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

geometry mechanisms and significance of extensional folds from examples in the rocky mountain basin and range province u s a abstract geologic mapping and structural analysis in paleogene half graben of idaho and montana have revealed over 70 extensional folds that form orthogonal sets with mostly ne and se trending axes on a regional scale they parallel or lie perpendicular to the strikes of the largest normal faults transverse folds at a high angle to the fault and folds oblique to the fault comprise more than half of the folds and reflect the highly three dimensional nature of the bulk strain detailed geometric analysis of 15 folds in the salmon horse prairie and medicine lodge basins shows that they typically have an upright to steeply inclined bisecting surface an interlimb angle of 141 and a plunge of 19 based on our synthesis of the eight common mechanisms of extensional folding and their distinguishing characteristics we were able to determine how some of the investigated folds formed fault bend folding above both fault parallel and fault perpendicular bends in the underlying normal fault produced most of the folds in the study area but displacement gradients along normal faults fault drag isostatic adjustments and transtension were also factors in the deformation most of the compound folds which result from more than one mechanism of folding are oblique to all adjacent normal faults recognition of such extensional folds is critical because they might be misinterpreted as contractional structures they influence sediment thickness patterns and dispersal in rift basins and may control the migration and trapping of petroleum and groundwater resources

calcite twin widths and intensities as metamorphic indicators in natural low temperature deformation of limestone abstract twins in naturally deformed coarse grained limestones of the northern subalpine chain france show a direct relationship of twin width and intensity number of twins per mm to the peak metamorphism under diagenetic and anchimetamorphic conditions the transition from dominant thin twinning to dominant thick twinning occurs between a mean vitrinite reflectance ro of 1 60 and 2 60 or temperatures of between 150 and 200 c below the transition temperature increasing strain is apparently accommodated by adding new twins and above the transition temperature increasing strain is accommodated by twin widening similar transitions from thin to thick twinning with increasing metamorphism are also present in the helvetic alps the prealps and the central appalachian valley and ridge province these data for naturally deformed limestones support the interpretation from experimental studies of a temperature effect on twin width and intensity and indicate that the twinning in the northern subalpine chain occurred at or near peak metamorphism

palinspastic reconstruction and topographic evolution of the eastern alps during late tertiary tectonic extrusion this paper presents a new palinspastic restoration of the eastern alps for neogene time and an attempt to reconstruct the neogene palaeogeology palaeotopography and palaeohydrography in connection with the structural evolution the eastern alps underwent radical horizontal displacement during the neogene due to large strike slip systems and formation of structural windows our palinspastic reconstruction considers a the rearrangement of tectonic units dismembered during tectonic extrusion b the tectonic denudation driven by displacement of the crystalline blocks c geochronological arguments and d the sedimentary record of the syn extrusion basins the rearrangement of tectonic blocks results in a remarkably good fit of highly dismembered zones both in crystalline and sedimentary areas and shows the pre miocene unstretched pattern of the eastern alps reduced to 65 of its present day e w elongation using this structural frame and considering the sedimentary record a set of palaeogeologic and palaeotopographic sketch maps with the palaeo river systems is presented for three time slices pre syn and post extension situation in late oligocene and early miocene times the western eastern alps were already mountainous whereas the eastern part of the orogen formed lowlands or hilly areas enhanced block movement in the course of the extrusion process around the early middle miocene boundary led to the formation of intramontane sedimentary basins and a fault induced reorientation of the drainage pattern which forms the basis of the modern river system in the area east of the tauern window this region where pre miocene land surfaces are preserved probably became a mountainous area not before late miocene time and never reached the elevations of the areas further west

geology and geochemistry of wall rock alteration at the carlin gold deposit nevada the carlin disseminated gold deposit occurs in an autochthonous sequence of paleozoic sedimentary rocks exposed in a structural window in the roberts mountains thrust in north central nevada the upper 175 m of the silurian roberts mountains formation hosts the majority of ore at carlin and is characterized by laminated fine grained calcareous and or dolomitic argillaceous siltstone with local coarser grained siltstones and 50 cm thick lenticular interbeds of sand and granule sized calcareous bioclastic debris or fossil hash detailed studies of drill core and exposures in the east pit of the carlin mine show that alteration and mineralization are zoned away from crosscutting fault conduits and these more permeable bioclastic beds indicating that these two features were major inflow zones for hydrothermal fluid in unoxidized rocks unaltered calcareous siltstone 1 containing quartz dolomite calcite illite k feldspar and pyrite is progressively converted to assemblages of 2 quartz dolomite calcite illite pyrite 3 quartz dolomite illite k mica pyrite 4 quartz illite k mica pyrite and 5 quartz kaolinite dickite pyrite adjacent to inflow zones where jasperoids are developed gold most consistently enriches the zone of calcite and dolomite removal 3 and 4 above though it occurs in all zones locally in high concentrations this zoned alteration was accomplished by a co 2 rich acidic fluid this acidic alteration enhanced the passage of fluids by extensive carbonate removal to form zones of higher permeability oxidation is wholly a supergene effect related to deep weathering because the oxidation is superimposed on both mineralized and altered rocks with only minor effect on the major element chemistry it has produced low temperature goethitic fe oxides rather than higher temperature hematite and is not spatially related to au distribution at the mine or on a district scale because of extensive carbonate removal leading to local volume reduction through collapse and or compaction geochemical effects are examined using ratios to relatively immobile elements such as al and ti extensive depletion of ca mg aud co 2 and introduction of si au and s have occurred potassium is depleted in the conversion of illite to dickite kaolinite in proximal silicified inflow zones and fe enriches some pyritized rock carbonate removal and silicification are two separate processes both of which are spatially associated with mineralization mineralized decarbonated rocks and barren footwall rocks commonly are not silicified and intensely silicified proximal alteration zones are generally low grade

kinematics of orocline and arc formation in thin skinned orogens the regional traces of folds faults and foliations found in many fold thrust belts are bent in plan view it proves valuable to distinguish between two types of bent orogens based on their kinematic evolution oroclines or rotational arcs are bent orogens in which segments of the orogen change strike during the evolution of the bend and nonrotational arcs are bent orogens in which segments of the orogen do not change strike during development of the bend the kinematic evolution of a bend that forms in a thin skinned orogen can be described in terms of three parameters the displacement path trajectories followed by points along the strike of the orogen the magnitude and distribution of tangential extension along the strike of the orogen and the change in position of the endpoints of the orogen with respect to a reference line only slight differences in the displacement path trajectory pattern determine whether an orogen evolves as a nonrotational arc or as an orocline thus the distinction between these two types of bends is not always of major tectonic significance compressional deformation along irregular continental margins or the impact of indentors on continental margins during collisional orogenies more likely leads to formation of nonrotational arcs as can be simulated with a sand wedge model interaction between a fold thrust wedge with obstacles in the foreland or with a wrench fault more likely leads to orocline formation orocline formation is also associated with noncoaxial reactivation of thrust faults tangential extension is a necessary consequence of certain displacement path trajectory patterns and can accompany development of either nonrotational arcs or oroclines two examples one from the umbrian apennines and one from the makran range demonstrate how movement on cross strike fault arrays can accommodate tangential extension in thin skinned bends

estimates of modern arc parallel strain rates in fore arcs deflections of slip vectors of interplate thrust earthquakes from expected directions are used to estimate arc parallel strain rates within the overriding plates at the world9s major convergent plate margins arc parallel extension strain rates between 10 8 yr and 10 7 yr and significant at 2 standard deviations are observed in the fore arcs of the aegean aleutian mariana sumatran southern kuriles new hebrides scotia and southern central american lat 8 to 12 n subduction zones and the himalayas northern chile lat 17 to 31 s and central america lat 11 to 18 n show arc parallel compression available geologic and geodetic estimates of fore arc slip and strain rates agree within a factor of two with slip vector estimates arc parallel strain in fore arcs is rapid enough to produce geologically significant effects such as unroofing of high grade metamorphic rocks and disruption of transported fore arc terranes fore arcs deform even where convergence is perpendicular to curved margins demonstrating that head on subduction can produce a three dimensional strain field

