subsurface failure in spherical bodies a formation scenario for linear troughs on vesta s surface abstract many asteroids in the solar system exhibit unusual linear features on their surface the dawn mission recently observed two sets of linear features on the surface of the asteroid 4 vesta geologic observations indicate that these features are related to the two large impact basins at the south pole of vesta though no specific mechanism of origin has been determined further the orientation of the features is offset from the center of the basins experimental and numerical results reveal that the offset angle is a natural consequence of oblique impacts into a spherical target here we demonstrate that a set of shear planes develops in the subsurface of the body opposite to the point of first contact these subsurface failure zones then propagate to the surface under combined tensile shear stress fields after the impact to create sets of approximately linear faults on the surface comparison between the orientation of damage structures in the laboratory and failure regions within vesta can be used to constrain impact parameters e g the approximate impact point and likely impact trajectory

chronology of internal drainage development and uplift southern puna plateau argentine central andes fundamental to understanding the late cenozoic evolution of the central andes is knowledge of the age that the puna altiplano plateau became a unique tectonomorphic entity within the orogen the two attributes that define the puna altiplano are 1 its average elevation of 3 km and 2 regions of high elevation that correspond spatially to regions with internal drainage the chronology of plateau uplift to elevations similar to modern ones is equivocal owing to an inability to define the absolute elevation of the plateau surface through time however because regions of the puna altiplano have nonmarine evaporites nad because such evaporites must accumulate within regions of internal drainage the chronology of these strata elucidates when the plateau became internally drained we show that for the southern puna between lat 24 and 26 s internal drainage was established between 14 1 and 24 2 ma and probably by 15 ma these results corroborate and refine other workers inferences that the puna altiplano plateau became uplifted and paleogeographically isolated from the adjacent fore arc and foreland during middle to late miocene time

mechanical and geological effects of impact cratering on ida asteroids respond to impact stresses differently from either laboratory specimens or large planets gravity is typically so small that seismic disturbances of a few cm s 1can devastate unconsolidated topography yet the presence of regolith and the likelihood that many asteroids are gravitational assemblages tell us that gravity cannot generally be ignored we use numerical models for impact fracture in solids to examine the initial stage of crater formation on asteroid 243 ida up to the cessation of fracture and the establishment of the cratering flow at this stage we can infer final crater diameters but not profiles we find that a modified strength scaling applies for craters up to a few 100 m in diameter forming in rock subject to ida s gravity and that gravity controls all craters larger than 1 km bright annuli around a number of intermediate craters may be the result of low velocity surface disturbances rather than bright proximal ejecta deposits we also consider large impactors to which ida presents a curved finite target surface with irregular gravity these can excavate asymmetrical concavities stresses from large events can refocus and cause fracture far from the crater using the shape of ida as a basis for 3d hydrocode simulations we show that impact genesis of the vienna regio concavity can cause fracture in pola regio where grooves are observed in spacecraft images other simulations indicate that the formation of the 10 km crater azzurra might have reopened these fractures which may account for their fresh appearance this mechanism of groove formation requires an interior which coherently transmits elastic stress while this precludes a classic rubble pile asteroid it does allow well joined fault planes and welded blocks or pores smaller than the stress pulse

critical slip surface and landslide volume of a soil slope under random earthquake ground motions finding the critical slip surface and estimating the landslide volume are of primary importance for slope seismic design however this may be difficult due to the uncertainty of ground motions to address this problem a new method for calculating uncertainties is recommended in this paper especially for the critical slip surface and landslide volume under random earthquake ground motions firstly a series of intensity frequency nonstationary random earthquake ground motions were generated based on an improved orthogonal expansion method a given number of potential slip surfaces were set in a soil slope subsequently the factor of safety fos of each slip surface for all ground motions was calculated and the minimum fos curves were obtained it was found that the critical slip surfaces and failure times are uncertain under different earthquakes the monte carlo method was used to verify the accuracy of probability density evolution method pdem and the results of the pdem and the monte carlo method are consistent meaning that the pedm has higher computational efficiency moreover the distributions of earthquake triggered landslide volume and landslide depth were analyzed by considering equivalent extreme events both landslide volume and depth exhibit a normal distribution for a homogeneous soil slope the framework of this study is meaningful for slope seismic design in engineering for example the location of critical slip surface can be used for slope reinforcement and the distribution of sliding volume can be used for disaster assessment

large scale cenozoic wind erosion in the puna plateau the salina del fraile depression wind erosion is integral to the evolution of arid landscapes on earth and mars but the nature of long term wind erosion of bedrock is poorly understood here we describe the salina del fraile sdf

accurate source depths and focal mechanisms of shallow earthquakes in western south america and in the new hebrides island arc synthetic seismograms are matched to long period p waveforms in order to obtain accurate depths of shallow earthquakes with known focal mechanisms accurate depths are obtained for a large sample of moderate sized 6 ms 7 events which produced relatively simple p waveforms and do not require intensive analysis the estimated error in the determination of depth is about 5 km or less for most of the events considered the procedure is applied to two suites of events one in south america 83 events and one in the new hebrides 61 events in these two areas of contrasting styles of subduction the accurate depths provide new information on the bending of descending plates near trenches the seismicity and tectonics of the upper plate and the geometry and structure of the zone of contact between the upper and descending plates depths of suboceanic earthquakes which occurred near the trench and which have either tensional and compressional horizontal stress axes agree well with chapple and forsyth s 1979 preferred model of a bending elastic perfectly plastic plate however an unusually deep event occurred beneath the fiji plateau in an area of young ocean floor supposed to have been formed by sea floor spreading processes only since late miocene time the calculated depth of the event 48 km places it below the lithosphere asthenosphere boundary derived from thermal models vertical cross sections through the shallow parts of the subduction zones show that most of the earthquakes with thrust faulting focal mechanisms can be interpreted to be interplate events located in a thin 10 km thick curved zone of contact dipping arcward or landward from the trench axis however a few events with focal mechanisms similar to the interplate events do not fit on the inferred thin zone of contact these events would be accommodated by an interplate zone 15 km thick in both subduction zones interplate events occur at depths between about 15 and 50 km however the downdip width of the south american interplate zone is 2 to 3 times larger than that in the new hebrides most of the intraplate events near the interplate boundary are located in the descending plate in the upper plate seismicity is more pronounced in the back arc regions of the two subduction zones thus our results support the generality of a relatively aseismic zone in the upper plate located trenchward of the magmatic arc depths of shallow earthquakes in the eastern andes and sub andean regions of south america range between about 9 and 36 km indicating that both the upper and middle crust are seismically active about 75 of the events occurred in the upper 25 km furthermore the focal mechanisms indicate that from ecuador to argentina the crust of the eastern side of the andean cordilleran is in a state of east west compression oriented nearly parallel to the direction of convergence between the nazca and south american plates the depths the orientations of the earthquake nodal planes and the close relationships to quaternary tectonics imply basement involved crustal shortening in much of the eastern andes and sub andean zones of peru and argentina

