polymetallic epithermal fissure vein mineralization topia durango mexico part ii silver mineral chemistry and high resolution patterns of chemical zoning in veins

catastrophic debris avalanche deposit of socompa volcano northern chile between 10 000 and 500 yr ago the socompa volcano in northern chile experienced a catastrophic collapse of a 70 deg sector of the original cone causing a debris avalanche that descended nearly 3000 m vertically and traveled more than 35 km from the volcano the deposits cover some 490 sq km and have a minimum volume of 15 cu km parts of the original cone slumped in a nearly coherent form and are now preserved as large blocks more than 400 m high the primary avalanche traveled northwestward over sloping ground before coming to rest transiently forming a prominent marginal ridge and then slid away northeastward to form a secondary flow overriding much of the primary avalanche deposit abundant prismatic jointed dacite blocks within the debris avalanche deposit and a thin fine grained pumiceous deposit beneath it suggest that the collapse was triggered by magmatic activity and may have been accompanied by a violent lateral blast collapse was followed by eruption of pumiceous pyroclastic flows and extrusion of voluminous dacite domes

seismotectonics of the san andreas fault system between point arena and cape mendocino in northern california implications for the development and evolution of a young transform the northernmost and relatively youthful segment of the san andreas fault system is situated within a 100 km wide zone of northwest trending strike slip faults that includes from west to east the san andreas maacama and bartlett springs faults although the san andreas fault is the principal strike slip fault in this system it has been virtually aseismic since the 1906 earthquake moderate levels of seismicity locate to the east along the maacama fault and to a lesser extent the bartlett springs fault at focal depths typical of other strike slip faults within the san andreas fault system in central california north of the san andreas fault system within the cape mendocino area earthquakes occur at depths of up to 40 km and primarily reflect internal deformation of the subducting gorda slab and slip along the mendocino fracture zone seismicity along the maacama and bartlett springs faults is dominated by right lateral to oblique reverse slip along fault planes that dip 50 75 to the northeast the northern extent of seismicity along these faults terminates near the surface projection of the southern edge of the gorda slab the onset of seismicity along these faults may be related to the abrupt change in the elastic thickness of the north american plate as it enters the asthenospheric window the maacama and bartlett springs faults are strike parallel with active reverse faults within the forearc region of the cascadia subduction zone this preexisting structural fabric of northwest trending reverse faults in the forearc area appears to have strongly influenced the initial slip and complexity of these faults continuation of the moderately dipping maacama fault to the southeast along the steeply dipping healdsburg and rodgers creek fault zones and the near vertical hayward and calaveras fault zones in the san francisco bay area suggests that these faults evolve toward a more vertical dip to minimize the shear stresses that tend to resist plate motion

structural controls on the auriferous shear zones of the saint yrieix district massif central france evidence from the le bourneix and laurieras gold deposits at least 2 000 occurrences of gold mineralization including four economic deposits occur in the saint yrieix district in the massif central france two of these the le bourneix and laurieras deposits are currently being mined the mineralization consists of quartz lenses with sulfides and native gold the lenses are several meters thick and 300 m in length and occur within a shear zone several kilometers long in para and orthogneisses of the hercynian basement metallogenic and structural investigations in the saint yrieix district and in le bourneix mine have made it possible to relate the various episodes of mineralization to the sequence of tectonic events the lenses of vein quartz were emplaced during phases of extension their distribution being controlled by anastomosing mylonitic bands formed during a preceding tectonic phase the mylonitization was accompanied by sericitization and the introduction of disseminated pyrite arsenopyrite rutile and organic matter along the planes of mylonitic foliation gold fine grained galena and sulfosalts were introduced later impregnating the microsaccharoidal quartz resulting from cataclasis of initially barren vein quartz subsequent partial remobilization of the gold bearing lenses during phases of extension caused coarsening of the gold with the appearance of argentiferous nuggets the emplacement of the quartz lenses in brittle ductile shear zones and the diachronism between the introduction of the vein quartz and that of the gold are characteristic of this mineralization and make it possible to propose guides for exploration

deposit scale structural controls on orogenic gold deposits an integrated physical process based hypothesis and practical targeting implications the application of deposit scale structural targeting to orogenic gold deposits is primarily concerned with locating volumes of higher grade mineralisation ore shoots within a broader host structural system although empirical correlations between structural heterogeneities and ore shoots have long been noted and certain key physical processes elucidated no integrated system based synthesis has been previously published however linking the existing understanding with important recent advances in understanding of the dynamic physical processes of emplacement of hydrothermal ore systems now provides the opportunity to develop such a synthesis the most important of these new concepts includes the idea that deposits form in vertically extensive conduit systems linking source to sink and the related concept of the phenomena of injection driven swarm behaviour integration of all of these elements leads to the proposal of a coherent physical process based model for the structural emplacement of orogenic gold systems and the local formation of gold enriched volumes this model considers that an orogenic gold deposit must be viewed from the perspective of three different geometric elements each with a particular meaning in terms of physical process and which typically relate to different scales these elements include a the integrated swarm volume isv which is the rock volume which bounds the limits of the mineralisation hosting structural system and is effectively the geological entity that defines a deposit b fluid pathways which are 3d pathways through the isv where dynamic ore fluid emplacement has been repeatedly strongly focused and c ore shoots which are localised volumes of high grade gold deposition that are interpreted to represent volumes of anomalous dilation or second order valve sites within host fluid pathways consideration of this new framework suggests significant potential to improve the effectiveness of structural targeting at the deposit scale in particular ore shoots are only expected within fluid pathways and equivalent dilatant zones outside such pathways are predicted to be barren a key theme is that the rheological architecture of the host rock mass is the most important predictive input to targeting both at the scale of emplacement of the ore hosting conduit system and the scale of localised ore shoots

archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec part i geologic relations and formation of the vein system the sigma deposit consists of an archean gold bearing vein system in metavolcanic rocks intruded by an irregular body of porphyritic diorite both affected by regional deformation and in turn cut by younger undeformed feldspar porphyry dikes the veins cut all rock types and are composed chiefly of quartz and tourmaline with minor amounts of carbonates pyrite chlorite scheelite and free gold they were emplaced during or after a greenschist facies metamorphism postdating the intrusion of feldspar porphyry dikes the veins were not significantly affected by any subsequent metamorphism or tectonism two principal types of veins occur in the mine subvertical veins and subhorizontal veins they are contemporaneous and cogenetic subvertical veins occur within steeply dipping east west trending ductile shear zones along which reverse subvertical displacements have taken place the veins occupy openings created by overriding of undulations and irregularities of foliation and slip planes during progressive shearing movements within the shears subhorizontal veins occupy extensional fractures which developed between ductile shear zones preferentially in the more competent host rocks these veins were formed by one or several episodes of open space filling geologic relations indicate that the mineralized veins were emplaced progressively in a dynamic tectonic environment during and shortly after the formation of ductile shear zones a structural analysis has shown that the ductile shear zones and the two vein types can be related to a single strain ellipsoid of the following orientation the z axis trending north south and horizontal the y axis plunging 10 degrees west and the x axis plunging 80 degrees east the deformation responsible for the generation of ductile shear zones and emplacement of the veins is ascribed to a late north south trending compression postdating regional deformation

fault zone connectivity slip rates on faults in the san francisco bay area california the slip rate of a fault segment is related to the length of the fault zone of which it is part in turn the slip rate of a fault zone is related to its connectivity with adjoining or contiguous fault zones the observed variation in slip rate on fault segments in the san francisco bay area in california is consistent with connectivity between the hayward calaveras and san andreas fault zones slip rates on the southern hayward fault taper northward from a maximum of more than 10 millimeters per year and are sensitive to the active length of the maacama fault

kilometre scale structural setting of ore shoots in the frasers gold deposit macraes mine new zealand abstract the hyde macraes shear zone in southern new zealand contains the circa 10 million ounce macraes gold deposit one of the larger phanerozoic orogenic gold deposits discovered to date globally approximately 50 of this 10 million ounce resource is hosted by 5 major ore shoots up to 400 m wide and 1500 m long in the frasers area at the southern end of the mine higher grade 1 5 g t au ore shoots are located along and immediately below the hangingwall shear the principal strand of the hyde macraes shear zone at the frasers deposit they typically trend parallel to the intersection of the shear and foliation in the underlying schist commonly where the foliation dips more steeply that the overlying hangingwall shear especially thick zones of higher grade mineralised rock are located between the hangingwall shear and underlying second order splay shears whose position correlates with minor right hand bends in the strike of the overlying hangingwall shear lower grade

