mesozoic tectonics and metamorphism in the pequop mountains and wood hills region northeast nevada implications for the architecture and evolution of the sevier orogen discussion and reply the pequop mountains wood hills east humboldt range region northeast nevada exposes a nearly continuous cross section of precambrian to mesozoic strata representing middle to upper crustal levels of the mesozoic hinterland of the sevier orogen these rocks preserve the transition from unmetamorphosed mesozoic upper crust to partially melted middle crust integration of new structural metamorphic and u pb thermochronologic data from the wood hills and pequop mountains coupled with a regional tectonic reconstruction reveals substantial cretaceous metamorphism contraction and extension in the sevier hinterland in northeast nevada we report two phases of contraction not previously recognized that are accommodated by top to the southeast thrust faults the windermere and independence thrusts contraction was succeeded by two phases of extension along west rooted normal faults the late cretaceous pequop fault and tertiary mary9s river fault system the earliest phase of thrust faulting resulted in as much as 30 km of crustal thickening and an estimated minimum of 69 km of shortening along an inferred fault called the windermere thrust the timing of this thrusting event is bracketed between late jurassic ca 153 ma and late cretaceous 84 ma relaxation of crustal isotherms following and perhaps during thrusting resulted in barrovian style metamorphism of footwall rocks and partial melting of metapelite at deep levels peak metamorphism was attained ca 84 ma and by this time hinterland crustal thickening had reached a maximum during 84 75 ma another minor pulse of shortening and thickening along the independence thrust was followed by partial exhumation of the metamorphic rocks and as much as 10 km of crustal thinning along the pequop fault thus the interval from 84 to 75 ma in northeast nevada marks a fundamental and apparently permanent change from horizontal contraction to extension in the upper to middle crust in the hinterland final exhumation of the metamorphic rocks was accomplished by the tertiary mary9s river fault system our data indicate that much of the metamorphism and some of the contraction in the sevier hinterland in northeast nevada which was previously thought to be largely late jurassic is actually cretaceous in age furthermore the data indicate that widespread metamorphism of the middle crust is a byproduct of tectonic burial and that hinterland and foreland thrust faulting were coeval suggesting that thrust faults in the sevier orogen do not form a simple foreland younging sequence

simulation of run off from the greenland ice sheet for planning hydro electric power ilulissat jakobshavn west greenland simulations of run off from the greenland ice sheet were made as part of a feasibility study for provision of hydroelectric power for liulissatfjakobshavn west greenland the aims were to see if the available short series of run off measurements are typical of those under present climatic conditions and to assess possible changes in run off likely to be caused by gross changes in drainage pattern on the ice sheet specific run off was calculated from climatological data whilst run off volumes were calculated by integrating specific run off over the area of the ice sheet there have been substantial year to year variations in run off but the 6 year measurement period is reasonabl y re prese ntati ve of present climatic conditions run off could be reduced by 21 as a result of changes in hydrauli c conditions on the ice sheet without this having a significant effect on the economy of the planned hydro electric power station

refrigeration of the western cordilleran lithosphere during laramide shallow angle subduction the laramide orogeny has generally been attributed to a shift from normal angle to shallow angle subduction beneath the western margin of the north american plate in addition to important mechanical effects this shift may have had important thermal effects on the lithosphere in the western cordillera before the laramide geothermal gradients in the western cordillera were probably normal to high because of the presence below of a hot asthenospheric wedge with the shift to shallow subduction the wedge was probably expelled and replaced by a cold subducting slab that extended just below the cordilleran lithosphere this would shift the western cordillera into a cold forearc like thermal setting dominated by the refrigeration effects of the subducting slab thermochronologic data from the sierra nevada great basin mojave desert western arizona and perhaps the colorado plateau record latest cretaceous early tertiary cooling that may be evidence of such refrigeration if regional refrigeration occurred it would have several important implications 1 metamorphism in the western cordillera would have waned with the start of the laramide orogeny 2 the crust in the western cordillera would have strengthened and become much more resistant to deformation e g gravitational collapse and extension of thickened crust in the sevier hinterland would have been impeded and 3 latest cretaceous early tertiary isotopic cooling ages may not be valid indicators of a period of major regional uplift and unroofng

mesozoic aseismic ridges on the farallon plate and southward migration of shallow subduction during the laramide orogeny we propose that an aseismic ridge of late jurassic and early cretaceous age on the farallon plate was subducted beneath the unites states cordillera during the laramide orogeny the relative buoyancy of the aseismic ridge caused shallow subduction which resulted in a magmatic lull 70 40 ma in the existing near trench magmatic arc and laramide uplift and faulting of crystalline basement 1000 1500 km inland from the trench the timing and latitudinal limits of the subduction of the aseismic ridge are similar to the timing and latitudinal limits of both the laramide orogeny and the magmatic lull in the western cordillera because the north northeast trending aseismic ridge was moving northeast relative to the north american plate the point of collision of the aseismic ridge with north america and therefore the locus of shallow subduction migrated southward with time causing the well documented southward migration of the magmatic lull

age inheritance and uplift history of the old woman piute batholith california and implications for k feldspar age spectra ion probe u pb zircon analyses yield a crystallization age of 74 pm 3 ma for both peraluminous and metaluminous plutons within the old woman piute batholith these analyses reveal that about 50 of the zircons from the peraluminous sweetwater wash granite are restitic inherited zircons from this batholith yield ages ranging from older than 1800 ma to 1100 ma and most are similar in age to proterozoic igneous rocks in the eastern mojave desert 40 ar 39 ar hornblende ages from the metaluminous granodiorite are 73 pm 2 ma and mica ages from both plutonic series are 70 pm 2 ma k feldspar age spectra yield age gradients with initial steps between 60 and 67 ma and rise to plateau ages of 70 pm 2 ma several k feldspars from the granodiorite yield elevated plateau ages after 50 39 ar released that result from excess argon uplift rates as high as 4 mm yr between 74 and 70 ma are required by the narrow age range for all minerals

late cretaceous extensional unroofing in the funeral mountains metamorphic core complex california new field and geochronologic data document the existence of late cretaceous extensional structures in the death valley region california nevada we have mapped two major low angle ductile shear zones that omit stratigraphy in the footwall of the funeral mountains metamorphic core complex intervening strata have been strongly attenuated although stratigraphic offset across the shear zones is only 1 5 km the presence of a large metamorphic discontinuity suggests that the amount of unroofing must be much greater the timing of shear zone formation attenuation and subsequent northwest vergent folding is constrained by u pb geochronology on 1 prekinematic or synkinematic and 2 postkinematic pegmatites deformation was taking place by 72 ma and had ended by 70 ma these results support earlier petrologic and geochronologic data that suggested substantial unroofing of the funeral mountains in late cretaceous time and add to a growing body of evidence for widespread mesozoic extension in the hinterland of the sevier thrust belt

synmagmatic deformation patterns in the old woman mountains se california abstract synmagmatic structures within granitic plutons may provide insight into the interplay between plutonism and regional deformation at mid crustal levels within the late cretaceous old woman pluton mojave desert se california synmagmatic structures include magmatic fabrics parallel to wall rock foliations melt filled shear zones folded gneissic xenoliths and schlieren with axial planar magmatic fabric deformation continued below granite solidus temperatures in shear zones on the pluton margins these structures permit reconstruction of a sheet like pluton that was emplaced under regional extension taken together with published thermochronometric data the synmagmatic structures constrain the role played by magmatic events in the collapse of a thickened crustal section the post plutonic cooling history suggests rapid uplift and denudation ending by 65 ma we argue that extensional deformation overlapped and post dated pluton emplacement and may have contributed to tectonic unroofing we suggest that magma emplacement can trigger subsequent extensional collapse in overthickened crustal blocks

strain path partitioning within thrust sheets microstructural and petrofabric evidence from the moine thrust zone at loch eriboll northwest scotland abstract quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the loch eriboll area of the moine thrust zone and are used to interpret the detailed processes involved in fabric evolution the intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude a correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the moine thrust within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate mutually interfering shear bands globular low strain detrital quartz grains whose c axes are aligned sub parallel to the principal finite shortening direction z and quartz c axis fabrics which are symmetric both in terms of skeletal outline and intensity distribution with respect to mylonitic foliation and lineation non coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation tectonic models offering possible explanations for the presence of kinematic strain path domains within thrust sheets are considered

polythermal three dimensional modelling of the greenland ice sheet with varied geothermal heat flux computations over 50 000 years into steady state with greve s polythermal ice sheet model and its numerical code are performed for the greenland ice sheet with today s climatological input surface temperature and accumulation function and three values of the geothermal heat flux 42 54 6 29 4 mw m 2 it is shown that through the thermomechanical coupling the geometry as well as the thermal regime in particular that close to the bed respond surprisingly strongly to the basal thermal heat input the most sensitive variable is the basal temperature field but the maximum height of the summit also varies by more than 100m furthermore some intercomparison of the model outputs with the real ice sheet is carried out showing that the model provides reasonable results for the ice sheet geometry as well as for the englacial temperatures

