first petrographic results on impactites from the yaxcopoil 1 borehole chicxulub structure mexico the icdp yaxcopoil 1 yax 1 borehole located 60 km south southwest of the center of the chicxulub impact structure intercepted an interval of allogenic impactites depth of 795 895 m petrographic analysis of these impactites allows them to be differentiated into five units based on their textural and modal variations unit 1 795 922 m comprises an apparently reworked poorly sorted and graded fine grained clast supported melt fragment bearing suevitic breccia the interstitial material similar to units 2 and 3 is permeated by numerous carbonate veinlets units 2 823 846 m and 3 846 861 m are groundmass supported breccias that comprise green to variegated angular and fluidal melt particles the groundmass of units 2 and 3 comprises predominantly fine grained calcite altered alkali element ca and si rich cement as well as occasional lithic fragments unit 4 861 885 m represents a massive variably devitrified and brecciated impact melt rock the lowermost unit unit 5 885 895 m comprises highly variable proportions of melt rock particles mrp and lithic fragments in a fine grained carbonate dominated groundmass this groundmass could represent either a secondary ydrothermal phase or a carbonate melt phase or both units 1 and 5 contain well preserved foraminifera fossils and a significantly higher proportion of carbonate clasts than the other units all units show diagnostic shock deformation features in quartz and feldspar clasts our observations reveal that most felsic and all mafic mrp are altered they register extensive k metasomatism in terms of emplacement we suggest that units 1 to 3 represent fallout suevite from a collapsing impact plume whereby unit 1 was subsequently reworked by resurging water unit 4 represents a coherent impact melt body the formation of which involved a significant proportion of crystalline basement unit 5 is believed to represent an initial ejecta ground surge deposit

the great extinction that never happened the demise of the dinosaurs considered the concept of a sudden extinction of the dinosaurs consequent upon the impact of some extraterrestrial object is so dramatic that it has taken hold upon the imaginations of many scientists as w

the petrogenesis of the neoarchean kukuluma intrusive complex nw tanzania abstract the kukuluma intrusive complex kic is a late archean igneous complex dominated by monzonite and diorite with subordinated granodiorite the monzonite and the diorite suites have low silica content sio2 62 wt moderate mg mg average 49 high sr y sr yaverage 79 and high la yb la ybaverage 56 ratios and strongly fractionated lan ybn 9 69 ree patterns their moderate ni niaverage 50 ppm cr craverage 85 ppm variable cr ni ratio 0 65 3 56 and low tio2 tio2average 0 5 wt indicate little to no interaction with the peridotitic mantle for most major elements al2o3 feot na2o tio2 and p2o5 the monzonite and the diorite suites display subparallel trends for the same sio2 content indicating they represent distinct melts intrusions belonging to the diorite suite have high na2o na2oaverage 4 2 wt dy ybn dy ybn average 1 6 a positive sr anomaly and uncorrelated nb la and zr sm ratios suggesting derivation from partial melting of garnet bearing amphibolite intrusions belonging to the monzonite suite have higher na2o na2oaverage 5 61 wt dy ybn dy ybn average 2 21 a negative sr anomaly and correlated nb la and zr sm ratios consistent with derivation from partial melting of eclogite with residual rutile small variations in the th u ratio and near chondritic morb average values th umonzonite 3 65 th udiorite 2 92 are inconsistent with a subducting slab signature and it is proposed that the monzonite and the diorite suites of the kic formed by partial melting of garnet bearing lower mafic crust of an oceanic plateau the granodiorite suite has lower mg mg average 41 moderately fractionated ree low sr y sr yaverage 20 la yb la ybaverage 15 dy ybn dyybn average 1 24 and small negative eu anomalies suggesting derivation from partial melting of amphibolite and plagioclase fractionation near morb th u th uaverage 2 92 and zr sm zr smaverage 30 21 ratios are consistent with intracrustal melting of amphibolite archean rocks with an adakitic geochemical signature have been used to argue in favour of a plate tectonics regime in the archean the results presented here suggest that tectonic models for the tanzania craton which invoke a subduction related setting for all greenstone belts may need revision

chicxulub impact the origin of reservoir and seal facies in the southeastern mexico oil fields stratigraphic and mineralogic studies of cretaceous tertiary k t boundary sections demonstrate that the offshore oil producing breccias and seals from oil fields in the campeche marine platform are of k t boundary age and that their mode of formation is probably related to the k t impact event at chicxulub the oil producing carbonate breccia and the overlying dolomitized ejecta layer seal found in several wells on the campeche marine platform contain typical chicxulub impact products such as shocked quartz and plagioclase and altered glass these offshore units are correlated with thick 50 300 m onshore breccia and impact ejecta layers found at the k t boundary in the guayal tabasco and bochil chiapas sections regionally the characteristic sequence is composed of from base to top coarse grained carbonate breccia covered by an ejecta bed and typical k t boundary clay the onshore and offshore breccia sequences are likely to have resulted from major slumping of the carbonate platform margin triggered by the chicxulub impact successive arrival times in this area 350 600 km from the crater of seismic shaking ballistic ejecta and tsunami waves fit the observed stratigraphic sequence the k t breccia reservoir and seal ejecta layer of the cantarell oil field with a current daily production of 1 3 million barrels of oil are probably the most important known oil producing units related to an impact event

gold mineralization in the mazowe area harare bindura shamva greenstone belt zimbabwe i tectonic controls on mineralization the mazowe group of mines principal mines mazowe bernheim and stori s golden shaft is situated within the harare bindura shamva greenstone belt of the zimbabwean archaean craton west of the chinamora batholith gold mineralization in the form of quartz sulfide reefs is structurally controlled in reverse shear zones that dip moderately north at mazowe mine and conjugate steep strike slip shear zones striking wnw dextral and ne sinistral at bernheim and stori s golden shaft mines the syn mineralization deformation d2 3 in all the mines has a northerly shortening direction this deformation is compatible with the regional late archaean d2 3 event which agrees with late archaean ages determined for the host rocks and for the mineralization the mineralization cannot be related to any major regional scale shear zones and it is incompatible with strains related to the intrusion of either the chinamora batholith or internal granitoid suites these observations show that significant gold deposits can form in relatively minor deformation events unrelated to either major shear zones or granitoid intrusions

decoupled fresh saline groundwater circulation of a coastal carbonate aquifer spatial patterns of temperature and specific electrical conductivity summary the coastal carbonate aquifer of the eastern yucatan peninsula discharges groundwaters to the caribbean sea temperature and specific electrical conductance sec are used as natural tracers to gain new insight into the fresh and saline groundwater circulation along an 80 km section of the caribbean coastline the aquifer is density stratified with a cooler freshwater lens overlying a warmer saline water zone non conduit sites generally have lower temperatures and sec in the freshwater lens than conduit sites in conduits 4 km inland within the shallow saline water zone temperature decreases exponentially with distance from the coast with near equilibrium with the fresh waters reached at 10 km inland a distance coincident with the known limit of conduit development this pattern is indicative of the progressive cooling of warm seawater moving inland from the coast a flow direction opposite to that of both the conventional freshwater flow entrainment and geothermal convection models of coastal aquifer circulation

chicxulub multiring impact basin size and other characteristics derived from gravity analysis the buried chicxulub impact structure in mexico which is linked to the cretaceous tertiary k t boundary layer may be significantly larger than previously suspected reprocessed gravity data over northern yucatan reveal three major rings and parts of a fourth ring spaced similarly to those observed at multiring impact basins on other planets the outer ring probably corresponding to the basin s topographic rim is almost 300 kilometers in diameter indicating that chicxulub may be one of the largest impact structures produced in the inner solar system since the period of early bombardment ended nearly 4 billion years ago

sedimentary signatures and processes during marine bolide impacts a review abstract studies of submarine impact craters resulting from impacts of comets or asteroids demonstrate that the presence of water and the physical properties of target rocks have a major influence on sedimentary processes associated with meteorite impacts this results in difference in sedimentary signature of bolide impacts in marine environments compared to subaerial impact craters in subaerial impacts the targets are commonly hard rocks frequently of igneous and or metamorphic origin whereas in submarine impacts the targets are mostly unconsolidated or poorly lithified sediments or sedimentary rocks with high volumes of pore water such differences result in variability in crater morphology and in sedimentary processes inside and outside the impact area impacts in shallow water marine neritic environments produced craters with low or absent rims and wide and shallow brims as characterize by both the montagnais on the scotian shelf the mjolnir in the barents sea 45 and 40 km in diameter respectively and the chesapeake bay 90 km in diameter lack of elevated rims is thought to be the result of current reworking and resurge of the water back into the excavated cavity as the water in the crater is vaporized during this process resurge gullies can be cut across the rim while mass and debris flows turbidites and other gravity deposits are produced as results of tsunami and crater wall and central high collapse during and after the crater excavation stage such deposits are found both within and outside the crater structure the only difference between gravity deposits triggered by an impact or other rare events such as earthquakes is the admixture of various melt particles and possible enrichments in iridium in the former impacts near the shelf edge may cause partial collapse of the continental margin as shown by the montagnais and chicxulub impacts some of the gravity and debris flows generated by margin collapse may be channelized with final deposits up to several hundred meters thick extending for hundreds of kilometers from the impact site other impact features such as shatter cones tektites spherules and ni spinels shocked quartz isotropication and partial melts are common to both submarine and subaerial impacts theoretical calculations of the destructive forces of mega tsunami waves triggered by meteorite impact in the ocean greatly exceed those based on geologic evidence a dearth of gigantic tsunami evidence for the chicxulub impact outside of the gulf of mexico where from theoretical modelling the maximum near bottom orbital velocity of water flow crossing the deep north atlantic basin should have been 1 m s could be the result of the mitigating effect of the bathymetry of surrounding area causing wave diffraction and interference calculated maximum horizontal orbital velocity near the seafloor at the shelf for the montagnais impact is 22 m s for a 200 m high wave 5 5 m s for a 50 m high wave at 500 km decreasing to 0 5 m s at 1000 km strong enough to scour the deep ocean bottom and produce distinct erosion surfaces and disconformities in marine sedimentary record however lack of cores across impact horizon prevents confirmation of occurrence of such bottom water flows

zircon u pb ages and lu hf isotope systematics from late tectonic granites geita greenstone belt implications for crustal growth of the tanzania craton granite plutons that intruded south of lake victoria and north of the geita greenstone belt have geochemical characteristics similar to high k granites when compared to the late high k granites from the musoma mara region they have lower sio2 content and higher tio2 al2o3 cao na2o p2o5 mgo and feot they also show higher values for v sr zr ba and hf concentrations all samples display high total ree abundances ree 139 393 and weak to moderate eu depletion eu eu 0 25 0 77 their chondrite normalized pattern indicates that the light ree are moderately fractionated 3 03 lan smn 6 24 whereas the heavy ree are weakly fractionated 1 63 gdn ybn 2 80 the granite plutons are generally undeformed with 207pb 206pb ages between 2660 and 2620 ma and intruded after the main ductile deformation that affected the geita greenstone belt the 176hf 177hf ratios for zircons from all samples are identical within error suggesting evolution from an isotopically uniform reservoir the hf values plot along the chur evolution line indicating that the granite represents juvenile crustal melts derived from greenstone material available zircon ages from the tanzania craton suggest that crustal growth occurred in three distinct periods 2850 2800 ma 2770 2730 ma and 2700 2620 ma with the dominant period of crustal growth around 2700 ma zircon ages from the sukumaland and musoma mara greenstone belts indicate that the greenstone sequences formed during the same three periods of crustal growth

