kink folding in an extended terrane tortilla mountains southeastern arizona structural analysis of early miocene metre scale kink folds in southeastern arizona shows that they formed in an extensional stress field and that they record horizontal extension the folds represent a previously unrecognized style of extensional fold

effect of lithospheric thickness on the formation of high and low angle normal faults it is accepted that extension of a homogeneous brittle layer should produce high angle normal faults the rotation of the upper parts of such a fault to a low dip requires fault offset that is greater than the local lithospheric thickness new model calculations indicate that a fault cutting lithosphere thicker than 10 20 km can build up only a few kilometres horizontal throw before a new fault replaces it such a small offset leaves the fault little rotated from its initial high dip angle for thinner lithosphere the fault offset may be much larger leading to rotation of the upper inactive parts of a high angle fault to a low dip angle in continents this can happen where thin upper crustal lithosphere is decoupled from mantle lithosphere by weak lower crust the calculations suggest that normal faults begin with high dip angles and only in areas of very thin lithosphere can they be offset enough to produce low angle fault structures this prediction is consistent with the occurrence of low angle faults in areas with higher than normal heat flow

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

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

cooling and uplift patterns in the lepontine alps south central switzerland and an age of vertical movement on the insubric fault line 96 new fission track ft apatite and zircon k ar and rb sr biotite and muscovite ages are presented for 19 samples mainly acid gneisses from a 40 km traverse through the lepontine alps in the maggia valley south central switzerland plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample the entire profile shows a fairly uniform late neogene recent mean uplift rate of 0 5 mm a confirmed by a gradient of ft apatite age with elevation cooling from higher temperatures occurred earlier in the south where uplift rates of 2 2 mm a in the steep belt root zone indicate 9 km early miocene uplift of the northern pennine block this uplift started before 23 ma and is interpreted as resulting from a major phase of backthrusting along the insubric line and as dating the formation of the mylonite belt estimated cooling rates constrain the timing of lepontine mid tertiary metamorphism 3 schematic models are proposed which also consider published rb sr k ar mica and hornblende and u pb monazite ages slow cooling differential initial heating and subsequent cooling of different parts of the central alps and post 38 ma cooling with syntectonic metamorphism at 27 ma are postulated as alternative interpretations of isotopic data and geologic evidence from extrapolation between k ar and rb sr mica ages and apatite ft ages 240 50 c is proposed as the closure temperature for the retention of fission tracks in zircon

tilting history of the san manuel kalamazoo porphyry system southeastern arizona the laramide san manuel kalamazoo porphyry system of arizona has been pivotal in concepts of both extensional tectonics and alteration mineralization zoning this paper reexamines the tilting history in light of new work in the region and reinterprets the geometry of the deposit the porphyry mineralization occurs in and near an intrusion of laramide san manuel porphyry in precambrian oracle granite the area has an extremely complicated history of tertiary crustal extension and fanglomerate deposition but the blocks containing the two main fragments of the original orebody separated by the san manuel fault were involved in only the later parts of this history and are less tilted than other nearby blocks originally horizontal features of mid tertiary age are tilted about 30 degrees those of laramide age about 35 degrees and those of pre laramide age about 45 degrees to the northeast paleomagnetism of the porphyry intrusion itself suggests tilting of about 33 degrees the data thus suggest that postemplacement tilt of the laramide porphyry system was 30 degrees to 35 degrees and that virtually all of it was mid tertiary in age an earlier interpretation of greater tilt magnitude was apparently based in part on correlation with adjacent areas showing greater tilt magnitude as in the tar wash allochthon and in part on a choice of cross section that gives the impression of a nearly recumbent attitude however lowell9s sections are oriented along the strike of both the orebodies and the productive porphyry intrusion which are dipping tablo cylindroidal bodies ore forms a hollow envelope whose inner margin has an aspect ratio of about 4 3 length 2 5 downdip height l limb separation the productive intrusion around which ore is wrapped has a similar aspect ratio in the same plane strike n 58 degrees e dip 47 degrees se reconstruction of the original attitudes of intrusion and ore yields a common strike near n 759 e and a steep dip to the south this is a common attitude for undisturbed productive porphyry dikes of laramide age