evolution of retreating subduction boundaries formed during continental collision retreating subduction boundaries formed where the rate of subduction exceeds the rate of overall plate convergence appear to be commonly developed features within regions of early or incomplete continent continent collision they are characterized by regional extension within the overriding plate and at their leading edge by thin skinned arcuate thrust belts that are concave towards the overriding plate as is illustrated by examples from the mediterranean region the formation of retreating subduction boundaries is intimately related to the process of continental collision during the early stages of collision retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary retreating plate boundaries can also form before the main collision and the associated thrust belts emplaced as precollisional accretionary assemblages because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab slab pull subduction usually ceases when and only when thick buoyant continental crust enters the subduction zone thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted in some cases retreating subduction boundaries may escape into the open ocean where they form nearly isolated local tectonic systems in these systems the rate of subduction is approximately compensated by the rate of upper plate extension and migration of the system across the oceanic region may be very rapid for example the horseshoe seamounts located about 800 km offshore in the eastern north atlantic may be the active expression of an east dipping westwardly migrating retreating subduction boundary that has evolved from the betic cordillera rif system active in miocene time and may now be progressing across the atlantic at approximately 50 mm yr an analogous situation may be represented by the scotia arc system a westward dipping retreating subduction system located between the south american and antarctic plates which may have escaped into the south atlantic ocean from a zone of crustal shortening in the andes and is now progressing across the atlantic at a rate of about 80 mm yr

thermomechanical modelling of northern hemisphere ice sheets with a two level mass balance parameterization a 3 d time dependent thermomechanical ice sheet model was used together with a two level snow accumulation runoff mass balance model to investigatethe quaternary ice sheets of the northern hemisphere the model freely generates the ice sheet geometry in response to specified changes of surfacetemperature and mass balance and includes bedrock adjustment basal sliding and a full temperature calculation within the ice the mass balanceparameterisation makes a distinction between snowfall and melting yearly snow fall rates depend on the present precipitation distribution and are variedproportional to changes of surface temperature and the moisture content of the air the ablation model is based on the positive degree day method anddistinguishes between ice and snow melting this paper discusses steady state characteristics conditions for growth and retreat and response time scales ofice sheets as a function of a prescribed lowering of summer temperature most notably the modelled extents of the eurasian ice sheet for a summertemperature lowering of 6 7 k and of the laurentide ice sheet for a cooling of 9 10 k are in reasonable agreement with most reconstructions based ongeological evidence except for the presence of a large ice sheet stretching from alaska across the bering strait to most of eastern siberia in addition wetbasal conditions turned out to be always confined to the margin whereas central areas in these reconstructions remained always cold based this is ofrelevance for processes involving reduced basal traction

ireteba pluton eldorado mountains nevada late deep source peraluminous magmatism in the cordilleran interior abstract the ireteba pluton is a 66 ma documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape mathrm biotite pm mathrm muscovite pm mathrm garnet end document granite emplaced at the eastern edge of the cordilleran plutonic belt in southeastern nevada in common with other cordilleran peraluminous granites its mineralogy major element chemistry isotopic composition and abundant proterozoic zircon inheritance document crustal origin distinctive trace element chemistry field relations and inherited components furth

positive degree day factors for ablation on the greenland ice sheet studied by energy balance modelling ice ablation is related to air temperature by the positive degree day factor variations of the positive degree day factor in west greenland are studied using an energy balance model to simulate ablation under different conditions degree day factors for simulated and measured ice ablation at nordbogletscher and qaman rssup sermia agree well with values around 8 mm d 1 c 1 degree day factors for snow are less than half those for ice energy balance modelling shows that degree day factors vary with summer mean temperature surface albedo and turbulence but there is only evidence of large positive degree day factors at lower temperatures and with low albedo 0 3 the greatest effect of albedo variations 0 3 0 7 is at lower temperatures while variations in turbulence have greater effect at higher temperatures current models may underestimate runoff from the greenland ice sheet by several tenths because they use a degree day factor for melting ice that is too small for the colder parts of the ice sheet i e the upper ablation area and the northerly margin

chapter 7 jurassic granitoids and related rocks of the southern bristol mountains southern providence mountains and colton hills mojave desert california

seasonal evolution of supra glacial lakes on the greenland ice sheet abstract a survey of supra glacial lakes on the western margin of the greenland ice sheet reveals a seasonally driven hydrological system culminating in widespread lake drainage in late summer we used satellite imagery to study the evolution of 292 lakes across two sites totalling 22 000 km 2 in area during 2001 the lakes combined area increased to 75 5 km 2 by the beginning of july over the following 25 days an area totalling 36 3 5 km 2 drained from 216 lakes at one study site we used meteorological data and a positive degree day model to calculate the volume of water generated by melting in the lake catchments based on this estimate the mean depth of filling lakes surveyed rose from 1 5 0 7 m on 7th july to 3 9 1 1 m on 1st august in agreement with a value for one lake of 4 4 0 9 m we have derived from airborne altimetry during this 25 day period we estimate that 38 18 10 7 m 3 of water drained from the surface at this site and that there was an average water flux of 1 3 0 3 m 3 s 1 passing through each lake that drained completely

rheological control on the initial geometry of the raft river detachment fault and shear zone western united states the strain exhumation history and field orientation of a well exposed shear zone and detachment fault in the raft river mountains of northwestern utah a cordilleran metamorphic core complex have been studied to determine the kinematics of ductile shearing and initial orientations of the shear zone and detachment fault mapping and strain and kinematic analysis indicate that the top to the east raft river shear zone initially developed parallel to an unconformity separating archean rocks from overlying proterozoic quartzite and schist for at least 24 km in the shear direction experimental rock deformation data from lithologies similar to the archean and proterozoic rocks suggest the unconformity represented a significant rheological boundary at the deformation temperatures the base of the shear zone was localized along the boundary between relatively weak quartzite above and stronger monzogranite below an extensive thermochronological database is used to reconstruct the position of the basement unconformity in temperature lateral distance coordinates the initial average dip of the shear zone and basement unconformity is estimated between 7 and 30 assuming subhorizontal isotherms and geothermal gradients of 20 40 c km the east dip of the unconformity at the onset of miocene extension is interpreted to have resulted from late eocene unroofing and flexure beneath a top to the wnw extensional shear zone in the western raft river grouse creek and albion mountains the observations from the raft river shear zone suggest that the orientation of some midcrustal shear zones may not reflect the predicted orientation for ductile faults according to ductile failure criteria but rather the orientation of rheological boundaries along which deformation is localized furthermore detachment faults that are superimposed on mylonite during progressive displacement and footwall unroofing may use an inherited mechanical anisotropy from the mylonite and their orientations may not reflect the predicted orientation of shear fractures in isotropic rock the common parallelism between detachment faults and mylonitic foliation may indicate a mechanical and kinematic preference for localization of throughgoing brittle faults parallel to preexisting mylonitic foliation because of this preference studies restricted to detachment faults which lack footwall mylonite or restricted to structural levels between the breakaway and mylonitic front have more bearing on the question of the initial dip of normal sense shear fractures faults within the seismogenic crust

the 40ar 39ar thermochronology of the eastern mojave desert california and adjacent western arizona with implications for the evolution of metamorphic core complexes mesozoic thickening and cenozoic extension resulted in the juxtaposition of upper and middle crustal rocks in the eastern mojave desert southeastern california and western arizona the application of 40ar 39ar thermochronology to rocks in this region provides information about the timing and nature of thrusting plutonism metamorphism denudation and detachment faulting the 40ar 39ar ages of 175 to 125 ma from the clipper piute turtle mohave bill williams and hualapai mountains are interpreted to be the result of a middle mesozoic thermal event s caused by crustal thickening the 40ar 39ar data from the clipper and piute mountains suggest that this thermal event was followed by a period of cooling at rates of 1 5 c m y orogenesis culminated during the late cretaceous when rocks exposed in the old woman piute chemehuevi and sacramento mountains attained temperatures 500 c which reset the k ar systems of minerals from proterozoic rocks high grade metamorphism in the old woman mountains area was caused by the intrusion of the old woman piute batholith at 73 1 ma cooling rates following batholith emplacement in the old woman mountains were 100 c m y between 73 and 70 ma and 5 10 c m y from 70 to 30 ma between 65 and 25 ma the entire eastern mojave desert underwent a period of cooling at a rate of 2 10 c m y by 30 ma rocks exposed in the old woman piute marble ship clipper and turtle mountains were below 100 c the 40ar 39ar ages from the sacramento mountains suggest that mylonitization caused by the onset of regional extension occurred at 23 1 ma when extension started in the chemehuevi mountains rocks exposed in the southwestern and northeastern portions of footwall to the chemehuevi detachment fault were at 180 c and 350 c respectively this suggests that the exposed part of the chemehuevi detachment fault initiated at a dip of 5 30 or as a series of higher angle faults that cut to a depth of 10 12 km and were later rotated to their present dip unroofing of the footwalls to detachment faults in the sacramento and chemehuevi mountains resulted in average cooling rates of 10 50 c m y between 22 and 15 ma

