from partial to full strain partitioning along the indo burmese hyper oblique subduction abstract the andaman nicobar trench and its onshore prolongation the indo burmese wedge is the least studied segment of the india australia subduction new offshore geological and geophysical data have recently been collected along the burma scarp during a marine survey conducted with the r v marion dufresne andaman cruise swath bathymetric mapping combined with shallow and deep seismic show that the dominant active tectonics is dextral strike slip faulting accompanied by various amounts of shortening the southern portion of the burma scarp is transpressive north andaman islands the central portion is pure strike slip on nne oriented segments southern myanmar the northern portion shows sedimentary wedge growth along nw segments both onshore and offshore arakan yoma prism further north the wedge bends to the west while dextral shear faults more or less parallel to the sagaing fault develop within the internal part of the wedge and elongated ns folds form in its external part abutting onto the shillong plateau we propose a simple kinematic model involving evolution from partial to full partitioning from south to north along the west burma scarp wbs and we test it quantitatively using the most recent geodetic results in the south half of the 3 5 cm year of india motion is taken at the trench itself the other half being accommodated onto a single shear fault the sagaing fault in myanmar in the north where the bangladesh fold system developed dextral strike slip faults are activated within the arakan yoma belt and at the accretionary prism backstop contact resulting in full partitioning there faults accommodating the oblique component of motion of india are progressively migrating in space from far field faults andaman sagaing to near trench faults arakan yoma belt and trench itself the burma sliver being buttressed by the eastern himalayas this kinematic extends back to 4 ma which is the time of initiation of the last pulse of oceanic accretion in the andaman basin

late cretaceous early palaeogene tectonic development of se asia abstract the late cretaceous early palaeogene history of the continental core of se asia sundaland marks the time prior to collision of india with asia when se asia from the tethys in the west to the palaeo pacific in the east lay in the upper plate of subduction zones in myanmar and sumatra subduction was interrupted in the aptian albian by a phase of arc accretion woyla and mawgyi arcs and in java eastern borneo and western sulawesi by collision of continental fragments rifted from northern australia subsequent resumption of subduction in the myanmar thailand sector explains 1 early creation of oceanic crust in the andaman sea in a supra subduction zone setting 95 ma 2 the belt of granite plutons of late cretaceous early palaeogene age starting 88 ma in western thailand and central myanmar and 3 amphibolite grade metamorphism between 70 and 80 ma seen in gneissic outcrops in western and central thailand and 4 accretionary prism development in the western belt of myanmar until glancing collision with the ne corner of greater india promoted ophiolite obduction deformation and exhumation of marine sediments in the early palaeogene the ranong strike slip fault and other less well documented faults were episodically active during the late cretaceous palaeogene time n to nw directed subduction of the palaeo pacific ocean below southern china vietnam and borneo created a major magmatic arc associated with rift basins metamorphic core complexes and strike slip deformation which continued into the late cretaceous the origin and timing of termination of subduction has recently been explained by collision of a large luconia continental fragment either during the late cretaceous or palaeogene evidence for such a collision is absent from the south china sea well and seismic reflection record and here collision is discounted instead relocation of the subducting margin further west possibly in response of back arc extension which created the proto south china sea is preferred lying between the two subduction related arcs the khorat basin is of predominantly late jurassic early cretaceous age but stratigraphic and apatite fission track data also indicates deposition of 1 2 km of late cretaceous sediments the synformal basin geometry probably arose due to the dynamic topography created by converging tethyan and palaeo pacific subduction zones the aptian albian slowing of basin subsidence and onset of evaporite deposition coincides with collision of the mawgyi and woyla island arcs extensive palaeogene deformation and exhumation 3 km in places affected all margins of the khorat plateau deformation includes folds of the phu phan uplift and strike slip faults thrusts and folds on the southern and eastern margins south of the khorat plateau outcrop and seismic reflection data from the ton le sap basin cambodia and the gulf of thailand indicate syn depositional fault controlled subsidence was important during cretaceous deposition the hot thickened crust developed during the late cretaceous palaeogene events follows the weak indosinian crustal scale inthanon and sukhothai zones which persistently guided the location of later structures including cenozoic extensional and post rift basins and influenced the widespread occurrence of low angle normal faults metamorphic core complexes and eastern gulf of thailand super deep post rift basins

late cretaceous through cenozoic strike slip tectonics of southwestern alaska abstract new geologic mapping and geochronology show that margin parallel strike slip faults on the western limb of the southern alaska orocline have experienced multiple episodes of dextral motion since 100 ma these faults are on the upper plate of a subduction zone 350 450 km inboard of the paleotrench in southwestern alaska dextral displacement is 134 km on the denali fault at least 88 94 km on the iditarod nixon fork fault and perhaps tens of kilometers on the dishna river fault the strike slip regime coincided with late cretaceous sedimentation and then folding in the kuskokwim basin and with episodes of magmatism and mineralization at 70 60 and 30 ma no single driving mechanism can explain all of the 95 million year history of strike slip faulting since 40 ma the observed dextral sense of strike slip has run contrary to the sense of subduction obliquity this may be explained by northward motion of the pacific plate driving continental margin slivers into and or around the oroclin