miocene crustal extension and uplift in southeastern nevada constraints from fission track analysis the gold butte block of southeastern nevada appears to be a cross section through the upper approximately 17 km of colorado plateau crust tilted eastward and tectonically denuded via top to the west normal faulting in tertiary time the east side of the block exposes a steeply dipping cratonic paleozoic section 2 to 3 km thick proterozoic basement to the west is continuously exposed for an apparent thickness of approximately 15 km structurally below the basal cambrian unconformity apatite fission track dating and confirmed track length distributions have been used to determine the time of unroofing of the gold butte block and hence the timing of one episode of extension in this part of the basin and range province samples collected in as east west transect along the block indicate an abrupt transition from a period of relative thermal and tectonic stability to the onset of rapid cooling the fission track data indicate that the unroofing of the gold butte block was rapid and that the crustal section was brought to within a few kilometers of the surface at ca 15 ma

structural and thermal constraints on the initiation angle of detachment faulting in the southern basin and range the chemehuevi mountains case study the cenozoic normal fault system exposed in the chemehuevi mountains of the southern cordillera provides constraints on the initiation angle and geometry of an extensional fault system that has accommodated extreme crustal stretching there three stacked brittle low angle normal faults that formed at depths as great as 10 12 km cut gently down section northeastward through deformed proterozoic and mesozoic crystalline basement hanging wall blocks are displaced relatively northeastward the upper crust above the chemehuevi detachment fault was pulled apart along high angle normal faults that rotated to more gentle dips through time in contrast rocks of originally mid crustal affinity in the footwall were only gently rotated and accommodated minor extension application of 40 ar 39 ar and fission track thermochronology to rocks in the footwall of the chemehuevi detachment fault system provides further constraints on the timing and initiation angle of regional detachment faulting at the onset of extension between 22 and 24 ma granitic rocks exposed in the southwestern and northeastern portions of the footwall were at 200 c and 350 400 c respectively separated by a distance of some 23 km down the known slip direction this gradual increase in temperature with original depth is attributed to the gentle southwest tilting of broadly planar pre extension isothermal surfaces and constrains the exposed part of the chemehuevi detachment fault to have had a regional dip initially of about 15 to 30 the fault system apparently cut gently down through the upper crust to a minimum depth of 10 12 km the deepest exposed parts of the system today and was domed from midcrustal depths and locally denuded during continued slip together the structural and thermochronologic data confirm the suggestion that faults accommodating large magnitude slip can be initiated and move within the seismogenic regime at moderate to low angles that is 30

tectonic evolution of the cenozoic olympic subduction complex washington state as deduced from fission track ages for detrital zircons

cenozoic thermal evolution and tectonics of the coast mountains of british columbia 1 fission track dating apparent uplift rates and patterns of uplift the high relief of the coast mountains of british columbia has been produced in part by rapid late neogene uplift the relationship of this uplift to the presently diverse and complex plate tectonic regime off the west coast of british columbia however is not clear cut to study this problem the areal patterns rates and total amounts of cenozoic uplift from 50 to 55 n have been determined by using fission track dating of zircon and apatite separated from rocks collected at high and low altitudes along several traverses through the mountains accepting closure temperatures of 105 c 10 c and 175 c 20 c for apatite and zircon respectively and a likely closure temperature difference between zircon and apatite of 70 15 c paleogeothermal gradients have been measured at kemano c km 35 ma ago and near ocean falls king island c km 20 ma ago subcrustal heat flow is suggested to have significantly increased in the late neogene near king island uplift rates in the axial part of the northern 52 55 n coast mountains were 0 1 0 2 km ma from 25 to 15 ma ago this uplift was temporally and tectonically related to part of the subsidence in the adjacent queen charlotte basin uplift rates increased somewhat in the late miocene to about 0 4 km ma probably due to passage of the anahim hot spot beneath the area late miocene recent erosion in the north 52 55 n has reduced summit altitudes and been more extensive than farther south despite this relicts of miocene river valleys and topography are still locally preserved partly beneath 7 10 ma basalts the area of the present southern 50 to 52 n coast mountains was near sea level and experienced very low uplift rates 0 5 km ma of the southern coast mountains the profile of this uplift was broadly plateaulike and this uplift led to the inversion of the miocene coastal topography middle cenozoic uplift from 52 to 55 n was probably the result of denudation and diminishing uplift after the terminal paleocene eocene orogenic episode which grossly thickened the crust this middle cenozoic erosion resulted in the restoration of a normal crustal thickness late neogene uplift in the coast mountains was the result of thermal expansion in the mantle caused by a mantle hot spot near 53 n and changes in the geometry of the subducted slab farther south