contribution to the glaciology of northern greenland from satellite radar interferometry interferometric synthetic aperture radar insar data from the ers 1 and ers 2 satellites are used to measure the surface velocity topography and grounding line position of the major outlet glaciers in the northern sector of the greenland ice sheet the mass output of the glaciers at and above the grounding line is determined and compared with the mass input we find that the grounding line output is approximately in balance with the input except for the three largest glaciers for which the mass loss is 4 3 km3 ice year 1 or 11 8 of the mass input along the coast we detect a systematic retreat of the grounding lines between 1992 and 1996 with insar which implies that the outlet glaciers are thinning the inferred coastal thinning is too large to be explained by a few warm summers glacier thinning must be of dynamic origin that is caused by spatial and temporal changes in ice velocity iceberg production from the glaciers is uncharacteristically low it accounts for only 8 of the ice discharge to the ocean about 55 of the ice is lost through basal melting 5 8 m ice year 1 on average from the underside of the floating glacier tongues that are in contact with warm ocean waters mass losses are highest in the first 10 km of floating ice where ice reaches the greatest depths and basal melting is 3 times larger than on average only a small increase in basal melting would suffice to disintegrate the floating glacier tongues copyright 2001 by the american geophysical union

evidence for deformation induced k feldspar replacement by myrmekite several examples of deformation induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths namely a muscovite poor s c mylonite and a single foliation muscovite poor mylonitic gneiss back scattered sem and conventional optical microscopy show that in both rock types syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of k feldspar grains that faced the local inferred incremental shortening direction for the mylonite myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction the replacement of k feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains we suggest that the ambient temperature pressure and chemical activities were such that the replacement reaction was favoured but the addition of extra strain energy along the high pressure sides of the grains localized the reaction at these sites this energy could arise from elastic strain or strain associated with tangled dislocations or twin boundaries the relative roles of stress and strain energy concentrations in driving the replacement reaction are not known but both were probably important

surface roughness and bulk heat transfer on a glacier comparison with eddy correlation more than 100 simultaneously measured profiles of wind speed and temperature were analyzed to determine the aerodynamic and thermal roughness lengths of glacier ice the results proved to be extremely sensitive to measurement height which is difficult to define precisely on the hummocky terrain of a glacier therefore aerodynamic roughness lengths were estimated from the microtopography of the surface yielding 2 46 mm for ice and 5 5 mm for snow despite the indeterminate nature of the profile analysis it did support the view that the thermal roughness length must decrease as the roughness reynolds number increases a relationship which has been modelled in the literature good agreement between bulk transfer estimates and eddy correlation measurements of fhe sensible heat flux density on melting ice and snow was obtained by combining the aerodynamic roughness length with a suitable model for thermal roughness length

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

variations of ablation albedo and energy balance at the margin of the greenland ice sheet kronprins christian land eastern north greenland a meteorological and glaciological experiment was carried out in july 1993 at the margin of the greenland ice sheet in kronprins christian land eastern north greenland within a small area about 100 m 2 daily measurements were made on ten ablation stakes fixed in light and dark ice and were compared to each other simultaneously the components of the energy balance including net radiation sensible heat flux latent heat flux and conductive heat flux in the ice were determined global radiation longwave incoming radiation and albedo were measured and longwave outgoing radiation was calculated by assuming that the glacier surface was melting sensible and latent heat fluxes were calculated from air temperature humidity and wind speed conductive heat flux in the ice was estimated by temperature profile measurements in the uppermost ice layer net radiation is the major source of ablation energy and turbulent fluxes are smaller energy sources by about three times while heat flux into the ice is a substantial heat sink reducing energy available for ice melt albedo varies from 0 42 to 0 56 within the experimental site and causes relatively large differences in ablation at stakes close to each other small scale albedo variations should therefore be carefully sampled for large scale energy balance calculations

mesozoic and cenozoic extension recorded by metamorphic rocks in the funeral mountains california the funeral mountains metamorphic core complex like so many of these complexes in the north american cordillera contains early ductile structures related to mesozoic compressional tectonics that have been overprinted by structures related to later extension the core bounding detachment system formed in late miocene time and ductile extensional fabrics in its footwall have been viewed as evidence of a higher temperature stage in the same event integrated structural analysis and geochronology however indicate that ductile fabrics with a transport direction similar to the northwest direction of movement on the detachment are part of an older extensional event of late cretaceous age this event produced mylonitic fabrics a dominant foliation and well developed mineral and stretching lineations mesoscopic folds and a widespread crenulation lineation that developed during a late stage in this event also are consistent with northwest directed shear late cretaceous mylonitic fabrics are related to extensional shear zones within the core complex that have small stratigraphic offsets but may represent significant structural and metamorphic discontinuities rocks in the footwall of the shear zones experienced mid crustal depths that cannot be explained by the overburden available if the funeral mountains core rocks represent an intact block of mesozoic crust instead it appears that a significant section of mid crustal material may have been excised in late cretaceous time by the combined monarch canyon and chloride cliff shear zones as the result of gravitational instability of an overthickened crust in our interpretation this early extension was made possible by a waning of compression within the mesozoic thrust belt combined with a high regional heat flow due to the sierran arc this early extension took place 50 m y before development of the core bounding detachment system movement on which took place at low enough temperatures that deformation was predominantly brittle with some calc mylonite fabric development the youngest structures in the core are normal faults that cut the detachment and indicate a southwest transport direction orthogonal to those of both the earlier extensional events

the mahogany peaks fault a late cretaceous paleocene normal fault in the hinterland of the sevier orogen the contact separating ordovician rocks from the underlying lower part of the raft river mountains sequence northwestern utah is reinterpreted as a large displacement low angle normal fault the mahogany peaks fault that excised 4 5 km of structural section high 13c values identified in marble in the lower part of the raft river mountains sequence suggest a proterozoic rather than cambrian age metamorphic conditions of hanging wall ordovician and footwall proterozoic strata are upper greenschist and middle amphibolite facies respectively and quantitative geothermometry indicates a temperature discontinuity of about 100 c a discordance in muscovite 40ar 39ar cooling ages between hanging wall and footwall strata in eastern exposures and the lack of a corresponding cooling age discordance in western exposures suggest a component of west dip for the fault the juxtaposition of younger over older and colder over hotter rocks the muscovite cooling age discordance with older over younger and top to

timing of detachment faulting in the bullfrog hills and bare mountain area southwest nevada inferences from 40ar 39ar k ar u pb and fission track thermochronology crustal extension in the bullfrog hills and bare mountain area of southwest nevada is associated with movement along a regional detachment fault normal faulting in the upper plate and rapid cooling denudation of the lower plate were coeval with miocene silicic volcanism and with west northwest transport along the detachment fault a west northwest progression of tilting along upper plate normal faults is indicated by ages of the volcanic rocks in relation to angular unconformities near the breakaway tilting in the upper plate occurred between 12 7 and 11 6 ma continued less strongly past 10 7 ma and was over by 8 2 ma ten to 20 km west of the breakaway tilting occurred between 10 7 and 10 33 ma continued less strongly after 10 33 ma and was over by 8 1 ma the cooling histories of the lower plate metamorphic rocks were determined by thermochronologic dating methods k ar and ar 40 39 a on muscovite biotite and hornblende ar 40 39 a on k feldspar u pb on apatite zircon and sphene and fission track on apatite zircon and sphene lower plate rocks 10 km west of the breakaway cooled slowly from early cretaceous lower amphibolite facies conditions through 350 50 degrees to 300 50 degrees c between 57 and 38 ma then cooled rapidly from 205 50 degrees to 120 5o degrees c between 12 6 1 6 and 11 1 1 9 ma lower plate rocks 20 km west of the breakaway cooled slowly from early cretaceous upper amphibolite facies conditions through 500 50 degrees c at 78 67 ma passed through 350 50 degrees to 300 50 degrees c between 16 3 0 4 and 10 5 0 3 ma then cooled rapidly from 285 50 degrees to 120 50 degrees c between 10 2 and 8 6 ma upper plate tilting and rapid cooling denudation of the lower plate occurred simultaneously in the respective areas the early slow cooling part of the lower plate thermal histories was probably related to erosion at the earth s surface which stripped off about 9 km of material in 50 to 100 m y the results indicate an initial fault dip greater than or equal to 30 degrees and a 12 mm yr 1 west northwest migration of the locus of rapid tilting in the upper plate

