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

crustal structure and dynamics of botswana we present the first nationwide crustal thickness and vp vs maps for botswana based on the analysis of new p wave receiver functions of nars botswana network integrated with previous receiver function results in botswana using h k analysis we found crustal thickness values ranging from 34 km for the okavango rift zone to 49 km at the border between the magondi belt and the zimbabwe craton for stations with significant sediment cover a sediment correction was applied based on sequential h k stacking we observed distinct differences for the kaapvaal craton the eastern part has a high vp vs ratio typical of a predominantly mafic composition suggesting lateral extension of the bushveld mafic complex on the other hand the western part with a vp vs ratio of 1 67 is felsic probably as a result of delamination caused by proterozoic rifting processes further to the west of the kaapvaal craton we found a crustal thickness of 42 km and a vp vs ratio of 1 76 for the nosop basin these values are similar to other cratonic regions in botswana suggesting the presence of a buried craton as proposed by previous studies we confirm a relatively thin crust 34 39 km compared to the rest of botswana and high vp vs ratio 1 84 underneath the okavango rift zone found by previous receiver function studies notably we also found a relatively thin crust 37 km and high vp vs ratio 1 84 in central botswana underneath the passarge basin

permo carboniferous paleoclimate of the congo basin evidence from lithostratigraphy clay mineralogy and stable isotope geochemistry abstract approximately 1000 m of strata in the upper paleozoic lukuga formation in the dekese core in the central congo basin provide lithostratigraphic mineralogical and isotopic evidence for substantial climatic variation within a long lived lacustrine basin lithostratigraphic indicators of cold climate include polymictic strata dropstone deposits and coupled laminations of fine clay size material and coarse silt glacial varves dropstones are concentrated in three stratigraphic zones in the lower 425 m of the lukuga formation and varved strata occur in two broad stratigraphic zones in the lower 700 m of the formation these sedimentological indicators suggest that the lower 2 3 of the lukuga formation was strongly influenced by frigid conditions and glacial like processes the clay size fraction of 97 samples is dominated by detrital minerals including quartz feldspar chlorite illite and poorly ordered expansible 2 1 phyllosilicates based on variation in the mineralogy of these samples the lukuga formation is divisible into three clay mineral zones cmzs numbered in ascending stratigraphic order cmz 1 and cmz 3 include several horizons of expansible 2 1 phyllosilicates that represent warmer wetter intervals intervening cmz 2 is a long 500 m zone of chlorite and illite with no expansible phyllosilicates and is interpreted as a continuous cold frigid interval there are numerous calcite cemented layers including spar filled veins that cross depositional bedding and represent postburial alteration radiaxial fibrous cements that displace detrital grains and micrite that crystallized near the time of deposition eighty two stable isotope analyses of micrite yield 13c values that range from 44 6 to 4 1 and 18o values that range from 20 0 to 5 0 vpdb the carbon isotope data likely reflect a range of local carbon sources derived from bacterial activity and are unrelated to paleoclimatic conditions in contrast stratigraphic patterns in the oxygen isotope data suggest five or six intervals of frigid conditions conducive to glacial processes

observations of regional phase propagation across the tibetan plateau we present observations of regional phase velocity and propagation characteristics using data recorded during a 1 year deployment of broadband digital seismic stations across the central tibetan plateau along the qinghai tibet highway from golmud to lhasa previous seismological studies within this region have had to rely on earthquakes recorded almost exclusively at stations outside of the plateau we have the opportunity to study numerous source receiver paths confined entirely within the tibetan plateau our analysis concentrates on travel time amplitude and frequency content measurements of the pg pn and sn phases pn can be clearly picked for all observed paths and propagates at an average velocity of 8 16 0 07 km s within the tibetan plateau sn however shows dramatic variations in propagation efficiency across the tibetan plateau that is strongly dependent on frequency we observe that sn rapidly decreases in frequency and amplitude as it passes through the northern portion of the plateau we show that in general sn propagation efficiency decreases with increasing frequency content we use 122 events from outside of the plateau and 61 from within to refine the boundaries of a region of inefficient high frequency sn propagation specifically we show that a larger portion of the northern tibetan plateau attenuates sn energy than was previously suggested in the southern plateau where high frequency sn is observed we computed an average velocity of 4 59 0 18 km s we also observed that the pn velocity within this region of inefficient high frequency sn propagation is nearly 4 slower than the pn velocity computed for paths restricted to the southern plateau and that the crust is about 10 km thinner than in the south the coincident locations of inefficient sn propagation and slow pn velocity is commonly observed in regions of active tectonics our results add constraints to the velocity structure of the lithosphere beneath the tibetan plateau and require first order lateral variations in the uppermost mantle structure despite the relatively uniform topography of the plateau

crustal and upper mantle shear wave velocity structure of botswana the 3 april 2017 central botswana earthquake linked to the east african rift system rayleigh wave group and phase velocity measurements obtained from ambient noise and earthquake data at 51 broadband stations were used to construct the first 3 d crustal and upper mantle shear wave velocity model of botswana the model shows low crustal velocities associated with the passarge and nosop sedimentary basins whereas the kaapvaal zimbabwe maltahohe and congo cratons are recognized by high mantle velocities the lowest upper mantle shear wave velocity beneath northeastern botswana is associated with the southwestern branch of the east african rift system this low velocity mantle anomaly appears to be linked to the crust of the okavango rift zone and the location of the 3 april 2017 mw 6 5 earthquake in central botswana we suggest that fluids or melt at the base of the crust from the southward continuation of the east african rift zone triggered the intraplate earthquake in an extensional tectonic setting

upper mantle p and s wave velocity structure of the kalahari craton and surrounding proterozoic terranes southern africa new broadband seismic data from botswana and south africa have been combined with existing data from the region to develop improved p and s wave velocity models for investigating the upper mantle structure of southern africa higher craton like velocities are imaged beneath the rehoboth province and parts of the northern okwa terrane and the magondi belt indicating that the northern edge of the greater kalahari craton lithosphere lies along the northern boundary of these terranes lower off craton velocities are imaged beneath the damara ghanzi chobe belt and may in part reflect thinning of the lithosphere beneath the incipient okavango rift lower velocities are also imaged to the north and northwest of the bushveld complex beneath parts of the okwa terrane magondi belt and limpopo belt indicating that cratonic upper mantle in some areas beneath these terranes may have been modified by the 2 05 ga bushveld and or 1 1 ga umkondo magmatic events

compressional velocity structure and anisotropy in the uppermost mantle beneath italy and surrounding regions travel times of about 39 000 pn arrivals recorded from regional earthquakes by the italian telemetered seismic network and by stations of nearby countries are inverted to image lateral variations of seismic velocity and anisotropy at subcrustal depth in italy and surrounding regions this method allows simultaneous imaging of variations of pn velocity and anisotropy as well as crustal thickness variations the po plain the adriatic sea and the ionian sea have normal to high pn velocities in contrast lower velocities 7 9 8 0 kin s are imaged in italy beneath the western alps the northern apennines and eastern sicily and nearby calabria as well as in northern albania and beneath the pannonian basin low pn velocities beneath the northern apennines correlate with present day extension and may have resulted from thermal anomalies in the uppermost mantle possibly due to delamination processes low velocities are consistent with the high attenuation zone inferred in the uppermost mantle beneath the internal apennine units and the tyrrhenian margin of the peninsula by mele et al 1996 1997 on the contrary low velocities beneath the western alps may be an apparent effect due to the abrupt thickening of the crustal roots pn velocity is anisotropic in the study area with a maximum amplitude of 0 2 km s the largest anisotropic velocity anomalies are observed along the major arc structures of italy i e the northern apennines and the calabrian arc indicating that these features are controlled by uppermost mantle processes the anisotropy anomaly along the calabrian arc extends as far as albania but ends abruptly north of this area suggesting that a lithospheric discontinuity is present along the northern albanian border