40ar 39ar thermochronology and thermobarometry of metamorphism plutonism and tectonic denudation in the old woman mountains area california discrimination of individual tectonometamorphic events in polymetamorphosed terranes requires a comprehensive understanding of the relative timing and conditions of metamorphism and plutonism we have applied a combination of 40 ar 39 ar thermochronology petrology and thermobarometry to reconstruct the complex early proterozoic through early cenozoic tectonic and metamorphic evolution of continental crust in the old woman mountains area south eastern california strong mesozoic thermal events obscure the earlier history in much of the old woman mountains area in those areas where early proterozoic rocks underwent only lower greenschist facies metamorphism during the mesozoic thermobarometry of pelitic schists indicates that proterozoic metamorphism occurred at 9 to 11 kbar and 700 c 40 ar 39 ar ages of hornblende from samples of interbedded proterozoic amphibolite indicate that this high grade metamorphism took place before 1600 ma the relatively high pressure conditions of early proterozoic metamorphism in the old woman mountains area contrast with the low pressure granulite facies metamorphism that occurred elsewhere in the mojave desert at this time 40 ar 39 ar analyses of hornblende from proterozoic rocks within mesozoic shear zones and hornblende barometry from jurassic intrusive rocks suggest that tectonism and burial of paleozoic strata to 10 km began between 170 and 150 ma this tectonism resulted in regional greenschist facies metamorphism late stage mineral assemblages in proterozoic and paleozoic pelitic rocks in the old woman mountains area indicate an increase in metamorphic grade from greenschist to upper amphibolite facies toward late cretaceous plutons of the 73 ma old woman piute batholith metamorphic temperatures from garnet biotite thermometry increase from 650 c and 40 ar 39 ar ages of hornblende from proterozoic rocks decrease from 1600 to 73 ma toward these plutons barometric calculations from garnet bearing metamorphic rocks suggest that this cretaceous metamorphism took place at 3 5 to 5 0 kbar in the old woman mountains similar pressures are obtained from hornblende barometry of late cretaceous granodiorite 40 ar 39 ar and apatite fission track data from the 73 ma batholith indicate that the rocks exposed in the old woman mountains cooled rapidly to below 350 to 300 c by 70 1 ma and that moderately rapid cooling at rates of 10 to 30 c m y continued until after 60 ma the presence of maximum mesozoic temperatures on the borders of plutons hornblende ages of ca 73 ma and similar pressure estimates for plutons and country rocks indicate that the high temperatures needed to cause ductile deformation and amphibolite facies metamorphism were related to heating from the intrusion of the 73 ma old woman piute batholith amphibolite facies metamorphism along pluton contacts was superimposed on the regional greenschist facies metamorphism related to thrusting and crustal thickening the extremely rapid cooling in the old woman mountains between 73 and 71 ma was due largely to conduction of heat into the country rocks whereas protracted rapid cooling to temperatures below ambient temperatures estimated for the depth of emplacement was due to tectonic denudation at 1 to 2 km m y until about 68 to 65 ma

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

denudation of metamorphic core complexes and the reconstruction of the transition zone west central arizona constraints from apatite fission track thermochronology apatite fission track thermochronologic results from transects across the basin and range and transition zone provinces in west central arizona provide constraints on the denudational history and structural framework of the region apatite fission track ages decrease from 21 to 14 ma in the harcuvar mountains and from 16 to 13 ma in the buckskin rawhide mountains in the slip direction sw ne of detachment faults in the lower plates of the metamorphic core complexes mean lengths of confined fission tracks from the core complexes are all 14 m indicating that the apparent apatite ages record rapid cooling through the apatite partial annealing zone 40 c m y for lower plate rocks apparent apatite ages in the transition zone province generally increase from 25 ma to 100 ma away from the basin and range province this trend of increasing apatite age is disrupted by faulting as many as seven times at the fronts of major mountain ranges and within valleys between the weaver mountains and the colorado plateau gradients of apatite fission track age and confined track length with elevation in the mountain ranges and in the phillips kirkland drill hole reveal parts of denuded mesozoic and cenozoic apatite partial annealing zones these paleopartial annealing zone profiles provide a reference datum for preextension reconstructions of fault blocks the reconstructions indicate that the major faults in the transition zone province have relative displacements of 1 km and that offset on them occurred mostly after 25 ma these data also indicate that most of the rocks now exposed in the transition zone of west central arizona were not exposed until miocene time