critical crater diameter and asteroid impact seismology if impact stress reverberation is the primary gradational process on an asteroid at global scales then the largest undegraded crater records an asteroid s seismological response the critical crater diameter dcrit is defined as the smallest crater whose formation disrupts all previous craters globally up to its size it is solved for by combining relationships for crater growth and for stress attenuation the computation for dcrit gives a simple explanation for the curious observation that small asteroids have only modest undegraded craters in comparison to their size whereas large asteroids have giant undegraded craters dcrit can even exceed the asteroid diameter in which case all craters are local and the asteroid becomes crowded with giant craters dcrit is the most recent crater to have formed on a blank slate when it is equated to the measured diameter of the largest undegraded crater on known asteroids peak particle velocities are found to attenuate with the 1 2 1 3 power of distance less attenuative than strong shocks and more characteristic of powerful seismic disturbances this is to be expected since global degradation can result from seismic cm s 1 particle velocities on small asteroids attenuation as modeled appears to be higher on asteroids known to be porous although these are also bodies for which different crater scaling rules might apply

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

three dimensional modeling of pull apart basins implications for the tectonics of the dead sea basin we model the three dimensional 3 d crustal deformation in a deep pull apart basin as a result of relative plate motion along a transform system and compare the results to the tectonics of the dead sea basin the brittle upper crust is modeled by a boundary element technique as an elastic block broken by two en echelon semi infinite vertical faults the deformation is caused by a horizontal displacement that is imposed everywhere at the bottom of the block except in a stress free shear zone in the vicinity of the fault zone the bottom displacement represents the regional relative plate motion results show that the basin deformation depends critically on the width of the shear zone and on the amount of overlap between basin bounding faults as the width of the shear zone increases the depth of the basin decreases the rotation around a vertical axis near the fault tips decreases and the basin shape the distribution of subsidence normalized by the maximum subsidence becomes broader in contrast two dimensional plane stress modeling predicts a basin shape that is independent of the width of the shear zone our models also predict full graben profiles within the overlapped region between bounding faults and half graben shapes elsewhere increasing overlap also decreases uplift near the fault tips and rotation of blocks within the basin we suggest that the observed structure of the dead sea basin can be described by a 3 d model having a large overlap more than 30 km that probably increased as the basin evolved as a result of a stable shear motion that was distributed laterally over 20 to 40 km

the late miocene quaternary antofalla volcanic complex southern puna nw argentina protracted history diverse petrology and economic potential abstract the antofalla volcanic complex avc is located on the southern puna plateau of nw argentina 100 km east of the main axis of the cordillera occidental volcanic arc it lies on the nw se archibarca lineament one of several transverse structures that cut across the andes from chile and divert arc magmatism in se trending fingers across the puna 40ar 39ar dating of volcanic products from the avc reveals a protracted magmatic history from 10 9 to 1 6 ma initial volcanism was characterized by eruption of voluminous rhyolitic ignimbrites deposited directly onto exposed crystalline basement and supracrustal sedimentary rocks these rhyolites record significant geochemical evidence for crustal contamination or derivation by crustal melting which decreases upwards through the pyroclastic sequence they were followed by extrusion of potassic lavas of the shoshonite latite trachydacite suite from 10 1 to 9 5 ma subsequent magmatism 9 1 1 6 ma was of basaltic andesite andesite dacite composition with localized smaller volume dacitic and rhyolitic ignimbrites monogenetic basaltic andesite cinder cones and lava flows were erupted in the quaternary in response to a change to transtensional tectonics in the puna the volcanic sequence is interpreted to record the early development of a lower crustal mash zone at 11 ma where mantle derived arc magmas interacted extensively with felsic lower crustal rocks to produce evolved shoshonitic compositions and crustal melts that erupted to form rhyolitic ignimbrites common at this time throughout the puna after establishment of a steady state mash zone by 9 ma subsequent basaltic andesite andesite magmas show less evidence of felsic crustal contamination and may have developed largely by interaction with previous ultramafic mafic fractionates of early magmas in the lower crust more felsic dacitic magmas appear to have evolved largely by fractional crystallization from basaltic andesites and andesites with widespread evidence of mixing and mingling between magmas of different degrees of differentiation some contamination by upper crustal components is also evident in these dacites but is shown more clearly by small volume rhyolites some of which may again represent predominantly crustal melts epithermal deposits of au and pb zn ag occur within the avc apparently associated with the formation of rhyolite domes the mineralization style is typical of other andean volcanic centres and may be underlain by deeper seated porphyry styles of mineralization extensive blanketing by younger lavas and limited erosion restricts exposure of these altered and mineralized sequences however although the potential for further discoveries beneath cover or at depth in this system appears high

