u pb ages of neoarchean granitoids from the black hills south dakota usa implications for crustal evolution in the archean wyoming province abstract zircons in basement rocks from the eastern wyoming province black hills south dakota usa have been analyzed by ion microprobe shrimp in order to determine precise ages of archean tectonomagmatic events in the northern black hills nbh near nemo phanerozoic and proterozoic meta sedimentary rocks are nonconformably underlain by archean biotite feldspar gneiss bfg and little elk gneissic granite leg both of which intrude older schists the archean granitoid gneisses exhibit a pervasive nw se trending fabric whereas an earlier ne sw trending fabric occurs sporadically only in the bfg which is intruded by the somewhat younger leg zircon crystals obtained from the leg and bfg exhibit double terminations oscillatory zoning and th u ratios of 0 6 0 3 thereby confirming a magmatic origin for both lithologies in situ analysis of the most u pb concordant domains yields equivalent 207 pb 206 pb ages upper intercept u pb concordia of 2559 6 and 2563 6 ma both 2 for the leg and bfg respectively which constrains a late neoarchean age for sequential pulses of magmatism in the nbh unzoned in bse patches of 2560 ma zircon commonly truncate coeval zonation in the same crystals with no change in th u ratio suggesting that deuteric fluid assisted recrystallization accompanied post magmatic cooling a xenocrystic core of magmatic zircon observed in one leg zircon yields a concordant age of 2894 6 ma 2 this xenocryst represents the oldest crustal material reported thus far in the black hills whether this older zircon originated as unmelted residue of 2900 ma crust that potentially underlies the black hills or as detritus derived from 2900 ma crustal sources in the wyoming province cannot be discerned in the southern black hills sbh the peraluminous granite at bear mountain bmg of previously unknown age intrudes biotite plagioclase schist zircon crystals from the bmg are highly metamict and altered but locally preserve small domains suitable for in situ analysis a u pb concordia upper intercept age of 2596 11 ma 2 obtained for zircon confirms both the late neoarchean magmatic age of the bmg and a minimum age for the schist it intrudes taken together these data indicate that the neoarchean basement granitoids were emplaced at 2590 2600 ma sbh and 2560 ma nbh most likely in response to subduction associated with plate convergence final assembly of supercontinent kenorland in contrast thin rims present on some leg bfg zircons exhibit strong u pb discordance high common pb and low th u ratios suggesting growth or modification under hydrothermal conditions as previously suggested for similar zircons from se wyoming the leg bfg zircon rims yield a nominal upper intercept date of 1940 2180 ma which may represent a composite of multiple rifting events known to have affected the nemo area between 2480 and 1960 ma together these observations confirm the existence of a paleoproterozoic rift margin along the easternmost wyoming craton moreover the 2480 1960 ma time frame inferred for rifting in the black hills nemo area corresponds closely to a 2450 2100 ma time frame previously inferred for the fragmentation of supercontinent kenorland

thermo rheological shear heating model for leucogranite generation metamorphism and deformation during the proterozoic trans hudson orogeny black hills south dakota abstract this paper evaluates thermotectonic models for metamorphism and leucogranite generation during the proterozoic trans hudson orogeny as recorded in rocks exposed in the black hills sd intrusion of the harney peak granite and associated pegmatites at 1715 ma occurred at the waning stages of regional deformation and staurolite grade regional metamorphism published consortium for continental reflection profiling cocorp results indicate that proterozoic sedimentary rocks were thrust over the archean wyoming province during the trans hudson collision isotopic compositions of the harney peak granite suggest that the exposed proterozoic and archean metasedimentary rocks in the black hills represent source rocks of the granites numerical simulations of the regional metamorphism and harney peak granite generation assuming crustal thickening by thrusting coupled with erosion show the following 1 doubling of the crust with normal distribution of radioactive elements does not yield sufficiently high temperatures to cause anatexis anywhere in the crust or growth of garnet in the now exposed part of the crust 2 a 35 km drop off length for internal heat production can yield sufficient temperature for garnet growth at the current erosion level it is however insufficient to produce staurolite and melting can occur only in the deepest parts of the crust 3 temperatures in crust with stable 70 km thickness for 40 ma and 35 km drop off length for heat production could become sufficient to produce staurolite at the current erosion level and subsequent rapid denudation of the crust could potentially trigger decompression melting of lower crustal rocks although this model could potentially explain the observed temporal relationship between regional metamorphism and leucogranite generation it is inconsistent with melting of upper crustal proterozoic source rocks that is indicated by isotopic compositions of the granites with lack of evidence for rapid denudation of the trans hudson orogen and with confinement of the leucogranites to the deformed proterozoic metapelitic rocks production of the harney peak granite and its relationship to regional metamorphism of the country rocks are best explained by shear heating at the interface between the wyoming province and overthrusted sedimentary rocks we suggest that with reasonable rheologic properties of metapelites and rates of plate convergence shear heating sufficiently perturbs locally the geotherms to cause anatexis in a deep shear zone system and growth of staurolite in the overlying crust modeling rheology of the lithologically stratified thickened crust with granitic basement and metapelitic upper plate shows that the currently exposed part of the crust and the granite source region were ductile through much of the orogeny which explains regional folding of the schists and predicts ductile shear zones in the granite source region because of the lithologic stratification the granitic basement is likely to become significantly weaker during crustal thickening than the upper crust dominated by schists a weak basement under a folded upper crust is likely to contribute to the observed relatively flat topography of high plateaus over thickened orogens

stuck in the mud earthquake nucleation and propagation through accretionary forearcs 1 subduction zone earthquakes can propagate to the surface causing large seafloor displacements resulting in tsunamis this requires the earthquake to rupture through clay rich sediments of the accretionary wedge which are largely aseismic as found previously the frictional properties of a range of wet clays at low slip velocity are velocity strengthening thus inhibiting earthquake nucleation however at high slip velocity the same materials weaken almost immediately resulting in a negligible critical slip weakening distance and fracture energy we interpret this behaviour as rapid thermal pressurization of the pore fluid within the clay gouge the lack of fracture energy can explain how a large rupture propagating from depth might not be arrested by clay rich velocity strengthening sediments as is commonly seen the results suggest that generally earthquakes may be difficult to nucleate on mature faults dominated by clay but the propagation of earthquakes through these zones is energetically very favourable

large displacement hydrothermal frictional properties of dfdp 1 fault rocks alpine fault new zealand implications for deep rupture propagation the alpine fault new zealand is a major plate bounding fault that accommodates 65 75 of the total relative motion between the australian and pacific plates here we present data on the hydrothermal frictional properties of alpine fault rocks that surround the principal slip zones psz of the alpine fault and those comprising the psz itself the samples were retrieved from relatively shallow depths during phase 1 of the deep fault drilling project dfdp 1 at gaunt creek simulated fault gouges were sheared at temperatures of 25 150 300 450 and 600 c in order to determine the friction coefficient as well as the velocity dependence of friction friction remains more or less constant with changes in temperature but a transition from velocity strengthening behavior to velocity weakening behavior occurs at a temperature of t 150 c the transition depends on the absolute value of sliding velocity as well as temperature with the velocity weakening region restricted to higher velocity for higher temperatures friction was substantially lower for low velocity shearing v 0 3 m s at 600 c but no transition to normal stress independence was observed in the framework of rate and state friction earthquake nucleation is most likely at an intermediate temperature of t 300 c the velocity strengthening nature of the alpine fault rocks at higher temperatures may pose a barrier for rupture propagation to deeper levels limiting the possible depth extent of large earthquakes our results highlight the importance of strain rate in controlling frictional behavior under conditions spanning the classical brittle plastic transition for quartzofeldspathic compositions

new constraints on the timing of early proterozoic tectonism in the black hills south dakota with implications for docking of the wyoming province with laurentia a question regarding the 1900 1600 ma assembly of laurentia is whether the wyoming province west central united states was part of the hearne province west central canada prior to the early proterozoic trans hudson orogeny or a separate entity welded later to a hearne superior continent central canada new 40 ar 39 ar mica dates help to address this question by extending a middle proterozoic geochronologic front long established along the southern wyoming province into the black hills of south dakota this suggests that previously unexplained north directed fold nappes f 1 in the black hills resulted from island arc accretion to the south ca 1780 ma north northwest trending upright f 2 folds which formed during east west collision of the wyoming and superior provinces must therefore be younger new 40 ar 39 ar hornblende dates recently published age data and crustal heat flow considerations further suggest that this collision began at or before ca 1770 ma and culminated with posttectonic magmatism beginning ca 1715 ma the harney peak granite this tectonic magmatic interval is 50 60 m y younger than that reported for the hearne superior collision trans hudson orogeny in canada comparably young metamorphic dates 1810 1710 ma also typify the eastern and northern wyoming province periphery western dakotas and southwestern montana collectively these data suggest that the hearne and wyoming provinces were once separate continents that were ultimately welded to the superior province and to each other during distinct early proterozoic orogenies regional relationships further suggest that final docking of the eastern wyoming province with laurentia began during the ca 1780 1740 ma interval of island arc accretion along the southern margin of the growing craton

anatomy structural evolution and slip rate of a plate boundary thrust the alpine fault at gaunt creek westland new zealand minimum slip rates calculated for plate vector parallel slickenside trends in cataclasite on the sole of the alpine fault at gaunt creek westland new zealand range from 18 to 24 mm yr between half and two thirds of the total relative motion between the pacific and australian plates is being accommodated by movement on a single structure the alpine fault during the past 14 ka the leading edge of the alpine fault has changed from a moderately southeast dipping oblique reverse fault to a shallowly dipping thrust the hanging wall pacific plate is composed of a gradational sequence from basal gouge through pseudotachylite bearing cataclasite to progressively more coherent schist derived mylonite which has been faulted against subhorizontally bedded fluvio glacial gravel in the footwall australian plate during uplift the hanging wall sequence has been internally sheared and imbricated producing duplex structures and retrogressively veined and altered by pervasive hydrothermal fluid flow erosion of the exhumed fault zone produced angular cataclasite and mylonite derived talus fan breccias building a west dipping apron beneath the fault scarp wood fragments from near the base off the talus breccias have been 14 c dated at 12 650 90 yr b p progressive tectonic shortening resulted in 180 m of overthrusting of a schist derived nappe across an irregular talus fan surface composed of its own erosional debris the structural history of the alpine fault at gaunt creek illustrates the importance of the interaction between fault induced topography and erosion and the control these processes exert on the continued tectonic geometric and geomorphic evolution of the fault zone