late cretaceous extension in the hinterland of the sevier thrust belt northwestern utah and southern idaho cover rocks of the raft river metamorphic core complex located in the sevier belt hinterland preserve a structural and metamorphic history that predates the middle tertiary extension of the region in the eastern raft river mountains cambrian permian rocks form two allochthons that occur in the hanging wall of the mid miocene raft river detachment fault dramatically attenuated metamorphosed cambrian pennsylvanian strata of the lower allochthon yield late cretaceous muscovite sup 40 ar sup 39 ar plateau ages ca 82 90 ma that are interpreted to date cooling during final stages of layer parallel extension devonian permian rocks exposed in the black pine mountains underwent east west extension approximately 160 associated with syntectonic growth of white mica along cleavage sup 40 ar sup 39 ar whole rock spectra of associated slates also indicate a late cretaceous metamorphism together field and sup 40 ar sup 39 ar results suggest that late cretaceous extension occurred in the sevier belt hinterland at the same time as shortening in the eastern foreland and at depth in the hinterland sufficient topography must have been present to drive upper crustal extension in the eastern hinterland

thermal effects of exhumation of a metamorphic core complex on hanging wall syn rift sediments an example from the rechnitz window eastern alps the removal of overburden by tectonic processes displaces the rock masses and their temperature with respect to the surface consequently high cooling rates and increased heat flow have been reported from rapidly exhuming terrains previous studies were mainly focused on the greenschist and amphibolite facies tectono thermal evolution of the footwall a two dimensional numerical thermal model presented in this study concentrates on the thermal history of the hanging wall and the footwall blocks and evaluates the variation of the near surface heat flow above an evolving metamorphic core complex based on structural thermobarometric and geochronological constraints from the rechnitz core complex of the eastern alps the modelling suggests that rapid movement along the extensional fault took place between 22 and 17 ma during this period the heat flow reached values of 140 150 mw m 2 in the hanging wall in a 10 15 km wide zone parallel to the fault trace the increased heat flow caused the high coal rank in the miocene syn rift sediments deposited on the hanging wall apatite fission track geochronology of the thermally overprinted sedimentary deposits gives the time at which the bottom of the syn rift sediment pile passed through 110oc 13 6 0 9 ma the vitrinite reflectance data rr 0 67 1 04 suggest significant burial of the margin of the hanging wall at the time of core complex formation considering the coal rank data and the thermal conductivity of the sedimentary deposits by modelling of the organic maturation we calculate syn rift early middle miocene burial of the hanging wall margin of the rechnitz core complex to be 1100 1600 m this value shows the importance of the post middle miocene erosion of the region which was related to the inversion of the pannonian basin 1998 elsevier science b v all rights reserved

a simple energy balance model to calculate ice ablation at the margin of the greenland ice sheet apres avoir etabli les relations donnees climatiques taux d enlevement de la glace dans 2 glaciers du groenland un modele simple de bilan energetique est effectue pour calculer le taux d enlevement de la glace aux bordures du glacier

isometamorphic variations in the reflectance and fluorescence of vitrinite a key to depositional environment abstract petrographic investigations of serial ply samples from five high to medium volatile bituminous coal seams from australia 4 and canada 1 reveal substantial in seam variations in the reflectance and monochromatic microfluorescence intensities of the maceral subgroup telovitrinite the variations consist of one case of reflectance enhancement and fluorescence suppression and four cases of reflectance suppression and fluorescence enhancement the single case of reflectance enhancement and fluorescence suppression is due to the oxidation of the vitrinite nuclei at the sequence boundary between the bayswater and upper wynn seams in new south wales the four cases of reflectance suppression and fluorescence enhancement result from the syn and epigenetic absorption by the vitrinite nuclei of hydrogen donated by presumably anaerobic bacteria generated lipids two of the coals are marine influenced the liskeard seam from the bowen basin by combined syngenetic and epigenetic effects and the greta seam from the sydney basin mainly by epigenetic contact with sea water for both coals the results are strong vitrinite reflectance suppression and fluorescence enhancement the remaining two coals the bulli seam from the sydney basin and a coal seam from the gates formation in british columbia show moderate epigenetic effects on the optical properties of telovitrinite by fresh water in the bulli seam which was studied in two adjacent localities the reflectance suppression and fluorescence enhancement of telovitrinite are stronger under sandstone roof than under shale roof in some cases the epigenetic effects are superimposed on syngenetic telovitrinite reflectance and fluorescence variations resulting from the cogeneration and mixing of different telovitrinite precursors for example autochthonous roots and hypautochthonous or allochthonous shoots a measure of the degree of dispersal and mixing is the coefficient of variation of telovitrinite reflectance and or fluorescence this coefficient correlates well with detrital minerals and dispersed macerals e g inertodetrinite and to a lesser extent sporinite some comments are made on slitted so called pseudovitrinite which is regarded as a telovitrinite that was subjected to very weak post coalification desiccation and possibly oxidation without losing much of its thermoplastic properties

steady state characteristics of the greenland ice sheet under different climates the greenland ice sheet is modelled to simulate its extent and volume in warmer climates and also to find out whether the ice sheet would re form on theice free bedrock under present climatic conditions the ice sheet model is a three dimensional thermo mechanical model with a fine resolution grid thebedrock surface beneath the ice sheet was mapped using radio echo sounding measurements by the electromagnetic institute copenhagen the modelexperiments show that increased temperature will result in ice margin retreat but the ice sheet is relatively stable it takes a rather high temperature rise of atleast 6ic for the ice sheet to disappear completely which indicates that the ice sheet probably survived the last interglacial also it appears that the greenlandice sheet is not a mere relict ice mass from a previously colder climate but that the ice sheet will still re form on the bare bedrock under the present or evenslightly warmer climatic conditions

the 40ar 39ar geochronology of the pelona schist and related rocks southern california seventeen 40ar 39ar ages for hornblende celadonitic muscovite and biotite from the pelona orocopia rand and portal ridge por schists range from 39 to 85 ma the 85 ma is from portal ridge consistent with the proposed correlation of that body with the schist of sierra de salinas two muscovites and one hornblende from the rand schist have ages of 72 to 74 ma indistinguishable from the k ar age of 74 ma for hornblende from a posttectonic granodiorite that intrudes the schist but younger than the 79 ma u pb age of the intrusion four muscovite and two hornblende ages for schist and mylonite from the east fork area of the san gabriel mountains range from 55 to 61 ma similar to k ar ages from nearby upper plate rocks a third hornblende has an age of 73 ma but is not well constrained concordance of schist and upper plate ages confirms structural and metamorphic evidence that the vincent thrust in the san gabriel mountains has not undergone significant postmetamorphic disruption ages from the orocopia mountains are 75 ma for hornblende from nonmylonitic upper plate 52 ma for muscovite from structurally high orocopia schist that is mylonitic and 41 ma for muscovite from nonmylonitic orocopia schist these are consistent with field evidence that the orocopia thrust is a postmetamorphic normal fault muscovite and hornblende from the gavilan hills have ages of 48 to 50 ma younger than ages from the san gabriel mountains but similar to schist ages from the orocopia mountains the geochronologic and structural complexities of the vincent chocolate mountains orocopia and rand thrusts imply that previously cited northeastward vergence may not relate to prograde metamorphism subduction of the por schists the 40ar 39ar data indicate substantial uplift of the por schists prior to middle tertiary detachment faulting which confirms other geochronologic evidence of uplift in southern california and southern arizona during the late cretaceous early tertiary

deformation of granitoids at low metamorphic grade i reactions and grain size reduction abstract deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones two from the alps corvatsch granodiorite and aiguilles rouges granite switzerland and one from eastern australia wyangala granite when examined by light and electron microscopy all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite quartz deformed plastically while both k feldspar and plagioclase fractured and recrystallized in conjunction with chemical change which had as its end point an assemblage albite quartz white mica caal silicates k feldspar and calcic plagioclase depending on fluid chemistry from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid since ca bearing plagioclase was not stable in these environments myrmekite did not replace k feldspar in any of the rocks examined recrystallization of feldspar which should perhaps be termed neocrystallization was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets in old feldspar grains particularly in plagioclases dislocations exist in wall like structures that are commonly associated with voids suggesting origins in microfractures these dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization we conclude that recrystallization mainly occurred via a classical nucleation mechanism with minor contributions from twin boundary migration we argue that the 450 550 c lower limit for recrystallization of feldspar referenced throughout the literature is not applicable to the rock systems under investigation and should be discarded as a universal limit microfracturing microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades see part ii of this paper neocrystallization fluid access and nucleation appear to have accompanied or immediately followed the first deformation increments and were primarily responsible for continued grain size reduction transgranular fracture was comparatively important only at the onset of deformation since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade in the presence of aqueous fluids in the earth s crust breakdown reactions and nucleation recrystallization in feldspars may be important influences upon localization of deformation in granitoids

