young as mid miocene the reconstructions suggest that local regional crustal stretching locally exceeded 100

the snake range decollement interpreted as a major extensional shear zone geological and geophysical constraints suggest that the snake range decollement of east central nevada is a major tertiary low angle normal fault zone this interpretation is consistent with all existing data and alleviates problems that result if large displacement across the decollement is excluded miller et al 1983 based on published surface geology and cocorp seismic reflection profiling we have constructed cross sections that suggest approximately 60 km of normal displacement on the decollement advantages of this interpretation over models that exclude large displacement are that it provides for overburden consistent with kyanite grade metamorphism of footwall rocks predicts reasonable crustal thicknesses before and after extension without invoking unexposed mantle derived tertiary intrusions at depth and explains contrasting metamorphism and structural styles of hanging wall and footwall without requiring an extreme geothermal gradient during regional metamorphism and extensional strain the present domal shape of the decollement is interpreted to result at least partly from reverse drag above an underlying younger detachment related to much of the extension in ranges to the west of the snake range

tertiary structural development of selected valleys based on seismic data basin and range province northeastern nevada reflexion seismic data in railroad diamond mary s river and goshute valleys provide information on their structural development that cannot be deduced solely from outcrop and well data these valleys contain tertiary sediments that in dip section define an asymmetrical basin bounded along the eastern flank by a major listric normal fault with about 3 0 4 5 km of displacement the west flank is defined by a gentle east dipping ramp seismically the trace of the listric fault is interpreted to dip westward and sole into the palaeozoic section exploiting regionally recognized mesozoic decollement surfaces the tertiary depocentre adjacent to this fault shifted from west to east with continued slippage through time the greatest movement occurring in the miocene and post miocene in the strike direction the valleys are separated into at least two subbasins by an east west structurally high axis the axis is postulated to be the result of a tear fault associated with movement along the listric normal fault tertiary stratigraphy varies between valleys and between sub basins in a given valley all the valleys contain miocene and younger rocks however not all sub basins contain the pre miocene section suggesting a complex scheme of structural development

forward modeling of non steady state deformations and the minimum strain path reply structural analyses of shear zones often rely on an assumption of steady state behavior i e that the ratio of pure shear strain rate s to simple shear strain rate s remains fixed throughout deformation however geological deformations are not necessarily steady state non steady state deformation paths can be theoretically modeled if certain deformation parameters such as strain or offset are specified we have analyzed a two dimensional case of specified offset and geometry termed the minimum strain path the minimum finite strain needed to produce a fixed offset across a shear zone is neither simple shear nor pure shear but a combination of the two the minimum strain path if this deformation accumulates with a steady state deformation the kinematic vorticity number w of the minimum strain path varies with the amount of finite offset although w approaches 0 7 at high offset values because of this relation between w and finite offset the minimum strain path is better modeled as a non steady state deformation in which case deformation history starts close to simple shear but rapidly changes to a more pure shear dominated deformation it is expected that the minimum strain path is applicable to geological deformation zones with relaxed boundary conditions such as basal parts of spreading nappes or extensional detachment systems 0 1997 elsevier science ltd

chemical characterization of some potsherd pavements from parts of yorubaland in southwestern nigeria several samples of potsherd pavement were recovered during excavation and archaeological reconnaissance of six sites along the trade and migration route of the yoruba of southwestern nigeria a significant feature of these pavements is the similarity to ife in design which consists of a herringbone design with a ritual pot embedded in their middle microscopic analysis of samples shows the presence of high proportions of non plastic grains mainly quartz micas minor plagioclases and traces of ferruginous grains tourmaline and amphibole interpreted as being produced by low temperature firing major trace and rare earth elements composition of 21 samples of sherds was determined using inductively coupled plasma icp ms technique the chemistry of the potsherd samples is generally similar to regolith derived from granitic rocks which are abundant in the area principal component analysis pca was used to reduce the number of variables and model based cluster analysis was used to find clusters in the pca scores model based clustering found one major cluster in the data set which corresponds to the local weathered granitic pegmatitic raw materials near the sites our work shows that the technology for the manufacture of these pavements might have originated from ile ife but the production was geologically controlled