high velocity frictional properties of alpine fault rocks mechanical data microstructural analysis and implications for rupture propagation abstract the alpine fault in new zealand is a major plate bounding structure that typically slips in m8 earthquakes every c 330 years to investigate the near surface high velocity frictional behavior of surface and borehole derived alpine fault gouges and cataclasites twenty one rotary shear experiments were conducted at 1 mpa normal stress and 1 m s equivalent slip velocity under both room dry and water saturated wet conditions in the room dry experiments the peak friction coefficient p p n of alpine fault cataclasites and fault gouges was consistently high mean p 0 67 0 07 in the wet experiments the fault gouge peak friction coefficients were lower mean p 0 20 0 12 than the cataclasite peak friction coefficients mean p 0 64 0 04 all fault rocks exhibited very low steady state friction coefficients ss room dry experiments mean ss 0 16 0 05 wet experiments mean ss 0 09 0 04 of all the experiments performed six experiments conducted on wet smectite bearing principal slip zone psz fault gouges yielded the lowest peak friction coefficients p 0 10 0 20 the lowest steady state friction coefficients ss 0 03 0 09 and commonly the lowest specific fracture energy values e g 0 01 0 69 mj m 2 microstructures produced during room dry and wet experiments on a smectite bearing psz fault gouge were compared with microstructures in the same material recovered from the deep fault drilling project dfdp 1 drill cores the near absence of localized shear bands with a strong crystallographic preferred orientation in the natural samples most resembles microstructures formed during wet experiments mechanical data and microstructural observations suggest that alpine fault ruptures propagate preferentially through water saturated smectite bearing fault gouges that exhibit low peak and steady state friction coefficients

frictional strength rate dependence and healing in dfdp 1 borehole samples from the alpine fault new zealand abstract the alpine fault in southern new zealand is a major plate boundary fault zone that according to various lines of evidence may be nearing the end of its seismic cycle and approaching earthquake failure in order to better characterize this fault and obtain a better understanding of its seismic potential two pilot boreholes have been completed as part of a larger drilling project samples representative of the major lithologic subdivisions of the fault zone at shallow 100 m depths were recovered and investigated in laboratory friction experiments we show here that materials from within and very near the principal slip zone psz tend to exhibit velocity strengthening frictional behavior and restrengthen heal rapidly compared to samples of the wall rock recovered near the psz fluid saturation causes the psz to be noticeably weaker than the surrounding cataclasites 0 45 and eliminates the velocity weakening behavior in these cataclasites that is observed in dry tests our results indicate that the psz has the ability to regain its strength in a manner required for repeated rupture but its ability to nucleate a seismic event is limited by its velocity strengthening nature we suggest that seismogenic behavior at depth would require alteration of the psz material to become velocity weakening or that earthquake nucleation occurs either within the wall rock or at the interface between the wall rock and the psz coseismic ruptures which propagate to the surface have been inferred for previous earthquakes on the alpine fault and may be facilitated by the slightly weaker psz material and or high rates of healing in the psz which support fault locking and eventual stress drops

nd o and sr isotopic constraints on the origin of precambrian rocks southern black hills south dakota abstract the nd o and sr isotopic characteristics of precambrian metasedimentary metavolcanic and granitic rocks from the black hills of south dakota are examined two late archean granites 2 5 2 6 ga have t dm ages of 3 05 and 3 30 ga suggesting that at least one of the granites was derived through the melting of significantly older crust early proterozoic metasedimentary rocks have t dm ages that range from 2 32 to 2 45 ga these model ages in conjunction with probable stratigraphic ages ranging from 1 9 to 2 2 ga indicate that mantle derived material was added to the continental crust of this region during the early proterozoic previous studies of the harney peak granite complex have reported u pb and rb sr ages of about 1 71 ga and most granite samples examined in this study have sr isotopic compositions consistent with that age two granite samples taken from the same sill however give two point rb sr and sm nd ages of 2 08 0 08 and 2 20 0 20ga 2200 nd 15 5 respectively in addition whole rock and apatite samples of the spatially associated tin mountain pegmatite give a sm nd isochron age of 2000 100 ma 2200 nd 5 8 1 8 the sm nd o and rb sr isotopic systematics of these granitic rocks have been complicated to some degree by both crystallization and post crystallization processes and the age of the pegmatite and parts of the harney peak granite complex remain uncertain processes that probably complicated the isotopic systematics of these rocks include derivation from heterogeneous source material assimilation mixing of ree between granite and country rock during crystallization via a fluid phase and post crystallization mobility of sr the nd isotopic compositions of the pegmatite and the harney peak granite indicate that they were not derived primarily from the exposed metasedimentary rocks

petrogenesis of early proterozoic pelitic schists of the southern black hills south dakota constraints on regional low pressure metamorphism the late archean to early proterozoic terrain exposed in the black hills south dakota records a history of deformation metamorphism and igneous activity extending from 2 5 to 1 7 ga metamorphism began during crustal thickening however maximum temperatures were attained after crustal thickening during a regional low pressure high temperature event coinciding with the emplacement of the harney peak granite at 1 7 ga metamorphic grade increased over a distance of 10 to 15 km from garnet grade in the northern part of the terrain to sillimanite k feldspar grade in the southern part isograds mapped in the terrain include the first appearance of staurolite biotite andalusite biotite sillimanite sillimanite biotite garnet and sillimanite k feldspar in the extreme western exposure of the terrain kyanite biotite occurs instead of andalusite biotite geobarometry indicates that metamorphism occurred between 2 0 and 4 4 kbar geothermometry indicates that temperature increased from 469 500 c in the garnet zone to 528 555 c in the sillimanite zone on the northern flank of the harney peak granite higher temperatures are recorded along the southern flank of the granite in the sillimanite biotite garnet zone 585 c and in the sillimanite k feldspar zone 662 c the distributions of isograds and isotherms indicate that the harney peak granite was at the locus of a regional thermal high and that doming during granite emplacement was partly responsible for exposure of the low pressure high temperature metamorphic terrain

hydraulic properties of a low permeable rupture zone on the yingxiu beichuan fault activated during the wenchuan earthquake china implications for fluid conduction fault sealing and dynamic weakening mechanisms abstract fluid transport properties of fault rocks are crucial parameters that affect earthquake nucleation and rupture propagation in this study we examined the internal structure mineral composition and fluid transport properties of fault rocks collected from two shallow boreholes penetrating a granitic rupture zone on the yingxiu beichuan fault ybf that was activated during the 2008 wenchuan earthquake fluid transport properties were measured using water as pore fluid at effective pressures p e ranging from 10 mpa to 165 mpa permeabilities of fault rocks exhibit a wide variation from 2 1 10 22 m 2 to 4 6 10 17 m 2 strongly depending on rock types and overburden pressure specifically at p e of 165 mpa the damage zone samples have permeabilities from 5 0 10 21 m 2 to 1 2 10 17 m 2 and the fault gouges are between 2 1 10 22 m 2 and 3 1 10 19 m 2 thus the ybf consists of a low permeability fault core acting as fluid barrier and surrounding high permeability damage zones acting as fluid conduits combining the structural and compositional results and transport data together we propose that the interplay between cataclasis and fluid rock interactions controls the hydraulic properties and their response to the fault zone evolution it is noteworthy that we measured extremely low permeabilities but high porosities and high specific storages for the gouges the cemented cataclasites which are inferred to be equivalent to the rocks in which the wenchuan earthquake nucleated also have low permeabilities suggesting the fault zone is a potential area for fluid storage and capable of generating high pore pressure at depths according to our laboratory data we found fluid pressurization could occur at depths below 2 7 km we suggest thermal pressurization has played an important role in causing the dynamic weakening of the wenchuan earthquake

petrogenesis and tectonic implications of paleoproterozoic mafic rocks in the black hills south dakota abstract four suites of mafic to intermediate intrusions and flows that occur in the precambrian core of the black hills were analyzed for major and trace elements to establish tectonic settings of magmatism during the late archean and early proterozoic history of the region most of the mafic rocks were later metamorphosed to amphibolites during the black hills orogeny the collision of the wyoming and superior cratons which began at 1760 ma compositions of the mafic suites define at least two distinct tectonic settings of magmatism between 2480 and 1880 ma the 2 48 ga blue draw intrusion located on the eastern edge of the precambrian core is differentiated with compositions ranging from gabbro to granodiorite it is a calcalkaline intrusion with relatively high sio2 from 51 8 to 60 5 wt and k2o mostly 1 wt ree patterns are slightly concave upward and show lree enrichment the intrusion is also enriched in ba rb th and zr these and other geochemical discriminants are typical of modern calcalkaline basalts the intrusion s calcalkaline character is similar to the neighboring slightly older 2 56 ga little elk granite gneiss these two intrusions support the presence of late archean early proterozoic arc magmatism on the eastern margin of the wyoming craton the three other mafic suites comprising metamorphosed flows and sills occur within a sequence of deep marine sediments including black shales limestones iron formations and cherts u pb zircon ages suggest their emplacement between 2 01 and 1 88 ga mafic rocks in the eastern half of the precambrian core in the mt rushmore and pactola dam quadrangles and near bear mountain on the western margin of the precambrian core have slightly depleted to enriched tholeiitic compositions with sio2 50 wt and k2o generally 2 5 wt and the lowest sio2 concentrations typically