the middle mountain shear zone southern idaho kinematic analysis of an early tertiary high temperature detachment the middle mountain shear zone is exposed in the idaho portion of the albion raft river grouse creek metamorphic core complex the structure strikes north northeast south southwest and dips shallowly to the west it is defined on the basis of well developed mylonitic fabrics and had an initial thickness of at least 4 km at least some movement on the shear zone occurred at depths of 22 26 km during eocene amphibolite facies metamorphism but shear zone fabrics may be found in some granite bodies of late oligocene age the zone may thus have been active in the ductile regime over a period of as much as 10 15 m y mylonitic rocks within the middle mountain shear zone display both type i and type ii s c fabrics augen structures pressure shadows and quartz c axis orientations are strongly asymmetric these fabrics and textures indicate that deformation within the zone was characterized by noncoaxial laminar flow during which the hanging wall of the structure moved down and to the west northwest relative to the footwall we interpret the middle mountain shear zone as a major detachment that played an important role in middle tertiary extension of this part of the eastern great basin the top to the west displacement sense contrasts with late cenozoic predominantly top to the east extension in this area implying a major change in extension geometry in early to middle miocene time

kinematic analysis of a large scale leading edge fold lost river range idaho abstract the mahogany creek buck creek structure within the lost river range idaho is a large scale leading edge fold which developed in late paleozoic outer shelf stratigraphy during the sevier orogeny incremental strain histories determined from antitaxial fibrous pressure shadows are used to quantify temporal variations in the magnitude and orientation of elongation as a function of structural position around the leading edge of this northward and eastward propagating blind thrust plane strain is indicated by coaxial up dip extension on cleavage planes on the backlimb of the structure the bluebird mountain formation a calcareous sandstone has deformed by flexural flow whereas the forelimb of the structure records spin through a fixed steeply plunging incremental extension direction the flat limb ahead of the anticline exhibits top to the foreland simple shear in contrast the underlying surrett canyon formation a thick bedded limestone has experienced flexural flow localized within thin zones on both limbs of the anticline samples from the anticlinal hinge zone suggests kinematic partitioning with pin lines which were variably distributed within each unit and which were temporally transient finite elongations are greatest in the hinge and above the foreland flat in the bluebird mountain formation but are negligible in the surrett canyon formation except within the mechanically active interbeds of the hanging wall and within the footwall adjacent to the fault along the northernmost exposures strain histories in the hanging wall do not vary significantly along strike while the footwall varies from undeformed in the south to penetratively deformed in the north thus the strain data are consistent with self similar along strike fold development in the hanging wall during strike parallel ramp propagation forward thrust propagation may have been blunted by a combination of fold accommodated shortening and footwall deformation to the north once the fold had tightened and fold shortening required greater differential stress forward fault propagation resumed transporting the anticline onto the upper flat with little change in hanging wall fold geometry or strain distribution

orogen parallel and orogen perpendicular extension in the central nepalese himalayas north dipping gravitationally driven extensional structures that unroof the high grade core of the himalayas have been recognized in several parts of the orogen recent work in west central nepal demonstrates that the extensional history at the top of the greater himalayan metamorphic sequence in this region is laterally variable in the upper marsyandi valley there is evidence for three generations of early to middle miocene west directed orogen parallel extensional structures the study area consists of two tectonostratigraphic sequences the lower sequence consists of a 12 km thick section of high grade metasedimentary rocks of the greater himalayan sequence the higher sequence consists of a 4 km thick section of medium grade marble of the tibetan sedimentary sequence it overlies the greater himalayan sequence in the marsyandi valley along a northwest dipping ductile shear zone the chame detachment mylonitic fabrics in footwall rocks show evidence of west directed displacement at sillimanite grade through greenschist grade conditions a cataclastic zone indicates progressively more brittle deformation at the eastern end of the outcrop trace of the chame detachment in the dudh khola valley 15 km west of the marsyandi valley the tibetan sedimentary sequence is juxtaposed with the greater himalayan sequence by an early miocene north dipping ductile shear zone the dudh khola detachment that has well developed north trending stretching lineations formed during sillimanite grade metamorphism north directed orogen perpendicular extension in the dudh khola region is interpreted as coeval with west directed orogen parallel extension in the marsyandi region along strike variations in the orientation of extensional strain at the top of the greater himalayan sequence may be explained by lateral crustal thickness gradients

alteration associated with gold deposition at the getchell carlin type gold deposit north central nevada wall rock alteration at the getchell underground deposit was examined to determine the effects of au bear ing fluids on host lithologies and the relationship between k bearing alteration minerals and au deposition the major minor and trace element geochemistry of highly altered and mineralized to unmineralized rocks from the getchell deposit was quantified for more than 50 samples collected along 13 transects through calcareous siltstone and carbonaceous limestone and along one transect through a rhyodacite dike each transect in sedimentary rocks was collected along a single homogeneous bed that could be followed from high grade ore to moderately altered rock or waste rock analyses were obtained for 39 elements 10 oxides and loss on ignition using multiple techniques petrographic studies were integrated with geochemistry and x ray diffraction and electron microbeam analyses to identify ore and alteration minerals and to correlate mineralogy with geochemical fluxes geochemical analyses indicate that ti al zr and th behave as immobile elements immobility isocon diagrams show that significant hg sb se te tl and cs are typically added to the wall rocks with au fe is im mobile in some transects added to wall rocks in others and removed from rhyodacite host rocks minor s and w are usually added to the rock whereas minor cu and mo are variably added or removed sio 2 appears to be either added during alteration or remain constant elements that are consistently removed during alteration include ca mn sr and sc and minor mg ba and k na was significantly removed from the rhyodacite au correlates positively with ag hg sb se si te and tl and negatively with ca mn and sc element fluxes correspond with observations of mineral abundances and show that sulfidation and decar bonitization are spatially associated with gold mineralization au hg sb tl cu and possibly te and w were incorporated in trace element rich pyrite as host rock fe was sulfidized fe was added to mineralized rocks in some transects indicating that pyritization occurred in addition to sulfidation cs is a component of galkhaite a trace mineral that is relatively common in some parts of the getchell system decarbonitization removed as much as 95 wt percent calcite from mineralized rocks in some transects and is responsible for loss of ca mn sr and sc although sio 2 as and s are significantly added to carlin type systems they do not appear as major added components in the isocon diagrams this is because the halos for these components are much larger than the areas examined by the sample transects x ray diffraction analyses identified minerals in ore and waste samples and quantified clay minerals including illite and montmorillonite kaolinite was not identified or is present in low abundances in both mineralized and unmineralized rocks montmorillonite does not correlate with gold and textures are consistent with pore filling deposition of montmorillonite following au deposition illite abundance exhibits a positive correlation with au r 2 0 80 intergrowths of illite with ore stage pyrite support a genetic relationship between the two minerals and indicate that illite formed as part of the au related hydrothermal event mineral abundances and textures suggest that illite formed by alteration of k feldspar during au deposition results show that dating of k bearing phases in sedimentary rocks other than illite would provide ages unrelated to gold deposition