predictive distribution of fault fill and extensional veins example from the sigma gold mine abitibi subprovince canada this paper presents the results of a geometric and structural study aimed at developing a predictive model to account for the distribution of gold bearing veins at the sigma gold deposit abitibi subprovince canada results indicate that third order shear zones define a well organized system comprising two sets of parallel striking moderately to steeply south dipping curviplanar structures intersecting at constant 15 to 25 angles fault fill veins are better developed within the curved segments of the shear zones that represent large zones of enhanced permeability most extensional veins were formed later as tension gashes mainly distributed along conjugate corridors of low reverse faults typical of andersonian type faulting

bridge structures as sense of displacement criteria in brittle fault zones abstract fault zones often occur as arrays of en echelon fractures there are two main types of en echelon patterns involving right or left stepping fractures in such arrays first generation fractures define compressive or tensile bridges the structural features of such natural bridges at any scale can be used as criteria to determine the sense of displacement small compressive bridges provide criteria based on the increase in linear density of solution seams related to the degree of asymmetry of the fault trace and to the sequence of formation of the different types of en echelon fractures the criteria deduced from large compressive bridges involve upward displacement of the material by folds and hill or mountain ranges asymmetry of the fault profile and relative chronology of the segments tensile bridges yield criteria based upon vein fillings asymmetry and relative chronolog in large scale structures some pull apart basins are shown to be bounded by normal faults parallel to but outside the strike slip segments from a comparison of analytical models with well known large scale fault zones a new criterion can be proposed in fault zones formed through coalescence of en echelon fractures the bridging segments are wide if facing leeward of the bulk movement sense and narrow if forward facing

ore controls in the charters towers goldfield ne australia constraints from geological geophysical and numerical analyses the approach taken in this paper namely synthesising a wealth of previous information with new data and a genetic model in combination with integrated numerical analyses led to new insights into the geological controls on the localisation of auriferous veins and residual prospectivity of the charters towers goldfield ne australia the method also has implications for the assessment of other mature goldfields worldwide despite a number of different ore controls having operated within the charters towers goldfield the controlling factors can be linked to a single genetic model for orogenic granitoid hosted lode gold mineralisation in a brittle deformation regime d4 of ne sw to nne ssw shortening under conditions of supralithostatic fluid pressure and low stress difference spatial autocorrelation results suggest district scale alignment of the auriferous veins parallel to and overlapping with the ese wnw to e w striking charters towers ravenswood lineament a major crustal boundary in the basement to the ravenswood batholith at the camp scale auriferous veins have abundance and proximity relationships with nw se nnw sse ne sw and ene wsw oriented lineaments suggesting that structures that controlled gold deposition in one camp did not necessarily control mineralisation in other camps fractal dimensions obtained with the box counting method range from 1 02 to 1 10 whereas veins in the charters towers city camp are characterised by a significantly higher fractal dimension of 1 28 this discrepancy may be taken to imply that most or all outcropping and near surface deposits within the charters towers city camp have been found and that new discoveries are more likely to occur at greater levels of depth or outside the boundaries of this camp the new understanding has implications for the assessment of the residual prospectivity of the charters towers goldfield where large areas of prospective rock types and structures e g approximately 40 of the charters towers ravenswood lineament are hidden under cover this parameter space was inaccessible to the historic prospectors and has received relatively little attention from recent explorers the following steps are suggested for the development of a targeting strategy for lode gold exploration in areas of the goldfield under cover 1 identify from geological and geophysical data the ene wsw 15 and nnw sse 15 striking structures and geological boundaries within a 20 km wide corridor parallel to and centred upon the charters towers ravenswood lineament the potential control on gold deposit distribution at the regional to district scale 2 interpret from geological and geophysical data the distribution of pre middle devonian granitoids within these areas that are the preferred host rocks of the payable gold deposits 3 deduce from geophysical data the ene wsw 15 and nnw sse 15 striking structures that cut or bound the intrusions identified in step 2 4 locate segments along the structures identified in step 3 that deviate most from the geometry of a straight line e g potential bends or splays and or intersect other structures or geological contacts or both and 5 define and rank potential targets within the prospective areas identified in step 4 and systematically test the best ones

melange and sediment hosted gold bearing quartz veins hodgkinson gold field queensland australia gold bearing quartz veins in the hodgkinson gold field are hosted in subgreenschist facies siluro devonian sandstone conglomerate carbonaceous shale clast in matrix rock chert and spilite mineralization occurs in brittle to brittle ductile shear zones which crosscut two previous deformations and melange vein systems are isolated in distinct structural and lithologic domains by postmineralization faulting scheelite quartz veins may locally overprint the gold quartz mineralization but stibnite gold quartz mineralization is restricted to separate domains the strongest mineralized areas define elongate elliptical ore shoots which plunge steeply down or across the dip of the host shear zones melange hosted lodes are parallel in strike but have opposite dips to the melange cleavage sandstone hosted shear zones lie within reactivated f 2 axial planes or are refracted through bedding planes splaying fissure intersections changes in attitude and lithologic contacts are the main ore shoot controls the gold quartz vein textural types are ribbon buck breccia assimilation textured and comb quartz multiple injection of quartz and multiple shear zone movement have produced complex zones of quartz separated by seams of gouge arsenopyrite galena sphalerite pyrite pyrrhotite and gold 600 840 fine constitute 5 percent of the ore shoots and occur as millimeter scale clusters in quartz fluid inclusions in quartz are liquid and liquid vapor types and give salinity ranges of 3 to 11 equiv wt percent nacl with a slight salinity decrease through time co 2 and other gases are not abundant homogenization temperatures of 170 degrees to 305 degrees c have been pressure corrected to 270 degrees to 355 degrees c based on estimated formation pressure of about 1 kbar measured and calculated stable isotope compositions of the fluid depositing gold quartz are delta 18 o 10 or 2 per mil delta d 100 or 20 per mil although meteoric water contributions cannot be discounted geologic characteristics isotopic data quartz types sulfide mineralogy and ore controls favor gold deposition from an upwardly migrating homogeneous metamorphic or distal magmatic fluid depletion of d is attributed to exchange reactions involving organic matter in the metasediments and reduced gas species rather than introduction of meteoric water gold mineralization accompanied brittle tectonism which was contemporaneous with regional plutonism and took place late in the evolution of the melange

volcanic evolution of the island of tenerife canary islands in the light of new k ar data new age determinations from tenerife together with those previously published 93 in all provide a fairly comprehensive picture of the volcanic evolution of the island the oldest volcanic series with ages starting in the late miocene are formed mainly by basalts with some trachytes and phonolites which appear in anaga teno and roque del conde massifs in anaga ne three volcanic cycles occurred one older than 6 5 ma a second one between 6 5 and 4 5 ma with a possible gap between 5 4 and 4 8 ma and a late cycle around 3 6 ma in teno nw after some undated units the activity took place between 6 7 and 4 5 ma with two main series separated by a possible pause between 6 2 and 5 6 ma in the zone of roque del conde s the ages are scattered between 11 6 and 3 5 ma between 3 3 and 1 9 ma the whole island underwent a period of volcanic quiescence and erosion the large canadas volcano made up of basalts trachytes and phonolites was built essentially between 1 9 and 0 2 ma to the ne of this central volcano linking it with anaga is a chain of basaltic emission centers with a peak of activity around 0 8 ma the canadas caldera had several collapse phases associated with large ignimbrite emissions there were at least an older phase more than 1 ma old on the western part of the volcano and a younger one less than 0 6 ma old in the eastern side the two large valleys of guimar and la orotava were formed by large landslides less than 0 8 ma ago and probably before 0 6 ma ago the present canadas caldera was formed by another landslide less than 0 2 ma ago this caldera was later filled by the huge teide volcano which has been active even in historic times during the same period a series of small volcanoes erupted at scattered locations throughout the island the average eruptive rate in tenerife was 0 3 km3 ka with relatively small variations for the different eruptive periods this island and la gomera represent a model of growth by discontinuous pulses of volcanic activity separated by gaps often coinciding with episodes of destruction of the edifices and sometimes extended for several million years the neighbouring gran canaria on the other hand had an initial rapid shield building phase during which more than 90 of the island was built and a series of smaller pulses at a much later period a comparison between these three central islands indicates that the previously postulated westward displacement in time of a gap in the volcanic activity is valid only as a first approximation several gaps are present on each island overlapping in time and not clearly supporting either of the models proposed to explain the evolution of the canaries