the nature of seismic reflections from deep crustal fault zones deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement perhaps the best example is the consortium for continental reflection profiling cocorp wind river line in wyoming which shows the wind river thrust fault to be a strong reflector from the surface to depths of about 30 km to identify the physical properties responsible for the seismic reflections from fault zones we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones these rocks are characterized by extensive ductile deformation of plastic minerals brittle deformation of the more rigid minerals grain size reduction and development of a strong fabric average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock seismic anisotropy of 7 or greater is present in several mylonites dependent upon their composition and fabric using our data on mylonite properties and models for the crustal structure near the wind river line we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the cocorp wind river reflection data when anisotropy and the finely laminated structure of the fault zone are considered our modeling indicates fairly strong reflections from mylonite zones in most cases we also show that elevated pore pressure in shear zones may produce strong reflections however a permeability of the order of 10 16 darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive

si isotope systematics of meteorites and terrestrial peridotites implications for mg si fractionation in the solar nebula and for si in the earth s core abstract high precision si isotope ratios have been measured for a series of meteorites and terrestrial samples using high resolution multi collector icp ms our results differ from those reported in an earlier study georg et al 2007 si in the earth s core nature 447 1102 1106 in two important aspects first our data set reveals systematic differences in 30 si between different chondrite groups that are correlated with their mg si elemental ratio second in agreement with the previous study 30 si for the terrestrial samples are higher than values for chondrites but the difference between the bulk silicate earth bse and the carbonaceous chondrites 30 si bse c arbonaceous chondrites 0 08 0 04 1 standard deviation is about a factor of 2 smaller than previously reported the 30 si versus mg si trend defined by the chondrite groups can be explained by reaction of olivine with a sio rich vapor to form enstatite starting from a carbonaceous chondrite composition in contrast the difference between the bse and carbonaceous chondrites must reflect a different process and can be explained by incorporation of si into the earth s core during metal silicate equilibration in a deep magma ocean the observed si isotope fractionation is consistent with the temperatures and pressures of metal silicate equilibration derived from siderophile element abundances in the earth s mantle

seismic reflectivity of mylonite zones in the crust deep fault zones are characterized by mylonitic rocks with strong preferred orientation of constituent minerals and retrograde mineral assemblages sonic velocities are consequently lower for propagation directions normal to mylonite foliation than in surrounding unmylonitized rocks synthetic seismograms computed for low velocity near horizontal thick mylonite zones of complex laminated geometry show multicyclic reflections with amplitudes up to twice those generated by a single interface seismic characteristics of mylonite zones are sufficient to produce reflections in crystalline rocks provided noise levels are relatively low

age and temperature constraints on metamorphism and exhumation of the syn orogenic metamorphic complexes of northern apennines italy abstract the apennine range is a young convergent orogen that formed over a retreating subduction zone syn orogenic metamorphic rocks are exposed in the mid tuscan ridge tuscan coast and tuscan archipelago and provide information about processes of continental subduction and exhumation associated with accretion and retreat apatite and zircon fission track analyses were applied to some of these syn orogenic metamorphic rocks the new low temperature thermochronological data helped to constrain the timing of the metamorphic peak and the age and modality of exhumation of these rocks that were structurally buried at 20 to 40 km depth during the main tectonogenic phase of the northern appenine belt in the giglio and elba islands and in the monti dell uccellina along the coast minimum ages of partially reset zircon samples yielded a first time constraint on the late oligocene age of the metamorphic peak the ca 27 ma obtained at the monti dell uccellina is coeval to that found in the alpi apuane located at the northern termination of the mid tuscan ridge the data from the tuscan archipelago point to a slightly older age in the monti pisani south of the alpi apuane and in the argentario promontary along the coast south of the monti dell uccellina completely reset zircon samples but composed by different populations were behaving as multiple thermochronometers and confirm the already proposed isothermal nature of the exhumation occurred after the metamorphic peak zircon p1 peak indicated that at 13 10 ma rocks were already at shallow crustal levels