chicxulub impact ejecta from albion island belize impact ejecta from the albion formation are exposed in northern belize the ejecta come from the outer portion of the continuous ejecta blanket of the chicxulub crater which is located 360 km to the northwest the basal unit of the albion formation is a 1 m thick clay and dolomite spheroid bed composed of up to four discrete flows the clay spheroids are altered impact glass and the dolomite spheroids are accretionary lapilli the upper unit is a 15 m thick coarse diamictite bed containing altered glass large accretionary blocks striated polished and impacted cobbles and rare shocked quartz the abundance of accretionary clasts evidence for atmospheric drag sorting and the presence of multiple flows in the albion formation indicate that atmospheres play an important role in the formation of the outer portions of continuous ejecta blankets of large craters

high spatial resolution geochemistry and textural characteristics of microtektite glass spherules in proximal cretaceous paleogene sections insights into glass alteration patterns and precursor melt lithologies abstract using laser ablation inductively coupled plasma mass spectrometry la icp ms we have conducted spatially resolved trace element analysis on fresh unaltered microtektite glasses linked to the cretaceous paleogene k pg boundary chicxulub crater and on their surrounding alteration phases this unique approach offers the opportunity to study in situ and at high spatial resolution both the mixing of different target lithologies and the variation of the major and trace element budget during the alteration process in addition two dimensional element distribution maps reveal important geochemical information beyond the capabilities of single spot laser drilling glasses from two localities in opposite quadrants from the source crater were studied at the beloc locality haiti the glass population is dominated by the presence of yellow high ca glass and black andesitic glass formed by admixture of carbonate dolomite anhydrite platform lithologies with crystalline basement these glasses alter according to the well established hydration palagonitization model postulated for mafic volcanic glasses rees become progressively leached from the glass to below the detection limit for the applied spot size while immobile zr hf nb and ta passively accumulate in the process exhibiting both inter element ratios and absolute concentrations similar to those for the original glass in contrast the arroyo el mimbral locality ne mexico is characterized by abundant green glass fragments high in si al and alkalis and low in mg ca fe low si black glass is less abundant though similar in composition to the black glass variety at beloc the alteration pattern of high si al green glass at the mimbral locality is more complex including numerous competing reaction processes ion exchange hydration dissolution and secondary mineral precipitation generally controlled by the ph and composition of the surrounding fluid all green high si al glasses are hydrated and variably enriched in sr ba and cs indicating preferred adsorption from seawater during hydration despite the onset of ion exchange reactions which only seem to have affected the alkalis the trace element composition of the green high si al glass is still largely representative of the original melt composition refining the geochemical signature of altered melt lithologies may advance our current understanding of glass stability in the natural environment and provide insight into the origin and emplacement of ejecta material during crater formation

yaxcopoil 1 and the chicxulub impact csdp core yaxcopoil 1 was drilled to a depth of 1 511 m within the chicxulub crater an organic rich marly limestone near the base of the hole 1 495 to 1 452 m was deposited in an open marine shelf environment during the latest cenomanian uppermost rotalipora cushmani zone the overlying sequence of limestones dolomites and anhydrites 1 495 to 894 m indicates deposition in various carbonate platform environments e g sabkhas lagoons a 100 m thick suevite breccia 894 794 m identifies the chicxulub impact event above the suevite breccia is a dolomitic limestone with planktic foraminiferal assemblages indicative of plummerita hantkeninoides zone cf1 which spans the last 300 ky of the maastrichtian an erosional surface 50 cm above the breccia dolomite contact marks the k t boundary and a hiatus limestones above this contact contain the first tertiary planktic foraminifera indicative of an upper p eugubina zone p1a 2 age another hiatus 7 cm upsection separates zone p1a 2 and hemipelagic limestones of planktic foraminiferal zone p1c planktic foraminiferal assemblages of zone plc to p3b age are present from a depth of 794 04 up to 775 m the cretaceous carbonate sequence appears to be autochthonous with a stratigraphic sequence comparable to late cretaceous sediments known from outside the chicxulub crater in northern and southern yucatan including the late cenomanian organic rich marly limestone there is no evidence that these sediments represent crater infill due to megablocks sliding into the crater such as major disruption of sediments chaotic changes in lithology overturned or deep dipping megablocks major mechanical fragmentation shock or thermal alteration or ductile deformation breccia units that are intercalated in the carbonate platform sequence are intraformational in origin e g dissolution of evaporites and dykes are rare major disturbances of strata by the impact therefore appear to have been confined to within less than 60 km from the proposed impact center yaxcopoil 1 may be located outside the collapsed transient crater cavity either on the upper end of an elevated and tilted horst of the terrace zone or even outside the annular crater cavity the chicxulub site thus records a large impact that predates the k t boundary impact and mass extinction

regional setting and nature of gold mineralization in tanzania and southwest kenya abstract the most productive gold deposits in tanzania and southwest kenya are in the nyanzian greenstone belts 2700 ma and the ubendian system 2000 ma the deposits occur in the form of massive or disseminated fe sulphide rich bodies in banded iron formation bif or tuffs mineralized shear zones in mafic rocks and shear zone controlled quartz reefs mineralization is mostly interpreted as epigenetic although for some deposits a syngenetic origin is possible there are also base metal rich massive sulphide deposits of probable syngenetic origin in the nyanzian greenstone belts iron rich mafic rocks are the most favourable hosts in the lower nyanzian belts as well as in the ubendian system whereas bif is the dominant host rock in the upper nyanzian system to date about two thirds of the gold production has come from shear zone controlled mineralization and about one third from stratiform stratabound deposits

petrology of impact melt rocks at the chicxulub multiring basin yucatan mexico compositions and textures of melt rocks from the upper part of the chicxulub structure are typical of melt rocks at other large terrestrial impact structures apart from variably elevated iridium concentrations less than 1 5 to 13 5 0 9 ppb indicating nonuniform dissemination of a meteoritic component bulk rock and phenocryst compositions imply that these melt rocks were derived exclusively from continental crust and platform sediment target lithologies modest differences in bulk chemistry among samples from wells located approximately 40 km apart suggest minor variations in relative contributions of these target lithologies to the melts subtle variations in the compositions of early formed pyroxene and plagioclase also support minor primary differences in chemistry between the melts evidence for pervasive hydrothermal alteration of the porous mesostasis includes albite k feldspar quartz epidote chlorite and other phyllosilicates as well as siderophile element enriched sulfides suggesting the possibility that chicxulub like sudbury may host important ore deposits

measurement of soil strength in simple shear tests during a simple shear test the axes of stress rotate and in a conventional apparatus in which the only stresses measured are the normal and shear stresses on horizontal planes it is not possible to define the stress state completely as a result the measured failure stresses may not represent the true strength of the soil examination of possible mohr s circles for soils at failure in simple shear tests demonstrates that the measured strength for a given soil depends on among other things the ratio of the horizontal and vertical effective stresses at failure results of laboratory tests on cowden till and on blue london clay show the differences between strengths measured in simple shear and triaxial tests a consequence of the conventional interpretation of the simple shear test is that effective stress failure envelopes have a false cohesion intercept with friction angles smaller than those measured in triaxial tests key words clays laboratory equipment shear strength shear tests triaxial tests

structural history of the manitouwadge greenstone belt and its volcanogenic cu zn massive sulphide deposits wawa subprovince south central superior province structural analysis of the manitouwadge greenstone belt integrated with detailed mapping and geochronological and petrographic studies reveals a complex early deformation history that significant

stress analysis post orogenic extension and 3 01 ga gold mineralisation in the barberton greenstone belt south africa gold deposits in the barberton greenstone belt bgb have formed along a common network of brittle ductile shear zones that developed during extension and emplacement of felsic porphyry dykes at 3015 ma at a deposit scale mineralised shears occur in en echelon arrays distributed as riedel p and anti riedel shears with envelopes that connect groups of mine workings along nnw ene ne and ese trends the dip of the mineralised shear envelopes is generally moderate to steep 50 80 the existence of the directional alignment of gold deposits at the scale of the greenstone belt has been demonstrated with fry analysis stress inversion for mineralised shear zones from workings near 28 mines using a bingham axial distribution and an optimized right dihedron approach provide near identical results with 1 vertical and 3 horizontal nw the stress regime is generally radial extensional or pure extensional at the scale of the greenstone belt the mineralised shear envelopes comprise two sets of near orthogonal trends positioned symmetrically with respect the measured far field stress 1 a nw and ne trending pair with the latter accommodating pure normal slip and the former both dextral or sinistral strike slip and 2 a nnw and ese trending pair accommodating dextral transtensional and sinistral transtensional movements respectively the fact that all mineralised shears developed systematically in response to a similar far field stress suggests that mineralisation formed during a single tectonic stage pb pb ages of 3017 1 17 8 ma and 3008 8 16 ma from a syn mineralisation porphyry dyke near golden quarry indicates gold deposition at 3015 ma i e 85 ma after the craton had stabilised older porphyry dykes intruded near sheba and consort mines have been dated at 3100 ma and 3260 ma respectively and provide a minimum and maximum age constraint on accretionary deformation in the area we propose that the main phase of gold enrichment in the bgb occurred after tectonic and thermal stabilisation of the crust at a time the craton was acting as a stable terrane mineralised brittle ductile shear zones allowed ingress of deeply sourced fluids during cratonic extension mine camps display a cluster spacing of 12 km which suggests a regular distribution of controlling shears possibly as a function of the elastic rigidity of the lithosphere rather than linking mineralisation to orogenic processes during greenstone belt accretion we link gold mineralisation to the opening of fault bounded intracratonic basins including the pongola and witwatersrand basins and concomitant igneous activity

new links between the chicxulub impact structure and the cretaceous tertiary boundary the 200 km diameter chicxulub structure1 3 in northern yucatan mexico has emerged as the prime candidate for the cretaceous tertiary k t boundary impact crater3 6 concentric geophysical anomalies associated with enigmatic occurrences of upper cretaceous breccias and andesitic rocks led penfield and camargo1 to suspect that this structure was a buried impact basin more recently the discovery of shocked quartz grains in a chicxulub breccia3 and chemical similarities between chicxulub rocks and k t tektite like glasses3 6 have been advanced as evidence that the chicxulub structure is a k t impact site here we present evidence from core samples that chicxulub is indeed a k t source crater and can apparently account for all the evidence of impact distributed globally at the k t boundary without the need for simultaneous multiple impacts or comet showers shocked breccia clasts found in the cores are similar to shocked lithic fragments found worldwide in the k t boundary ejecta layer7 8 the chicxulub melt rocks that we studied contain anomalously high levels of iridium up to 13 5 parts per 109 also consistent with the iridium enriched k t boundary layer9 our best estimate of the crystallization age of these melt rocks as determined by 40ar 39ar analyses is 65 2 0 4 1 myr in good agreement with the mean plateau age of 64 98 0 05 myr recently reported10 furthermore these melt rocks acquired a remanent magnetization indicating that they cooled during an episode of reversed geomagnetic polarity the only such episode consistent with40ar 39ar constraints is chron 29r which includes the k t boundary

flash vaporization during earthquakes evidenced by gold deposits fluids flowing through cavities in earth s crust can deposit gold thermo mechanical modelling of a fluid filled cavity that expands suddenly during an earthquake shows that the drop in pressure would cause the fluid to vaporize and deposit the gold almost instantaneously

