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

tectonic implications of rapid cooling of lower plate rocks from the buckskin rawhide metamorphic core complex west central arizona lower plate rocks exposed in the buckskin rawhide metamorphic core complex west central arizona underwent a pronounced increase in cooling rate 7 12 m y after the onset of extensional tectonic denudation at ca 27 ma a detailed 40 ar 39 ar study of lower plate rocks exposed near planet peak in the southwest part of the core complex indicates that the highest level mylonitic rocks exposed in the complex cooled below 350 c prior to 20 1 ma at an average rate of 15 c m y thereafter the cooling rate of these rocks increased progressively peaking at 280 80 c m y prior to initial exposure of the core between 15 1 and 13 3 ma published k ar 40 ar 39 ar and fission track ages for the most recently unroofed lower plate rocks exposed at the northeastern end of the core complex are consistent with a similar cooling history although the abrupt increase in cooling rate occurred as many as 5 m y later in this area because the increase in the cooling rate appears to have occurred progressively later toward the northeast it is unlikely to reflect changes in slip rate it appears to have coincided with cooling below 350 300 c and thus is interpreted to reflect steepening of the detachment system above the brittle ductile transition thermochronological data from lower plate rocks in the planet peak area together with constraints on their initial exposure at the surface are consistent with an increase in the dip of the detachment from le 5 at midcrustal depths where it was represented by a shear zone to at temperatures below 200 150 c lower plate rocks at both ends of the core complex probably cooled at rates 130 c m y which can not be reconciled with the average dip le 25 and slip rate 6 12 mm yr for the detachment fault given the likely geothermal gradient 25 c km during the early stages of extension this implies that cooling did not keep pace with denudation at upper crustal levels the very high cooling rates are interpreted to have primarily reflected the development of a pronounced thermal discontinuity across the detachment fault possibly 100 c as hot lower plate rocks were transported to shallow crustal levels rapid cooling of the lower plate at these levels may have been enhanced by interaction with fluids derived from the upper plate consistent with models that link the shallow level

on estimating crystal shape for crystal size distribution analysis abstract the accurate determination of three dimensional crystal habit long intermediate short axis is a requirement to enable correct stereological conversion of 2d crystal size measurements to true 3d crystal size distributions csds in this contribution we introduce a database and spreadsheet program which provides an objective estimate of true crystal habit from raw 2d measurements the database compares the sample s 2d measurements with 2d shape curves for random sections through 703 different habits the output gives the five best match curves and corresponding crystal habits based on a least squares fit between sample and database the minimum sample size required to give good shape estimates from 2d data is tested using random runs of sections through known shapes with different population densities at minimum 75 crystal sections are required to robustly determine crystal habit for csd measurements if crystals are tabular in shape with more acicular shapes requiring a minimum of 250 sections suggesting a sample size of 250 sections to be used for shape determination in natural examples where the true 3d shape is unknown the crystal habit of the best match shape can then be used to provide a more robust csd analysis which in turn can be used to investigate magmatic processes with more certainty

asymmetrical quartz crystallographic fabrics formed during constrictional deformation numerical simulations predict unique quartz crystallographic fabric patterns for plane strain flattening and constriction multiple studies support the predictions for plane strain and flattening to test predictions for constriction this paper analyzes five examples of quartz crystallographic fabrics from a 1km wide domain of l tectonites in the pigeon point high strain zone klamath mountains california u s a these samples were deformed under greenschist to amphibolite facies conditions quartz c axis fabrics are similar to the predicted double girdle fabrics except that amphibolite facies samples exhibit caxis maxima and are distinctly asymmetrical about the elongation lineations activation of different slip systems combined with small deviations from pure constriction account for the c axis maxima and noncoaxial flow accounts for the fabric asymmetry the simple shear component is randomly oriented in geographic coordinates throughout the domain of l tectonites these data confirm that numerical simulations predict the quartz c axis fabric geometry developed during constriction for some deformation conditions and they confirm the quartz a axis patterns predicted for constriction for the first time these data also demonstrate that the relationship between quartz crystallographic fabrics and strain geometry is not straightforward and they indicate that a axis fabrics may be more useful indicators of strain geometry variations

the death valley turtlebacks reinterpreted as miocene pliocene folds of a major detachment surface determining the origin of extension parallel folds in metamorphic core complexes is fundamental to understanding the development of detachment faults an excellent example of such a feature occurs in the death valley region of california where a major undulatory detachment fault is exposed along the well known turtleback antiformal surfaces of the black mountains in the hanging wall of this detachment fault are deformed strata of the copper canyon formation new age constraints indicate that the copper canyon formation was deposited from 6 to 3 ma the formation was folded during deposition into a se plunging syncline with an axial surface coplanar with that of a synform in the underlying detachment this relation suggests the turtlebacks are a folded detachment surface formed during large scale extension in an overall constrictional strain field the present more planar black mountains frontal fault system may be the result of out stepping of a normal fault system away from an older detachment fa

detachment faults evidence for a low angle origin the origin of low angle normal faults or detachment faults mantling metamorphic core complexes in the southwestern united states remains controversial if sigma sub 1 is vertical during extension the formation of or even slip along such low angle normal faults is mechanically implausible no records exist of earthquakes on low angle normal faults in areas currently undergoing continental extension except from an area of actively forming core complexes in the solomon sea papua new guinea in light of such geophysical and mechanical arguments w r buck and b wernicke and g j axen proposed models in which detachment faults originate as high angle normal faults but rotate to low angles and become inactive as extension proceeds these models are inconsistent with critical field relations in several core complexes the rawhide fault an areally extensive detachment fault in western arizona propagated at close to its present subhorizontal orientation late in the tertiary extension of the region neither the wernicke and axen nor buck models predict such behavior in fact both models preclude the operation of low angle normal faults the authors recommend that alternative explanations or modifications of existing models are needed to explain the evidence that detachment faults form and operate with gentle dips