geochronology and geochemistry of precambrian gneisses metabasites and pegmatite from the tobacco root mountains northwestern wyoming craton montanathis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh t e krogh deceased april 2008 the middle mountain metamorphic domain of the montana metasedimentary terrane northwestern wyoming craton within the northwestern tobacco root mountains mainly comprises migmatized tonalitic gneiss interlayered with amphibolitic hornblende gneiss both of which are cut by metamorphosed mafic rocks together these gneisses are defined as middle mountain gneiss archean tonalitic gneiss from west of and amphibolitic gneiss from east of the bismark fault give from chemically and air abraded zircon grains u pb id tims ages of 3325 5 1 7 and 3340 ma respectively these results reflect primary magmatic ages and show that the middle mountain gneiss extends into the northern area of the central fault block between the bismark and mammoth faults older crustal processes in the tonalitic gneiss are evidenced by inherited grains the oldest of which is 3460 ma a metabasite hosted in tonalitic gneiss in the bismark fault selvage zone yields a zircon age of 2468 ma which is interpreted as the time of

mantle and slab contributions in arc magmas destructive plate margins are major sites of terrestrial magmatism that have long had a key role in models for the generation of continental crust and the development of chemical heterogeneities in the upper mantle in these models it is important to evaluate the nature and size of contributions from preexisting crust in the generation of magmas above subduction zones and to constrain the size of the fluxes of different elements from the subducted crust and the overlying mantle wedge in arc magmas kay 1 980 hawkesworth ellam 1 989 hawkesworth et al 1 99 1 a mcculloch gamble 1 99 1 the more mobile components of the subducted oceanic crust are the products of element fractionation processes in the upper continental crust the ocean basins and hydrothermal systems on the ocean floor they therefore have distinctive isotope and even trace element ratios that in principle should be readily distinguished from those in peridotite in the mantle wedge prior to subduction in practice however element ratios are almost certainly fractionated during shallow level dehydration of sub ducted material and at greater depths where material from the slab appears to contribute in the generation of arc magmas moreover the isotope signal of slab derived material may be heavily diluted by exchange with

damaged beyond repair characterising the damage zone of a fault late in its interseismic cycle the alpine fault new zealand dfdp 1 was funded by gns science victoria university of wellington the university of otago the university of auckland the university of canterbury deutsche forschungsgemeinschaft and the university of bremen natural environment research council grants ne j024449 1 ne g524160 1 and ne h012486 1 and the university of liverpool and the marsden fund of the royal society of new zealand

significance of brittle deformation in the footwall of the alpine fault new zealand smithy creek fault zone abstract the smithy creek fault represents a rare exposure of a brittle fault zone within australian plate rocks that constitute the footwall of the alpine fault zone in westland new zealand outcrop mapping and paleostress analysis of the smithy creek fault were conducted to characterize deformation and mineralization in the footwall of the nearby alpine fault and the timing of these processes relative to the modern tectonic regime while unfavorably oriented the dextral oblique smithy creek thrust has kinematics compatible with slip in the current stress regime and offsets a basement unconformity beneath holocene glaciofluvial sediments a greater than 100 m wide damage zone and more than 8 m wide extensively fractured fault core are consistent with total displacement on the kilometer scale based on our observations we propose that an asymmetric damage zone containing quartz carbonate chlorite epidote veins is focused in the footwall damage zone asymmetry likely resulted from the fact that the hanging wall was mostly deformed at greater depth than the footwall rather than resulting from material contrasts across the fault plane kinematic inversions on mineralized fractures within the damage zone suggest veins formed in the current stress regime from fluids comparable to those now circulating in the footwall the smithy creek fault zone is therefore a rare exhumed example of the modern footwall hydrothermal system and of a structure actively accommodating footwall deformation near the alpine fault zone two significantly less mature subvertical faults having narrow 20 cm or less damage zones and similar orientations to nearby strike slip segments of the alpine fault crosscut the mineralized zone at smithy creek we envisage that hydrothermal mineralization strengthened the fault core causing it to widen as later slip was partitioned into the now weaker surrounding damage zone with progressive alteration formation of favorably oriented faults became preferable to reactivation of the existing structure

permeability and seismic velocity and their anisotropy across the alpine fault new zealand an insight from laboratory measurements on core from the deep fault drilling project phase 1 dfdp 1 the alpine fault a transpressional plate boundary between the australian and pacific plates is known to rupture quasi periodically with large magnitude earthquakes mw 8 the hydraulic and elastic properties of fault zones are thought to vary over the seismic cycle influencing the nature and style of earthquake rupture and associated processes we present a suite of laboratory permeability and p vp and s vs wave velocity measurements performed on fault lithologies recovered during the first phase of the deep fault drilling project dfdp 1 which sampled principal slip zone psz gouges cataclasites and fractured ultramylonites with all recovered lithologies overprinted by abundant secondary mineralization recording enhanced fluid rock interaction core material was tested in three orthogonal directions orientated relative to the down core axis and when present foliation measurements were conducted with pore pressure h2o held at 5 mpa over an effective pressure peff range of 5 105 mpa permeabilities and seismic velocities decrease with proximity to the psz with permeabilities ranging from 10 17 to 10 21 m2 and vp and vs ranging from 4400 5900 m s in the ultramylonites cataclasites and 3900 4200 m s at the psz in comparison with intact country rock protoliths the highly variable cataclastic structures and secondary phyllosilicates and carbonates have resulted in an overall reduction in permeability and seismic wave velocity as well as a reduction in anisotropy within the fault core these results concur with other similar studies on other mature tectonic faults in their interseismic period

physical properties of surface outcrop cataclastic fault rocks alpine fault new zealand 1 we present a unified analysis of physical properties of cataclastic fault rocks collected from surface exposures of the central alpine fault at gaunt creek and waikukupa river new zealand friction experiments on fault gouge and intact samples of cataclasite were conducted at 30 33 mpa effective normal stress n using a double direct shear configuration and controlled pore fluid pressure in a true triaxial pressure vessel samples from a scarp outcrop on the southwest bank of gaunt creek display 1 an increase in fault normal permeability k 7 45 10 20 m2 to k 1 15 10 16 m2 2 a transition from frictionally weak 0 44 fault gouge to frictionally strong 0 50 0 55 cataclasite 3 a change in friction rate dependence a b from solely velocity strengthening to velocity strengthening and weakening and 4 an increase in the rate of frictional healing with increasing distance from the footwall fluvioglacial gravels contact at gaunt creek alteration of the primary clay minerals chlorite and illite muscovite to smectite kaolinite and goethite accompanies an increase in friction coefficient 0 31 to 0 44 and fault perpendicular permeability k 3 10 10 20 m2 to k 7 45 10 20 m2 comminution of frictionally strong 0 51 0 57 cataclasites forms weaker 0 31 0 50 foliated cataclasites and fault gouges with behaviors associated with aseismic creep at low strain rates combined with published evidence of large magnitude mw 8 surface ruptures on the alpine fault petrological observations indicate that shear failure involved frictional sliding within previously formed velocity strengthening fault gouge

sources of solutes and heat in low enthalpy mineral waters and their relation to tectonic setting new zealand abstract approximately 90 of the low enthalpy spring systems in new zealand are distributed along the present day convergence of the pacific and australian plates which varies from subduction to oblique plate collision forearc subducted waters rising in the hikurangi accretionary prism consist of pore waters containing labile organic material water of hydration from clays and seawater these subducted waters are traceable in the north island axial ranges and the northern marlborough fault system in the south island albeit modified by temperature mixing with surface and heated meteoric waters and protracted interaction with the rock the signature of subducted marine sediments in the hikurangi trench is retained in the chemical and isotopic compositions of the taupo volcanic zone magmatic arc type waters the concentrations of b ba br ca cl i k li mg na rb and sr solutes found in high concentrations in forearc subducted waters increase from northeast to southwest towards the incipient subduction zone in the puysegur trench southwest of south island most thermal mineral waters in the south island are expelled along the alpine fault zone where plate collision results to rapid uplift of the southern alps elevated geothermal gradients and fault shearing of rocks in the alpine fault zone variations in temperatures 175 30 c and mineral water compositions are affected by elevation hydrological gradient intensity of tectonic activity permeability fluid volume and rate of throughput and rates of uplift as the main trace of the alpine fault zone is approached concentrations of cl and other solutes generally increase suggesting increasing contributions from ascending cl rich metamorphic waters outside the active convergent zone low enthalpy springs discharge diagenetically modified ancient forearc waters na cl sedimentary formation waters and heated seawater na hco 3 cl waters and na hco 3 deep circulating heated meteoric waters the total surface heat flow in the north island low enthalpy mineral waters is more than 10 higher than in the south island with projected subsurface temperatures as high as 250 c this is because of the widespread occurrence of shallow magmatic and mantle sources associated with pliocene to holocene volcanism and back arc continental rifting the sources of heat and solutes in low enthalpy mineral waters outside the taupo volcanic zone and ngawha are decoupled except for rapidly ascending hot pore waters in the hikurangi accretionary prism and low temperature serpentinization ca cl waters in fiordland

tectonic tremor and deep slow slip on the alpine fault 1 tectonic tremor is characterized by persistent low frequency seismic energy seen at major plate boundaries although predominantly associated with subduction zones tremor also occurs along the deep extension of the strike slip san andreas fault here we present the first observations of tectonic tremor along new zealand s alpine fault a major transform boundary that is late in its earthquake cycle we report tectonic tremor that occurred on the central section of the alpine fault on 12 days between march 2009 and october 2011 tremor hypocenters concentrate in the lower crust at the downdip projection of the alpine fault coincide with a zone of high p wave attenuation low qp and bright seismic reflections occur in the 25 45 km depth range below the seismogenic zone and may define the deep plate boundary structure extending through the lower crust and into the upper mantle we infer this tremor to represent slow slip on the deep extent of the alpine fault in a fluid rich region marked by high attenuation and reflectivity these observations provide the first indication of present day displacement on the lower crustal portion of the australia pacific transform plate boundary

