structural history and granite emplacement in the rottenstone domain during closure of the trans hudson orogen davin lake northern saskatchewan the rottenstone domain at davin lake northern saskatchewan exhibits structural and granite emplacement evidence for crustal thickening and possible himalayan style extrusion overprinted by transpressional strain increasing toward the contact with the wathaman batholith three discrete rottenstone subdomains parallel the regional strike of the trans hudson orogen i the southeast rottenstone subdomain sersd with gently northwest dipping migmatitic straight gneiss s1 and white granitoid rocks with pinch and swell structures parallel to the straight gneissosity ii the central rottenstone subdomain crsd which partly preserves the same nw dipping fabric s1 but is overprinted at its margins by tight upright f2 folds and includes a stockwork of pink monzogranitic aplites and pegmatites and iii the northwest rottenstone subdomain nwrsd in which the f2 folds are generally tighter and penetrative and its network of narrow white granitoid rocks is deformed and transposed by the f2 folds but in

coeval sedimentation magmatism and fold thrust development in the trans hudson orogen propagation of deformation into an active continental arc setting wekusko lake area manitoba early structures in the southeastern part of the trans hudson orogen tho wekusko lake area manitoba comprise northwest verging folds and a set of steep southeast dipping faults that occur at a low angle to bedding kinematic indicators are rare however the vergence of the associated folds supports reverse movement on the faults these folds and faults formed prior to the peak of regional metamorphism and therefore must correlate with the widely recognised deformational d2 event which developed pre to syn metamorphism and involved development of a regional south to southwest directed fold thrust belt the d2 folds and reverse faults in the study area are interpreted to have originally formed as gently dipping southwest verging structures i e as inclined folds and thrust faults which were then rotated into a steep northwest verging orientation during later deformation d3 in the southeastern tho regional d2 fold and thrust structures are cut by ca 1839 1830 ma plutons indicating that

paleoproterozoic arc and ophiolitic rocks on the northwest margin of the trans hudson orogen saskatchewan canada their contribution to a revised tectonic framework for the orogen abstract a new lithotectonic framework for the northwestern reindeer zone of the trans hudson orogen divides rocks into five northwest to north dipping volcano sedimentary assemblages 1 at the structural base the 1 92 1 87 ga largely sedimentary levesque bay assemblage partly equivalent to former maclean lake gneisses which lies within the confines of the kisseynew domain and is tectonically imbricated with metasedimentary rocks of the the lawrence point assemblage is interpreted as a dismembered supra subduction zone ophiolite high mgo refractory harzburgite group 1 ultramafic rocks at the structural base of the assemblage is geochemically identical to the upper mantle section of selected supra subduction zone ophiolites and mantle tectonites chromite and olivine compositions of the group 1 ultramafic rocks are also comparable to those of ophiolitic harzburgite and mantle tectonite mafic metavolcanic rocks of the assemblage are classified as subalkaline tholeiitic basalts their trace element patterns and hf ta th y nb and la element ratios resemble those of modern back arc basin basalts the reed lake assemblage represents a subduction generated arc complex that was built on top of the lawrence point assemblage its mafic metavolcanic rocks are subalkaline basalts with calc alkaline trends and elevated th and ce concentrations and negative nb anomalies feldspar porphyry dykes intruding the lawrence point and duck lake assemblages constrain timing of lawrence point ophiolite emplacement onto the duck lake assemblage to 1 86 1 84 ga the trace element geochemistry of the dykes suggests continued arc volcanism after ophiolite emplacement mafic metavolcanic rocks of the levesque bay assemblage are geochemically similar to those of the lawrence point assemblage other ultramafic rocks peridotite to pyroxenite are abundant in the lawrence point assemblage but have similar geochemistry to small ultramafic bodies intruding the reed lake duck lake and levesque bay assemblages they represent a separate later phase 1 86 ga of ultramafic plutonism which post dates ophiolite emplacement timing of lawrence point ophiolite emplacement between 1 86 and 1 84 ga and geochemistry of later felsic and mafic ultramafic volcanism suggest that the lawrence point ophiolite and overlying reed lake arc assemblage were not accreted to the hearne craton prior to 1 86 ga but were first accreted to the flin flon glennie complex after 1 86 ga

geologic structure and tectonics of the inner continental borderland of northern baja california detailed marine geophysical surveys of the inner california continental borderland west of northern baja california show that the region is underlain by two major northwest trending quaternary dextral wrench fault systems the san clemente fault system lies along the western part of the inner borderland and is delineated by the san clemente and san isidro fault zones together these fault zones connect to form a long 300 km narrow 5 10 km continuous zone of faulting that is very similar to the larger san andreas fault system onshore the agua blanca fault system is a complex zone of shear delineated by three or more subparallel wrench fault zones in the eastern part of the inner borderland the westernmost san diego trough bahia soledad fault zone consists of relatively long 50 km continuous main fault traces which cut the quaternary sediments of the nearshore basin trough the coronado bank agua blanca fault zone is more complicated with numerous discontinuous subparallel right and left stepping anastomosing fault traces which are associated with significant structural relief a nearshore zone of faults marked by the newport inglewood rose canyon fault zone in the north and the estero descanso fault zone in the south parallels the coast and defines the eastern boundary ofmore the california continental borderland structural province all of these eastern fault zones merge into the transpeninsular agua blanca fault and their n30 sup 0 w trend differs substantially from the trend of the major peninsular ranges fault zones less

an approximation for the xu white velocity model in 1995 s xu and r e white described a method for estimating compressional and shear wave velocities of shaley sandstones from porosity and shale content their model was able to predict the effect of increasing clay content on compressional wave velocity observed in laboratory measurements a key step in the xu white method estimates dry rock bulk and shear moduli for the sand shale mixture this step is performed numerically by applying the differential effective medium method to the kuster toksoz equations for ellipsoidal pores this step is computationally intensive using reasonable assumptions about dry rock elastic properties this step can be replaced with a set of approximations for dry rock bulk and shear moduli numerical experiments show an extremely close match between velocities obtained with these approximations and velocities computed with the differential effective medium method these approximations simplify the application of the xu white method and make the method computationally more efficient they also provide insight into the xu white method for example these approximations show how the xu white model is related to the critical porosity model

alteration mineralogy and stable isotope geochemistry of paleoproterozoic basement hosted unconformity type uranium deposits in the athabasca basin canada unconformity type uranium deposits are characterized by mineralization developed along the contact between younger sandstone cover and underlying crystalline basement rocks mineralization may extend up to 400 m into the underlying basement rocks whereas sandstone hosted unconformity type deposits have been well studied deposits hosted primarily in the basement have not this study examines the deposits at rabbit lake dawn lake and mcarthur river in the athabasca basin of canada which are hosted by the metamorphic archean and early paleoproterozoic rocks forming the basement to younger late paleoproterozoic sandstones alteration is similar in the three deposits and is characterized by three distinct paragenetic stages 1 preore alteration involving illitization of plagioclase and amphibole followed by chloritization of biotite and illite which formed at ca 230 c 2 ore stage alteration characterized by uraninite and coarse grained illite which formed at ca 240 c 3 postore alteration comprising spherulitic dravite vein chlorite quartz calcite and fe cu co and pb sulfides which formed at ca 135 c fluid circulation associated with emplacement of later mackenzie dikes initiated partial recrystallization of uraninite a later stage of alteration includes kaolinite and iron hydroxide precipitation formed at much lower temperatures of ca 50 c stable isotope compositions of the alteration minerals in conjunction with their paragenesis indicate that oxidized basinal fluids d 43 to 21 18o 3 8 were derived primarily from evolved seawater and leached uranium from the overlying sandstones of the athabasca formation and transported it into the basement via infiltration along fracture zones associated with reverse faults graphitic units in the basement and preore alteration served as both physical fractured zones and chemical reductants traps for the uranium mineralization the basinal fluids were responsible for the preore illite chlorite synore uraninite illite and the early postore alteration events this differs from many other sandstone hosted deposits where both oxidized basinal and reduced basement derived fluids were responsible for uranium precipitation

age constraints on the paleoproterozoic tectonometamorphic history of the committee bay region western churchill province canada evidence from zircon and in situ monazite shrimp geochronology in situ u pb sensitive high resolution ion microprobe shrimp analyses of monazite from upper amphibolite facies paragneiss of the committee bay supracrustal belt central rae domain canada reveal three age populations ca 2350 1850 and 1780 ma the ca 1850 ma age also corresponds to growth of low th u zircon as indicated by u pb shrimp analyses of zircon separates from melanosome and leucosome the contextual advantage of the in situ monazite analysis allows evaluation of the geochronological data in terms of the regional structural and metamorphic evolution the region is dominated by a northeast striking s2 s1 fabric axial planar to tight northwest vergent f2 folds early garnet is enveloped by this biotite sillimanite cordierite s2 fabric garneti hosts ca 1850 ma monazite inclusions with ca 2350 ma cores placing a maximum age on garneti growth and s2 development d2 metamorphic conditions progressed through 3 5 kbar 1 kbar 100 mpa and 600 c to near peak conditions of 5 kbar and 675 c a minimum age for s2 is provided by unstrained ca 1820 ma monzogranite that locally and regionally truncates s2 dominantly ca 1780 ma matrix monazite is interpreted to date post s2 garnetii and cordierite which record 5 kbar and 675 c these data indicate that the committee bay region experienced penetrative d2 tectonometamorphism at ca 1850 1820 ma with a subsequent static overprint the absence of a ca 1 85 ga plutonic suite in the region suggests that low pressure metamorphism was a response to thick skinned crustal thickening initiated at ca 1870 ma the new data highlight the importance of paleoproterozoic reworking of the central rae domain in the hinterland of the trans hudson orogen