tertiary structure and thermal history of the harquahala and buckskin mountains west central arizona implications for denudation by a major detachment fault system the harquahala and buckskin mountains lie in the footwall of the whipple buckskin bullard detachment system in the harquahala mountains mesozoic fabric and structure are progressively more intensely overprinted by penetrative tertiary deformation toward the northeastern pan of the range tertiary mylonitic deformation is recognized by the presence of deformed miocene mafic dikes and characteristic textural features lineations in the mylonite trend 040 060 and megascopic kinematic indicators mostly indicate top to the northeast sense of shear the crest of an antiform defined by tertiary mylonitic fabric does not coincide with the topographic crest of the range reorientation of mesozoic foliation in the vicinity of this antiform suggests that it originated as two shear zones as opposed to a single zone that was then bent the northeastern part of the brown s canyon granite acts as a large low strain lozenge in the southeast limb of the foliation arch mylonitic foliation along the southeast side of the range above this lozenge is strongly oblique to the trend of the detachment fault but the relative orientation is consistent with left oblique normal shear evidence of tertiary plastic deformation is absent southwest of sunset pass minor low angle normal faults in the southwestern harquahala mountains and little harquahala mountains dip northeast the low angle normal faults are cut by nw trending high angle right oblique faults in the central and southwestern harquahala mountains 40ar 39ar age spectra from k feldspar muscovite and hornblende and total gas ages from biotite indicate late cretaceous to early tertiary cooling to argon closure temperatures biotite and k feldspar from the area northeast of sunset pass record rapid early to middle miocene cooling reconstruction of the original geometry of the detachment system based on thermal differences indicated by contrasting cooling histories and the orientation of an early miocene dike swarm indicate that the initial dip of the detachment fault was most probably between 30 and 40 thus the harquahala mountains are a lilted block exposing of the order of a 10 km section through the pre tertiary crust heterogeneous proterozoic gneiss sparse paleozoic and mesozoic metasedimentary rocks and an oligocene plutonic complex are extensively overprinted by tertiary mylonitic fabrics in the buckskin mountains hornblende 40ar 39ar cooling ages suggest that most of these rocks were below hornblende closure temperature by early tertiary time except in the vicinity of the oligocene plutonic complex feldspar and biotite 40ar 39ar cooling ages suggest that the footwall of the whipple detachment system experienced a more uniform cooling history in the buckskin mountains than in the harquahala mountains cooling ages between about 13 and 20 ma are recorded throughout the range with no consistent spatial pattern of ages the eagle eye detachment fault southeasternmost extension of the whipple buckskin bullard fault system becomes a transfer fault along the southeastern side of the harquahala mountains this fault separates two distinct extensional terranes on the northwest approximately 50 km of crustal extension has been accommodated on the whipple fault system while on the southeast a similar total magnitude of crustal extension is accommodated by a series of smaller normal faults in the adjacent big horn lake pleasant extended terrane as the deformation becomes more distributed maximum slip on any one normal fault decreases to the point where no single fault has enough slip to denude crustal levels at which tertiary plastic deformation has occurred

the evolution of bubble size distributions in volcanic eruptions we review observations of bubble size distributions bsds generated during explosive volcanic eruptions and laboratory explosions as inferred from vesicle size distributions found in the end products unimodal polymodal exponential and power law bsds are common even in the absence of coalescence and both power law and exponential distributions have been generated in the same eruption to date theoretical models have proposed incompatible mechanisms for producing the various distributions we here present a unifying mechanism data from our laboratory analogue experiments suggest that power law distributions are associated with highly non equilibrium degassing a numerical model is developed in which bubbles nucleate repeatedly and grow in the spaces between those of previous generations where in a non equilibrium degassing scenario the volatile concentration remains high this process causes the bsd to evolve from unimodal through exponential into a power law the exponent of the power law is a measure of the number of nucleation events or the duration of the nucleation period compared with the timescale of bubble growth the mathematical inevitability of the evolution from unimodal poissonian to power law is discussed the findings may resolve the apparent contradiction between the equilibrium degassing conduit flow models and the non equilibrium degassing conditions derived from bubble growth models of explosive volcanic eruptions the process of ongoing nucleation is the mechanism whereby the volcanic system maintains near equilibrium in the case of rapid depressurisation and slow volatile diffusion

the alichur dome south pamir western india asia collisional zone detailing the neogene shakhdara alichur syn collisional gneiss dome complex and connection to lithospheric processes neogene syn collisional extensional exhumation of asian lower middle crust produced the shakhdara alichur gneiss dome complex in the south pamir the 1 km thick mylonitic brittle top nne normal sense alichur shear zone asz bounds the 125 25 km alichur dome to the north the shakhdara dome is bounded by the 4 km thick mylonitic brittle top sse south pamir normal sense shear zone spsz to the south and the dextral gunt wrench zone to its north the alichur dome comprises cretaceous granitoids gneisses cut by early miocene leucogranites its hanging wall contains non weakly metamorphosed rocks the 22 17 ma alichur dome injection complex leucogranites transition from foliation parallel centimeter to meter thick sheets within the asz into discordant intrusions that may comprise half the volume of the dome core secondary fluid inclusions in mylonites and mylonitization temperature constraints suggest alichur dome exhumation from 10 15 km depth thermochronologic dates bracket footwall cooling between 410 130 c from 16 4 ma tectonic cooling exhumation rates 42 c myr 1 1 km myr contrast with erosion dominated rates in the hanging wall 2 c myr 0 1 km myr dome scale boudinage oblique divergence of the asz and spsz hanging walls and dextral wrenching reflect minor approximately e w material flow out of the orogen we attribute broadly southward younging extensional exhumation across the central south pamir between 20 4 ma to i mostly northward foreland directed flow of hot crust into a cold foreland during the growth of the pamir orocline and ii contrasting effects of basal shear related to underthrusting indian lithosphere enhancing extension in the underthrust south pamir and inhibiting extension in the non underthrust central pamir

very high slip rates on continental extensional faults new evidence from u th he thermochronometry of the buckskin mountains arizona abstract new u th he in apatite ages suggest rapid slip rates on the tertiary buckskin rawhide detachment fault which crops out in the buckskin and rawhide mountains of western arizona single crystal u th he in apatite ages were determined for a suite of crystalline rock samples from the footwall to this detachment the samples were collected from a transect parallel to the fault slip direction and proximal to the fault zone they show a linear decrease in age with distance from 14 to 11 ma over 15 km the age versus distance relationship is interpreted to show a slip rate of 4 2 1 2 0 8 mm yr continuing for a period of 3 myr this rate is more or less in agreement with but more precise than previously published slip rates from this and other large offset extensional detachments such slip rates are alarmingly high being roughly 5 10 times faster than rates commonly observed on active structures at the earth s surface these rapid slip rates suggest that significant seismic hazard might be posed by some extensional structures and they raise questions regarding the relationship between such rapidly slipping large offset structures and the slower smaller offset faults that are more commonly observed at the earth s surface

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

fold fault relationships in low angle detachment systems a close kinematic and timing relationship between low angle extensional faulting and upright to moderately inclined folding with fold axes parallel to the extension direction is established for two well exposed neogene detachment systems in the simplon region of the alps and in the death valley region of california fold amplitude is largest in the oldest mylonitic foliation of the footwall these folds are transected by younger mylonitic foliation s and cataclastic detachment faults which are themselves folded about similar axes in general the amplitude decreases and the wavelength increases for progressively younger fold structures consistent with a history of progressive folding during faulting the interplay between folding and faulting in the footwall reflects a component of shortening perpendicular to fold axial surfaces during extension parallel to fold axes this strain pattern can develop in regions with a transcurrent component or during lateral extrusion and does not necessarily require overall crustal thinning