high velocity frictional properties of clay rich fault gouge in a megasplay fault zone nankai subduction zone 1 we conducted high velocity friction experiments on clay rich fault gouge taken from the megasplay fault zone in the nankai subduction zone under dry and wet conditions in the dry tests dehydration of clay minerals occurred by frictional heating and slip weakening is related to thermal pressurization associated with water vaporization resulting in a random distribution of clay clast aggregates in the gouge matrix in the wet tests slip weakening is caused by pore fluid pressurization via shear enhanced compaction and frictional heating and there is a very weak dependence of the steady state shear stress on the normal stress the resulting microstructure reflects the grain size segregation in a granular fluid shear flow at high shear rates these results suggest that earthquake rupture propagates easily through clay rich fault gouge by high velocity weakening potentially leaving the microstructures resulting from the frictional heating or the flow sorting at high slip rates

major earthquakes occur regularly on an isolated plate boundary fault the scarcity of long geological records of major earthquakes on different types of faults makes testing hypotheses of regular versus random or clustered earthquake recurrence behavior difficult we provide a fault proximal major earthquake record spanning 8000 years on the strike slip alpine fault in new zealand cyclic stratigraphy at hokuri creek suggests that the fault ruptured to the surface 24 times and event ages yield a 0 33 coefficient of variation in recurrence interval we associate this near regular earthquake recurrence with a geometrically simple strike slip fault with high slip rate accommodating a high proportion of plate boundary motion that works in isolation from other faults we propose that it is valid to apply time dependent earthquake recurrence models for seismic hazard estimation to similar faults worldwide

advanced seismic imaging techniques characterize the alpine fault at whataroa new zealand the plate bounding alpine fault in new zealand is a 850 km long transpressive continental fault zone that is late in its earthquake cycle we have acquired and processed reflection seismic data to image the subsurface around the main drill site of the deep fault drilling project dfdp 2 the resulting velocity models and seismic images of the upper 5 km show complex subsurface structures around the alpine fault zone the most prominent feature is a strong reflector at depths of 1 5 2 2 km with an apparent dip of 48 to the southeast below the dfdp 2 borehole which we assume to be the main trace of the alpine fault above the main reflector parallel reflectors suggest the presence of a 600 m wide damage zone additionally subparallel reflectors are imaged that we interpret as secondary branches of the main fault zone conjugate faults imaged by the data show the complexity of the subsurface the derived p wave velocity model reveals a 300 600 m thick sedimentary layer with velocities of 2 3 km s above a schist basement with velocities of 4 5 5 5 km s a low velocity layer can be observed within the basement at 0 8 2 km depth a small scale low velocity anomaly appears at the top of the basement that can be correlated to the fault zone the results provide a reliable basis for a seismic characterization of the dfdp 2 drill site that can be used for further structural and geological investigations of the architecture of the alpine fault in this area

influence of dilatancy on the frictional constitutive behavior of a saturated fault zone under a variety of drainage conditions we use numerical simulations to investigate how fault zone dilatancy and pore fluid decompression influence fault strength and friction constitutive behavior dilatant hardening can change the frictional response and the effective critical stiffness kcr which determines the transition from stable to unstable sliding in velocity weakening fault zones we study the frictional shear strength response to numerical velocity stepping experiments and show that when the duration of pore fluid decompression is long compared to the time necessary for frictional evolution as dictated by rate and state friction both the effective critical slip distance dc and the effective shear strength direct effect a are increased we investigate the role of fault zone permeability k dilatancy coefficient varepsilon and the magnitude of shearing velocity of the fault zone vlp and compare results using the dieterich and ruina state evolution laws over the range from k 10 15 to 10 21 m2 dc increases from 25 m to 2 mm and a increases from 0 15 to 5 mpa we vary varepsilon from 10 5 to 10 3 and the size of the velocity perturbation from 3x to 1000x and find large increases in the values of dc and a which may lead to inhibition of unstable sliding our results indicate that spatial variations with either depth or lateral extent in varepsilon and k could result in significant changes in the drainage state in fault zones such variation may lead to spatial variation of the nucleation and propagation of earthquakes based upon the drainage capabilities of the fault zone

appearance of de geer moraines in southern and western finland implications for reconstructing glacier retreat dynamics abstract lidar digital elevation models dems from southern and western finland were investigated to map and discriminate features of de geer moraines sparser and more scattered end moraines and larger end moraine features i e ice marginal complexes the results indicate that the occurrence and distribution of de geer moraines and scattered end moraine ridges in finland are more widespread than previously suggested this is probably attributed to the ease of detecting and mapping these features with high resolution dems indicating the efficiency of lidar applications in geological and geomorphological studies the variable appearance and distribution of moraine ridges in finland support previous interpretations that no single model is likely to be appropriate for the genesis of de geer moraines at all localities and for all types of end moraines de geer moraine appearances and interdistances probably result from a combination of the general rapidity of ice margin recession during deglaciation the proglacial water depth in which they were formed and local glacier dynamics related to climate and terrain topography the correlation between the varved clay based rate of deglaciation and interdistances of distinct and regularly spaced de geer moraine ridges indicates that the rate of deglaciation is probably involved in the de geer ridge forming process but more thorough comparisons are needed to understand the extent to which de geer interdistances represent an annual rate of ice margin decay and the rapidity of regional deglaciation

acoustic evidence for a broad hydraulically active damage zone surrounding the alpine fault new zealand abstract we present an analysis of the wireline sonic logs acquired through alpine fault hanging wall rocks during the second phase of the deep fault drilling project dfdp 2 due to inaccuracy in the initial tool processed slowness logs a more robust reprocessing of the data using the slowness time coherence method was required the reprocessing of the data yielded slowness logs that provide a significantly better estimate of compressional and shear slowness the average slowness values of compressional and shear waves are 68 s ft and 110 s ft respectively corresponding to velocities of 4 5 km s and 2 8 km s with some excursions related to fractures borehole conditions and tool settings but no systematic downhole trends the log values are compared to laboratory measurements of p and s wave velocity for samples of alpine schist and mylonitic rocks to examine the scale dependence of the measurement the average log velocity is 2 to 18 slower than the laboratory measurements due to the presence of mesoscale fractures that are present in situ but are not sampled at the laboratory scale using effective medium theory we estimate the crack density in the hanging wall host rock to be 0 4 comparison to field scale seismic studies indicates that there is a broad 500 600 m wide outer damage zone surrounding the intensely deformed inner damage zone of the alpine fault this fractured outer zone facilitates fluid flow through the hanging wall and around the alpine fault

hydraulic and acoustic properties of the active alpine fault new zealand laboratory measurements on dfdp 1 drill core abstract we report on laboratory measurements of permeability and elastic wavespeed for a suite of samples obtained by drilling across the active alpine fault on the south island of new zealand as part of the first phase of the deep fault drilling project dfdp 1 we find that clay rich cataclasite and principal slip zone psz samples exhibit low permeabilities 10 18 m 2 and that the permeability of hanging wall cataclasites increases from c 10 18 m 2 to 10 15 m 2 with distance from the fault additionally the psz exhibits a markedly lower p wave velocity and young s modulus relative to the wall rocks our laboratory data are in good agreement with in situ wireline logging measurements and are consistent with the identification of an alteration zone surrounding the psz defined by observations of core samples the properties of this zone and the low permeability of the psz likely govern transient hydrologic processes during earthquake slip including thermal pressurization and dilatancy strengthening

electron probe ultrachron microchronometry of metamorphic monazite unraveling the timing of polyphase thermotectonism in the easternmost wyoming craton black hills south dakota a metapelite from the easternmost wyoming craton black hills south dakota has been analyzed by microstructural methods to unravel polyphase deformational history associated with 1800 1700 ma assembly of southern laurentia three deformational fabrics are recognized in oriented thin sections an ene trending s 1 fabric preserved as oblique inclusion trails in garnet porphyroblasts a nnw trending s 2 fabric preserved as microlithons in the rock matrix and a flattening fabric s 3 which transposed s 1 s 2 and dominates the matrix a complex monazite porphyroblast has been analyzed in situ with the electron microprobe ultrachron to constrain the timing of s 1 s 3 fabric formation associated with monazite growth the core of this grain uniquely preserves the s 1 s 2 fabrics as sigmoidal inclusion trails the mean total pb age of this domain is 1750 10 ma all dates reported at 95 confidence n 39 spots which is equivalent to the published 2 0 7 pb 2 0 6 pb age for the same domain these results validate the total pb dating method in general and the ultrachron in particular for reliable age determination in low th monazite and are interpreted as 1750 ma minimum ages for the s 1 s 2 fabrics and sequential d 1 d 2 collisional events that imposed them n directed arc accretion and e w continental collision respectively a higher th y rim of this same rosetta grain truncates the s 1 s 2 sigmoid and is associated with resorption textures in garnet porphyroblasts coupled release of y and an s 3 fabric that pervasively overprinted s 1 s 2 in the rock matrix the mean ultrachron date of this domain is 1692 5 ma n 17 spots which is slightly younger that the published isotopic age for all monazite rims combined these results support a 1715 1690 ma timeframe for localized doming d 3 related to granite magmatism the onset of which has been dated independently at 1715 3 ma the timing of post d 3 cooling through 350 and 300 c is constrained by 4 0 ar 3 9 ar dates of 1610 and 1480 ma obtained for separates of d 3 matrix muscovite and biotite respectively which are interpreted as closure ages this study shows that fabrics in poly deformed rocks can be dated by linking monazite spot ages to key microtextures further the results of this micrometer scale study enhance previous knowledge of local thermotectonism black hills and regional terrane assembly laurentia