structural controls on the distribution of gold bearing quartz veins in the arltunga gold field northern territory australia the arltunga gold field in central australia is a carboniferous epigenetic vein type gold deposit which is hosted by quartzite quartzofeldspathic gneiss calc silicate and mica schist it is situated within a large ductile shear zone that forms the floor thrust for a series of nappes the arltunga nappe complex gold and other minerals were deposited within quartz veins by fluids that were channeled along the shear zone during the waning stages of greenschist facies metamorphism a geometric and genetic relationship existed between the evolving structure of the cooling shear zone and progressively emplaced quartz veins with associated gold and sulfide mineralization southward thrusting was inhibited late in the history of the shear zone and subsequent strain was partitioned along north verging kink zones the kinked anisotropy occurred at high angles to the kink boundary within the shortening field of the regionally distributed strain pattern this resulted in the development of simple kink geometries associated with minor buckling which rotated the laminated rock along the kink zones into the extensional field the accompanying dilatancy caused the deposition of extensional gold barren quartz veins along the kink zones that can be divided into two types en echelon arrays of dilational veins that formed as tension gashes in relatively incompetent mica schists and tensional veins parallel to the kink zone boundaries with progressive deformation and waning metamorphic temperatures strain was further localized within the kink zones as an anastomosing system of narrow steep south dipping reverse faults these faults reflect the transition from ductile to brittle structures and mainly occur in the more competent rock types such as the quartzite the mica poor quartzofeldspathic gneiss and the extensional quartz veins where the fracture system transects these lithologies extensive brecciation caused dilatant zones that host deposits of gold together with quartz pyrite chalcopyrite calcite and minor silver many of the earlier extensional veins that contained vugs filled in with comb quartz became mineralized the preferred occurrence of gold in the monomineralic quartzites and the conspicuous lack of wall rock alteration in all other lithologies suggests only limited control of the wall rock on gold deposition general retrograde metamorphic conditions in particular the fall in ambient temperatures probably forced the gold out of solution in the same temperature range in which brittle structures were initiated as a result of the irregular distribution of the anastomosing fault pattern and competent units within the preexisting ductile shear gold mineralization is highly irregular and discontinuous making exploration of the arltunga gold field difficult

submarine landslides around the canary islands the morphology and structure of the submarine flanks of the canary islands were mapped using the gloria long range side scan sonar system bathymetric multibeam systems and sediment echosounders twelve young 2 ma giant landslides have been identified on the submarine flanks of the canary islands up to now older landslide events are long buried under a thick sediment cover due to high sedimentation rates around the canary islands most slides were found on the flanks of the youngest and most active islands of la palma el hierro and tenerife but young giant landslides were also identified on the flanks of the older 15 20 ma but still active eastern islands large scale mass wasting is an important process during all periods of major magmatic activity the long lived volcanic constructive history of the islands of the canary archipelago is balanced by a correspondingly long history of destruction resulting in a higher landslide frequency for the canary islands compared to the hawaiian islands where giant landslides only occur late in the period of active shield growth the lower stability of the flanks of the canaries is probably due to the much steeper slopes of the islands a result of the abundance of highly evolved intrusive and extrusive rocks another reason for the enhanced slope instability is the abundance of pyroclastic deposits on canary islands resulting from frequent explosive eruptions due to the elevated volatile contents in the highly alkalic magmas dike induced rifting is most likely the main trigger mechanism for destabilization of the flanks flank collapses are a major geological hazard for the canary islands due to the sector collapses themselves as well as triggering of tsunamis in at least one case a giant lateral blast occurred when an active magmatic or hydrothermal system became unroofed during flank collapse

caldera formation at volcan colima mexico by a large large holocene volcanic debris avalanche abstract about 4 300 years ago 10 km 3 of the upper cone of ancestral volcan colima collapsed to the southwest leaving a horseshoe shaped caldera 4 km in diameter the collapse produced a massive volcanic debris avalanche deposit covering over 1550 km 2 on the southern flanks of the volcano and extending at least 70 km from the former summit the avalanche followed a steep topographic gradient unobstructed by barriers resulting in an unusually high area volume ratio for the colima deposit the apparent coefficient of friction fall height distance traveled for the colima avalanche is 0 06 a low value similar to those of other large volume deposits the debris avalanche deposit contains 40 75 angular volcanic clasts from the ancestral cone a small proportion of vesicular blocks that may be juvenile and in distal exposures rare carbonate clasts plucked from the underlying surface by the moving avalanche clasts range in size to over 20 m in diameter and are brecciated to different degrees pulverized and surrounded by a rock flour matrix the upper surface of the deposit shows prominent hummocky topography with closed depressions and surface boulders a thick coarse grained compositionally zoned scoria fall layer on the upper northeastern slope of the volcano may have erupted at the time of collapse a fine grained surge layer is present beneath the avalanche deposit at one locality apparently representing an initial blast event most of the missing volume of the ancestral volcano has since been restored at an average rate of 0 002 km 3 yr through repeated eruptions from the post caldera cone as a result the southern slope of volcan colima may again be susceptible to collapse over 200 000 people are now living on primary or secondary deposits of the debris avalanche and a repetition of this event would constitute a volcanic disaster of great magnitude ancestral volcan colima grew on the southern trenchward flank of the earlier and larger volcano nevado de colima trenchward collapse was favored by the buttressing effect of nevado the rapid elevation drop to the south and the intrusion of magma into the southern flank of the ancestral volcano other such trenchward younging paired volcanoes are known from mexico guatemala el salvador chile and japan the trenchward slopes of the younger cones are common sites for cone collapse to form avalanche deposits as occurred at colima and popocatepetl in mexico and at san pedro volcano in chile

k ar chronology of the volcanic eruptions in the canarian archipelago island of la gomera a geochronological study of the island of la gomera canaries has been carried out by the k ar method the 26 new ages obtained together with the 17 previous determinations show that above the main unconformity of the island separating the basal complex from the later volcanic series there is a unit of lower old basalts more than 10 m y old polymictic volcanic breccias were emplaced between 10 and 9 m y ago the upper old basalts above them were formed between 9 and 6 m y ago with a peak of activity around 7 m y after a period of erosion 6 5 m y a thick series of young basalts associated with trachytic and phonolitic domes and flows were rapidly emplaced between 4 5 4 m y ago finally local basaltic activity took place 2 8 m y ago the age of the basal complex is not well known although three ages 14 6 15 5 and 19 3 m y have been obtained for some alkaline intrusives which seem to represent the youngest events in the complex

gold quartz veins and auriferous granite at the oriental mine alleghany district california in the vicinity of serpentinite wall rocks the three fissure veins at the oriental mine alleghany district california contain small high grade ore shoots carrying hundreds of troy ounces of gold per ton in addition minable zones of albitized wall rock granite ore contain an average of 0 2 ounces of gold per ton in the veins most native gold is associated with arsenopyrite but in the granite it occurs as microscopic inclusions within the pyrite various geothermometers and paragenetic relations suggest that vein gold deposition occurred during the end stages of mineralization at temperatures between approximately 200 degrees and 300 degrees c at pressures between 670 and 2 500 bars and at depths exceeding 2 5 km during the early stages of mineralization when most hydrothermal alteration and gold deposition in granite probably occurred temperatures were above 340 degrees c and possibly as high as 491 degrees c muscovite paragonite calcite dolomite dawsonite and albite fluid inclusion daughter minerals have been identified in situ by refractive index measurements and by x ray powder diffraction the near neutral to alkaline character of the mineralizing fluids implied by the presence of 0 34 m dissolved dawsonite and the low equivalent nacl contents of fluid inclusions suggest that gold may have been complexed with sulfur or arsenic rather than occurring as chloride complexes wall rock alteration effects include envelopes of intense carbonatization and desilication surrounding the principal gold quartz veins but also occurring along splits and fractures emanating from the main vein structures for example in granite the main alteration effects dissolution of original quartz and formation of albite occur along nearly flat lying fractures chiefly in the footwall of the oriental vein chemical analyses suggest that desilication of granite and removal of silica during ankeritization of mafic minerals in the other rock types liberated sufficient amounts of silica to form the vein quartz at the oriental mine beyond the fringes of the main alteration envelopes is an outer essentially pervasive alteration zone characterized by sericitization and chloritization which grades into greenschist facies country rocks and may predate mineralization gold in high grade ore shoots may have been precipitated by reactions involving reducing fluids derived from the wall rock serpentinite near which all high grade vein ores occur limited fluid inclusion data for ore shoot quartz however seem to be consistent with the alternative possibility of temperature controls for ore deposition