modeling the formation of the k pg boundary layer abstract in this paper we investigate the formation of the cretaceous paleogene k pg boundary layer through numerical modeling the k pg layer is widely agreed to be composed of meteoritic material and target rock from the chicxulub impact site that has been ejected around the globe and mixed with local material during final deposition the observed composition and thickness of the k pg boundary layer changes with azimuth and distance from the impact site we have run a suite of numerical simulations to investigate whether we can replicate the observational data with a focus on the distal k pg layer and the impact glasses at proximal sites such as beloc haiti previous models of the k pg ejecta have assumed an initial velocity distribution and tracked the ejecta to its final destination here we attempt to model the entire process from impact to the arrival of the ejecta around the globe our models replicate the observed ejecta thickness at proximal sites and the modeled ejecta is composed of sediments and silicate basement rocks in agreement with observational data models that use a 45 impact angle are able to replicate the total ejecta and iridium volume at distal sites and the majority of the ejecta is composed of meteorite and target sediments sub vertical impacts generate too little iridium and oblique impacts of 30 degrees generate too much however in contrast to observations models that involve ballistic transport of ejecta lead to ejecta thickness decreasing with increasing distance and are unable to transport shocked minerals quartz and zircon from the chicxulub basement rocks around the globe we suggest that much of the k pg ejecta is transported non ballistically and that the most plausible mechanism is through re distribution from a hot expanding atmosphere the results are important for future investigations of the environmental effects of the chicxulub impact

the k t boundary at beloc haiti compared stratigraphic distributions of the boundary markers at beloc haiti the cretaceous tertiary boundary ktb is characterized by a spherule bed containing glass particles these particles are considered by some authors as remains of tektites resulting from a nearby impact however because of the stratigraphic complexity of the beloc sections the genetic link between the ktb cosmic event and the spherule bed is not obvious in this paper we report new data on shocked quartz and ni rich spinels at haitian ktb sites the detailed stratigraphy of these minerals shows that there is no empty gap in the sedimentary sequence the first and largest shocked quartz is found in the upper part of the spherule layer they are abundant and size graded over the 25 30 cm of carbonate rich sediments overlying this layer the first ni rich spinels which are also rich in cr appear in the carbonate sediments the size grading of the spherules and shocked quartz and the stratigraphical overlapping of their distributions suggests that these two components were derived from the same event although different from typical impact glasses tektites beloc glass particles must be considered as impact derived products the enormous fluence of shocked quartz 104 grains cm2 is consistent with a proximal event the upper part of the sequence is more complex a second distribution of shocked quartz associated with ni rich spinels of different compositions appears in the layer containing the maximum ir concentration we propose that these features which are not easily explained by a sedimentary artifact result from a second collisional event

structural and lithological controls of gold bearing quartz breccia zones in archean metaturbidites gordon lake northwest territories canada gold lode deposits hosted within shear zones of archean metavolcanic belts have produced some of the world s largest gold mines consequently exploration has focussed on regional lineaments with

extension and gold mineralization in the archean kalgoorlie terrane yilgarn craton abstract this paper focuses on two extensional events affecting late archean crust of the kalgoorlie terrane in the eastern goldfields and which are associated with gold mineralization these events bracket the duration of the crustal shortening event known as the kalgoorlie orogen the early extension phase is preserved around the raeside batholith and gave rise to the earliest recognized structures d 1e this event is characterized by multi directional horizontal extension in contrast to unidirectional extension associated with core complexes extension uplifted the batholith in relation to the overlying greenstone sequence and produced recumbent folds gold including the large deposits at leonora was deposited in normal shear zones around the batholith we propose that extension triggered doming of gravitationally unstable granitic rocks buried under a greenschist sequence and amplified the growth of extensional nappes the late phase of extension is evident in a number of gold deposits in the kalgoorlie terrane this phase is characterized by a overprinting crustal shortening structures that suggests it is the latest structure in all localities b gently dipping foliation associated with recumbent folds which are dragged into moderately dipping normal shear zones c maximum stretching axis nw se or n s at high angles to the maximum shortening axis inferred from regional shortening phases of the kalgoorlie orogen and d hosting gold this phase records relaxation of the kalgoorlie orogen and the findings suggest that gold mineralization was not restricted to a single deformation phase but started before the kalgoorlie orogen and waned at the end of the orogeny

alteration paragenesis and the timing of mineralised quartz veins at the world class geita hill gold deposit geita greenstone belt tanzania abstract the world class geita hill deposit is one of the largest gold deposits located within the geita greenstone belt in nw tanzania the deposit is hosted within a complexly deformed sedimentary package dominated by ironstone and intruded by diorite dykes the gold mineralisation is spatially associated with the geita hill shear zone which is a ne trending moderately nw dipping deformation zone consisting of a network of discontinuous shear fractures that record early thrusting overprinted by later strike slip and normal events the regional metamorphic assemblage in the meta sediments is characterised by biotite chlorite actinolite k feldspar magnetite pyrrhotite pyrite indicating upper greenschist facies conditions the gold related alteration overprints the regional metamorphic assemblage and is characterised by silicification and sulfidation fronts that generally extend out from the mineralised zone by no more than one meter the alteration assemblage includes sub vertical mineralised quartz veins that trend approximately e w the mineralised quartz veins are accompanied by alteration halos of quartz biotite k feldspar pyrite which overprints the peak metamorphic assemblage gold is closely associated with secondary pyrite and occurs as free gold and gold tellurides sylvanite calaverite and nagyagite it occurs mainly as inclusions in pyrite and as invisible gold in pyrite but also as gold inclusions in biotite and along quartz grain boundaries two distinct textural styles of auriferous pyrite can be distinguished inclusion rich subhedral pyrite hosting invisible gold and inclusion free euhedral pyrite hosting visible gold grains it is common for the inclusions rich pyrite to have thick rims of inclusion free pyrite the mineralising alteration is overprinted by barren multiphase quartz carbonate and carbonate chlorite veins this alteration is characterised by the assemblage calcite siderite chlorite quartz pyrite barite palaeostress analysis of mineralised shear fractures along the geita hill shear zone are indicative of sigma 1 being vertical and sigma 3 trending n s indicating n s extension which is consistent with the orientation of the mineralised quartz veins

chicxulub crater a possible cretaceous tertiary boundary impact crater on the yucatan peninsula mexico we suggest that a buried 180 km diameter circular structure on the yucatan peninsula mexico is an impact crater its size and shape are revealed by magnetic and gravity field anomalies as well as by oil wells drilled inside and near the structure the stratigraphy of the crater includes a sequence of andesitic igneous rocks and glass interbedded with and overlain by breccias that contain evidence of shock metamorphism the andesitic rocks have chemical and isotopic compositions similar to those of tektites found in cretaceous tertiary k t ejecta a 90 m thick k t boundary breccia also containing evidence of shock metamorphism is present 50 km outside the crater s edge this breccia probably represents the crater s ejecta blanket the age of the crater is not precisely known but a k t boundary age is indicated because the crater is in a thick carbonate sequence shock produced co2 from the impact may have caused a severe greenhouse warming

evidence for a meteoritic component in impact melt rock from the chicxulub structure abstract the chicxulub structure in yucatan mexico has recently been recognized as a 200 km diameter multi ring impact crater of k t boundary age crystalline impact melt rocks and breccias from within the crater which have compositions similar to those of normal continental crustal rocks and which show shock metamorphic effects have been studied for trace element and re os isotope compositions re os isotope systematics allow the sensitive and selective determination of an extraterrestrial component in impact derived rocks a melt rock sample shows elevated indium concentrations an osmium concentration of 25 ppb and a low 187 os 188 os ratio of 0 113 which are incompatible with derivation from the continental crust even though the 187 os 188 os ratio is slightly lower than the range so far measured in meteorites a mantle origin seems unlikely for mass balance reasons and because the cratering event is unlikely to have excavated mantle material the data support the hypothesis of a heterogeneously distributed meteoritic component in the chicxulub melt rock a sample of impact glass from the haitian k t boundary at beloc yielded about 0 1 ppb osmium and an 187 os 188 os ratio of 0 251 indicating the presence of a small meteoritic component in the impact ejecta as well

late archean granitoid hosted lode gold deposits yilgarn craton western australia deposit characteristics crustal architecture and implications for ore genesis granitoid hosted lode gold deposits constitute a sub group of late archean lode gold deposits in the yilgarn craton of western australia they share a number of common characteristics including 1 a strong structural control on a variety of scales 2 in most instances the presence of mineralisation in adjacent supracrustal sequence 3 wallrock alteration and vein assemblages consistent with the addition of sio2 k2o co2 s na2o but dependent on the p t conditions of the host environment and ore fluid 4 a metal association comprising au ag as bi te w mo and low concentrations of cu pb and zn and 5 deposition from a low moderate salinity near neutral h2o co2 ch4 fluid over a temperature and pressure range from 250 to 600 c and 1 to 4 kbar which corresponds to a range of crustal environments from lower greenschist to upper amphibolite facies these characteristics suggest that the granitoid hosted lode gold deposits represent a coherent genetic group that is equivalent to the more abundant greenstone hosted lode gold deposits of the craton in combination lode gold deposits irrespective of their host rocks and host terrane formed over a range of crustal levels and developed late in the tectonic sequence of the craton granitoids which host lode gold mineralisation are older than the inferred age of mineralisation by at least 10 million years and in combination with thermodynamic and isotope studies indicate that the host granitoids are not the dominant source of the ore fluid however the host granitoids do provide a structural and to a lesser extent chemical trap for gold bearing fluids in combination trace element and isotope studies of greenstone and granitoid hosted deposits suggest that the ore fluid originated from a deep mid to lower crustal source and at least interacted with granitic rocks at some stage the ore fluid composition is quite constant over the range of crustal levels from deep to shallow implying that the extent of modification by interaction with fluid conduit rocks is variable but not substantial

lithostratigraphic and tectonic setting of gold mineralization in the archean cratons of tanzania and uganda east africa abstract the archean cratons of tanzania and uganda are reviewed in terms of their lithostratigraphy and tectonometamorphic events the earliest of which is dated at 2900 ma set in granitoid cratonic terrane the greenstone belts of the nyanzian system 2700 2500 ma form the main host rocks to syngenetic and epigenetic types of gold mineralization stratabound gold mineralizations occurring in the banded ironstone formation of the greenstone belts may have been remobilized during major metamorphic magmatic events at 2500 ma and partly redistributed in the form of hydrothermal auriferous quartz veins subsequently metasomatic gold mineralization in the form of impregnations occurred the location of gold mineralization is controlled by lithology metamorphism and tectonics and is discussed in the context of crustal evolution

crustal scale auriferous shear zones in the central superior province canada a proposed correlation of major archean shear zones across different metamorphic terranes of the central superior province implies that the same shear zones are exposed at greenschist to granulite facies coalignment of the shear zones is recognized after systematic removal of regional proterozoic dextral transcurrent and shortening deformation associated with the intracratonic kapuskasing uplift subvertical in the upper crust the structures become shallowly north dipping at deeper levels the connection of these archean shear zones provides limits on any sinistral transcurrent component associated with the tectonic evolution of the kapuskasing uplift

fault fracture density and mineralization a contouring method for targeting in gold exploration abstract a widely observed correlation between high fracture density and mineralization throughout terranes and geological time indicates a fundamental underlying ore forming process in archaean greenstone hosted deposits high density fracturing was accompanied by enhanced fluid flow during fault fracture network development producing regional scale fluid pressure gradients that focussed hydrothermal fluids into preferentially fractured areas fracture density is both increased and decreased during faulting and fault healing and fracture density accumulates over time in zones of high palaeo fluid flow localised zones where the density of fracturing is increased by deformation become permeability nodes for migrating hydrothermal fluids leading to large zones of alteration and gold precipitation the ora banda mining centre in western australia contains significant gold deposits that appear to demonstrate a close association between high density fracturing and gold precipitation fracture density in the ora banda mines was enhanced by fault fault intersections fault contact intersections and changes in fault geometry the mine scale relationships between fracture density and gold mineralization are repeated at smaller and larger scales hence these relationships may be used in targeting for gold exploration contouring the density of fracturing in a region provides a semi quantitative way to rank areas for exploration and uses data from mapping drilling and high quality geophysical data as a basis for analyses fracture density contouring is complementary to other prospectivity analysis methods