evolution of strength recovery and permeability during fluid rock reaction in experimental fault zones abstract physical and chemical fluid rock interactions are implicated in controlling earthquake nucleation and recurrence in particular interseismic compaction sealing and healing of fractured fault rocks can lead to strength recovery and stabilisation of fault zones in contrast these same processes can also assist increases in pore fluid pressures and consequent destabilisation of faults here we present high temperature hydrothermal experiments designed to assess the evolution of strength of fault zones in previously intact rock and also characterise the associated changes to porosity and permeability cores of fontainebleau sandstone were initially loaded to failure in a high pressure gas medium apparatus the failed specimens were then hydrothermally reacted at 927 c for variable duration under isostatic conditions and subsequently re fractured to determine the interseismic strength recovery in the most extreme case hydrothermally induced gouge compaction cementation and crack healing resulted in 75 strength recovery after reaction for 6 h isostatic hydrothermal treatment also resulted in dramatic reduction in porosity and permeability strength of the fault zone following hydrothermal reaction appears to be closely correlated to porosity consistent with previous studies on brittle failure of porous aggregates the experimental results show how hydrothermal reactions in fault zones may lead to two competing time dependent effects fault strengthening due to increased cohesion in the fault zone and fault weakening arising from elevated pore pressures within a well cemented low permeability gouge layer

late interseismic state of a continental plate bounding fault petrophysical results from dfdp 1 wireline logging and core analysis alpine fault new zealand 1 we present a geophysical characterization at 0 1 100 m scales of a major plate bounding continental fault in a late interseismic state the alpine fault produces mw 8 earthquakes every 200 400 years and last ruptured in 1717 ad wireline geophysical logs and rock cores extending from one side of the alpine fault to the other were acquired in two boreholes drilled in 2011 at gaunt creek during the first phase of the deep fault drilling project dfdp 1 these data document ambient conditions under which the next alpine fault earthquake will occur principal component analysis of the wireline logging data reveals that 80 of the variance is accounted for by electrical acoustic and natural gamma properties and preliminary multivariate classification enables the lithologies of sections of missing core to be reconstructed from geophysical measurements the fault zone exhibits systematic variations in properties consistent with common processes of progressive alteration and comminution near the principal slip zone superimposed on different protolith compositions our observations imply that the fault zone has the opposite sense of elastic asymmetry at 0 1 100 m scales to that of the crustal scale orogen imaged by remote geophysical methods on the basis of the fault zone scale asymmetry the bimaterial interface model of preferred earthquake rupture directions implies a northeastward direction of preferred alpine fault rupture on going characterization of the structural and hydraulic architecture of the alpine fault will improve our understanding of the relationship between in situ conditions earthquake rupture processes and the hazards posed by future earthquakes

the generation and crystallization conditions of the proterozoic harney peak leucogranite black hills south dakota usa petrologic and geochemical constraints the mineralogy petrology and geochemistry of the proterozoic harney peak granite black hills south dakota were examined in view of experimentally determined phase equilibria applicable to granitic systems in order to place constraints on the progenesis of peraluminous leucogranites and commonly associated rare element pegmatites the granite was emplaced at 3 4 kbar as multiple sills and dikes into quartz mica schists at the culmination of a regional high temperature low pressure metamorphic event principally along the periphery of the main pluton and in satellite intrusions the sills segregated into granite pegmatite couplets the major minerals include quartz k feldspar sodic plagioclase and muscovite biotite mg no molar mgo mgo feo 0 32 0 38 is the predominant ferromagnesian mineral in the granite s core whereas at the periphery of the main pluton and in the satellite intrusions tourmaline mg no 0 18 0 48 is the dominant ferromagnesian phase almandine spessartine garnet is also found in the outer intrusions there is virtually a complete overlap in the wide concentration ranges of sio2 cao mgo feo sr zr w of the biotite and tourmaline bearing granite suites with no discernable differentiation trends on harker diagrams precluding the derivation of one suite from the other by differentiation following emplacement this is consistent with the oxygen isotope compositions which are 11 5 0 6 for the biotite granites and 13 2 0 8 for the tourmaline granites suggesting derivation from different sources the concentrations of tio2 and possibly ba are higher and of mno and b are lower in the biotite granites the normative orthoclase albite ratio is extremely variable ranging from 0 26 to 1 65 in the biotite granites to 0 01 1 75 in the tourmaline granites very few sample compositions fall near the high pressure watersaturated haplogranite minima eutectic trend indicating that the granites for the most part are not minimum melts generated under conditions with a text h text 2 text o 1 instead most biotite granites are more potassic than the water saturated minima and eutectics and in analogy with experimentally produced granitic melts they are best explained by melting at 6 kbar a text h text 2 text o 1 and temperatures 800 c such high temperatures are also indicated by oxygen isotope equilibration among the constituent minerals nabelek et al 1992 several of the tourmaline granite samples contain virtually no k feldspar and have oxygen isotope equilibration temperatures 716 775 c therefore they must represent high temperature accumulations of liquidus minerals crystallized under equilibrium conditions from melts more sodic than the water saturated haplogranite minima or during fractionation of intruded melts into granite pegmatite couplets accompanied by volatile aided differentiation of the alkali elements the indicated high temperatures a text h text 2 text o 1 the relatively high tio2 and ba concentrations and the relatively low partial 18 text o values of the biotite granites suggest that they were generated by high extent biotite dehydration melting of an immature archean metasedimentary source the ascent of the hot melts may have triggered low extent muscovite dehydration melting of schists higher in the crust producing the high b low ti melts comprising the periphery of the main pluton and the satellite intrusions alternatively the different granite types may be the result of melting of a vertical section of the crust in response to the ascent of a thermal pulse with the low partial 18 text o biotite granites generated at a deeper hotter region and the high partial 18 text o tourmaline granites at a higher cooler region of the crust the low ti and high b concentrations in the high partial 18 text o melts resulted in the crystallization of tourmaline rather than biotite which promoted the observed differentiation of the melts into the granitic and pegmatitic layers found along the periphery of the main pluton and the satellite intrusions

geophysical properties within the san andreas fault zone at the san andreas fault observatory at depth and their relationships to rock properties and fault zone structure 1 we examine the relationships between borehole geophysical data and physical properties of fault related rocks within the san andreas fault as determined from data from the san andreas fault observatory at depth borehole geophysical logs cuttings data and drilling data from the region 3 to 4 km measured depth of the borehole encompass the active part of the san andreas fault the fault zone lies in a sequence of deformed sandstones siltstone shale serpentinite bearing block in matrix rocks and sheared phyllitic siltstone the borehole geophysical logs reveal the presence of a low velocity zone from 3190 to 3410 m measured depth with vp and vs values 10 30 lower than the surrounding rocks and a 1 2 m thick zone of active shearing at 3301 3303 m measured depth seven low velocity excursions with increased porosity decreased density and mud gas kick signatures are present in the fault zone geologic data on grain scale deformation and alteration are compared to borehole data and reveal weak correlations and inverse relationships to the geophysical data in places vp and vs increase with grain scale deformation and alteration and decrease with porosity in the fault zone the low velocity zone is associated with a significant lithologic and structural transition to low velocity rocks dominated by phyllosilicates and penetratively foliated sheared rocks the zone of active shearing and the regions of low sonic velocity appear to be associated with clay rich rocks that exhibit fine scale foliation and higher porosities that may be a consequence of the fault related shearing of foliated and fine grained sedimentary rocks

late quaternary slip rates and slip partitioning on the alpine fault new zealand abstract published geological data on late quaternary offsets on the alpine fault new zealand have been assembled into a common format and analysed with respect to uncertainties uncertainties arise mainly from measurement of offset features relating apparent offsets to actual fault slip and dating the offset features despite the considerable uncertainties the data form a coherent set consistent with a relatively constant rate of strike slip of 27 5 mm year between milford sound and hokitika this rate represents 70 75 of the fault parallel interplate motion north of the confluence with the hope fault the rate drops substantially dip slip rates on the other hand show considerable variation along strike rising to a maximum of more than 10 mm year in the central section and decreasing to zero at the southern end partitioning of c 25 of the interplate slip on to structures east of the alpine fault occurs in the central section consistent with predictions from critical wedge models the partitioning of all the fault normal component of displacement on to other structures in the south may be related in part to a doubling in width of the deforming wedge to the east most probably however the fault normal displacement is mainly accommodated by underthrusting of the australian plate offshore due to a change in the nature of the crust from continental to oceanic

archean rocks of the black hills south dakota reworked basement from the southern extension of the trans hudson orogen the little elk and the bear mountain terranes on the margin of the precambrian core of the black hills provide the only known exposures of archean rocks between the archean wyoming and superior provinces the little elk terrane consists of a supracrustal sequence dominated by biotite feldspar gneiss meta arkose or metatuff that is intruded by the slightly metaluminous to peraluminous calc alkaline little elk granite which has a u pb intercept age of 2 549 11 m y a biotite plagioclase schist metagraywacke dominates the bear mountain terrane and is intruded subparallel to foliation by the peraluminous granite at bear mountain an isochron calculated from six severely discordant u pb zircon analyses yields an age of 2 393 230 m y the archean terranes participated in the trans hudson orogenic event suggested by a published rb sr age for the little elk granite 1 850 m y 1 87 1 97 gaigneous activity in the black hills and the similarity in orientation of the proterozoic deformational fabric with geophysical anomalies interpreted to be associated with the trans hudson orogen the metamorphic conditions estimated for the archean rocks may be the result of a thermal event that produced the proterozoic harney peak granite and thermal high northeast of lead south dakota the unexposed precambrian basement is probably dominated by granites similar to the little elk granite and supracrustal rocks consisting of continental margin deposits based on the ree contents of the archean schists and on the heavy mineral assemblages in the uraniferous nature of and the metasedimentary clasts in the lower proterozoic box elder creek formation the ages and the relatively long lived crustal sources for these archean granites and the similarity of the inferred basement characteristics to those of rocks in the wyoming province suggest that the black hills archean rocks may be the easternmost exposures of this province the probable trans hudsonian reworking of the black hills proterozoic rocks and their similarity to those in northern saskatchewan imply that the black hills occupies a tectonic position comparable to that of rocks described from the cree lake zone of saskatchewan