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

postmylonitic deformation in the raft river metamorphic core complex northwestern utah evidence of a rolling hinge much of the recent debate over low angle normal faults exposed in metamorphic core complexes has centered on the rolling hinge model the model predicts tilting of seismogenic high angle normal faults to lower dips by footwall deformation in response to isostatic forces caused by footwall exhumation this shallow brittle deformation should visibly overprint the mylonitic fabric in the footwall of a metamorphic core complex the predicted style and magnitude of rolling hinge strain depends upon the macroscopic mechanism by which the footwall deforms two end members have been proposed subvertical simple shear and flexural failure each mechanism should generate a distinctive pattern of structures that strike perpendicular to the regional extension direction subvertical simple shear svss should generate subvertical faults and kink bands with a shear sense antithetic to the detachment for an svss hinge the hinge related strain magnitude should depend only on initial fault dip rolling hinge structures should shorten the mylonitic foliation by 13 for an initial fault dip of 30 in flexural failure the footwall behaves as a flexed elastic beam that partially fails in response to bending stresses resulting structures include conjugate faults and kink bands that both extend and contract the mylonitic foliation extensional sets could predominate as a result of superposition of far field and flexural stresses strain magnitudes do not depend on fault dip but depend on the thickness and radius of curvature of the flexed footwall beam and vary with location within that beam postmylonitic structures were examined in the footwall of the raft river metamorphic core complex in northwestern utah to test these predictions observed structures strike perpendicular to the regional extension direction and include joints normal faults tension gash arrays and both extensional and contractional kink bands aside from the subvertical joints the extensional structures dip moderately to steeply and are mainly either synthetic to the detachment or form conjugate sets range wide the extensional structures accomplish about 4 elongation of the mylonitic foliation contractional structures dip steeply mainly record shear antithetic to the detachment and accomplish 1 contraction of the foliation these observations are consistent with the presence of a rolling hinge in the raft river mountains but a rolling hinge that reoriented a high angle normal fault by svss is excluded the pattern and magnitudes of strain favor hinge related deformation mainly by flexural failure with a subordinate component of svss

age constraints on jerritt canyon and other carlin type gold deposits in the western united states relationship to mid tertiary extension and magmatism carlin type gold deposits are difficult to date and a wide range of ages has been reported for individual deposits therefore several methods were employed to constrain the age of the gold deposits in the jerritt canyon district dated igneous rocks with well documented crosscutting relationships to ore provided the most reliable constraints k ar and 40 ar 39 ar dates on igneous rocks are as follows andesite dikes 324 ma sericitic alteration in andesite dikes 118 ma basalt dikes 40 8 ma quartz monzonite dikes 39 2 ma and calc alkaline ignimbrites 43 1 to 40 1 ma of these only the andesite and basalt dikes are clearly altered and mineralized the gold deposits are therefore younger than the 40 8 ma basalt dikes the sericitic alteration in the andesite dikes is unrelated to the gold deposits a number of dating techniques did not work k ar and 40 ar 39 ar dates on mica from mineralized ordovician to devonian sedimentary rocks gave misleading results the youngest date of 149 ma from the smallest 187 os 188 os initial 0 9 3 6 but did not yield a meaningful isochron due to very low re and os concentrations and large analytical uncertainties paleomagnetic dating techniques failed because the hydrothermal fluids sulfidized nearly all of the iron in the host rocks leaving no remnant magnetism when published isotopic dates from other carlin type deposits in nevada and utah are subject to the rigorous evaluation developed for the jerritt canyon study most deposits can be shown to have formed between 42 and 30 ma k ar and 40 ar 39 ar dates on the youngest preore igneous rocks range from 41 to 32 ma whereas the oldest postore igneous rocks range from 35 to 33 ma hydrothermal adularia from the twin creeks deposit yields similar 40 ar 39 ar dates of 42 ma k ar dates on supergene alunite range from 4 to 30 ma k ar and 40 ar 39 ar dates on micas separated from sedimentary 395 43 ma and igneous 145 38 ma rocks are usually much older than the gold deposits and most are suspect because they are from incompletely reset preore micas or from mixtures of preore and ore stage mica fission track dates on zircons are also generally older than the deposits 169 35 ma and are not completely reset by mineralization apatites are likley to be reset by the hydrothermal systems and by younger thermal events and yield dates 83 22 ma that are younger than those from zircon independent support of a mid tertiary age is provided by the low delta d sub h 2 o values of hydrothermal fluids from 10 widely separated carlin type deposits 134 or 23ppm the low delta d sub h 2 o values are consistent with a mid tertiary age when the climate was cool but are inconsistent with the cretaceous and jurassic ages favored by some workers when the climate was warm the age and distribution of carlin type deposits coincides with areas that underwent extension and calc alkaline magmatism over the same time period despite this relationship the deposits lack clear spatial or genetic relationships to mid tertiary epizonal plutons rather many deposits are localized along preexisting crustal fault zones in the getchell trend structures tapped deep sourced metamorphic or magmatic fluids and subsequently variably exchanged meteoric water in the other trends and districts only variably exchanged meteoric water has been detected these relationships suggest either that all of the deposits formed from deep sourced fluids and were subsequently flooded by meteoric water or that fluids from different sources evolved to produce deposits with similar charateristics

strain variations in an ancient accretionary complex implications for forearc evolution strain magnitudes and orientations and incremental strain histories are characterized throughout the kodiak formation a portion of a late cretaceous early tertiary accretionary prism the results of these analyses are used to evaluate the history of underplating penetrative strain and unroofing in the forearc of the southwest alaskan convergent margin five major deformation events are recognized in the kodiak formation 1 layer parallel extension and layer parallel shear d1 related to underthrusting 2 seaward verging fold and thrust deformation d2 with development of both a slaty cleavage s2 and a downdip stretching lineation l2 related to underplating and early penetrative strain within the wedge 3 development of a regional scale antiform d3 4 development of a shear zone along the northwest limb of the d3 antiform d4 and 5 right lateral strike slip faulting and penetrative right lateral simple shear in the seaward half of the kodiak formation d5 based on sharp differences in structural style and the magnitude and orientation of strain associated with d2 the kodiak formation has been subdivided into three structural units informally called the landward central and seaward belts the structural variations observed in these three belts correspond to differences in structural relief of 1 5 km associated with the d3 antiform the lowest d2 structural level is in the core of the antiform central belt and progressively higher structural levels are exposed on both limbs landward and seaward belts the regional distribution of incremental strain histories associated with d2 d3 d4 and d5 has been quantified by using syntectonic fibers in pressure shadows around spherical pyrite framboids from the highest to the lowest structural level d2 strain is characterized by 1 an increase in the magnitude of finite strain 2 a change in the orientation of finite extension from nearly vertical to gently northwest plunging 3 a change in the incremental extension direction from nearly vertical to moderately northwest plunging and finally 4 a change in the strain history from coaxial to noncoaxial with a sense of rotation consistent with southeast directed thrusting we interpret these d2 strain patterns to reflect a strain field that was fixed relative to the basal decollement with coaxial subhorizontal shortening in the interior of the prism and noncoaxial strain probably simple shear near the base subsequent variations in the orientation of incremental extension associated with d3 are consistent with relatively rapid vertical translation of the accreted sequences relative to the d2 strain field we specifically propose two periods of wedge thickening in response to a combination of underplating and penetrative strain these two wedge thickening events correspond with episodes of subaerial exposure and erosional unroofing in the late cretaceous and middle tertiary

behavior of the brittle crust in wide plate boundary zones in wide and active continental plate boundary zones ductile flow in the deeper and strong parts of the lithosphere may control crustal deformation this is likely if average resistive shear stresses on faults in the brittle crust are much less than 108 pa and the underlying bulk effective viscosity is much greater than 1021 pa s in this case a simple model of distributed deformation referred to as the floating block model may be useful this treats the crust as an array of rotating and translating rigid blocks which are floating on an underlying continuous flow with a constant rheology the model is analyzed in detail in this paper because it has the potential to link detailed observations of crustal deformation with the large scale flow crustal blocks are defined by at least two sets of faults the kinematics of crustal deformation can be described in terms of the motions of these blocks both the relative motion on block boundaries faults and block tilting about a horizontal axis can be described in terms of the underlying flow and block rotation about a vertical axis however rotations about a vertical axis which are an important component of the crustal deformation will depend not only on the underlying flow but also on the shape orientation and arrangement of the crustal blocks the average rotation rate about a vertical axis over finite rotations will be significantly different from that predicted at any instant also the rotation history is considerably complicated if as is likely the underlying flow field or block shape has changed with time these aspects of crustal deformation are discussed with reference to real zones of active deformation in new zealand greece and western north america