tectonic framework of a paleoproterozoic arc continent to continent continent collisional zone trans hudson orogen from geological and seismic reflection studies vertical incidence seismic data were collected along a 300 km long profile across the northwestern flank of the trans hudson orogen in saskatchewan line s2b the present study integrates the seismic data with previously published geological maps and recent results from the la ronge lynn lake bridge project that provide new constraints on the lithological structural and tectonic framework of this collisional zone an interpretative section based on the seismic reflectivity and surface geological constraints is presented the integrated data suggest the following key elements 1 although an early foreland vergent accretion history is suggested by surficial geological constraints the predominant fabric is reflected by penetrative hinterlandward vergence 2 a zone of more reflective lower crust with hinterland verging fabric may represent fossil reflectors from a pre collisional accretionary phase 3 the archean to paleoproterozoic sask craton may extend in the lower middle crust to at least

structure of a paleoproterozoic continent continent collision zone a lithoprobe seismic reflection profile across the trans hudson orogen canada abstract an 800 km reflection seismic profile across the trans hudson orogen northern saskatchewan and manitoba images crustal scale tectonic imbrication in an unprecedented picture of paleoproterozoic crustal accretion and continent continent collisional tectonism the profile is crudely symmetric about a crustal scale culmination in the western part of an accreted juvenile collage reindeer zone geologic and isotopic data suggest that this culmination is cored by microcontinental archean basement west of the culmination highly reflective juvenile crustal elements dip westward into the lower crust beneath the wathaman batholith and archean continental crust of hearne craton to the east strong reflections in the juvenile reindeer zone crust and reworked archean foreland of the thompson belt have eastward dips persisting into the middle crust and extending beneath the superior craton a continuous reflection moho well defined for 500 km in the western part of the profile shows marked relief 12 15 s including a prominent root below the crustal culmination these imaged structures give evidence of substantial crustal shortening and thickening via large scale imbrication consistent with collisional orogeny w dipping structures below the wathaman batholith and reworked hearne craton may reflect subduction polarity in this part of the orogen however geological evidence suggests that e dipping structures below superior craton are largely related to late post collisional deformation rather than to prior oceanic subduction polarity

high resolution mapping of two large scale transpressional fault zones in the california continental borderland santa cruz catalina ridge and ferrelo faults new mapping of two active transpressional fault zones in the california continental borderland the santa cruz catalina ridge fault and the ferrelo fault was carried out to characterize their geometries using over 4500 line km of new multibeam bathymetry data collected in 2010 combined with existing data faults identified from seafloor morphology were verified in the subsurface using existing seismic reflection data including single channel and multichannel seismic profiles compiled over the past three decades the two fault systems are parallel and are capable of large lateral offsets and reverse slip during earthquakes the geometry of the fault systems shows evidence of multiple segments that could experience throughgoing rupture over distances exceeding 100 km published earthquake hypocenters from regional seismicity studies further define the lateral and depth extent of the historic fault ruptures historical and recent focal mechanisms obtained from first motion and moment tensor studies confirm regional strain partitioning dominated by right slip on major throughgoing faults with reverse oblique mechanisms on adjacent structures transpression on west and northwest trending structures persists as far as 270 km south of the transverse ranges extension persists in the southern borderland a logjam model describes the tectonic evolution of crustal blocks bounded by strike slip and reverse faults which are restrained from northwest displacement by the transverse ranges and the southern san andreas fault big bend because of their potential for dip slip rupture the faults may also be capable of generating local tsunamis that would impact southern california coastlines including populated regions in the channel islands

geology u pb zircon geochronology and geochemistry of pge bearing neoarchean and paleoproterozoic gabbroic rocks of the peter lake domain southern hearne craton canada the peter lake domain a component of the hearne craton in saskatchewan includes several intrusive complexes ranging in age from 2 70 to 1 83 ga two gabbroic complexes swan river and porter bay together represent one of the largest accumulations of precambrian mafic intrusion both metamorphosed at amphibolite facies sulfide poor pge occurrences are found in both complexes and share many textural and lithological characteristics with magmatic contact type pge ni cu deposits such as the chaotic lac des iles pluton and with layered stratiform deposits of large layered intrusions such as bushveld or stillwater lithologically both complexes are dominated by leucocratic gabbronorite and gabbro locally characterized by cumulate layering cross bedding brecciated and pegmatitic textures u pb zircon crystallization ages between 2562 and 2560 ma were obtained for the swan river complex sulfide formation is interpreted to be of magmatic origin rather than remobilized as had been speculated by explora

tectonic entrapment and its role in the evolution of continental lithosphere an example from the precambrian of western canada new geophysical data acquired over the buried crystalline basement of western canada provide constraints on the history of tectonic assembly of the western canadian shield in the interval 1 75 1 85 ga specifically these data provide new perspectives on the evolution of an archean continental fragment heame province that was trapped in a tectonic vise between coeval orogenic belts that dipped beneath the hearne province the trans hudson orogen developed along the southeastern margin of the hearne province as a series of ocean floor oceanic arc and arc marginal basins were telescoped and thrust obliquely beneath the hearne along the northwest edge of the hearne collapse and subduction of a narrow marginal basin now marked by the subsurface extension of the snowbird tectonic zone led to formation of magmatic arc and collision of older proterozoic terranes the hearne province itself is characterized by regional granulite grade metamorphism and evidence of extensive and pervasive partial melting of the crust the internal character of the hearne province seen on crustal seismic reflection profiles is that of a crustal scale structural fan with reflection fabrics that verge toward the bounding orogens the deformation of the hearne is predominantly of paleoproterozoic age and constitutes a thorough reworking of this formerly archean crustal domain over a distance of more than 600 km across strike entrapment and thermal weakening of the hearne resulted from mechanical coupling of inferred buoyant subduction collision zones and removal of or modification of archean lithospheric mantle that may have originally formed the keel to the hearne long period magnetotelluric profiles show that anomalously conductive mantle lithosphere underlies the present day hearne province which is attributed to metasomatic modification of the subcontinental lithosphere following collisional thickening and delamination convective removal of thickened lithosphere in the proterozoic tectonic entrapment as illustrated by the proterozoic structural and thermal evolution of the hearne province crust and subcontinental mantle may be an example of the lithospheric consequences of opposing collisional polarity during assembly of continents

the wathaman batholith an early proterozoic continental arc in the trans hudson orogenic belt canada the wathaman batholith a major element of the trans hudson orogenic belt in northern saskatchewan and manitoba has an exposed strike length of 900 km this immense body separates dominantly archean rae hearne provinces to the northwest from early proterozoic arc related terranes of the orogen to the southeast major and trace element data for 42 samples from the saskatchewan part of the batholith are similar to those of phanerozoic arc batboliths for example sierra nevada and andean batholiths data plotted on trace element discrimination diagrams show that the batholith has continental arc affinities u pb zircon geochronology of five main phase granite samples suggests that most of the batholith formed between 1865 and 1850 ma and is coeval with smaller plutons of the la rouge arc the main deformation of the batholith is limited by the 1830 ma age of a late granite sheet that crosscuts folds and foliation of main phase granites the 15 to 25 m y crystallization span of the batholith is similar to the histories of many more recent continental magmatic arcs

early partitioning of quebec microcontinent formation in the paleoproterozoic the distribution of allochthonous versus parautochthonous carbonate platforms combined with the timing of initial continental rift magmatism versus the timing of subsequent rifting of the continental margin provides tectonostratigraphic evidence for the formation and isolation of the oldest documented microcontinent perhaps as a consequence of the impingement of a mantle plume onto a cratonic margin during the paleoproterozoic initial rifting of the archean nucleus of north america in eastern canada is constrained to have been diachronous between 2 17 and 2 03 ga renewed rifting of a segment of the continental margin 292 to 120 m y later was accompanied by the emplacement of ultramafic layered sills the accumulation of komatiitic and alkalic basalts the deposition of banded iron formations and the isolation of a microcontinent and its 1 93 ga continental shelf succession subsequently accreted to the telescoped continental margin during collisional orogenesis

geochronological constraints on the legs lake shear zone with implications for regional exhumation of lower continental crust western churchill province canadian shield the legs lake shear zone marks the southeastern boundary of an extensive region 20 000 km2 of high pressure 0 8 1 5 gpa granulite facies rocks in the western churchill province canada the shear zone is one of the largest exhumation related structures in the canadian shield and coincides with the central segment of the 2 800 km long snowbird tectonic zone the movement history of this shear zone is critical for the development of models for the exhumation history of the high pressure region we used electron microprobe u th pb dating of monazite with supplemental id tims u pb geochronology to place constraints on the timing of shear zone activity combining these and other data we suggest that regional exhumation occurred during at least three distinct phases over an 150 million year period the first phase involved high temperature decompression from 1 0 to 0 8 0 7 gpa shortly following 1 9 ga peak metamorphism possibly under an extensional regime the second phase involved rock uplift and decompression of the hanging wall to 0 5 0 4 gpa during east vergent thrusting across the legs lake shear zone at ca 1 85 ga this phase was likely driven by early collision related convergence in the trans hudson orogen the final phase of regional exhumation involving the removal of 15 20 km of overburden from both footwall and hanging wall likely occurred after 1 78 ga and may have been related to regional extensional faulting

structural controls on hypozonal orogenic gold mineralization in the la ronge domain trans hudson orogen saskatchewan the la ronge domain is a granite greenstone belt in the saskatchewan segment of the ca 1 9 1 8 ga trans hudson orogen the la ronge volcanic arc was accreted to the archean hearne craton from ca 1 87 to 1 86 ga subduction of oceanic lithosphere beneath the accreted la ronge hearne margin produced a voluminous suite of continental arc intrusions in the waddy lake area the 1852 6 1 5 ma corner lake stock and 1859 4 ma and 1861 2 ma feldspar porphyry dykes crystallized from magmas generated from melting of the subducted oceanic slab during the ca 1 83 1 80 trans hudson collision of the hearne craton with the archean sask and superior cratons a penetrative regional foliation and a steeply plunging lineation formed within the la ronge domain during further contraction across the domain the deformation became localized in dextral and oblique slip shear zones that generally follow contacts between more competent and less competent rock units orogenic gold mineralization is associated with quar