upper mantle velocity temperature conversion and composition determined from seismic refraction and heat flow 1 we compile upper mantle pn velocities from seismic refraction wide angle reflection surveys in the southern superior province of the canadian shield and compare them with temperatures at the moho deduced from heat flow data calculated moho temperatures and pn velocities correlate well showing that in this area pn depends primarily on temperature the obtained values of v pn t depend weakly on the assumed value of moho heat flow and are on the order of 6 0 10 4 10 km s 1 k 1 within the range of temperature derivatives obtained in laboratory studies of ultramafic rocks comparison between observed pn velocities and predicted values for several mineralogical models at moho temperatures allows constraints on both the moho heat flow and the shallow mantle composition for all moho heat flows undepleted clinopyroxene rich mantle compositions do not allow a good fit to the data for depleted mantle compositions temperatures consistent with the observed pn velocities correspond to values of moho heat flow larger than 12 mw m 2 for our preferred moho heat flow of 15 mw m 2 the best fit mantle composition is slightly less depleted than models for average archean subcontinental lithospheric mantle this may be due to rejuvenation by melt related metasomatism during the keweenawan rifting event the similarity in pn t conversion factors estimated from this empirical large scale geophysical study and those from laboratory data provides confidence in the absolute temperature values deduced from heat flow measurements and seismic studies

evidence from dwyka tillite cobbles of archaean basement beneath the kalahari sands of southern africa we have found a new source of information about what lies beneath the kalahari sands the kheis and rehoboth provinces of southern africa were thought to be underlain by either an similar to 1800 ma orogenic belt or a northern branch of the similar to 1200 ma namaqua natal province glacial diamictites of the permocarboniferous dwyka group exposed at rietfontein west of the kalahari sands carry cobbles plucked from the bedrock by the ice sheet which covered the gondwana supercontinent about 300 ma ago despite altered mineralogy the cobbles are not severely altered geochemically their normative mineral compositions give classifications as trondhjemites and granites supported by rare earth element diagrams microbeam u pb zircon dating of the granitic cobbles shows that they contain no evidence of crustal growth or orogeny at either 1800 or 1200 ma rather they testify to the presence of 2500 to 2900 ma archaean trondhjemitic and granitic crust beneath the kalahari with a lesser similar to 2050 ma granite component the pebble assemblages from the diamictites we sampled lack the diagnostic banded iron formation bif stromatolitic limestone and other supracrustal pebbles which characterise diamictites derived from the kaapvaal craton thus we envisage shorter transport distances and derivation from the region now beneath the kalahari sands three of the archaean granite cobbles have unusual less than mantle zircon oxygen isotope values around 3 delta 18 o vsmow which may reflect interaction of their source with high temperature originally meteoric water before melting to produce the granites the mafic cobbles described in a companion paper are much younger and are related to intrusions of the 1 1 ga umkondo large igneous province probably located on the kalahari line or rehoboth province five trondhjemitic granites from the westernmost outcrops of the kaapvaal craton were dated the oldest being 3061 9 ma and four others between 2882 7 ma and 2854 7 ma reflecting the cratonisation of the kimberley terrane four of the archaean dwyka cobbles we dated are younger than the 2 7 ga kaapvaal cover sequence and are thus too young to be derived from the craton all the dwyka cobbles described here are most likely derived from either the rehoboth province or the kalahari line with origins from the kheis province kaapvaal craton or further afield considered unlikely we envisage the rehoboth province to consist of an archaean core supplemented by palaeoproterozoic granitoids which was joined to the kaapvaal craton at an early stage of crustal development and played an important role during later tectonic events this has important implications not only for the tectonic framework and assembly of southern africa but also for exploration for diamonds and other ore deposits c 2011 elsevier b v all rights reserved

flexural isostasy and uplift of the sierra nevada of california the sierra nevada of california has experienced several kilometers of uplift in the last 10 m y but its present elevation is isostatically compensated by a crustal root that has probably existed since late cretaceous flexural isostasy can reconcile these observations basin and range faulting and extension starting 10 m y ago allowed rapid uplift of an eroded and regionally compensated magmatic arc underlain by an excess crustal root a quantitative model of an unbroken elastic plate buoyed up by the excess root provides an explanation for the estimated 10 m y paleotopography of the range the same model fits the present topography when broken shear and fiber stresses not transmitted at the location of the owens valley fault zone the broken plate model predicts the appropriate amount of westward tilt of the sierra nevada and can explain the pronounced eastward tilt of the white and inyo mountains the position where paleo and present drainages in the sierra cross over in elevation is the most sensitive indicator of the elastic thickness required of the lithosphere an effective elastic thickness of 50 km best fits the observed crossover distances and is consistent with the amount of post 10 m y uplift calculated fiber and shear stresses suggest that actual extension of the crust is more important in permitting the uplift than the mere presence of normal faults part of the residual isostatic gravity anomaly pattern high gravity over the eastern great valley and western sierra nevada low residual gravity over the high sierra may be explained by the continuing flexural support of departure from local isostatic equilibrium the model implies that before 10 m y ago the sierra should have been marked by a negative isostatic gravity anomaly of more than 100 m gal flanked by isostatic gravity highs such large isostatic anomalies have not been observed to date over eroded arcs but the sierra has uniquely low heat flow and presumably therefore high lithospheric strength and the sierran root is at the upper limit of size that could be isostatically suppressed

crust mantle structure of the central north island new zealand based on seismological observations abstract two geophysical features characterise the anomalous crust mantle structure beneath central north island of new zealand the apparent thinning of the felsic crust by a factor of at least 50 and upper mantle p wave velocities pn phase about 10 less than normal 7 3 7 4 km s pn velocities increase slowly to a maximum of 7 8 km s at a depth of about 80 100 km above a depth of 15 km seismic p wave velocities vp are 6 km s or less representing what is likely to be a mix of the original greywacke crust that has been stretched along with an unknown volume of intruded igneous rocks that are in various stages of cooling defining the moho is problematic in this dynamic region below a depth of 15 km there is a continuous increase in vp such that velocities 7 km s are reached at a depth as shallow as 20 km on a east west line and 7 4 km s on a north south line we interpret these wave speeds as a transition zone into low pn velocities rather than high lower crustal values of vp pn wave speeds as low as 7 4 km s are detected by tomographic methods to depths of at least 65 km and laterally over an area of at least 10 000 km2 a 1 4 partial melt in the mantle wedge beneath the central north island is estimated based on qp seismic anisotropy low values of pn and amplitudes of deep reflections these upper mantle features are more indicative of asthenosphere than lithosphere and coupled with the geological evidence for rapid uplift here at 5 ma are consistent with convective removal of the mantle lithosphere from western and central north island in late miocene times

physical changes in the lithosphere associated with thermal relaxation after rifting we define the terms modem rift and paleorift with respect to the thermal state of the lithosphere the former being rifted lithosphere not in thermal equilibrium and the latter being thermally stabilized rifted lithosphere a comparison of the physical characteristics of modern rifts with paleorifts shows that they share many of the same geomorphic structural and magmatic features but have important differences in their geophysical signatures using a simple model for the thermal relaxation of the lithosphere in the transition from modern rift to paleorift we show that differences in upper mantle seismic velocities and density surface heat flow thermal structure and curie point depth and magnetotelluric properties of the lithosphere between modern and paleorifts can be explained by lithospheric cooling cooling alone cannot explain the complex subsidence histories associated with and following rifting however nor the presence of high velocity crustal layers and sometimes thickened crust in paleorifts these features can be explained by consideration of lithospheric flexural strength and magmatic thickening of the crust associated with rifting we speculate that the anomalous upper mantle beneath many modern rifts may be associated with the process of magmatic thickening of the rifted crust