absolute timing of sulfide and gold mineralization a comparison of re os molybdenite and ar ar mica methods from the tintina gold belt alaska new re os molybdenite dates from two lode gold deposits of the tintina gold belt alaska provide direct timing constraints for sulfide and gold mineralization at fort knox the re os molybdenite date is identical to the u pb zircon age for the host intrusion supporting an intrusive related origin for the deposit however 40ar 39ar dates from hydrothermal and igneous mica are considerably younger at the pogo deposit re os molybdenite dates are also much older than 40ar 39ar dates from hydrothermal mica but dissimilar to the age of local granites these age relationships indicate that the re os molybdenite method records the timing of sulfide and gold mineralization whereas much younger 40ar 39ar dates are affected by post ore thermal events slow cooling and or systemic analytical effects the results of this study complement a growing body of evidence to indicate that the re os chronometer in molybdenite can be an accurate and robust tool for establishing timing relations in ore systems

the crustal heritage of silver and gold ratios in arizona ores the ratios of produced grades of silver and gold in ore deposits of arizona representing nearly a century of mining correlate with their geologic setting the deposits occur south and west of the colorado plateaus where two geological domains are recognizable on the basis of type and age of rocks and geological history the eastern or naco domain has a basement of proterozoic pinal schist formed from a protolith of mostly clastic sedimentary rocks and widely scattered younger proterozoic granites overlain by paleozoic platform strata and mesozoic volcanic and clastic rocks the western deserts domain is characterized by an older proterozoic section of largely submarine volcanic and volcaniclastic strata together with proterozoic granites it is largely devoid of paleozoic strata but mesozoic clastic and volcanic strata are widely exposed in both domains middle and late tertiary volcanic rocks are extensive ore deposits of many genetic types of proterozoic nevadan laramide and younger tertiary ages are present deposits of laramide and younger tertiary ages occur in both domains but deposits of proterozoic age are confined mostly to the western deserts and those of nevadan age are restricted to the naco domain the ore deposits evolved at different times under different conditions of tectonic stress the produced grades of silver and gold and the silver to gold ratios reveal a bimodal distribution most deposits of all epochs and genetic type in the naco domain show ag enrichment relative to the ratio of the ag au crustal abundance clarke ratio 17 5 1 except for two laramide porphyry copper deposits and the proterozoic massive cu pb zn ores whose ratios are close to the clarke ratio most other districts of the western deserts domain are enriched in gold with respect to the ratio the regional zoning transgresses ore deposit type time tectonic style and reported igneous rock types and metal associations it does not appear to be related to the hypothetical results of subduction processes wherein metals and magmas are directly derived from partial melting of a descending slab of oceanic rocks rather the precious metal characteristics may represent inheritance from specific kinds of crust

mesozoic aseismic ridges on the farallon plate and southward migration of shallow subduction during the laramide orogeny we propose that an aseismic ridge of late jurassic and early cretaceous age on the farallon plate was subducted beneath the unites states cordillera during the laramide orogeny the relative buoyancy of the aseismic ridge caused shallow subduction which resulted in a magmatic lull 70 40 ma in the existing near trench magmatic arc and laramide uplift and faulting of crystalline basement 1000 1500 km inland from the trench the timing and latitudinal limits of the subduction of the aseismic ridge are similar to the timing and latitudinal limits of both the laramide orogeny and the magmatic lull in the western cordillera because the north northeast trending aseismic ridge was moving northeast relative to the north american plate the point of collision of the aseismic ridge with north america and therefore the locus of shallow subduction migrated southward with time causing the well documented southward migration of the magmatic lull

origin of lode gold deposits of the juneau gold belt southeastern alaska mesothermal gold bearing quartz veins along the juneau gold belt formed in the early tertiary from h 2 o co 2 n 2 h 2 s ch 4 rich low salinity fluids at temperatures of 200 to 325 c and pressures in excess of 1 1 5 kbar isotopically heavy ore forming fluids with 18 o values between 8 and 12 and d values of 20 to 30 indicate a deep seated fluid source the data are compatible with a model of ore fluid generation via metamorphic devolatilization within material subducted below the southern alaska continental margin prograde metamorphic fluids infiltrated up zones of mechanically enhanced permeability associated with the coast range megalineament and formed the gold bearing veins within brittle units at higher retrograding crustal levels deposition of gold was accomplished by several mechanisms including boiling fluid wall rock reactions and decreases in pressure and temperature of the hydrothermal fluids