kinematics and timing of tertiary extension in the western lake mead region nevada explanation of the origin of the complex array of structures in some extensional terranes including folds and normal strike slip and reverse faults includes many models that implicitly assume kinematic compatibility between and contemporaneous operation of these structures we present new stratigraphic and age data from the highly extended western lake mead region nevada together with an analysis of fault kinematics technique of marrett and allmendinger 1990 to test the assumptions of kinematic compatibility and contemporaneity of structures in an area of excellent exposure and superb stratigraphic control our analysis indicates an overlapping but clearly distinct chronology of deformation early regional extension 18 13 5 ma is marked by development of a basin into which the middle miocene lower horse spring formation was deposited in the western lake mead region this basin was disrupted by more areally restricted post 13 ma normal and kinematically coupled right slip faulting along the las vegas valley shear zone kinematic analysis of faults indicates an average regional extension direction of nearly due west for the middle and late miocene extension and right slip faulting was followed by development of dominantly southvergent contractional structures including tight east plunging folds and east striking reverse faults these structures deform the post 8 5 ma muddy creek formation the muddy creek formation is not cut by the las vegas valley shear zone older faults including the eastern las vegas valley shear zone reactivated as south vergent reverse faults northeast striking left slip faults cut folds and reverse faults these observations show that north south shortening and left slip faulting postdate the major phase of extension and right slip faulting in the western lake mead area dynamic models that invoke either a single stress field or rotating stress fields to explain development of structures in the western lake mead area are inconsistent with the kinematic and age data similarly kinematic models that view all structures in the context of a single strain field are precluded by systematic cross cutting relationships that demonstrate at least partial diachroneity of deformational styles large magnitude extension south of the las vegas valley and lake mead fault zones appears to have been followed by north south contraction that was highly localized near the region of greatest extension we suggest that lateral pressure gradients arising from differential crustal thinning at the northern end of the colorado river extensional corridor may have provided the driving mechanism for localized contractional deformation

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

normal faulting in the upper continental crust observations from regions of active extension abstract observations of present day normal faulting in regions of active continental extension may be helpful when interpreting the geological record in older extensional basins the most obvious manifestation of active extension is normal faulting in earthquakes earthquake foci are mostly confined to the upper seismogenic continental crust whose thickness imposes a scale on the observed deformation large earthquakes move faults with lengths similar or large compared with the thickness of the seismogenic layer large seismogenic normal faults on the continents appear to be restricted to a dip range of around 30 60 dips significantly gentler than 20 have not been observed in fault plane solutions of large earthquakes large seismographic normal faults are approximately planar in cross section and cut through the base of the upper seismogenic layer rotating about a horizontal axis as they move a reasonable estimate of the regional extension can often be made from the dip of the large normal faults and the tilt of the blocks they bound the domino model such faults are rarely continuous for more than 15 20 km but commonly change strike or step in an en echelon fashion this segmentation may occur on a scale controlled by the thickness of the seismogenic layer and is an important influence on sedimentation and drainage these observations are common to large seismogenic normal faults from a variety of tectonic settings and suggest that the kinematics of normal faulting may not be strongly influenced by the forces responsible for the extension which can vary widely in nature and magnitude small earthquakes which move faults that are small compared with the seismogenic layer thickness show no simple pattern and often can be interpreted as internal deformation of blocks bounded by large faults in some places the seismogenic basement faults do not reach the surface but are decoupled from the sedimentary cover by layers of weak lithology faults in the sedimentary cover may be strongly curved in cross section requiring their hangingwalls to deform internally estimating extension from such faults is not straightforward and requires a knowledge of the hangingwall deformation estimates made in this way are often very non unique and prone to large errors we postulate that the thickness of the seismogenic upper crust controls both the maximum length of large normal fault segments along strike and the maximum size of blocks that can rotate coherently about a horizontal axis

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

quantitative discrimination of magma fragmentation and pyroclastic transport processes using the fractal spectrum technique abstract the morphology of volcanic particles can yield insight into magma fragmentation transport processes and style of eruption however the complexity and variability of volcanic particle shapes make quantitative characterization difficult the technique applied in this study is based on fractal geometry which has been successfully used to characterize a wide variety of particles and shapes here fractal data is produced by dilation of the 2 d particle boundary to produce a full spectrum of fractal dimensions over a range of scales for each particle multiple fractal dimensions which can be described as a fractal spectrum curve are calculated by taking the first derivative of data points on a standard richardson plot use of multiple fractal dimensions results in more effective discrimination than expressions of shape based on one or two fractal dimensions quantitative comparisons are carried out using multivariate statistical techniques such as cluster and principal components analysis applications to samples from well documented eruptions e g mt st helens 1980 tambora 1815 surtsey 1963 64 indicate that the fractal spectrum technique provides a useful means of characterizing volcanic particles and can be helpful for identifying the products of specific fragmentation processes volatile exsolution phreatomagmatic quench granulation and modes of volcanic transport deposition tephra fall pyroclastic flow blast surge

the effects of lower crustal strength and preexisting midcrustal shear zones on the formation of continental core complexes and low angle normal faults to investigate the formation of core complexes and low angle normal faults we devise thermo mechanical simulations on a simplified wedge like orogenic hinterland that has initial topography moho relief and a preexisting mid crustal shear zone that can accommodate shear at very low angles 20 we mainly vary the strength of the lower crust and the frictional strength of the preexisting mid crustal shear zone we find that the strength of the lower crust and the existence and strength of a preexisting shear zone significantly affect the formation and evolution of core complexes with increasing lower crustal strength we recognize varying extensional features with decreasing exhumation rate these are characterized by bivergent metamorphic massifs classic cordilleran metamorphic core complexes multiple consecutive core complexes or boudinage structures and a flexural core complex underlined by a large subsurface low angle detachment fault with a small convex curvature topographic loading and mantle buoyancy forces together with divergent boundaries drive a regional lower crustal flow that leads to the exhumation of the lower crust where intensive upper crustal faulting induces strong unloading the detachment fault is a decoupling zone that accommodates large displacement and accumulates sustained shear strain at very low angle between upper and lower crust though the regional stress is largely andersonian we find non andersonian stress in regions adjacent to the preexisting shear zone and those with high topographic gradient our new models provide a view that is generally consistent with geological and geophysical observations on how core complexes form and evolve

growth of an emergent tuff cone fragmentation and depositional processes recorded in the capelas tuff cone sao miguel azores abstract the capelas tuff cone is an emergent surtseyan type tuff cone that erupted in shallow seawater off the coast of sao miguel azores in this paper we present a detailed stratigraphic study which is used to infer depositional processes and modes of fragmentation for the capelas tuff cone deposits the growth of the tuff cone can be divided into three stages based on variations in depositional processes that are probably related to differences in water magma w m ratios the first stage corresponds well to wet surtseyan type activity where wet fallout is the dominant depositional process with only minor representation of pyroclastic surge deposits the second stage of the eruption is suggested to be the result of alternating wet and slightly drier periods of surtseyan activity with an overall lower w m ratio compared to the first stage the drier surtseyan periods are characterized by the presence of minor grain flow deposits and undulating pyroclastic surge deposits that occasionally display relatively dry structures such as strongly grain segregated layers and brittle behavior when impacted by ballistic ejecta the first deposits of the second stage show an intense activity of pyroclastic surges but fallout commonly modified by surges is still the dominant depositional process during the second stage the third stage represents a final effusive period with the build up of a scoria cone and ponded lava flows inside the tuff cone crater phreatomagmatic fragmentation as seen by studies of the fine ash fraction