an experimental examination of the critical state and other similar concepts for granular soils measuring the critical state parameters of dense granular soil by drained triaxial tests is problematic as significant nonhomogeneous deformations can develop prior to approaching a critical state

shallow seismic velocity structure of the chicxulub impact crater from modelling of rg dispersion using a genetic algorithm abstract a 20 element seismic array was deployed onshore across the 65 ma chicxulub impact crater yucatan mexico as part of an integrated onshore offshore seismic experiment fundamental and higher mode group velocity dispersion curves have been calculated through the application of multiple filter and phase match techniques to high frequency 0 2 5 hz rayleigh waves recorded on the array and originating from local quarry blasts several 1 d shear wave velocity depth models across the crater have been produced through the optimisation of these dispersion curves using a genetic algorithm these models are resolved to a depth of circa 3 5 km and provide information on the velocity structure of the upper crater sequences and post impact sedimentation placing constraints on crater morphology and post impact sedimentation patterns shear wave velocities of 0 8 2 2 km s 1 are modelled within the interpreted tertiary sedimentary sequence an inverted velocity gradient is observed over the upper 300 m of the sediments with the exception of a circular region across the crater centre suggesting the crater influenced sedimentation throughout the tertiary the base of the tertiary sequence is modelled at circa 1 1 5 km depth where a thin layer with velocities up to 30 higher than the overlying sediments is observed immediately below this high velocity layer a circa 200 m thick low velocity zone with velocities of 1 0 2 3 km s 1 is interpreted as a layer of suevitic impact breccia a shallowing to the base of this layer to circa 1 3 km depth between 30 and 40 km radius from the crater centre would be consistent with the existence of a peak ring occurring as a topographic high above the crater floor

genetic aspects of the geita and jubilee reef archean bif hosted gold deposits tanzania the sukumaland greenstone belt new name which is located in north western tanzania hosts a large number of gold deposits prospects and occurrences the archean stratigraphy comprises an older basement of granitic gneisses dodoman system overlain by mafic and ultramafic volcanics which are succeeded by felsic volcanic rocks and a banded iron formation bif predominantly as oxide facies the volcanics and bif together make up the nyanzian system the uppermost part of the archean stratigraphy kavirondian system consists of conglomerates and quartzites which rest unconformably on the older units the sequence has been intruded by both syn and post orogenic granitoids and by several generations of felsic and intermediate dykes and sills gold mineralisation is abundant in all stratigraphic units except for the granitic rocks six types of mineralisation can be distinguished these are bif hosted hear zone type quartz vein type clastic sedimenthosted alluvial eluvial and massive sulphide type deposits the last is represented by only one deposit in this area

yucatan subsurface stratigraphy implications and constraints for the chicxulub impact much of the discussion about the effects of an end of cretaceous impact by a large extraterrestrial body in northwestern yucatan has been done in the context of limited and partly erroneous published data on the mesozoic stratigraphy of that area reexamination of cores and geophysical logs taken in several pemex wells has produced improved lithologic and biostratigraphic correlation of the jurassic to maastrichtian section across the northern yucatan peninsula these data suggest that major disturbance of strata by an impact would have been confined to within about 100 km of the proposed impact center near chicxulub the only unusual lithologic unit is polymict breccia which apparently was penetrated at or near the top of the cretaceous section in all the deep wells of northern yucatan this breccia in pemex wells yucatan 1 2 4 5a and 6 is composed predominantly of detrital dolomite limestone and anhydrite clasts set in dolomitized carbonate mud matrix which contains upper maastrichtian foraminifers these constituents mixed with fragments of altered glass or melt rock shocked quartz and feldspar and basement rock suggest an impact as the most likely origin for the breccia the timing of brecciation is poorly constrained by biostratigraphic data there is some evidence however that the breccia unit is overlain by about 18 m of uppermost maastrichtian marls suggesting an impact before the cretaceous tertiary boundary in addition there may have been more than one episode of breccia deposition

impact winter and the cretaceous tertiary extinctions results of a chicxulub asteroid impact model abstract the chicxulub impact crater in mexico is the site of the impact purported to have caused mass extinctions at the cretaceous tertiary k t boundary 2 d hydrocode modeling of the impact coupled with studies of the impact site geology indicate that between 0 4 and 7 0 1017 g of sulfur were vaporized by the impact into anhydrite target rocks a small portion of the sulfur was released as so3 or so4 which converted rapidly into h2so4 aerosol and fell as acid rain a radiative transfer model coupled with a model of coagulation indicates that the aerosol prolonged the initial blackout period caused by impact dust only if the aerosol contained impurities a larger portion of sulfur was released as so2 which converted to aerosol slowly due to the rate limiting oxidation of so2 our radiative transfer calculations combined with rates of acid production coagulation and diffusion indicate that solar transmission was reduced to 10 20 of normal for a period of 8 13 yr this reduction produced a climate forcing cooling of 300 wm 2 which far exceeded the 8 wm 2 greenhouse warming caused by the co2 released through the vaporization of carbonates and therefore produced a decade of freezing and near freezing temperatures several decades of moderate warming followed the decade of severe cooling due to the long residence time of co2 the prolonged impact winter may have been a major cause of the k t extinctions

mapping chicxulub crater structure with gravity and seismic reflection data the chicxulub crater buried under the yucat n platform of mjxico affords one of the most favourable opportunities on earth for structural studies of a large complex impact crater due to its preservation and accessibility the crater was buried soon after rim erosion in an environment with only minor subsequent tectonic disturbance it is also of interest as the chicxulub impact apparently ended the cretaceous period and the crater s ejecta blanket is known globally at hundreds of localities the buried crater is half overlain by a coastal plain and half by a shallow sea allowing exploration by both marineand land based techniques as well as hybrid combinations the radial symmetry of the crater allows application of data acquired in either terrain to the entire structure using the common data sets of magneticand gravity field anomalies gravity data sets covering the yucat n platform e g 1 2 reveal an 180 km diameter low with internal concentric structure e g 3 4 that is the expression of the chicxulub crater impact lithologies and crater fill the gravity signature is complicated by regional anomalies and early modifications to the crater rim and the northern half of the crater is not well imaged due to the scarcity and imprecision of marine data also an 20 km wide gap in survey coverage due to shallow water occurs between the land data and the nearest seismic profile of camargo and suarez 5 fig 1 which roughly corresponds to the beginning of offshore coverage during september and october 1996 a marine gravity survey was successively conducted on 32 and 10 5 m vessels to survey this gap and offshore seismic lines of the imperial college birps u of texas seismic project offshore coverage was 2 400 line km followed by 1 500 line km inshore with 1 3 and 1 9 mgal unadjusted crossovers respectively data precision was strongly influenced by sea conditions preliminary results from the marine surveys confirm the provisional ties of gravity gradient features to the seismic line the perimeter fault crater fill margin slump faults and collapsed transient cavity margin may all be linked to gradient features unfortunately the clearest crater expression in the gravity field is on the east side where the seismic lines lose resolution at the basin margin to the west the strong peripheral gradient splits with one component corresponding to the perimeter slump fault and a larger feature corresponding to the basin margin this splitting is interpreted as resulting from prompt backwash of crater ejecta into the northwestern part of the crater regional gradients also obscure the crater signature to a greater extent to the west asymmetry of the central high is indicated although a weak feature may extend to the ne the surrounding sharp low continues until it abuts the regional high to the nw additional land surveys support the correlation of the cenote ring to gravity gradient features and suggest that a weak high may correlate to the rim uplift in some places

influences of principal stress direction and intermediate principal stress on the stress strain strength behaviour of completely decomposed granite the measurement and study of the stress strain strength behaviour of soils in general stress states involving the change of magnitudes and direction of the principal stresses are necessary and important to investigate the strength behaviour under such conditions consolidated undrained tests on remoulded hollow cylinder specimens of completely decomposed granite cdg were carried out using a hollow cylinder apparatus tests were conducted by maintaining a fixed principal stress direction with angle from the vertical direction together with a fixed value of intermediate principal stress coefficient b it is observed that the value of the friction angle decreases with an increase in and the failure surface is anisotropic there is an increase in friction angle with an increase in b value up to b 0 25 and the friction angles are almost the same for b 0 25 in addition the behaviour of the soil in an undrained simple shear condition was examined the simple shear condition is very near to the cond

the cretaceous tertiary boundary at beloc haiti no evidence for an impact in the caribbean area abstract the k t boundary at beloc in haiti is characterized by a bed containing more or less spheroidal globules composed of either smectite or calcite a small fraction of these objects still contains well preserved glass particles which have been interpreted as the remains of tektites resulting from a bolide impact in the caribbean area however beloc glasses are markedly different from tektites and microtektites their structure and composition suggest that they are not impact generated products but are likely of volcanic origin on the other hand in sections which have probably preserved their original stratigraphy the globule bed showing well developed grading appears first in the sedimentary column followed by the ubiquitous cretaceous tertiary boundary ktb clay layer containing ir and ni rich spinels granulometric considerations indicate that the globule bed and the ir and spinel rich clay layer correspond to two distinct events we conclude that the globules at the ktb depositional sequence of beloc represent a local volcanic event slightly preceding the great ktb cosmic catastrophe our observations do not provide any unequivocal evidence that a cratering event occurred nearby

deposition of channel deposits near the cretaceous tertiary boundary in northeastern mexico catastrophic or normal sedimentary deposits investigation of cretaceous tertiary k t boundary channel deposits in three sections in northeastern mexico reveal three lithologically and mineralogically distinct units separated by three or more disconformities the units include a spherule rich layer unit 1 laminated sandstone unit 2 and rippled sand and shale layers unit 3 on the basis of paleontological mineralogical and geochemical evidence the k t boundary is just above the top of unit 3 although the deposits have previously been interpreted as a tsunami deposit related to a presumed k t impact at chicxulub on yucatan we have found no evidence of a nearby impact we suggest that deposition occurred in an incised valley setting and over an extended period of time

trace element concentrations in the mexico belize ejecta layer a link between the chicxulub impact and the global cretaceous paleogene boundary four exposures of chicxulub impact ejecta along the mexico belize border have been sampled and analyzed for major and trace element abundances the ejecta deposits consist of a lower spheroid bed containing clay and dolomite spheroids and an upper diamictite bed with boulders and clasts of limestone and dolomite the matrix of both beds is composed of clay and micritic dolomite the rare earth element ree compositions in the matrix of both units show strong similarities in concentrations and pattern furthermore the zr tio2 scatter plot shows a linear correlation indicating one source these results indicate that the basal spheroid bed has the same source and was generated during the same event as the overlying diamictite bed which lends support to a single impact scenario for the albion formation ejecta deposits the elevated concentrations of non meteoritic elements such as sb as u and zn in the matrix of the lower spheroid bed are regarded to have been derived from the sedimentary target rocks at the chicxulub impact site the positive eu and ce anomalies in clay concretion and in the matrix of the lower part of the spheroid bed in albion island quarry is probably related to processes involved in the impact such as high temperature and oxidizing conditions analogous trace element anomalies have been reported from the distal cretaceous paleogene k t boundary clay layer at different sites thus the trace element signals reported herein are regarded to support a genetic link between the chicxulub impact the ejecta deposits along the mexico belize border and the global k t boundary layer