india and sunda plates motion and deformation along their boundary in myanmar determined by gps using a regional gps data set including 190 stations in asia from nepal to eastern indonesia and spanning 11 years we update the present day relative motion between the indian and sundaland plates and discuss the deformation taking place between them in myanmar revisiting measurements acquired on the main boundary thrust in nepal it appears that points in southern nepal exhibit negligible deformation with respect to mainland india including these points using a longer time span than previous studies and making an accurate geodetic mapping in the newest reference frame allows us to refine the present day indian motion our results confirm that the current motion of india is slower than predicted by the nuvel 1a model and in addition our india eurasia motion is significantly 5 mm yr slower than previous geodetic determinations this new indian motion combined with a refined determination of the sundaland motion gives way to a relative india sunda angular velocity of 20 2 n 26 1 e 0 370 myr in itrf2000 predicting a relative motion of 35 mm yr oriented n10 at the latitude of myanmar there the sagaing fault accommodates only 18 mm yr of right lateral strike slip only half of the shear component of motion we present two models addressing how and where the remaining deformation may occur a first model of distributed deformation implies convergence on the arakan subduction the northern continuation of the now famous sumatra andaman trench and wrench faulting in the arakan wedge the second model uses localized deformation where deformation observed west of the sagaing fault is entirely due to elastic loading on a faster and oblique arakan subduction 23 mm yr this latter model predicts that a major earthquake of mw 8 5 may occur every century on this segment of the subduction

the real southern basin and range mid to late cenozoic extension in mexico much of northern and central mexico underwent east northeast extension in the mid to late cenozoic this area constitutes a major but little recognized part of the basin and range province the extended region is bounded on the east by the laramide thrust front of the sierra madre oriental on the west the relatively unfaulted sierra madre occidental separates the extended area in central mexico from that around the gulf of california extension occurred as far south as what is now the trans mexican volcanic belt and in oaxaca south of the belt the basin and range province in mexico constitutes approximately half of the 19 x 10 5 km 2 of western north america that underwent mid to late cenozoic extension north northwest orientations of numerous epithermal vein systems indicate that east northeast extension began as early as 30 ma in areas north of the volcanic belt major episodes of faulting began at about 23 to 24 ma and 12 to 13 ma both in mexico and in the southwestern united states faulting was commonly accompanied by eruption of alkali basalts typical of intraplate rifting widespread quaternary fault scarps and alkali basalts indicate that extension continues to the present in the region north of the trans mexican volcanic belt in contrast the tectonics in and south of the belt are now probably related to subduction of the rivera plate contemporaneity of the 12 ma episode with early extension around the gulf of california attributed to pacific north american plate boundary reorganization suggests that extension is related predominantly to plate boundary effects the beginning of extension at 30 ma may be related to initial encroachment of the east pacific rise upon the trench that lay off western north america

first global positioning system results in northern myanmar constant and localized slip rate along the sagaing fault we present the first recorded global positioning system gps data from myanmar measured at the northern tip of the sagaing fault this area is in a very complex geodynamic context where rigid and semirigid plates interact the 12 gps sites measured in 2005 and 2008 in northern myanmar show that the slip rate is 18 mm yr and is localized along a single active narrow fault trace 20 km wide the same rate was previously demonstrated and remeasured 500 km southward in central myanmar despite the geodynamic regional complexity induced by interaction between the sunda and india plates the burma microplate and the highly deformable eastern himalayan syntaxis the slip rate remains surprisingly constant along this fault however the modeled locking depth varies from 20 km in central myanmar to 5 km in the north

granite and gabbrodiorite associated skarn deposits of nw iran abstract field and laboratory studies show that there are two types of skarn deposits in nw iran granite associated type i and gabbrodiorite associated type ii granite associated deposits are accompanied by cu and fe mineralisation whereas mn and fe are the main ore metals in gabbrodiorite associated skarn deposits there are some differences in the mineralogy of these skarn deposits bixbyite piemontite and cr bearing garnet are found only in gabbrodiorite associated skarns whereas idocrase occurs only in granite associated deposits type ii skarns show exoskarn features whereas some type i skarns have developed endoskarn as well evidence of boiling of hydrothermal fluid can be seen in both types and seems to be a common mechanism of mineral deposition gabbrodiorite associated skarns show higher f o 2 than granite associated deposits based on mineralogical and textural evidence mineralisation in both groups has started from about 550 c early formed anhydrous minerals have begun to be replaced by hydrous minerals from about 400 c it seems that due to low fluid content in the gabbrodioritic magma heated meteoritic water in the surrounding volcanoclastic and tuffaceous rocks was the main source of hydrothermal solution in the gabbrodiorite associated skarn system