high pressure melting experiments on fe si alloys and implications for silicon as a light element in the core abstract we carried out melting experiments on fe si alloys to 127 gpa in a laser heated diamond anvil cell dac on the basis of textural and chemical characterizations of samples recovered from a dac a change in eutectic liquid composition in the fe fesi binary system was examined with increasing pressure the chemical compositions of coexisting liquid and solid phases were quantitatively determined with field emission type electron microprobes the results demonstrate that silicon content in the eutectic liquid decreases with increasing pressure to less than 1 5 0 1 wt si at 127 gpa if silicon is a single light element in the core 4 5 to 12 wt si is required in the outer core in order to account for its density deficit from pure iron however such a liquid core whose composition is on the si rich side of the eutectic point crystallizes less dense solid cscl b2 type phase at the inner core boundary icb our data also show that the difference in silicon concentration between coexisting solid and liquid is too small to account for the observed density contrast across the icb these indicate that silicon cannot be the sole light element in the core previous geochemical and cosmochemical arguments however strongly require 6 wt si in the core it is possible that the earth s core originally included 6 wt si but then became depleted in silicon by crystallizing sio2 or mgsio3

paleomagnetic results for eocene volcanic rocks from northeastern washington and the tertiary tectonics of the pacific northwest the direction of remanent magnetization for 102 sites in eocene volcanic and volcaniclastic rocks of the o brien creek formation sanpoil volcanics and klondike mountain formation suggests approximately 25 of clockwise rotation of a 100 by 200 km area in northeastern washington the volcanic rocks consist chiefly of rhyodacite and quartz latite flows with intercalated ash flow tuff and volcaniclastic layers these rocks have been sampled at 102 sites distributed among five volcanotectonic depressions the toroda creek republic keller and first thought grabens and the spokane enterprise lineament the volcanic rocks probably range in age from 55 m y to about 48 m y and the 50 to 48 m y old volcanic rocks within this suite appear to be rotated as much as the older rocks previous investigators have shown that 40 m y old and younger plutonic rocks of northwestern washington are not rotated hence we infer that the north central washington rocks were rotated to their present declination between 48 and 40 m y b p during the middle and or late eocene during early eocene time this region was extended in a westward direction through crustal necking gneiss doming diking and graben formation internal deformation of the region related to this crustal extension was extreme but most bedrock units that were formed concurrent with the crustal extension were probably in place prior to the rotation hence we infer that the rotation was chiefly accommodated by movement on faults peripheral to the sampled area faults active during paleogene time appear to define boundaries of a triangular crustal block the sanpoil block encompassing much of northeastern washington northern idaho northwestern montana and adjacent parts of british columbia the faults include the laramide thrusts of the rocky mountain thrust belt the strike slip faults of the lewis and clark line and strike slip faults of the straight creek fraser zone we suggest that during early eocene time the sanpoil block was extended westward through crustal necking and dilation and then during the middle eocene was rotated clockwise and thrust over the craton in a final stage of laramide thrusting the motor driving these deformations presumably was interaction of north america with oceanic lithosphere off its western margin such interaction probably involved right oblique underthrusting and dextral shear

experimental determination of the si isotope fractionation factor between liquid metal and liquid silicate abstract the conditions of core formation and the abundances of the light elements in earth s core remain debated silicon isotope fractionation provides a tool contributing to this subject we present experimentally determined si isotope fractionation factors between liquid metal and liquid silicate at 1450 c and 1750 c which allow calibrating the temperature dependence of si isotope fractionation experiments were performed in a centrifuging piston cylinder at 1 gpa employing both graphite and mgo capsules tin was used to lower the melting temperature of the metal alloys for experiments performed at 1450 c tests reveal that neither sn nor c significantly affects si isotope fractionation an alkaline fusion technique was employed to dissolve silicate as well as metal phases prior to ion exchange chemistry and mass spectrometric analysis the results show that metal is consistently enriched in light isotopes relative to the silicate yielding average metal silicate fractionation factors of 1 48 0 08 and 1 11 0 14 at 1450 c and 1750 c respectively the temperature dependence of equilibrium si isotope fractionation between metal and silicate can thus be described as 30 si metal silicate 4 42 0 05 10 6 t 2 the si isotope equilibrium fractionation is thus about 1 7 times smaller than previously proposed on the basis of experiments a consequence of this smaller fractionation is that the calculated difference between the si isotope composition of the bulk earth and that of the bulk silicate earth generated by core formation is smaller than previously thought it is therefore increasingly difficult to match the si isotope composition of the bulk silicate earth with that of chondrites for metal silicate equilibration temperatures above 2500 k this suggests that si isotopes were more sensitive to the early stages of core formation when low oxygen fugacities allowed significant incorporation of si into metal