tsunami backwash deposits with chicxulub impact ejecta and dinosaur remains from the cretaceous palaeogene boundary in the la popa basin mexico the la popa basin in north eastern mexico features outstanding continuous three dimensional exposures of the cretaceous palaeogene boundary event deposit in shallow shelf environments pierced by salt stocks in the area to the south east of the el papalote diapir the cretaceous palaeogene deposit consists of two superimposed sedimentary units and erosively overlies upper maastrichtian sand siltstones with soft sediment deformation and liquefaction structures the basal unit 1 is an up to 8 m thick chaotic carbonate rich bed that discontinuously fills incised gutters and channels besides abundant silicic and carbonate ejecta spherules from the chicxulub impact unit 1 includes large sandstone boulders and abundant shallow water debris for example mud clasts algae bivalve shells gastropod shells and vertebrate remains unit 1 is conformably overlain by unit 2 distal to the diapir unit 2 consists of a centimetre to decimetre thick conglomeratic coarse bioclast and spherule bearing sandstone bed closer to the diapir unit 2 becomes a metre thick series of four to eight conglomeratic to fine grained graded sandstone beds rich in shell debris and ejecta spherules unit 2 is conformably overlain by structureless to parallel laminated sandstone beds that may mark the return to the pre event depositional regime the sedimentary characteristics of the cretaceous palaeogene deposit including its erosive base its sheet like geometry the presence of multiple graded beds evidence for upper flow regime conditions and the absence of bioturbation support an origin by a short term multiphase depositional event the occurrence of soft sediment deformation structures for example liquefaction below the cretaceous palaeogene deposit suggests that earthquakes were the first to occur at la popa then shelf collapse and strong backflow from the first tsunami waves may have triggered erosion and deposition by violent ejecta rich hyperconcentrated density flows unit 1 subsequently a series of concentrated density flows resulting from tsunami backwash surges may have deposited the multiple graded bedding structures of unit 2 the specific depositional sequence and the fe mg rich as well as si k rich composition of the ejecta spherules both provide a critical link to the well known deep marine cretaceous palaeogene boundary sites in the adjacent burgos basin in north eastern mexico moreover the pulse like input of chicxulub ejecta material at the base of the event deposit allows for correlation with other cretaceous palaeogene boundary sites in the gulf of mexico and the atlantic as well as in central and northern america the presence of diverse dinosaur and mosasur bones and teeth in the event deposit is the first observation of such remains together with chicxulub ejecta material these findings indicate that dinosaurs lived in the area during the latest maastrichtian and suggest that the tsunami waves not only eroded deltas and estuaries but the coastal plain as well

horizontal tectonic deformation geometries in a late archaean sedimentary sequence belingwe greenstone belt zimbabwe in the belingwe greenstone belt of zimbabwe structural evidence from the circa 2 65 ga old sedimentary cheshire formation which overlies and is imbricated with a mafic volcanic unit is consistent with thrusting of the greenstone sequence the cheshire formation consists of a karstified carbonate ramp sequence overlain by siliciclastic turbidite deposits that formed in a southeast deepening basin the earliest deformational structures formed during a syndepositional to postdepositional thin skinned thrusting event d1 that affected poorly consolidated sediments and is recorded in bedding parallel ductile shear zones boudins folds and block in matrix structures d1 shear zones separate the volcanic sequence and the overlying sediments and occur between the carbonate and siliciclastic units syntectonic sulphide mineralization and silicification of mainly fine grained sediments along the main thrust faults gave rise to the formation of rocks similar in appearance to banded iron formations stratigraphic units were locally duplicated along the d1 thrust faults including a tectonic slice of mafic volcanics that was emplaced onto carbonates along a chaotic unit similar to a tectonic melange the elongation of inclusions in the melange zone and lineations in ironstones together with kinematic indicators suggest that stratigraphic duplication resulted from northwestward directed tectonic transport deposition of the cheshire formation took place in an asymmetric foreland type basin contemporaneously with thrusting soon after deposition the formation was incorporated into the thrust stack subsequent deformation events include tight upright folding gentle cross folding and dextral strike slip faulting of the greenstone succession evidence for early thrusting suggests that horizontal tectonic processes played an important role in the evolution of the belingwe greenstone belt

fiber bundle model for multiscale modeling of hydromechanical triggering of shallow landslides 1 sudden and rapid mass movements associated with landslides and snow avalanches present a hazard to life and infrastructure yet their predictions and triggering mechanisms remain poorly understood statistical methods and correlative studies have been used to produce landslide or avalanche susceptibility maps often with limited physical basis mechanistic approaches based on factor of safety computation seldom represent the progressive transition from local failure events to a landslide and in general do not include heterogeneities associated with land cover or with subsurface material properties and hydrologic pathways focusing on rainfall induced shallow landslides we propose the use of the fiber bundle model fbm a generic yet powerful and adaptable model used in modeling fatigue and fracture of complex and disordered materials the primary strength of the fbm is its ability to represent the progressive failure of cracks and shear zones and the ruptures of highly heterogeneous bonding elements that are present in soils at all scales the model also provides a natural framework for interpretation of acoustic emission signatures from failing slopes which may form the basis of a monitoring and warning system

structural event framework for the eastern yilgarn craton western australia and its implications for orogenic gold abstract the nnw trending tectonic grain of the eastern yilgarn craton eyc was established as a result of predominantly ene wsw directed extension d1 and d3 and e ene w wsw d2 d4 to ne sw directed d5 contraction the result has been a succession of nnw striking temporally discrete fabric elements which can be difficult to interpret reliably at any single location despite this many past workers interpreted the nnw striking fabric as the result of only one regional contractional event and used it as a marker for correlating structural events across the region in order to unravel the complexity this paper presents a new sixfold d1 d6 deformation nomenclature based on 10 000 new mesoscale structural observations including their kinematic analysis and cross cutting relationships these mesoscale data were referenced with regional 3d map patterns stratigraphic magmatic metallogenic considerations and deep seismic reflection images this integrated geodynamic architectural approach is applicable to solving structural event histories in other polydeformed terrains gold mineralisation occurred during the first five events but was particularly concentrated from d3 onwards the d3 event marked the most profound change in the tectonic evolution of the eyc with changes in greenstones granites and tectonic mode lithospheric extension and core complexes with the first significant gold deposited within extensional shear zones that dissect the crust later contraction d4 was imposed at a high angle to the previously established anisotropic architecture the outcome was the creation of a new dynamic permeability framework which resulted in gold mineralisation during nnw striking sinistral strike slip faulting and associated thrusting a further stress switch d5 further modified the architecture resulting in n to nne striking dextral strike slip faulting and the final period of gold mineralisation before late stage extension d6

the geology and mineralisation at the golden pride gold deposit nzega greenstone belt tanzania the golden pride gold deposit 3 moz is located in the central part of the nzega greenstone belt at the southern margin of the lake victoria goldfields in tanzania it represents an inferred late archaean orogenic gold deposit and is hosted in intensely deformed meta sedimentary rocks in the hanging wall of the approximately e w striking golden pride shear zone the hanging wall sequence also includes felsic quartz porphyritic to mafic lamprophyric intrusions as well as banded iron formations hydrothermal alteration phases associated with mineralisation are dominated by sericite and chlorite two main ore types can be distinguished chlorite and silica ore both occupying dilational sites and structural intersections in the hanging wall of the main shear zone sulphide minerals in both ore types include pyrrhotite arsenopyrite pyrite and accessory sphalerite galena sulphosalts and ni co bi sulphides gold and tellurides are late in the paragenetic sequence and associated with a secondary phase of pyrrhotite deposition sulphur isotope compositions range from 6 to 7 per mil and are interpreted to reflect contributions from two distinct sources to the mineralising fluids in the golden pride gold deposit a redox change potentially induced by the intrusion of mafic melts together with structural elements in the hanging wall of the golden pride shear zone are interpreted to be the main controls on gold mineralisation in this deposit

archaean crustal growth through successive partial melting events in an oceanic plateau like setting in the tanzania craton the detrital zircon population in quartzitic conglomerates from the northern tanzania craton yield ages between 2640 ma and 2790 ma which includes most of the igneous history from this part of the craton the igneous evolution is characterised by mafic volcanism with an oceanic plateau like geochemical signature at 2800 ma followed by diorite and tonalite trondhjemite granodiorite dominated magmatism between 2790 and 2700 ma which transitioned into more evolved high k magmatism between 2700 and 2620 ma the ehf values of the detrital zircons range from 2 4 to 1 4 and change with time from radiogenic hf pre 2700 ma 98 positive ehf to unradiogenic hf post 2700 ma 41 positive ehf the petrological progression from mafic to felsic crust is reflected in the detrital age distribution and ehf isotopes and is consistent with juvenile mafic crust slowly maturing into more evolved felsic crust through a series of successive partial melting events in an oceanic plateau like environment

ringed structural zones with deep roots formed by the chicxulub impact the chicxulub meteorite comet impact of 65 ma occurred on the present northeast coast of the yucatan peninsula of mexico and is considered by many to have caused a mass extinction at the end of the cretaceous period impact craters are traditionally described in terms of ring structures deeply rooted faults underlying four previously recognized ring zones of the chicxulub impact crater as revealed on new deep seismic reflection profiles demonstrate the catastrophic and diverse deformations that produced these zones the innermost annular zone at chicxulub coincides with a central peak ring the seismic data show that the peak ring is composed of low velocity 5 km s 1 chaotically reflective material interpreted as impact breccia lying on top of blocks of pre impact strata that were downdropped 4 6 km normal faults within the cretaceous strata and deep inward dipping reflections characterize two deformation zones at radial distances of 55 65 and 85 98 km these normal faults accommodated the collapse during the modification stage of crater development of large radial width of 5 25 km blocks of pre impact strata uplifted during the excavation stage the structures root in the lower crust at depths of 15 30 km blind thrusts indicated by monoclines in pre impact sedimentary rocks and deep inward dipping reflections within the crystalline basement mark the outermost ring of deformation these monoclines correlate with small topographic changes on the cretaceous tertiary boundary layer at radial distances of 120 135 km palinspastic restoration of coherent blocks of downdropped pre impact strata enabled improved estimates of the size and shape of the hole formed by the impact in a prominent reflector at 3 to 4 km depth this hole is slightly elliptical along a se nw major axis radius of 68 versus 63 km

the role of early formed structures on the development of the world class st ives goldfield yilgarn wa a revised structural interpretation for the victory to kambalda area of the world class st ives goldfield in the archean yilgarn craton has mapped out the distribution of wnw trending faults within this area of the field these previously cryptic wnw trending structures had been identified in gravity data and also by isopach thickness variations the wnw trending faults acted as transfers syn gold mineralization although only discrete segments of these faults were active during the main stage of gold mineralization where mineralized the faults transferred strain from a complex combination of block on block movement associated with thrusting and strike slip movement on nw and n trending faults along some segments n trending mineralized faults terminate against the wnw trending faults many of the wnw trending faults correlate with major strike changes on regional and camp scale faults and they are domain boundaries for the critical n trending fault segments that host high grade gold within contractional jogs the wnw trending faults also show evidence for an older deformation history prior to main stage gold which may extend back to early basin development associated with ultramafic and mafic volcanism they are inferred to have been a series of early wnw trending normal faults and breached relay ramps associated with oblique rifting along an older nnw trending basement boundary