pn velocity and cooling of the continental lithosphere average upper mantle p sub n velocities and heat flow were computed within continental physiographic provinces in north america from published data p sub n velocity and heat flow data display an inverse relationship and were found to correlate with a statistically significant correlation coefficient temperatures at the crust mantle boundary were estimated from the heat flow values and these were used to demonstrate a correlation between p sub n velocity and temperature of upper mantle material the value of partialp sub n partialt p thus obtained 4 4 x 10 sup 4 to 8 1 x 10 sup 4 km s sup 1 sup 0 c sup 1 is within the range of temperature derivatives determined from laboratory studies of ultramafic rocks this dependence of p sub n velocity on temperature implies that one possible explanation for the observed geographical distribution in upper mantle seismic velocity is that the variation in p sub n velocity is primarily a temperature effect combined with the relationship between heat flow and crustal age for continents the p sub n versus heat flow correlation suggests a relationship between p sub n velocity and crustal age probably due to progressive cooling of the continental lithosphere after a thermotectonic event

accreted terranes of northwestern british columbia canada lithospheric velocity structure and tectonics 1 lithospheric structure beneath the accreted terranes of the northwestern canadian cordillera is interpreted from analyses of p wave refraction and wide angle reflection data the 535 km long profile extends from the deformed and displaced ancestral north american margin across the omineca belt suture zone and intermontane superterrane to the coast belt this region is interpreted to have grown westward through a progression from thin skinned to thick skinned accretionary tectonics large lateral variations of velocities 2 0 6 4 km s within the upper 15 km correlate with the mapped geological and terrane elements fault offsets and the imbricated base of an overlap basin are clearly identified inboard of the stikine accreted terrane a zone of slow upper crustal velocities thins to the east consistent with an interpretation of thin skinned accretion leaving slivers of terranes overlying a wedge of metamorphosed proterozoic continental margin sediments however in the lower crust the proposed underlying cratonic ramp is not distinguished from outboard or overlying accreted crust by lateral p velocity contrasts or wide angle reflections the moho remains nearly horizontal beneath the intermontane and omineca belts gradually thinning to the east from 36 5 to 35 km to the southwest below the coast belt and to the northeast beneath the tintina fault the crust thins to 32 km upper mantle velocities across the coast intermontane and omineca belts are 7 8 7 9 km s the slow upper mantle and lower crustal velocities are indicative of high temperatures that may permit the flat moho or weak lower crust to partially decouple the crust from the mantle reflections in the upper mantle between 50 70 km depth may be related to the base of the lithosphere or to heterogeneities introduced through melting or accretionary subduction events

relationships between the petrophysical properties density seismic velocity heat generation and mineralogical constitution the petrophysical properties density p compressional wave velocity v p and heat generation a exhibit considerable variation with rock type to characterize a given rock quantitatively a single parameter the cation fraction expressed by the k value has been found to be suited best 100 crystalline rocks have been sampled to cover the widest possible range in k value from silicic types like granophyres and granites through granodiorites tonalites diorites gabbros amphibolites hornblendites pyroxenites to ultrabasic rocks p k a and v p were determined experimentally the latter at 0 5 kbar 50 mpa the interrelations between these properties were established by regression analysis the relation k and v p k show a linear dependence whereas a k a and a v p follow exponential laws in a w m 3 12 20 230 5 k mole cm 3 in a 22 5 8 15 g cm 3 and in a 16 5 2 74 v p km s all correlation coefficients are better than 0 9

the yellowstone snake river plain seismic profiling experiment crustal structure of the eastern snake river plain seismic refraction profiles recorded along the eastern snake river plain esrp in southeastern idaho during the 1978 yellowstone snake river plain cooperative seismic profiling experiment are interpreted to infer the crustal velocity and attenuation q 1 structure of the esrp travel time and synthetic seismogram modeling of a 250 km reversed refraction profile as well as a 100 km detailed profile indicate that the crust of the esrp is highly anomalous approximately 3 to 6 km of volcanic rocks with some interbedded sediments overlie an upper crustal layer compressional velocity approx 6 1 km s which thins southwestward along the esrp from a thickness of 10 km near island park caldera to 2 to 3 km beneath the central and southwestern portions of the esrp an intermediate velocity approx 6 5 km s layer extends from approx 10 to approx 20 km depth a thick approx 22 km lower crust of compressional velocity 6 8 km s a total crustall thickness of approx 42 km and a p sub n velocity of approx 7 9 km s is observed in the esrp similar to the western snake river plain and the rocky mountains provinces high attenuation is evident on the amplitude corrected seismic data due to low q values in the volcanic rocks q sub p more 20 to 200 and throughout the crust q sub p 160 to 300 based on these characteristics of the crustal structure and volcanic age progression data it is suggested that the esrp has resulted from an intensitive period of intrusion of mantle derived basaltic magma into the upper crust generating explosive silicic volcanism and associated regional uplift and caldera collapse this activity began about 15 m y ago in southwestern idaho and has migrated northeast to its present position at yellowstone subsequent cooling of the intruded upper crust results in the 6 5 km s velocity intermediate layer crustal subsidence and periodic basaltic volcanism as represented by the esrp complete the sequence of crustal evolution less

geochronology of mesoproterozoic hybrid intrusions in the konkiep terrane namibia from passive to active continental margin in the namaqua natal wilson cycle abstract this work investigates a number of gabbro granite hybrid rocks exposed in the great escarpment between the coastal namib desert and the inland plateau of namibia they form part of the tectonostratigraphic konkiep terrane which lies between the mesoproterozoic namaqua natal province and largely palaeoproterozoic rehoboth province in southern africa microbeam u pb and lu hf analyses of zircon were used to investigate the geochronology of these rocks which have previously been assigned to the mooirivier metamorphic complex or the kairab formation the oldest hybrid rocks at neuhof valley are u pb dated at 1359 6 ma and cut by 1347 8 ma unfoliated granite dykes but have the youngest lu hf crustal residence ages between 1690 and 1430 ma together with nearby volcanic rocks of the 1333 6 ma neuhof and 1327 10 ma welverdiend formations which we dated they are thought to reflect rifting of the rehoboth province followed by the ocean development by seafloor spreading phase of the namaqua wilson cycle the hybrid rocks at goede hoop foothills are u pb dated at 1230 6 ma with a number of xenocrysts up to 1532 ma similar hybrid rocks at hauchabfontein have 20 30 ma younger u pb ages of 1211 19 ma and 1202 7 ma two samples they formed by gabbro intruding an 1195 5 ma granite within error but older from field relations and contain many xenocrysts with u pb ages 1755 29 ma n 7 1367 and 1260 17 ma n 24 all the goede hoop foothills and hauchabfontein hybrid rocks have old lu hf crustal residence ages from 1935 to 2000 ma and one sample up to 2320 ma reflecting involvement of palaeoproterozoic rehoboth province crust this 1230 to 1200 ma group of hybrid rocks corresponds in age to the arc magmatism recognised in the namaqua natal province to the west and south of the konkiep terrane the hybrid rocks probably formed during the closure of the namaqua ocean by eastward subduction under the rehoboth province corresponding to the previously proposed rehoboth magmatic arc and forming the konkiep terrane by massive intrusions into the margin of the rehoboth province the hybrid rocks we investigated in the konkiep terrane are thus related to the rifting phase and the pre collisional subduction phase of the namaqua wilson cycle the arc related magmatism ended abruptly at 1200 ma as the namaqua natal terranes were assembled by collisions and subduction ceased we did not find evidence for the 1200 to 1150 ma post collisional metamorphic cycle and magmatism which prevailed in the adjacent grunau terrane of the namaqua province suggesting that the konkiep terrane did not experience crustal thickening during the namaqua collision and terrane assembly an undeformed felsic lava at sukses which we dated at 1105 10 ma corresponds in age to the langberg formation in the rehoboth terrane it is coeval with the keimoes suite of post tectonic intrusions and younger koras lavas in the namaqua natal province and these 1100 ma rocks all extend the area of the plume related umkomdo large igneous province documented on the kaapvaal craton this part of the konkiep terrane originated in the rehoboth province and was then modified by magmatic intrusions coupled to the evolution of the namaqua natal province