the cheyenne belt analysis of a proterozoic suture in southern wyoming abstract the cheyenne belt of southeastern wyoming is a major shear zone which separates archean rocks of the wyoming province to the north from 1800 1600 ma old eugeoclinal gneisses to the south miogeoclinal rocks 2500 2000 ma old unconformably overlie archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift formed continental margin intrusive tholeiitic sills and dikes are interpreted as rift related intrusions and a date of 2000 ma on a felsic differentiate of these intrusions gives the approximate age of rifting there are no known post 2000 ma felsic intrusions north of the cheyenne belt volcanogenic gneisses and abundant syntectonic calc alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks the volcanics are a bimodal basalt rhyolite assemblage plutons include large gabbroic complexes and quartz diorite 1780 ma syntectonic granitoids 1730 1630 ma and post tectonic anorthosite and granite 1400 ma there is no evidence for archean crust south of the cheyenne belt structural data thrusts in the miogeoclinal rocks vertical stretching lineations and the same fold geometries north and south of the shear zone suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane island arc over the northern terrane craton and miogeocline probably as a continuation of the south dipping subduction which generated calc alkaline plutons of the southern terrane a metamorphic discontinuity across the shear zone with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south also suggests thrusting of the southern terrane deeper crustal levels over the northern terrane shallower levels the cheyenne belt may be a deeply eroded master decollement perhaps analogous to a ramp in the master decollement in the southern appalachians this interpretation of the cheyenne belt as a proterozoic suture zone provides an explanation for the geologic geochronologic geophysical metallogenic and metamorphic discontinuities across the shear zone

geochemistry and origin of archean granites from the black hills south dakota the little elk granite 2549 ma and granite at bear mountain bmg 2 5 ga of the black hills formed as a result of a collisional event along the eastern margin of the wyoming province during th

earthquake slip between dissimilar poroelastic materials 1 a mismatch of elastic properties across a fault induces normal stress changes during spatially nonuniform in plane slip recently rudnicki and rice showed that similar effects follow from a mismatch of poroelastic properties e g permeability within fluid saturated fringes of damaged material along the fault walls in this case it is pore pressure on the slip plane and hence effective normal stress that is altered during slip the sign of both changes can be either positive or negative and they need not agree both signs reverse when rupture propagates in the opposite direction when both elastic and poroelastic properties are discontinuous across the fault steady sliding at a constant friction coefficient f is unstable for arbitrarily small f if the elastic mismatch permits the existence of a generalized rayleigh wave spontaneous earthquake rupture simulations on regularized slip weakening faults confirm that the two effects have comparable magnitudes and that the sign of the effective normal stress change cannot always be predicted solely from the contrast in elastic properties across the fault for opposing effects the sign of effective normal stress change reverses from that predicted by the poroelastic mismatch to that predicted by the elastic mismatch as the rupture accelerates provided that the wave speed contrast exceeds about 5 10 the precise value depends on the poroelastic contrast and skempton s coefficient for faults separating more elastically similar materials there exists a minimum poroelastic contrast above which the poroelastic effect always determines the sign of the effective normal stress change no matter the rupture speed

microseismicity and stress in the vicinity of the alpine fault central southern alps new zealand 1 we investigate present day microseismicity associated with the central alpine fault and the zone of active deformation and uplift in the central southern alps using 14 months of data robust hypocenter locations have been obtained for 1800 earthquakes of magnitudes between 0 3 and 4 2 we derived a magnitude scale with a frequency dependent attenuation factor f 0f where 0 1 89 0 02 10 3 s km that enables magnitudes to be calculated consistently for earthquakes of different sizes and frequency contents the maximum depth of the seismicity varies systematically with distance from the alpine fault from 10 2 km near the fault to 8 2 km within 20 km and 15 2 km further southeast this distribution correlates with lateral variations in crustal resistivity earthquake hypocenters are concentrated in areas of strong resistivity gradients and restricted to depths of resistivities 100 m rocks at greater depth are too hot too fluid saturated or too weak to produce detectable earthquakes focal mechanism solutions computed for 211 earthquakes ml 0 44 exhibit predominantly strike slip mechanisms we obtain a maximum horizontal compressive stress direction of 115 10 from focal mechanism inversion this azimuth is consistent with findings from elsewhere in the central and northern south island and indicates a uniform crustal stress field despite pronounced variations in crustal structure and topographic relief our stress estimates suggest that the alpine fault with a mean strike of 055 is poorly oriented in an andersonian sense but that individual thrust and strike slip segments of the fault s surface trace have close to optimal orientations

material contrast does not predict earthquake rupture propagation direction 1 earthquakes often occur on faults that juxtapose different rocks the result is rupture behavior that differs from that of an earthquake occurring on a fault in a homogeneous material previous 2d numerical simulations have studied simple cases of earthquake rupture propagation where there is a material contrast across a fault and have come to two different conclusions 1 earthquake rupture propagation direction can be predicted from the material contrast and 2 earthquake rupture propagation direction cannot be predicted from the material contrast in this paper we provide observational evidence from 70 years of earthquakes at parkfield ca and new 3d numerical simulations both the observations and the numerical simulations demonstrate that earthquake rupture propagation direction is unlikely to be predictable on the basis of a material contrast

field characterization of elastic properties across a fault zone reactivated by fluid injection we studied the elastic properties of a fault zone intersecting the opalinus clay formation at 300m depth in the mont terri underground research laboratory switzerland four controlled water injection experiments were performed in borehole straddle intervals set at successive locations across the fault zone a three component displacement sensor which allowed capturing the borehole wall movements during injection was used to estimate the elastic properties of representative locations across the fault zone from the host rock to the damage zone to the fault core young s moduli were estimated by both an analytical approach and numerical finite difference modeling results show a decrease in young s modulus from the host rock to the damage zone by a factor of 5 and from the damage zone to the fault core by a factor of 2 in the host rock our results are in reasonable agreement with laboratory data showing a strong elastic anisotropy characterized by the direction of the plane of isotropy parallel to the laminar structure of the shale formation in the fault zone strong rotations of the direction of anisotropy can be observed the plane of isotropy can be oriented either parallel to bedding when few discontinuities are present parallel to the direction of the main fracture family intersecting the zone and possibly oriented parallel or perpendicular to the fractures critically oriented for shear reactivation when repeated past rupture along these plane has created a zone of weakness

the reappearance of the huronian in wyoming rifting and drifting of ancient continents striking stratigraphic and sedimentological similarities between the early proterozoic huronian supergroup of the canadian shield and the snowy pass supergroup of wyoming suggest that they were deposited in a single broad epicratonic basin developed atop a large archean continent that included the superior and wyoming geological provinces breakup of the continent after the 2 2 ga intrusion of widespread gabbro sheets and dykes resulted in the separation of the archean superior and wyoming cratons and their early proterozoic covers these crustal fragments were subsequently reassembled during early proterozoic 1 85 ga orogenesis the end result being the present 2000 km separation of the huronian and snowy pass supergroups and their archean basements

the geochemistry of mafic gneisses from the renzy terrane western grenville province quebec implications for the geodynamic setting of the early mesoproterozoic laurentian margin abstract this study presents new geochemical data on mafic gneisses from the renzy terrane rt located in the grenvillian parautochthonous belt of western quebec this region is of particular interest since it contains one of the rare ni cu co massive sulfide deposits ever mined in the grenville province rt mafic gneisses have been divided into 3 groups based mainly on major element compositions trace element profiles normalized to the primitive mantle and incompatible trace element ratios the behaviour of high field strength elements hfse ta zr hf ti th and large ion lithophile elements lile ba rb are particularly useful for discriminating rt mafic gneisses major and trace element compositions of group 1 mafic ultramafic gneisses are very similar to those of the rt ultramafic sheets the mafic gneisses are therefore interpreted as melagabbroic sills cogenetic with these sheets the trace element composition of group 1 mafic gneisses is typical of magmas formed from a previously metasomatized depleted mantle source these rocks have high th ta ratios and show strong to moderate negative anomalies in ti zr and hf on multi element plots normalized to the primitive mantle mafic gneisses of group 2 show geochemical features transitional between extensional and compressional settings these rocks generally have low concentrations of hfse and high concentrations of lile typical features of mafic rocks formed in a compressional environment however some of them are fe ti rich with minor or no negative anomalies in hfse and low to moderate concentrations of lile on multi element diagrams normalized to primitive mantle features more typical of an extensional environment mafic gneisses of group 3 have a geochemical signature typical of extensional environments or of ocean island basalt oib these gneisses have no negative anomalies in hfse and have low concentrations of lile rt mafic gneisses are similar in composition to mafic granulites in the bondy gneiss complex in the central metasedimentary belt and to shawanaga domain amphibolites from the central gneiss belt two paleoarc back arc environments in the grenville province based on the contrasting geochemical signatures found in the rt mafic gneisses and the similarities with known arc back arc settings in the grenville province we propose that the rt was formed by arc back arc magmatic activity most likely associated with a period of arc magmatism along the proterozoic laurentian margin oib like mafic gneisses found in the rt are similar to those from the kipawa region formed by intraplate continental magmatism in the parautochthonous belt of the grenville province