yucatan subsurface stratigraphy implications and constraints for the chicxulub impact comment and reply 92 geology january 1997david j and syme e c 1994 u pb geochronology of the late neoarcheantonalites in the flin flon belt trans hudson orogen surprise at the surface canadian journal of earth sciences v 31 p 1785 1790 david j bailes a h and mac hado n 1996 evolution of the sno w lake portionof the paleoproterozoic flin flon and kisseynew belts trans hudson orogen manitoba canada precambrian research in press gale d lucas s b and dixon j m 1996 structural geology of the flin flonarea manitoba and saskatchewan geological survey of canada current re search 1996 c p 93 103 gordon t m and lemk ow d r 1987 geoc hemistr y of the missi gr oup v olcanicrocks wekusk o lake manitoba geological survey of canada open filereport1442 gordon t m hunt p a bailes a h and syme e c 1990 u pb ages from theflin flon and kisse ynew belts manitoba chronology of crust formation at anearly proter ozoic accr etionar y mar gin in lewry j f and stauf fer m r eds the ear ly proter ozoic trans hudson or ogen of nor th amer ica geolo gical as socia tion of canada special p aper 37 p 177 199 machado n and zwanzig h v 1995 u pb geochronology of the kisseynew do main in manitoba provenance ages for metasediments and timing of magma tism in trans hudson orogen transect lithoprobe report 48 p 133 138 zwanzig h v 1995 alternative structural restorations of the flin flon beltkisseynew belt boundary zone in trans hudson orogen transect lithoprobereport 48 p 135 142

a new locality of palygorskite rich clay from the southeastern yucatan a potential material source for environmental applications a palygorskite unit was discovered in a road cut of undifferentiated tertiary limestone between the villages of el pariso and san roman 18 49 309n 88 37 861w in the southeastern yucatan peninsula mexico this is the southern most locality of a clay rich sedimentary unit reported in the literature for the tertiary carbonates of the yucatan peninsula this occurrence indicates a much wider range of palygorskite rich clay deposition than previously recognized the lithology is 99 clay and 1 sand to silt size diagenetic quartz grains the clay consists of approximately 85 palygorskite 15 montmorillonite and trace amounts of titanium oxides eds analyses on palygorskite are largely consistent with sedimentary palygorskites from other coastal marine settings however palygorskite has a low total fe content average 0 40 wt expressed as fe2o3 compared to many other sedimentary palygorskites montmorillonite chemical compositions are typical and compared to the palygorskite have substantially higher fe2o3 concentrations average 3 90 wt the low percentage of coarse grains in the lithology combined with a high proportion of palygorskite and lack of detrimental trace minerals suggest the deposit is of industrial grade however it has limited reserves 6 000 m3 the unit could be potentially used in a wide array of environmental applications which are needed in the region including liners for landfills and constructed wetlands the unit is in a geographic location which would serve the expanding economy of the region this resource has the potential to have great impact on the quality of the local environment and the economy of a region under great environmental threat

diversity within a unified model for archaean gold mineralization in the yilgarn craton of western australia an overview of the late orogenic structurally controlled gold deposits abstract the archaean yilgarn craton has produced 3000 tonnes of gold mainly from structurally controlled deposits that formed during the latest stages of an orogenic event that affected the entire craton and culminated in the period 2 66 2 63 ga as a group these late orogenic deposits encompass a wide range of host rocks structural settings and structural styles and alteration types however several consistent features justify grouping the deposits into a single class of mineralization these features include timing relative to orogenesis high gold to base metal ratios an association with potassium metasomatism and carbonation of host rocks low sulfide contents and a low salinity h 2 o co 2 ch 4 ore fluid most deposits formed in domains of low mean stress during east west regional compression and preserve evidence in the form of quartz vein arrays and hydraulic breccias for high fluid pressures and rock dilation the range of ore depositional conditions implied by alteration assemblages at various deposits makes the term mesothermal an inappropriate description for this class of mineralization the term late orogenic structurally controlled gold deposits is proposed as one which more completely encompasses the class of mineralization described in this overview late orogenic structurally controlled gold deposits formed in thermal equilibrium with high grade metamorphic rocks at the margins of the greenstone belts but were retrograde with respect to lower grade host rocks in the cores of greenstone belts these relationships probably reflect diachronous metamorphism and a single gold mineralization event which was contemporaneous with the later high grade metamorphism geochronological data indicate gold deposition at ca 2 63 ga in the western part of the yilgarn craton isotopic data suggest similar depositional ages in the eastern part of the craton eastern goldfields superterrane but this timing is difficult to reconcile with field relationships that suggest broad contemporaneity between regional metamorphism and gold mineralization at ca 2 66 ga the main competing genetic models for late orogenic structurally controlled gold mineralization in the yilgarn craton propose an orthomagmatic deeply sourced or metamorphogenic ore fluid none of these models can account satisfactorily for all the features of the late orogenic structurally controlled gold deposits

synchronous vertical and horizontal tectonism at late stages of archean cratonization and genesis of hemlo gold deposit superior craton ontario canada the hemlo gold deposit 700 t au northwestern ontario canada is hosted in a shear zone in the hemlo greenstone belt which is a synclinal keel between two granitoid domes the dome and keel structure formed by diapirism and sagduction vertical tectonism that was synchronous with regional horizontal shearing horizontal tectonism mineralization occurred during shearing and mineralizing fluids probably had a magmatic source that is interpreted to be related to syntectonic mantle derived granodiorite sanukitoid intrusions the gold deposit the intrusions and the hosting shear zone all developed within a regime characterized by synchronous vertical and horizontal tectonism they were all possibly linked to a range of processes associated with the accretionary growth and stabilization of the craton in particular slab breakoff and the associated extensional orogenic collapse following terrane accretion

acid neutralizing scenario after the cretaceous tertiary impact event acid rain from the cretaceous tertiary k t boundary impact event should have caused significant damage to freshwater life but only minor extinctions of freshwater species are actually observed we propose a mechanism to neutralize the acid using larnite ca 2 sio 4 produced as a result of the specific lithology at the chicxulub impact site the impact vapor plume must have been enriched in calcium from the carbonate rich target leading to the crystallization of larnite the acid neutralizing capacity of the larnite grains would have been high enough to consume acid produced after the k t event within several hours reducing it to a level at which freshwater life would not have been affected even if all the acid had precipitated instantaneously after the k t impact this scenario can explain some of the extinction selectivity at the k t boundary

a dual layer chicxulub ejecta sequence with shocked carbonates from the cretaceous paleogene k pg boundary demerara rise western atlantic abstract an up to 2 cm thick chicxulub ejecta deposit marking the cretaceous paleogene k pg boundary the k t boundary was recovered in six holes drilled during odp leg 207 demerara rise tropical western atlantic stunning features of this deposit are its uniformity over an area of 30 km2 and the total absence of bioturbation allowing documentation of the original sedimentary sequence high resolution mineralogical petrological elemental isotopic sr nd and rock magnetic data reveal a distinct microstratigraphy and a range of ejecta components the deposit is normally graded and composed predominantly of rounded 0 1 to max 1 mm sized spherules spherules are altered to dioctahedral aluminous smectite though occasionally relict si al rich hydrated glass is also present suggesting acidic precursor lithologies spherule textures vary from hollow to vesicle rich to massive some show in situ collapse others include distinct fe mg ca ti rich melt globules and lath shaped al rich quench crystals both altered glass spherules and the clay matrix site 1259b display strongly negative e nd t 65 ma values 17 indicating uptake of nd from contemporaneous ocean water during alteration finally fe mg rich spherules shocked quartz and feldspar grains few lithic clasts as well as abundant accretionary and porous carbonate clasts are concentrated in the uppermost 0 5 0 7 mm of the deposit the carbonate clasts display in part very unusual textures which are interpreted to be of shock metamorphic origin the preservation of delicate spherule textures normal grading with lack of evidence for traction transport and sub millimeter scale compositional trends provide evidence for this spherule deposit representing a primary air fall deposit not affected by significant reworking the odp leg 207 spherule deposit is the first known dual layer k pg boundary in marine settings it incorporates compositional and stratigraphic aspects of both proximal and distal marine sites its stratigraphy strongly resembles the dual layer k pg boundary deposits in the terrestrial western interior of north america although there carbonate phases are not preserved the occurrence of a dual ejecta layer in these quite different sedimentary environments separated by several thousands of kilometers provides additional evidence for an original sedimentary sequence therefore the layered nature of the deposit may document compositional differences between ballistic chicxulub ejecta forming the majority of the spherule deposit and material falling out from the vapor ejecta plume which is concentrated in the uppermost part

deformation progressive or multiphase abstract complexly deformed rocks that exhibit refolded folds and overprinting planar fabrics are often interpreted as the result of polyphase deformation and described in a scheme of deformation phases d 1 d 2 d n at the same time a number of models have been presented over the last decades that explain how complex and overprinting structures can form by single phase progressive deformation as strain accumulates strain generally accumulates heterogeneously in a deforming region depending on strain intensity and geometry flow parameters rheology and lithology the result is regions of different structural complexity fabrics and fabric orientations that can be sorted into different structural facies such structural pattern should be mapped and presented before defining a scheme of deformation phases and attempts should be made to explain them by a single structural kinematic model such as partitioned transpression only if this is not possible or if independent geochronologic data reveals a more punctuated history should we define an additional tectonic regime and deformation phase multiphase and progressive deformation histories are not mutually exclusive and it is important that both possibilities are considered when describing the deformation history of any complexly deformed region when the concept of deformation phases is employed the number of phases should be minimized and be related to major tectonic event of external character uncritically defining deformation phases can easily generate a complicated discrete deformation history with no link to tectonic reality

the structural history and mineralization controls of the world class geita hill gold deposit geita greenstone belt tanzania the geita hill gold deposit is located in the archean geita greenstone belt and is one of the largest gold deposits in east africa the geita greenstone belt experienced a complex deformation and intrusive history that is well illustrated and preserved in and around the geita hill gold deposit deformation involved early stages of ductile shearing and folding d1 to d5 during which episodic emplacement of large diorite intrusive complexes sills and dykes occurred these ductile deformation phases were followed by the development of brittle ductile shear zones and faults d6 to d8 the last stages of deformation were accompanied by voluminous felsic magmatism involving the intrusion of felsic porphyry dykes within the greenstone belt and the emplacement of large granitic bodies now forming the margins of the greenstone belt early folded lamprophyre dykes and later lamprophyre dykes crosscutting the folded sequence are common although volumetrically insignificant the gold deposit formed late during the tectonic history of the greenstone belt post dating ductile deformation and synchronous with the development of brittle ductile shear zones that overprinted earlier structural elements the main mineralizing process involved sulfide replacement of magnetite rich layers in ironstone and locally the replacement of ferromagnesian phases and magnetite in the diorite intrusions the intersection between the brittle ductile d6 geita hill shear zone and different structural elements of ductile origin e g fold hinges and the contact between banded ironstone and folded diorite dykes and sills provided the optimal sites for gold mineralization