eruptive response of oceanic islands to giant landslides new insights from the geomorphologic evolution of the teide pico viejo volcanic complex tenerife canary large sector collapses are a major component of oceanic islands evolution here we show that voluminous events such as the icod landslide on tenerife canary islands cause dramatic changes on the magma feeding system and control the subsequent volcanic and geomorphologic evolution of the eruptive complex over a period of more than 150 kyr instantaneous unloading by the icod landslide is marked by the development of a large phonolitic explosive eruption dated at 175 3 ka and interpreted as reflecting the immediate emptying of a shallow pre existing magma chamber geochronological geomorphological and geochemical analyses carried out on the post landslide volcanic succession sampled in a 4 4 km long underground water recovery gallery provide further evidence for an enhanced extrusion of primitive lavas starting in the 10 kyr time interval following the failure rapid construction 40 kyr of a thick basaltic volcano in the landslide scar at high eruptive rates up to 8 km3 kyr 1 increased the lithostatic pressure which then favored the intermittent storage of basic magma under the edifice this resulted in more episodic construction evidenced by a significant decrease in output rates and the increasing occurrence of lavas with intermediate composition from 117 7 to 52 7 ka an apparent volcanic gap is observed between 52 7 and 18 1 ka after which highly differentiated lavas have been dominantly erupted we propose that part of the gap can be explained by the individualization of a shallow magma reservoir a few kilometers below the base of the teide volcano during recent periods vertical and lateral extrusions of trachytic and phonolitic viscous bodies from this storage area contributed to increase the slope of the main edifice up to 35 overall favoring its present day instability

erosion of the submarine flanks of the canary islands surveying with multibeam echo sounders around old 1 ma volcanic ocean islands reveals that their submarine flanks contain a strong downslope oriented ridge and valley corrugation which modifies the original volcanic morphology of lava terraces and cones by analogy with canyons in other settings this corrugation was probably caused by channel incision by erosive sedimentary mass flows such as turbidity currents and debris flows we adapt a method first used in subaerial geomorphology to isolate the erosion depth exhumation and apply it to the eroded flanks of the 6 8 ma anaga massif of tenerife the channels formed around this massif divert around local topographic highs these highs which are probably original volcanic cones are therefore preferentially preserved during erosion so that their elevations can be used to construct an artificial reference surface terrain depth was calculated by subtracting this reference surface from measured bathymetry comparison of the terrain depth of the old eroded submarine flank of anaga with that of the young mostly unaltered submarine flank of el hierro allows us to infer the mean depth of anaga s submarine erosion which is 100 m volcanic terrains can be dated by radiometric methods so they also provide a way of quantifying long term denudation rates we infer that submarine denudation of anaga has occurred at comparable rates to that of subaerial lowlands and much slower than denudation of highlands illustrated locally by the more extensive erosion of the subaerial anaga edifice

geodetic measurement of crustal deformation on the san andreas hayward and calaveras faults near san francisco california analysis of a geodetic network of 115 lines crossing the san andreas hayward and calaveras faults in the vicinity of san francisco bay and measured repeatedly between 1970 and 1980 has revealed details about the accommodation of relative plate motion in this area the most striking result is that the deformation is not uniformly distributed across the area in the east bay along the hayward and calaveras faults all motion appears to take place as slip directly on the fault with no accumulation of strain in the adjacent crust on both the calaveras and the hayward faults the rate obtained for the 1970 to 1980 period agrees with geologic rates spanning a few million years and with creep rates spanning a few decades the hayward fault slip rate is 7 1 mm yr the calaveras fault slip rate is 7 1 mm yr with perhaps half of this slip distributed across a zone a few kilometers wide probably as inelastic deformation of weak near surface material the absence of strain accumulation in the east bay is surprising since the hayward and calaveras faults have been the site of large earthquakes in the past a block located east of the calaveras fault and south of the las positas fault has been rotating clockwise at a rate of 0 3 0 1 rad yr with very little internal deformation along the san francisco peninsula no detectable slip occurs less than 1 5 mm yr at the surface but appreciable strain is accumulating near fault shear strain rates are 0 6 0 1 strain yr engineering with direction n47 w 9 the slip rate near the san andreas fault is 12 2 3 9 mm yr distributed across a broad zone the relative motion across the whole region during the period 1970 1980 is 32 1 7 4 mm yr supplement available with entire article on microfiche order from the american geophysical union 2000 florida avenue n w washington d c 20009 document j81 011 1 00 payment must accompany order

fault zone reactivation kinematics and mechanisms the kinematics and mechanisms of fault zone reactivation are reviewed reactivation is dependent upon fault zone orientation and the existence of weak mylonites along these zones the recognition of reactivation within mylonite zones and the softening processes that first concentrate deformation into these and secondly provide a weak medium for reactivation are discussed attention is given to the darling mobile zone western australia and the redbank zone see obee white this symposium as examples of reactivated zones

the role of fluids in syntectonic mass transport and the localization of metamorphic vein type ore deposists abstract in low to medium grade metamorphic environments the mobilization of species is closely associated with the operation of solution precipitation creep processes during foliation development such deformation processes involve dissolution of material from source sites which are usually discrete dissolution surfaces having orientations controlled by the applied stress and displacement fields mass transfer occurs via solute diffusion in an essentially static fluid network or by transport of dissolved species in a migrating fluid precipitation of material removed from dissolution sites occurs in a variety of sink sites including pore surfaces grain scale microcracks and veins sink sites may be close to or distantly removed from source sites the transport of material during deformation involving solution precipitation creep is largely controlled by the mobility of metamorphic fluids in regions of high fluid mobility significant mass transfer may occur over large distances fluid flow mass transfer paths and the development of sink sites in such cases are constrained by the structurally controlled development of high fluid pressure domains and the consequent mechanical enhancement of permeability by hydraulic fracture processes and the development of grain scale dilatancy these processes may result in focussed fluid flow and precipitation of mobilized species in restricted regions a proportion of syntectonically mobilized material is precipitated to form vein type deposits in hydraulic fracture arrays which develop in high fluid pressure environments the microstructures developed in minerals deposited in these sites reflect the interaction of a range of nucleation and growth processes which usually involve multiple generations of fracture growth and mineral crystallization the formation of syntectonic gold quartz vein deposits in central victoria australia illustrates the importance of structural controls on both the large scale migration of metamorphic fluids and the development of sites suitable for the precipitation of metamorphically mobilized species

dykes and structures of the ne rift of tenerife canary islands a record of stabilisation and destabilisation of ocean island rift zones many oceanic island rift zones are associated with lateral sector collapses and several models have been proposed to explain this link the north east rift zone nerz of tenerife island spain offers an opportunity to explore this relationship as three successive collapses are located on both sides of the rift we have carried out a systematic and detailed mapping campaign on the rift zone including analysis of about 400 dykes we recorded dyke morphology thickness composition internal textural features and orientation to provide a catalogue of the characteristics of rift zone dykes dykes were intruded along the rift but also radiate from several nodes along the rift and form en echelon sets along the walls of collapse scars a striking characteristic of the dykes along the collapse scars is that they dip away from rift or embayment axes and are oblique to the collapse walls this dyke pattern is consistent with the lateral spreading of the sectors long before the collapse events the slump sides would create the necessary strike slip movement to promote en echelon dyke patterns the spreading flank would probably involve a basal decollement lateral flank spreading could have been generated by the intense intrusive activity along the rift but sectorial spreading in turn focused intrusive activity and allowed the development of deep intra volcanic intrusive complexes with continued magma supply spreading caused temporary stabilisation of the rift by reducing slopes and relaxing stress however as magmatic intrusion persisted a critical point was reached beyond which further intrusion led to large scale flank failure and sector collapse during the early stages of growth the rift could have been influenced by regional stress strain fields and by pre existing oceanic structures but its later and mature development probably depended largely on the local volcanic and magmatic stress strain fields that are effectively controlled by the rift zone growth the intrusive complex development the flank creep the speed of flank deformation and the associated changes in topography using different approaches a similar rift evolution has been proposed in volcanic oceanic islands elsewhere showing that this model likely reflects a general and widespread process this study however shows that the idea that dykes orient simply parallel to the rift or to the collapse scar walls is too simple instead a dynamic interplay between external factors e g collapse erosion and internal forces e g intrusions is envisaged this model thus provides a geological framework to understand the evolution of the nerz and may help to predict developments in similar oceanic volcanoes elsewhere