the behaviour of so called immobile elements in hydrothermally altered rocks associated with volcanogenic submarine exhalative ore deposits evidence is available that some elements notably zr tio2 y sc ce and nb are largely immobile during the alteration of volcanic rocks owing to metamorphism hydrothermal events and weathering e g floyd and winchester 1978 however it is shown by reference to analyses of rocks from the environment of five volcanogenic massive sulphide bodies that while zr tio2 and ce are mostly immobile even during intense hydrothermal alteration y and particularly sc and nb may be extremely mobile when elements are removed by solution in a hydrothermal fluid it seems that reaction rates are such that these elements are almost totally removed from the rock therefore of the so called immobile trace elements only zr and tio2 may be used with any reliability to identify the degree of magmatic differentiation in an hydrothermally altered rock however if an element has been mobile it is usually readily identified as having moved

slip vectors and stretching of the sumatran fore arc slip vectors from thrust earthquakes at the java trench southwest of sumatra demonstrate that the sumatran fore arc is not a single rigid plate that is translated to the northwest by oblique plate convergence instead they indicate arc parallel stretching of the fore arc at a uniform strain rate of 3 4 x 10 8 yr the northwestward motion of the fore arc relative to the upper plate southeast asia increases from near zero at the sunda strait to 45 60 mm yr in northwest sumatra and should result in variable slip rates on the sumatran fault

criteria for identifying structures related to true crustal extension in orogens faults and shear zones in orogens may be related to crustal shortening or extension or both true extensional structures should ultimately intersect the earth s surface when traced out in a direction opposite to that of hangingwall transport the present dip of a fault in an orogen and its relation to local layering are unreliable guides the observation of low pressure metamorphic rocks structurally above high pressure metamorphic rocks is not sufficient to diagnose crustal extension a valid metamorphic criterion is the difference in pressure time histories between footwall and hangingwall if the footwall was decompressing faster than the hangingwall during shear zone movement then the shear zone was truly extending the crust the pressure time history can be constrained via the temperature time and pressure temperature histories ideally crustal extension inferred from structural data should be confirmed using diagnostic ptt information

change of direction of overthrust shear in the helvetic nappes of western switzerland abstract the lowest of the helvetic nappes in western switzerland the morcles and the doldenhorn nappes are large scale recumbent folds in equivalent tectonic positions in the alpine chain a comparison of the orientations of stretching lineations in both nappes shows that the lineations in the two stratigraphically normal sequences trend n s whereas those in the two stratigraphically inverted sequences trend nw se a similar geometrical relationship has been found from an analysis of the calcite c axis directions of these limestones the two normal sequences show nw se directed c axes whereas the two inverted sequences show w e directed c axes these observations lead to the following conclusions 1 regionally consistent shear movements occurred over a strike distance of more than 60 km and these gave rise to similar deformation patterns in both nappes 2 a progressive rotation of these shear movements from a northerly direction in the earlier formed limbs to northwesterly in the later formed inverted limbs can be deduced the finite strain stretching lineations lag behind the crystallographic fabric axes formed in the later part of the deformation history

role of oblique convergence in the active deformation of the himalayas and southern tibet plateau noting similarities with subduction along curved oceanic trenches and using a simple block model we show that radial vergence evident in earthquake slip vectors along the himalayan deformation front east west extension on north trending normal faults in the himalayas and southern tibet and right lateral strike slip on the karakorum jiali fault zone can all result from basal shear caused by the indian plate sliding obliquely beneath tibet along a gently dipping arcuate plate boundary within the framework of this mechanism the normal faults in the himalayas and southern tibet are not proxies for the uplift history of tibet the distribution and style of the faults in the himalayas and southern tibet suggest that the basal drag from the underthrusting indian lithosphere extends northward beneath most of southern tibet

prograde metamorphism strain evolution and collapse of footwalls of thick thrust sheets a case study from the mesozoic sevier hinterland u s a abstract analysis of prograde metamorphic fabrics in the exhumed footwall of the mesozoic windermere thrust northeast nevada reveals fabrics and style of ductile flow developed during tectonic burial metamorphism of a sedimentary footwall of a thick thrust sheet in response to structural burial footwall strata underwent barrovian metamorphism synchronous with development of bedding parallel or near parallel s and s l tectonites footwall rocks range from unmetamorphosed at high structural levels and progressively increase in metamorphic grade to upper amphibolite facies at deep levels attenuation of footwall stratigraphic units accompanied tectonite development whereby the amount of attenuation varies with metamorphic grade from 0 in lower greenschist facies to 30 50 in upper amphibolite facies rocks microstructures in metacarbonate metapsammite and metapelite and analysis of crystallographic preferred orientations of quartz c axes in quartzite suggest a dominantly coaxial strain path with flattening strain predominating at lower metamorphic grade and plane strain more dominant at higher metamorphic grade the microstructural data indicate that dominantly coaxial deformation during metamorphism accommodated extension of the footwall ductile extension most likely records footwall collapse induced by a loss of strength due to relative upward migration of isotherms during and or following burial extension may act as a mechanism that in part facilitates isostatic accommodation or sinking of the overlying load resulting in a reduction of topography attenuation of footwalls of thick thrust sheets and production of predominantly coaxial layer parallel or near parallel fabrics may be an intrinsic process once sufficient structural overburden is developed and thermal relaxation and prograde metamorphism ensues

kilometre scale sheath fold at mattmark and implications for transport direction in the alps folds were the first tectonic features of rocks to be studied and as such are taken as a reference for the interpretation of other structures intuitively fold axes were assumed to be subperpendicular to the direction of transport nevertheless many studies of metamorphic belts showing intense ductile deformation describe folds with axes close to the transport direction in particular sheath folds1 9 are highly curvilinear folds elongated parallel to the direction of transport we discuss here a spectacular large sheath fold situated in a crustal shear zone in the monte rosa nappe swiss alps our analysis implies that both the geometry and significance of folds in the internal alps must be re examined we further demonstrate the existence of crustal ductile shear zones in an area of the alps where previously emphasis was generally placed on fold nappes the movements on these shear zones is often tens of kilometres

clay alteration and gold deposition in the genesis and blue star deposits eureka county nevada the genesis and blue star sedimentary rock hosted gold deposits occur within the 40 mile long carlin trend and are located in eureka county nevada the deposits are hosted within the devonian calcareous popovieh formation the siliciclastic rodeo creek unit and the siliciclastic vinini formation the host rocks have undergone contact metamorphism decalcification silicification argillization and supergene oxidation detailed characterization of the alteration patterns mineralogy modes of occurrence and associated geochemistry of clay minerals resulted in the following classifications least altered rocks found distal to the orebody consisting of both metamorphosed and unmetamorphosed host rock that has not been completely decalcified and altered rocks found proximal to the orebody that have been decalcified altered rocks are classified further into the following groups based on clay mineral content silicic 1 to 10 percent clay silicic argillic 10 to 35 percent clay and argillic 35 to 80 percent clay clay species identified are 1m illite 2m 1 illite kaolinite halloysite and dioctahedral smectite an early hydrothermal event resulted in the precipitation of euhedral kaolinite and at least one generation of silica this event occurred contemporaneously with decalcification which increased rock permeability and porosity a second clay alteration event resulted in the precipitation of hydrothermal 1m illite which replaced hydrothermal kaolinite and is associated with gold deposition silver and silica deposition is also associated with this phase of hydrothermal alteration hydrothermal alteration was followed by supergene alteration which resulted in the formation of supergene kaolinite halloysite and smectite as well as the oxidation of iron bearing minerals supergene clays are concentrated along faults dike margins and within rocks containing carbonate gold mineralization is not associated with supergene clay minerals within the genesis and blue star deposits rocks classified as silicic argillic in the popovich formation represent the most significant gold host silicic argillic rocks commonly exhibit bedding parallel alteration zones this pattern of alteration indicates that stratigraphy as well as northwest trending structures played a significant role in the migration of gold bearing fluids based on k ar age determinations of hydrothermal 1m illite associated with gold the main event of mineralization in the genesis and blue star deposits occurred between 93 and 100 ma during mid cretaceous time