regional aeromagnetic and stratigraphic correlations of the kalahari copperbelt in namibia and botswana abstract the late mesoproterozoic to neoproterozoic kalahari copperbelt kcb in namibia and botswana is widely covered by kalahari sand which precludes direct correlations between known stratabound sediment hosted cu ag districts we use a combination of review of literature data and newly processed and interpreted high resolution aeromagnetic maps in both countries to provide a new correlative cross border interpretation lithostratigraphic control on the aeromagnetic response allows detailed indirect mapping of the kalahari copperbelt lithotectonic domains below the sand cover this enabled us to redefine the width and lateral extent of the kcb as two continuous magnetic domains the rehoboth and ghanzi chobe domains extending from central namibia to northern botswana and helped in resolving problems of stratigraphic correlations across the international border the rehoboth magnetic domain in the western part of the kcb in namibia records continental arc magmatism at 1200 ma during orogenic events along the northwestern edge of the kalahari craton this was followed at 1110 1090 ma by widespread magmatism identified within the entire kcb and related to the 1112 1106 ma umkondo large igneous province the basal parts of the tsumis group in namibia and ghanzi group in botswana were deposited in shallow water environments after a period of erosion and peneplanation subsequently and prior to the sturtian glaciation the host rocks of the cu ag deposits formed by the deposition of chemically reduced shales and siltstones that formed in deeper water and overlie chemically oxidised shallow water sandstones this regional interface which is both a permeability barrier and redox boundary played a critical role in the formation of the stratabound sediment hosted cu ag deposits of the kalahari copperbelt and the interface with its strong magnetic contrast can be followed through the entire ghanzi chobe magnetic domain of the copperbelt the whole kcb was affected by the damara orogeny during early cambrian times which resulted in the formation of a ne sw trending 250 km wide fold and thrust belt

crustal structure of the eastern snake river plain determined from ray trace modeling of seismic refraction data ray trace travel time modeling of the seismic refraction record sections for a profile from near soda springs idaho to near mckay idaho was used to derive a crustal model across the eastern snake river plain esrp the derived crustal model is consistent with the velocity structure interpreted from a profile along the axis of the esrp the interpretation also indicates that significant lateral inhomogeneities exist in the upper crust beneath the esrp when compared with the upper crust beneath the adjoining northern rocky mountain and basin and range provinces the most prominent features of the crustal structure inferred by the ray trace modeling are as follows 1 the northwest margin of the esrp was modeled as a fault structure downthrown on the srp side se with an offset of greater than 4 km the southeast margin conversely appears to be downwarped with possible minor faulting paleozoic sedimentary rocks are present beneath the cenozoic volcanics of the esrp possibly to and beyond 40 km from the southeastern margin of the esrp near blackfoot idaho 2 the modeling indicates no abrupt variation in the depth to the top of the lower crustal layer near the margins of the esrp 3 an intermediate 6 5 km s layer occurring beneath the esrp interpreted from a refraction profile located along the axis of the esrp was found to be localized within the esrp margins this layer is interpreted as a pervasive intrusion of higher velocity material from the upper mantle into the highly fractured upper crustal layer in this region 4 a density model of the crust across the esrp was prepared with the densities selected using the interpreted seismic velocities as a constraint the gravity field calculated from this model resulted in a good match to observed gravity data over the eastern snake river plain

first local seismic tomography for red river shear zone northern vietnam stepwise inversion employing crustal p and pn waves abstract the 900 km long red river shear zone rrsz lends a compelling support to the continental extrusion model for the tectonic evolution of southeastern asia but has been challenged by many of views as some new records mainly from northern vietnam suspecting the dimensions of rrsz neither in depth nor in displacement are as large as we expected before however compared to the northwestern half of the rrsz in yunnan province better studied by many fields the southeastern half in northern vietnam is relatively poorly constrained by seismic study due to insufficient stations and data in the past this study using a newly deployed portable broadband seismic network obtained the first local seismic tomography with a stepwise inversion using p and p n phases surface geology major structures and rock properties are well correlated and identified in our model suggesting the rrsz is a lithospheric structure at least penetrating to the uppermost mantle with mantle thermal anomalies in general the crust of northern vietnam appears to be weak and sits on a relatively hot uppermost mantle showing a long and complex thermo tectonic history a mid lower crustal segmentation of rrsz is also proposed to compromise the discrepancies recently observed between yunnan province and northern vietnam

high velocities in the uppermost mantle of the siberian craton abstract anomalous high seismic velocities up to 8 8 km s have been reported suvorov et al 1985 in the mantle beneath the kimberlite areas of siberia high mantle velocities were also observed on some geon center profiles carried out using nuclear explosions egorkin et al 1987 to obtain clear general signatures of the mantle structure in siberia and the role of the high velocity blocks the geon data were analyzed and compared with other geophysical information this analysis showed that the largest blocks of 300 400 km width with velocities up to 8 6 km s are observed beneath the tunguss depression and in the southern part of the craton near its boundary with the baikal rift zone the blocks penetrate to depths of 100 110 km where they are underlain by low velocity zones or a horizontally homogeneous layer there is no evidence of these blocks in the gravity field velocity anisotropy is proposed as a main cause of the anomalous high velocities although the crossing profiles show that such velocities are observed in different directions the yakutia kimberlite fields are located in the region with average sub moho velocity of 8 3 8 4 km s but with higher velocity at depths of 70 80 km

tomography of the western united states from regional arrival times first arrival times from regional distances 200 1200 km in the western united states extracted from the international seismological centre isc data set were inverted in a tomographic study to map the laterally varying pn velocity structure of the uppermost mantle i e the mantle lid and to estimate the crustal static delay at each seismograph station synthetic data were used to evaluate resolution results correlate well with major tectonic features we find low uppermost mantle velocities v 7 9 km s these low apparent pn velocities are primarily due to the effects of isostasy hotspot volcanism and crustal extension we interpret the low apparent pn velocities beneath the sierra nevada 7 6 7 7 km s to be the result of time delays introduced by ray paths tunneling through the deep crustal root the yellowstone hotspot migrating along the snake river plain has left low velocity 7 6 7 8 km s hot upper mantle in its wake lower pn velocities 7 8 km s beneath the extensional basin and range are also presumably due to hotter uppermost mantle based on station static delays for the basin and range we infer that moho depths there do not vary significantly from 30 km normal continental pn velocities 8 0 km s as well as thicker crust 40 50 km underlying the rocky mountains wyoming basin province colorado plateau and the northern columbia plateau indicate the presence of relatively cool uppermost mantle beneath these regions

pn anisotropy in the northern apennine chain italy pn travel times recorded by the stations of the italian seismic network have been used to image the azimuthal variations of seismic velocity in the uppermost mantle beneath the northern part of the apennine chain the azimuthal variation of pn velocity is interpreted here in terms of seismic anisotropy we have found that about 5 anisotropy characterizes the uppermost mantle of the studied area and that the fastest direction of pn follows the arcuate trend of the chain this suggests that seismic anisotropy is strongly related to the tectonic regime that originated the northern apennine arc