the geology of gaspra the surface of gaspra can be divided into several facets separated by ridges superimposed on the facets and ridges are two populations of craters type 1 craters are crisply defined and constitute a production function of impact origin type 2 craters are mostly irregular shallow depressions and probably of various origins some of the type 2 craters may have formed by impact and be remnants of a crater population that predates the type 1 population the number of type 1 craters suggests that they started to accumulate 2 107 to 3 108 years ago the freshest craters have a depth diameter ratio of 1 7 as compared with 1 5 typically observed on other bodies the craters appear to have become shallower with age at a rate of 10 6 to 10 7 m year the shallow depth of newly formed craters and the progressive degradation with age are ascribed to downslope movement of poorly coherent surface materials the movement being aided by seismic shaking as a result of impact the surface shows subtle color differences the most prominent differences are observed around craters on ridges where the surface has a stronger 1 m absorption than elsewhere the contrast is not observed around craters on the facets the color differences are attributed to alteration of the materials brought to the surface the unaltered material is estimated to be located at least 50 m below the surface on the facets only on the ridges is unaltered material at a shallow enough depth to be excavated by the observed craters the rate of degradation of craters the rounded form of the ridges the depth of altered material on the facets and the presence of old degraded craters all suggest that gaspra is covered with a regolith a few tens to several tens of meters thick

extinction of pull apart basins numerous pull apart basins are present along the haiyuan fault zone in northwestern china detailed geological mapping of these basins reveals an evolution of faulting and subsidence that leads toward extinction of some pull apart basins the most common cause of pull apart basin extinction appears to be the development of new strike slip faults along one of the basin margin normal faults or diagonally across the basin the strike slip faults may develop together with normal faults to form a complex zone within the basin the migration of boundary strike slip faults toward the basin center can also cause the extinction of pull apart basins the extinction of these pull apart basins suggests that there is probably a tendency for a strike slip fault zone to straighten itself and the examples show how this takes place

impact craters on asteroids does gravity or strength control their size abstract the formation of kilometer size craters on asteroids is qualitatively different from the formation of meter size laboratory and weapons scale craters on earth a numerical hydrocode model is used to examine the outcomes of various size cratering impacts into spheres and half spaces a shock wave fractures the target in advance of the crater excavation flow thus for impactors larger than 100 m impacting at typical asteroid impact velocities target tensile strength is irrelevant to the impact outcome this result holds whether the target is initially intact or a rubble pile even ignoring the effects of gravity because of the shock induced fracture crater excavation is controlled by gravity at smaller sizes than would otherwise be predicted determining the strength gravity transition by comparing the physical strength of the material to the force of gravity will not work because strength is eliminated by the shock wave

the precordilleran fault system of chuquicamata northern chile evidence for reversals along arc parallel strike slip faults abstract the chilean precordillera situated between the longitudinal valley and the western cordillera of northern chile was the site of the andean magmatic arc from the late cretaceous to the eocene oligocene boundary magmatism came to an end during the incaic tectonic phase 38 ma which caused arc normal shortening and the development of longitudinal dextral strike slip faults precordilleran fault system this magmatic arc tectonism is also related to the formation of the chuqicamata porphyry copper ore deposit as well as of other important deposits of this type in the precordillera structural investigations in and around the chuqicamata open pit mine have shown that wrench tectonics determined the kinematics of the area the n s striking west fissure which separates a 35 ma old non mineralised pluton to the west from a central late paleozoic basement ridge containing the mineralization became a sinistral fault along with other parallel faults in the area the central part is separated from similar paleozoic rocks to the east by the messabi este fault and a narrow faulted and sheared syncline of mesozoic cenozoic sediments this fault bears structures indicating dextral movements which probably are of an age that is similar to the mylonites 34 8 ma in the western pluton the dextral movements preceded the sinistral shear thus the fault system of chuqicamata displays a reversal of arc parallel shear movements according to the orientation of quartz veins in the mineralized body it is presumed that the sense of displacement of these strike slip motions reversed when mineralization started at about 32 ma during this time the stress field must have changed fundamentally the incaic phase dextral transpression is supposed to have been induced by the oblique vector of plate motion the following sinistral transtension corresponds to a time of reduced convergence rate and possibly reduced plate coupling as however the vector of plate motion remained unchanged during that time oblique subduction cannot be used as an argument for arc parallel sinistral shear movements

geodynamic and magmatic stages on a traverse through the andes between 20 and 24 s n chile s bolivia nw argentina metamorphic precambrian basement is exposed in the chilean pre cordillera in the form of a narrow horst and also in the argentinian puna miogeosynclinal pelites and psammites of devonian age dominate in the western part of the andes while early palaeozoic clastics occur in the e insignificant triassic sediments were followed by wide spread shallow marine jurassic deposition cretaceous and tertiary strata are predominantly terrestrial deposits whereas the precambrian and palaeozoic series were intensely deformed by folding block faulting dominates in the volcano sedimentary cover magmatic activity has been significant since the precambrian while s type intrusives and extrusives occurred mainly in the palaeozoic i type magmatic rocks predominated in the mesozoic the cenozoic volcanism in the cordillera occidental originated in the lower and upper crust magnetotelluric and seismic investigations on the traverse show extremely low electric resistivity values coinciding with a strong absorption of seismic waves below the chilean pre cordillera and the cordillera occidental this phenomenon can best be explained by the existence of partial melts at depths of more than 10 km the bouguer values decrease by about 430 mgal from the coast to the cordillera occidental in the precambrian and palaeozoic the continent apparently extended farther to the w the change from a passive to an active continental margin took place at the palaeozoic mesozoic boundary with the break up of pangea in order to grasp better the tectonic and magmatic evolution as well as the recent structure of the crust in the central andes a combined geological geophysical investigation has been made along a traverse through the andes between 20 and 24 s fig 1 the following pages present a review of the geological development together with the first results of work from the chilean section of the traverse

analogue models of pull apart basins sandbox analogue models of pull apart basins that developed in sedimentary strata above releasing steps in underlying basement faults are characterized by rhombic basins that are flat bottomed box grabens with a subhorizontal synkinematic basin infill steep to nearly vertical sigmoidal oblique slip and segmented oblique extensional faults are the dominant bounding structures of the pull apart basins cross basin short cut faults link the offset principal displacement zones that are characterized by flower structure development the structural architectures of the physical models compare directly in form and dimensions to natural examples of strike slip pull apart basins