composition of the earth s inner core from high pressure sound velocity measurements in fe ni si alloys article i nfo we performed room temperature sound velocity and density measurements on a polycrystalline alloy fe0 89ni0 04si0 07 in the hexagonal close packed hcp phase up to 108 gpa over the investigated pressure range the aggregate compressional sound velocity is 9 higher than in pure iron at the same density the measured aggregate compressional vp and shear vs sound velocities extrapolated to core densities and corrected for anharmonic temperature effects are compared with seismic profiles our results provide constraints on the silicon abundance in the core suggesting a model that simultaneously matches the primary seismic observables density p wave and s wave velocities for an inner core containing 4 to 5 wt of ni and 1 to 2 wt of si

simultaneous sound velocity and density measurements of hcp iron up to 93 gpa and 1100 k an experimental test of the birch s law at high temperature we performed sound velocity and density measurements on polycrystalline hexagonal close packed hcp iron at simultaneous high pressure and high temperature up to 93 gpa and 1100 k by inelastic x ray scattering and x ray diffraction our experimental results indicate that high temperature anharmonic corrections are negligible at least up to 1100 k and that the aggregate compressional velocity vp scales linearly with density over the pressure and temperature range of the investigation birch s law the new results are compared with literature studies and we discuss the extrapolation schemes commonly used in experimental mineral physics with specific regard to extrapolations to the earth s core conditions

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

cainozoic evolution of the state of stress and style of tectonism of the basin and range province of the western united states cainozoic evolution of the modern plate boundary along the western united states from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western united states extensions tectonism responsible for the modern basin and range province appears to represent a unique late stage episode of a much longer period of extension initiated in an in tra arc setting contemporaneously with calc alkaline magmatism basin range extension is distinguished from early extension on the basis of angular unconformities differences in fault trends and spacing and associated magmatism basaltic prebasin range extension i e extension preceding the break up of the region into ranges resembling the modern ones was under way locally by at least 30 ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks by large regions of the crust underlain by passively emplaced subvolcanic batholiths and by the thickness and distribution of stratigraphic units locally high strain rates that accompanied early extensions of as much as 50 100 are implied data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform wsw ene least principal stress orientation in the period ca 20 10 ma during this early extension the change from early extension to basin range style faulting of the upper 15 km of crust which resulted in broadly spaced ranges 25 35 km crest crest spacing was time transgressive and probably not abrupt locally both types occurred concurrently southern basin and range block faulting occurred largely in the period 13 10 ma in response to a stress field orientated similarly to that responsible for the early extension in contrast northern basin and range block faulting developed after 10 ma and continues to the present in response to a stress field orientated approximately 45 clockwise to the earlier stress field this modern stress field with a wnw ese to e w directed least principal stress characterizes the entire modern basin and range province and rio grande rift region the 45 change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the san andreas transform fault inclusion of pre basin range extension may help resolve the discrepancy between estimates of 15 30 for basin range block faulting and total extension estimates of 100 300 for the basin and range province

extensional tectonics in the death valley area california transport of the panamint range structural block 80 km northwestward in the hypothesis presented here the panamint range is a structural block that was detached during late cenozoic time from underlying rocks that now form the black mountains and was transported tectonically an estimated 80 km to the northwest transport occurred along a single low angle westward dipping detachment fault or perhaps along a system of such faults the estimate of 80 km of transport distance is based on the apparent right lateral offset of late precambrian and paleozoic facies and thickness trends along the furnace creek fault zone that bounds the detached block on the northeast the death valley turtleback surfaces are considered to be gigantic mullions related to the detachment surface