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

three magmatic components in the 1973 eruption of eldfell volcano iceland evidence from plagioclase crystal size distribution csd and geochemistry abstract the 1973 eruption of eldfell volcano iceland appears to have been a short simple event but textural and geochemical evidence suggest that it may have had three different magmatic components the first erupted fissure magmas were chemically evolved rich in plagioclase 18 and had shallow straight crystal size distribution csd curves the early lavas were less evolved chemically had lower plagioclase contents 13 and steeper slightly concave up csds the late lavas were chemically similar to the early lavas but even richer in plagioclase than the initial magmas 24 and had the steepest csds there was no chemical evidence for plagioclase fractionation but compositional diversity could be produced by clinopyroxene fractionation which must have occurred at depth we propose that the eruption started with old coarsened ostwald ripened magma left over from a previous eruption possibly that which produced surtsey island ten years earlier the early flows may be mixtures of small amounts of this old magma with a new low crystallinity uncoarsened magma or a completely new magma the late flows are another new magma from depth chemically similar to the early flows but which has grown plagioclase under increasing saturation undercooling perhaps during its ascent all three magmatic components may have originated from the same parent but had varying degrees of clinopyroxene fractionation plagioclase nucleation and growth and coarsening

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

kinematics and age of syn intrusive detachment faulting in the southern alps evidence for early permian crustal extension and implications for the pangea a versus b controversy permian basin formation and magmatism in the southern alps of italy have been interpreted as expressions of a wsw ene trending dextral megashear zone transforming early permian pangea b into late permian pangea a between 285 and 265 ma in an alternative model basin formation and magmatism resulted from n s crustal extension to characterize permian tectonics we studied the grassi detachment fault a low angle extensional fault in the central southern alps the footwall forms a metamorphic core complex affected by upward increasing top to the southeast mylonitization two granitoid intrusions occur in the core complex the synmylonitic val biandino quartz diorite and the postmylonitic valle san biagio granite u pb zircon dating yielded crystallization ages of 289 1 4 5 ma for the former and 286 8 4 9 ma for the latter consequently detachment related mylonitic shearing took place during the early permian and ended at 288 ma but kinematically coherent brittle faulting continued considering 30 anticlockwise rotation of the southern alps since early permian the extension direction of the grassi detachment fault was originally n s even though a dextral continental wrench system has long been regarded as a viable model at regional scale the local kinematic evidence is inconsistent with this and rather supports n s extensional tectonics based on a compilation of 200 u pb zircon ages we discuss the evolution and tectonic framework of late carboniferous to permian magmatism in the alps

development of extension parallel corrugations in the buckskin rawhide metamorphic core complex west central arizona extension parallel detachment fault corrugations are one of the defining features of metamorphic core complexes yet the origin of these corrugations remains disputed structural data from the buckskin rawhide metamorphic core complex in west central arizona demonstrate that prominent ne trending detachment fault corrugations are folds produced by extension perpendicular nw se shortening during the middle to late stages of core complex extension ca 18 10 ma kinematic data indicate that dominant ne directed slip on the buckskin rawhide detachment fault was locally overprinted by nw and se directed slip associated with corrugation folding the last motion recorded along several nw and se dipping flanks of the detachment fault is reverse slip directed toward the corrugation antiform hinges orientation patterns of upper plate bedding across the corrugations are compatible with folding about a ne trending axis extension perpendicular shortening in the lower plate is recorded by synmylonitic constriction and folding constriction and grain scale y axis shortening were most important during the later stages of mylonitization in the synextensional swansea plutonic suite which typically form l s tectonites upright meter and kilometer scale lower plate folds parallel the detachment fault corrugations and are best developed in well layered pre tertiary rocks most lower plate folding occurred by postmylonitic flexural slip that was coeval with detachment faulting the total amount of nw se shortening across the lower plate is 10 but the amount of nw se shortening recorded by the younger detachment fault is only 1 the relatively late stage development of corrugations in the buckskin rawhide metamorphic core complex suggests that the constriction and corrugation folding developed primarily in response to crustal thinning and tectonic denudation

neogene structural evolution of the sierra san felipe baja california evidence for proto gulf transtension in the gulf extensional province abstract the sierra san felipe located in the gulf extensional province of northeastern baja california experienced a complex deformation history of integrated normal and strike slip faulting block rotations and extension parallel folding as a result of neogene transtensional plate margin shear between the pacific and north american plates the eastern range front of the sierra san felipe is defined by three left stepping en echelon detachment faults that are linked by dextral and sinistral transfer faults and accommodation zones the las cuevitas santa rosa and huatamote detachments comprise ne to se dipping moderate to low angle normal faults that accommodate between 1 5 and 9 km of broadly e to se directed extension and show strong along strike displacement gradients hanging wall half grabens are infilled with northwest tilted miocene pliocene volcanic and sedimentary rocks that have been deposited nonconformably onto the batholithic basement stratigraphic relationships indicate that faulting on the las cuevitas and santa rosa detachment faults initiated synchronously as a kinematically linked fault system before 7 ma 9 8 ma based on the timing of footwall exhumation during the so called proto gulf phase of rifting paleostress calculations suggest a transtensional stress regime with ne to se directed extension and permutating vertical to n s trending subhorizontal contraction fault kinematics and paleostress orientations of the fault array do not vary through time but reflect the spatial distribution of fault planes with respect to a transtensional stress regime that lasted throughout the entire slip history of the detachments our data indicate that clockwise vertical axis block rotations and constrictional folding were an integral part of the deformation history in the sierra san felipe since rifting began in the late miocene and may have played an important role in facilitating the transfer of deformation between the main gulf escarpment and offshore faults in the gulf of california these observations support the hypothesis that middle miocene to present oblique divergent plate motion was accommodated by a single phase of integrated transtensional shearing in the gulf extensional province

miocene low angle normal faulting and dike emplacement homer mountain and surrounding areas southeastern california and southernmost nevada homer mountain and surrounding regions are within or adjacent to the western part of a broad region of low angle normal faults exposed within the lower colorado river trough during middle miocene time upper plate rocks in the homer sacramento dead and newberry mountains moved eastward or northeastward relative to the lower plate above single or multiple low angle normal faults deposition of coarse clastic sedimentary rocks occurred during extensional faulting and was accompanied by and closely followed by eruption of basaltic volcanics upper plate fault blocks of miocene volcanic and sedimentary rocks and older underlying crystalline rocks are tilted gently to steeply to the west or southwest low angle normal faults have complex sinuous traces due to the irregular form of fault surfaces antiformal warping and uplift of the lower plate about a north south trending axis due to tectonic denudation and isostatic rebound divided the regionally east thickening extensional allochthon into separate synformal and wedge shaped components the homer mountain area lies within or adjacent to the synformal component lower plate rocks are intruded by numerous middle miocene east west trending dikes which are in turn intruded by subhorizontal dikes both sets of dikes are cut and displaced by low angle normal faults except on the west flank of the newberry mountains where an east west trending dike cuts the basal low angle normal fault k ar geochronologic and field data establish the approximate synchroneity of dike emplacement and low angle faulting the state of stress in the lower plate as inferred from dike orientations differed radically from the state of stress in the upper plate as inferred from fault geometry low angle faulting and east northeast west southwest distension of upper plate rocks reflect regional reduction of compression in the east northeast west southwest direction and associated large scale east northeast west southwest crustal extension in contrast concave upward flexure of the lower plate in response to tectonic denudation and resultant isostatic uplift is inferred to have produced local subhorizontal compression at shallow crustal levels in the lower plate that overwhelmed the regional extensional stress and prevented emplacement of dikes oriented perpendicular to the direction of regional extension north south crustal extension during dike emplacement appears to have resulted from minor divergence of lower plate crustal blocks as they were displaced in a west southwest direction away from the colorado plateau