new insights into the geology of the namaqua tectonic province south africa from ion probe dating of detrital and metamorphic zircon abstract an ion microprobe study in the western part of the namaqua natal tectonic province of south africa provides new insights into ages of sedimentation magmatism and metamorphism in the bushmanland terrane concordant u pb ages and discordia regressions are reported for three samples and in combination with secondary electron microscope images th u ratios and rare earth element analyses of different zircon age domains are used to distinguish several igneous and metamorphic events detrital zircon grains from the tectono stratigraphic lowermost part of the kamiesberg subgroup louisrus formation yielded ages between 1200 and 1250 ma and formed before a first tectono thermal event at 1187 ma in contrast detrital zircon grains from a metapelitic granulite from the top of the subgroup bitterfontein formation were deposited during a previously unrecognized sedimentation stage after 1157 ma and after this first tectono thermal event therefore at least parts of the supracrustal rocks in the bus

an analysis of p wave amplitudes recorded by seismological stations in the ussr summary short period p wave amplitudes at soviet seismic stations are analysed to determine an amplitude distance curve in the range 0 180 from the ussr station amplitude terms corrected for crustal amplification effects are determined from seismic sources located between 30 and 90 a relationship between these station terms and regional p wave speed and regional heat flow is determined and is found to be in close agreement with similar observations in north america station amplitude terms were found to be reasonably uniform across the continental portion of the ussr but the baikal rift valley is characterized by very low p wave amplitudes and is the most significant feature discernible from the p wave amplitude terms

postglacial vegetation climate and fire history along the east side of the andes lat 41 42 5 s argentina abstract the history of the low elevation forest and forest steppe ecotone on the east side of the andes is revealed in pollen and charcoal records obtained from mid latitude lakes prior to 15 000 cal yr bp the vegetation was characterized by steppe vegetation with isolated stands of nothofagus the climate was generally dry and the sparse vegetation apparently lacked sufficient fuels to burn extensively after 15 000 cal yr bp a mixture of nothofagus forest and shrubland steppe developed fire activity increased between 13 250 and 11 400 cal yr bp contemporaneous with a regionally defined cold dry period huelmo mascardi cold reversal the early holocene period was characterized by an open nothofagus forest shrubland mosaic and fire frequency was high in dry sites and low in wet sites the data suggest a sharp decrease in moisture eastward from the andes a shift to a surface fire regime occurred at 7500 cal yr bp at the wet site and at 4400 cal yr bp at the dry site preceding the expansion of austrocedrus by 1000 1500 yr the spread of austrocedrus is explained by a shift towards a cooler and wetter climate in the middle and late holocene the change to a surface fire regime is consistent with increased interannual climate variability and the onset or strengthening of enso the present day mixed forest dominated by nothofagus and austrocedrus was established in the last few millennia

a crustal seismic refraction study in west central arizona a reversed seismic refraction profile was recorded in the southern basin and range province between parker and globe arizona interpretation of both refracted and reflected phases indicates a two layered crust 23 25 km thick and a low pn velocity 7 67 km s pmp pn amplitude analysis on individual traces and synthetic seismogram modeling indicate that a positive velocity gradient may exist in the upper mantle the low pn velocity is indicative of partial melting in the upper mantle and implies that the crust is identical with the lithosphere the heat flow data suggest that the lithosphere was about 40 km thick approximately 5 m y ago on the basis of this and a negative free air gravity anomaly we propose that the lithosphere has been thinned from about 40 to 24 km over the past 5 m y and will probably thin and uplift further in the southern basin and range in arizona

tectonics and thermal structure of western satpura india abstract increased seismicity and occurrences of hot springs having surface temperature of 36 58 c are observed in the central part of india 74 81 e 20 25 n where the ne trending middle proterozoic aravalli mobile belt meets the ene trending satpura mobile belt earlier deep seismic sounding dss studies along thuadara sendhwa sindad profile in the area has showed mesozoic sediments up to around 4 km depth covered by deccan trap and the moho depth with a boundary velocity p n of 8 2 km s in the present study surface heat flow of 48 4 mw m 2 has been estimated based on p n velocity which agrees with the value of heat flow of 52 4 mw m 2 based on curie point isotherms estimates the calculated temperature depth profile shows temperature of 80 120 c at the basement which is equivalent to oil window temperature in mesozoic sediments and around 570 635 c at moho depth of 38 43 km and the thermal lithosphere is about 110 km thick which is comparatively higher than those of adjoining regions the present study reveals the brittle ductile transition zone at 14 41 km depth temperature around 250 600 c where earthquake nucleation takes place

sub crustal lvz below dharwar craton india an evidence for mantle metasomatism and tectonothermal activity in the archean crust abstract improved knowledge of seismic structure of the continental crust provides deep insights into the mechanism of the crustal growth since the archean times reprocessing of deep seismic sounding data acquired over western dharwar craton india reveals presence of i a high velocity magmatic layer 7 30 km s at the base of the crust ii a higher pn velocity 8 35 km s at a deeper depth of 45 km than in an usual archean crust and iii an unusually low velocity layer 7 40 km s at a depth range of 48 60 km in the upper mantle such shallow subcrustal lithospheric lvzs are rarely found in a stable archean craton which is associated with low surface heat flow and devoid of recent tectonic activities considering various geological tectonic and geothermal scenarios occurrence of such a low velocity zone has been ascribed to chemical alteration caused by mantle metasomatism rather than mafic partial melts or high temperature sub solidus the derived velocity structure and distribution of volcanic rocks of different ages in the studied area indicate multiple crust mantle thermal interaction in the archean crust

integrated depositional model for the cenomanian turonian organic rich strata in north africa abstract during the late cenomanian early turonian c t oceanic anoxic event oae2 organic rich strata was deposited in rift shelf basins and slopes across north africa and in deep sea basins of the adjacent oceans based on a review of published and unpublished information this paper documents the distribution and organic richness of c t strata across the whole region within a palaeogeographic framework and systematically analyses the conditions and processes which controlled their deposition previously the c t in north africa has been most intensively studied in southern morocco tarfaya and tunisia only little data is availabe for other parts of north africa namely algeria libya and egypt because distribution of c t corg strata there becomes more patchy a general decrease in peak organic richness and black shale thickness occurs from west to east partly as a result of upwelling along the moroccan atlantic coast and the absence of upwelling in the eastern mediterranean area furthermore in the confined central atlantic the oxygen minimum zone omz in many places reached down to the deep sea floor 3 4 km while the lower limit of the omz along the north african tethys was much shallower and underlain by oxic water masses as documented by high resolution biostratigraphic and chemostratigraphic data c t black shale deposition in most areas outside the upwelling zone are restricted to a strong eustatic latest cenomanian transgressive phase triggered by this sea level rise the omz impinged onto the north african continental shelf and the margins of intrashelf basins which mostly formed during the early cretaceous as halfgrabens important units containing c t organic rich strata in the region are the atlantic tarfaya black shales morocco western sahara black shales and phtanites in the moroccan and algerian atlas the bahloul fm in the se constantine basin and in northern and central tunisia the etel fm in the sirte basin the al hilal fm in cyrenaica libya the abu roash fm in the abu gharadig and fayium basins western desert egypt the daliyya fm along the ne sinai palestine isreal coast and the shueib fm in west central jordan