relation between extensional geometry of the northern grant range and oil occurrences in railroad valley east central nevada in the northern grant range heterogeneous neogene extension was dominated by synchronous arching and attenuation attenuation was accomplished along a stacked set of attenuation faults that formed at low angles to bedding as the paleozoic carbonate and paleogene rocks arched about a north northwest axis the style and amount of attenuation was controlled by lithologic character and structural depth of rock units and by geometry of the arch on the steeper west side of the grant range arch the curviplanar low angle attenuation faults converge into a single shallowly west dipping fault zone along which the stratigraphic juxtaposition of mississippian units over middle cambrian units and late cretaceous granite marks the zone of maximum attenuation the distinct geometry o the arched westward converging low angle fault array is seen in windows into the deeper structure low on the west side of the range we conclude that arching and heterogeneous extension resulted from uplift of the grant range relative to the structural basin of railroad valley to the west this structural differentiation is expressed as a complex zone of subparallel to bedding shallow dipping attenuation faults rather than as a simple high angle range front fault seismic and drill hole data indicate that low angle attenuation faults in the range extend into railroad valley and control the structure buried in the valley mississippian and paleocene to eocene petroleum source rocks and devonian to oligocene reservoir rocks in railroad valley oil fields are in extensively fractured rocks of the upper plate to the major extensional fault system thus relatively cold upper plate rocks immature with respect to hydrocarbon generation were brought relatively down into contact with hotter lower plate rocks by neogene attenuation faulting oil in railroad valley which is sourced from rocks as young as eocene was probably generated by this juxtaposition during neogene crustal attenuation and subsequently migrated into upper plate fractured reservoi s

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

tertiary extension in the old woman mountains area california evidence from apatite fission track analysis apatite fission track ages and confined track length distributions from the old woman mountains area southeastern california record evidence for periods of rapid cooling during the late cretaceous following mesozoic orogenesis and during miocene crustal extension apatites from the clipper marble and turtle mountains give fission track ages between 62 and 69 ma as a result of cooling of these upper crustal rocks after regional late cretaceous plutonism and metamorphism in the piute mountains apatite fission track ages decrease from 70 ma with mean track lengths of 13 5 m to 14 m from west to east intermediate aged samples from the piute mountains have mean track lengths between 11 and 13 m due to their extended residence in the apatite annealing zone during the early tertiary this pattern is the result of 20 30 of westward tilting of the piute mountains block as indicated by the dip of tertiary strata the apatite data indicate that tilting occurred after 18 ma and caused rapid cooling of rocks exposed on the eastern side of the mountain range knowing the amount of tilting and the apparent paleotemperatures across the piute mountains allows us to estimate that the synextensional geothermal gradient was 50 20 c km apatites from the northeastern old woman mountains yield fission track ages of 68 ma with mean track lengths of 13 8 m these samples record cooling after batholith emplacement at 73 ma in the northwestern old woman mountains apatite fission track ages decrease to 20 ma and have mean track lengths 15 m indicating very rapid cooling we interpret this age pattern to be the result of minor eastward tilting of the old woman mountains block after 18 19 ma or flexural unroofing of the northwestern old woman mountains due to movement on the ship mountains fault extension in the old woman mountains area occurred between 19 and 16 ma which is coincident with extension in the colorado river extensional corridor and later than major extension in the mojave extensional belt

a pseudotachylyte geothermometer abstract pseudotachylytes invariably contain a conspicuous concentration of clasts and lithic fragments in their matrices which reflect brittle wear processes during frictional melting the ratio of clasts to matrix is interpreted as the ratio of wear to melt w m and measurements of this ratio show a range of 0 1 0 7 based on models of melting and wear processes it is shown that the ratio w m is independent of fault displacement stress fault area and mineralogy thermodynamic considerations indicate that w m corresponds to the thermodynamic efficiency of the conversion of work to heat and is defined as w q thigh tlow thigh where thigh refers to the melt temperature and tlow is the ambient country rock temperature this provides a simple new technique for estimating the ambient crustal temperature tcrust in degrees kelvin t crust 1 w m t melt assuming a reasonable value for the temperature of the melt estimates of country rock temperatures tcrust for pseudotachylytes from four different localities in the usa indicate a range of 123 387 c eclogite facies pseudotachylytes from western norway yield a mean country rock temperature of 658 c in good agreement with independent fe mg exchange geothermometry this new geothermometer appears to be applicable throughout the entire crust and may provide a better understanding of melting and wear processes during seismic faulting

plutonism and the origin of metamorphic core complexes the formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension pulses of ductile deformation have taken place during short lived thermal events initiated by the heat input from intruded plutons sills or dikes such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes fast cooling inferred from 40 ar 39 ar apparent ages may have taken place after periods of magma arrival and need not be the result of rapid erosional or tectonic denudation heterogeneity of 40 ar 39 ar apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills or sill swarms under such circumstances apparently isothermal decompression paths may actually link pressure temperature points set during periods of transient mineral growth during and after periods of igneous activity