self induced deformation on the fault plane during an earthquake part ii continuous tangential displacements the problem of the coseismic deformation induced by an earthquake on its own fault plane is investigated here the analysis concentrates on the on fault displacement field accompanying the occurrence of an earthquake in response to a prescribed uniform shear slip and it is carried out on the basis of the classical analytical model by okada 1992 for a rectangular fault buried in an elastic homogeneous half space delimited by a planar free surface the analysis is subdivided into two separate papers the first dealing with normal and the second with tangential on fault displacements in this first paper concerning the study of the normal displacement component the contributions of the source and of the correction introduced by the free surface are investigated separately and their dependence on the fault characteristics is thoroughly discussed particular attention is also devoted to the effects of the normal displacement on the fault surface geometry it will be shown that the main effect is that of deforming the fault itself with deformation consisting chiefly in a rotation of the plane and in a bending of the fault edges the rotation angle is negligibly small on the order of 1 10 rad for a single earthquake although repeated seismic events occurring on the same fault might result in rotations of several degrees over geological time scales

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

explosive energy during volcanic eruptions from fractal analysis of pyroclasts abstract despite recent advances by means of experiments and high resolution surveys and the growing understanding of the physical processes before and during volcanic eruptions duration and type of eruptive activity still remain highly unpredictable this uncertainty hinders appropriate hazard and associated risk assessment tremendously in an effort to counter this problem experimentally generated pyroclasts have been studied by fractal statistics with the aim of evaluating possible relationships between eruption energy and fragmentation efficiency rapid decompression experiments have been performed on three differently porous sample sets of the 1990 1995 eruption of unzen volcano japan at 850 c and at initial pressure values above the respective fragmentation threshold u kueppers b scheu o spieler d b dingwell fragmentation efficiency of explosive volcanic eruptions a study of experimentally generated pyroclasts j volcanol geotherm res 153 2006 125 135 o spieler b kennedy u kueppers d b dingwell b scheu j taddeucci the fragmentation threshold of pyroclastic rocks epsl 226 2004 139 148 the size distribution of generated pyroclasts has been studied by fractal fragmentation theory and the fractal dimension of fragmentation df a value quantifying the intensity of fragmentation has been measured for each sample results show that size distribution of pyroclastic fragments follows a fractal law i e power law in the investigated range of fragment sizes indicating that fragmentation of experimental samples reflects a scale invariant mechanism in addition df is correlated positively with the potential energy for fragmentation pef while showing a strong influence of the open porosity of the samples results obtained in this work indicate that fractal fragmentation theory may allow for quantifying fragmentation processes during explosive volcanic eruptions by calculating the fractal dimension of the size distribution of pyroclasts it emerges from this study that fractal dimension may be utilised as a proxy for estimating the explosivity of volcanic eruptions by analysing their natural pyroclastic deposits

the transition from large magnitude extension to distributed dextral faulting in the buckskin rawhide metamorphic core complex west central arizona brittle fault data from the buckskin rawhide metamorphic core complex west central arizona document the miocene transition from large magnitude ne directed extension to distributed e w extension and dextral faulting the buckskin rawhide detachment fault locally records a clockwise rotation of the slip direction from dominant top ne directed slip to ene and e directed slip during the last stages of activity postmylonitic brittle deformation is dominated by normal and dextral slip along moderately to steeply ne dipping faults relative timing relationships indicate that most postmylonitic normal faulting was coeval with middle miocene detachment fault slip whereas dextral and oblique dextral faulting postdates detachment fault slip post 11 ma based on the persistent sw dip of lower plate fabrics and ne dip of abundant normal faults the amount of ne sw brittle extension across the mylonitic lower plate is estimated to be 20 30 in several areas the end of detachment faulting in the buckskin rawhide metamorphic core complex was marked by a transition to dominantly e w extension and associated dextral normal faulting at least 10 ne dipping postdetachment faults record 0 1 1 km dextral or oblique dextral displacement the cumulative amount of dextral shear across the core complex is probably 7 9 km which is the amount needed to restore the topographic trend of lower plate corrugations into alignment with the dominant extension direction postdetachment dextral transtensional faulting across the core complex reflects the increasing influence of the diffuse pacific north america transform plate boundary toward the end of the middle miocene

mechanisms for the formation of domal and basinal detachment faults a three dimensional analysis detachment faults i e regional low angle normal faults in metamorphic core complexes of the north american cordillera commonly exhibit doubly plunging antiformal domal and synformal basinal geometries these geometries have been previously attributed to the superposition of two processes 1 the antiforms and synforms with axes parallel to the extension direction originated as primary fault undulations or corrugations developed coevally with fault slip and 2 the antiforms and synforms with axes perpendicular to the extension direction were produced by isostatic uplift due to tectonic denudation however the coaxial relationship between undulations of detachment faults and the sedimentary beds and metamorphic foliations in both the upper and lower plates of some detachment fault systems in the u s cordillera suggests that the synforms and antiforms with axes parallel to the extension direction may have formed as folds and that the domal and basinal geometry of detachment faults may have formed synchronously by a single process a three dimensional elastic thin plate model is developed to investigate possible mechanisms for the formation of domal and basinal detachment faults this model explores the interactions among the vertical horizontal and basal shearing forces during the formation of warped detachment faults the model considers the role of compression perpendicular to the extension direction and stress reduction parallel to the extension direction the results of the model suggest that the stress reduction itself in the extension direction is insufficient to cause buckling of a thin elastic crust two mechanisms are most likely to explain the formation of domal and basinal geometries of detachment faults 1 an upward pushing by undulatory crustal roots or buoyant synextensional plutons beneath the extensional belt and 2 buckling caused by a compression perpendicular to the extension direction both mechanisms are consistent with the geologic constraints during the development of cordilleran core complexes in addition to isostatic uplift due to tectonic denudation upward warping of detachment faults can also be caused by buckling basal shearing and buoyant forces

north south contraction of the mojave block and strike slip tectonics in southern california the mojave block of southern california has undergone significant late cenozoic north south contraction this previously unappreciated deformation may account for part of the discrepancy between neotectonic and plate tectonic estimates of pacific north american plate motion and for part of the big bend in the san andreas fault in the eastern mojave block contraction is superimposed on early miocene crustal extension in the western mojave block contractional folds and reverse faults have been mistaken for extensional structures the three dimensional complexity of the contractional structures may mean that rigid block tectonic models of the region based primarily on paleomagnetic data are unreliable