lithospheric boundaries and upper mantle structure beneath southern africa imaged by p and s wave velocity models we report new p and s wave velocity models of the upper mantle beneath southern africa using data recorded on seismic stations spanning the entire subcontinent beneath most of the damara belt including the okavango rift our models show lower than average velocities 0 8 vp 1 2 vs with an abrupt increase in velocities along the terrane s southern margin we attribute the lower than average velocities to thinner lithosphere 130 km thick compared to thicker lithosphere 200 km thick immediately to the south under the kalahari craton beneath the etendeka flood basalt province higher than average velocities 0 25 vp 0 75 vs indicate thicker and or compositionally distinct lithosphere compared to other parts of the damara belt in the rehoboth province higher than average velocities 0 3 vp 0 5 vs suggest the presence of a microcraton as do higher than average velocities 1 0 vp 1 5 vs under the southern irumide belt lower than average velocities 0 4 vp 0 7 vs beneath the bushveld complex and parts of the mgondi and okwa terranes are consistent with previous studies which attributed them to compositionally modified lithosphere resulting from precambrian magmatic events there is little evidence for thermally modified upper mantle beneath any of these terranes which could provide a source of uplift for the southern african plateau in contrast beneath parts of the irumide belt in southern and central zambia and the mozambique belt in central mozambique deep seated low velocity anomalies 0 7 vp 0 8 vs can be attributed to upper mantle extensions of the african superplume structure

the 1978 yellowstone eastern snake river plain seismic profiling experiment crustal structure of the yellowstone region and experiment design in 1978 a major seismic profiling experiment was conducted in the yellowstone eastern snake river plain region of idaho and wyoming fifteen shots were recorded that provided coverage to distances of 300 km in this paper travel time and synthetic seismogram modeling was used to evaluate an average p wave velocity and apparent q structure of the crust from two seismic profiles reversed across the yellowstone national park region this area includes the well known hydrothermal features of yellowstone national park geysers fumeroles etc a large collapse caldera and extensive silicic volcanism of quaternary age features attributed to shallow crustal sources of magma the averaged crustal structure for this region as interpreted from the seismic data consists of 1 a highly variable near surface layer approximately 2 km thick with variable velocities of 3 0 to 4 8 km s and a low apparent q of 30 that is interpreted to be composed of weathered rhyolites and sedimentary infill 2 an upper crustal layer 3 to 4 km thick with variable velocities of 4 9 to 5 5 km s and apparent q of 50 to 200 that is thought to represent the accumulation of the pleistocene quaternary rhyolite flows ash flow tuffs and possible paleozoic and precambrian metamorphic equivalents 3 the crystalline upper crust that is characterized by a laterally inhomogeneous layer that varies in velocity from 4 0 to 6 1 km s averaging 5 km thick with a q of 300 this layer appears to be a cooling but still hot body of granitic composition beneath the yellowstone caldera it is thought to be a remnant of the magma chambers that produced the quaternary silicic volcanic rocks of the yellowstone plateau and may still be a major contributor to the high heat flow 4 a laterally homogeneous intermediate crustal layer 8 to 10 km thick with a velocity of 6 5 km s and apparent q of 100 to 300 5 a homogeneous 25 km thick lower crust with a velocity of 6 7 to 6 8 km s and an apparent q of 300 and 6 a total crustal thickness of 43 km the upper crustal layer 5 5 to 6 0 km s is thought to be the thermally altered equivalent of the continental crystalline basement that is normally 15 to 20 km thick in the surrounding thermally undisturbed archean crust an interpretation from these results suggests that mafic melts from the mantle have penetrated the lower crust without significant variations in the velocity structure but produce the main source of heat that drives the volcanic and hydrothermal systems of yellowstone the high apparent attenuation and large lateral velocity variations in the upper crust are consistent with a model in which partial fractionation partial melting and metamorphism differentiate the original upper crust to produce silicic melts that were extruded as rhyolites and ash flow tuffs across the yellowstone plateau this seismic model is consistent with the evidence for a systematic northeastward propogation of silicic volcanic centers along the eastern snake river plain to their present location beneath the yellowstone hydrothermal system while these findings do not bear directly on the origin and source of the heat i e mantle lumes lithospheric fractures mantle radiogenetic heat basal lithospheric shearing etc they provide a constraint on the configuration and lateral extent of crustal layers that reflect thermal and compositional boundaries

subduction and back arc activity at the hikurangi convergent margin new zealand the hikurangi margin is a region of oblique subduction with northwest dipping intermediate depth seismicity extending southwest from the kermadec system to about 42 s the current episode of subduction is at least 16 20 ma old the plate convergence rate varies along the margin from about 60 mm a at the south end of the kermadec trench to about 45 mm a at 42 s the age of the pacific lithosphere adjacent to the hikurangi trench is not known

petrogenesis of the granitic donkerhuk batholith in the damara belt of namibia protracted syntectonic short range crustal magma transfer the areally extensive 5000 km2 syn tectonic ca 520 ma mainly s type donkerhuk batholith was constructed through injection of thousands of mainly sheet like magma pulses over 20 25 myr it intruded schists of the southern zone accretionary prism in the damara belt of namibia each magma pulse had at least partly crystallised prior to the arrival of the following batch however much of the batholith may have remained partially molten for long periods close to the h2o saturated granite solidus the batholith shows extreme variation in chemistry while having limited mineralogical variation and seems to be the world s most heterogeneous granitic mass the nd model ages of 2 ga suggest that eburnean rocks of the former magmatic arc structurally overlain by the accretionary wedge are the most probable magma sources crustal melting was initiated by mantle heat flux probably introduced by thermal diffusion rather than magma advection the granitic magmas were transferred from source to sink with minimal intermediate storage the whole process having occurred in the middle crust resulting in feeble crustal differentiation despite the huge volume of silicic magma generated source heterogeneity controlled variation in the magmas and neither mixing nor fractionation was prominent however due to the transpressional emplacement regime local filter pressing formed highly silicic liquids as well as felsic cumulate rocks the case of the donkerhuk batholith demonstrates that emplacement level tectonics can significantly influence compositional evolution of very large syn tectonic magma bodies

lithospheric structure of the marmara and aegean regions western turkey we have simulated the waveforms of three aftershocks of the izmit 17 august 1999 earthquake with magnitudes greater than m 5 0 to determine the lithospheric structure of the gulf of izmit marmara region using the discrete wavenumber technique bouchon 1981 different layered crustal models have been tested for the simulation of the waveforms and the best crustal model was determined by a best fit criterion between the observed and simulated seismograms our results indicate that the average thickness of the crust is 32 km and that pn and s velocities are 8 0 and 4 60 km sec respectively in the gulf of izmit marmara region we also computed synthetic waveforms of the 21 april 2000 denizli honaz and the 9 july 1998 izmir doganbey earthquakes in the aegean in order to compare the lithospheric structures of the marmara and the aegean regions the aegean region has an average crustal thickness of 33 km and pn and s velocities of 7 85 and 4 53 km sec respectively although thicknesses of the crusts are comparable and suggest an approximately equal amount of e w stretching in the marmara and n s stretching in the aegean regions a patchy midcrustal low velocity zone exists in the aegean the upper mantle pn velocity variation between the marmara and the aegean regions is interpreted as the effect of a thinning continental lithosphere toward the active aegean arc and the establishment of a consequent upper mantle temperature gradient increasing from the north to the south additionally the black sea oceanic lithosphere that steeply dips southward beneath the marmara region as evidenced by the seismic tomographic results obtained by gulen and kuleli 1995 can contribute to the advective cooling of the upper mantle that causes relatively high pn velocities in the marmara region manuscript received 20 august 2000