crustal history of the rae and hearne provinces southwestern canadian shield saskatchewan constraints from geochronologic and isotopic data abstract the cree lake zone hearne province and western granulite domain rae province in northern saskatchewan are contrasting parts of the variably reworked upper plate western hinterland of the paleoproterozoic trans hudson orogen their junction is defined by the virgin river shear zone a part of the more extensive snowbird tectonic zone subsurface extension of the 2 0 ga thelon orogen lies west of the western granulite domain we present u pb zircon ages sm nd and rb sr isotopic data from the cree lake zone and western granulite domain and from the junction granite which intrudes the virgin river shear zone data from the western granulite domain indicate that prevalent granulite facies metamorphism occurred between 2 3 and 2 0 ga but is imposed on archean protoliths 2 8 3 0 ga old low amphibolite facies retrogression which increases toward the virgin river shear zone probably postdates 2 1 ga data from the cree lake zone also yield archean protolith ages as old as 3 0 ga and some rocks yield interpreted high grade metamorphic overprint ages of 2 3 2 0 ga however two samples yield data indicating hudsonian high grade metamorphic overprint 1 87 to 1 81 ga the junction granite has a u pb zircon age of 1 82 0 30 ga but relict zircons and sm nd data show that it was derived by melting of archean crust high grade mid crustal thermal resetting in the western granulite domain may be coeval with the 1 9 2 0 ga thelon orogen although the data do not preclude an earlier event or events similar 2 3 2 1 ga reworking extends into the cree lake zone but high grade hudsonian 1 87 1 84 ga reworking is also evident in parts of this zone well to the west of the wollaston domain and may extend entirely across the zone to the virgin river shear zone relative importance of the thelon and hudsonian overprints in cree lake zone is unknown data from the junction granite and western granulite domain suggest that all recorded high strain events in the virgin river shear zone postdate 2 3 ga and are thus unrelated to deep crustal archean granulite facies mylonites in the tantato black lake segment of the snowbird tectonic zone north of the athabasca basin

timing constraints of orogeny to cratonization thermochronology of the paleoproterozoic trans hudson orogen manitoba and saskatchewan canada abstract a comprehensive thermochronometric investigation across a 600 km transect of the paleoproterozoic trans hudson orogen tho in canada has resolved two first order thermal events 1 widespread peak metamorphism at 1 82 1 79 ga associated with terrane amalgamation and 2 a late collisional metamorphic episode at ca 1 77 ga that is characterized by significant strike slip deformation and rapid cooling geophysical observations of the lithosphere beneath the tho reveal crust that is markedly different on either side of the crustal scale tabbernor sturgeon weir tsw strike slip fault system which offsets lower crustal reflectors the tho east of the fault system records u pb titanite and total pb monazite domain ages that are relatively indistinguishable from 40ar 39ar hornblende and mica cooling ages indicating rapid cooling and perhaps exhumation following 1 77 ga metamorphism seismic imaging indicates the eastern orogen is an irregular package of stacked crustal blocks and our thermochronometric data demonstrate it behaved as a single coherent element shortly 20 30 m y following cessation of convergence in contrast rocks located west of the tsw fault system cooled distinctly later preserving 40ar 39ar muscovite and biotite ages between 1 74 and 1 72 ga this crustal architecture delineated from seismic surveys illustrates a more homogeneous slightly 5 km thinner crust in the case of this orogen apparent stabilization and cratonization occurred ca 100 m y after peak terrane amalgamation and notably the orogen was likely cratonized as two blocks east and west of the tsw fault system

paleomagnetism anisotropy of magnetic susceptibility and argon argon geochronology of the clearwater anorthosite saskatchewan canada abstract the clearwater anorthosite in saskatchewan is a major gabbro anorthosite layered intrusion lying within the archean rae province along the northwestern hinterland of the paleoproterozoic trans hudson orogen tho the intrusion which could be at least 2 4 ga old carries a well defined remanent magnetization with a direction of declination 112 and inclination 38 k 316 95 3 1 n 8 the corresponding paleomagnetic pole plat 6 5 n plong 48 2 w dp 2 2 dm 3 7 is the first one obtained from the archean western granulite domain of the rae province field observations supplemented by anisotropy of magnetic susceptibility data show that the magnetization post dates ductile deformation of the anorthosite argon argon plateau dates of 1917 7 ma for hornblende and 1839 5 ma for biotite are interpreted as cooling ages with the hornblende age approximating the time of remanence the pole position and others of comparable age from the northwestern margin of the tho form a loose cluster the mean of which is about 90 of arc away from that of a broadly contemporaneous group of poles from the superior province this relation supports previous interpretations of paleomagnetic data that prior to the tho at about 1 9 ga ago a major ocean the manikewan ocean at least 5000 km across separated the superior and rae hearne provinces

lithoprobe reflection studies of archean and proterozoic crust in canada abstract lithoprobe canada s national collaborative earth science research project established to develop a comprehensive understanding of the evolution of the north american continent is a multidisciplinary program spearheaded by seismic reflection studies five recently recorded seismic lines discussed in this paper are located in precambrian regions the mesoproterozoic grenville province the paleoproterozoic trans hudson orogen tho the paleoproterozoic archean basement of alberta and the archean superior province data acquired across the grenville orogen in eastern quebec show strong reflectivity throughout the crust upper crustal reflections can be correlated with exposed structural elements including extensional shear zones and packages of deformed high pressure rocks eclogites in a marine survey across the grenville orogen off southeastern labrador seismic images show variably dipping reflections and a structural high associated with a major gravity anomaly data acquired across central alberta show crustal scale thrust stacking and imbrication of the archean hearne craton to the east across the trans hudson orogen images of similar collisional features are observed geochronologic constraints indicate contemporaneity of tectonic activity between the two regions at 1 8 ga suggesting that collisional tectonic activity was coeval over a broad crustal region ca 1000 km across strike in the superior province seismic data across a collision zone involving the northern abitibi greenstone belt and the arc related opatica plutonic belt show spectacular crustal reflectivity and dipping reflections that extend 8 s 30 km into the mantle the latter feature is interpreted as representing a relict 2 69 ga old suture associated with subduction providing the first direct evidence that plate tectonics was active in the late archean these five examples supported by other lithoprobe results refute a number of generalizations about crustal reflectivity that have been made in the past and illustrate how reflection studies combined with other geoscience studies can lead to a better understanding of precambrian tectonics reflectivity persists throughout the crust there is no general separation into a poorly reflective upper crust and a reflective lower crust crustal reflectivity in archean and proterozoic regions is as pervasive as that in areas of more recent tectonism the precambrian reflection moho is generally well defined but shows a range of characteristics relative ages of reflectors can be discerned and tectonic significance can be attached to characteristic features of the crustal reflectivity

the sask craton and hearne province margin seismic reflection studies in the western trans hudson orogen a three dimensional model of the regional crustal architecture of the western trans hudson orogen based on the interpretation of 590 km of deep sounding seismic reflection data and a comparable length of existing seismic reflection information is presented the seismic images identify the regional geometry of the basal detachment zone pelican thrust that separates juvenile allochthonous terranes from the underlying archean microcontinent sask craton the sask craton is inferred to have a minimum spatial extent of over 100 000 km2 with an associated crustal root that extends for 200 km along strike during terminal collision complete convergence of the rae hearne and superior continental blocks was precluded by the presence of the sask craton resulting in the preservation of anomalous amounts of oceanic and associated sedimentary juvenile material along regional tectonic strike consistency of crustal structure across the rae hearne margin reindeer zone boundary is established several phases of

constraints on the timing of crustal imbrication in the central trans hudson orogen from single zircon 207pb 206pb ages of granitoid rocks from the pelican thrust zone saskatchewan the pelican thrust is a major ductile high strain zone in the reindeer zone trans hudson orogen northern saskatchewan it is interpreted as the main sole thrust separating stacked juvenile paleoproterozoic allochthons and underlying archean microcontinental crust in this central part of the orogen exposed nonmylonitic rocks in the footwall of the thrust consist of the sahli monzocharnockite and the smaller more highly retrograded macmillan point granite protomylonitic to ultramylonitic gneisses in the thrust zone derive from a variety of prethrust protoliths a footwall internal suite mainly comprises quartzofeldspathic orthogneisses q gneisses and high grade migmatitic paragneisses hanging wall external suite mylonitic gneisses include feldspar porphyroclastic hornblendic grey gneisses probably derived from arc plutons and laminated amphibolites derived from volcanic rocks the overlying allochthon mainly comprises protoliths equivalent to those of the porphyroclastic orthogneisses and lam

crustal evolution and age of thermotectonic reworking in the western hinterland of the trans hudson orogen northern saskatchewan abstract the hearne province in northern saskatchewan forms the western hinterland of the trans hudson orogen it is a fault bounded terrane lying structurally and geographically between the rae province to its west and the wathaman batholith and orogenic internides to its east the hearne province is comprised primarily of high grade metamorphic archean rocks structurally overlain by archean or proterozoic metasedimentary rocks and intruded by paleoproterozoic granites whereas the rae terrane on its west boundary is comprised primarily of still higher grade archean rocks the calc alkaline wathaman batholith was intruded at ca 1 855 ga and the orogenic internides to the east of it include mostly high grade mafic and intermediate volcanic rocks crystallized between 1 92 and 1 86 ga terminal collision and orogenesis in the internides have been constrained to be later than 1 82 ga and earlier than 1 78 ga mineral u pb data presented here unequivocally demonstrate that high grade metamorphism and large volume anatexis across the southern hearne province were approximately synchronous with terminal collision and high grade metamorphism across the trans hudson orogen zircon data for most samples form complex arrays in concordia space but demonstrate a high grade metamorphic overprint at 1810 1800 ma zircons typically have inherited components of archean age suggesting the presence of extremely old material in the hearne and a potential archean connection with the rae province sm nd data indicating ca 3500 ma model ages for similar parageneses in both terranes further support the proposed archean connection between hearne and rae province and the great antiquity of hearne crust sm nd data also elucidate the similarity of the ca 1 80 ga anatectic granites of the hearne to the 1 82 ga junction granite which intruded the virgin river shear zone and constrains the timing of final motion on that structure monazite 207 pb 206 pb ages are not older than 1812 ma or younger than 1769 ma and rutile 207 pb 206 pb ages and u pb ages are tightly constrained at about 1750 ma these data in combination with results of standard thermobarometric analysis permit determination of post orogenic cooling and denudation rates high cooling 7 c ma and denudation 240 m ma rates suggest fault accomodated uplift during post orogenic collapse