biotite chemistry and the role of halogens in archaean greenstone hosted gold deposits a case study from geita gold mine tanzania abstract geita hill is one of the largest tanzanian gold deposits located southwest of lake victoria and 2 km along strike from the giant nyankanga deposit the deposit is hosted by metasedimentary rocks dominated by ironstones and intruded by numerous diorite dykes textural structural and geochronological observations indicate that the gold mineralisation overprints all the ductile fabrics and the late orogenic d6 reverse faulting event suggesting that gold was introduced during later d7 and d8 events the metamorphic background is characterized by biotite k feldspar actinolite pyrhotite which is overprinted by the gold related alteration consisting of sulfidation fronts veins and micro fractures dominated by quartz biotite k feldpar pyrite the almost identical mineralogy of the mineralised assemblage and the metamorphic background makes it difficult to identify the source the effect and the spatial extent of the mineralising fluid as part of this study detailed analyses of biotite were conducted in order to identify the nature of the mineralising fluid and its spatial effect across the geita hill deposit results show that the mineralised assemblage is slightly more oxidized pyrite magnetite compared to the metamorphic background pyrrhotite magnetite the intense sulfidation within the ore zone resulted in the formation of mg rich biotite which grades into more fe rich biotite away from the ore zone this change in biotite chemistry causes a shift in the swir fe oh 2250 nm absorption feature to lower wavelengths which is also correlated with an increase in gold grade within the mineralised zone the abundance of f in biotite significantly increases within the ore zone indicating the presence of f rich hydrothermal fluids during mineralisation late timing of mineralisation synchronous with the emplacement of voluminous high k granites and the presence of k and f bearing fluids indicate a genetic connection between the mineralising fluids and the late granites biotite grains from different deposits types porphyry iocg lode au appear to have distinct composition suggesting that the source of the fluids involved in the formation of each deposit type has a characteristic halogen chemistry

geology and tectonostratigraphy of the onverwacht suite barberton greenstone belt south africa abstract a synthesis of new and existing data reveals that at least seven major shear zones and an unconformity separate the rocks of the southern barberton greenstone belt into seven units with different geological histories current views that these units constitute a continuous stratigraphic sequence and their subdivision into successive formations cannot be maintained we argue here that the status of the traditional formations of the onverwacht group of the barberton greenstone belt be changed to complexes and collectively these complexes be referred to as the onverwacht suite the total age range and tectonostratigraphic thickness of the suite is about 120 million years and 15 km respectively the precise age range of the rocks present in each complex is largely unknown the original spatial relationships between the complexes can in most cases only be inferred all of the complexes include significant volumes of intrusions that were emplaced into the volcanic and volcaniclastic host rocks the ages of the intrusions in the different complexes vary in some complexes the intrusions and volcanic rocks have similar geochemistry and are probably co magmatic others provide stitching ages between complexes four of the complexes display internal relationships that resemble ophiolites basaltic pillowed and massive lavas are the most common rock types sheeted dykes are generally absent whilst multiple mafic sills are common and dominate one of the complexes the volcanology and geochemistry of the complexes suggest that they formed predominantly in oceanic spreading and intra oceanic arc environments at water depths of 1 7 4 km the complexes were tectonically stacked during the final stages of their development and during two subsequent orogenic events one complex the hooggenoeg complex was uplifted and deeply eroded between about 3460 and 3470 ma with a minimum uplift rate ca 0 5 mm yr similar to that during emplacement of phanerozoic ophiolites unconformably overlying sub aerial coarse clastic sedimentary rocks part of the newly distinguished noisy complex overlap in age with felsic intermediate igneous activity dated between 3445 and 3460 ma that in turn is genetically related to tonalite trondjemite plutons exposed farther to the south the emergence of the deep water hooggenoeg complex above sea level the start of its erosion and fluvial sedimentation on top of it at ca 3458 ma represents the onset of its obduction onto older complexes some of which are now preserved as enclaves in the granitoid terrains to the south other oceanic slabs the kromberg and the mendon complexes were subsequently emplaced onto the noisy complex during latitudinal displacements ca 12 cm yr that were not notably faster than today exhumation with up to 18 km of unroofing along the southern margin of the greenstone belt took place during a second orogenic episode of subduction and accretion between 3140 and 3230 ma this episode was associated with folding thrusting and molasse type sedimentation within the greenstone belt coupled to extension and ductile faulting at a lower crustal level during this period younger complexes were assembled and the older complexes re cycled and eroded at rates and with geotherms comparable to those in modern orogenic belts in the barberton region between 3 2 and 3 5 ga tectonic processes appear to have operated at rates similar to those within modern evolving plate boundaries

the tectonic history of a crustal scale shear zone in the tanzania craton from the geita greenstone belt nw tanzania craton abstract in this contribution we present for the first time field based evidence of a crustal scale shear zone from the southern margin of geita greenstone belt the geita shear zone is a broad 800 m wide ductile high strain deformation zone that can be traced for at least 50 km along the southern margin of the geita greenstone belt it is near vertical trends e w and separates the mafic volcanics of the kiziba formation to the north from the ttg gneisses that crop out south of the shear zone the shear zone is hosted almost entirely by the ttg gneisses and is characterised by a well developed mylonitic foliation near the greenstone margin that transitions into a gneissic foliation and eventually becomes a weakly developed foliation further south it contains approximately equal amounts of dextral sinistral and asymmetric shear sense indicators suggesting that the shear zone accommodated mainly flattening strain while the mineral stretching lineation defined by quartz and feldspar ribbons and stretched biotite selvages plunges shallowly w a series of younger sub vertical nw trending brittle ductile strike slip shear zones truncate and displace the geita shear zone with dextral displacement in the order of 2 4 km deformed tonalite interpreted to predate the shear zone yielded u pb zircon ages of 2710 ma while synshearing granodiorite samples have zircon ages between 2680 ma and 2660 ma the 2630 ma age of the undeformed nyankumbu granite is interpreted to mark the minimum age of movement on the shear zone the presence of 3000 ma and 3200 ma zircon xenocrysts in the tonalite and granodiorite opens the possibility that older basement rocks underlie the greenstone belts in the northern half of tanzania craton whether or not the greenstone belts were erupted on older basement thrusted on top of older basement rocks or incorporated older basement fragments has profound implications for the tectonic framework and evolution of the tanzania craton

gravity and magnetic field modeling and structure of the chicxulub crater mexico the chicxulub crater is an 180 km diameter peak ring crater based on drill hole logs and samples potential fields seismic reflection profiles and surface fracture patterns a structural cross section produced based on these constraints has the features expected for a large complex impact crater the bouguer gravity anomaly consists of a broad 90 km radius 30 mgal low with a central 20 km radius 20 mgal high and two 5 mgal concentric lows at 35 and 60 km radius the gravity anomaly is disrupted by large scale basement anomalies and possibly by large scale slumping and backwash erosion effects the magnetic field anomaly over the crater consists of three zones an outer zone from 45 to 90 km radius of low amplitude short wavelength anomalies with an irregular perimeter a middle zone from 20 to 45 km radius of high amplitude short wavelength anomalies slightly elongated nnw sse and an inner 20 km radius single large amplitude anomaly magnetic field modeling indicates that the melt pool averages 90 km in diameter and the melt volume in the crater is estimated at 20 000 km3 the melt pool size constrains the collapsed transient cavity diameter to 90 km gravity and magnetic field modeling indicate that the structural uplift is irregular in shape but 40 km in diameter and underlies or protrudes into the melt pool the preliminary structural cross section indicates that the inferred peak ring is decoupled from the structural uplift the geometry and bouguer gravity signature of the crater indicate that no significant uplift of the moho or relaxation of the crater has occurred

near k t age of clastic deposits from texas to brazil impact volcanism and or sea level lowstand near k t boundary clastic deposits from texas mexico haiti guatemala and brazil often described as impact generated tsunami deposits are stratigraphically below well defined k t boundary horizons and appear not to be causally related to the k t boundary event stratigraphic evidence indicates that their deposition began during the last 170 200 kyr of the maastrichtian coincident with a major eustatic sea level lowstand that lowered sea level by as much as 70 100 m clastic deposition ended a few tens of thousands of years before the k t boundary during a rapidly rising sea level the presence of glass in clastic deposits in haiti northeastern mexico and yucatan suggests that the sea level lowstand coincided with a time of major volcanism or pre k t boundary bolide impact

the geology of the giant nyankanga gold deposit geita greenstone belt tanzania nyankanga is the largest gold deposit in the geita greenstone belt of the northern tanzania craton the deposit is hosted within an archean volcano sedimentary package dominated by ironstones and intruded by a large diorite complex the nyankanga intrusive complex the supracrustal package is now included within the intrusive complex as roof pendants the ironstone fragments contain evidence of multiple folding events that occurred prior to intrusion the supracrustal package and nyankanga intrusive complex are cut by a series of ne sw trending moderately nw dipping fault zones with a dominant reverse component of movement but showing multiple reactivation events with both oblique and normal movement components the deposit is cut by a series of nw trending strike slip faults and e w trending late normal faults the nyankanga fault zone is a major nw dipping deformation zone developed mainly along the ironstone diorite contacts that is mineralised over its entire length the gold mineralization is hosted within the damage zone associated with nyankanga fault zone by both diorite and ironstone with higher grades typically occurring in ironstone ore shoots dip more steeply than the nyankanga fault zone the mineralization is associated with sulfidation fronts and replacement textures in ironstones and is mostly contained as disseminated sulphides in diorite the close spatial relationship between gold mineralization and the ironstone diorite contact suggests that the reaction between the mineralising fluid and iron rich lithotypes played an important role in precipitating gold intense brecciation and veining mainly in the footwall of nyankanga fault zone indicates that the fault zone increased permeability and allowed the access of mineralising fluids the steeper dip of the ore shoots is consistent with mineralization during normal reactivation of the nyankanga fault zone

geochemistry of drill core samples from yaxcopoil 1 chicxulub impact crater mexico the chemical composition of suevites displaced cretaceous target rocks and impactgenerated dikes within these rocks from the yaxcopoil 1 yax 1 drill core chicxulub impact crater mexico is reported and compared with the data from the yucatn 6 y6 samples within the six suevite subunits of yax 1 four units with different chemical compositions can be distinguished a upper lower sorted and upper suevite depth of 795846 m b middle suevite depth of 846861 m c brecciated impact melt rock depth of 861885 m and d lower suevite depth of 885895 m the suevite sequence a b and d display an increase of the cao content and a decrease of the silicate basement component from top to bottom in contrast the suevite of y6 shows an inverse trend the different distances of the yax 1 and y6 drilling sites from the crater center 60 and 47 km respectively lead to different suevite sequences within the cretaceous rocks of yax 1 a suevitic dike depth of 916 m does not display chemical differences when compared with the suevite while an impact melt rock dike depth of 1348 m is significantly enriched in immobile elements a clastic breccia dike depth of 1316 m is dominated by material derived locally from the host rock while the silicate rich component is similar to that found in the suevite significant enrichments of the k2o content were observed in the yax 1 suevite and the impact generated dikes all impactites of yax 1 and y6 are mixtures of a crystalline basement and a carbonate component from the sedimentary cover an anhydrite component in the impactites is missing yax 1 or negligible y6

high resolution planktonic foraminiferal analysis from the cretaceous tertiary boundary at ain settara tunisia evidence of an extended mass extinction the ain settara section located in the kalaat senan area of north central tunisia spans the cretaceous tertiary k t boundary which is characterized by a red layer and a thin non bioturbated boundary clay sediment accumulation across the k t boundary at ain settara was probably interrupted by three short hiatuses and or condensed sedimentation the first hiatus occurs at the top of the cf1 zone the second hiatus condensation occurred just below the boundary clay and the third hiatus at the p0 p1a boundary in the earliest danian these hiatuses are marked by weak unconformities bioturbation and sudden disappearances appearances of species which are known to disappear evolve sequentially in continuous sections quantitative high resolution planktonic foraminiferal analysis across zones cf1 upper p0 and p1a 1 reveals an extended and selective mass extinction all 41 of the species which disappeared at or below the k t boundary are rare to very rare and primarily ecologically specialized keeled deeper dwelling tropical subtropical forms globotruncana globotruncanita gublerina planoglobulina rosita contusotruncana racemiguembelina their combined relative abundance varies between 10 and 15 of the total population at the end of the maastrichtian the k t crisis thus appears more catastrophic when viewed in tropical subtropical assemblages and based on analysis of larger species 200 m which preferentially includes the more specialized forms though in fact the k t mass extinction actually involved a relatively small part of the foraminiferal population in terms of relative abundance the pattern of extinction and changes in dominant population at ain settara appear to be very similar to the planktonic foraminiferal turnover of the other north central tunisian sections el kef elles the selective mass extinction pattern suggests that the catastrophic effects of the bolide impact superimposed those related to long term environmental changes such as variations in temperature sea level and associated water mass changes