kinematics of decollement folding in the lost river range idaho decollement folding of massive 2 m thick carbonate beds of the scott peak formation within the willow creek anticline northern lost river range idaho was associated with the development of thin 0 25 m widely spaced deformation zones incremental strain histories determined from fibrous overgrowths in strain fringes from the deformation zones provide a basis for kinematic analysis of the anticline plane strain is indicated by coaxial up dip extension on cleavage xy planes and a lack of extension in the y direction in xz sections coaxial incremental strain histories characterize fold hinges and document pinned hinges during folding the sense of rotation on opposing limbs indicates that the fiber curvature records primarily the spin of layers through fixed moderately steep 66 12 dg l diverging extension directions the magnitude of the external rotation for each sample is typically less than the bedding dip suggesting that the external rotation also includes a component of internal rotation related to layer parallel shear toward the upper flat panel or the hinge pin line the magnitude of this internal rotation varies within and between layers

orogen parallel extension and oblique tectonics the relation between stretching lineations and relative plate motions many collisional orogens exhibit two distinct deformation phases the first and relatively early phase forms within an obliquely subducting continent and is characterized by regionally consistent stretching lineations subparallel to the orogen and to associated isoclinal and sheath folds second phase folds and stretching lineations develop during and after imbrication of the subducting plate into the overriding plate and they become progressively disharmonic with stretching lineations oriented perpendicular to the orogen structural morphology and stretching lineations indicate that a fundamental kinematic transition occurs as material is imbricated from footwall to hanging wall kinematic modeling based on modern oblique convergent boundaries and considerations of buoyancy forces suggest that only the relatively early structures reflect the relative plate motion but that the correspondence is not simple in particular the orientation of stretching lineations is a function of finite strain kinematics of deformation and angle of subduction and convergence and it will consistently underestimate the angle of oblique convergence second phase structures reflect isostatic emergence of the hanging wall and are independent of relative plate convergence examples are drawn from the south central canadian rockies the western alps the variscides of ireland and france and the himalaya

the relationship between strain rates crustal thickening palaeomagnetism finite strain and fault movements within a deforming zone the deformation of continental lithosphere generally does not occur on a single fault but is distributed over a zone up to a few hundred kilometers wide if the deformation rate is constant throughout the zone and if the velocity is continuous everywhere and does not change with time the deformation gradient tensor can be obtained analytically this tensor directly determines the amount of crustal thickening and is related in a complicated manner to the finite strain tensor the declination of remnant magnetization measured within the zone can also be rotated by the deformation because palaeomagnetic measurements are generally made on rigid blocks which have not been deformed there is no simple relationship between such rotations and the finite strain expressions for all these quantities are obtained in terms of the velocity gradients which are taken to be constant the history of western north america is used to illustrate the relationship between the present deformation by strike slip and thrust motions the rotations of the remnant magnetization vectors and the shape of accreted terranes at the surface distributed deformation is taken up by motion on faults which separate the zone into blocks the displacement rate on the faults their strike and the finite rotations of the blocks are all controlled by the velocity gradients analytic expressions relating these quantities are obtained a deforming zone containing normal faults which crosses central greece takes up both strike slip and extension and illustrates the relationship between the structures within and the motion across the zone a similar but more complicated zone in northeastern iran involves both strike slip and shortening

neogene lateral extension during convergence in the central alps evidence from interrelated faulting and backfolding around the simplonpass switzerland abstract the simplon fault zone occurs at the western end of the lepontine metamorphic dome in the central alps and represents a major low angle 25 dip normal fault associated with wsw ene directed lateral extension movement began in the early miocene and continued until at least 3 ma accumulating a total vertical throw of around 10 km corresponding to 24 km displacement parallel to the fault this displacement is partitioned between a broad zone of ductile mylonites in the footwall and a more discrete cataclastic detachment fault the simplon line in the central region simplon village to zwischbergen some 3 km of the total vertical displacement is accommodated by the discrete cataclastic zone due to the higher exposed structural level the importance of the cataclastic component increases towards the northwest in the northern steep belt around the simplonpass retrograde greenschist facies mylonite formation and younger cataclastic faulting associated with lateral extension occurred contemporaneously with sse vergent large scale backfolding representing nnw sse shortening and crustal thickening the combined effect was an overall shortening across the orogen accommodated by an extension parallel to the orogen allowing lateral escape of continental material normal faulting and the resultant lateral escape was not just superficial it extended to depths of at least 15 km and probably to depths exceeding 25 km the overall geometry and kinematics of the simplon fault zone and in particular the interplay between folding and faulting is very similar to that described from the metamorphic core complexes and detachment faults of the cordillera of the southwestern usa within the central alps extension was associated with rapid exhumation of the lepontine metamorphic dome as evidenced by very rapid cooling rates determined from mineral ages in the footwall similar to the cenozoic tectonics of the himalayas and tibet orogen parallel extension in the mountain belt may have been promoted by an overall reduction in the horizontal compressive stress in this direction more limited extension of the same age and orientation is known from a large area of the european foreland greater extension in the mountain belt compared with the foreland was accommodated by major strike slip transfer fault systems and was linked to coeval overthrusting of the lowlands towards the southwest in the external southwestern alps of haute provence france coeval folding and orogen parallel extension such as at the simplonpass were developed in a dextral transpressive regime due to oblique wnw ese convergence across the alps during the neogene

dating gold deposition in a carlin type gold deposit using rb sr methods on the mineral galkhaite significant effort has been expended in an attempt to date hydrothermal activity that generated carlin type gold deposits ctds in the great basin of nevada thus far these efforts have been only partially successful because the relationship s between the dated mineral and hydrothermal activity are equivocal in many cases galkhaite a trace component of at least four ctds in nevada contains significant amounts of rb and virtually no sr making it an ideal candidate for radiometric dating at the getchell deposit galkhaite is paragenetically late but clearly associated with gold mineralization our data place gold mineralization at getchell at 39 0 2 1 ma this is the first unequivocally gold related date produced for any of the carlin type systems galkhaite also has been reported at the carlin rodeo and betze deposits and is likely present in other ctds in nevada this mineral may provide a solution to the conundrum of dating of ctds

strain variation in thrust sheets across the sevier fold and thrust belt idaho utah wyoming implications for section restoration and wedge taper evolution abstract although strain in fold and thrust belts is typically inhomogeneous on the largest scale strain within thrust sheets shows regular patterns the strain which is a quantitative measure of the deformation of any thrust sheet is independent of the translation of the sheet the internal deformation can be represented by a deformation profile the change in shape during thrusting of a fault perpendicular line which can then be used to unstrain the sheet during restoration in cross section balancing deformation profiles in well studied thrust sheets show regular geometric patterns and compatibility constraints can be used to minimize the data needed to define the profiles in other sheets from the geometry of deformation profiles and direct field observations the most important component of internal deformation of thrust sheets in many fold and thrust belts is early layer parallel shortening lps this can be removed in a straightforward manner during restoration and significantly affects the geometry i e taper of the restored section original basin wedge taper may have important implications for the mechanical evolution of fold and thrust belts thus changes in original basin taper resulting from strain removal may affect significantly the way in which we interpret the evolution of a fold and thrust belt in an area