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

the pliocene seamount series of la palma canary islands a pliocene submarine series of alkali basaltic pillow lavas hyaloclastites and breccias a a sheeted dike swarm b and a basal suite of gabbro and ultramafic rocks c from la palma canary islands is interpreted as a cross section through an uplifted seamount this series has been tilted to its present orientation of 50 230 plunge and azimuth probably by upwarping due to intrusions in the central portion of the island the basal plutonic complex c also includes intrusives coeval with up to 2000 m of younger subaerial alkali basaltic lavas unconformably overlying the submarine series the plutonic suite c is overlain abruptly by more than 1800 m of sills b 0 4 1 m thick on average with minor screens of lavas and breccias extrusives a form a 1750 m thick sequence of pillow lavas breccias and hyaloclastites the clastic rocks increase in abundance upward and are of four main types 1 breccias consisting of partly broken pillows formed nearly in situ 2 heterolithologic pillow fragment breccias 3 hyaloclastites composed dominantly of highly vesicular lapilli and ash sized shards the latter thought to have formed by near surface explosive eruptions and been subsequently transported downslope by mass flows 2 and 3 being interpreted to have been resedimented and 4 pillow scoria breccias from the upper 700 m of the extrusive section consisting of amoeboidal highly vesicular pillows and lava stringers and local bombs probably formed by cracking and bleeding of gas rich expanding pillow lava and some shallow submarine subaerial lava fountaining the extrusive series is chemically and mineralogically crudely zoned with the most differentiated rocks metatrachytes and mugearites at the base and most picritic lavas occurring near the top of the series subsequent to emplacement the entire extrusive and intrusive series has been hydrothermally altered the lower part to greenschist and the upper part to smectite zeolite facies mineral assemblages the la palma succession combined with evidence from surface studies of seamounts suggests that seamounts are formed by intrusive and extrusive processes in approximately equal portions the nature of eruptive clastic and depositional mechanisms changes drastically during growth of a seamount if the critical depth for major magmatic degassing is surpassed and especially if magmatic explosive processes can occur at very shallow water depth the critical depth depending on magma and thus volatile composition changes in slopes of a seamount influence depositional processes based on these factors at least three major depositional sites develop as a seamount grows summit flank and apron facies nonexplosive extrusive processes prevail in the deep water stage dominantly producing pillow lavas 75 these consist of individual pillow volcanoes up to 200 m high with large pillows near the base and decreasing pillow size toward the top of a volcano pillow breccias and pillow fragment breccias comprise approximately 20 of this facies the deep water flank and apron facies are characterized by debris flow deposits with possibjy rather dense matrix material the shallow water shoaling stage is reached when the seamount top reaches the critical depth for drastic increase in exsolution of magmatic volatiles about 800 m for the alkali basaltic seamount of la palma resulting in formation of mainly clastic rocks 70 in situ pillow rind breccias and scoriaceous amoeboidal breccias and pillows are formed in the summit regions of the seamount by repeated expansion and leaking of frothy pillow lava possibly by lava fountaining resedimented heterolithologic pillow fragment breccias lapilli breccias and hyaloclastites are deposited on the flanks and aprons of the seamount pillow lavas comprise 30 of these deposits

the primitive volcano of mount pelee its construction and partial destruction by flank collapse abstract the geology of mount pelee the active volcano of martinique lesser antilles has been actively studied during the past decade this paper reviews the current knowledge about the oldest least well known part of the volcano here called paleo pelee and the relations between paleo pelee and the mont conil volcanic complex paleo pelee consists mostly of volcanic breccias with a few lava flows it differs in this respect from neighbouring active volcanoes such as the grande decouverte soufriere in guadeloupe and the soufriere in st vincent the oldest products of which are mainly lava flows paleo pelee initially formed a regular cone which was comparable in size with the present edifice it is now largely eroded and its products form prominent radial ridges on the western flank of mount pelee these are buried under younger deposits on the eastern flank the uniformly andesitic breccias of paleo pelee are for the most part brecciated lava flows it is inferred that the activity of mount pelee was less explosive at this stage than during following periods neo pelee the paleo pelee formations overlap those of mont conil the geology of mont conil is poorly known however its lowest exposed products are submarine andesitic breccias cut by many dikes this contrasts with paleo pelee where all breccias are of subaerial origin pleistocene limestones are interbedded within the mont conil breccias between 120 and 150 m above sea level though mont conil seems to be closely linked to mount pelee on the basis of available petrological data a major uplift must have occurred sometime during or after the construction of mont conil and forms a recognizable geological boundary between the two volcanic systems the paleo pelee edifice was partially destroyed by a flank collapse event that occurred on the southwest flank during the early stage 40 000 20 000 y b p of neo pelee this event is evidenced by a large 6 2 5 km horseshoe shaped structure which is limited by well preserved morphological scarps on land and by a 300 m offshore scarp as shown by seabeam bathymetry paleo pelee breccias and lava flows are truncated by the boundary of the collapse structure lava domes occupied the upslope part of the structure at a later time these domes are intersected by the present crater etang sec which has probably been the active vent for at least 2000 years the active crater is located at the intersection between the flank failure depression and an older caldera formed at the paleo pelee stage but probably enlarged later the flank failure structure appears as a major structure of the volcano and creates a strong structural asymmetry in the edifice this fact should be considered when dealing with such various aspects as the hydrogeology of the volcano the structural control of past and future eruptions and the volcanic hazards

regional dome evolution and its control on ore grade distribution insights from 3d implicit modelling of the navachab gold deposit namibia abstract we introduce a novel approach to analyse and assess the structural framework of ore deposits that fully integrates 3d implicit modelling in data rich environments with field observations we apply this approach to the early palaeozoic navachab gold deposit which is located in the damara orogenic belt namibia compared to traditional modelling methods 3d implicit modelling reduces user based modelling bias by generating open or closed surfaces from geochemical lithological or structural data without manual digitisation and linkage of sections or level plans instead a mathematically defined spatial interpolation is used to generate 3d models that show trends and patterns that are embedded in large drillhole datasets in our 3d implicit model of the navachab gold deposit distinctive high grade mineralisation trends were identified and directly related to structures observed in the field the 3d implicit model and field data suggest that auriferous semi massive sulphide ore shoots formed near the inflection line of the steep limb of a regional scale dome where shear strain reached peak values during fold amplification this setting generated efficient conduits and traps for hydrothermal fluids and associated mineralisation that led to the formation of the main ore shoots in the deposit both bedding parallel and highly discordant sets of auriferous quartz sulphide veins are interpreted to have formed during the later lock up stage of the regional scale dome additionally pegmatite dykes crosscut and remobilise gold mineralisation at the deposit scale and appear to be related to a younger joint set we propose that kilometre scale active folding is an important deformation mechanism that influences the spatial distribution and orientation of mineralisation in ore deposits by forming structures traps and pathways for fluids at different preferred sites and orientations we also propose that areas that experience high shear strain located along the inflection lines of folds can act as preferred sites for syn deformational hydrothermal mineralisation and should be targeted for regional scale exploration in fold and thrust belts our research also suggests that examination of existing drillhole datasets using 3d implicit modelling is a powerful tool for spatial analysis of mineralisation patterns when combined with fieldwork this approach has the potential to improve structural understanding of a variety of ore deposits

nomenclature concepts and classification of oreshoots in vein deposits abstract oreshoots are discrete hypogene masses usually hosted within a planar channel surface lode or conduit which may be either a shear zone fissure fault zone or lithologic bed or unit such as a contact oreshoots are characterized by breadth strike 1000 m and dip and plunge 100 500 m lengths and have higher metal contents than the adjacent parts of the host conduit the mass of most oreshoots ranges between 1 10 6 and 2 10 4 tonnes there is a tendency for oreshoots to be thicker and richer in the center rather than to have uniform grade distributions the thickness of the oreshoots may be between 0 25 and 1 75 m in shear zone hosted deposits to up to 60 m in replacement deposits several conduits may connect to form vein systems vein systems have common fluid sources which result in general homogeneity of alteration mineralization types and oreshoot control and therefore commonly share the same plumbing system the internal constituents usually reflect unique episodes relating to ore formation the main intern constituents in oreshoots are mineralization gangue and alteration these constituents usually mix with each other in complex patterns the relationships between which may be used to interpret the processes of oreshoot formation the term ground preparation represents the effect of various events in the geologic history of an ore district or oreshoot area that have assisted in enhancing the rocks so that oreshoots can preferentially form in certain areas or geometries several types of ground preparation can be recognized 1 sequential deformation that produces a grain in the rock 2 severe faulting and jointing which augments permeability and areas where ore minerals can precipitate and 3 interplay between ore fluid and deformation to produce an oreshoot controls of oreshoot location and shape are usually due to dilatant zones caused by changes in attitude splays lithologic contacts and intersections in addition conceptual parameters such as district fabric magic distances and stacking are also used to describe the geometry of oreshoots controls in vein systems and the location and geometry of oreshoots within vein systems can be predicted by a number of qualitative concepts such as internal and external plunges district plunge district stacking conduit classification gradients and warps these concepts have a practical and empirical application in most districts where they are useful in the exploration for ore but are of such broad and general application that they can rarely be explained definitively