mapping high pn velocity beneath the colorado plateau constrains uplift models the massive international seismological centre data set of the past 20 years and the two station method are used to determine pn velocities in the mantle lid beneath the colorado plateau in this method the event is located at distances where pn is the first arrival 2 16 and the path is in or very near the azimuth of the two station pair and crosses the plateau this technique to a large extent minimizes the hypocenter mislocation effect and possible errors due to variations in the crustal structure near the source since only the difference in travel times at the two stations is used however this technique has a few underlying assumptions and possible sources of errors such as the quality of the pn data base and station delays caused by varying crustal structure that require an extremely careful application of the method a detailed study of the source of errors and a methodology of selection of the data are presented application of this method to the colorado plateau using all possible two station pairs from 53 stations located within or along the margin of the plateau yields an average high pn velocity of 8 12 0 09 km s this value is considerably larger than the average value of 7 83 km s based on available but very limited seismic refraction profiles but is remarkably similar to the average value of 8 1 km s for the relatively stable midcontinent region our new pn velocity for the colorado plateau eliminates the paradox in the literature that emphasizes the rather close similarity between average pn velocities beneath the colorado plateau and the basin and range province while their tectonic and magmatic cenozoic history is dramatically different previous models for the structure and evolution of the plateau have used the low pn velocity as an important constraint on density and thermal state of the lithosphere hence such models should be reexamined on the basis of this new uppermost mantle pn velocity determination there are two main models that have been proposed to explain the 2 km uplift of the colorado plateau one is based on a combination of thermal thinning of the lithosphere and crustal thickening and the other involves a combination of the delamination of the subducted subhorizontal farallon oceanic plate from the overriding north american plate and crustal thickening we show that the delamination model is more readily consistent not only with our velocity determination and the elevation of the plateau but also with varied geological observations reported in the literature that concern the cenozoic evolution of western north america

the lateral variation of pn velocity gradient under eurasia mantle lid p wave velocity gradient or pn velocity gradient reflects the depth and lateral variations of thermal and rheological state of the uppermost mantle mapping the pn velocity gradient and its lateral variation helps us gain insight into the temperature composition and dynamics of the uppermost mantle in addition because pn velocity gradient has profound influence on pn propagation behavior an accurate mapping of pn velocity gradient also improves the modeling and prediction of pn travel times and amplitudes in this study i used measured pn travel times to derive path specific pn velocity gradients i then inverted these velocity gradients for two dimensional 2 d pn velocity gradient models for eurasia based on the assumption that a path specific pn velocity gradient is the mean of laterally varying pn velocity gradients along the pn path result from a monte carlo simulation indicates that the assumption is appropriate the 2 d velocity gradient models show that most of eurasia has positive velocity gradients high velocity gradients exist mainly in tectonically active regions most tectonically stable regions show low and more uniform velocity gradients strong velocity gradient variations occur largely along convergent plate boundaries particularly under overriding plates

the velocity structure of the crust and upper mantle under china from broad band p and pp waveform analysis summary we model 45 p and pp waveforms at epicentral distances of 14 to 40 primarily sampling western and central china to determine regionalized p wave velocity variations for the crust and upper mantle in this region observed p and pp waveforms are matched by 1 d forward modelling using the reflectivity technique our approach is to first find a homogeneous average velocity model for china which can match the observed broad band waveforms filtered with a long period instrument response this model is then used as a starting model for the matching of the broad band waveforms to investigate the lateral velocity variations our preferred average model wch has a 50 km thick crust with a velocity of 6 4 km s a p velocity of 8 15 km s and a low velocity zone between 100 km and 175 km depth it has intermediate characteristics between an active tectonic region and a stable shield compatible with previous s wave modelling results modelling of the broad band waveforms indicates significant lateral deviations from model wch for the tibetan plateau we observe larger crustal thickness 55 70 km and p velocity 8 15 8 25 km sc in combination with a low velocity zone that varies in magnitude throughout the tibetan region lower velocities than our average model may be present below 200 km depth our modelling indicates a thin lid of about 30 45 km under tibet we infer from our results and previous work that crustal shortening and thickening is the most likely process responsible for the central and northern tibetan plateau s thick crust and high average elevation and that the indian plate has probably only underplated the southernmost part of the plateau mantle convection associated with the crustal shortening process has led to strong lateral heterogeneity in the upper mantle under tibet higher p velocities in western tibet might indicate that this region is presently undergoing crustal shortening paths traversing eastern china require a constant velocity lid of about 100 km thickness a p velocity of 8 12 8 14kms and a crustal thickness between 27 and 35 km a preliminary model for northern china suggests a crustal thickness between 43 and 50km a p velocity of about 8 0kms and a 100km thick lid with a positive velocity gradient for this region

role of crustal contribution in the early stage of the damara orogen namibia new constraints from combined u pb and lu hf isotopes from the goas magmatic complex abstract the tholeiitic to calc alkaline goas complex represents the earliest magmatic activity of the inland branch of the damara orogen and reflects the pan african convergence between the congo and kalahari cratons new results from laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb geochronology coupled with single zircon lu hf isotopic data constrain the age of emplacement and allow the identification of a crustal contribution to the magma sources no temporal gap is registered between the first tholeiitic intrusion at ca 575 ma and the main diorite to granite episodes invariably sub chondritic ehf t values 3 8 to 34 4 attest to a significant crustal residence time with long term reworking of multiple and mixed crustal components no significant juvenile magma was involved in the magmatogenesis hf model ages coupled with an exhaustive dataset of u pb geochronological data suggest that the crustal components which fed the goas sources might have formed during specific orogenic events with a major contribution of material derived from the central western african paleoproterozoic eburnean orogen geochemistry u pb geochronology and hf isotope systematics however do not unequivocally clarify the long debate regarding whether or not the pan african orogenic cycle included ocean closure and subduction as the involvement of crustal melting processes during the early stage of the damaran event seems to have played a major role

is extension in death valley accommodated by thinning of the mantle lithosphere beneath the sierra nevada california while relatively rapid and large scale extension occurred in the death valley region of california during neogene time little or no shallow extension occurred in the adjacent sierra nevada this contrast in tectonic history has often been extrapolated to include the entire lithosphere but geophysical and geologic observations indicate that more extension of the mantle lithosphere has occurred under the sierra than under the death valley region upper mantle seismic velocities observed beneath the high sierra are lower than those observed in other regions with comparable surface heat flow this discrepancy could be resolved if the mantle lithosphere beneath the high sierra had become warmer presumably by tectonic thinning in the last 10 m y upper mantle seismic velocities averaged topography and bouguer gravity anomalies all are consistent with the presence of thinner mantle lithosphere beneath the high sierra than beneath the california portion of the basin and range province to the east this suggests that extension of the crust near death valley might be accommodated at a deeper level by thinning of the mantle lithosphere beneath the sierra nevada the extension in the crust of the california basin and range province and the thinning of the mantle lithosphere under the high sierra appear to share the same bounds in time and space the uplift of the high sierra occurred over the past 9 m y which coincides with most of the extension that occurred in the california basin and range province because the orientation of extension in the california basin and range province is inferred to be approximately n60 w from geologic geodetic and in situ stress measurements the northern and southern edges of the death valley extensional subprovince may extend n60 w from the inferred northern and southern limits of west dipping low angle normal faults of the death valley region pronounced changes in the averaged topography and bouguer gravity anomaly across these two bounds both in the basin and range province and in the sierra nevada support a connection between the tectonics of both regions the geomorphic history of the southern sierra suggests an up to the north warp of the sierra across this southern bound during latest cenozoic time hence extension near death valley may be localized in the crust and may be laterally connected to thinning of the mantle lithosphere beneath the sierra nevada this geometry requires