continental arc magmatism along the southeast hearne craton margin in saskatchewan canada comparison of the 1 92 1 91ga porter bay complex and the 1 86 1 85ga wathaman batholith abstract detailed mapping coupled with geochronological and geochemical investigations has revealed the presence of a 1917 1913 ma gabbro monzodiorite monzonite suite along the southeast margin of the hearne craton in northern saskatchewan canada the predominantly plutonic suite is also characterised by 1915 ma old trachyandesitic subvolcanic and volcaniclastic inclusions the rocks are hornblende epidote titanite augite bearing and collectively termed the porter bay complex the plutonic rocks cut the 2569 ma lueaza river granitoid suite a component of the hearne craton and are themselves intruded by 1859 ma pegmatitic diorite 1856 ma layered gabbro anorthosite and 1853 ma quartz diorite belonging to the wathaman batholith one of the world s largest paleoproterozoic andean type continental arcs wholerock major element geochemistry characterises the porter bay complex as calc alkalic to alkali calcic metaluminous and variable from ferroan to magnesian trace element concentrations are characterised by negative high field strength element anomalies suggesting emplacement along a destructive plate margin the geochemical signatures of the wathaman batholith and the porter bay complex are largely identical the geographic location map relationships and geochronological geochemical and petrographic constraints are consistent with the porter bay complex having formed in a subduction related continental arc setting the southeastern margin of the hearne craton was therefore a long lived active continental margin with two separate periods of continental arc magmatism between 1 92 1 91 ga and 1 86 1 85 ga

origin of the archean sask craton and its extent within the trans hudson orogen evidence from pb and nd isotopic compositions of basement rocks and post orogenic intrusions u pb zircon ages from the exposed sask craton are 2450 3100 ma from the peter lake domain 2575 2640 ma and from rocks of the trans hudson orogen 1840 1880 ma u pb monazite and zircon ages of post orogenic pegmatites and aplites are 1770 1800 ma common pb and sm nd isotopic compositions of post orogenic intrusions as probes of crust beneath the orogen were compared to sask craton rocks and ca 1850 ma orogenic rocks to infer the origin and subsurface distribution of the sask craton within the internides of the trans hudson orogen results show that post orogenic intrusions within most of the glennie domain and hanson lake block were derived at least in part from archean source materials demonstrating that the sask craton lies beneath paleoproterozoic orogenic rocks present at the surface in contrast common pb and sm nd isotopic compositions from pegmatites and aplites of the la ronge domain are essentially identical with those of the paleoproterozoic orogenic rocks into which they are intruded indicating derivation by partial melting of similar rocks thus if the sask craton extended to the west beneath the la ronge domain it was beneath the zone of melting that produced the post orogenic intrusions making it unlikely that the sask craton is a detached part of the hearne craton many samples from the sask craton have elevated pb 208 pb 204 ratios unlike superior craton or hearne craton rocks suggesting that the sask craton was derived from an exotic source such as the wyoming craton which shares similar elevated pb 208 pb 204 ratios

seismic inversion combining rock physics and multiple point geostatistics a novel inversion technique combines rock physics and multiple point geostatistics the technique is based on the formulation of the inverse problem as an inference problem and incorporates multiple point geostatistics and conditional rock physics to characterize previously known geologic information the proposed implementation combines elements of sampling from conditional probabilities and elements of optimization the technique provides multiple solutions all consistent with the expected geology well log data seismic data and the local rock physics transformations a pattern based algorithm was selected as the multiple point geostatistics component rock physics principles are incorporated at the beginning of the process defining the links between reservoir properties e g lithology saturation and physical quantities e g compressibility density making it possible to predict situations not sampled by log data results for seismic lithofacies inversion on a synthetic test and a real data application demonstrate the validity and applicability of the proposed inversion technique

estimation of net to gross from p and s impedance in deepwater turbidites the limitations of using low frequency 3 d seismic data for detailed reservoir characterization are well recognized nonetheless these data are very well suited for quantitative estimates of the net to gross n g which is a reservoir scale property critical in deepwater turbidite reservoir characterization and development projects the acoustic impedance inversion of 3 d seismic has been routinely used with varying success to estimate n g in many clastic reservoirs indeed when the shale baseline impedance can be estimated the departure from this trend can be argued to represent a certain measure of the hydrocarbon saturated sand volume on the reservoir scale we take this technique a step further by using both acoustic and shear impedance ai and si and applying them to horn mountain deepwater reservoirs gulf of mexico gom where six appraisal wells were available for calibration

the 1830 ma trans hudson hairpin from paleomagnetism of the wapisu gneiss dome kisseynew domain manitoba the wapisu gneiss dome is located in the northeastern part of the kisseynew domain of the trans hudson orogen tho in north central manitoba the dome is circular about 6 km in diameter with steeply dipping flanks it is composed of upper amphibolite facies gneisses derived from turbiditic sediments with leucogranitic sill like intrusions that were metamorphosed starting at about 1830 ma alternating field and thermal step demagnetization of 153 specimens from 17 sites around the perimeter of the dome isolated a characteristic remanent magnetization chrm direction of d 346 6 i 78 9 95 5 3 k 46 n 17 that gives a pole of 119 5 w 75 8 n dp 9 5 dm 10 0 unblocking temperatures and saturation isothermal remanence analyses show that the chrm resides mostly in single to pseudosingle domain magnetite or titanomagnetite with minor hematite commonly present and with minor pyrrhotite present in the leucogranites the chrm is found to be postfolding with 99 9 confidence indi

crustal suture preserved in the paleoproterozoic trans hudson orogen canada lithoprobe seismic reflection data acquired in 1994 within the trans hudson orogen include a 180 km profile that crosses the boundary between a collapsed marginal basin kisseynew domain and an older volcanic plutonic arc terrane lynn lake belt the seismic data image a prominent band of northeast dipping anastomosing reflections that extend to 10 s two way traveltime from the surface location of a geological boundary where interpreted oceanic crust is preserved and separate thick crust 16 s of the kisseynew domain from the abruptly thinner crust 12 s of the lynn lake belt highly reflective lower crust 10 16 s of the kisseynew domain dips gently northward and extends laterally for at least 40 km beneath the adjacent lynn lake belt in a manner reminiscent of the western alps complementary gravity modeling shows that the thickness of metasedimentary rocks within the kisseynew domain is 20 3 km the seismic image is interpreted as portraying a crustal scale collisional suture that formed as the end result of northward subduction of backarc basin crust ancestral kisseynew basin floor and subsequent contraction and thrusting of the remnant kisseynew basin fill under the margin of the lynn lake belt

le batholite de de pas nouveau quebec infrastructure composite d arc magmatique proterozoique the de pas batholith in new quebec is a mass of granitoids elongated in a n s direction covering 7000 km2 in the southeastern part of the archean rae province it is located between the paleopro

proterozoic tectonic accretion and growth of western laurentia results from lithoprobe studies in northern alberta the western canadian shield of northern alberta is composed of a series of continental slivers that were accreted to the margin of the archean rae hinterland ca 1 9 2 0 ga preserving a unique record of continental evolution for the interval 2 1 2 3 ga this part of laurentia owes its preservation to the accretionary style of tectonic assembly south of the great slave lake shear zone which contrasts with indentation escape processes that dominate the paleoproterozoic record farther north the buffalo head and chinchaga domains form the central core of this region comprising a collage of ca 2325 2045 ma crustal elements formed on an archean microcontinental edifice and similar age crust is preserved as basement to the taltson magmatic zone the distribution of magmatic ages and inferred collision and subduction zone polarity are used to indicate closure of intervening oceanic basins that led to magmatism and emplacement of continental margin arc and collisional belts that formed from ca 1998 to1900

paleoproterozoic arc magmatism imposed on an older backarc basin implications for the tectonic evolution of the trans hudson orogen canada compositional data from 72 granitoids and associated mafic rocks in the paleoproterozoic kisseynew domain trans hudson orogen canada were used to evaluate the tectonic setting of magma generation between 1840 and 1820 ma these data thus provide insights into the development of the trans hudson orogen just prior to amphibolite grade metamorphism and the final stages of continental collision the granitoid rocks are hornblende or orthopyroxene bearing medium to high k calc alkalic and weakly peraluminous the trends on harker variation diagrams are consistent with crystal fractionation processes although the range in values is related to the temporal and spatial distribution of samples the extent of lree light rare earth element enrichment increases with increasing sio 2 as does variability in hree heavy rare earth element fractionation and magnitude of eu anomalies most samples have gd yb n 10 suite ia although five have gd yb n 17 24 suggesting the presence of garnet residua suite ib the range in rock types calc alkalic character and negative nb and ti anomalies are all characteristic of arc environments magma generation within the trans hudson orogen including the kisseynew domain between 1 84 and 1 82 ga was related to interaction between the sask craton and the flin flon glennie complex a decrease in subduction angle likely resulted from northward directed subduction of more buoyant crust with associated changes in thermal conditions leading to melting over a wide area most magmas formed in a slab dehydration dominated regime although localized slab melts may have been generated ultimately subduction ceased and the slab foundered into the mantle potentially resulting in the generation of alkalic and potassic magmas within the rae and hearne cratons