western old woman mountains shear zone evidence for late ductile extension in the cordilleran orogenic belt rocks within the 1 km thick western old woman mountains shear zone wsz contain ductilely deformed quartz and ductilely and brittlely deformed feldspars indicating greenschist to lower amphibolite facies mylonitization foliation measured in 73 ma mid crustal granitoids within the zone generally dips west southwest and sense of shear indicators demonstrate top to the west sense of movement parallel to gently southwest plunging lineation the tip of the shear zone is not exposed but 10 km to the west unmetamorphosed late cretaceous age upper crust is present the wsz is thus most simply interpreted as a normal sense low angle shear zone timing of deformation is constrained to the interval 73 to ca 65 ma by the age of deformed granitoids and sup 40 ar sup 39 ar chronology this interval coincides with a period of rapid cooling of rocks now exposed in the old woman mountains that probably resulted from 4 to 8 km of unroofing the authors suggest that movement along the wsz is responsible for at least some of this unroofing the proposed history involves tectonic denudation along a low angle ductile shear zone and is similar to that demonstrated for tertiary cordilleran metamorphic core complexes

avhrr surface temperature and narrow band albedo comparison with ground measurements for the greenland ice sheet an ice surface temperature retrieval algorithm for the greenland ice sheet was developed using noaa 11 thermal radiances from channels 4 and 5 temperature pressure and humidity profiles cloud observations and skin temperatures from the swiss federal institute of technology eth camp located at the equilibrium line altitude at 49 deg17 min w 69 deg 34 min n were used in the lowtran 7 model through a statistical analysis of daily clear sky profiles the coefficients that correct for the atmospheric effects were determined for the eth camp field season may to august surface temperatures retrieved by this method were then compared against the in situ observations with a maximum difference of 0 6 k the noaa 11 narrow band planetary albedo values for channels 1 and 2 were calculated using pre launch calibration coefficients scattering and absorption by the atmosphere were modelled with lowtran 7 then narrow band albedo values for the avhrr visible and near infrared channels were compared with in situ high resolution spectral reflectance measurements in the visible band 580 680 nm avhrr derived narrow band albedo and the in situ measurements corrected with radiative transfer model lowtran 7 showed a difference of less than 2 for the near infrared channel 725 1100 nm the difference between the measured and modelled narrow band albedo was 14 these discrepancies could be either the result of inaccurate aerosol scattering modelling lack of the in situ observation or the result of sensor drift due to degradation

paleogeographic and paleotectonic setting of laramide sedimentary basins in the central rocky mountain region in the rocky mountain region between central montana and central new mexico sedimentologically isolated nonmarine basins were produced by basement deformation during the laramide orogeny within the area formerly occupied by a broad late cretaceous foreland basin in which laterally continuous marine facies had accumulated previously laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere laramide basins include perimeter basins along the cratonic periphery of the arcuate laramide province axial basins along a north south intramontane trend and ponded basins located farther west closer to the overthrust belt twelve specific stratigraphic and sedimentologic criteria for the onset duration and termination of laramide deformation allow the chronology of basin development to be inferred independently for each basin maastrichtian initiation of laramide deformation was approximately synchronous throughout the laramide province but termination of laramide deformation was systematically diachronous from north to south between early and late eocene time widespread eocene erosion surfaces truncate syntectonic laramide sequences and are overlain by largely volcanic and volcaniclastic post laramide strata of eocene age in the north and oligocene age in the south fluvial depositional systems draining toward the great plains were dominant in perimeter and axial basins but ponded basins were occupied at times by large lakes that served as regional sediment traps paleocene drainages from ponded basins also led eastward toward the continental interior but partly interconnected lakes that developed within several ponded basins by mid eocene time were either closed hydrologically or else they drained westward into the tyee paleoriver of the pacific northwest

u th pb rb sr and ar ar mineral and whole rock isotopic systematics in a metamorphosed granitic terrane southeastern california mesozoic structural domes are developed in an older proterozoic crystalline basement of granitic to granodioritic foliate metaplutonic rocks in the halloran hills southeastern california isotopic analyses of whole rocks and mineral separates from these rocks by u th pb rb sr and ar ar techniques yield a complex pattern of discordance that is the result of a fairly simple geologic history individual mineral isotopic systems have variably equilibrated with each other in response to mesozoic regional metamorphism and locally to later heating during mesozoic batholith emplacement discordant u th pb zircon data indicate that the granitic core rocks are 1 710 ma and that one dioritic phase may be slightly older rb sr whole rock model dates scatter about 1 700 ma rb sr amphibole whole rock and u th pb amphibole dates are also proterozoic potassium feldspars retain a 207 pb 206 pb signature of their proterozoic age ar ar amphibole spectra from the flank of the main dome reveal disturbed dates of 1 450 ma to 1 100 ma and the dates become younger toward the structurally deeper core of the dome all remaining isotopic determinations yield mesozoic or younger dates for mineral whole rock systems rb sr whole rock apatite feldspar biotite analyses show nonequilibration of strontium isotopes with resultant mineral pair dates from 4 foliate plutonic rocks ranging from 200 to 50 ma no single metamorphic age is indicated by the rb sr data rb sr whole rock biotite dates are consistently younger than any other determinations and may be reduced by weathering or gain of nonradiogenic strontium from ground water u pb sphene and apatite analyses from rocks that yield 1 710 ma zircon dates are nearly concordant at 140 ma an amphibole from the structurally deepest rocks of the main dome that yield 140 to 150 ma u pb sphene dates has an ar ar plateau date of 144 ma the u pb sphene and ar ar amphibole analyses are believed to be the best age estimate for the end of the highest temperature phase of regional metamorphism th pb sphene and apatite dates and ar ar biotite dates cluster at 90 5 ma as a consequence of regional cooling during late cretaceous time following extensive mesozoic plutonism in the region at 97 to 90 ma we interpret the discordant mineral date patterns to have resulted from metamorphism of 1 700 ma plutonic rocks during the jurassic 140 50 ma and subsequent uplift and cooling to 200 c at about 90 ma on the basis of this study the isotope dating systems ranked in decreasing order of resistance to resetting are u th pb zircon concordia intercept rb sr whole rock rb sr amphibole u th pb amphibole pb pb whole rock ar ar amphibole rb sr sphene u pb sphene and apatite rb sr plagioclase potassium feldspar apatite th pb sphene and apatite ar ar biotite u pb feldspars rb sr biotite

the pinto shear zone a laramide synconvergent extensional shear zone in the mojave desert region of the southwestern united states the pinto shear zone is one of several late cretaceous shear zones within the eastern fringe of the mesozoic magmatic arc of the southwest cordilleran orogen that developed synchronous with continued plate convergence and backarc shortening we demonstrate an extensional origin for the shear zone by describing the shear zone geometry and kinematics hanging wall deformation style progressive changes in deformation temperature and differences in hanging wall and footwall thermal histories deformation is constrained between w74 and 68 ma by 40 ar 39 ar thermochronology of the exhumed footwall including multi diffusion domain modeling of k feldspar we discount the interpretations applied in other areas of the mojave desert region that widespread late cretaceous cooling results from refrigeration due to subduction of a shallowly dipping laramide slab or to erosional denudation and suggest alternatively that post intrusion cooling and exhumation by extensional structures are recorded widespread crustal melting and magmatism followed by extension and cooling in the late cretaceous are most consistent with production of a low viscosity lower crust during anatexis and or delamination of mantle lithosphere at the onset of laramide shallow subduction q 2005 published by elsevier ltd

relationship between polarity of extensional fault arrays and presence of detachments abstract extensional fault arrays are often dominated by a single fault vergence forming domino fault blocks the polarity of faults can be defined as synthetic or antithetic in relation to the overall shear sense across the fault system observations of the geometry of a number of field examples of extensional fault arrays vs the lithologies as a proxy for strength profile show that synthetic extensional arrays from the north sea and bristol channel basin detach on salt layers whereas examples of antithetic arrays from the north sea are found to pin out downwards and no basal detachment is present in the sacramento mountains core complex of the basin and range the major low angle detachment fault post dates antithetic shears that evolved at mid crustal level and are preserved in the granitic footwall these antithetic shears were cut by the main low angle detachment whose hanging wall disintegrated into an array of synthetic faults the kinematics of synthetic vs antithetic arrays dictate that faults within synthetic arrays must branch onto a basal detachment whereas faults within antithetic arrays may die out downwards therefore the results emphasise that fault polarity in domino arrays may be related to the boundary conditions of the fault blocks and therefore may be diagnostic of the strength profile of the faulted stratigraphy

degree day glacier mass balance modelling with applications to glaciers in iceland norway and greenland a degree day glacier mass balance model is applied to three glaciers in iceland norway and greenland for which detailed mass balance measurements are available over a period of several years model results are in good agreement with measured variations in the mass balance with elevation over the time periods considered for each glacier in addition the model explains 60 80 of the year to year variance in the elevation averaged summer season mass balance measurements on the glaciers using a single parameter set for each glacier the increase in ablation on the glaciers due to a warming of 2 c is predicted to range from about 1 m w e year 1 at the highest elevations to about 2 5 m w e year 1 at the lowest elevations predicted changes in the winter balance measured between fixed date are relatively small except at the lowest elevations on the icelandic and norwegian glaciers where the winter balance is significantly reduced equilibrium line altitudes are raised by 200 300 m on the icelandic and norwegian glaciers except at the highest elevations the winter balance of the icelandic and norwegian glaciers is predicted to decrease even if the warming is accompanied by a 10 increase in the precipitation no firm evidence of a climate related variation in the degree day factors or in the temperature lapse rate on the same glacier could be found the model furthermore reproduces large variations in the mass balance with elevation and from year to year on each glacier using the same parameter set we assume therefore that these parameters will not change significantly for the climate scenarios considered here