sedimentary architecture structural setting and late cenozoic depocentre migration of an asymmetric transtensional basin lake izabal eastern guatemala abstract lake izabal located in eastern guatemala lies in an ew trending basin located along the transform margin between the north american and caribbean plates this plate boundary consists of two main left lateral strike slip faults known as the polochic and motagua fault system pmfs the basin is a 100 20 km lens shaped depression in which the lake occupies its eastern half to the north the basin is confined by a principal deformation zone pdz which is recognized in this area as the most important upper crustal branch of the polochic fault the eastern part of the basin has been uplifted and dissected exposing the basin infill along the slopes of the montana del mico analysis of a set of multichannel seismic reflection profiles most of them acquired within the lake combined with well and outcrop information allowed to outline the structure and evolution of the basin and its stratigraphic architecture the basin is strongly asymmetrical both in cross and along strike directions and is filled up by neogene to quaternary sediments with the deepest side of the basin being to the north close and parallel to the pdz the base of the infill is a mid tertiary unconformity whose differential subsidence through time brought the basin to its present structural setting five main evolutionary stages are recognized based on seismo stratigraphic and structural features which are related to the development and activity of the bordering faults development of the basin began in its easternmost sector a small half graben tilted to the south developed in response to the activity of an oligocene or early middle miocene growth fault in the second stage late miocene the basin tilts to the north causing the sedimentary sequences to thicken towards the northern pdz this stage is dominated by oblique extension and by progressive migration of the basin depocenter towards the west the third stage early pliocene records a strong acceleration of the strike slip activity which causes a rapid westward migration of the depocenter the fourth stage late pliocene although still dominated by regional transtension is related to the development of a transpressional event along the eastern side of the basin as evidenced by the occurrence of folds and reverse faults parallel to the northern bend of the polochic fault during the fifth stage quaternary the eastern part of the basin remains relatively quiescent while the western part undergoes subsidence with localized transtension still related to the activity in this sector of the polochic fault left lateral movement along the polochic fault caused the depocenter to migrate 75 80 km at different velocities with slip rates ranging between 2 1 and 16 3 mm yr this study suggests that the polochic fault developed in the early middle miocene in connection to the deposition of the early sediments of the izabal basin even though an older age oligocene could also be possible to our knowledge this is one of the best examples of sedimentary architecture documenting depocentre migration due to fault movement along a transform system

the agost basin betic cordillera alicante province spain a pull apart basin involving salt tectonics the agost basin is characterized by a miocene quaternary shallow marine and continental infilling controlled by the evolution of several curvilinear faults involving salt tectonics derived from triassic rocks from the serravallian on the area experienced a horizontal maximum compression with a rotation of the maximum stress axis from e w to n s the resulting deformation gave rise to a strike slip fault whose evolution is characterized progressively by three stages 1 stepover releasing bend with a dextral motion of blocks 2 very close to pure horizontal compression and 3 restraining bend with a sinistral movement of blocks in particular after an incipient fracturing stage faults generated a pull apart basin with terraced sidewall fault and graben subzones developed in the context of a dextral stepover during the lower part of late miocene p p the occurrence of triassic shales and evaporites played a fundamental role in the tectonic evolution of the study area the salty material flowed along faults during this stage generating salt walls in root zones and salt push up structures at the surface during the purely compressive stage middle part of late miocene p p the salt walls were squeezed to form extrusive mushroom like structures the large amount of clayish and salty material that surfaced was rapidly eroded and deposited into the basin generating prograding fan clinoforms the occurrence of shales and evaporites both in the margins of the basin and in the proper infilling favored folding of basin deposits faulting and the formation of rising blocks later in the last stage upper part of late miocene p p the area was affected by sinistral restraining conditions and faults must have bent to their current shape the progressive folding of the basin and deformation of margins changed the supply points and finally caused the end of deposition and the beginning of the current erosive systems on the basis of the interdisciplinary results the agost basin can be considered a key case of the interference between salt tectonics and the evolution of strike slip fault zones the reconstructed model has been compared with several scaled sandbox analogical models and with some natural pull apart basins

discovery geology and origin of the fruta del norte epithermal gold silver deposit southeastern ecuador fruta del norte is a completely concealed and extraordinarily well preserved epithermal gold silver deposit located in the remote cordillera del condor mountain ranges of southeastern ecuador currently defined resources in a single steep relatively small body displaying exceptional grade continuity contain 9 81 million ounces moz of gold and 15 0 moz of silver with an indicated resource grade of 9 59 g t au and 12 9 g t ag the deposit was discovered in early 2006 during a greenfields program conducted by aurelian resources a canadian junior explorer discovery resulted from systematic drill testing of a conceptual geologic model which predicted that auriferous veins would be present in andesitic volcanic rocks inferred to underlie a steep silicified rib cutting a fluvial conglomerate sequence the rib is highly anomalous in arsenic antimony and mercury but contains low order gold values the second drill test the discovery hole intersected 230 m of ore grade gold silver mineralization beneath 200 m of the barren conglomerate cover the host andesitic volcanic rocks crosscutting feldspar porphyry and associated phreatic breccias are assigned to the piuntza unit of the santiago formation which along with the zamora batholith and a series of porphyry copper stocks was generated in a continental margin magmatic arc during the middle to late jurassic the deposit is located near the northeastern extremity of the 10 km long suarez pull apart basin where it is linked to the steep west and central faults part of the regionally extensive las penas strike slip fault zone the pull apart basin was progressively filled by fluviatile conglomerate dacitic ignimbrite flows finer grained siliciclastic sedimentary rocks and finally andesite flows all assigned to the chapiza formation the fruta del norte deposit consists of a 1 3 km long and up to 300 m wide vein stockwork associated with quartz illite pyrite alteration the deposit comprises two principal vein types one in the south dominated by quartz manganoan carbonates and abundant base metal sulfides and the other in the north dominated by manganese and base metal poor quartz and calcite adularia is a minor main stage gangue mineral in each both types are abruptly transitional upward and westward to a third ore type marked by intense silicification and chalcedony veining with disseminated and veinlet marcasite pyrite the uppermost part of this silicic zone is sulfide deficient probably the result of a short lived supergene oxidation event prior to initial conglomerate deposition the deposit is notable for the widespread occurrence of fine to coarse visible gold which gives rise to bonanza grades and is closely associated with quartz chalcedony carbonate and sulfide gangue the sulfide free silicic zone is overlain by an extensive silica sinter horizon which may either directly overlie the piuntza volcanic rocks and or occur as interbeds in the basal 20 m of the conglomerate which are invariably silicified and marcasite bearing otherwise the conglomerate above the orebody lacks silicification with one critical exception the steeply inclined zone that is exposed as the silicified rib that led to discovery the sinter horizon containing localized mud pool deposits and hydrothermal eruption breccias is in unusual proximity to the underlying gold silver orebody the northern and southern parts of the fruta del norte deposit possess characteristics that are usually considered typical of low and intermediate sulfidation epithermal deposits respectively they may have required two discrete mineralizing fluids both of which are suspected to have ascended via the west and central faults the upward change to the silicified zone is attributed to cooling of the ascendant fluid on approach to the paleosurface sinter progressive burial of the deposit beneath the suarez basin fill may have contributed to the suppression and eventual extinction of the epithermal system with gold mineralization of the conglomerate being inhibited by rheologic and chemical factors formation of the fruta del norte deposit and complete filling of the suarez basin took place in 3 to 6 m y during the late jurassic 157 154 ma