a new velocity model for clay sand mixtu res1 none of the standard porosity velocity models e g the time average equation raymer s equations is satisfactory for interpreting well logging data over a broad depth range clays in the section are the usual source of the difficulty through the bias and scatter that they introduce into the relationship between porosity and p wave transit time because clays are composed of fine sheet like particles they normally form pores with much smaller aspect ratios than those associated with sand grains this difference in pore geometry provides the key to obtaining more consistent resistivity and sonic log interpretations a velocity model for clay sand mixtures has been developed in terms of the kuster and toksoz effective medium and gassmann theories in this model the total pore space is assumed to consist of two parts 1 pores associated with sand grains and 2 pores associated with clays including bound water the essential feature of the model is the assumption that the geometry of pores associated with sand grains is significantly different from that associated with clays because of this porosity in shales affects elastic compliance differently from porosity in sand stones the predictive power of the model is demonstrated by the agreement between its predictions and laboratory measurements and by its ability to predict sonic logs from other logs over large depth intervals where formations vary from unconsolidated to consolidated sandstones and shales

seismic studies on the trans hudson orogen of western canada abstract the discovery of diamondiferous kimberlites in the late 1980 s within the central saskatchewan segment of the trans hudson orogen prompted an aggressive diamond exploration program in the region the presence of this apparently uncharacteristic mineralization within the proterozoic orogen was initially a puzzle recent seismic reflection and geochronology evidence however suggests that the glennie domain of the orogen may be cored by a microcontinental archean block while geochemical results imply that segments of the precambrian basement of southern saskatchewan mainly comprise archean crust which has been reworked by thermotectonism we present the initial results of two seismic programs designed to elucidate the lithospheric structure associated with the economically targeted segment of the orogen preliminary interpretation of wide angle reflection data along a 730 km north south profile in north central saskatchewan reveals a very complex structure with the crustal thickness varying from a maximum of 50 km to a minimum of 37 km over a distance of 200 km beneath the glennie domain a teleseismic feasibility study with an 8 station array with individual seismographs operating for 4 to 6 months supports and extends this image of complexity relative travel time differentials across a 500 km array are as large as 700 ms while key features of these delay times correlate with the crustal structure a significant portion of the variations must be assigned to deeper levels of the lithosphere receiver function analysis for crustal structure is shown to be feasible in this environment where the phanerozoic sediment thickness varies from 0 to 2 1 km and provides evidence for large variations 7 km in crustal thickness in the southwestern area of the province sks analysis at a single station also adds to the evidence which shows rapid variations in anisotropy within the orogen a teleseismic program with seventeen 3 component stations is now underway to clarify the nature of this complex crustal and upper mantle structure

earthquake locations in the inner continental borderland offshore southern california the inner continental borderland region offshore southern california is tectonically active and contains several faults that are potential seismic hazards to nearby cities however fault geometries in this complex region are often poorly constrained due to a lack of surface observations and uncertainties in earthquake locations and focal mechanisms to improve the accuracy of event locations in this area we apply new location methods to 4312 offshore seismic events that occurred between 1981 and 1997 in seven different regions within the borderland the regions are defined by either temporal or spatial clustering of seismic activity in the southern california seismic network scsn catalog obtaining accurate locations for these events is difficult due to the lack of nearby stations the limited azimuthal coverage and uncertainties in the velocity structure for this area our location procedure is based on the l 1 norm grid search waveform cross correlation method of shearer 1997 except that we use a nearest neighbor approach astiz et al 2000 to identify suitable event pairs for waveform cross correlation and we explore the effect of different velocity models on the locations and associated station terms in general our relocated events have small estimated relative location errors and the events are more clustered than the scsn catalog locations a quarry on the south tip of catalina island provides a test of our location accuracy and suggests that under ideal conditions offshore events can be located to within 1 to 2 km of their true locations our final locations for most clusters are well correlated with known local tectonic features we relate the 1981 santa barbara island m l 5 3 earthquake with the santa cruz fault the 13 july 1986 oceanside m l 5 3 sequence with the san diego trough fault zone and events near san clemente island with the known trace of the san clemente fault zone over 3000 of the offshore events during this time period are associated with the 1986 oceanside earthquake and its extended aftershock sequence our locations define a northeast dipping fault plane for the oceanside sequence but in cross section the events are scattered over a broad zone about 4 km thick this could either be an expression of fault complexity or location errors due to unaccounted for variations in the velocity structure events that occur near coronado bank in the scsn catalog are relocated closer to the san diego coast and suggest a shallow angle northeast dipping fault plane at 10 to 15 km depth manuscript received 12 february 1999

lithospheric structure of the trans hudson orogen from seismic refraction wide angle reflection studies the trans hudson orogen tho is the world s largest paleoproterozoic orogenic belt data from three refraction profiles are used to investigate its lithospheric structure in saskatchewan and manitoba r1 crosses the orogen from the hearne craton on the west to the superior craton on the east r2 and r3 are along the orogen p wave velocity structural models are generated using a ray based technique on line r1 higher crustal velocities in its eastern part coincide with rocks of the flin flon namew gneiss complex depth to moho is in the 40 45 km range and equates to that from the reflection data including a small crustal root below the sask minicontinent along lines r2 and r3 depth to moho varies from about 40 km up to 55 km at the north end of r2 and south end of r3 in general variations in crustal velocity and depth to moho do not correlate with the location and extent of geological domains they appear to reflect the complex deformation and metamorphic history of the crustal rocks mantle velo

geodynamic evolution and thermal history of the central flin flon domain trans hudson orogen constraints from structural development 40ar 39ar and stable isotope geothermometry shear zones in the flin flon area record five episodes of development these have been studied using structural analysis and 40ar 39ar thermochronology deformation occurred as a result of collision between the archean rae hearne and archean superior plates during the trans hudson orogeny 1900 to 1700 ma the geodynamic evolution encompasses a transition from island arc supracrustal development 1900 ma to tectonic mountain building under brittleductile conditions 1860 ma two episodes of ductile lineation schistosity l s shear zones on the prograde metamorphic path 1860 1810 ma and two episodes of retrograde displacement between 1790 and 1690 ma during final suturing of the archean cratons with paleoproterozoic assemblages metamorphism evolved in three stages contact aureoles to granitoid bodies formed at 1860 1840 ma hornblende homfels facies 40ar 39ar ages indicate cooling through 500 c at 1840 ma this was followed by regional heating burial which produced regional peak thermal recrystallization at 1820 1790 ma under lower greenschist to lower amphibolite conditions lastly retrograde metamorphism took place during unroofing between 1790 and 1690 ma with a regional cooling rate of 2 c m y

thermochronology of the needle falls shear zone a post collisional high strain zone of the trans hudson orogen the needle falls shear zone is a major paleoproterozoic discontinuity separating an archean craton from accreted paleoproterozoic terranes in the western trans hudson orogen it is a 1 km wide subvertical mylonite zone forming the southwestern sector of a northeast trending shear system extending for more than 400 km kinematic data indicate oblique east side up dextral strike slip displacement throughout its evolution from ductile through late brittle deformation for most of its exposed length it marks the northwestern margin of the wathaman batholith the oldest pb evaporation ages of zircons from mylonite average 1857 ma consistent with published zircon ages of the wathaman batholith confirming the batholith is protolith for much of the shear zone the synkinematic metamorphic assemblage of amphibole biotite quartz feldspar signifies deformation under amphibolite facies conditions 500 630 c oxygen isotope fractionations between quartz hornblende and quartz biotite pairs in textural equilibrium

sedimentary basin controls on orogenic gold deposits new constraints from u pb detrital zircon and re os sulphide geochronology lynn lake greenstone belt canada abstract sedimentary basins that open and close during the last stages of mountain building represent an important exploration criterion for orogenic gold deposits however the genetic and or preservation controls of these synorogenic or timiskaming type sedimentary basins and their controlling fault systems on orogenic gold deposits remain unclear herein we address that knowledge gap and report new u pb detrital zircon and re os sulphide arsenopyrite and pyrite geochronology and sulphide pb isotope results for the paleoproterozoic lynn lake greenstone belt llgb manitoba canada the youngest detrital zircon from all six meta conglomerate and psammite samples of the synorogenic sickle group and previously reported u pb zircon ages for post sickle group intrusions are used to constrain its depositional timing from 1836 15 to 1831 4 ma replicate analyses of one highly radiogenic arsenopyrite sample from an auriferous vein at the maclellan gold deposit yield a weighted average re os model age of 1824 12 ma which is identical to previously published in situ u pb xenotime ages at the same deposit 1827 8 ma each of these hydrothermal ages demonstrate that early stage auriferous veins immediately post date deposition of the sickle group and most likely occurred prior to peak metamorphism 1814 1801 ma this sequence of events is very similar to the abitibi greenstone belt suggesting that a synorogenic phase of extension and rapid burial of auriferous veins by timiskaming type sickle group sediments may have played an important genetic and or preservation control on early stage gold mineralization in the llgb however unlike the abitibi greenstone belt none of the known gold deposits within the llgb are hosted within the sickle group younger re os model arsenopyrite ages at 1782 16 ma from the maclellan gold deposit also post date synorogenic sedimentary basins by ca 50 myr these late stage auriferous veins are unrelated to the synorogenic extensional phase and more likely reflect repeated fluid focusing along reactivated structures during a post peak metamorphic phase of hydrothermal activity the multi stage hydrothermal history of orogenic gold deposits in the llgb also provides a possible explanation for the mixture of depleted mantle like and highly radiogenic fluid components that are inferred from age corrected sulphide pb isotope compositions 1 8 ga 8 9 10 6 reworked cratonic margins and their associated greenstone belts thus represent favourable depositional settings for auriferous fluids at multiple stages throughout the lifespan of an orogen