gold butte crustal section south virgin mountains nevada the gold butte block consists of a complex of proterozoic medium to high grade metamorphic and plutonic rocks unconformably overlain by steeply east dipping paleozoic sedimentary rocks on its east side this complex is one of the largest continuous exposures of crystalline rocks in the southwestern united states that lie structurally beneath a steeply tilted section of phanerozoic strata and it may represent part of an intact proterozoic miocene crustal section reaching paleodepths of approximately 15 km the section was exposed during tectonic denudation in miocene time at about 15 ma and lies directly adjacent to the colorado plateau to the east proterozoic and tertiary geological features support the hypothesis that the block represents a relatively intact crustal section a large exposure of proterozoic rapakivi granite the gold butte granite circa 1 45 ga crops out over most of the crystalline complex and displays strong zoning in phenocryst abundance which increases toward the top of a 10 to 12 km thick magma chamber the wallrocks of the granite are predominantly garnet bearing gneisses circa 1 7 ga which display increasing pressures of final equilibration westward away from the basal phanerozoic unconformity thermobarometry of gneisses immediately beneath the unconformity indicate pressures of 2 3 kbar whereas those 12 13 km west of the unconformity yield pressures of 5 6 kbar suggesting the crustal section was established by 1 7 ga the style of deformation displayed by denudation structures also varies from east to west with brittle faulting only in eastern portions of the block and retrograde metamorphism mylonitization and chlorite brecciation of the garnet gneiss observed in western areas

role of tectonic denudation in warping and uplift of low angle normal faults large displacement on a low angle normal fault results in isostatic uplift of the lower plate in response to tectonic denudation simple models of the denudation process predict warping of the lower plate into a broad antiform or antiform synform pair with axes perpendicular to the direction of extension the amount of warping is strongly influenced by initial fault geometry surface topography the amount of extension and the distribution of extension within the upper plate late stage processes that augment antiform growth include one sided denudation of the antiform reverse faulting due to concave upward flexure and wholesale detachment and reverse drag folding of the antiform antiformal uplifts now exhumed by erosion form mountain ranges in the southwestern united states where conditions were favorable for warping during mid tertiary crustal extension four major structural domains are apparent in transects across these uplifts and adjacent areas 1 unextended area 2 synformal upper plate 3 antiformal uplift and 4 wedge shaped upper plate each domain is separated from adjacent domains by the traces of a master low angle normal fault

prediction of normal fault geometries a sensitivity analysis several workers have developed cross section balancing schemes for extensional structures using these geometric models structural interpreters can predict normal fault geometries at depth by specifying the shape of a folded bed in the hanging wall of a normal fault the deformation mechanism that accommodates the folding the footwall counterpart of the folded hanging wall bed and the dip and location of the fault segment between the hanging wall and footwall beds we have systematically determined how uncertainties in these input parameters affect the prediction of normal fault shape and detachment depth our sensitivity analysis shows that reasonable uncertainties in the fold shape near the normal fault the deformation mechanism the footwall correlation and the fa lt dip produce large uncertainties in the predicted fault geometry reasonable uncertainties in the fold shape far from the normal fault and the fault location however produce only small uncertainties in the predicted fault geometry our work suggests that if seismic and well data are lacking or of poor quality then geometric models cannot provide unique answers to questions about the geometry of a normal fault at depth geometric models however can provide a range of possible fault geometries

intrusion of horizontal dikes tectonic significance of middle proterozoic diabase sheets widespread in the upper crust of the southwestern united states initially horizontal sheet intrusions of middle proterozoic diabase are abundant in a region 650 by 300 km across in arizona and california the diabase forms discordant sheets in basement granite and gneiss and sills in overlying shelf sedimentary sequences massive granite is the most common basement host for the sheets probably because it fractured more easily than foliated hosts during sheet emplacement steep feeder dikes are rare compared to the sheets the diabase in many places is exposed in fault blocks that were tilted during tertiary tectonic extension structure sections restored from the map patterns of upended blocks show that the sheets were intruded at levels throughout the upper crust to depths of at least 13 km sheet intrusion implies a vertical orientation of the least compressive stress so i conclude that the crust was under tectonic compression or in an isotropic state of stress at the time of diabase intrusion about 1 1 ga magma overpressures water encountered by rising magma vertical changes in the crustal stress regime and flotation of low density granite all may be important factors for sheet intrusion the stress conditions suggested by the presence of the sheets argue against an extensional tectonic regime earlier proposed for the diabase event the diabase province contrasts structurally with similar age provinces of basaltic magmatism elsewhere in north america that show evidence of tectonic extension such as the midcontinent rift the consistent orientations of the sheets commonly allow them to be used as structural markers for postdiabase deformation of basement blocks sheet intrusions in the geologic record may be seriously underreported their recognition is important for the interpretation of seismic reflection profiles of continental crust sheets in arizona may be responsible for the bagdad reflection sequence which extends to depths of at least 15 km