footwall rocks to the mid tertiary chemehuevi detachment fault a window into the middle crust in the southern cordillera crystalline rocks unroofed from the middle crust during tertiary crustal extension by the chemehuevi detachment fault record a complex history of comprises both subhorizontal and subvertically foliated mylonitic gneisses sharing a common gently sw plunging stretching lineation which are intruded by members of the voluminous laccolith shaped chemehuevi mountains plutonic suite proterozoic and mesozoic gneisses migmatites and early members of the chemehuevi mountains plutonic suite in the eastern chemehuevi mountains are deformed into a gently southwest dipping zone of protomylonite mylonite and locally ultramylonite in excess of 1 5 km thick a belt of steeply foliated mylonitic gneiss up to 5 km across traceable for 12 km along strike ne sw crops out in the northern part of the range this belt passes abruptly downward into the subhorizontal shear zone exposed in the eastern chemehuevi mountains a preliminary shear sense study indicates ne directed shear within the sub horizontal mylonitic rocks and sinistral shear parallel to the steeply foliated mylonites deformation in these two zones took place under upper greenschist to middle amphibolite facies conditions mylonitic deformation associated with the two shear zones was spanned by intrusion of the chemehuevi mountains plutonic suite cross cutting contacts within the suite indicate that ductile deformation occurred after intrusion of hornblende biotite granodiorite four zircon fractions from this unit are discordant but define a chord with a lower intercept of around 90 ma and an upper intercept of 1900 ma thirteen zircon fractions from two separate undeformed members of the suite biotite granodiorite and porphyritic biotite granodiorite are variably discordant but fractions for each unit define chords that yield latest cretaceous crystallization ages with upper intercepts of 1600 ma together these data constrain the timing of plutonism and ductile deformation with associated thick zones of fabric reorientation and mylonitic transposition to the late cretaceous superimposed on this fabric and involving all rocks types in the footwall are the localized brittle and ductile effects of the mid tertiary extensional deformation movement direction and the timing of mylonitic deformation in the chemehuevi mountains suggest that the effects of the cordilleran fold and thrust belt extends into the chemehuevi mountains region where it in part overlapped with and was heated by plutonism along the eastern margin of the mesozoic magmatic arc during late cretaceous time

new precipitation and accumulation maps for greenland annual total precipitation and the annual accumulation on the greenland ice sheet are evaluated and presented in two maps the maps are based on accumulation measurements of 25 1 pits and cores obtained from the upper accumulation zone and precipitation measurements made at 35 meteorological stations in the coastal region to construct the accumulation map the annual precipitation was split into solid and liquid precipitation components annual total precipitation exceeding 2500 mm w e occurs on the southeastern tip of greenland while the minimum precipitation is estimated to occur on the northeastern slope of the ice sheet the mean annual precipitation for all of greenland is 340 mm w e the largest annual accumulation of about 1500 mm w e is found on the glaciers in the southeastern corner of greenland while the smallest accumulation is found on the northeastern slope of the ice sheet west of danmarkshavn the mean accumulation on the greenland ice sheet is estimated at 310 mm w e the regional difference in accumulation is examined with respect to the 850 hpa m bar level circulation the present surface topography is found to play an important role in determining regional accumulation on the ice sheet

isotopic complexities and the age of the delfonte volcanic rocks eastern mescal range southeastern california stratigraphic and tectonic implications combined u pb zircon rb sr 40 ar 39 ar laser fusion and conventional k ar geochronology establish a late early cretaceous age for the delfonte volcanic rocks u pb zircon analyses define a lower intercept age of 100 5 2 ma that is interpreted as the crystallization age of the delfonte sequence argon studies document both xenocrystic contamination and postemplacement ar loss rb sr results from mafic lavas at the base of the sequence demonstrate compositionally correlated variations in initial 87 sr 86 sr ratios sr i from 0 706 for basalts to 0 716 for andesitic compositions this covariation indicates substantial mixing of subcontinental lithosphere with proterozoic upper crust correlations between rb sr and sr i may result not only in pseudoisochrons approaching the age of the crustal component but also in reasonable but incorrect apparent ages approaching the true age ages obtained in this study require that at least some of the thrust faulting in the mescal range clark mountain portion of the foreland fold and thrust belt occurred later than ca 100 ma and was broadly contemporaneous with emplacement of the keystone thrust plate in the spring mountains to the northeast comparison of the age and rb sr systematics of ash flow tuff boulders in the synorogenic lavinia wash sequence near goodsprings nevada with those of the delfonte volcanic rocks supports a delfonte source for the boulders the 99 ma age of the lavinia wash sequence is nearly identical to the delfonte age requiring rapid erosion transport and deposition following delfonte volcanism

magmatic evidence for neogene lithospheric evolution of the central andean flat slab between 30 s and 32 s abstract geochemical data from andean miocene magmas erupted through the now volcanically inactive flat slab between 30 s to 32 s coupled with geological and geophysical data illuminate details on magmatic and continental lithospheric evolution over a progressively shallowing subduction zone pb sr and nd isotopic and trace element data show that main cordilleran precordilleran and pampean magmas were contaminated in both the mantle and crust and that the nature of the contaminants varied in time and space reflecting tectonic events contaminants included both enriched high lil element high sr and pb and low nd isotopic ratios and depleted low lile element low sr and pb and high nd isotopic ratios types in the western region main cordilleran earlier miocene lavas had contaminants with less enriched signatures than later miocene lavas progressive enrichment is attributed to a increasing amounts of sediment and tectonically eroded crust being subducted into the mantle wedge and b contamination in a thickening main cordilleran lower crust whose composition was progressively enriched this enrichment occurred through addition of upper crust by an intracrustal mixing process driven by a propagating wedge tip associated with westward wedging heating and deformation of crust from beneath the shortening precordillera thrust belt to the east further east magmas erupted through back arc crust have more depleted signatures those erupted in the central part through the evolving precordilleran thrust belt were contaminated by an older thinner grenville 1100 ma basement whose depleted signature is unique among central andean terranes late miocene pocho lavas erupted further east in conjunction with uplift of the sierras pampeanas show enrichment through time arguably these magmas could contain a component mechanically removed from the base of the thinning continental lithosphere to the west and progressively incorporated into the convecting asthenosphere first order mass balance calculations comparing the lithospheric geometry proposed for 20 ma with that today suggests that 60 of the lithosphere within 600 km of the trench at 20 ma has been lost this lithospheric loss is required to accommodate simultaneous shallowing of the oceanic slab and thickening and shortening of the crust continental lithosphere recycled into the asthenosphere could contribute to the sources of the enriched em1 and em2 mantle components in oceanic island basalts

aerodynamic stability and turbulent sensible heat flux over a melting ice surface the greenland ice sheet the turbulent sensible heat flux to a melting ice surface is calculated from wind speed and air temperature at 2 m over the ice surface assuming a certain wind profile with the appropriate surface roughness the aerodynamic stability of the boundary layer over melting ice is examined by comparing sensible heat fluxes for logarithmic and log linear wind profiles where the logarithmic profile is strictly valid only for neutral conditions increasing stability reduces the sensible heat flux to the glacier surface and introduces a non linear relation between heat flux and air temperature the stability effect is greatest at low wind speeds and fairly small at the high wind speeds that are common over the ice sheet earlier estimates of ablation by energy balance modelling may be too large due to neglect of stability but this was almost offset by using a surface roughness that was too small the log linear wind profile should be used in future energy balance models to take account of stability but more research is needed on the parameters of the profile as well as on the surface roughness

vorticity analysis in a metachert from the sanbagawa belt sw japan abstract common tectonic features of deformed metachert include both sets of extended and shortened veins and quartz c axis preferred orientation patterns in such rocks information is also commonly available on the orientation and magnitude of the finite strain these features can be used to derive two independent estimates of the degree of non coaxiality during deformation in an example from the sanbagawa belt sw japan the two methods give consistent results and show that deformation was intermediate between simple and pure shear such types of progressive deformation distributed over a broad region could cause substantial subvertical thinning and be a significant factor in the exhumation of high p t rocks in the region