shear behavior of dfdp 1 borehole samples from the alpine fault new zealand under a wide range of experimental conditions the alpine fault is a major plate boundary fault zone that poses a major seismic hazard in southern new zealand the initial stage of the deep fault drilling project has provided sample material from the major lithological constituents of the alpine fault from two pilot boreholes we use laboratory shearing experiments to show that the friction coefficient of fault related rocks and their precursors varies between 0 38 and 0 80 depending on the lithology presence of pore fluid effective normal stress and temperature under conditions appropriate for several kilometers depth on the alpine fault 100 mpa 160 c fluid saturated a gouge sample located very near to the principal slip zone exhibits 0 67 which is high compared with other major fault zones targeted by scientific drilling and suggests the capacity for large shear stresses at depth a consistent observation is that every major lithological unit tested exhibits positive and negative values of friction velocity dependence critical nucleation patch lengths estimated using representative values of the friction velocity dependent parameter a b and the critical slip distance d c combined with previously documented elastic properties of the wall rock may be as low as 3 m this small value consistent with a seismic moment m o 4 1010 for an m w 1 earthquake suggests that events of this size or larger are expected to occur as ordinary earthquakes and that slow or transient slip events are unlikely in the approximate depth range of 3 7 km

chemostratigraphy of paleoproterozoic carbonate successions of the wyoming craton tectonic forcing of biogeochemical change the archean wyoming craton is flanked on the south and east by belts of paleoproterozoic supracrustal successions whose correlation is complicated by lack of geochronologic constraints and continuous outcrop however carbonate units in these successions may be correlated by integrating carbon isotope stratigraphy with lithostratigraphy the 10 km thick paleoproterozoic snowy pass supergroup in the medicine bow mountains was deposited on the present day southern flank of the wyoming craton it contains three discrete levels of glacial diamictite correlative with those in the huronian supergroup on the southern margin of the superior craton the nash fork formation of the upper snowy pass supergroup is significantly younger than the uppermost diamictite and was deposited after the end of the paleoproterozoic glacial epoch carbonates at the base of the nash fork formation record remarkable 13 cenrichment up to 28 v pdb whereas those from overlying members of the lower nash fork formation have d 13 c values between 6 and 8 carbonates from the upper nash fork formation above the carbonaceous shale have carbon isotope values ranging between 0 and 2 5 the transition from high carbon isotope values to those near 0 in the nash fork formation is similar to that at the end of the ca 2 2 2 1 ga carbon isotope excursion in fennoscandia this chemostratigraphic trend and deposition of bifs mn rich lithologies carbonaceous shales and phosphorites at the end of the global ca 2 2 2 1 ga carbon isotope excursion are likely related to ocean overturn associated with the final breakup of the kenorland supercontinent correlative carbonates from the slaughterhouse formation in the sierra madre wy and from the whalen group in the rawhide creek area in the hartville uplift wy have highly positive carbon isotope values in contrast carbonates from other exposures of the whalen group in the hartville uplift and all carbonate units in the black hills sd have carbon isotope values close to 0 combined with existing geochronologic and stratigraphic constraints these data suggest that the slaughterhouse formation and the succession exposed in the rawhide creek area of the hartville uplift are correlative with the lower and middle nash fork formation and were deposited during the ca 2 2 2 1 ga carbon isotope excursion the estes and roberts draw formations in the black hills and carbonates from other exposures in the hartville uplift postdate the ca 2 2 2 1 ga

distribution of present day vertical deformation across the southern alps new zealand from 10 years of gps data 1 we use ten years of gps data from nine continuous and six semi continuous gps stations in a transect across the southern alps to measure rates of relative vertical movement with typical 1 uncertainties of 0 3 0 5 mm yr the estimated vertical rates define a fairly smooth profile across the range with the highest rates of 5 mm yr found near the crest of the mountains and 20 30 km southeast of the alpine fault the distribution of vertical rates supports models based on horizontal velocities from gps surveys in which interseismic coupling on the alpine fault decreases from 100 near the surface to zero by 13 18 km depth

the east bull lake intrusive suite remnants of a 2 48 ga large igneous and metallogenic province in the sudbury area of the canadian shield the 2 49 to 2 475 ga intrusions of the east bull lake intrusive suite occur in an east northeast trending discontinuous belt along the presently exposed boundary between the archean superior and the proterozoic southern provinces of the canadian shield near sudbury ontario the east bull lake intrusive suite is part of a regional paleoproterozoic magmatic event extending from 2 49 to 2 44 ga which also includes bimodal volcanic rocks felsic plutons and the regionally extensive hearst and matachewan dike swarms this regional magmatic event is thought to be the result of a mantle plume driven intracontinental rifting event that led to the development of a major basin to the south that was subsequently filled by sedimentary rocks of the huronian supergroup the east bull lake intrusive suite appears to have been emplaced along a major axial rift fault related to this rifting event compelling field and geochemical evidence summarized here indicates that the three largest east bull lake suite intrusions the east bull lake agnew lake and river valley intrusions crystallized from similar low ti high al tholeiitic parent magmas that originated in deeper more basic chambers it is proposed that the primary magmas for the east bull lake suite intrusions were second stage melts derived from a sublithospheric depleted mantle source that had been modified by neoarchean subduction and accretion events most of the petrological characteristics of the east bull lake suite intrusions reflect plagioclase dominated fractional crystallization that generated a pronounced fe enrichment trend in the residual magmas orthopyroxene is the high temperature pyroxene and occurs as a cumulus phase through most of the stratigraphy of each intrusion of the suite local olivine rich cumulates present in all of the major east bull lake suite plutons in some instances appear to be comagmatic with associated plagioclase rich cumulates a distinctive marginal facies comprising brecciated and locally thermally recrystallized and or partially melted footwall rocks is a common feature of the east bull lake suite plutons and indicates that a high energy flow regime existed during the initial stages of emplacement this marginal unit typically grades into a heterolithic inclusion rich gabbronorite that contains erratically distributed mafic autoliths and footwall derived xenoliths chemical contamination of the mineralized matrix of these inclusion bearing rocks can be extreme at a local scale the inclusion rich zone is overlain by a thick interval of undifferentiated plagioclase rich cumulates that locally display spectacular glomerocrystic textures rhythmically and irregularly modally layered leucogabbronorite and gabbronorite make up much of the middle parts of the stratigraphy massive to crudely layered ferrogabbro and ferrosyenite only occur in the uppermost part of the agnew lake intrusion varitextured gabbro and pegmatitic gabbro occur as irregular layers and local veins and pods throughout the stratigraphy the east bull lake suite is currently being explored for contact type platinum group element pge cu ni mineralization that has a clear spatial association with orthopyroxene rich cumulates which are otherwise poorly represented in these intrusions disseminated pge and cu rich sulfide mineralization is concentrated within the inclusion bearing zones but extends upward into overlying plagioclase cumulates and appears to have collected along the lower margins of these intrusions through the combined action of vigorous convection of the resident magma and downward percolation of dense mafic residual liquid the resultant narrow zones of sulfide mineralization were thus derived from a much larger volume of parent magma feeder dikes to the mineralized parts of the intrusions appear to have been saturated in sulfide upon emplacement and had relatively high background pge contents compared to those for other known magmatic pge deposits however the economic potential of the contact type mineralization hinges on the efficacy of the physical concentration process which upgraded the metal content and size of the original sulfide particles inside the magma chambers based on the proposed basal accumulation model the best prospective mineralization is most likely to occur in embayments along the sidewall and floor of the intrusions

new on fault evidence for a great earthquake in a d 1717 central alpine fault new zealand the dextral reverse alpine fault is the major onshore plate boundary structure between the australian and pacific plates in new zealand no previous study of the central portion of the 200 km long central segment has provided on fault evidence for the most recent event mre using lidar light detection and ranging data coupled with field mapping we recognized the main trace of the alpine fault north of gaunt creek south island as a north striking fault scarp we enhanced a natural exposure that revealed evidence for repeated late holocene thrust fault movement the north northwest striking fault zone is characterized by a distinct 5 50 cm thick clay fault gouge layer juxtaposing hanging wall bedrock mylonites and cataclasites over unconsolidated late holocene footwall colluvium the bedrock is cut by a strath terrace and overlain by mid holocene ca 5400 calibrated 14 c yr b p alluvial terrace which has been faulted repeatedly and is conformably overlain by undeformed late holocene colluvium and alluvium an unfaulted peat at the base of the scarp is buried by post mre alluvium and yields a calibrated 2 radiocarbon age of a d 1710 1930 which dates the mre as post 1709 our data are consistent with sparse on fault data and validate earlier off fault records that suggest an a d 1717 mre the 1717 event had a moment magnitude of m w 8 1 0 1 based on the 380 km long surface rupture because the fault has not ruptured for 300 yr it is likely approaching the end of its seismic cycle and poses a significant seismic hazard to new zealand

proterozoic collisional tectonism in the trans hudson orogen saskatchewan isotopic and structural data from the juvenile reindeer zone of the trans hudson orogen northern saskatchewan indicate a pre 1 85 ga thermotectonic event possibly reflecting arc continent collision followed by a more extensive nappe forming ca 1 83 1 80 ga thermotectonism during terminal continent continent collision preliminary data from the adjacent ensialic cree lake zone suggest high grade reworking of archean crust by the pre 1 85 ga event in the rae province to the west high grade metamorphism and reworking of archean crust occurred about 2 0 ga and may be related to the formation of the coeval taltson magmatic zone

shear heating induced thermal pressurization during earthquake nucleation 1 we model earthquake nucleation in 2d on narrow faults with coupled rate state friction and shear heating induced thermal pressurization including diffusive transport of heat and pore pressure thermal pressurization increases pore pressure p decreasing frictional resistance observed fault core permeability is generally too low to mitigate thermal pressurization at subseismic slip speeds under drained isothermal conditions nucleation with the aging law is crack like with the interior of the slip zone always near maximum slip speed when thermal pressurization is included it can dominate weakening at speeds of 0 02 20 mm s for hydraulic diffusivities chyd from 10 8 to 10 3 m2 s and nominal material parameters well before seismic radiation occurs dramatic along strike localization of slip occurs due to feedback in which the area of maximum slip experiences the greatest weakening which in turn favors more slip with the slip law however nucleation is pulse like with slip speed decaying behind the pulse tip thermal pressurization is diminished relative to the aging law case since most weakening occurs in locations with limited slip yet we find that it can overwhelm frictional weakening at slip speeds in the range of 1 100 mm s for chyd from 10 8 to 3 10 5 m2 s at higher slip speeds the finite thickness of the shear zone becomes significant reducing thermal pressurization even if not the dominant weakening mechanism thermal pressurization is likely to be significant at or before the onset of seismic radiation

near surface clay authigenesis in exhumed fault rock of the alpine fault zone new zealand o h ar isotopic xrd and chemical analysis of illite and chlorite abstract exhumed fault rock of the central alpine fault zone south island new zealand shows extensive clay mineralization and it has been the focus of recent research that aims to describe the evolution and frictional behavior of the fault using quantitative x ray powder diffraction 40 ar 39 ar geochronology hydrogen isotope d geochemistry and electron microbeam analysis we constrain the thermal and fluid conditions of deformation that produced two predominant clay phases ubiquitous to the exposed fault damage zone illite and chlorite illite polytype analysis indicates that most end member illite and chlorite material formed in equilibrium with meteoric fluid d 55 to 75 but two locations preserve a metamorphic origin of chlorite d 36 to 45 chlorite chemical geothermometry constrains crystal growth to t 210 296 c isotopic analysis also constrains illite growth to t

seismic p wave velocity model from 3 d surface and borehole seismic data at the alpine fault dfdp 2 drill site whataroa new zealand