unusually cu rich magmas associated with giant porphyry copper deposits evidence from bingham utah mass balance constraints indicate that formation of giant porphyry copper deposits pcds requires either highly efficient collection of cu from large volumes of magma or unusually cu rich parent magmas support for the second of these possibilities has been discovered in the form of mafic enclaves with abundant bornite and chalcopyrite in the last chance stock one of the parent intrusions of the giant bingham pcd utah united states mineral assemblages and compositions indicate that the last chance enclaves are autoliths consisting of phases that crystallized from the intrusion and that the intrusion was unusually enriched in cu one possible mechanism for generating cu rich magmas is fractional crystallization during pyrrhotite undersaturated conditions the high fo 2 conditions observed for the last chance stock may have allowed such an evolution alternatively if the magma has not undergone significant fractionation early crystallization of chalcopyrite and bornite from the magma would indicate that the lower crustal source region for the magma probably contained cu fe sulfides possible cu rich source regions are a subcrustal mafic intrusion with sulfide cumulates or a deeply buried metamorphic terrane containing cu deposits such as those in the curaca valley brazil or okiep south africa heterogeneous distribution of cu fe sulfides in an okiep type source terrane would produce local pcds such as bingham or large accumulations of cu fe sulfides possibly in the form of cumulates in subcrustal intrusions at convergent margins could produce giant pcd provinces such as those in indonesia papua new guinea and central chile

isotopic evidence for the sources of cretaceous and tertiary granitic rocks east central alaska implications for the tectonic evolution of the yukon tanana terrane magnetotelluric traverses across the southern yukon tanana terrane ytt reveal the presence of a thick conductive layer or layers beneath paleozoic crystalline rocks these rocks have been interpreted to be flysch of probable mesozoic age on the basis of the occurrence of jurassic cretaceous flysch in the kahiltna assemblage and gravina nutzotin belt flanking the ytt to the southwest and southeast respectively the pb nd sr and o isotopes in cretaceous and tertiary granitic rocks that crop out throughout the ytt were measured to determine if these rocks do in fact contain a component of flysch previous limited analyses indicated that the pb isotopes of the granitic rocks could be a mixture of radiogenic pb derived from paleozoic crystalline rocks of the ytt with an increasing component of relatively nonradiogenic pb with decreasing age our nd sr and o data along with additional pb isotope data eliminate flysch as a likely source and strongly suggest that the nonradiogenic end member was deri

an autochthonous geological model for the eastern andes of ecuador abstract we describe a traverse across the cordillera real and sub andean zone of ecuador poorly known areas with very little detailed mapping and very little age control the spine of the cordillera comprises deeply eroded triassic and jurassic plutons the roots of a major arc emplaced into probable palaeozoic pelites and metamorphosed volcanic rocks the w flank comprises a jurassic submarine basaltic andesitic volcanic sequence which grades up into mixed jurassic cretaceous volcanic and sedimentary rocks of the inter andean valley the sub andean zone on the e flank of the cordillera comprises a newly recognized cretaceous basin of cleaved mudrocks quartz arenites and limestones east of the syndepositional cosanga fault the cretaceous basin thins into a condensed sequence that is indistinguishable from the rocks of the adjacent hydrocarbon bearing oriente basin the principal penetrative deformation of the cordillera real was probably latest cretaceous palaeocene it telescoped the magmatic belts but shortening was largely partitioned into the pelites between plutons the plutons suffered inhomogenous deformation some portions completely escaped tectonism the pelites conserve two foliations the earliest comprises slaty cleavage formed under low or sub greenschist conditions the later is a strong schistosity defined by new mica growth it largely transposed and obliterated the first both foliations may have developed during a single progressive deformation we find inappropriate recent terrane models for the cordillera real and sub andean zone of ecuador instead we find remarkable similarities from one side of the cordillera to the other including a common structural history in place of sutures we find mostly intrusive contacts between major plutons and pelites triassic to cretaceous events occurred on the autochthonous western edge of the archaean guyana shield the latest cretaceous paleocene deformation is interpreted as the progressive collision of an oceanic terrane s with the south american continent young fault movements have subsequently juxtaposed different structural levels through the cordillera real orogen