geologic structural and thermochronologic constraints on the tectonic evolution of the sierra mazatan core complex sonora mexico new insights into metamorphic core complex formation 1 the sierra mazatan in northwestern mexico is the southernmost metamorphic core complex in the north american cordillera large magnitude tertiary extension at sierra mazatan involved both ductile and brittle slip along a major normal fault that presently dips 10 15 west extension was polyphase and involved an early period of extension from 25 to 23 ma followed by major slip from 21 to 16 ma total slip was 20 km and occurred at rates of 3 4 mm a this extension predated the plate boundary change to transtension at 12 ma and was largely decoupled from relative pacific north american plate motion numerous lines of evidence suggest that the presently low angle normal fault initiated at a steep dip 50 60 and was rotated to lower angles during slip when corrected for this tilting fault corrugations at sierra mazatan had a similar geometry to the segmentation of many active normal faults which is compatible with their origin as primary fault features many aspects of the sierra mazatan are comparable to large active normal faults indicating that this core complex formed owing to prolonged extension on an otherwise typical high angle normal fault therefore core complexes need not represent a fundamentally unique mode of crustal extension

fault and bed rotation during continental extension block rotation or vertical shear abstract for almost a century the view has existed that the tilting of blocks between closely spaced planar normal faults is rigid body rotation this interpretation requires only simple geometry and has consequently found widespread application however it is not consistent with the deformation expected around normal faults given the present knowledge of stress fields and rheology in basement in the brittle upper crust which is better regarded instead as distributed vertical simple shear rigid body rotation and vertical shear involve different relations between fault and bed tilting and thus predict different initial fault dips for particular present day dips of faults and beds these two schemes also predict different amounts of extension and it is consequently important to establish which is correct with this aim in mind we examine normal faults associated with neogene extension in western turkey and the western united states and with mesozoic extension in the north sea except where extension and the associated tilting are minimal rigid body rotation predicts unrealistically steep initial fault dips some extensional basins also exhibit reversals of normal fault polarity and tilt polarity of beds which are incompatible with rigid body rotation we therefore conclude that the general cause of the tilting is vertical shear not rigid body rotation this has three main observational consequences first the heave on any normal fault equals the amount of extension across it second no feature near a normal fault can rotate through the vertical a normal fault thus cannot rotate through the vertical and appear as a reverse fault third any initially vertical feature near a normal fault will remain vertical a vertical dyke in the tilted surroundings of a normal fault is thus not necessarily younger than the extension that caused the tilting

rapid extension recorded by cooling age patterns and brittle deformation naxos greece the metamorphic core complex exposed as the island of naxos in the aegean sea records an unusually complete sequence of structures developed as a result of continental extension the structures formed during miocene rise and cooling from ductile upper amphibolite facies and anatectic conditions to brittle near surface conditions beneath the naxos detachment fault top to the north ductile fabrics in the footwall which initially developed during amphibolite facies prograde metamorphism were overprinted by a succession of north directed normal sense lower temperature brittle structures as the footwall was tectonically unloaded and unroofed pseudotachylite and cataclasite formation brittle faulting alteration and erosion of the footwall occurred during continued slip and tectonic denudation neogene conglomerate and megabreccia in part derived from exhumation of this footwall lie structurally above the peripheral naxos fault published k ar and 40ar 39ar ages for hornblende white mica and biotite in the footwall decrease northwestward apparent ages 17 50 ma in the southeast correspond to areas of low metamorphic grade where preextension argon was partially retained published ages 16 10 ma in higher grade rocks of the domal core in the north are cooling ages that for each of the three minerals show a component of younging in the nne direction of extension assuming this is the direction of unroofing we interpret the rate of this younging as the fault slip rate as the footwall rocks moved 20 km ssw relative to their hanging wall along the base of the naxos detachment fault the calculated rates of slip average 5 8 mm yr comparable to maximum rates reported in the basin and range province