spatial distribution of the seismicity parameters in the red sea regions the spatial distribution of the seismicity parameters of the magnitude frequency relation has provided preliminary quantitative basis for establishing different zones of seismic activities in the southern red sea regions four zones were constructed from application of proposed models that yielded approximate b value range for each zone thus providing relative bases for preliminary classification as follows zone iii has b value 1 0 the zones were characterized through correlation to seismotectonic and some geophysical configuration in the study area narrow contour spacing among zones is observed to occur along intersections of areas of probably different seismic source zones while wider spacing occurs seemingly for a prominently dominating source of seismic activity corresponding spatial distribution of estimated maximum magnitude and expected magnitude at 90 non exceedance in 50 years was prepared likely occurrence of a major earthquake is spread over a wider area of coverage characterized by presence of rift zones structural discontinuities and dislocations in the study area broad confidence limits for the parameter values are encountered but correlation to primary tectonic structures is seen to be possible comparison of the present results to previous relevant studies indicates general agreement with regards to tectonics and related phenomena the spatial distribution of the parameters seemed to provide appropriate basis of analysis to the observed complexities of seismic occurrences in the red sea and adjacent shield areas 1998 elsevier science ltd all rights reserved

computer experiments to investigate complex fibre patterns in natural antitaxial strain fringes antitaxial non deforming strain fringes from lourdes france show complex quartz calcite and chlorite fibre patterns that grew around pyrite in a slate during non coaxial progressive deformation development of these fringes was modelled using a computer program fringe growth 2 0 which can simulate incremental growth of crystal fibres around core objects of variable shape it uses object centre paths as input which are obtained from fibre patterns in thin section the numerical experiments produced fibre patterns that show complex intergrowth of displacement controlled face controlled and intermediate fibres similar to those in the natural examples the direction of displacement controlled growth is only dependent on the relative movement between core object and fringe so that core object rotation with respect to the fringe influences the fibre patterns and produces characteristic asymmetric fibre curvature object centre paths should be used for kinematic analysis of strain fringes instead of single fibres since these paths represent the fringe as a whole the length along the path can be interpreted in terms of finite strain and path curvature in terms of rigid body rotation of fringes with respect to an external reference frame

kinematic model for the curvature of the northern subalpine chain france abstract analyses of cross sectional shortening mapped faults and calcite twin strain are used in conjunction with geometric kinematic modeling to interpret the mechanism of curvature formation in the northern subalpine chain of france there is a 45 change in strike around the chain that is convex towards the foreland northwest five models for fold thrust belt curvature are applied to the northern subalpine chain they can reproduce the geometry and the kinematic history of a curved fold thrust belt and can be used to predict the magnitude and orientations of strains and the three dimensional geometry of the curvature the models are pure bending radial thrusting curve parallel simple shear uniform displacement uniform shortening and transport parallel simple shear two principal criteria for distinguishing among the models geometrically are the horizontal tangential strain extension or contraction parallel to strike and cross sectional shortening the transport parallel simple shear model predicts no tangential extension in the central domain and greater radial shortening and significant tangential extension in the northeastern and southern domains this is the pattern observed in the northern subalpine chain therefore the curvature of the northern subalpine chain is attributed to a transport parallel simple shear mechanism

kinematics of compressional and extensional ductile shearing deformation in a metamorphic core complex of the northeastern basin and range abstract analysis of shear criteria enables the kinematics of two main ductile shearing events d 1 and d 2 to be established in the raft river grouse creek and albion metamorphic core complex the first event d 1 is a nne thrusting and corresponds to mesozoic shortening a well developed non coaxial ductile deformation d 2 of cenozoic age is marked by the occurrence of opposing eastward in raft river and westward shear criteria in albion grouse creek these characterize an arch structure where the shear strain increases outwards in the axial zone of the complex d 2 seems coaxial cenozoic extension is considered to be related to gravitational instability induced by mesozoic overthickening of the crust involving uplift erosion and abnormal heating brittle extension occurs in the upper part of the uplifted domain it is transformed laterally above undeformed basement towards stretched domains of the middle and lower crust through the ductile shear zones localized at the precambrian paleozoic interface this extension of the middle and lower crust occurred in the vicinity of the root zones of the former mesozoic thrusts which may thus have been reactivated as ductile normal faults during cenozoic extension

seismic hazard in the northwestern apennines a study of the seismic hazard for the northwestern apennines based on seismotectonic and seismicity data is presented

extensional shearing deformation and kilometer scale a type folds in a cordilleran metamorphic core complex raft river mountains northwestern utah cordilleran metamorphic core complexes are characterized by ductile mylonitic deformation and regional stretching lineations in the raft river mountains a mylonitic zone and a late superimposed detachment fault separate precambrian crystalline and metasedi mentary basement rocks from an upper plate made of allochthonous paleozoic rocks which suffered a brittle extensive deformation we suggest that the mylonitic ductile deformation is the result of tertiary extensional tectonics strain analysis shows that 1 strain results from large scale progressive shearing deformation in a non coaxial regime 2 e w regional stretching lineations represent the direction of shearing and transport 3 shear criteria indicate an east directed movement 4 strain magnitude is increasing eastward 5 kilometer scale folds parallel to the lineations are interpreted as a type folds formed by eastward shearing of a previously northward dipping layering measurement of striated faults in the upper plate gives an e w direction of extension parallel to the lineations thus ductile and brittle deformations are closely related to the same regional scale extensional tectonics a model is proposed to explain the observed geometry and kinematics of the structures the mesozoic to paleocene cordilleran compressional tectonics induced an overthickening of the crust in the hinterland belt at the future emplacement of the complexes important uplift abnormal heating and granite emplacement occurred later during the collapse of the former belt the combined effects of thermal and gravitational instabilities during tertiary extensional tectonics enabled the development of a ductile shear zone localized at the precambrian basement paleozoic cover interface the brittle extension of the upper crust is displaced laterally along this flat shear zone and transformed farther east into the extended regions of the middle and lower crust

timing of gold and arsenic sulfide mineral deposition at the getchell carlin type gold deposit north central nevada pregold mineralization at the getchell carlin type gold deposit includes quartz and base metal vein mineralization associated with intrusion of a cretaceous granodiorite stock the veins contain minor pyrite and trace chalcopyrite arsenopyrite galena and sphalerite the pyrite is moderately coarse and in thin section has high relief is well polished and is fractured and locally cemented by the gold ore assemblage white micas are associated with veins near the granodiorite intrusion gold was not observed or detected by fire assay analyses of samples or electron microprobe analyses of pyrites microprobe analyses show that pregold pyrites have near stoichiometric compositions variable low arsenic is present in pyrite in samples overprinted by gold mineralization secondary ion mass spectrometry sims analyses detected trace gold in the coarse near stoichiometric pyrite in overprinted samples the pregold vein assemblage was fractured and cemented by gold ore stage mineralization the gold ore stage assemblage consists of gold and arsenic enriched pyrite and marcasite encompassed by jasperoid and drusy quartz and local late fluorite orpiment and galkhaite the consistent spatial association of jasperoid and ore pyrite reflects their near contemporaneous formation the ore pyrite occurs as either fine irregularly shaped grains or rims on earlier gold free pyrite in thin section the pyrite is visibly distinct and has a low polishing relief and a poor polish sims and electron microprobe analyses show that ore pyrites commonly contain 8 to 11 wt percent arsenic and as much as 2 400 ppm gold near the end of the ore stage fluorite orpiment and galkhaite precipitated locally in open space created by faulting or limestone dissolution late ore stage mineralization consists dominantly of open space filling realgar and calcite with minor quartz stibnite and framboidal pyrite realgar conforms to euhedral crystal faces of ore stage quartz fluorite and galkhaite calcite filled most remaining open space and conforms to euhedral crystal faces of quartz fluorite and realgar significant textural observations argue against an earlier interpretation that the gold mineralization and arsenic minerals formed as two discrete events separated by 40 m y in a few areas where calcite and realgar are in contact with ore stage quartz containing gold bearing pyrite realgar and calcite enclose gold bearing pyrite grains massive realgar and calcite distal from these contacts do not contain the gold bearing pyrite the presence of ore pyrite in realgar calcite and jasperoid requires close timing of these minerals and shows that there is no major time break separating ore stage and late ore stage mineral deposition textures indicate instead that the ore stage and late ore stage formed as part of a single evolving hydrothermal system the consistent successive overgrowth of younger minerals on perfectly preserved euhedral faces of older minerals supports the continuous evolution of the gold ore stage into the late ore stage these results are consistent with fluid inclusions that indicate that paragenetically successive minerals precipitated from an aqueous ore fluid with consistent salinity and gas contents but at declining temperatures results show that within error the 34 39 and 42 ma ages determined for fluorite and galkhaite at getchell and adularia at the nearby twin creeks mine respectively most closely approximate the timing of gold deposition at getchell