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

rb sr k ar and stable isotope evidence for the ages and sources of fluid components of gold bearing quartz veins in the northern sierra nevada foothills metamorphic belt california gold bearing quartz veins occur in and near major fault zones in deformed oceanic and island arc rocks west of the main outcrop of the sierra nevada composite batholith veins typically occupy minor reverse faults that crosscut blueschist to amphibolite grade metamorphic rocks whose metamorphic ages range from early paleozoic to jurassic vein micas and carbonate quartz mica assemblages that formed by hydrothermal metasomatism of ultramafic wall rocks in the alleghany grass valley washington and mother lode districts yield concordant k ar and rb sr ages the dated veins are significantly younger than prograde metamorphism penetrative deformation and accretion of their host rocks to the continental margin new and previously published mineralization ages from 13 localities in the sierra foothills range from about 140 to 110 m y ago with mean and median between 120 and 115 m y the age relations suggest that mineralizing fluids were set in motion by deep magmatic activity related to the resumption of east dipping subduction along the western margin of north america following the late jurassic nevadan collision event co 2 bearing fluids responsible for metasomatism and much of the vein mica carbonate albite and quartz deposition in several northern mines were isotopically heavy delta 18 o asymp 8 14ppm delta d between about 10 and 50ppm and do not resemble seawater magmatic or meteoric waters metasomatic and vein filling mica dolomite magnesite and quartz in altered ultramafic rocks generally formed from fluids with similar sr and o isotope ratios at a given locality consistent quartz mica delta 18 o fractionations delta 18 o sub q m 4 5 4 9ppm from various localities imply uniform equilibration temperatures probably between 300 degrees and 350 degrees c on a local mine scale fluids responsible for both carbonate alteration of mafic and ultramafic wall rocks and albitic alteration of felsic and pelitic rocks had similar sr isotope ratios samples from three veins in the central alleghany district fit a 115 7 or 3 m y rb sr isochron with a 87 sr 86 sr i value of approximately 0 7119 inferred 87 sr 86 sr ratios of metasomatic fluids from mines in different parts of the foothills region vary considerably 0 704 0 718 suggesting that sr was derived from sources ranging from western assemblage mesozoic ophiolitic or arc volcanic rocks to early paleozoic continent derived clastic rocks of the shoo fly complex systematic geographic variations in both sr and o isotopes can be rationalized by assuming extensive fluid interaction with rocks similar to the ones that are exposed within a few kilometers of the veins but the ultimate sources of the fluids and of au and other constituents may be independent of these isotopically lighter meteoric fluids deposited some late quartz overgrowths and occupied secondary fluid inclusions in earlier vein quartz

crustal lineament control on magmatism and mineralization in northwestern argentina geological geophysical and remote sensing evidence abstract the relationship between long lived deep crustal lineaments and the locations of magmatic centers and associated mineral deposits has been investigated in the puna region of northwestern argentina through the analysis of regional aeromagnetic surveys landsat images and geological information the good exposure and excellent preservation of basement and supracrustal geology in this region makes it particularly suitable for such a study at a regional scale several contrasting magnetic domains are recognized which correlate with crustal geology two basement domains are separated by a nne trending boundary which is believed to correlate with a paleozoic suture zone between the pampia to the southeast and arequipa antofalla terranes to the northwest locally overlying these basement terranes is the cenozoic magmatic domain which is best developed in the n s trending volcanic arc at the western edge of the puna the cordillera occidental in addition four southeast trending volcanic zones extend for several hundred kilometers across the puna many important mineral deposits and areas of hydrothermal alteration are associated with these volcanic breakouts and we have selected three such areas for more detailed study bajo de la alumbrera argentina s largest porphyry copper deposit cerro galan the largest ignimbrite caldera in argentina with associated hydrothermal alteration zones and el queva a historic polymetallic district located within a major volcanic range a comparison of lineament maps generated from aeromagnetic and landsat tm images reveals broad correlation between these different remote sensing techniques which respectively highlight subsurface magnetic and surface geological features in addition the locations of magmatic and hydrothermal centers can be related to the interpreted structural framework and are seen to occur near the intersections of major lineament zones it is suggested that in three dimensions such intersection zones form trans lithospheric columns of low strength and high permeability during transpressional or transtensional tectonic strain and may thereby serve as conduits for magma ascent to the shallow crust pooling of large volumes of deeply derived magma in shallow crustal magma chambers may then result in voluminous devolatilization and the formation of hydrothermal mineral deposits it is important to note that in this model structural intersections serve as facilitators for magma ascent and volatile exsolution but do not in themselves cause this process other factors such as magma supply rate and tectonic stress are essential primary ingredients and local magmatic and volcanic processes affect the ultimate potential for ore formation nevertheless we suggest that lineament analysis provides a valuable framework for guiding the early stages of mineral exploration other regional and local geological considerations must then be applied to identify priority targets within this framework

a fluid inclusion and stable isotope study of au quartz veins in the klondike district yukon territory canada a section through a mesothermal vein system gold bearing quartz veins in the klondike district of yukon territory are of earliest cretaceous age and are hosted by permian middle to upper greenschist facies metasedimentary and meta igneous lithologies belonging to the yukon tanana terrane the veins occur in brittle extensional structures which may have developed during extensional deformation during uplift of the klondike region in the late jurassic to early to middle cretaceous following thrust imbrication in the early jurassic the quartz veins are massive and unbanded with localized carbonate flooding and or pyritization of the wall rocks fluid inclusion studies have delineated significant regional p t x trends in primary inclusion fluids from the au quartz veins the fluids have low salinities 6 wt nacl equiv but variable co 2 concentrations decreasing from approximately 0 13 to 0 18 mole fraction co 2 in the southeastern klondike through variable concentrations in the central klondike x sub co 2 0 01 to asymp 1 00 to dominantly aqueous fluids x sub co 2 0 01 in veins from the northwest portion of the region estimated trapping pressures decrease from 2 300 or 700 bars to between 300 to 900 bars from southeast to northwest estimated fluid temperatures range from 300 degrees to 350 degrees c in central and southern lodes to 200 degrees to 250 degrees c in the northwest klondike the p x trends are interpreted as evidence for 1 effervescence of co 2 from the mineralizing fluid most likely in response to regional uplift during the life of the hydrothermal system and 2 that the au quartz vein system represents a section through a mesothermal vein system with deeper structural levels exposed in the southeastern klondike convection of the hydrothermal fluid may have been driven by an elevated geothermal gradient resulting from the rapid uplift the calculated isotopic composition of the au quartz vein fluids delta 18 o fluid 8 3 or 1 2ppmsmow delta d fluid asymp 150 or 18ppmsmow indicates that the mineralizing fluid was meteoric in origin but underwent considerable isotopic evolution prior to mineralization additionally the delta d fluid values of extracted inclusion fluids 166 or 11ppm and from hydrothermal muscovites 174 or 3ppm from the sheba vein system are in good agreement and indicate that the deuterium signature reflects that of the primary mesothermal fluid