displacement gradients on fault surfaces abstract the maximum displacements and the dimensions of single faults are systematically related and the variation in displacement from a maximum at the centre of a fault to zero at an elliptical tip line loop is described by a simple theoretically derived expression these relationships are used to examine the theoretical ranges and distributions of rates of change of displacement on fault surfaces the dimensions and maximum displacement of a fault can be estimated from a limited number of displacement gradient measurements estimates are further constrained by a knowledge of the effective shear modulus of the rocks containing a fault an understanding of displacement gradients can be applied to problems commonly encountered in mining operations and in the interpretation and use of seismic reflection data in addition to field problems a comparison of theoretical displacement gradients with measured displacement gradients from british coalfield faults shows that i fault surface ellipticities are between 1 5 and 2 5 and ii gross displacement gradients expressed as the ratio of maximum displacement long axis radius of the fault ellipse range from 0 02 to 0 002 the measured displacement gradients indicate that the shear moduli of the faulted rocks range from 3 to 11 gpa which is within the known range for coal measure rocks displacement gradients along an array of fault segments are similar to those of single faults for analysis of displacement gradients it is necessary to sum the discontinuous and continuous components of displacement in extreme cases all the displacement may be accommodated by continuous deformation

late neogene and quaternary tectonics associated with northward growth of the san andreas transform fault northern california strain patterns within the forearc at a convergent margin adjacent to a passing fault fault trench triple junction show a systematic evolution we describe late neogene and quaternary deformation in northern coastal california in the last 3 m y associated with the migration of the mendocino triple junction deformation in the north american forearc near humbolt bay consists of a 30 km wide zone of on land contraction with a narrower zone of translation to the east the zone of subaerial contraction is part of the relatively wide southernmost extent of forearc contraction within the cascadia subduction zone net quaternary northeast southwest contraction across forearc thrust faults is at least 7 9 km and minimum fault slip rates on the six major thrust faults are 0 8 2 3 mm yr net right slip within the zone of translation is a minimum of 3 km south of the forearc translation is predominant in the north american plate and is accommodated along two major right lateral fault zones of the san andreas transform boundary the lake mountain and garberville fault zones discontinuous exposures of late neogene sediments along these faults near covelo and garberville respectively indicate that crustal contraction expressed by thrust faults and associated folding predated translational deformation at each site inception of contraction near covelo was related to inception of internal deformation of the gorda plate at 3 ma this contraction due to coupling of the convergent plates migrated northward in the forearc in advance of and at the rate of migration of the triple junction present day deformation near humboldt bay thus reflects contraction associated with subduction of the gorda plate and translation farther east associated with oblique convergence at the plate boundary as the triple junction migrates the strike slip faults of the san andreas transform boundary will extend to the north northwest into the present forearc and forearc strike slip faults will become faults of the san andreas system contractional structures near humboldt bay will then cease to be active and will be preserved in the geologic record in the same way as contractional structures are preserved farther south

the structure of shear related vein type gold deposits a review abstract the structure of vein type gold deposits is defined by the shapes and geometrical relationships of mineralized bodies the form of the mineralization making up these bodies and the sequence of vein forming events most mineralized zones occur within or are spatially associated with shear zones especially shear zones in larger systems of intersecting shear zone sets they range in shape from tabular to linear and in form from disseminated to breccia to stockwork or sheeted veinlet zones to single veins there typically is a complex history of mineral deposition which overlaps and is genetically related to the deformation that generated the host structural zone the purpose of this paper is to consider the structural features of vein type deposits in relation to the geometrical properties and evolution of veinhosting fractures particularly shear zones the evidence indicates that veins are localized by tectonically generated dilatancy in an environment of low mean stress caused by high fluid pressure it is proposed that a major cause of tectonic dilation of shear zones is the interference between intersecting shears during bulk inhomogeneous flattening by movement on systems of intersecting shear zone sets

offshore continuation of volcanic rift zones el hierro canary islands el hierro is the youngest and most southwesterly of the seven canary islands the established view based on subaerial geology is that el hierro is a classic example of an oceanic island with 120 spaced volcanic rift arms vrzs forming a mercedes star however new offshore data do not support this simple interpretation instead of the discrete ridges of vrzs we observe to the nw and ne broad areas of irregular morphology which suggest that rifting activity might not be confined to narrow zones furthermore our data suggest that the anomalously long and steep flanked southern ridge could be part of an older eroded volcanic edifice that predates much of the other submarine flanks of el hierro the southern ridge has a distinctive gullied morphology which strongly contrasts with adjacent flanks there is also a 400 m deep saddle in its longitudinal profile 15 km from the coastline which we interpret as evidence that the southern ridge did not form by continuous dyke intrusion from the el hierro volcanic centre south of the saddle mean flank slopes are 10 steeper 30 with a sharp slope break at 3700 m between the ridge and smoothly sedimented seafloor these steeper slopes and lack of landslide scars to the south of the saddle indicate that the southern ridge is a stable edifice relative to the rest of el hierro surrounding sediments to the southeast appear to onlap the southern ridge a large landslide deposit el julan estimated age 200 ka occurs to the west of the ridge this landslide appears to have been constrained from spreading southeastwards by the southern ridge resulting in an elevation difference of 300 m for the seafloor on either side of the ridge

age distribution of cinder cones within the bandas del sur formation southern tenerife canary islands the quaternary bandas del sur formation in the south of tenerife comprises a complex sequence of pyroclastic rocks and lavas in contrast to the nw and ne rift zone on tenerife the s rift zone comprises a number of characteristics with respect to the morphological features eruption cyclicity and the geochemistry of the volcanic deposits various flank eruptions of the las canadas volcano associated with basaltic lavas and the formation of cinder cones within the bandas del sur are important volcanic units for understanding the explosive volcanic cycles during the pleistocene on tenerife a number of palaeomagnetic studies as well as major and trace element geochemistry and two radio isotope dates k ar have been carried out on prominent cinder cones in order to discover their stratigraphic position combining our results with previous k ar data the cones and lavas can be subdivided into three stratigraphic units the first unit contains cinder cones with reverse magnetization and y nb ratios between 0 37 and 0 41 cinder cones which belong to the second unit show normal magnetization and y nb ratios of c 0 948 0 779 ma and 0 323 0 300 ma these units define volcanic cycles ending in violent plinian eruptions the third and youngest unit possibly marks the beginning of a further volcanic cycle that started c 0 095 ma ago

diverse pliocene quaternary tectonics in a transform environment san francisco bay region california the san francisco bay region occupies part of the diffuse transform boundary zone between the pacific and north american plates although dextral strike slip faulting is dominant the plate motion is expressed in a variety of ways some strike slip faults are parallel with the plate boundary but some are slightly oblique the major strike slip faults are zones in which an interplay occurs between strands some of which are en echelon this interplay may be responsible for some apparent pull apart basins and presumed normal faults along the calaveras fault zone in extensive areas between strike slip fault zones there are many compressional structures folds and reverse faults that are pliocene quaternary in age hence largely coeval with the strike slip faults some of the compressional structures are oblique to and others are parallel with the major strike slip faults we have divided the san francisco bay region into domains on the basis of the types orientations and relationships of structures or in one case lack of strong deformation each domain has responded in its own way to the regional northwest southeast dextral shear imposed by the relative plate motion in an attempt to understand the origin of various structures and the interrelationships among them we compared established models with our observations some observed geometric relationships agree quite well with classical coulomb anderson and simple shear models but others require further explanation models based on fault interaction seem to apply to certain cases for example the east bay hills domain which is between the left stepping calaveras and hayward rodgers creek fault zones is under compression resulting from interaction between the two strike slip zones

a correlation between the evolution of mineralizing fluids and the geomechanical development of a shear zone as illustrated by the henderson 2 mine quebec two ore zones one enclosed within a shear zone zone b and the other in a subsidiary structure zone d at onset of the shear zone subsidiary fracture patterns are developed in second order faults in the following sequence at peak strength riedel shears are formed which propagate out into the walls of the shear zones producing the zone d structure after peak strength restraint p shears are developed in the thrust attitude within the shear zone principal displacement shears d developed toward residual strength in the direction of movement the continuation of the shear displacement gives rise to schistosities within the main shear zone while the main shear zone and the p and d shears within the main shear zone were in continual movement the subsidiary zone d structure once formed retained a simple fracture pattern and moved little in comparison to the main shear zone mineralizing fluids introduced into this shear environment show a similar pattern of evolution with a wider range of homogenization and halite disappearance temperatures in the more active zone b than within zone d systematic variations of ca na and ca mg indicate migration modified journal abstract