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

the pyrite type strain fringes from lourdes france indicators of alpine thrust kinematics in the pyrenees a new method is developed to reconstruct polyphase deformation histories from pyrite grains with displacement controlled fibrous strain fringes it decomposes the relative displacement between a pyrite grain and a fringe into incremental rotations and pull apart directions application to a classic specimen from lourdes france shows that the rotations determine the fibre curvature sense and strongly depend on local strain parameters as well as the initial orientation and shape of pyrite grains in contrast pull apart directions appear to be relatively constant and straight although curved pull apart trajectories were expected from the heterogeneous non coaxial deformation in the matrix the discrepancy is due to the localization of shear strain components around pyrite fringe complexes the complexes themselves represent ellipsoidal domains of relatively coaxial strain that parallel a regional crenulation cleavage even in zones lacking this cleavage due to stretching and shearing of a pre existing foliation s1 thus strain fringes consistently record bulk shortening orientations regardless of local variations in strain parameters and microstructural development the analysed pyrite fringe complexes from lourdes were previously considered as classic shear sense indicators but yield hook shaped pull apart trajectories in the new method this indicates two deformation events that can be related to a sub horizontal s2 and steep regional crenulation cleavage s3 the recorded polyphase deformation history is tentatively interpreted in terms of a change from gravitational spreading to a compressive emplacement of a thrust sheet in the north pyrenean foreland

40 ar 39 ar k ar and fission track geochronology of sediment hosted disseminated gold deposits at post betze carlin trend northeastern nevada the post betze deposit of nevada is the largest sediment hosted disseminated gold deposit presently known both dimensionally and in terms of contained metal ore occurs primarily as submicron sized gold that is disseminated in altered sedimentary rocks of the lower paleozoic roberts mountains formation however significant portions of the ore are present in altered monzonite of the goldstrike stock alteration and mineralization were controlled by both structure and stratigraphy alteration began with early decarbonatization and was followed by silicification and finally argillization phyllosilicate mineral zoning grades from proximal kaolinitc to kaolinitc sericite to unaltered rock based on geochronologic studies utilizing the k ar 40 ar 39 ar and fission track techniques we suggest that the age of gold mineralization is approximately 117 ma the premineralization goldstrike stock was emplaced at about 158 ma and a postmineralization sill was emplaced at 39 ma these events place clear limits on the age of mineralization fine grained sericite that is interpreted to have formed during hydrothermal events which also generated gold ore was dated by either the k ar method or a modification of the standard 40 ar 39 ar fusion technique age determinations on several samples of fine grained sericite from altered host rocks both sedimentary and igneous yield ages near 117 ma younger dates are interpreted to be the result of either thermal disturbance of the k ar system or mixing of sericites of two ages one sample of coarser grained sericite that was stepheated gave discordant spectra fission track analyses of zircon and apatite from the postore sill clearly document the lack of significant thermal or hydrothermal activity younger than 39 ma although less definitive fission track data on zircon from preore sedimentary and igneous hosts suggest that no widespread hydrothermal activity having temperatures above about 100 degrees c has occurred since approximately 110 ma the 117 ma age is consistent with one of the major magmatic pulses recognized in the northern great basin it is also similar to within analytic uncertainty a k ar date of 109 ma from the welches canyon stock 20 km south of post betze an intrusion of similar age unexposed at present could have been the driving force for the hydrothermal system at post betze published geochronologic data though less well tied to mineralization suggest similar mid cretaceous ages for other sediment hosted disseminated gold deposits in nevada based on these ages we suggest that sediment hosted disseminated gold deposits are not necessarily products of extensional environments and in fact appear to be associated with compressional environments in nevada during cretaceous time the alignment of sediment hosted disseminated gold deposits and other features in the basin and range province of the western united states suggests that zones of long term crustal weakness may have controlled igneous intrusion and consequent hydrothermal gold mineralization although not all igneous episodes necessarily generated sediment hosted disseminated gold ore deposits the strong association between sediment hosted disseminated gold deposits the precambrian craton and these deep seated structures also suggests the possibility of a deep source for gold and other components of these systems

lower paleozoic host rocks in the getchell gold belt several distinct allochthons or a sequence of continuous sedimentation the lower paleozoic rocks that host gold deposits along the getchell gold belt in humboldt county nevada represent several allochthonous terranes rather than a sequence of continuous deposition the term terrane is used only in a descriptive sense evidence for allochthonous terranes in this area includes fault boundaries and differences in age lithology and structural style among several rock sequences the two most widespread and distinct terranes in the area are 1 the osgood terrane which consists of intensely deformed regionally metamorphosed marine rocks lower cambrian osgood mountain quartzite lower cambrian to lower ordovician preble formation and some rocks currently mapped as comus formation and 2 the getchell terrane which consists of less deformed chert clastic sedimentary rocks and volcanic rocks rocks mapped as valmy and vinini formations including lower and upper ordovician and lower silurian strata in this region osgood terrane bedding and foliation dip predominantly eastward and folds verge westward getchell terrane folds verge southeastward the comus formation which is middle ordovician at its type locality on edna mountain represents a third terrane iron point terrane situated structurally between the osgood and getchell terranes use of the unit name comus formation outside the type locality has created confusion and needs reexamination some of the rocks currently mapped as comus formation might really be part of the other terranes

isotopic dating of strain fringe increments duration and rates of deformation in shear zones the time scales over which deformation in the earth9s crust remains localized in shear zones are poorly known as are the associated strain rates we have determined the longevity and rates of deformation using rubidium strontium rb sr microsampling dating of increments of fibrous strain fringes from a pyrenean shear zone the fibers grew quasi continuously through a protracted deformation history between 87 and 50 million years ago over a period comparable to that of an orogeny during a short interval between 66 and 62 million years a rise in strain rate from 1 1 10 15 to 7 7 10 15 seconds 1 occurred this acceleration correlates with an abrupt change in fiber growth direction and a stress field inversion from gravitational collapse to renewed horizontal crustal shortening