2480 ma mafic magmatism in the northern black hills south dakota a new link connecting the wyoming and superior cratons the laramide black hills uplift of southwest south dakota exposes a precambrian crystalline core of 2560 2600 ma basement granitoids nonconformably overlain by two paleoproterozoic intracratonic rift successions in the northern black hills a 1 km thick layered sill the blue draw metagabbro that intrudes the older rift succession provides a key constraint on the timing of mafic magmatism and of older rift basin sedimentation ion microprobe spot analyses of megacrysts of magmatic titanite from a horizon of dioritic pegmatite in the uppermost sill portion yield a 207pb 206pb upper intercept age of 2480 6 ma all age errors 2 comparable to two point 207pb 206pb errorchron ages obtained by pb stepwise leaching of the same titanites nearly concordant domains in coexisting magmatic zircon yield apparent spot ages ranging from 2458 16 to 2284 20 ma i e differentially reset along u pb concordia and hornblende from an associated metadiorite yields a partially reset date with oldest apparent a

40ar 39ar evidence for middle proterozoic 1300 1500 ma slow cooling of the southern black hills south dakota midcontinent north america implications for early proterozoic p t evolution and posttectonic magmatism 40ar 39ar total gas and plateau dates from moscovite and biotite in the southern black hills south dakota provide evidence for a period of middle proterozoic slow cooling early proterozoic 1600 1650 ma mica dates were obtained from metasedimentary rocks located in a synformal structure between the harney peak and bear mountain domes and also south of bear mountain metamorphic rocks from the dome areas and undeformed samples of the 1710 ma harney peak granite hpg yield middle proterozoic mica dates 1270 1500 ma two samples collected between the synform and bear mountain dome yield intermediate total gas mica dates of 1550 ma we suggest two end member interpretations to explain the map pattern of cooling ages 1 subhorizontal slow cooling of an area which exhibits variation in mica ar retention intervals or 2 mild folding of a middle proterozoic 1500 ma 300 c isotherm according to the second interpretation the preservation of older dates between the domes may reflect reactivation of a preexisting synformal structure and downwarping of relatively cold rocks during a period of approximately east west contraction and slow uplift during the middle proterozoic the mica data together with hornblende data from the black hills published elsewhere indicate that the ambient country rock temperature at the 3 4 kbar depth of emplacement of the hpg was between 350 c and 500 c suggesting that the average upper crustal geothermal gradient was 25 40 c km prior to intrusion the thermochronologic data suggest hpg emplacement was followed by a 200 m y period of stability and tectonic quiescence with little uplift we propose that crust thickened during the early proterozoic was uplifted and erosionally thinned prior to 1710 ma and that the hpg magma was emplaced into isostatically stable crust of relatively normal thickness we speculate that uplift and crustal thinning prior to hpg intrusion was the result of differential thinning of the subcrustal lithosphere beneath the black hills if so this process would have also caused an increase in mantle heat flux across the moho and triggered vapor absent melting of biotite to produce the hpg magma this scenario for posttectonic granite generation is supported in part by the fact that in the whole of the black hills the hpg is spatially associated with the deepest exposed early proterozoic country rock

effects of temperature on the frictional behavior of material from the alpine fault zone new zealand abstract temperature is widely believed to act as a primary control on fault rheology and therefore on the distribution of seismicity along plate boundary faults however there are few detailed measurements of the frictional strength and stability of natural fault gouges at elevated temperatures here we report on a suite of shearing experiments designed to investigate the frictional behavior of fault rocks sampled from depths of 111 5 142 9 m along the alpine fault in new zealand obtained by the international continental scientific drilling program icdp deep fault drilling project dfdp we tested five samples from the dfdp 1b pilot hole two hanging wall chloritic cataclasites two footwall granitic cataclasites and a fault gouge from the principal slip zone psz 1 each sample was sheared at a range of temperatures from 23 to 500 c and at an effective normal stress of 80 mpa the wall rock cataclasites exhibit an increase in the friction coefficient with temperature from 0 45 0 64 at 23 c to 0 87 at 500 c the psz 1 gouge exhibits lower friction coefficient values than the wall rock at temperatures 180 c 0 35 0 46 vs 0 45 0 65 but comparable values 0 87 0 90 at 500 c the variation in frictional strength is accompanied by a transition from velocity strengthening to velocity weakening behavior at temperatures 180 c for all materials extrapolation of the experimentally defined rheological critical stiffness of the fault material and the estimated in situ stiffness of the surrounding crust suggests upper and lower stability boundaries at 1 8 2 5 km and 8 5 8 8 km depth respectively the upper stability boundary is also consistent with the observed depth frequency distribution of earthquakes

the significance of heat transport by shallow fluid flow at an active plate boundary the southern alps new zealand fluid flow can influence fault behavior here we quantify the role of groundwater heat advection in establishing the thermal structure of the alpine fault a major tectonic boundary in southern new zealand that accommodates most of the motion between the australian and pacific plates convergence on the alpine fault has rapidly uplifted the southern alps resulting in high geothermal gradients and a thin seismogenic zone a new equilibrium temperature profile from the 818 m deep deep fault drilling project 2b dfdp 2b borehole has been interrogated using one dimensional analytical models of fluid and rock advection models indicate a total heat flux of 720 mw m 2 results from groundwater flow with darcy velocities approximating to 7 8 10 10 m s 1 groundwaters advect significantly more heat than rock advection in the shallow orogen 6 km depth and are the major control on the subsurface temperature field

frictional properties of actinolite chlorite gouge at hydrothermal conditions abstract exhumed subduction zones frequently show widespread actinolite chlorite act chl dominated compositions suggesting that this may play an important role in controlling megathrust fault slip we investigate the frictional behavior of simulated act chl 85 15 gouge mixtures derived from natural mafic metamorphic rocks under hydrothermal conditions using a ring shear deformation apparatus experiments were performed at effective normal stresses neff of 50 200 mpa pore fluid pressures pf of 50 200 mpa at temperatures t of 23 600 c in each experiment we applied a shear displacement x of 10 mm at a constant sliding velocity v of 10 m s followed by v stepping in the range 0 3 100 m s and slide hold slide shs tests with hold times t ranging from 3 s to 3000 s we quantified the rate and state dependent friction parameter a b and investigated the effect of t on fault healing pk the results showed no effects of temperature on the coefficient of friction 0 6 0 7 or on a b with some experiments showing persistent displacement hardening or weakening trends nonetheless effects of v and of normal stress n neff pf on a b fall into three temperature regimes 1 t 23 100 c 2 t 200 400 c and 3 t 500 600 c in regimes 1 and 3 a b 0 for all conditions tested whereas in regime 2 a b 0 at neff pf 50 mpa and v 0 3 3 m s we discuss on the origin of persistent displacement hardening or weakening trends observed and assess the implications of our data for subduction zone seismogenesis extrapolation of a b data using multiple linear regression suggests that high pore pressure ratios 0 9 are needed to promote seismogenesis in faults cutting actinolite chlorite compositions

lead isotopic evidence for mixed sources of proterozoic granites and pegmatites black hills south dakota usa abstract the lead isotopic compositions of k feldspars separated from the ca 1700 ma harney peak granite complex and spatially associated granitic pegmatites indicate that these rocks were derived from at least two sources it has been reported previously that the core of the harney peak granite complex is dominated by relatively lower gd18o avg 11 5 granites whereas higher gd18o avg 13 2 granites occur around the periphery of the complex the higher 18 o granites and one simple pegmatite have low values of 207 pb 204 pb for their 206 pb 204 pb thus they likely were derived from a source with a short crustal residence time this source may have been the pelitic schists into which the harney peak granite complex and pegmatites were intruded feldspars from granites with lower gd18o values have significantly higher 207 pb 204 pb for their 206 pb 204 pb the data define a linear array with a slope equivalent to an age of ca 2 6 ga with t2 defined to be 1 7 ga such a slope could represent a mixing array or a secondary isochron for the source these low 18o granites could have been derived from a source with a high u pb and with a crustal residence beginning before the proterozoic the source s of these granites may have been a sediment derived from late archean continental crust the highly evolved tin mountain pegmatite has lead isotopic systematics intermediate between those of the two granite groups suggesting either a mixed source or contamination two late archean granites the little elk granite and the bear mountain granite had precursors with high u pb and low th u histories the th u history of the bear mountain granite is too low for this rock to have been an important component of the source of the proterozoic granites however crustal rocks with lead isotopic compositions similar to those of the little elk granite were an important source of lead for some of the proterozoic granitic rocks