role of tectonic denudation in warping and uplift of low angle normal faults large displacement on a low angle normal fault results in isostatic uplift of the lower plate in response to tectonic denudation simple models of the denudation process predict warping of the lower plate into a broad antiform or antiform synform pair with axes perpendicular to the direction of extension the amount of warping is strongly influenced by initial fault geometry surface topography the amount of extension and the distribution of extension within the upper plate late stage processes that augment antiform growth include one sided denudation of the antiform reverse faulting due to concave upward flexure and wholesale detachment and reverse drag folding of the antiform antiformal uplifts now exhumed by erosion form mountain ranges in the southwestern united states where conditions were favorable for warping during mid tertiary crustal extension four major structural domains are apparent in transects across these uplifts and adjacent areas 1 unextended area 2 synformal upper plate 3 antiformal uplift and 4 wedge shaped upper plate each domain is separated from adjacent domains by the traces of a master low angle normal fault

crustal velocity structure beneath the eastern flank of the ruby mountains metamorphic core complex results from normal incidence to wide angle seismic data abstract p wave velocity structure along the eastern side of the ruby mountains metamorphic core complex determined using 2 d ray inversion modeling shows that 1 the crust can be divided into three layers corresponding to the upper middle and lower crust 2 from east of ruby dome to the northern end of the seismic profile velocities of the upper crust range from 6 12 0 1 to 6 20 0 1 km s and near surface velocities range from 1 90 0 1 to 4 80 0 1 km s from this point to the southern end of the profile upper crustal velocities range from 5 80 0 1 to 6 25 0 1 km s and near surface velocities range from 3 01 0 1 to 4 80 0 1 km s 3 middle crustal velocities range from 6 35 0 15 to 6 45 0 15 km s 4 lower crustal velocities range from 6 60 0 15 to 6 80 0 15 km s and 5 depths to the moho vary irregularly between 30 5 and 33 5 km from interpretation of these results we conclude that 1 the transition in metamorphic grade from deep upper crustal rocks south of harrison pass to middle crustal rocks north of harrison pass does not correlate with an increase in seismic velocity near surface basement seismic velocities increase in the central part of the range near ruby dome well north of the major metamorphic transition 2 depths to the moho do not reflect local surface relief isostatic balance is achieved predominantly in the crust rather than by formation of a local crustal root and 3 the crust has been modified during extension by intracrustal processes rather than by large scale magmatic underplating

cenozoic and mesozoic structure of the eastern basin and range province utah from cocorp seismic reflection data cocorp seismic reflection data collected from the eastern basin and range in west central utah provide information on cenozoic extensional tectonics mesozoic thrusting and their interrelationships those data show a series of remarkably continuous low angle reflectors that extend more than 120 km perpendicular to strike and can be traced as deep as 15 20 km over that distance none of these events are significantly cut by any high angle normal faults a major detachment beneath the sevier desert can be traced from a surface zone of normal faulting to a depth of 12 15 km with a regional apparent westward dip of 12 tentative correlation of upper and lower plate events suggests 30 60 km of extensional displacement on this detachment whether this structure is a reactivated mesozoic thrust is uncertain west steepening splays off the end of the detachment reach depths of 20 km and may represent a major mesozoic ramp or zones of distributed ductile shearing during extension some events are interpreted to be mesozoic thrusts of which at least one beneath the house range has been reactivated during the cenozoic the snake range decollement dips gently east and has a sense of cenozoic displacement opposite to that of other cenozoic detachments farther east deep events are most numerous beneath the east side of the sevier desert where they occur to depths of 30 km at the top of or perhaps partly within the anomalously low velocity upper mantle

brittle deformation in the columbia river fault zone near revelstoke southeastern british columbia the brittle columbia river fault zone forms part of the eastern margin of the shuswap metamorphic complex and for much of its 230 km length it coincides with the eastern exposure of the ductile monashee decollement the eocene brittle fracturing and displacement are broadly parallel with but cut and disrupt the middle jurassic ductile mylonite zone excavations for a hydroelectric dam and a highway have facilitated a detailed analysis of fracture patterns at several localities along the fractured zone regionally the brittle fault zone strikes 350 locally 310 030 and dips shallowly eastward over most of the studied part of the zone the distributions of subsidiary fractures and displacement vectors demonstrate a normal dip slip displacement consistent with subhorizontal east west extension within the fractured zone irrespective of the local trend of the zone at revelstoke damsite where the zone trends 030 for 2 km motion was towards the southeast possibly indicating a localized clockwise rota