age constraints on jerritt canyon and other carlin type gold deposits in the western united states relationship to mid tertiary extension and magmatism carlin type gold deposits are difficult to date and a wide range of ages has been reported for individual deposits therefore several methods were employed to constrain the age of the gold deposits in the jerritt canyon district dated igneous rocks with well documented crosscutting relationships to ore provided the most reliable constraints k ar and 40 ar 39 ar dates on igneous rocks are as follows andesite dikes 324 ma sericitic alteration in andesite dikes 118 ma basalt dikes 40 8 ma quartz monzonite dikes 39 2 ma and calc alkaline ignimbrites 43 1 to 40 1 ma of these only the andesite and basalt dikes are clearly altered and mineralized the gold deposits are therefore younger than the 40 8 ma basalt dikes the sericitic alteration in the andesite dikes is unrelated to the gold deposits a number of dating techniques did not work k ar and 40 ar 39 ar dates on mica from mineralized ordovician to devonian sedimentary rocks gave misleading results the youngest date of 149 ma from the smallest 187 os 188 os initial 0 9 3 6 but did not yield a meaningful isochron due to very low re and os concentrations and large analytical uncertainties paleomagnetic dating techniques failed because the hydrothermal fluids sulfidized nearly all of the iron in the host rocks leaving no remnant magnetism when published isotopic dates from other carlin type deposits in nevada and utah are subject to the rigorous evaluation developed for the jerritt canyon study most deposits can be shown to have formed between 42 and 30 ma k ar and 40 ar 39 ar dates on the youngest preore igneous rocks range from 41 to 32 ma whereas the oldest postore igneous rocks range from 35 to 33 ma hydrothermal adularia from the twin creeks deposit yields similar 40 ar 39 ar dates of 42 ma k ar dates on supergene alunite range from 4 to 30 ma k ar and 40 ar 39 ar dates on micas separated from sedimentary 395 43 ma and igneous 145 38 ma rocks are usually much older than the gold deposits and most are suspect because they are from incompletely reset preore micas or from mixtures of preore and ore stage mica fission track dates on zircons are also generally older than the deposits 169 35 ma and are not completely reset by mineralization apatites are likley to be reset by the hydrothermal systems and by younger thermal events and yield dates 83 22 ma that are younger than those from zircon independent support of a mid tertiary age is provided by the low delta d sub h 2 o values of hydrothermal fluids from 10 widely separated carlin type deposits 134 or 23ppm the low delta d sub h 2 o values are consistent with a mid tertiary age when the climate was cool but are inconsistent with the cretaceous and jurassic ages favored by some workers when the climate was warm the age and distribution of carlin type deposits coincides with areas that underwent extension and calc alkaline magmatism over the same time period despite this relationship the deposits lack clear spatial or genetic relationships to mid tertiary epizonal plutons rather many deposits are localized along preexisting crustal fault zones in the getchell trend structures tapped deep sourced metamorphic or magmatic fluids and subsequently variably exchanged meteoric water in the other trends and districts only variably exchanged meteoric water has been detected these relationships suggest either that all of the deposits formed from deep sourced fluids and were subsequently flooded by meteoric water or that fluids from different sources evolved to produce deposits with similar charateristics

the 2011 mw 6 8 burma earthquake fault constraints provided by multiple sar techniques we used two tracks of alos palsar images to investigate the focal mechanism and slip distribution of the 2011 march 24 m w 6 8 burma strike slip earthquake three different sar techniques namely conventional interferometry sar pixel offsets spo and multiple aperture insar mai were employed to obtain the coseismic surface deformation fields along the 30 km length of the fault rupture along track measurements from spo and mai techniques show a high correlation and were subsequently used to precisely determine the location and extent of the surface fault trace the best fitting fault model geometry derived from an iterative inversion technique suggests that the rupture occurred on a near vertical sinistral strike slip fault west of the nam ma fault with a strike of 70 a maximum slip of 4 2 m occurs at a depth of 2 5 km with significant slip constrained only to the upper 10 km of the crust