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

structural and stratigraphic development of the whipple chemehuevi detachment fault system southeastern california implications for the geometrical evolution of domal and basinal low angle normal faults recent debates on the development of detachment faults during continental extension have been centered on whether they were active at low angles and initiated as relatively planar faults in addition intense investigations have concentrated on how the detachment fault geometry evolved in time and space and how the extension was accommodated in their upper and lower plates the superb exposure of the mid tertiary whipple mountains detachment fault system in southeastern california provides an unusual opportunity to address these questions analysis of the stratigraphy and structures of upper plate rock units constrains the three dimensional kinematic evolution of the mid tertiary whipple chemehuevi detachment fault system in southeastern california tertiary volcanic strata in the northwestern whipple mountains and central chemehuevi valley consist of similar rock types that have undergone about 15 extensional strain in contrast a different succession of tertiary volcanic and sedimentary rocks underlies the northeastern whipple mountains along the structural trend of central chemehuevi valley these strata have been extended by 40 differences in magnitudes of extension and mid tertiary stratigraphies between these two areas argue for the presence of a synformal gently east dipping normal fault above the whipple fault in the central chemehuevi valley this fault accommodates 15 km of the total 40 km extension along the whipple chemehuevi detachment fault system in addition tilting patterns of upperplate strata and the presence of an areally extensive syn extension landslide 70 km2 show that 1 the whipple detachment fault was active at low angles 70 km2 from the north side of the whipple mountains its emplacement occurred before the end of detachment faulting

crustal xenoliths in the 6220 bp saefell tuff cone south iceland evidence for a deep diatreme forming surtseyan eruption abstract three types of xenoliths were collected from a 80 m high vertical section of the saefell tuff cone on heimaey iceland these types are alkali basaltic type i sedimentary type ii and cpx bearing basalts type iii they are distinct in hand specimen thin section and geochemistry near the base of the eruptive succession only juvenile material and type i xenoliths are present the middle section has type ii xenoliths in addition to type i xenoliths of type iii first appear in the uppermost xenolith rich horizons where they are found together with all other types juvenile saefell clasts are present in various amounts throughout the investigated section a downward migration of explosion foci during the eruption is established based on the distribution of the different types of xenoliths in the tuff cone i e the appearance of type i type ii type iii xenoliths with increasing stratigraphic height in combination with the observed relations in basement stratigraphy beneath heimaey in the beginning of the eruption a combination of high eruption rate and abundant seawater resulted in shallow seated explosions depositing only juvenile material and type i xenoliths the downward migration of explosion foci is suggested to be the result of lowered influx of seawater due to the formation of a cone that reached 50 75 m a s l in combination with lowered eruption rate as the basement stratigraphy beneath heimaey is well documented by a 1565 m deep drill hole the depth of the explosions can be constrained to vary between less than 170 m in the beginning of the eruption and exceeding 820 m in the final phases this is much deeper than previously reported for any surtseyan tuff cone forming eruption and consistent with the downward penetration of a diatreme

tertiary sedimentary rocks associated with the harcuvar core complex in arizona u s a insights into paleogeographic evolution during displacement along a major detachment fault system abstract geologic mapping and analysis of oligocene miocene sedimentary rocks in the upper plate of the buckskin rawhide detachment fault system west central arizona reveals a complex paleogeographic history during fault displacement involving shifting sediment source areas and multiple drainage reversals within the study area four upper plate fault blocks are capped by homoclinal sedimentary sections that display fanning dip relationships indicating concurrent tilting and sedimentation during deposition of these strata upper plate sedimentation was intermittently confined to separate half graben while at other times rapid rates of aggradation relative to fault displacement resulted in burial of ridges separating sub basins four sedimentary assemblages recognized within the study area can be correlated between fault blocks on the basis of lithologic similarity stratal position provenance paleocurrent data and sparse geochronologic constraints the basal assemblage consists of lacustrine rocks and interfingering fluvial strata composed of detritus derived from the granitic terrane surrounding the northern part of the study area these sediments were deposited during the earliest stages of tilting of upper plate fault blocks and inferred displacement along the buckskin rawhide detachment fault system the lower assemblage which directly overlies the basal assemblage consists of fine grained lacustrine deposits sandy conglomerate and breccia during lower assemblage deposition mass flow dominated alluvial fans were derived from source areas consisting mainly of mesozoic and paleozoic rocks exposed to the south of the study area these source areas may have been associated with the breakaway fault of the detachment system or with fault blocks stranded above structurally elevated parts of the footwall tilting of fault blocks continued during deposition of the lower assemblage and strongly affected dispersal patterns and lithofacies distributions lacustrine sediments were deposited mainly within large deep sub basins and with few exceptions lower assemblage streams followed half graben axes the middle assemblage consists of conglomerate and sandstone deposited by an extensive south directed stream system that probably flowed off undistended parts of the hanging wall across extended parts and locally onto the footwall tilting of upper plate fault blocks continued during middle assemblage deposition but rapid burial of escarpments by sediment permitted relatively unobstructed southward drainage the upper assemblage consists of sandy conglomerate deposited by a northeast directed system of broad shallow streams these deposits display a variety of clast types including tertiary mylonitic sedimentary and volcanic rocks that were eroded from the upwarped footwall of the core complex and overlying klippen upper assemblage streams maintained relatively consistent northeasterly flow even during the early stages of deposition when some fault blocks were still rotating

linking variable explosion style and magma textures during 2002 at stromboli volcano italy microtextural characteristics of fresh ejecta from stromboli volcano were examined from three periods of differing eruption style and intensity in 2002 activity shifted from relatively weak and infrequent ash charged explosions during january through may into two broad cycles of waxing activity in june through late september and late september through december followed by the onset on 28 december of the 2002 2003 effusive eruption analyzed sets of lapilli from may september october and 28 december show contrasts in the physical properties of magma resident in the shallow conduit during this range of activity three distinct textures are observed among the analyzed pyroclasts low density ld with an abundance of subspherical bubbles the presence of large irregularly shaped bubbles and a light to transparent glass matrix transitional texture tt with an intermediate number of subspherical bubbles a high frequency of large irregularly shaped bubbles and a honey colored glass matrix and high density hd with sparse relatively small bubbles conspicuous large irregular bubbles and a dark glass matrix observational and quantitative data density vesicle size indicate that these textures are linked through variable residence time in stromboli s shallow conduit with an ongoing evolution from ld to hd magma calculations suggest that residual ld magma will evolve to hd texture in a period of hours to days contrasting amounts of the ld tt and hd magmas are present in each sample with the most tt in may the most ld in september october and the most hd in december this implies that the shallow magma had a different rheology at each collection period the viscosity of ld and hd magmas are calculated to be in the range of 2 000 to 2 600 and 3 000 to 5 000 pa s respectively which with their changing proportions must have implications for rates of bubble slug ascent and processes of fragmentation this study suggests that an increasing maturity of magma in stromboli s shallow conduit with resultant increase in viscosity feeds back to reduce the intensity of explosions whereas a steady flux of ld magma favors more powerful explosions