present and past ice sheet mass balance simulations for greenland and the tibetan plateau net annual mass balance was evaluated for greenland and the tibetan plateau using the meteorological forcings from the ncep reanalysis and two gcms foam1 0 and csm1 4 for modern climate and for different time periods extending back to the beginning of the holocene 11 000 years ago for the climate models the ice sheet budget calculations using the degree day methodology were performed on a finer grid than the model output by interpolating monthly precipitation and surface temperature and correcting the latter to account for the gcm s smoothed topography the computed net mass balance for greenland in the present day is positive and it ranges between 290 300 mm water equivalent w e year for the two models values close to the ncep estimate of 250 mm year the past climate simulations show that the greenland mass balance has become slightly more positive since the beginning of the holocene the tibetan plateau s present day area average net mass balance is negative and ranges between 1200 and 2000 mm w e year for the two models values bracketing the ncep estimate of 1700 mm year although the balance is positive over small regions of the plateau consistent with the existence of small ice caps and glaciers the calculated past mass balance shows an increasingly less negative value for foam from 11 000 years ago towards the present and expansion of the positive mass balance areas mainly due to decreased snow ablation as the summertime insolation decreases with the changes in orbital forcing in csm the opposite trend occurs but changes are smaller and less systematic the result from foam shows that the likelihood of ice sheets developing on the tibetan plateau may have increased since 11000 years ago which is consistent with some glacial records

significance of seismic reflections beneath a tilted exposure of deep continental crust tehachapi mountains california the focus of this article is a process whereby lower crustal crystalline and schistose rocks can rise to the surface with the tehachapi mountains in california being the case in point as a prime example of the lower crust these mountains expose cretaceous gneisses that formed 25 30 km down in the sierra nevada batholith and appear to be underlain by the ensimatic rand schist integrated geophysical and geological studies by the calcrust program have produced a cross section through this post mid cretaceous structure and suggest a general model for its development seismic reflection and refraction profiles show that the batholithic rocks dip northward as a tilted slab and extend beneath the southern end of the san joaquin basin s tejon embayment two south dipping reverse faults on the rim of the tejon embayment were discovered in the reflection data and verified in the field the faults have a combined separation of several kilometers and cut through an upper crustal reflection zone that projects to the surface outcrop of the rand schist the upper and lower crusts are separated by a zone of laterally discontinuous reflectors reflections from the lower crust form a wedge the base of which is a nearly flat moho at 33 km regional geological relations and gravity models both suggest that the reflective zone corresponds to the rand schist and the newly recognized faults account for its neogene exposure alternatively the reflective zone maybe part of the gneiss complex suggesting that the schist either lies deeper or is not present under the gneisses if the rand schist underlies the tehachapi mountains and mojave region to their south a model for their evolution can be constructed from regional geological relations it seems that during late cretaceous laramide subduction the protolith of the schist was thrust eastward beneath the mojave along this portion of the cordilleran batholithic belt the subduction was evidently at very low angles the bottom of the batholith was removed and replaced by a thick section of schist fluids from which weakened the overlying batholith this thickened crust collapsed by horizontal flow in the schist and faulting of the upper crust into flat lying slabs when emplacement of the schist ended in latest cretaceous earliest paleocene the underlying mantle rose compensating for the extension and providing material for magmatic underplating in the neogene transpression and rotation of the upper crust along the san andreas and garlock faults resulted in the exposure of the schist

the origin and evolution of the southern snake range decollement east central nevada regional and local stratigraphic metamorphic and structural constraints permit reconstruction of the southern snake range extensional deformational system in east central nevada the dominant structure of the range the southern snake range decollement ssrd operated during oligocene and miocene extensional deformation to exhume a footwall of multiply deformed metasedimentary and plutonic rocks intrusion of three plutons 160 ma 79 1 0 5 ma and 36 1 ma respectively and development of two cleavages preceded the onset of extensional deformation plastic deformation of lower plate metasedimentary rocks accompanied the early phases of regional extension and produced bedding parallel grain shape foliations and wnw trending stretching lineations these fabrics parallel the ssrd even in low strain domains suggesting that a significant component of pure shear strain probably accompanied noncoaxial deformation associated with motion on the ssrd consistent with other lines of evidence meanwhile hanging wall rocks were greatly extended by at least two generations of tilt block style normal faults soling into the ssrd with the earlier faults antithetic to the ssrd and the later faults dipping in the same direction as the ssrd a retrodeformed regional cross section sequence illustrates plausible alternative schemes for reconstructing the southern snake range extensional system in one scheme the ssrd forms as a crustal scale stretching shear zone separating an upper plate that extends on steeply inclined normal faults from a lower plate that stretches by penetrative flow in the other lower plate deformation incorporates a component of coaxial stretching but the ssrd also functions as a conventional shear zone accommodating through going displacement between opposing plates in either case as tectonic unroofing proceeds differential isostatic unloading induces the ssrd to rotate to steeper dips as it migrates into the frictional sliding regime thus enabling it to remain active as a brittle normal fault until it finally rotates to its present shallow inclination in either scenario cross section constraints suggest that total extension accommodated by the ssrd was probably between 8 km and 24 km

strain and kinematic analysis in general shear zones we present a unifying theory for the development and distribution of strain markers and kinematic indicators in zones of general shear and thus provide a framework in which data previously considered contradictory may be understood rigid and deformable porphyroclast systems including and complex grains are potential indicators of both strain and flow the shapes and distribution of such porphyroclast systems may be used to distinguish among different tectonic regimes general shear is divided into two fields sub simple shear in which the rotational component of the strain is less than that for simple shear and super simple shear in which the rotational component is greater than for simple shear sub simple shear may involve narrowing or broadening of shear zones super simple shear regimes are possible in local regions such as the vicinities of deformable porphyroclasts but must be enclosed by regimes of sub simple shear the polar mohr constructions for finite deformation and flow are useful to analyze general shear in theory the hyperbolic net is employed for practical plotting of real data and derivation of the kinematic vorticity number wn this number represents the relative contributions of pure and simple shear in steady flow in nature deformation is thought to build up and decay by processes that may invalidate the assumption of constant flow regime we therefore introduce the concept of accelerating deformation and analyze the implications of non steady flow for the shearing histories of deformed objects

jurassic magmatism in the central mojave desert implications for arc paleogeography and preservation of continental volcanic sequences exposures of jurassic magmatic rocks in the west central mojave desert provide insight into the changing character of volcanism throughout jurassic time and the paleogeography of the continental arc middle jurassic explosive volcanism lower sidewinder volcanic series resulted in collapse of multiple calderas this was followed by north south extension broadly coeval with batholith emplacement late jurassic effusive volcanism upper sidewinder volcanic series appears to reflect transtension over a larger region during intrusion of the independence dike swarm the lower sidewinder volcanic series consists of a subaerial nested caldera complex with an aggregate thickness of 4 km the first caldera formed during the eruption of crystal poor rhyolite ignimbrite outflow and intracaldera facies are intercalated with quartzose sandstone of probable cratonal provenance the second caldera formed during the eruption of crystal rich rhyolite to dacite ignimbrite this ignimbrite exhibits complex mineralogical zoning and contains two units of probable collapse related mesobreccia within the 1 400 m thick intracaldera sequence a third caldera nested within the second is filled with crystal rich biotite dacite ignimbrite and tuff breccia a fourth caldera largely coincident with the second formed during eruption of lithic and pumice lapilli dacite ignimbrite eruption and collapse appear to have been multistage as several units of reworked tuff and fallout tuff occur within the 1 750 m thick intracaldera sequence and caldera collapse breccias occur at the base and near the middle of the sequence the top of this sequence contains reworked tuffs and epiclastic rocks suggesting that the fourth caldera provided a posteruptive depocenter for accumulation of sediment normal faulting tilting and erosion followed explosive volcanism and was broadly contemporaneous with intrusion of middle jurassic porphyritic quartz monzonite plutons plutons and tilted lower sidewinder volcanic series are intruded and unconformably over lain by latest jurassic volcanic rocks upper sidewinder volcanic series the upper sidewinder volcanic series is characterized by alkalic basalt to basaltic andesite and rhyolite lavas hypabyssal intrusions and dikes including the 148 ma independence dike swarm regional northeast southwest extension is suggested by the presence of the northwest striking dike swarm and by bimodal volcanism the absence of debris flow deposits and epiclastic rocks suggests an intra arc region of low relief although local caldera related subsidence was the principal control on accumulation of lower sidewinder volcanic rocks preservation was likely enhanced by middle jurassic extension late jurassic extension was of broader regional extent but lesser magnitude because it did not create basins or cause significant tilting of strata the geometry timing and regional setting of middle and late jurassic extension and magmatism suggest a sinistral oblique subduction regime for the jurassic arc of the southern u s cordillera

subduction and exhumation of the pelona orocopia rand schists southern california the pelona orocopia and rand schists of southern california and southwestern arizona are thought to have formed in either the same east dipping subduction zone as the franciscan complex or in a southwest dipping subduction zone related to collision of an outboard continental fragment with north america the principal justification for the collision model has been the observation that continental rocks overlying the schists locally show transport to the northeast field and petrographic studies now confirm that the northeast movement occurred during exhumation of the schist not during subduction combined with regional geologic relations this evidence makes the collision model unlikely