southward younging apatite u th he ages in the northern california coast ranges due to a northward migrating crustal welt abstract u th he in apatite dating of franciscan formation samples from between 39 47 n and 39 08 n in the northern california coast ranges nccr reveals a southward and downward younging pattern of ages 21 8 0 6 ma to 7 9 0 9 ma these ages are interpreted to represent a northward or northeastward dipping fossil he partial retention zone that was tilted and exhumed by a northward migrating pulse of crustal thickening that begins 150 200 km ahead of the migrating mendocino triple junction mtj k p furlong r govers ephemeral crustal thickening at a triple junction the mendocino crustal conveyor geology 27 1999 127 130 the 8 ma ages from the southern end of the sample transect are interpreted to come from rocks that were immediately below the he partial retention zone when crustal thickening began 5 my before the mtj passed by these rocks have now reached the surface 3 my after passage of the mtj the older ages from the central and northern portions of the sample transect are interpreted to represent rocks that were within the he partial retention zone prior to crustal thickening ahead of the mtj and which therefore record ages transitional from older subduction related cooling and exhumation to more recent transform related cooling and exhumation in addition to providing support for the model of a migrating crustal welt associated with the mtj the existence of an essentially intact gently tilted fossil partial retention zone in the northern california coast range raises the possibility of using isochronous surfaces as neotectonic structural markers in this geologically complex and heavily vegetated region

strain refraction in layered systems abstract strain refraction across competence contrasts is presented as a simple model consisting of two components a homogeneous strain and a heterogeneous simple shear for newtonian materials the ratio of the layer parallel simple shear component in adjacent layers is the inverse of their viscosity ratio strong changes in ellipsoid size shape and orientation are predicted across viscosity contrasts the geological implications of strain refraction theory are considered within the context of the cleavage strain debate the particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies and kinematic histories should appear to be subparallel to the xy planes of measured strain ellipsoids competent rocks should develop dominantly layer orthogonal strain and incompetent layers shear dominated deformation a variety of structural features ranging from cleavage refraction changing lineation orientations folds transected by cleavage changes from coaxial to non coaxial deformation and ramp flat fault geometry may be the result of stress and strain refraction in rocks

structural controls in the distribution of gold at how mine bulawayo zimbabwe the how mine is situated in the southeastern quadrant of the bulawayo greenstone belt within sediments and volcaniclastics of the late archean dacitic greenstone formation of the upper bulawayan group sited on the northern limb of a northeast verging syncline the mine comprises several ore zones which occur as an en echelon array of parallel steeply plunging linear shoots these shoots are confined within an extensional right stepping northerly trending strike slip duplex faulting has followed preferred lithologic contacts mineralization is dominated by a pyrite gold association and occurs as disseminations within altered mylonites felsites and tuffs as fabric replacement within tuffs and associated with veins in siltstones and iron formations accessory magnetite in tuffs and magnetite layers in iron formation are replaced by pyrite and ore textures suggest an epigenetic origin to the mineralization alteration is widespread and dominated by carbonation sulfidization and propylitic alteration permeability within the duplex was generated by slip and dilation of the synclinal axialplanar cleavage which lies at a high angle to the strike of the duplex early ductile deformation is overprinted by later brittle faulting which cuts early mineralization the linear ore zones parallel the intersection lineation between the preexisting cleavage and the duplex fault shear zones in the north area of the mine ore zones are developed within the fault bounded tuff unit e g the north 180 ore zone but in the south the ore zones parallel the hanging wall fault shear zone and transgress major lithologic contacts

reconstruction of the fossil hydrothermal system at lake city caldera colorado u s a constraints for caldera hosted geothermal systems abstract reconstruction of the physiochemical characteristics of fossil hydrothermal systems can help guide exploration for modern geothermal or mineral resources in similar settings the 22 9 ma lake city caldera in colorado u s a is well exposed and contains an exhumed fossil shallow hydrothermal system in this study alteration mineralogy vein textures and fluid inclusions are used to characterise the temperature and composition spatiotemporal variability and structural controls of the hydrothermal system at paleo depth equivalent between 1 and 2 km the hydrothermal system was dominantly moderate temperature up to 290 c low salinity 20 wt nacleq fluid inclusions within the caldera centre we interpret that the resurgent syenite intrusions provided little magmatic fluid input and had cooled significantly by the time the hydrothermal system had established in contrast in the eastern portion of the caldera late distinct magma batches provided high temperature up to 540 c and hypersaline up to 65 nacl eq magmatic fluid input above intrusions our conceptual hydrothermal model emphasizes the importance of discontinuity intersections in facilitating permeability in caldera settings we also recognize the contrasting hydrothermal manifestations of a waning degassed magma batch left over from a caldera forming eruption compared to fresh volatile rich magma

tectonic geomorphology of the san andreas fault zone in the southern indio hills coachella valley california geomorphic investigation of the san andreas fault zone in the indio hills indicates many tectonically produced landforms including beheaded streams right lateral deflected and offset streams sags shutter ridges pressure ridges and fault scarps near biskra palms an alluvial fan pediment complex has an apparent cumulative offset of about 0 7 km along the mission creek fault zone north branch san andreas fault many of the tectonic landforms as well as the fracture pattern that has developed during the pleistocene are explainable by simple shear or uplift associated with a small left bend in the main trace of the mission creek fault the ratio of vertical to horizontal displacement in the vicinity of the bend is about 0 04 exposed in pushawalla canyon 5 km northwest of the alluvial fan pediment complex are 1 a sequence of stream terraces 2 folded plio pleistocene fanglomerates and 3 an example of stream capture following a right lateral deflection or offset of several hundred metres a left step of the mission creek fault in pushawalla canyon is a probably cause of folds and sporadic uplift that produced the stream terraces possible cause of recent stream capture are 1 juxtaposition of pushawalla canyon with a relict canyon moving northwestward along the mission creek fault or 2 right lateral deflection or offset of pushawalla canyon along the fault with simultaneous headward erosion of a shorter steep stream flowing toward the coachella valley estimation of a slip rate and identification of paleoseismicity for the san andreas fault in the indio hills is difficult however degree of topographic dissection formation and preservation of desert pavement and relative soil profile development suggest that the age of the offset fan may be as old as 70 000 yr but that most likely it is on the order of 20 000 to 30 000 yr these age estimates for the offset fan indicate a minimum slip rate for the san andreas fault of 10 to 35 mm yr with about 23 to 35 mm yr the most likely the pattern of observed offset drainages is complex but suggests that during the past few thousand years creep events or moderately large earthquakes have periodically produced several metres of right lateral displacement

structural control of greenstone hosted gold mineralization by transcurrent shearing a new interpretation of the kalgoorlie mining district western australia abstract the kalgoorlie mining district encloses the golden mile and mt charlotte mining areas and has produced 1200 metric tons of gold it is classed as one of the largest archaean gold deposits in the world the district is located in the norseman wiluna greenstone belt of the yilgarn craton western australia this greenstone belt is characterized by nnw trending transcurrent shear zones some of which are traceable over more than 500 km strike length mineralization in the district is controlled by second third and fourth order structures located between principal shear zones of regional extent an early deformation phase of regional folding d1 associated with prograde regional metamorphism of low grade preceded two phases of wrenching the geometry of mineralized shear zones in both mining areas as well as field and microstructural studies allow us to distinguish an early phase of transpressional sinistral shearing d2 from a later phase of transcurrent dextral shearing d3 the earlier sinistral shear system in the golden mile area was in part reactivated but its gross original geometry was preserved these observations are consistent with clay model experiments carried out by the authors hydrothermal carbonate sericite alteration and gold mineralization commenced during the sinistral shear event and continued into the dextral event mineralization within individual shear zones forms tabular orebodies whereas pipe shaped ore shoots occur at the intersection of shear zones the ore shoots contain high grade telluride mineralization locally relative timing relationships show that mineralization postdates the peak of regional metamorphism hydrothermal activity in the district was preceded by the intrusion of calc alkaline porphyry dykes of granodioritic to tonalitic composition and accompanied by the intrusion of lamprophyre dykes