textural variation in juvenile pyroclasts from an emergent surtseyan type volcanic eruption the capelas tuff cone sao miguel azores abstract here i present textural data i e vesicularity vesicle size distributions vsd plagioclase crystallinity crystal size distributions csd combined with fractal analyses of particle outlines from a natural succession of alternating fall and surge deposits in the emergent capelas tuff cone azores the textural variation in the capelas succession is surprisingly small considering the wide variety of fragmentation processes vent activity and emplacement mechanisms that are characteristic of emergent eruptions the plagioclase crystal content varies between 24 and 33 vol csd analyses of plagioclase show near linear trends with a slight increase in time for the smallest crystal sizes with surge deposits having more groundmass plagioclase when compared with fall deposits this is consistent with crystallization induced by degassing and decompression at lower eruption rates the vesicularities of the capelas pyroclasts are more variable 18 to 59 vol with vsds displaying kinked trends characteristic of coalescence this is especially evident in the fall deposits and consistent with being formed in continuous uprush jetting with an overall shallow fragmentation level within the conduit bubble coalescence can also be identified in the surge deposits although to a much lesser extent the amount of bubble coalescence is negatively correlated with the amount of groundmass crystallization i e plagioclase in the capelas deposits a relatively broad range of fractal dimensions with average dbox 1 744 and 0 032 for the outlines of pyroclastic fragments emplaced by fall or as surges indicate that there is little difference in the fragmentation process itself at capelas in addition to this the fact that the fractal dimensions for both the fall and surge end members completely overlap suggests that shape modification due to abrasion and chipping of grain edges was minor during emplacement of base surges these results are consistent with emergent eruptions building tuff cones to be a relatively low energy phreatomagmatic landform e g at least when compared with more energetic phreatomagmatic eruptions producing tuff rings and maar volcanoes

development of oceanic detachment and asymmetric spreading at the australian antarctic discordance 1 the largest known oceanic detachment terrains occur in segment b3 of the australian antarctic discordance aad using newly collected bathymetry magnetic and gravity data we show that segment b3 is divided into two contrasting second order segments the western subsegment b3w is characterized by well ordered ridge parallel abyssal hills and low mantle bouguer gravity anomalies the eastern subsegment b3e displays rough chaotic morphology and includes several megamullions characterized by high mantle bouguer gravity anomaly values the crust is estimated to be thinner by a maximum of 3 km in southern b3e the combination of chaotic morphology with thinner crust supports the idea that the megamullions are exposed footwalls of oceanic detachments megamullion terrains are characterized by higher magnetization than adjacent terrains most likely as a result of serpentinization of peridotite exposed at the detachment surfaces detachment surfaces constitute up to 70 of the total area of both ridge flanks younger than 2 ma in b3e indicating that oceanic detachments have played a major role in its development spreading in b3e has been extremely asymmetric with higher apparent rates associated with the large detachment surfaces where up to 75 of the total extension occurred similar asymmetric spreading on oceanic detachments is also recognized in segment b4 suggesting that this is the dominant mode of extension associated with cold mantle and low magma supply in this deepest part of the aad where it is confined to a mere 100 km long section of the aad spreading axis

late stage slip history of the buckskin rawhide detachment fault and temporal evolution of the lincoln ranch supradetachment basin new constraints from the middle miocene sandtrap conglomerate

constrictional strain in a non coaxial shear zone implications for fold and rock fabric development central mojave metamorphic core complex california finite strain and fold analyses of footwall mylonites in the central mojave metamorphic core complex cmmcc reveal two distinct deformation paths that generated non coaxial constrictional strain the first path recorded in the waterman hills accomplishes constrictional strain through the formation of l tectonites at the grain scale the second path recorded in the mitchel range involves a combination of plane strain at the grain scale and y axis shortening through syn mylonitic folding the ductile shear zone in the waterman hills is approximately 500 m thick and is developed entirely within a miocene syn kinematic granodiorite strain magnitude decreases structurally downward the mylonites contain abundant ne directed non coaxial microstructures yet are l tectonites rf o analysis of quartz ribbon grain shapes indicate prolate strains k 6 and bulk chemical data suggest that mylonitization was isochemical or involved volume loss therefore ductile shearing in the waterman hills involved true constrictional strain at the grain scale immediately to the southeast in the mitchel range the lateral continuation of the shear zone is more than 1000 m thick and primarily involves lithologically heterogeneous pre tertiary basement the mylonitic fabric comprises a well developed foliation and ne trending stretching lineation rf o strain analysis was applied to grain shapes of cataclasized and strongly altered garnet porphyroclasts in a peraluminous granite in contrast to the waterman hills finite strains in these l s tectonites approximate plane strain k 1 1 however the mylonitic fabric is strongly folded about axes uniformly oriented subparallel to the stretching lineation the coaxial folds range from open to isoclinal we interpret the folds to have nucleated with axes parallel to the stretching lineation during mylonitization and to have accomplished y axis shortening in the shear zone macroscopic folds of the brittle detachment spatially correspond to open folds in the ductile shear zone suggesting that y axis shortening continued after the fault zone had reached the brittle regime corrugations and folds oriented subparallel to the transport direction are characteristic features of fault zones in cordilleran metamorphic core complexes therefore constrictional strain may be common in core complexes strain compatibility with upper plate rocks in core complexes may be maintained by y axis shortening along conjugate strike slip faults a nearly uniaxial stress field 3 2 1 is consistent with most of the structures formed in basin and range extension and could generate the observed constrictional strain

progressive back warping of a rider block atop an actively exhuming continental low angle normal fault abstract can continental low angle normal faults lanfs initiate and remain active at shallow although the mai iu fault is still active in the west to the east it dips 15 22 at the surface and has been abandoned in favor of the younger 42 7 dipping gwoira fault fault abandonment has caused part of the former hanging wall to be captured by the footwall this rider block s structure records progressive shallowing of the mai iu fault concurrent with slip we present 26al 10be cosmogenic nuclide burial dating along with a recently published u th sm he apatite and zircon age these provide the first direct dating of the gwoira conglomerate these data imply inception of the mai iu fault before 3 12 6 04 ma and inception of the gwoira fault between 2 29 and 5 62 ma bedding fault cut off angles suggest the mai iu fault did not initiate as a lanf rather it initially dipped 44 at the surface and was subsequently back rotated while active to its present 15 22 surface dip this suggests that continental lanfs initiate at moderate dips and evolve according to the rolling hinge process

large magnitude continental extension an example from the central mojave metamorphic core complex the central mojave metamorphic core complex is defined by a belt of miocene brittle ductile extension and coeval magmatism the brittle ductile fault zone defines a basin and dome geometry that results from the interference of two orthogonal fold sets that we infer to have formed by mechanically independent processes one fold set contains axes that lie parallel to the extension direction of the shear zone and has a maximum characteristic wavelength of about 10 km the axial surfaces of these folds can be traced from the footwall mylonites through the brittle detachment and into hanging wall strata however folds of mylonitic layering have smaller interlimb angles than those of the brittle detachment suggesting that the folds began to form during ductile shearing and continued to amplify in the brittle regime possibly after movement across the fault zone ceased mesoscopic fabrics related to the transport parallel fold set indicate that the folds record true crustal shortening perpendicular to the extension direction we interpret these folds to form in response to elevated horizontal compressive stress perpendicular to the extension direction and suggest that this stress regime may be a natural consequence of large magnitude extension the other fold set has axes perpendicular to the extension direction and a characteristic maximum wavelength of about 50 km mesoscopic fabrics related to these folds include northwest striking joints kink bands and en echelon tension gash arrays these fabrics formed after mylonitization and record both layer parallel extension and northeast side up subvertical shear the postmylonitic fabrics are kinematically compatible with rolling hinge style isostatic rebound of the footwall following tectonic denudation the relative timing of extension related magma intrusion and ductile deformation varies through the central mojave metamorphic core complex on the scale of the small mountain ranges that make up the central mojave metamorphic core complex no correlation was observed between either shear zone thickness or intensity of ductile deformation and either the proximity or relative volume of extension related igneous rocks this suggests that models that invoke a single upper crustal genetic relationship such as magmatism triggering extension or vice versa do not apply to the central mojave metamorphic core complex systematic variation in the relative timing of dike emplacement and mylonitization suggests that at the time of dike emplacement rocks in the mitchel range were below the brittle ductile transition while those in the hinkley hills were above it the hinkley hills and mitchel range are separated by 2 km in the dip direction of the fault zone which suggests that the vertical thickness of the brittle ductile transition probably was between 100 and 950 m