trace element discrimination of arc slab failure and a type granitic rocks abstract although widely used classification schemes for granitic rocks have until now failed to separate slab failure from arc magmatism in collisional orogens our previous work showed that cretaceous cordilleran type batholiths are composed of a pre collisional belt of mesozonal to epizonal arc plutons commonly emplaced into their own cover and a post accretion tract of what we interpret as slab failure plutons emplaced into the tectonically thickened collisional hinterland histograms based on new geochemical compilations of pre and post collisional igneous rocks were employed to derive trace element criterion to distinguish between these two plutonic types using rocks over the sio2 range of 55 70 wt and aluminum saturation index of 20 nb y 0 4 ta yb 0 3 la yb 10 gd yb 2 and sm yb 2 5 also we found that a type magmas can be distinguished from other post collisional igneous rocks based on ta yb 6 ppm and nb y 60 ppm these trace element differences between our pre collisional arc and post collisional slab failure plus a type groups allowed us to build composite discrimination diagrams for destructive margin igneous rocks although most s type granites are also post collisional they should not be plotted on our diagrams for some of their geochemical features overlap those of our slab failure group unlike our arc and slab failure groups a type granites are formed in diverse tectonic settings such that their classification is more related to geochemical signature and magma sources than to tectonic setting lastly we show how our discrimination diagrams delineate examples of slab failure magmatism through time and suggest links with porphyry cu au deposits

earthquake locations in southern california obtained using source specific station terms we relocate 297 400 events recorded by the southern california seismic network scsn between 1975 and 1998 using spatially varying station terms to improve relative location accuracy our method uses existing scsn p and s picks a smooth one dimensional velocity model and an iterative grid search approach based on the l1 norm we apply empirical corrections for three dimensional structure by computing station timing corrections that continuously vary as a function of source position station terms for each event are obtained by smoothing the residuals from nearby events using a natural neighbor delaunay tessellation of the seismicity and then iterating until a stable set of locations and station terms is achieved our approach achieves relative location accuracy comparable locally to master event methods but can be applied uniformly over large regions median estimated standard errors for our final locations are 328 m in horizontal position and 741 m in depth our locations exhibit much less scatter particularly in depth than those of the scsn catalog and a greater tendency to align into linear and planar features suggestive of fault structures our results appear comparable to and in some cases better than previous scsn relocation studies using joint hypocenter velocity inversion techniques plots of daytime versus nighttime events permit discrimination between clusters of natural and artificial seismicity we observe no simple relationship between the maximum depth of seismicity and surface geology

tectonics and surface effects of the supercontinent columbia abstract assembly of the supercontinent columbia at about 1 85 1 90 ga coincided with several events that affected the entire earth the oldest worldwide network of orogenic belts formed at the same time although some granite granodiorite gg suites had formed earlier the gg suites became common in the 1 8 1 9 ga orogenic belts these suites succeeded the older tonalite trondhjemite granodiorite ttg suites which were not produced after 1 8 ga changes on the earth s surface at 1 8 1 9 ga include rapid increase in the concentration of oxygen in the atmosphere and oceans and probably the evolution of eukaryotes all of these surface changes occurred as columbia accreted and the assembly of columbia may have contributed to the drastic changes in the earth s surface environment as well as to the evolution of primitive life forms

the pelican thrust zone basal detachment between the archean sask craton and paleoproterozoic flin flon glennie complex western trans hudson orogen the pelican thrust zone is a 3 7 km wide recrystallized mylonite zone along which paleoproterozoic arc volcano plutonic and derived sedimentary rocks of the flin flon glennie complex were thrust over an archean package jan lake complex consisting of ca 3 1 ga calc alkaline orthogneisses pelitic migmatites and a ca 2 45 ga tholeiitic charnockite norite intrusive suite a regional northeast plunging stretching lineation and a variety of kinematic indicators imply southwesterly transport matching that of other coeval shallower crustal level structures observed throughout the northern flin flon domain and southern flank of the kisseynew domain e g sturgeon weir and annabel lake shear zones subsequent east side up displacement on the tabbernor fault together with domal fold interference has exposed the pelican window and mantling mylonite zone where it has been seismically profiled along lithoprobe transects tectonic windows also expose archean rocks in the glennie domain to the west where similar southwest verging recrystallized mylonite zones have been documented together these zones record collision and underplating of the flin flon glennie complex by the archean sask craton during prolonged protocontinent continent collision over a minimum 1826 1805 ma interval resume la zone de chevauchement pelican est une zone de mylonite recristallisee d une largeur de 3 7 km le long de laquelle des roches volcano plutoniques d un arc datant du paleoproterozoique et des roches sedimentaires derivees du complexe de flin flon glennie ont ete chevauchees par dessus un ensemble archeen le complexe de jan lake qui est compose d orthogneiss calco alcalins et de migmatites pelitiques gees d environ 3 1 ga ainsi que d une suite intrusive charnockite norite tholeiitique gee d environ 2 45 ga une lineation regionale d etirement a pendage nord est et divers indicateurs cinematiques correspondent a un transport vers le sud ouest rejoignant celui d autres structures contemporaines de niveaux crustaux moins profonds observees a travers tout le domaine de flin flon nord et le flanc sud du domaine kisseynew p ex les zones de cisaillement de sturgeon weir et d annabel lake un deplacement subsequent le cote est vers le haut le long de la faille de tabbernor avec une interference de plis de dome a fait affleurer la fenetre pelican et la zone mylonitique qui la recouvre elle a ete determinee par des profils sismiques le long des transects lithoprobe des fenetres tectoniques permettent aussi l affleurement de roches archeennes du do maine de glennie vers l ouest la ou ont ete documentees des zones semblables de mylonites recristallisees a pendage sud ouest ensemble ces zones enregistrent la collision et le sous placage du complexe de flin flon glennie par le craton archeen sask au cours d une collision prolongee entre le protocontinent et le continent au cours d un intervalle d au moins 1826 a 1805 ma

composition age and origin of granitoid rocks in the davin lake area rottenstone domain trans hudson orogen northern saskatchewan the rottenstone domain of the trans hudson orogen is a 25 km wide granitic migmatitic belt lying between the la ronge volcanic plutonic island arc 1890 1830 ma to the southeast and the ensialic wathaman batholith 1855 ma to the northwest the rottenstone domain consists of three lithotectonic belts parallel to the orogen i southeast gently folded migmatized quartzo feldspathic metasedimentary and mafic metavolcanic rocks intruded by small concordant and discordant white tonalite monzogranite bodies ii central intensely folded and migmatized metasedimentary rocks and minor metavolcanic rocks intruded by largely discordant xenolith rich pink aplite pegmatite monzogranite bodies and iii northwest steeply folded migmatized metasedimentary rocks cut by subvertical white tonalite monzogranite sheets emplacement of granitoid rocks consists predominantly of contiguous orogen parallel steeply dipping syntectonic and post tectonic sheets with prominent magmatic schlieren bands overprinted by parallel solid state deformation features the white granitoid rocks have a cnk mol al2o3 mol cao na2 o k 2o 1 14 1 22 k rb 500 ree sum of rare earth elements 1 87 sr 86 sri 0 7032 and endi 2 the pink monzogranites have a cnk 1 11 1 16 k rb 500 ree 90 ppm eu eu 1 87 sr 86 sri 0 7031 and endi 2 the white granitoid rocks show a wider compositional range and more compositional scatter than the pink monzogranites reflecting some combination of smaller volume melts less homogenization and less control by crystal melt equilibria all metavolcanic metasedimentary and granitic rocks in the rottenstone domain have the distinctive geochemical signatures of an arc environment new sensitive high resolution ion microprobe shrimp u pb geochronology on the rottenstone granitoid rocks reveals complex growth histories for monazite and zircon variably controlled by inheritance magmatism and high grade meta morphism monazite ages for the granitoid bodies and migmatites cluster at 1834 and 1814 ma whereas zircon ages range from 2480 ma rare cores to 1900 1830 ma cores and mantles but also 1818 1814 ma for low th u recrystallized rims overgrowths and rare discrete euhedral prisms these results demonstrate that at least some source material for the granitic magmas included earliest paleoproterozoic crust sask craton or its derived sediments and that rottenstone granitic magmatism postdated plutonism in the bounding la ronge arc and wathaman batholith we estimate the age of terminal metamorphism in the davin lake area to be 1815 ma petrogenetically the rottenstone migmatites and granitoid rocks appear for the most part locally derived from their metasedimentary and metavolcanic host rocks shed from the la ronge arc sask craton and possibly the hearne craton the rottenstone domain was the least competent member in the overthrust stack and probably underwent a combination of fluid present melting and fluid absent decompression melting resulting in largely syntectonic granitoid magmatism 1835 1815 ma analogous to granite production in the high himalayan gneiss belt

geochemical and nd isotopic compositions of the metasedimentary rocks in the la ronge domain trans hudson orogen canada implications for evolution of the domain abstract metasedimentary rocks of greenschist to amphibolite facies occur in various belts in the paleoproterozoic la ronge domain of the trans hudson orogen metasedimentary rocks in the central metavolcanic greenstone belt are low to intermediate in sio2 contents have nd values of 2 3 to 4 5 with two rare earth element ree patterns of la yb n 8 5 respectively the predominant component meta arkoses of the overlying mclennan lake belt have distinctly high k2o and sio2 contents high la yb n ratios 11 46 large negative eu anomalies eu eu 0 1 0 58 and uniformly low nd values 0 8 to 0 2 the maclean lake belt is comprised of a variety of metasedimentary gneisses the volumetrically minor amphibole and diopside rich gneisses have high ferromagnesian element sc and co concentrations high nd values of 4 1 to 5 7 and flat ree patterns with no eu anomalies the predominant lithological unit in the maclean lake belt the psammitic gneisses is geochemically more variable the mafic psammitic gneiss samples in the maclean lake belt and the metasedimentary rocks in the central metavolcanic belt are similar in trace and major element geochemistry and nd isotope composition whereas those more felsic psammitic gneisses are similar to the mclennan lake meta arkoses geochemical data combined with lithological and geochronological evidence indicate that sediments in the central metavolcanic belt were derived mainly from contiguous volcanic debris with minor addition of felsic components from exotic sources in contrast the mclennan lake meta arkoses were derived from a distinct felsic source whereas the psammitic gneisses in the maclean lake belt are mixtures of volcanic materials in the central metavolcanic belt and meta arkoses like felsic components nd isotopic data and ages of detrital zircon suggest that proterozoic juvenile materials were dominant sources for metasedimentary rocks in the la ronge domain comparison of geochemical signatures of the metasedimentary rocks with those of the felsic plutonic rocks in the central metavolcanic belt suggests that the felsic materials in the mclennan lake and maclean lake belts could not have been derived predominantly from the plutonic rocks implying more distal sources topographic changes during collision may have resulted in a shift from mixing of both local and distal sources for the rocks in the maclean lake belt to the dominance of a more distal source for the mclennan lake meta arkoses