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

variations of near surface firn density in the lower accumulation area of the greenland ice sheet pâkitsoq west greenland firn density variations have been studied in the lower accumulation area of the greenland ice sheet 1440 1620 m a s l near p kitsoq west greenland the main control on density in the near surface firn layer of 5 10 m thickness is the formation of ice layers by the refreezing of meltwater that reaches depths of 2 4 m below the surface the density variations are described by the ratio of annual surface melt m to the annual accumulation c the ratio m c is about 0 6 at the run off limit at about 1400 m a s l in the study area where refreezing of meltwater transforms snow into impermeable ice the mean density of near surface firn decreases with elevation reflecting a decrease in melt with elevation there is a surprising decrease in firn density at depths of more than about 4 m below the 1991 summer surface which reflects lower melt rates and or higher accumulation in the early 1980s and late 1970s when this firn was passing through the surface layer the formation of such low density firn may have partially contributed to the 1978 85 thickening of the ice sheet observed by satellite radar altimetry near surface firn density is therefore very sensitive to climate change and might be an attractive target for climate monitoring

experimental modelling of mantled porphyroclasts abstract mantled porphyroclasts are commonly used as sense of shear indicators in mylonites analogue experiments have been carried out to model the development of such objects rigid spheres mantled by a viscous plasticene putty mixture with power law behaviour were suspended in an optically transparent newtonian polymer matrix the samples were deformed between two cylinders one placed within the other and with coinciding axes this induces a non coaxial flow between the cylinders and allows the achievement of high finite strain the viscous mantle around the rigid sphere remained undeformed deformed into an ellipsoid or developed wings the final geometry depends mainly on the induced viscosity contrast between the mantle and the matrix which is a function of the flow induced stress on the surface of the mantle winged mantles only develop if the induced viscosity contrast between the mantle and the matrix polymer is sufficiently small if this critical induced viscosity contrast is exceeded the mantled sphere remains undeformed or obtains a stable ellipsoidal shape even at high finite strain shaped winged mantles only develop in simple shear flow if the mantle around a rigid sphere is thin

laramide crustal thickening event in the rocky mountain foreland and great plains the rocky mountain foreland and great plains of the western united states were formerly part of a continental platform adjusted by erosion and deposition in cambrian through jurassic time to near mean sea level and therefore to a near uniform crustal thickness of approximately 33 km today the region stands at regional elevations up to 2 km isostatically supported by a crust exceeding 50 km in thickness reasonable estimates of tertiary sedimentation and laramide strain do not account for more than 15 of the implied thickening however from approximately 70 40 m y b p this region was underlain by a horizontally subducting slab of farallon plate lithosphere moving northeast this slab could have been the cause of the thickening finite difference thermal models with specified kinematics show only a temporary cooling of the base of the north american lithosphere by this slab the excess weight of the slab would have depressed the region plate bending calculations show a quantitative agreement of predicted depression with upper cretaceous isopachs since depression by this slab lasted until the eocene at least the latest cretaceous regression was probably caused by a laramide crustal thickening event the farallon slab might have caused crustal thickening in two ways its excess weight would have drawn in ductile lower crust from surrounding regions however calculations show that this effect is too slow too local and too reversible to explain most of the crustal thickening therefore it seems likely that ductile lower crust was transported from sw to ne by shear stresses which the farallon plate exerted on the base of the north american lithosphere a preliminary finite element calculation based on this hypothesis shows the correct general pattern of crustal thickening an unexpected but encouraging result is that predicted principal compression directions are orthogonal to many laramide basement uplifts

development of normal faults during emplacement of a thrust sheet an example from the lewis allochthon glacier national park montana u s a abstract geologic mapping in southern glacier national park montana reveals the presence of widespread e dipping normal faults within the basal portion of the lewis allochthon the displacement along the normal faults increases downward from less than 1 m at the highest exposure 200 300 m above the lewis thrust to a maximum of 200 m near or at the lewis thrust the normal faults are located below discrete bedding parallel shear zones associated with mesoscopic structures characterized by ne or sw trending striations on bedding surfaces and asymmetric e verging folds these shear zones lie directly below the e directed brave dog fault a major bedding subparallel fault within the lewis allochthon the shear zones are interpreted to have formed during the development of the brave dog fault striations on the brave dog fault normal faults and shear surfaces in the shear zones are consistent with the transport direction of the lewis thrust the kinematic compatibility of the normal faults with the lewis thrust the concentration of the normal faults along the basal part of the lewis plate and the increase in displacement along them toward the lewis thrust all suggest that their development was synkinematic with eastward emplacement of the lewis allochthon the normal faults may have formed as riedel shears r that accommodated a bulk simple shear strain within the thrust plate between the simultaneously moving subhorizontal brave dog and lewis faults

petrologic constraints on the unroofing history of the funeral mountain metamorphic core complex california this is the published version copyright 1990 american geophysical union all rights reserved

staircase 40 ar39ar spectra of fine grained white mica timing and duration of deformation and empirical constraints on argon diffusion staircase 40 ar 39 ar spectra of 2 to 6 m white micas in ductile carbonate mylonites morcles doldenhorn nappe switzerland record ages between 32 and 13 ma these ages are within independent geochronologic and biostratigraphic time limits for the beginning and ending of deformation of this fold thrust nappe we interpret these data to record protracted white mica neocrystallization and growth during low temperature ductile deformation such that the differences between the maximum and minimum ages in the 40 ar 39 ar spectra record the time span over which mica formed and grew we have modeled the argon concentrations in these micas taking into account the thermal history of the nappe mica growth and diffusional loss of argon our results indicate that available experimental data underestimate the activation energy for argon diffusion in muscovite and that the retentivity of argon in white mica is greater than generally thought

new age controls on initiation and timing of foreland belt thrusting in the clark mountains southern california new geochronologic data from the clark mountain area eastern california better define the regional development of the cordilleran magmatic arc and the timing of hinterland and foreland deformation at this latitude the new ages show that sevier belt thrusting started in the clark mountain thrust complex in late jurassic time this is consistent with other studies that suggest a similar age of initiation of the sevier belt in areas to the north the clark mountain thrust complex is located at the intersection of the mesozoic magmatic arc and the east sierran and sevier thrust belts new u pb zircon ages for granitic rocks establish the timing of deformation within the thrust complex the pachalka thrust structurally the highest fault in the complex carries plutonic rocks eastward above cambrian rocks hanging wall and footwall rocks are mylonitic near the thrust hanging wall plutonic rocks previously considered precambrian are dated at 146 62 ma and thus are late jurassic folds kinematically related to thrusting in footwall metasedimentary rocks are overturned eastward and cut by the pachalka pluton dated at 142 7 ma this brackets thrusting and related deformation in structurally high levels of the complex to be latest jurassic the ivanpah and striped mountain plutons in the ivan pah mountains have been dated at 147 7 ma and 141 11 ma respectively these plutons are cut by the keaney mollusk mine and morning star mine thrusts which are in turn cut by the middle to late cretaceous teutonia batholith these data indicate that these thrusts are early to middle cretaceous in age

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

climate and the initiation of maritime ice sheets the lower latitude extension of polar water during quaternary glaciations has two opposing effects on the mass balances of adjacent man time ice sheets cooler air temperatures reduce ablation and increase the fraction of precipitation falling as snow but also lead to reduced atmospheric moisture content and reduced precipitation the effects of these contrasting processes on ice sheet initiation are investigated using a coupled ice sheet atmospheric moisture model of the loch lomond ice sheet in scotland 1 0000 a a p there is a delicate balance between the degree of cooling required to initiate ice accumulation and that which restricts the flow of moisture over the area at the regional scale of scotland the combined effects of orographic enhancement and the inherent feedback between the rising ice surface elevation and increasing mass balance are dominant and ice sheet growth accelerates comparison of the results from models using different wind directions suggests that south westerly winds were prevalent during the loch lomond glaciation in contrast to the dominant westerly winds of the present day the modelling experi ments demonstrate the sensitivity and complexity of the links between ocean surface cooling and ice sheet growth particularly at the early stages of a glacial cycle

thrust timing growth of structural culminations and synorogenic sedimentation in the type sevier orogenic belt western united states most of the regional shortening in the type area of the sevier orogenic belt in central utah was accommodated by displacement on the canyon range neocomian aptian pavant aptian albian paxton cenomanian campanian and gunnison late campanian paleocene thrust systems inception of each thrust system generated synorogenic sediment associated with frontal thrust tip anticlines or triangle zones and older thrust sheets that were elevated above major ramps farther toward the hinterland the sevier culmination a large antiformal duplex cored by crystalline basement rocks developed during paxton and gunnison thrusting west of and structurally beneath the canyon range and pavant thrusts growth of the sevier culmination was coeval with reactivation of the canyon range and pavant thrust systems and produced a second culmination in proterozoic lower cambrian rocks in the canyon range these structural highs provided much of the sediment to the adjacent foreland basin from late cenomanian to late paleocene time and may have helped to maintain critical taper in the thrust belt