structure of the hanmer strike slip basin hope fault new zealand hanmer basin 10 x 20 km located in northern south island new zealand is evolving where two major segments of the dextral strike slip hope fault are projected to converge across a 6 to 7 km wide releasing step over the structural geometry and development of hanmer basin does not conform to traditional pull apart basin models the respective fault segments do not overlap but are indirectly linked along the southwest margin of the basin by an oblique normal fault the hope river segment terminates in an array of oblique normal faults along the northwestern basin range front and east west striking normal faults on the west hanmer plain faulted holocene alluvial fan surfaces indicate west hanmer basin is actively subsiding evolving under north south extension the conway segment along the southeastern margin of the basin terminates in a complex series of active fault traces small pop up ridges and graben depressions early basin fill sediments of pleistocene age are being folded elevated and dissected as the eastern part of hanmer basin is progressively inverted and destroyed by north south contraction the north margin of the basin is defined by a series of topographic steps caused by normal faulting outside of the area of the releasing step over these normal faults we interpret to reflect large scale upper crustal collapse of the hanging wall side of the hope fault new seismic reflection data and geologic mapping reveal a persistent longitudinal and lateral asymmetry to basin development four seismic stratigraphic sequences identified in the eastern sector of the basin thicken and are tilted southward with insequence lateral onlaps occurring to the north and east and also onto basement near the fault controlled basin margins the basin depocenter currently contains 1000 m of sediment adjacent to the south margin and is disrupted by faulting only at depth in the western part of the basin the sediment fill is thinner today the rate of basin deepening under transtension at the western end is matched by its progressive inversion and destruction under transpression in the eastern sector with the oldest basin fill now being recycled we propose a hybrid model for hanmer strike slip basin one in which geometric elements of a fault wedge basin downward and upward tipped spindle shaped ends are combined with those of a pull apart basin step over region between the major fault segments we also conclude that changes in fault geometry releasing and restraining bends and step overs at a variety of scales and over short distances control the development of the extensile and contractile parts of the basin and three dimensional basin asymmetry strain partitioning is complex and cannot be related simply to local reorientation of the regional stress field

crater ejecta scaling laws fundamental forms based on dimensional analysis a model of crater ejecta is constructed using dimensional analysis and a recently developed theory of energy and momentum coupling in cratering events general relations are derived that provide a rationale for scaling laboratory measurements of ejecta to larger events specific expressions are presented for ejection velocities and ejecta blanket profiles in two limiting regimes of crater formation the so called gravity and strength regimes in the gravity regime ejecta velocities at geometrically similar launch points within craters vary as the square root of the product of crater radius and gravity this relation implies geometric similarity of ejecta blankets that is the thickness of an ejecta blanket as a function of distance from the crater center is the same for all sizes of craters if the thickness and range are expressed in terms of crater radii in the strength regime ejecta velocities are independent of crater size consequently ejecta blankets are not geometrically similar in this regime for points away from the crater rim the expressions for ejecta velocities and thickness take the form of power laws the exponents in these power laws are functions of an exponent that appears in crater radius scaling relations thus experimental studies of the dependence of crater radius on impact conditions determine scaling relations for ejecta predicted ejection velocities and ejecta blanket profiles based on measured values of are compared to existing measurements of velocities and debris profiles

impact fragmentation from the laboratory to asteroids abstract in this paper we study the effect of target size on the fragmentation outcome of rock targets using a 2d numerical hydrocode after comparing our hydrocode calculations to laboratory data including explosive disruption experiments to validate the results we use the code to calculate how the critical specific energy q needed to catastrophically fracture a body varies with target size in the regimes not accessible to experiment impact velocity is generally kept constant at about 2 0 km s 1 although some higher velocity 5 km s 1 simulations were run to determine a velocity dependence for the fragmentation outcome to reflect the asteroid population target diameters range from 10 cm to 1000 km spanning the regimes where strength and self gravity radially varying lithostatic stress each dominate resistance to fragmentation we find that there is a significant difference in fragmentation outcome when the lithostatic stress is included in the computations as expected surface layers fragment more easily while the strength of the central regions is greatly enhanced we derive the q versus size relationship for three materials basalt strong and weak cement mortar each having different static compressive strengths and representing a range of asteroid materials the hydrocode results showed that q decreased with increasing target size in the strength regime with slopes of 0 43 0 59 and 0 6 for basalt strong and weak mortar respectively this decrease is directly related to the decrease in strain rate as target size grows in the gravity regime q increases with increasing target size with a slope equal to 2 6 for all three of the materials modeled these values are much steeper than those previously derived from scaling theories ejecta velocity distributions as a function of target size are examined as well for large bodies resultant ejecta speeds tend to be well below escape velocity implying that these asteroids are likely to be reaccumulated rubble piles in simulating the creation of the asteroid family eos we find that the code calculated fragment size distribution is similar in character to the observed data but secondary fragment sizes are significantly underestimated more importantly the determined ejecta speeds were too low for these fragments to have achieved escape velocity and thus we fail to actually form the separate bodies comprising the eos family and are left instead with a single rubble pile conglomerate