cretaceous cenozoic tectonic events in the western myanmar burma assam region correlation of late mesozoic mafic and ophiolitic rocks suggests that in the late lower cretaceous a northeasterly facing mafic arc was emplaced onto southeast borneo western sumatra western myanmar burma then 1100 km south of its present position the mogok belt of myanmar and the denqen bangong co ophiolite zone in tibet all then situated on the southwestern margin of asia reversal in tectonic polarity and eastward subduction generated a late cretaceous magmatic arc in western myanmar and sumatra and its continuation through the mogok belt into the gandise batholith of tibet magmatism interrupted in the latest cretaceous probably resumed in the western myanmar tibet and sumatra arc segments in the early palaeogene it was again interrupted in the mid eocene when an east vergent ophiolite nappe overrode the indo burman ranges deforming lower eocene turbidites derived from the magmatic arc to the east following renewed northward and eastward subduction of ocean floor in the latest eocene india collided with northern myanmar and with tibet along a suture south of the jurassic triassic flysch and adjacent yarlung ophiolite in the mogok segment of the late cretaceous arc crust thickened in the early cretaceous orogeny underwent extension to form a metamorphic core complex as myanmar west of the sagaing fault moved northward more than half of this northward movement preceded the well documented 450 km of post early miocene dextral displacement on the sagaing fault and related spreading in the andaman sea generation of late cretaceous to early eocene i and s type granites in and east of the mogok belt was related either to crustal shortening or more probably to subsequent extension in the mogok belt

petrochemistry and tectonic significance of cretaceous island arc rocks cordillera oriental dominican republic abstract cretaceous island arc rocks of the caribbean island arc system have been exposed by cenozoic faulting in the cordillera oriental in eastern hispaniola high k 2 o intermediate to felsic volcanic rocks loma la vega volcanics are interbedded with marine epiclastic sedimentary rocks and tuffs las guajabas tuffs and unconformably overlie pre aptian sedimentary rocks low k 2 o volcanic rocks guamira volcanics and a granodioritic to tonalitic intrusion el valle pluton the petrology and geochemistry of these units in conjunction with regional stratigraphic data are used to speculate on the tectonics of the newly developing caribbean island arc system during early and late cretaceous time the loma la vega volcanics are characterized by the presence of large phenocrysts of sanidine and minor amounts of clinopyroxene opaque oxides and rare leucite in a devitrified matrix of chlorite and clay although the volcanic rocks have undergone some low temperature alteration metamorphism which redistributed some major elements and large ion litho phile trace elements the high field strength elements rare earth elements and radiogenic isotopes appear to have been minimally affected based on abundances of the relatively immobile elements trace element enrichment patterns and isotopic compositions the loma la vega volcanics are considered part of the high k calc alkaline ca or shoshonitic island arc volcanic series in contrast pre aptian early cretaceous volcanic and plutonic rocks of the underlying los ranchos formation have chemical characteristics similar to rocks in the island arc tholeiitic or primitive island arc pia series that form coeval and along strike sections of the early cretaceous caribbean island arc in other parts of present day hispaniola cuba puerto rico and the virgin islands an abrupt and regional change in composition from island arc tholeiites to high k calc alkaline rocks is coincident with a hypothesized reversal in subduction polarity in pre aptian time as inferred from previously published tectonic models polarity reversal may have been triggered by attempted subduction of the caribbean oceanic plateau beneath this segment of the caribbean island arc the observed magmatic and tectonic effects of the proposed cretaceous caribbean arc reversal are similar to the better documented neogene subduction reversal event in the solomon islands arc in the southwest pacific

active tectonics and earthquake potential of the myanmar region this paper describes geomorphologic evidence for the principal neotectonic features of myanmar and its immediate surroundings we combine this evidence with published structural geodetic and seismic data to present an overview of the active tectonic architecture of the region and its seismic potential three tectonic systems accommodate oblique collision of the indian plate with southeast asia and extrusion of asian territory around the eastern syntaxis of the himalayan mountain range subduction and collision associated with the sunda megathrust beneath and within the indoburman range and naga hills accommodate most of the shortening across the transpressional plate boundary the sagaing fault system is the predominant locus of dextral motion associated with the northward translation of india left lateral faults of the northern shan plateau northern laos thailand and southern china facilitate extrusion of rocks around the eastern syntaxis of the himalaya all of these systems have produced major earthquakes within recorded history and continue to present major seismic hazards in the region