tektite bearing deep water clastic unit at the cretaceous tertiary boundary in northeastern mexico the hypothesis of cretaceous tertiary k t boundary impact on yucatan mexico predicts that nearby sites should show evidence of proximal impact ejecta and disturbance by giant waves an outcrop along the arroyo el mimbral in northeastern mexico contains a layered clastic unit up to 3 m thick that interrupts a biostratigraphically complete pelagic marl sequence deposited at more than 400 m water depth the marls were found to be unsuitable for determining magnetostratigraphy but foraminiferal biostratigraphy places the clastic unit precisely at the k t boundary we interpret this clastic unit as the deposit of a megawave or tsunami produced by an extraterrestrial impact the elastic unit comprises three main subunits 1 the basal spherule bed contains glass in the form of tektites and microtektites glass spherules replaced by chlorite smectite and calcite and quartz grains showing probable shock features this bed is interpreted as a channelized deposit of proximal ejects 2 a set of lenticular massive graded laminated beds contains intraclasts and abundant plant debris and may be the result of megawave backwash that carried coarse debris from shallow parts of the continental margin into deeper water 3 at the top several thin ripple beds composed of fine sand are separated by clay drapes they are interpreted as deposits of oscillating currents perhaps a seiche an iridium anomaly 921 23 pg g is observed at the top of the ripple beds our observations at the mimbral locality support the hypothesis of a k t impact on nearby yucatan

synchronous development of type 2 and type 3 fold interference patterns evidence for recumbent sheath folds in the allendale area broken hill nsw australia abstract synchronous development of type 2 and type 3 fold interference patterns occur in the poly deformed broken hill inlier the interference patterns have resulted from the superposition of recumbent f2 folds and n s oriented upright f3 folds the synchronous development of type 2 and type 3 fold interference patterns is attributed to variation in the hinge of f2 folds by as much as 90 suggesting the development of a regional scale sheath like fold geometry during d2 overprinting relationships along the eastern limb of the pap synform suggests that it formed part of a recumbent fold hinge that was flattened during horizontal crustal shortening this has resulted in the development of a modified type 3 fold interference pattern the results of this study show that the type 2 and type 3 fold interference patterns can develop during the same deformation event in an evolving orogen

constraints on archaean crustal evolution of the zimbabwe craton a u pb zircon sm nd and pb pb whole rock isotope study the u pb ages of zircons from seven felsic volcanic and plutonic rocks from northern zimbabwe combined with field data and pb pb and sm nd whole rock isotope data constrain the timespan of development of the harare shamva granite greenstone terrain and establish the relative involvement of juvenile mantle derived and reworked crustal material basement cover field relationships and isotope and geochemical data demonstrate that the greenstones were deposited onto 3 2 2 8 ga basement gneisses in ensialic continental basins geodynamic models for the generation of the areally extensive bimodal magmatic products and growth of the pre existing crustal nucleus consistent with our interpretations are rift related 1 intracontinental rifting related to mantle plume activity or 2 rifting in a back arc environment related to a marginal volcanic arc the data in conjunction with field evidence do not indicate the presence and accretion of an older ca 2 70 ga and a younger ca 2 65 ga greenstone sequence in the harare part of the greenstone belt as was recently postulated on the basis of shrimp zircon ages zircon ages for basal felsic volcanics 2715 15 ma and a subvolcanic porphyry 2672 12 ma constrain the initiation and termination of deposition of the greenstone sequence the timespan of deposition of the upper bulawayan part of the greenstone sequence corresponds well with radiometric ages for upper bulawayan greenstones in the central and southern part of the craton and supports the concept of craton wide lithostratigraphic correlations for the late archaean in zimbabwe zircon ages for a syn tectonic gneiss 2667 4 ma and a late syn tectonic intrusive granodiorite 2664 15 ma pinpoint the age of deformation of the greenstone sequence and compare well with a pb pb age of shear zone related gold mineralization 2659 13 ma associated with the latter intrusive phase the intimate timing relation of greenstone deformation and granitoid emplacement but also the metamorphic evidence for a thermal effect of the batholiths on the surrounding greenstone belts and the structural and strain patterns in the greenstone sequence around and adjacent to the batholiths imply that the intrusion of the granitoids had a significant influence on the tectono thermal evolution of the greenstone belt prolonged magmatic activity is indicated by the zircon ages of small post tectonic plutons intrusive into the greenstone belt with a mineralized granodiorite dated at 2649 6 ma and an unmineralized tonalite at 2618 6 ma the 2601 14 ma crystallization age of an external chilimanzi type granite agrees well with existing radiometric ages for similar granites within the southern part of the craton demonstrating a craton wide event and heralding cratonization the similarity between u pb zircon ages and tdm model ages 2 65 2 62 ga and the positive ndt values 3 to 2 for the late syn tectonic and post tectonic intrusive plutons within the greenstone belt indicate magmatism was derived directly from the mantle or by anatexis of lower crustal sources with very short crustal residence times and minor contamination with older crust the rather high model 1 values 8 2 8 6 are unlikely to indicate the involvement of significant amounts of older crust and may be inherited from a high u pb mantle source as was suggested by previous workers for the archaean mantle beneath southern africa the older tdm ages for the felsic volcanics 3 0 2 8 ga and the porphyries 2 8 2 7 ga suggest that these felsic magmas were derived by partial melting of a source that was extracted from the mantle ca 200 ma prior to volcanism or may indicate interaction between depleted mantle derived melts and older crustal material

gauss s theorem mass deficiency at chicxulub crater yucatan mexico and the extinction of the dinosaurs using gauss s theorem we estimated the mass deficiencies of the chicxulub impact structure yucatan mexico from its gravity anomaly the mass deficiency obtained from the residual gravity anomaly map ranges between 1 06 1016 and 1 67 1016 kg because the gravity anomaly has approximately radial symmetry we also estimated the mass deficiency from selected profiles in this way we obtained slightly lower mass deficiency 6 16 1015 to 1 35 1016 kg the central gravity high which is supposed to be associated with the central structural high has a mean excess mass of 1 93 1014 kg by assuming a mean density contrast of 100 kg m3 between the country rock and the sedimentary and brecciated rocks we estimated the equivalent total mass 1 60 1017 to 4 34 1017 kg and volume 6 16 1013 to 1 67 1014 m3 of breccias and sedimentary rocks responsible for the gravity anomaly these figures represent lower bounds on the mass and volume ejected from the impact crater they represent estimates m

orebody geometry in lode gold deposits from zimbabwe implications for fluid flow deformation and mineralization the geometry of some orebodies can be described simply and accurately by three orthogonal axes u v w the ratios between these axes can be expressed as a parameter j u v 1 v w 1 and represented by a graph of u v plotted against v w analogous to the treatment of strain ellipsoids the orientations of orebodies can be plotted simply on projections using the uvw axes measurements of ore bodies from two examples of lode gold deposits from the zimbabwe craton show that most of these orebodies are oblate however orebodies can have significant u v ratios implying a component of pipe like fluid flow during mineralization pipe flow is demonstrated to be orders of magnitude more conductive than flow in planar veins and faults there are significant variations in orebody geometry between deposits and within different sections of a single deposit w values appear to be influenced by host rock more permeable rocks have higher w a negative trend of j value with orebody volume indicates that orebodies do not evolve in a self similar way but tend to more oblate shapes with increasing volume

effect of volatiles and target lithology on the generation and emplacement of impact crater fill and ejecta deposits on mars impact cratering is an important geological process on mars and the nature of martian impact craters may provide important information as to the volatile content of the martian crust terrestrial impact structures currently provide the only ground truth data as to the role of volatiles and an atmosphere on the impact cratering process recent advancements based on studies of several well preserved terrestrial craters have been made regarding the role and effect of volatiles on the impact cratering process combined field and laboratory studies reveal that impact melting is much more common in volatile rich targets than previously thought so impact melt rocks melt bearing breccias and glasses should be common on mars consideration of the terrestrial impact cratering record suggests that it is the presence or absence of subsurface volatiles and not the presence of an atmosphere that largely controls ejecta emplacement on mars furthermore recent studies at the haughton and ries impact structures reveal that there are two discrete episodes of ejecta deposition during the formation of complex impact craters that provide a mechanism for generating multiple layers of ejecta it is apparent that the relative abundance of volatiles in the near surface region outside a transient cavity and in the target rocks within the transient cavity play a key role in controlling the amount of fluidization of martian ejecta deposits this study shows the value of using terrestrial analogues in addition to observational data from robotic orbiters and landers laboratory experiments and numerical modeling to explore the martian impact cratering record

the archean lode gold deposits at wiluna western australia high level brittle style mineralization in a strike slip regime the wiluna lode hosted gold deposits comprise a group of individual archean lode hosted gold deposits located in the northern part of the norseman wiluna belt western australia collectively they are the fifth largest gold producer in the yilgarn block the deposits are hosted in a series of very low grade metamorphosed prehnite pumpellyite facies mafic and ultramafic lava flows with abundant primary igneous textures preserved lode hosted gold mineralization is controlled by the wiluna strike slip fault system which consists of north and northeast trending and west and east dipping sets of principal displacement faults and associated subsidiary structures the movement on all faults was dextral with minor oblique reverse or oblique normal motion and maximum apparent displacement of 1 350 m the lode deposits are centered on structural inhomogeneities in the principal displacement faults e g divergent bends and dilational jogs including horsetail splays and or subsidiary structures e g overstepping and intersecting faults the plunge of the ore shoots is generally moderately to steeply to the south and is controlled by the intersections between 1 lithologic layering and fault planes 2 two or more fault planes 3 a fault plane and an earlier formed barren or weakly mineralized quartz reef that is semiconcordant to the regional stratigraphy and 4 a fault plane and a felsic intrusion the shape and orientation of ore shoots becomes more complex downdip where they are controlled by additional features such as drag folds and link structures fault hosted ore breccias including implosion and net veined breccias and brecciated veins are significant loci of gold mineralization in all lode deposits they contain mainly quartz chalcedony carbonates fuchsite and arsenopyrite pyrite and or stibnite mineralization occurs also in shear veins and extensional veins and finely disseminated along foliation planes of the mineral fabric formed oblique to the principal displacement faults textures of implosion breccias at wiluna indicate a history of incremental extensional opening across dilational sites and multiple episodes of brecciation and hydrothermal recementation quartz textures such as vugs cavities comb textures and cockade textured quartz are ubiquitous indicating the preservation of open space due to low effective confining pressure geologic relations indicate that the mineralized strike slip fault system was generated late in the tectonic history of the greenstone belt and that high grade ore zones formed late in the strike slip deformation event the very low grade metamorphic facies and the brittle style of deformation in the host rocks combined with the style of mineralization indicate that the fault and mineralizing system formed in a shallow near surface crustal environment textural evidence combined with the tectonic setting of the lode deposits favors the suction pump mechanism as a fluid infiltration mechanism for the lode hosted gold deposits within the wiluna strike slip fault system