ejecta megaregolith accumulation on planetesimals and large asteroids megaregolith accumulation can have important thermal consequences for bodies that lose heat by conduction as vacuous porosity of the kind observed in the lunar megaregolith lowers thermal conductivity by a factor of 10 i have modeled global average ejecta accumulation as a function of the largest impact size with no explicit modeling of time in conjunction with an assumed cratering size distribution exponent b the largest crater constrains the sizes of all other craters that significantly contribute to a megaregolith the largest impactor mass ratio is a major fraction of the catastrophic disruption mass ratio and in general the largest crater s diameter is close to the target s diameter total accumulation is roughly 1 5 of and proportional to the target s radius global accumulations estimated by this approach are higher than in the classic housen et al 1979 study by a factor of roughly 10 this revision is caused mainly by higher typical largest crater size for b 2 the single largest crater typically contributes close to 50 of the total of new nonrecycled ejecta megaregolith can be destroyed by sintering a process whose pressure sensitivity makes it effective at lower temperature on larger bodies planetesimals 100 km in diameter may be surprisingly well suited about as well suited as bodies two to three times larger in diameter for attaining temperatures conducive to widespread melting a water rich composition may be a significant disadvantage in terms of planetesimal heating as the shallow interior may be densified by aqueous metamorphism and will have a low sintering temperature

tectonic and magmatic evolution of the andes of northern argentina and chile abstract two orogenic cycles both with different evolution are developed in the western margin of the south american continent in northern argentina and chile the paleozoic hercynic cycle and the meso cenozoic andean cycle the hercynic cycle a wide marine basin extending westward of the cordillera oriental which developed in cambrian ordovician times marks the beginning of this cycle in contrast to peru and bolivia where this basin developed between two precambrian blocks the western margin of this basin in northern argentina and chile is still unknown the ordovician sedimentation and accompanied volcanism ends with the ocloyic deformation phase and its synkinematic granitic plutonism two basins were developed in the silurian devonian separated by the puna arc uplifted during this ocloyic phase the shallow water marine terrigeneous sediments which were deposited in them were deformed by a new tectono magmatic associated phase chanic phase carboniferous to lower permian marine carbonates were deposited west of the puna arc and red beds east of it later on during the permian to triassic a magmatic belt was developed along the cordillera occidental the rhyolites ignimbrites and the granitic to granodioritic related intrusives are well represented in chile although the overall geologic history of this period is known many problems concerning its origin and relations to plate tectonics are still unsolved the andean cycle during this cycle a series of magmatic arc systems related to the subduction of the pacific crust was built up along the western margin of south america huge volumes of calc alkaline lavas and related plutons were emplaced since the jurassic along belts parallel to the present coastline showing a general eastward migration trend up to the lower cretaceous an ensialic back arc basin was formed east of this magmatic arc thousands of meters of marine and continental sediments were deposited in it this basin disappeared during the middle cretaceous probably as a result of the final opening of the atlantic and the active westward movement of the south american plate since middle cretaceous times the magmatic arc has been the fundamental paleogeographic element the arc progressively migrated stepwise eastward each step marked by a tectonic phase subduction related crustal erosion could explain the lack of fore arc petrotectonic assemblages the eastward magmatic polarity which characterizes this section of the andes could also be explained by such a mechanism

impact origin of the vesta family the compelling petrographic link consolmagno and drake 1977 gaffey 1983 between basaltic achondrite meteorites and the 530 km diameter asteroid 4 vesta has been tempered by a perceived difficulty in launching rocks from this asteroid s surface at speeds sufficient to bring them to earth wasson and wetherill 1979 without obliterating vesta s signature crust i address this impasse in response to recent imaging zellner et al 1996 dumas and hainaut 1996 of a 450 km impact basin across vesta s southern hemisphere thomas et al 1997 and model the basin forming collision using a detailed two dimensional hydrocode with brittle fracture including self gravitational compression cf asphaug and melosh 1993 a 42 km diameter asteroid striking vesta s basaltic crust atop a denser mantle and iron core at 5 4 km s launches multikilometer fragments up to 600 m s without inverting distal stratigraphy according to the code this modeling together with collisional dynamical rheological and exposure age timescales marzari et al 1996 welten et al 1996 and observations of v type asteroids binzel and xu 1993 suggests a recent 1 ga impact origin for the vesta family and a possible vesta origin for earth approaching v type asteroids cruik shank et al 1991

galileo encounter with 951 gaspra first pictures of an asteroid galileo images of gaspra reveal it to be an irregularly shaped object 19 by 12 by 11 kilometers that appears to have been created by a catastrophic collisional disruption of a precursor parent body the cratering age of the surface is about 200 million years subtle albedo and color variations appear to correlate with morphological features brighter materials are associated with craters especially along the crests of ridges have a stronger 1 micrometer absorption and may represent freshly excavated mafic materials darker materials exhibiting a significantly weaker 1 micrometer absorption appear concentrated in interridge areas one explanation of these patterns is that gaspra is covered with a thin regolith and that some of this material has migrated downslope in some areas