phase relations in the fe fesi system at high pressures and temperatures the earth s core is comprised mostly of iron and nickel but it also contains several weight percent of one or more unknown light elements which may include silicon therefore it is important to understand the high pressure high temperature properties and behavior of alloys in the fe fesi system such as their phase diagrams we determined melting temperatures and subsolidus phase relations of fe 9 wt si and stoichiometric fesi using synchrotron x ray diffraction at high pressures and temperatures up to 200 gpa and 145 gpa respectively combining this data with that of previous studies we generated phase diagrams in pressure temperature temperature composition and pressure composition space we find the b2 crystal structure in fe 9si where previous studies reported the less ordered bcc structure and a shallower slope for the hcp b2 to fcc b2 boundary than previously reported in stoichiometric fesi we report a wide b2 b20 two phase field with complete conversion to the b2 structure at 42 gpa the minimum temperature of an fe si outer core is 4380 k based on the eutectic melting point of fe 9si and silicon is shown to be less efficient at depressing the melting point of iron at core conditions than oxygen or sulfur at the highest pressures reached onlymore the hcp and b2 structures are seen in the fe fesi system we predict that alloys containing more than 4 8 wt silicon will convert to an hcp b2 mixture and later to the hcp structure with increasing pressure and that an iron silicon alloy in the earth s inner core would most likely be a mixture of hcp and b2 phases less

the moho in the northern basin and range province nevada along the cocorp 40 n seismic reflection transect cocorp seismic reflection profiles across nevada at about 40 n image a prominent essentially continuous band of reflectors at a two way traveltime of 9 to 11 s the approximate correspondence of this reflection time with estimates of the two way traveltime to the moho in this area provided by seismic refraction data suggests that the prominent reflections are from the moho the relief on these reflectors the reflection moho beneath nevada across a latitudinal transect of

silicon in the earth s core the earth and moon have iron isotopic abundances that are slightly heavy compared with mars the asteroid vesta and primitive meteorites the reason for this has been unclear one suggestion is that there was a large scale isotopic equilibration during the giant impact the collision between the early earth and a mars sized body that created the moon but this has seemed unlikely given that most other elements do not show the effect an analysis of 44 meteorite and terrestrial samples now shows that the silicon isotopic compositions of basaltic rocks from the earth and moon are also distinctly heavy the finding is consistent with the isotopic equilibration model and it suggests that silicon was already partitioned into the earth s core as a light element before the moon formed the silicon isotopic compositions of basaltic rocks from the earth and moon are distinctly heavy the similar isotopic composition of the bulk silicate earth and the moon are consistent with large scale isotopic equilibration during the giant impact indicating that si was already a light element in the earth s core before the moon formed small isotopic differences between the silicate minerals in planets may have developed as a result of processes associated with core formation or from evaporative losses during accretion as the planets were built up basalts from the earth and the moon do indeed appear to have iron isotopic compositions that are slightly heavy relative to those from mars vesta and primitive undifferentiated meteorites1 2 3 4 chondrites explanations for these differences have included evaporation during the giant impact that created the moon when a mars sized body collided with the young earth however lithium5 and magnesium6 lighter elements with comparable volatility7 8 9 reveal no such differences rendering evaporation unlikely as an explanation here we show that the silicon isotopic compositions of basaltic rocks from the earth and the moon are also distinctly heavy a likely cause is that silicon is one of the light elements in the earth s core we show that both the direction and magnitude of the silicon isotopic effect are in accord with current theory10 based on the stiffness of bonding in metal and silicate the similar isotopic composition of the bulk silicate earth and the moon is consistent with the recent proposal11 that there was large scale isotopic equilibration during the giant impact we conclude that si was already incorporated as a light element in the earth s core before the moon formed