thermochronologic constraints on the tectonic evolution of active metamorphic core complexes d entrecasteaux islands papua new guinea metamorphic core complexes in the d entrecasteaux islands papua new guinea formed as the result of active extension at the western end of the propagating woodlark basin spreading center domes of high grade metamorphic rocks i e amphibolites eclogites and migmatites intruded by large granodiorite bodies comprise the lower plate of the d entrecasteaux metamorphic core complexes the domes are transected by kilometer scale shear zones a thermochronologic study of the d entrecasteaux islands utilizing k ar 40ar 39ar and fission track techniques has documented the unroofing history of these active metamorphic core complexes gneisses in the cores of the domes cooled rapidly 100 c m y as indicated by hornblende and biotite 40ar 39ar apparent ages of 2 7 to 3 0 ma and 1 6 to 1 7 ma respectively and apatite fission track ages of 0 4 to 0 9 ma 40ar 39ar apparent ages on white mica biotite and potassium feldspar and fission track ages on apatites from shear zone gneisses indicate extremely rapid cooling in some cases 500 c m y and suggest shear zones were active from 4 0 to 3 5 ma and 1 9 to 1 4 ma in general 40ar 39ar mineral ages for retrogressed core zone gneisses shear zone gneisses and granodiorites are 2 0 to 3 0 ma amphiboles 1 5 to 1 7 ma muscovites and 1 4 to 1 8 ma biotites and 1 0 to 2 0 ma k feldspars apatite fission track ages from core zone gneisses shear zone gneisses and granodiorites range from 0 4 to 1 0 ma thermochronologic results indicate that emplacement of granodiorites closely coincided with retrogression of the metamorphic basement and movement on the outer shear zones bounding the gneiss domes the granodiorite bodies associated with the d entrecasteaux islands domes represent syn kinematically emplaced granitoids intruded into an area of active continental extension

annealing kinetics of fission tracks in zircon an experimental study a series of laboratory annealing experiments on zircon fission tracks has been carried out under heating conditions of 350 750 c for 10 1 103 hr i e 4 5 min 40 days variation in the confined lengths of spontaneous fission tracks was determined using zircon grains from nisatai dacite the fading contours of normalized mean track length r on the arrhenius diagram showed as sets of straight lines we performed a series of model fittings called the parallel and fanning models in order to describe the decrease in r with increasing temperature or heating time the lowest temperature limit of the zircon partial annealing zone zpaz was defined as r 0 95 and the highest as r 0 4 which approximately corresponded to the total fading of surface tracks extrapolation of the results of the laboratory experiments to the geological timescale gives for a heating duration of 106 yr estimated values of the zpaz of 210 320 c 60 c 2 with the parallel model 190 350 c 50 c 2 with the fanning model critical temperature t0 and 170 390 c 50 c 2 with the fanning model t0 the temperatures of the zpaz decrease by 20 c for an annealing duration that is an order of magnitude longer because the estimated closure temperature of zircon fission track analysis approximately corresponds to the middle of the zpaz these results support the previously estimated closure temperature of 240 c by varying the etching time it was revealed that significant removal of radiation damage occurs at r 0 93

thermochronologic evidence of major tectonic denudation associated with detachment faulting northern ruby mountains east humboldt range nevada fission track ft thermochronology studies on igneous and metamorphic rocks from the ruby mountains east humboldt range metamorphic core complex provide important constraints on the timing and nature of major mid tertiary extension in northeast nevada rocks from within the ruby east humboldt detachment brittle ductile normal sense shear zone were analyzed the dominant lithology studied was variably mylonitic mafic orthogneiss nonmylonitic amphibolite from the top of the structurally lower migmatitic core and porphyritic biotite granodiorite from the oligocene circa 36 ma harrison pass pluton were also studied ft ages on apatite zircon and sphene except one 18 4 ma apatite from the basal portion of the detachment zone are concordant and range in age from 26 5 ma to 23 6 ma all ages overlap at 1 between 25 4 ma and 23 4 ma these data suggest that rocks of the upper and middle portions of the detachment zone cooled rapidly from temperatures above 285 c sphene closure temperature to below 70 c lower temperature limit of track stability in apatite near the beginning of the miocene minimum cooling rate of 40 c ma the length distribution of confined fission tracks in apatites from these exhumed middle crustal rocks is strikingly similar to those of rocks known to have cooled rapidly fish canyon ash flow tuff the lower part of the detachment zone as well as the underlying migmatitic core also cooled through sphene and zircon closure temperatures during this time interval but apparently only cooled to a temperature within the zone of partial annealing of apatite 70 130 c residence of these rocks in this zone led to a partial retention of fission tracks thus resulting in a reduced age confined track length distributions from apatite provide independent evidence of very rapid cooling but also indicate that the rocks of the lower detachment zone experienced a more protracted cooling history remained hotter longer than structurally higher levels of the zone ft data firmly establish the lower limit on the timing of mylonitization during detachment faulting in this area as 23 4 ma rapid cooling of the region is considered to reflect large scale tectonic denudation intracrustal thinning the vertical complement to crustal extension rocks originating in the middle crust 10 15 km were quickly brought near the surface along the ruby east humboldt detachment fault brittle ductile simple shear zone and juxtaposed against brittlely extended rocks deformed under upper crustal conditions