development of active low angle normal fault systems during orogenic collapse insight from tibet active north trending rifts in tibet vary significantly in their character as a function of extension magnitude most rifts are characterized by internally drained basins bounded by high angle normal faults with paleogene or older rocks in the footwall however the central part of the yadong gulu rift near lhasa and the newly documented lunggar rift in west central tibet are bounded by low angle normal faults detachments with mylonitic rocks and miocene granites in the footwall and exhibit active basin incision and intrabasin topographic highs in areas of inferred maximum extension we suggest that tibetan rifts initiate as high angle normal fault and half graben or graben basin systems and evolve in response to increasing extension and footwall isostatic rebound into detachment systems that are active at uppermost crustal levels and above which rift basin fill is being uplifted and eroded

geologic continuous casting below continental and deep sea detachment faults and at the striated extrusion of sacsayhuaman peru in one common type of industrial continuous casting partially molten metal is extruded from a vessel through a shaped orifice called a mold in which the metal assumes the cross sectional form of the mold as it cools and solidifies continuous casting can be sustained as long as molten metal is supplied and thermal conditions are maintained i propose that a similar process produced parallel sets of grooves in three geologic settings as follows 1 corrugated metamorphic core complexes where mylonitized mid crustal rocks were exhumed by movement along low angle normal faults known as detachment faults 2 corrugated submarine surfaces where ultramafic and mafic rocks were exhumed by normal faulting within oceanic spreading centers and 3 striated magma extrusions exemplified by the famous grooved outcrops at the inca fortress of sacsayhuaman in peru in each case rocks inferred to have overlain the corrugated surface during corrugation genesis molded and shaped a plastic to partially molten rock mass as it was extruded from a moderate to high temperature reservoir

tectonics of mid tertiary extension along a transect through west central arizona large magnitude miocene extension in west central arizona occurred primarily along three imbricate northeast dipping normal faults the structurally highest of these faults the gently dipping buckskin rawhide detachment fault accommodated approximately 66 km of crustal extension whereas the two structurally lower faults accommodated a total of about 20 km extension due to this large magnitude extension an area at the earth s surface that was 10 to 20 km wide is now over a 100 km wide and crystalline rocks with mid tertiary mylonitic fabrics uncovered by detachment faulting are exposed over roughly 2000 km in the harcuvar metamorphic core complex most of the upper plate of the buckskin rawhide detachment fault was largely undeformed by internal extension only the thin tapered end of the upper plate was highly extended during extension the lower plate must have flexed to conform to the listric underside of the upper plate and to have flattened to its present subhorizontal form as it was uncovered grooves on the underside of the upper plate were apparently imposed on the pliable lower plate as it was denuded forming extension parallel folds in the lower plate low flexural strength characterized the lower plate during denudation and a highly mobile low viscosity deeper crust must have effectively decoupled the upper crust from the mantle lithosphere

mylonitization in the lower plate of the buckskin rawhide detachment fault west central arizona implications for the geometric evolution of metamorphic core complexes abstract in metamorphic core complexes it is commonly unclear whether lower plate mylonites formed as the down dip continuation of a detachment fault or whether they represent a subhorizontal shear zone that was captured by a more steeply dipping detachment fault detailed microstructural fabric and strain data from mylonites in the buckskin rawhide metamorphic core complex west central arizona constrain the structural development of the lower plate shear zone widespread exposures of 22 21 ma granitoids of the swansea plutonic suite enable us to separate miocene strain coeval with core complex extension from older deformation mylonites across the lower plate consistently record top to the ne directed shear miocene quartz and feldspar deformation recrystallization mechanisms indicate 450 500 c mylonitization temperatures that were relatively uniform across a distance of 35 km in the extension direction quartz dynamically recrystallized grain sizes do not systematically vary in the extension direction strain recorded in the swansea plutonic suite is also relatively uniform in the extension direction which is incompatible with models in which lower plate mylonites form as the ductile root of a major detachment fault altogether these data suggest the mylonitic shear zone initiated with a 4 dip and was unroofed by a more steeply dipping detachment fault system lower plate mylonites in the buckskin rawhide metamorphic core complex thus represent a captured subhorizontal shear zone rather than the down dip continuation of a detachment fault

timing rate and magnitude of slip on the buckskin rawhide detachment fault west central arizona we present thermochronologic and geochronologic data that constrain the slip history of the buckskin rawhide detachment fault in west central arizona one of the largest extensional fault systems in the north american cordillera u th he zircon and apatite thermochronology integrated with 40ar 39ar geochronology of postdetachment volcanic rocks indicate that large magnitude extension associated with the detachment fault initiated at 21 20 ma and continued until 12 11 ma in the southwestern portion of the buckskin rawhide metamorphic core complex u th he footwall cooling ages from the breakaway zone in the western bouse hills to upper greenschist facies mylonites in the southern buckskin mountains indicate that the slip rate on the detachment fault was 3 1 5 1 km myr during the early miocene space time patterns of hanging wall tilting suggest that at 17 16 ma a secondary detachment fault breakaway developed 12 km northeast of the primary detachment fault breakaway proximal conglomerates deposited in a supradetachment basin adjacent to the secondary breakaway scarp were displaced 6 11 km northeast in the middle miocene by the buckskin rawhide detachment fault at a slip rate of 1 2 2 7 km myr the total displacement across the detachment fault in the southwestern portion of the core complex is 24 10 km well short of the previous estimate of 66 8 km across the entire core complex based on these data and new observations we propose that total displacement on the buckskin rawhide detachment fault system increases in the slip direction to 40 50 km at the northeastern end of the exposed footwall corresponding to time averaged slip rates that ranged from 2 km myr to 6 km myr across the entire core complex

extension parallel folding in the scandinavian caledonides implications for late orogenic processes abstract structural analyses of the devonian basins and basement rocks in the scandinavian caledonides show the existence of large scale e w oriented folds which affect the para autochthonous western gneiss region of the baltic craton these folds mostly occur in areas of widespread and intense late orogenic extension chronology based on structural relationships both within the devonian sediments and caledonian rocks shows that folding has been active from the first stages of the ductile extension and ended after devonian basin sedimentation folding evolved through time towards crustal scale transcurrent left lateral faulting in the zones of maximum deformation such an evolution is explained by progressive unloading of the western gneiss region due to extension which involved a permutation of the maximum stress from a vertical position at the onset of extension to a horizontal position in the subsequent stages folding parallel to late orogenic extension as well as left lateral translation on east to northeast trending crustal scale faults suggest the occurrence of a regional direction of compression perpendicular to the direction of extension this in turn argues for an external force triggering the late orogenic extensional process in the scandinavian caledonides