magnitude of crustal extension in the southern great basin strike slip faults in the southern great basin separate areas of cenozoic upper crustal extension from relatively stable tectonic blocks linear geologic features offset along the garlock fault las vegas valley shear zone and lake mead fault system allow reconstruction of the southern great basin to a pre extension configuration the sierra nevada mojave desert spring mountains and colorado plateau are treated as stable unextended blocks that have moved relative to each other in response to crustal extension with the spring mountains held fixed to the mojave block our reconstruction indicates a minimum of 65 extension 140 km between the southern sierra nevada and colorado plateau

the structure of fe si alloy in earth s inner core abstract phase relations of iron silicon alloy fe 6 5 wt si and fe 9 wt si were investigated up to 407 gpa and 5960 k in a laser heated diamond anvil cell which likely covers the entire pressure and temperature conditions of the earth s inner core synchrotron x ray diffraction measurements show that fe 9 wt si with a hexagonal close packed hcp structure is stable to 4800 k at 330 gpa corresponding to the pressure at the inner outer core boundary and decomposes into a mixture of si poor hcp and si rich cscl type b2 phases at higher temperatures we also found that the solubility of silicon in solid iron is relatively insensitive to temperature decreasing from 9 to 6 5 wt over a range of 1500 k at 70 gpa these suggest that the inner core is composed solely of the hcp phase when the silicon content is up to 7 wt that likely accounts for the inner core density deficit as well as for the mg si ratio and the si isotopic composition of the mantle additionally the present experiments demonstrate that the incorporation of silicon in iron expands the stability of hcp with respect to that of fcc

phase relations in the system fe ni si to 200 gpa and 3900 k and implications for earth s core abstract phase relations in fe 5 wt ni 4 wt si alloy was examined in an internally resistive heated diamond anvil cell under high pressure p and temperature t conditions to about 200 gpa and 3900 k by in situ synchrotron x ray diffraction the hexagonal close packed hcp structure was observed to the highest p t condition supporting the idea that the stable iron alloy structure in earth s inner core is hcp the p t locations of the phase transition between the face centred cubic fcc and hcp structures were also constrained to 106 gpa the transition occurs at 15 gpa and 1000 k similar to for pure fe the clausius clapeyron slope is however 0 0480 gpa k which is larger than reported slopes for fe 0 0394 gpa k fe 9 7 wt ni 0 0426 gpa k and fe 4 wt si 0 0394 gpa k stabilising the fcc structure towards high pressure thus the simultaneous addition of ni and si to fe increases the d p d t slope of the fcc hcp transition this is associated with a small volume change upon transition in fe ni si the triple point where the fcc hcp and liquid phases coexist in fe 5 wt ni 4 wt si is placed at 145 gpa and 3750 k the resulting melting temperature of the hcp phase at the inner core outer core boundary lies at 550 k lower than in pure fe

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

crustal evolution by arc magmatism arc magmas generated at depths near 100 km by dehydration of subducting slabs are olivine rich melabasalts but the magmas that reach the surface in mature continental magmatic arcs have an average composition near that of rhyodacite depth varying fractionations and equilibrations profoundly modify the initial magmas plate tectonics has operated throughout proterozoic and phanerozoic time much as it does now and so many deeply eroded terrains must expose products of arc magmatism most exposed middle and deep continental crust consists of igneous rocks plus older rocks equilibrated at magmatic temperatures variably deformed and metamorphosed subsequently each crustal level displays a typical assemblage of magmatic rock types which are here deduced to be products mostly of arc magmatism pre arc mantle consists of depleted dunite and harzburgite rising arc magmas precipitate much additional olivine and add the equivalent of 10 or 15 of basalt in clinopyroxene orthopyroxene garnet and spinel the seismic m discontinuity may be controlled primarily by the shallow limit of the depth zone in which crystallization is mostly of ultramafic components and the fractionated magmas that reach the crust are mostly of gabbroic to granodioritic compositions the lower crust is characterized by differentiated layered complexes which often contain mafic or ultramafic basal cumulates medial floated plagioclase anorthosite and upper quartz poor pyroxene mesoperthite granites and by other magmatic rocks magmatic rocks are more voluminous than pre existing rocks supracrustal rocks are in middle granulite facies and much granitic material has been melted from them by magmatic heat the magmatically modified middle crust consists primarily of migmatites in lower granulite facies in the deeper part and upper amphibolite facies in the shallower part much dissociation of hydrous mineral assemblages is caused by the magmatic heat producing water rich aluminous magmas assimilation and anatexis the high water contents restrict rise of the equilibrated magmas voluminous pegmatites are expelled into the wall rocks and crystallization is forced sheets of two mica granites characterize the upper part of the middle crust the comparatively dry magmas that rise into the upper crust are mostly tonalite to adamellite these magmas spread out in steep sided batholiths above the migmatites erupt as ash flow sheets from calderas and produce voluminous far travelling volcanic ash inverted metamorphic gradients and outward verging structures are produced beneath the spreading batholiths magmatic arcs are extensional at all crustal levels