dynamic tensile strength of lunar rock types the dynamic tensile strengths of four rocks have been determined a flat plate impact experiment is used to generate 1 s duration tensile stress pulses in rock samples by superposing rarefaction waves to induce fracture a gabbroic anorthosite and a basalt were selected because they are the same rock types as occur on the lunar highlands and mare respectively although these have dynamic tensile strengths which lie within the ranges 153 174 mpa and 157 179 mpa whereas arkansas novaculite and westerly granite exhibit dynamic tensile strengths of 67 88 mpa and 95 116 mpa respectively the effect of chemical weathering and other factors which may affect application of the present results to the moon have not been explicitly studied the reported tensile strengths are based on a series of experiments on each rock where determination of incipient spallation is made by terminal microscopic examination these data are generally consistent with previous determinations at least one of which was for a significantly chemically altered hydroxylated but physically coherent rock the tensile failure data do not bear a simple relation to compressive results and imply that any modeling involving rock fracture consider the tensile strength of igneous rocks under impulse loads distinct from the values for static tensile strength generally the dynamic tensile strengths of nonporous igneous rocks range from 100 to 180 mpa with the more basic and even amphibole bearing samples yielding the higher values

experimental study on impact induced seismic wave propagation through granular materials abstract impact induced seismic waves are supposed to cause movements of regolith particles resulting in modifications of asteroidal surfaces the imparted seismic energy is thus a key parameter to determining the scale and magnitude of this seismic shaking process it is important to study the propagation velocity attenuation rate and vibration period of the impact induced seismic wave to estimate the seismic energy hence we conducted impact cratering experiments at kobe university using a 200 m glass beads target to simulate a regolith layer and measured the impact induced seismic wave using three accelerometers set on the target surface at differences ranging from 3 2 to 12 7 cm the target was impacted with three kinds of projectiles at 100 m s 1 using a one stage gas gun the propagation velocity of the seismic wave in the beads target was 108 9 m s 1 and the maximum acceleration gmax in the unit of m s 2 measured by each accelerometer showed good correlation with the distance from the impact point normalized by the crater radius x r irrespective of projectile type they also were fitted by one power law equation gmax 102 19 x r 2 21 the half period of the first peak of the measured seismic waves was 0 72 ms and this duration was almost consistent with the penetration time of each projectile into the target according to these measurements we estimated the impact seismic efficiency factor that is the ratio of seismic energy to kinetic energy of the projectile to be almost constant 5 7 10 4 inside the crater rim while it exponentially decreased with distance from the impact point outside the crater rim at a distance quadruple of the crater radius the efficiency factors were 4 4 10 5 for polycarbonate projectile and 9 5 10 5 for alumina and stainless steel projectiles

a laboratory impact study of simulated edgeworth kuiper belt objects abstract this paper reports on a series of laboratory impact experiments designed to provide basic data on how simulated edgeworth kuiper belt objects ekos fragment in an impact event in september october 1997 we carried out 20 low velocity airgun shots at the ames vertical gun range into porous and homogeneous ice spheres using aluminum fractured ice and solid ice projectiles we found that the porous ice targets behaved as strongly as solid ice in collision energy is apparently well dissipated by the void spaces within the target such that these fragile ice structures respond as if they were strong in impacts therefore it would appear that if ekos are porous they are not collisionally weak also our data show that collisional outcomes for low velocity impacts into ice targets depend on the type of projectile used as well as the properties of the target we observed that the degree of fragmentation for a given type of target increases as the strength of the projectile increases aluminum projectiles are far more damaging to the target at the same collisional energy than are solid ice projectiles which in turn are more damaging than fractured ice projectiles one possible explanation for this behavior is the variable depth of penetration of the projectile for the different cases stronger projectiles penetrate more deeply and couple more energy into the target than do weak projectiles based on this if we assume that there has not been significant heating or differentiation in the edgeworth kuiper e k belt the most applicable impact strength for the low velocity e k belt collisions is likely to be that derived from similar target projectile materials impacting each other the laboratory data from this analysis indicate that a value for impact strength 5 10 5 erg cm 3 is appropriate for porous ice targets impacted with solid porous ice projectiles

giant evaporite belts of the neogene central andes large volumes of continental evaporites accumulated within the central andes during neogene uplift of the altiplano puna plateau and development of the andean volcanic arc halite and gypsum are dominant minerals along with local and economically important borates playa conditions have existed since ca 15 ma halite and borate deposition has occurred for the past 7 to 8 m y evaporites formed in salar environments e g playa lakes and are characterized by complex mineral assemblages occurrence zonation and geochemistry evaporite deposition was controlled by volcanism geothermal activity closed drainage and climate these andean deposits and their controls differ from evaporites in other continental and marine environments

a mid pleistocene deformation transition in the hula basin northern israel implications for the tectonic evolution of the dead sea fault the dead sea fault dsf plate boundary has accommodated relative sinistral motion between the sinai and arabian plates since the neogene geologically based models point to a long term 5 0 ma n trending relative motion roughly parallel to the central sector of the dsf gps based calculations of present day relative motion between the sinai and arabian plates indicate a northward increasing convergent component across the dsf suggesting a kinematic change over the plio pleistocene period we study the evolving deformation of the hula rhomb shaped graben situated along the central sector of the dsf in order to examine the possible kinematic change the hula graben is widely accepted as a pull apart basin part of a series of basins extending southward we use a 3 d approach combining mapped surface structures with subsurface seismic reflection profiles 173 km and borehole data 38 boreholes gathered here into a single geographic information systems database results indicate that during the mid pleistocene a major tectonic transition modified the structure of the basin fill a subvertical nnw trending left lateral throughgoing strike slip fault developed diagonally across the basin consequently the basin entered a new geodynamic phase where subsidence was controlled by both the basin bordering faults and the diagonal fault while the vertical displacement across the transverse faults is minor synchronous structural changes recorded in additional localities along the dsf attests to a mid pleistocene 1 ma regional tectonic transition that was associated with a northward increasing convergent component across the central and northern sectors of the dsf