archean rocks in the southern rottenstone domain significance for the evolution of the trans hudson orogen an augened strongly flasered and multiply folded monzonitic gneiss occurs in a structural dome in the black bear island lake region of northern saskatchewan within the ca 1850 ma paleoproterozoic southern rottenstone domain of the trans hudson orogen tho a sample of this rock has yielded thermal ionization mass spectrometry tims and sensitive high resolution ion microprobe shrimp zircon data consistent with a formation age of at least 2500 ma zircons also show somewhat younger ca 2380 ma overgrowths indicating a complex history the nd model age tdm of 2726 ma also confirms the late archean age of the rock as does the isotopic composition of common pb from a k feldspar sample u pb analyses of titanites yield ages of ca 1800 ma indicating recrystallization during terminal closure of the tho it is unlikely that the archean rocks are a part of the archean sask craton in the glennie domain for lithoprobe seismic sections indicate that the sask craton dips westward beneath the la ronge a

continental shelf morphology and stratigraphy offshore san onofre california the interplay between rates of eustatic change and sediment supply abstract new high resolution chirp seismic data acquired offshore san onofre southern california reveal that shelf sediment distribution and thickness are primarily controlled by eustatic sea level rise and sediment supply throughout the majority of the study region a prominent abrasion platform and associated shoreline cutoff are observed in the subsurface from 72 to 53 m below present sea level these erosional features appear to have formed between melt water pulse 1a and melt water pulse 1b when the rate of sea level rise was lower there are three distinct sedimentary units mapped above a regional angular unconformity interpreted to be the holocene transgressive surface in the seismic data unit i the deepest unit is interpreted as a lag deposit that infills a topographic low associated with an abrasion platform unit i thins seaward by downlap and pinches out landward against the shoreline cutoff unit ii is a mid shelf lag deposit formed from shallower eroded material and thins seaward by downlap and landward by onlap the youngest unit iii is interpreted to represent modern sediment deposition faults in the study area do not appear to offset the transgressive surface the newport inglewood rose canyon fault system is active in other regions to the south e g la jolla where it offsets the transgressive surface and creates seafloor relief several shoals observed along the transgressive surface could record minor deformation due to fault activity in the study area nevertheless our preferred interpretation is that the shoals are regions more resistant to erosion during marine transgression the cristianitos fault zone also causes a shoaling of the transgressive surface this may be from resistant antecedent topography due to an early phase of compression on the fault the cristianitos fault zone was previously defined as a down to the north normal fault but the folding and faulting architecture imaged in the chirp data are more consistent with a strike slip fault with a down to the northwest dip slip component a third area of shoaling is observed off of san mateo and san onofre creeks this shoaling has a constructional component and could be a relict delta or beach structure

regional setting and kinematic features of the needle falls shear zone trans hudson orogen needle falls shear zone is the southern part of a major northeast trending ductile shear system within the paleoproterozoic trans hudson orogen in saskatchewan throughout its exposed length of 400 km the shear zone separates reworked archean continental crust and infolded paleoproterozoic supracrustals of the cree lake zone to the northwest from mainly juvenile paleoproterozoic arc terrains and granitoid plutons of the reindeer zone to the southeast it also defines the northwest margin of the ca 1855 ma wathaman batholith which forms the main protolith to shear zone mylonites although not precisely dated available age constraints suggest that the shear zone formed between ca 1855 and 1800 ma toward the end of peak thermotectonism in this part of the orogen in the needle falls study area shear zone mylonites exhibit varied sequentially developed ductile to brittle fabric features including c s fabrics winged porphyroclasts especially delta type small scale compressional and extensional

stochastic fault stress implications for fault dynamics and ground motion up to the present time ground motion calculations for future earthquakes have almost exclusively been made from kinematic models however dynamic faulting models offer many benefits over kinematic models including the assurance the faulting models obey at least the general rules of elastodynamics and friction and contain a natural relationship between stress drop slip rise time and rupture velocity dynamic models also offer insight into the physics of the rupture and slip processes and can show how these processes lead to patterns of ground motion in the current work we use the 3d finite difference method and a suite of stochastic stress patterns with variable assumptions on strength and stress inhomogeneity to investigate two issues 1 the effect of assumptions about stress pattern on the evolution of rupture and slip on the fault and 2 the effect of these assumptions on the resultant ground motion we find that stress drop has a complicated relationship with slip rise time and rupture velocity especially in faults with strongly heterogeneous strength we also find that these inhomogeneous strength faults can produce highly inhomogeneous slip even without any form of frictional restrengthening at healing time finally we find that smoother strength models produce a better fit than the coarse models to the directivity pulse often observed on the surface the results help to shed light on the transition of the faulting system between locally controlled and more globally controlled rupture and also show how dynamic models may be used to generate ground motion estimates for seismic hazard calculations

seismic constraints on the architecture of the newport inglewood rose canyon fault implications for the length and magnitude of future earthquake ruptures the newport inglewood rose canyon fault zone nirc is an active strike slip fault system within the pacific north american plate boundary in southern california located in close proximity to populated regions of san diego orange and los angeles counties prior to this study the nirc fault zone s continuity and geometry were not well constrained nested marine seismic reflection data with different vertical resolutions are employed to characterize the offshore fault architecture four main fault strands are identified offshore separated by three main stepovers along strike all of which are 2 kilometers or less in width empirical studies of historical ruptures worldwide show that earthquakes have ruptured through stepovers with this offset models of coulomb stress change along the fault zone are presented to examine the potential extent of future earthquake ruptures on the fault zone which appear to be dependent on the location of rupture initiation and fault geometry at the stepover these modeling results show that the southernmost stepover between the la jolla and torrey pines fault strands may act as an inhibitor to through going rupture due to the stepover width and change in fault geometry across the stepover however these results still suggest that rupture along the entire fault zone is possible

deep seismic profile across a proterozoic collision zone surprises at depth collisions between continents generate the world s most striking mountains the deep structure of these collision zones inferred from seismic reflection profiles1 3 can be equally dramatic if less well understood improved knowledge of the structure of ancient collisional belts now known to have developed through plate tectonic processes4 5 can provide important new insights into the mechanisms of crustal growth although many such belts have been partly destroyed by severe post collisional deformation6 7 or magmatism8 some have been preserved in the stable interiors of continents9 one example is the early proterozoic trans hudson orogen9 10 in western canada here we present seismic reflection images across the entire orogen which reveal a broadly symmetric structure with reflections linked to island arc rocks dipping beneath both bounding archaean cratons the observed reflection geometries imply a doubling of crustal thickness during the collision along crust penetrating faults the presence of a significant amount of archaean crust at depth the extent of which could not have been determined from surface mapping suggests the need for caution in inferring crustal growth rate from the surface area of juvenile crust11 12

development of a transverse to orogen parallel extension lineation in a complex collisional setting trans hudson orogen manitoba canada abstract in the southeastern trans hudson orogen tho the wide dispersion in orientation of the l3 extension lineation from orogen parallel to transverse is considered primary as it cannot be accounted for by simple overprinting of a single precursor orientation interpretation of the origin of this extension lineation is complicated by the fact that its gently northeast plunging orientation at the north end of the study area is inconsistent with the evidence for sinistral east side up movement of the steep oblique slip d3 faults the hypothesis presented here to explain the complicated lineation pattern is based on evidence that the preceding period d2 southwest directed transport overlapped temporally and spatially with d3 northwest southeast transpressional shortening d2 and d3 are attributed to successive but overlapping collisional events d3 transpressional shortening is interpreted to have propagated westward through the tho resulting in a prolonged and complex transition between d2 and d3 deformation regimes the dispersion of the l3 extension lineation is interpreted to reflect the competing effects of both deformation regimes which appear to have been partitioned in the crust with d2 effects best developed in higher metamorphic grade rocks and d3 effects best developed in lower metamorphic grade rocks

forward and inverse three dimensional p wave velocity models of the southern california crust we construct a three dimensional p wave velocity model of the southern california crust by combining existing one dimensional models each describing a region defined by surface geology and calibrate the model with travel times from three explosions the model is expressed as blocks each of a given slowness the variance of the p wave travel time residuals of 1000 earthquakes relocated in and near the los angeles basin where the model is most detailed is half that of the catalog locations in the standard one dimensional model for southern california starting from the forward model we invert 21 000 p wave arrivals from earthquakes for hypocenters and block slownesses using the technique of roecker 1981 the variance of these p wave travel time residuals decreases 47 during the inversion many of the blocks representing the upper crust and midcrust are well sampled and well resolved the resulting model is useful both for locating earthquakes and for comparing the geologies of the different regions for example the velocity structure of the los angeles basin represents seismically slow sediments on top of basement rocks having velocities similar to the granitic rocks under the peninsular ranges moho is between 26 and 32 km depth in contrast the ventura basin has mostly slower sediments above a deeper higher velocity basement compared to catalog locations relocations in the final three dimensional model of 98 ml 4 earthquakes throughout southern california tend to deepen below sediment filled valleys and basins shallow in regions without sedimentary cover and have a 44 lower p wave travel time residual variance