displacement and strain patterns of extensional orogens rock fabrics that result from displacement in extensional orogens provide a means of identifying geometric models responsible for extension of continental crust strain compatibility arguments indicate that a finite extension can be accommodated by displacements across 1 planar nonrotating faults or ductile shear zones 2 shear zones which rotate above a horizontal detachment the domino model or 3 shear zones which rotate as a result of a horizontally oriented pure shear stretching component the plastic model listric normal shear zone geometries may develop as the result of a depth dependent change in the pure shear component in the plastic model or the area loss component in the planar shear zone model within the geometric framework of these various models the effects of superposed simple shear pure shear extension and area change on the rock fabric are investigated these displacement components which may be superposed sequentially or simultaneously determine the state of finite strain associated with a given magnitude of tectonic extension the relationships between displacement and strain are expressed as graphs of foliation dip or strain ratio versus shear strain and as graphs of foliation dip or strain ratio versus tectonic extension the shear strain graphs illustrate the effects of the different displacement components on the rock fabric in a single shear zone whereas the tectonic extension graphs better illustrate these effects on the regional scale the shear strain and tectonic extension graphs can be used by field geologists 1 to determine tectonic extension across a region from measurements of strain ratio or orientation and fluctuation of foliation 2 to distinguish between possible extension models from field data and 3 to evaluate the influence of shear zone attitude and thickness upon the amount of displacement applications of these methods are illustrated for field areas in metamorphic core complexes of arizona and in the basin and range province of the north america cordillera

dynamics of rifting and modes of extension on icy satellites 1 a simple numerical model of extension in icy satellite shells is developed thinning of the ice weakens the shell promoting further extension if lateral flow in the lower part of the shell is unimportant extension is opposed and wide rifts are generated if lateral flow is rapid localized extension is favored and narrow rifts are produced thick shells or high strain rates favor the development of narrow rifts low strain rates favor wide rifting it is proposed that bands extensional features on europa are narrow rifts while groove lanes on ganymede are wide rifts the existence of wide rifting on ganymede is consistent with previous estimates of a conductive shell thickness at the time of rifting of 4 8 km and a strain rate of 10 15 s 1 strain rates at the time of rifting whether this shell thickness applies to present day europa depends on the age of band formation which is poorly known the difference between rifting behavior on ganymede and europa is due to either higher strain rates or higher shell thicknesses on europa during rifting the mean stresses required to cause the observed rifting are 0 2 mpa for ganymede and 0 3 mpa for europa these values are comparable to estimates derived from flexural features index terms 5475 planetology solid surface planets tectonics 8149 6218 planetology solar system objects jovian satellites 8109 tectonophysics continental tectonics extensional 0905 8160 tectonophysics rheology general keywords europa ganymede extension faulting ice

metamorphic core complexes of cordilleran type in the cyclades aegean sea greece the islands of ios and naxos in the cycladic archipelago greece contain elongate domes over which a pervasive north to north northeast stretching lineation is warped these islands may be metamorphic core complexes of cordilleran type involving middle crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south from ca 25 ma onward during uplift the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow dipping ductile shear zone this shear zone may have connected with low angle normal faults in its upper levels because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks this model can be extended to explain the exhumation of the high pressure low temperature metamorphic rocks on crete which with the cyclades forms a distended paired metamorphic belt separated by an active sedimentary basin