lithospheric structure evolution and diamond prospectivity of the rehoboth terrane and western kaapvaal craton southern africa constraints from broadband magnetotellurics abstract a 1400 km long 2 d magnetotelluric mt profile consisting of 69 sites at 20 km intervals across the western part of the archaean kaapvaal craton the proterozoic rehoboth terrane and the late proterozoic early phanerozoic ghanzi chobe damara belt reveals significant lateral heterogeneity in the electrical resistivity structure of the southern african lithosphere the lithospheric structures of the rehoboth terrane and ghanzi chobe damara belt have not been imaged previously by geophysical methods temperature is the primary control on the resistivity of mantle minerals and the mt derived lithospheric thicknesses therefore provide a very reasonable proxy for the thermal thickness of the lithosphere i e the thickness defined by the intersection of a conductive geotherm with the mantle adiabat allowing approximate present day geotherms to be calculated the work indicates the following present day average lithospheric thicknesses to a precision of about 20 km for each of the terranes traversed inferred geotherms in brackets eastern kimberley block of the kaapvaal craton 220 km 41 mw m 2 western kimberley block 190 km 44 mw m 2 rehoboth terrane 180 km 45 mw m 2 and ghanzi chobe damara belt 160 km 48 mw m 2 a clear relationship between the electrical resistivity structure of the lithosphere and the tectonic stabilisation age of the terrane is evident good agreement between the inferred present day lithospheric geotherms and surface heat flow measurements indicate the latter are strongly controlled by variations in lithospheric thickness a significant difference in lithospheric thickness is observed between the eastern and western kimberley blocks and is consistent with previous seismic tomography images of the kaapvaal craton the present day lithospheric thickness and reduced depth extent into the diamond stability field accounts for the absence of diamondiferous kimberlites in the gibeon and gordonia kimberlite fields in the rehoboth terrane previously published mantle xenolith p t arrays from the gibeon gordonia and kimberley fields however suggest that the rehoboth terrane had equilibrated to a cooler conductive palaeo geotherm 40 42 mw m 2 very similar to that of eastern kimberley block of the kaapvaal craton at some time prior to the mesozoic eruption of the kimberlites the timing and nature of both the thermal equilibration of the rehoboth terrane and the subsequent lithospheric heating thinning event required to account for its present day lithospheric structure are not well constrained a model consisting of the penetration of heat transporting magmas into the lithosphere with associated chemical refertilisation at an early stage of mesozoic thermalism appears to be the most plausible model at present to account for both the present day lithospheric structure of the rehoboth terrane and an earlier cooler palaeo geotherm some problems however remain unresolved in terms of the isostatic response of the model based on a compilation of xenocryst cr ca in pyrope barometry observations the extent of depleted mantle in the rehoboth terrane is found to be significantly reduced with respect to the eastern kimberley block 117 km versus 138 167 km it appears most likely that the depletion depth in both terranes at least in the vicinity of kimberlite eruption is explained by refertilisation of the lower lithospheric mantle

why is it downhill from tonopah to las vegas a case for mantle plume support of the high northern basin and range a major abrupt southward decline in elevation 800 m with an accompanying increase in bouguer gravity anomaly 80 mgal crosses a seismically active region of southern nevada at about 37 n the steepness of the gravity gradient requires significant crustal density contrast some of which is probably caused by the plutonic roots of voluminous tertiary volcanism to the north but the amplitude of the anomaly requires additional contrast within the mantle the topographic step probably developed in mid miocene time coincident with the arrival of the starting head of the yellowstone thermal plume at the base of the lithosphere a plausible combination of crustal and deep buoyancy sources related to the heat and melt input of an anomalously hot asthenospheric source is consistent with gravity seismic heat flow and isotopic observations and explains the origin of the topographic step

measurements of coda buildup and decay rates of western pacific p po and so phases and their relevance to lithospheric scattering we use empirical functions to characterize the shape of band passed p po and so coda envelopes of 93 earthquakes the data sets consist of earthquake data from the western and northwestern pacific recorded by arrays of several stations and include both hydrophone and geophone recordings on analog and digital instruments each earthquake was band passed into six octave wide frequency bands between 0 8 and 50 hz the empirical function a poisson function pn t a n 1 tn e t n has parameters a a measure of energy falloff rate and nc time of the maximum the study showed that these parameters varied in systematic ways po so area a ratios were found to be considerably higher for hydrophone data in the 20 35 range than for geophone and hydrophone data in the 8 20 range the difference was significantly larger than between vertical and hydrophone data in the same range in some instances a strong frequency dependence or reverse dispersion of the time of maximum amplitude n was observed with the lower 1 2 hz frequencies arriving up to 25s later than the higher 10 20 hz frequencies p po and so average falloff rates a vary systematically with frequency where the lowest frequencies most commonly have the minimum falloff rates and the middle frequencies have the maximum falloff rates the variation in falloff between earthquakes is large with the trends only becoming apparent upon averaging many measurements a strong increase in falloff rates at about 3 hz may be associated with a decrease in the amount of scatterers with scale lengths of about one half the wavelength of a 3 hz compressional wave which is about 1 3 km falloff rates were seen to decrease with epicentral range reflecting an increased scattering with length of propagation path the scattering that produces the coda is thus not concentrated in the source region but distributed along the propagation path falloff rates were seen to decrease with increasing age of crust the difference may be due to the different average age of the lithosphere in these two geographical areas earthquakes at a depth of 100 300 km have systematically lower po falloff values than shallower events this effect is clearly seen in trench earthquakes and indicates that significant scattering occurs within or near the deeper buried part of the subducting plate the so po falloff ratio is a factor of 2 lower for the hydrophone data than for the geophone measurements this difference is most likely due to conversion of shear waves at or below the seafloor if we assume that the coda decay is controlled by intrinsic attenuation then measurements of coda decay can be converted into estimates of the quality factor q at 10 hz our falloff rates give qpo 1250 which is a reasonable number for a compressional wave quality factor in the uppermost mantle and qso qpo 2 the low snear wave q is in disagreement with inferences made by other observers however the fact that the falloff rates are fairly constant in the frequency range 5 30 hz but vary with propagation distance suggests that attenuation cannot be the only phenomenon determining the decay of the coda other factors such as reverberations in the water column may play an important part in the coda formation as well and may bias these estimates q

crustal thickness and upper mantle velocities in the tibetan plateau region from the inversion of regional pnl waveforms evidence for a thick upper mantle lid beneath southern tibet the crustal thickness and upper mantle velocity variation in tibet and surrounding regions was mapped through the inversion of 130 individual long period pnl waveforms the 130 paths crisscross the tibetan plateau hindu kush karakoram southwest china burma and northern india regions we regionalized these areas on the basis of topographic and geologic expression and determined the average upper mantle velocity and crustal thickness value for each block or region through a linear least squares inversion of the 130 parameter values crustal thicknesses in the tibetan plateau karakoram and hindu kush regions range from 63 to 72 km and pn velocities are 0 10 0 20 km s faster beneath these regions than those beneath northern india we observed impulsive first arrivals evidence for positive upper mantle gradients on seismograms recorded at distances between 1000 and 1400 km the paths of these waves crossed portions of tibet the hindu kush and east india regions simple modeling of the long period pn wave form and amplitude requires fairly high upper mantle gradients of 0 18 0 25 km s per 100 km beneath the pamir hindu kush region and requires an upper mantle lid in excess of 100 km thickness the southern half of tibet also requires a lid of about 100 km thickness a thinner upper mantle lid 50 km provided a poor match to the tibetan observations even with a very high gradient of 0 2 km s per 100 km the upper mantle structure of east india appears to be similar to the southern half of tibet 100 km thick positive gradient zone both the increase in pn velocity from india into tibet and the evidence for a thick upper mantle lid beneath tibet are consistent with the model in which indian lithosphere has underthrust southern tibet due to poor path coverage the structure beneath northern tibet cannot be resolved and thus underthrusting beyond central tibet cannot be constrained the rapid increase in upper mantle velocity from northern india into tibet can be explained by the pressure increase alone produced by the double thick crust and indicates that the indian lithosphere which underlies tibet is out of thermal equilibrium