seismic reflection images of high angle faults and linked detachments in the trans hudson orogen postcollisional 1 8 1 7 ga intracontinental deformation in the trans hudson orogen canada produced a series of orogen parallel high angle faults and folds in seismic reflection profiles the faults are imaged by subvertical zones of diffractions and truncated reflections that extend to 4 8 s 12 24 km the folded and faulted upper part of the crust is underlain by laterally coherent shallow dipping reflections that are locally bounded by discrete highly reflective zones these zones are interpreted as detachments shear zones and can be traced from the upper to lower crust where some of them appear to pass into laterally continuous reflections that define the moho two distinct regimes of postcollisional crustal deformation are inferred from the seismic images high angle faulting and lateral block extrusion in the upper crust and low angle ductile shearing in the mid lower crust the surface geology indicates that the faults resulted in southwest orogen parallel extrusion of the orogen s internal zone relative to the bounding archean hearne and superior cratons faulting was concurrent with the development of upright folds with trends that are subparallel to the extrusion direction the seismic images suggest that the high angle fold fault structures are kinematically linked to low angle detachments represented by laterally coherent highly reflective zones the detachment shear zones are inferred to have a top to the southwest sense of shear associated with a subhorizontal northeast southwest extension direction parallel to those observed for 1 83 1 80 ga collisional shear zones exposed in major postcollisional fold culminations long lived orogen parallel extension is interpreted as a consequence of the boundary conditions imposed by the northeast trend of both the superior and hearne margins

thermochronological history of the northern canadian shield abstract the northern canadian shield is comprised of multiple archean cratons that were sutured by the late paleoproterozoic to form the canadian component of supercontinent nuna more than 2000 combined k ar and 40ar 39ar cooling ages from across the region reveal a stark contrast in upper and lower plate thermal responses to nuna forming events with the churchill province in particular revealing near complete thermal reworking during the late paleoproterozoic we review the detailed cooling history for five regions that span the churchill province and trans hudson orogen tho thelon tectonic zone south rae reindeer zone south hall peninsula and the cape smith belt the cooling patterns across churchill province are revealed in two 1500 km transects at the plate scale churchill s cooling history is dominated by tho accretionary and collisional events during which it formed the upper plate cooling ages generally young from west to east across both southern and central churchill and latest cooling in the tho is 50 myr older in southernmost churchill reindeer zone compared to eastern churchill hall peninsula indicating diachronous thermal equilibration across 2000 km strike length of the tho churchill exhibits relatively high post terminal tho cooling rates of 4 c myr which support other geological evidence for widespread rapid exhumation of the tho upper plate following terminal collision potentially in response to lithospheric delamination

reservoir property inversion a method for quantitative interpretation of seismic inversion results abstract with improved seismic data quality prestack inversion has become a routine process for quantitative seismic interpretation however direct products from traditional seismic inversion usually are p impedance pi s impedance si and in some cases density these elastic properties are only an indirect description of subsurface geology a bridge must be established from inverted pi si and density to more understandable reservoir properties lithology porosity and water saturation reservoir property inversion is a model based inversion process to transform pi si and density to lithology porosity and water saturation the inversion is performed in two steps 1 well log inversion on log data to estimate optimal elastic properties of rock grain constituents and 2 reservoir property inversion to estimate reservoir properties from pi si and density the underlying rock physics models are the same for both inversions including mass balance equation gassmann equation voigt reuss hill

thermotectonic studies in the paleoproterozoic glennie domain trans hudson orogen canada abstract the glennie domain of the trans hudson orogen tho is characterized by paleoproterozoic island arc volcanogenic assemblages and granitoid rocks thrust as sheets over the archean saskatchewan craton prior to terminal collision between the archean rae hearne and superior cratons four discrete deformation events have been recorded in the domain the d1 and d2 events are associated with the southwestward directed imbrication of paleoproterozoic allochthons over the saskatchewan craton while the d3 and d4 events are associated with terminal collision and transpression respectively discernable metamorphic episodes are associated with the d2 and d3 events with peak thermal metamorphism during d3 using inveq thermobarometry metamorphic conditions during d3 in the santoy lake area were ca 655 705 c and 6 1 7 3 kbar and ca 530 595 c and 3 2 4 8 kbar for the brownell lake area 40ar 39ar hornblende and biotite ages suggest the santoy lake area remained above ca 490 c until ca 1720 ma when it underwent rapid cooling through the biotite ar closure temperature of ca 330 c 40ar 39ar biotite ages from the brownell lake area suggest moderate cooling from peak thermal metamorphism at ca 1810 1755 ma when it passed through the ca 310 c biotite ar closure temperature differences between cooling rates for the two areas are interpreted as differential exhumation of smaller lithotectonic blocks bound by domain wide deep crustal faults and shear zones zircon pb evaporation ages of variably deformed granitoid rocks from the santoy lake area show that the d2 event occurred after ca 1857 ma the oldest granitoid in the santoy lake area provides a minimum age of mafic volcanism in the area of ca 1886 ma as well as a significant hiatus ca 50 ma between deposition of the two supracrustal assemblages in the domain

velocity structure of the pamir hindu kush region possible evidence of subducted crust the arrival times of compressional p and shear s waves from approximately 580 microearthquakes recorded by a temporary array in the pamir hindu kush region in central asia are used to deduce one and three dimensional velocity structures of this region the results for one dimensional structures imply that the moho is at 70 5 depth also there is a velocity reversal near 160 km depth which is inferred to be the beginning of the low velocity zone this reversal continues to depths of approximately 230 km below 230 km velocities are somewhat higher than those of normal mantle at similar depths 9 3 km s versus 8 4 km s for p waves the outstanding feature of the results for three dimensional velocity structures is a broad 40 km centrally located region with 8 to 10 lower velocities than those in the surrounding regions this low velocity region envelopes the seismic zone at depths between 70 and 150 km the region may actually extend beyond these depths but the results for shallower and deeper structure lack sufficient resolution to decide several tests using both hypothetical and real data were performed to estimate the reliability of the three dimensional solutions the results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks although one must be cautious of the effects of averaging in interpreting the solution the low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust therefore while subduction has occurred in the pamir hindu kush the results of the three dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km appendices are available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 013 1 00 payment must accompany order

an earthquake explosion reversed refraction line in the peninsular ranges of southern california and baja california norte abstract an approximate reversed refraction profile has been obtained for the center of the peninsular ranges of southern california and baja california norte using arrival times from corona blasts to obtain the nw se profile and arrival times from the well located pino solo earthquake of 17 july 1975 to obtain the reversing se nw profile the results indicate a relatively high velocity crust with p velocities of 6 57 to 6 95 km sec similar to the high velocities found by hadley and kanamori 1979 a crustal thickness of about 40 km was found for the axis of the peninsular ranges significantly greater than was found by hadley and kanamori 1979 for the average crustal thickness of the northern part of the province this suggests that the thick crust may be confined to a relatively narrow zone along the axis of the province the crustal thickness found here is approximately 10 km less than found for the deeper crust of the sierra nevada bateman and eaton 1967 pakiser and brune 1980

nd isotope and geochemical constraints on the depositional setting of paleoproterozoic metasedimentary rocks along the margin of the archean hearne craton saskatchewan canada abstract the cree lake zone in the southwestern part of the hearne craton reworked during the paleoproterozoic trans hudson orogeny includes the wollaston group a sequence of paleoproterozoic metasedimentary rocks as these rocks have been multiply deformed and metamorphosed to high grade a geochemical and isotopic study was initiated to see through the overprinting events to better constrain the provenance and evolution of the wollaston group the wollaston group consists of thick sedimentary successions representing different depositional and tectonic environments that can be subdivided into two subgroups the lower subgroup is represented by deep water turbidite to shallow water marine deposits the upper subgroup separated from the lower one by a regional unconformity comprises molasse type assemblages ranging from talus to shallow marine lacustrine with subordinate volcanogenic rocks major and trace element data obtained from 31 psammopelitic rocks indicate that the clastic material was derived from sources consisting of both felsic and mafic material although felsic sources are more important in the samples from the lower subgroup sedimentary tectonic discrimination diagrams demonstrate that most of the wollaston group was deposited on an active continental margin rather than a passive margin the e nd values for the wollaston group psammopelites calculated at 1 92 ga the approximate age of the boundary between the lower and upper subgroups vary from 3 4 to 6 8 which yields an approximate 70 30 mixture between juvenile paleoproterozoic detritus and archean detritus from the hearne craton a secondary source of detritus may be 1 95 2 0 ga rocks of the taltson orogen which is exposed to the west of the archean rae hearne craton the wollaston group records the complete evolution of the western margin of the trans hudson orogen during the paleoproterozoic the rifting of an archean continent at ca 2 1 ga led to the deposition of the passive margin siliciclastic sediments comprising the lowermost but poorly preserved part of wollaston group the formation of a continental arc rottenstone along the hearne margin at ca 1 92 ga resulted in a change in tectonic environment from a passive margin to a back arc basin and detritus in most of the lower subgroup was then derived from both basement and newly formed magmatic arc collision with the juvenile la ronge arc led to uplift along the hearne margin which transformed the back arc basin into a retroarc foreland basin at ca 1 88 ga sedimentary starvation and shallowing of the basin led to widespread accumulation of carbonate and evaporite deposits in the west whereas rapidly uplifted orogenic belts in the east provided sources of coarse grained clastic detritus forming the wollaston upper subgroup closure and inversion of the sedimentary basin occurred by ca 1 86 ga coincident with the emplacement of mafic to felsic intrusive rocks in the cree lake zone and the emplacement of the voluminous ca 1860 ma andean type wathaman batholith into the continental margin closure was manifested by the onset of foreland directed thrusting of the wollaston group