strike slip deformation in the confidence hills southern death valley fault zone eastern california usa the confidence hills form a well exposed composite positive flower structure developed in pliocene to recent lacustrine and alluvial fan sediments the structure has developed along the current trace of the southern death valley dextral strike slip fault zone california usa nw se striking fault zones bound the confidence hills in 3d these fault segments are inferred to link at depth to a common basal fault system the flower structure is formed by doubly plunging anticlines that roughly parallel the bounding fault segments fold development was aided by a the presence of a basal salt deposit and numerous detachment horizons within the sedimentary pile and b buttressing and uplift along and against the bounding oblique slip reverse faults structural and palaeomagnetic evidence indicates that the anticlines developed with axial surfaces parallel and sub parallel to the trace of both fault zones folding appears to have initially developed adjacent to and above the southeastern fault segment and then propagated outwards and to the northwest the latest displacements along the current southern death valley fault zone are probably only in the orders of hundreds of metres and as young as 0 9 ma to recent this young fault zone is highly segmented along the length of the floor of southern death valley the high proportion of oblique slip faulting consistent with the strike slip zone being immature the confidence hills structure displays close geometric similarities with other natural examples of positive flower structures and to the features found in scaled analogue sandbox experiments of strike slip faulting

first images of asteroid 243 ida the first images of the asteroid 243 ida from galileo show an irregular object measuring 56 kilometers by 24 kilometers by 21 kilometers its surface is rich in geologic features including systems of grooves blocks chutes albedo features crater chains and a full range of crater morphologies the largest blocks may be distributed nonuniformly across the surface lineaments and dark floored craters also have preferential locations ida is interpreted to have a substantial regolith the high crater density and size frequency distribution 3 differential power law index indicate a surface in equilibrium with saturated cratering a minimum model crater age for ida and therefore for the koronis family to which ida belongs is estimated at 1 billion years older than expected

active tectonics of the andes nearly 90 mm a 1 of relative plate convergence is absorbed in the andean plate boundary zone the pattern of active tectonics shows remarkable variations in the way in which the plate slip vector is partitioned into displacement and strain and the ways in which compatibility between different segments is solved along any traverse across the plate boundary zone the sum of relative velocities between points must equal the relative plate motion we have developed a kinematic synthesis of displacement and strain partitioning in the andes from 47 s to 5 n relevant for the last 5 ma based upon 1 relative plate motion deduced from oceanic circuits giving a roughly constant azimuth between 075 and 080 2 moment tensor solutions for over 120 crustal earthquakes since 1960 3 structural studies of deformed plio pleistocene rocks 4 topographic geomorphic studies 5 palaeomagnetic data and 6 geodetic data we recognize four neotectonic zones with subzones and boundary transfer zones that are partitioned in different ways these zones are not coincident with the classic zones defined by the presence or absence of a volcanic chain or differences in finite displacements and strains and tectonic form the long term segmentation and finite evolution of the andes may not occur in constantly defined segments in space and time in segment 1 47 39 s the slip vector is partitioned into roughly orthogonal benioff zone slip with large magnitude large slip surface earthquakes and both distributed dextral shear giving clockwise rotations of up to 50 and dextral slip in the curved liquine ofqui fault system giving 5 10 of anticlockwise fore arc rotation in segment 2 39 20 s the slip vector is partitioned into benioff zone slip roughly parallel with the slip vector andean crustal shortening and a very small component of dextral slip including that on the atacama fault system between 39 and 34 s a cross strike dextral transfer which deflects the chile trench and the volcanic arc absorbs the shortening contrast between segments 1 and 2 in segment 3 20 6 s the slip vector is partitioned into roughly orthogonal benioff zone slip crustal shortening sinistral trench parallel faulting and northeast southwest extension compatibility between segments 2 and 3 is maintained by the sinistral east southeast trending cochabamba shear zone and north trending dextral faults in segment 4 6 s to 5 n the slip vector is partitioned into roughly orthogonal benioff zone slip and dextral strike slip faulting in the fore arc and volcanic chain

impacts on asteroids fragmentation regolith transport and disruption abstract we use a numerical hydrocode model to examine the outcomes of various size impacts into targets the sizes of asteroids 951 gaspra and 243 ida which were imaged by the galileo spacecraft a shock wave fractures the asteroid in advance of crater excavation flow thus for impactors larger than 100 m impacting at 5 3 km s 1 tensile strength is unimportant in these bodies whether they are initially intact or are rubble piles because of the shock induced fracture impact results are controlled by gravity therefore these asteroids are much more resistant to catastrophic disruption than predicted by previous estimates which had assumed that strength was controlling these processes for rock targets the rubble and regolith produced by this fracture can be jolted by the impact redistributing surface material and globally erasing craters the crater ejecta can produce tens of meters of regolith per large event likely consisting of 100 m size boulders mixed with smaller particles the response of kilometer size asteroids to impacts is qualitatively different from that of few centimeter targets in terrestrial experiments making prediction based on such experiments difficult the compositional distribution of delivered meteorites depends on the outcomes of such asteroid impacts

tectonic development southeastern border of the puna plateau northwestern argentine andes the puna altiplano a 500 000 km2 area at an average elevation of 3 7 km in the central andes occurs in a non collisional plate setting over a 30 east dipping segment of the benioff zone and is bounded to the north and south by nearly flat segments of the subducted nazca plate a 120 km long portion of the southeastern boundary of the puna in northwestern argentina was studied to gain insight into the structures associated with uplift of the plateau and into the nature of structures over the transition from flat to steeper subduction the structural geometry and kinematic evolution of the region was determined from geologic mapping and kinematic analysis of minor fault arrays the present southeastern margin of the puna was uplifted between 5 10 and 2 ma northwest southeast horizontal shortening and vertical extension during uplift were produced by northeast striking en echelon thrust faults dipping 17 55 northwest four to seven en echelon structural blocks each 20 30 km long constitute the border of the puna their structural geometry precludes large scale overthrusting as an important mechanism of plateau uplift along the studied segment in the past 2 m y there has been a 45 change in the direction of shortening and locally a 90 change in the extension direction indicating that the structures accompanying uplift are no longer active this change could be due to a change either in the geometry of the subducted plate or in convergence directions although neither of these hypotheses is testable given presently available data