evaluation of radioisotope dating of carlin type deposits in the great basin western north america and implications for deposit genesis a variety of techniques have been used in attempts to date the mineralization in carlin type deposits in the great basin with highly variable results these techniques and results are reviewed in this paper along with presentation of two new dates for the rodeo deposit on the carlin trend and for the barneys canyon deposit in utah complete resetting of sericite by hydrothermal fluids of the temperature and duration of hydrothermal activity that form carlin type deposits is considered highly unlikely therefore dates from sericite are generally of questionable value unless that sericite can be shown to have formed during the same hydrothermal event during which au was deposited for the deposits that have been investigated to date sericite dates rarely if ever record the age of au mineralization however sericite ages do appear to record pre au events in some districts such events may have contributed to ground preparation and to a much lesser extent to the tenor of the ore fission track and u th he techniques provide important age constraints on mineralization in some districts but also suffer from a less than clear association with au rb sr dating of galkhaite an hg sulfosalt provides the only direct age of mineralization but galkhaite has been recognized in only a few locations and dated in only two deposits in a similar manner adularia is contemporaneous with au at twin creeks but twin creeks is the only carlin type deposit where adularia has been reported several other techniques u pb reos sm nd have been used in attempts to date the deposits but with limited success the best available ages are those from preore and postore igneous events and those from galkhaite galkhaite from getchell indicates a mineralization age of 39 ma and that from rodeo carlin trend indicates a mineralization age of 40 ma a new u th he age 34 ma for apatite at barneys canyon suggests contemporaneity with the nearby bingham porphyry cu system the most reliable data from several deposits and districts in the great basin indicate that carlin type mineralization occurred in a relatively narrow time interval between 33 and 42 ma this interval coincides with the transition from a compressional to an extensional tectonic environment in the region although similar tectonic environments have existed at different times and places in western north america no other carlin type mineralization is known to exist in the region therefore although the age and tectonic environment of these deposits is now reasonably well constrained it is clear that tectonic environment alone is insufficient to explain the genesis of carlin type deposits additional perhaps unique factors such as host rock geochemistry must have contributed to the development of these exceptional deposits

40 ar 39 ar dating and mineral paragenesis for carlin type gold deposits along the getchell trend nevada evidence for cretaceous and tertiary gold mineralization the orebodies at the getchell and twin creeks mines were studied through mineral paragenesis geologic relationships and 40 ar 39 ar dating mineral paragenetic relationships are based on observations made during the logging of 18 000 m of drill cuttings and core and crosscutting relationships recognized in the field and main pit at the getchell mine ages for igneous and mineralizing events were determined through 40 ar 39 ar incremental heating analyses of 15 samples of biotite k feldspar sericite and vein adularia a thermal history for the area was also developed using k feldspar multiple diffusion domain results and age determinations on cogenetic minerals which have different argon closure temperatures mineral paragenetic relationships and 40 ar 39 ar dating at the getchell and twin creeks deposits document that gold mineralization occurred in five stages between 95 and 42 ma stage 1 pyrrhotite quartz arsenopyrite chalcopyrite biotite gold mineralization in skarn is associated with 95 ma dacite dikes and granodiorite plugs stage 2 chalcopyrite sphalerite galena pyrite arsenopyrite or gold and silver mineralization in skarn is related to the 92 ma osgood mountains stock secondary k feldspar and sericite in stage 3 quartz pyrite kaolinite gold mineralization formed at 83 ma and are not related to 95 or 92 ma igneous activity stage 4 low grade quartz pyrite gold mineralization formed at about 75 ma in the matrix of a breccia pipe that cuts 83 ma silicic and argillic ores adularia intergrown with stibnite orpiment pyrite quartz gold or realgar mineralization yields precise 40 ar 39 ar plateau ages of 42 11 or 0 43 and 41 90 or 0 25 ma for stage 5 this study documents the temporal complexity of mineralization and igneous activity at the getchell and twin creeks deposits minor gold in skarn type mineralization stages 1 and 2 associated with 95 and 92 ma igneous intrusions preceded and was not genetically related to carlin type gold mineralization stages 3 and 5 the carlin type deposits at getchell and twin creeks therefore represent a composite orebody formed by the overprinting of 83 ma stage 3 quartz pyrite gold by 42 ma stage 5 orpiment stibnite pyrite gold mineralization

alternating contraction and extension in the hinterlands of orogenic belts an example from the raft river mountains utah combined macroscopic to microscopic structural analyses detailed geologic mapping and thermochronology were used to study the structural evolution of midcrustal rocks of the sevier orogenic belt hinterland in northwestern utah these results when combined with previous studies provide new insight into the structural history of this region and suggest alternating tectonic contraction and extension during mesozoic to early cenozoic time two allochthons form the upper plate of the miocene raft river detachment fault in the eastern raft river mountains the lower allochthon comprises neoproterozoic ordovician and pennsylvanian strata and is bounded below by the raft river detachment fault and above by the middle detachment fault strata within the lower allochthon were dramatically attenuated by two episodes of ductile deformation the first deformation d1 took place at metamorphic temperatures of 500 c and resulted in penetrative fabrics throughout these rocks that record combined flattening and top to northeast shearing strains the second deformation d2 resulted in significant stratigraphic attenuation along discrete top to the west shear zones that are generally parallel to lithologic contacts separates of synkinematic muscovite from the penetrative fabric yield 40ar 39ar cooling ages that indicate that d1 deformation occurred prior to cooling ca 90 ma both fabrics were subsequently folded about d3 kilometer scale recumbent folds the areally extensive middle allochthon composed chiefly of pennsylvanian and permian rocks metamorphosed in the greenschist facies was emplaced d4 along the low angle middle detachment fault this fault cuts across various structural levels of the recumbently folded lower allochthon in its footwall and juxtaposes greenschist facies over amphibolite facies metamorphic rocks the lower and middle allochthon were subsequently deformed d5 into open folds with north trending axes neogene extension d6 produced an 200 m thick top to east ductile shear zone in precambrian rocks and formed the younger raft river detachment fault which forms the present upper contact of the ductile shear zone northeast vergent d1 fabrics probably record shortening deformation on the basis of fabric correlations with the grouse creek and albion mountains deformation kinematics and synkinematic prograde metamorphism d2 attenuation faults have been interpreted to record crustal extension of late cretaceous age if d3 recumbent folds developed during extension then the deformation sequence records early shortening followed by protracted extensional unroofing with variable structural styles the favored alternative is that d3 recumbent folds developed during shortening in this case d1 through d4 record episodic alternations of contraction and extension such alternations are consistent with observations from analog and theoretical models of contractional mountain belts and suggest that the hinterland of the sevier orogenic belt underwent dynamic adjustments in crustal thickness and deformation kinematics in response to changes in the boundary conditions of the orogenic belt

pressure temperature paths from garnet zoning evidence for multiple episodes of thrust burial in the hinterland of the sevier orogenic belt garnets are present in two horizons of the schist of stevens spring from the basin creek area of the grouse creek mountains in northwest utah the two horizons possess different bulk compositions which resulted in garnet growth by different reactions along the same pressure temperature p t path garnet in the upper horizon grew from the breakdown of chlorite at upper greenschist facies conditions and garnet in the lower horizon grew from the breakdown of staurolite at upper amphibolite facies conditions from the upper horizon five garnets from three samples were analyzed all display growth zoning ragged morphologies associated with secondary rim consumption and yield garnet biotite geothermometry temperatures of 460 490 c from the lower horizon one garnet from each of two samples was analyzed these also display growth zoning but differ from garnet in the upper horizon in that they are dominantly idioblastic and yield garnet biotite geothermometry temperatures of 635 c garnet biotite geothermometry calculated for every point along detailed compositional traverses across the garnets revealed localized reequilibration along rims cracks and inclusions in both generations of garnet garnets from the upper horizon display prograde reequilibration and the garnets from the lower horizon display retrograde reequilibration numerical simulations of garnet growth using the gibbs method with duhem s theorem were carried out to determine p t paths the p t path defined by the modeling of five garnets from the upper horizon is an isothermal pressure increase of 1 7 kbar the p t path defined by the modeling of two garnets from the lower horizon has a steep p t trajectory d p d t 32 bars c and a total pressure change of 0 9 kbar both paths are indicative of thrust burial however the two paths cannot be reconciled as products of a single thrust episode these data are interpreted to indicate two different episodes of thrust burial during the sevier orogeny separated by 150 c of heating and partial exhumation