hologene activity and tectonic setting of the maacama fault zone mendocino county california abstract the maacama fault zone is a north northwest trending right lateral zone of structural weakness in eastern sonoma and mendocino counties california and fault related landforms along its trace suggest that it may well have been active during holocene time urbanization and dam construction in the vicinity of the zone have made it especially desirable to determine the extent and history of such geologically recent movements in order to better understand the seismic capability of faults within the zone the maacama fault zone is a segment of one of two north northwest trending zones of right lateral faulting that diverge as major branches from the easterly side of the san andreas fault zone north of hollister these branches extend at least as far north as eureka and may mark the eastern boundaries of slivers of a separate tectonic plate this investigation was conducted to evaluate holocene activity along the part of the maacama fault zone in mendocino county the investigation shows that this zone comprises discontinuous subparallel fault strands which extend from points south of the sonoma mendocino county line to points north of laytonville some fault segments are expressed by alignments of topographic features clearly caused by holocene strike slip movements other topographic features are less distinct and may well represent fault segments with pre holocene activity the maacama fault zone creeps at a current rate of possibly as high as 2 mm yr the total amounts of holocene offset are unknown evidence obtained from exploratory trenches indicates that at least two surface rupture events have occurred within the past 16 200 years that at least one has occurred within the past 8 310 years and that there probably has been no surface rupturing within the past 1 140 years i conclude that surface offsets have occurred along major portions of the maacama fault zone during holocene time and that this fault zone should be regarded as capable of producing a moderate to strong future earthquake with accompanying surface rupture in mendocino county

archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec part ii vein paragenesis and hydrothermal alteration the sigma deposit consists of an archean gold bearing vein system in deformed volcanic rocks intruded by two generations of porphyries the veins cut all rock types and wall rock alteration is superposed on greenschist facies metamorphic assemblages there are two related vein sets subvertical veins in ductile shears and subhorizontal veins in extensional fractures between shear zones this paper reports on the paragenesis of extensional veins and related hydrothermal alteration tension veins are composed chiefly of quartz and tourmaline and subordinate amounts of carbonates chlorite pyrite pyrrhotite scheelite and free gold the veins formed by one or more episodes of open space filling the paragenetic sequence consists essentially of successive deposition of carbonate scheelite tourmaline pyrite pyrrhotite carbonate chlorite and quartz most of the gold and tellurides was introduced into fractures and along grain boundaries of recrystallized vein minerals during a postfilling episode of deformation veins are surrounded by zoned alteration envelopes consisting of an outer zone of cryptic alteration characterized by a chlorite carbonate white mica assemblage and an inner zone of visible alteration characterized by a carbonate white mica outer subzone and a carbonate albite inner subzone alteration zones represent reaction fronts advancing into the wall rocks and resulted from progressive interaction of wall rocks with a single hydrothermal fluid the chlorite carbonate white mica zone is characterized by replacement of epidote by calcite and white mica accompanied by fixation of co 2 and k and by release of small amounts of si in carbonate white mica and carbonate albite subzones of visible alteration chlorite biotite and white mica were replaced by calcite albite and quartz variable amounts of gold tourmaline pyrite pyrrhotite and ilmenite were also introduced visible alteration zones are characterized by additions of ca co 2 na b s p and au and by important losses of mg fe al and h 2 o mass balance calculations show that ca na si co 2 s and b in vein minerals were contributed from the hydrothermal fluid whereas fe mg and al were derived from the walls the development of alteration envelopes began with the infiltration of hydrothermal fluids into open extensional fractures and it ceased before complete filling of the veins when continuous mineral coatings formed along vein walls deposition of carbonate accompanied development of cryptic alteration including initial co 2 additions to the wall rocks subsequent deposition of tourmaline pyrite and pyrrhotite carbonate chlorite accompanied the progressive development of visible alteration which encroached on cryptically altered rocks visible alteration is characterized by additions of ca and co 2 to the wall rocks and by release of mg fe and al later bulk quartz deposition which filled the vein openings generally took place after complete development of the alteration envelopes except for this late quartz deposition of vein minerals is attributed to chemical changes in the fluid resulting from progressive wall rock alteration bulk quartz deposition is attributed to progressive cooling of the hydrothermal fluid

structural controls on gold bearing quartz mineralization in a duplex thrust system hyde macraes shear zone otago schist new zealand the hyde macraes shear zone in east otago new zealand is a low angle north to northeast dipping thrust system in which gold bearing quartz veins have been deposited mineralization accompanied deformation in the shear zone which is preferentially developed in graphitic pelitic schist the shear zone is a duplex system dominated by steep hinterland dipping thrust faults between well defined shallow dipping upper and lower bounding thrusts imbrication and stacking have occurred in the shear zone especially where intrashear schist is psammitic and relatively brittle this stacking contributes to lateral thickness changes from 2 up to 125 m mineralized veins formed mainly in dilational jogs or localized extensional zones on the relatively shallow dipping parts of the intrashear thrust faults late stage shallow south dipping extensional veins crosscut the duplex structures but are related to the same overall deformation from analysis of the shear zone structure it is possible to make predictions about the geometry of mineralized schist within unexposed parts of the shear zone this is a potentially useful tool for further prospecting and mining in hydrothermally mineralized duplex systems

archean lode gold deposits fluid flow and chemical evolution in vertically extensive hydrothermal systems abstract the range of conditions of formation of lode gold deposits from the sub greenschist to the lower granulite facies in archean greenstone belts and the generally steeply plunging vertically continuous pipe like or tabular geometries of individual deposits indicate long distance hydrothermal fluid advection along well defined channelways in the upper and middle crust from presently available gold solubility data destabilisation of gold bisulphide complexes through h 2 s loss from the fluid to the wallrock was the dominant gold precipitation mechanism within these hydrothermal systems as a whole this inference is supported by the s au ratios of ores sulphur and au precipitation in the hydrothermal system is estimated to be relatively inefficient with only 10 50 of s or au contained in the fluid precipitated over any kilometre length of fluid channelway the relative inefficiency of gold precipitation allowed mineralisation over a significant depth range in a crustal profile

neogene non marine sedimentation and tectonics in small pull apart basins of the san andreas fault system sonoma county california the san andreas fault system in northern california forms an 80 90 km wide zone of right lateral shear extensional tectonism within this broad shear zone is indicated by both neogene silicic volcanic rocks that gradually young in the direction of shear propagation to the north west and by numerous neogene faultbounded structural basins filled with thick non marine sequences the little sulphur creek basins three well exposed 1 5 2 km wide pull apart basins within this shear system have sedimentation patterns analogous to those of much larger pull apart basins they were formed and subsequently deformed by east west extension and by north west to south east orientated right slip concurrently with basin filling palaeocurrent and maximum clast size data indicate both lateral sediment transport from fault bounded basin margins and longitudinal transport down the basin axes the basins are filled primarily with coarse alluvial fan and streamflow deposits derived from a surrounding igneous sedimentary and metamorphic provenance two of the basins contain basin plain type lacustrine turbidites that grade laterally into distal alluvial fan fan delta and sublacustrine delta deposits talus deposits along the south west margin of the basins contain megabreccia indicative of active uplift structures indicative of dewatering liquefaction and slumping suggest penecontemporaneous tectonism

vertical axis rotation of the upper portions of the north east rift of tenerife island inferred from paleomagnetic data abstract paleomagnetic sampling sites were established in 82 dykes along an 8 km long section of the north east rift zone nerz of tenerife canary islands spain of the 70 interpretable sites 16 are of normal polarity and 54 of reversed polarity four normal polarity sites and fifteen reverse polarity sites were excluded from the grand mean calculation for statistical reasons after inverting the reverse polarity sites through the origin the in situ grand mean yields a declination d 023 8 an inclination i 42 3 95 3 2 39 0 n 51 that is discordant to the expected late miocene to pleistocene field direction d 357 6 i 38 8 95 4 7 this discordance can be explained as either a 26 clockwise vertical axis rotation or a 28 wnw side down tilt about an average 009 horizontal tilt axis the sampled section is composed of numerous semi vertical dykes cutting mainly lava flow units that are sub horizontal and cross cut by steeply dipping faults 70 90 field evidence is therefore more compatible with a vertical axis rotation rather than a horizontal axis tilt of the drilled units we argue that this clockwise vertical axis rotation is likely related to strike slip movements that occurred along the edges of the collapse scars and accommodate the emplacement and growth of the underlying intrusive core and associated dykes six new 40ar 39ar age determinations constrain the main interval of dyke emplacement within the nerz between 0 99 ma and 0 56 ma the intrusive activity in the sampled section of the rift appears to have been almost continuous with several intrusion pulses that are probably related to flank destabilisation event s during the mid pleistocene our study thus demonstrates a long lived multi faceted history that shaped the nerz