black mountains crustal section death valley extended terrain california recent reconstructions of cenozoic extension in the southern great basin juxtapose the panamint range above the black mountains and indicate major tectonic denudation of the black mountains during tertiary extension structural and geologic characteristics of the black mountains and consideration of the regional geology of surrounding ranges suggest that the black mountains block may represent a 10 to 30 km preextensional crustal section that has been uplifted along a major detachment zone geobarometry and ductile deformation features in a synrift intermediate to mafic batholith and metamorphism of eocambrian sedimentary rock on the western side of the range appear to support this hypothesis these data suggest that the black mountains block is one of the youngest and perhaps among the deepest exposed examples of cordilleran metamorphic core complexes

thermal and unroofing history of a thick tilted basin and range crustal section in the tortilla mountains arizona we estimate here a geothermal gradient of only 17 5 c km 1 for the tilted grayback fault block in southeastern arizona when extension began 25 ma this gradient is lower than preextension gradients estimated elsewhere in the basin and range is only about 50 of typical gradients in the basin and range today and needs to be accounted for in models of continental extension the grayback block exposes a 12 km thick crustal section of proterozoic and cretaceous granitoids which was tilted 90 during extension between 25 and 15 ma zircon fission track ages decrease structurally downward westward across the block and were all within a zone of partial track annealing prior to tilting and quenching the zircon age gradient suggests that the 220 240 c isotherm migrated downward 5 6 km during paleogene erosion and regional cooling apatite fission track ages decrease westward from 83 ma in the structurally highest crystalline rocks to 24 ma at 6 km paleodepth and then to 15 ma another 6 km farther west track length analysis confirms that apatites above the break in slope in age at 5 7 km paleodepth resided in a zone of partial annealing prior to tilting and deeper apatites record rapid cooling upon tilting and unroofing beginning 25 ma at that time the 110 10 c isotherm determined by the depth at which tracks in apatite were fully erased was at a basement paleodepth of 5 7 km and the 220 30 c isotherm as estimated from zircon data resided at a pretilting basement depth of 12 15 km from consistent values of paleogeothermal gradient for two depth intervals we estimate the pretilt gradient was 17 5 c km 1 from 25 to 15 ma the rotating grayback block cooled rapidly as higher westward moving blocks unroofed it tectonically at a rate of 1 km m y 1

stacked late devonian lowstand shorelines and their relation to tectonic subsidence at the cordilleran hingeline western utah abstract the upper devonian lower famennian cove fort quartzite and associated strata of western utah comprise three stratigraphic sequences each containing a thick as much as 65 m lowstand systems tract coarse grained shoreface quartz arenite and lagoonal dolomicrite of the lowstand systems tracts display the following features characteristic of forced regression 1 they contain coarse grained proximal shoreline deposits that overlie an erosional surface sequence bounding unconformity cut into fine grained distal marine carbonate facies 2 they lie seaward of a zone of sediment bypass and are spatially detached and separated from the previous highstand shoreface 3 they are confined geographically to a relatively narrow zone less than 30 km wide 4 they are onlapped and overlain by transgressive systems tract deposits consisting of deep subtidal carbonates west range limestone and 5 landward of the lowstand shoreline pinch out the sequence bounding unconformity beneath them merges with a transgressive surface of erosion above them a thick wedge of siliciclastic forced regression deposits is present at the base of each depositional sequence the stratigraphic position of each wedge above the basal sequence boundary was controlled by eustatic sea level fall the forced regression deposits are geographically restricted to a narrow strike belt along the eastern margin of the pilot basin which coincides with the trace of the cordilleran hingeline a zone of crustal discontinuity the geographic confinement and unusual stratal thickness of barrier bar complexes in the lowstand systems tract as well as the stratigraphic stacking of the forced regression deposits were controlled by flexurally induced high subsidence rates focused at the cordilleran hingeline