origins of large volume rhyolitic volcanism in the antarctic peninsula and patagonia by crustal melting voluminous rhyolitic volcanism along the palaeo pacific margin of gondwana was marked by three principal episodes of magmatism the first of these v1 is essentially coincident with the main episode of karoo ferrar magmatism at 184 ma a younger v2 episode occurred at 168 ma and a third episode v3 occurred in the interval 157 153 ma we evaluate the origin of v1 and v2 rhyolites from the antarctic peninsula using major and trace element and isotopic sr nd o data an isotopically uniform 87sr 86sri 0 707 endi 3 andesite dacite magma was generated as a result of anatexis of grenvillian age hydrous mafic lower crust linked to earlier arc related underplating the lower crustal partial melts would have mixed with fractionated components of the mafic underplate followed by subsequent storage and homogenization early jurassic v1 rocks of the southern antarctic peninsula are interpreted as melts of upper crustal paragneiss which have mixed with the isotopically uniform magma in upper crustal magma chambers the v2 rhyolites are the result of assimilation fractional crystallization of the isotopically uniform magma this occurred in upper crustal magma chambers involving assimilants with similar isotopic composition to that of the magma a continental margin setting was crucial in developing hydrous readily fusible lower crust lower crustal anatexis was in response to mafic underplating associated with the discovery shona bouvet group of plumes thought to be responsible for the karoo magmatic province the progression old to young of volcanism from ne to sw in patagonia and south to north in the antarctic peninsula is consistent with migration away from the mantle plumes towards the proto pacific margin of gondwana during rifting and break up

present day crustal deformation around sagaing fault myanmar 1 global positioning system gps measurement campaigns in myanmar conducted in 1998 and 2000 allow quantifying the present day crustal deformation around the sagaing fault system in central myanmar both a regional network installed at four points within the country and a local 18 station network centered on the city of mandalay across the sagaing fault demonstrate that active deformation related to the northward motion of india is distributed across myanmar in a platelet that extends from the western edge of the shan plateau in the east to the andaman trench in the west in this platelet deformation is rather diffuse and distributed over distinct fault systems in the east the sagaing shan scarp fault system absorbs 10 mm yr this gps study combined with an on land geotectonic survey demonstrate that oblique slip of india along the rigid sundaland block is accommodated by a partitioned system characterized by distribution of deformation over a wide zone index terms 1206 geodesy and gravity crustal movements interplate 8155 1243 geodesy and gravity space geodetic surveys 8110 tectonophysics continental tectonics general 0905 8158 tectonophysics plate motions present and recent 3040 keywords tectonics gps fault southeast asia

shallow rupture of the 2011 tarlay earthquake mw 6 8 eastern myanmar we use l band advanced land observation satellite palsar data to infer the distribution of subsurface fault slip during the tarlay earthquake m w 6 8 in eastern myanmar we find the total length of surface rupture is approximately 30 km with nearly 2 m maximum surface offset along the westernmost section of the nam ma fault the tarlay segment finite fault inversions constrained by interferometric synthetic aperture radar insar and pixel tracking data suggest that fault slip is concentrated within the upper 10 km of the crust maximum slip exceeds 4 m at a depth between 3 and 5 km comparison between field measurements and near fault deformation obtained from the insar range offset result suggests about 10 80 of displacement occurred within a 1 km wide zone off the main surface fault trace this off fault deformation may explain the shallow slip deficit that we observed during this earthquake we estimate a recurrence interval for tarlay like events to be 1600 6500 yrs at this section of the nam ma fault a detailed paleoseismological study is essential to clarify the slip behavior and the earthquake recurrence interval of the nam ma fault

contribution of mafic melt to porphyry copper mineralization evidence from mount pinatubo philippines and bingham canyon utah usa mount pinatubo in the philippines known for its cataclysmic eruption in 1991 hosts several porphyry copper deposits and active geothermal systems an underlying mafic melt supplied much of the sulphur for the dacitic magma and its injection into the dacitic magma chamber triggered the eruption the eruption caused purging of the sulphur rich fluid from the dacite to the atmosphere and extensive fracturing similar events took place at bingham canyon utah site of the largest copper and gold deposit in north america at 38 ma the bingham canyon mineralization took place beneath an active stratovolcano and pyroclastic flows contemporaneous with the mineralization show evidence for magma mingling ascent of mafic melt supplied sulphur and chalcophile elements to the felsic magma which consolidated to form the bingham stock and its underlying magma chamber injections of the mafic melt caused periodic eruptions of felsic magma to form the stratovolcano and deposition of sulphide minerals in highly fractured rocks in and around the stock

age and paleotectonic setting of volcanogenic massive sulfide deposits in the guerrero terrane of central mexico constraints from u pb age and pb isotope studies the guerrero terrane of central and western mexico comprises volcanic and sedimentary strata of mainly mesozoic age with geochemical signatures indicating formation in juvenile back arc to slightly more evolved arc environments at least 60 volcanogenic massive sulfide vms deposits have been discovered thus far within the guerrero terrane they comprise two main belts the coastal belt mainly comprising deposits in the zihuataneo subterrane including the cuale bramador district and the central belt which includes deposits in the teloloapan subterrane campo morado tlanilpa azulaquez tizapa rey de plata etc the guanajuato subterrane el gordo las gavalinas and the zacatecas subterrane san nicolas and el salvador deposits the age of host rocks for the vms mineralization in the various subterranes was poorly constrained prior to this study eleven new u pb zircon ages for volcanic host rocks from the campo morado tlanilpa azulaquez leon guanajuato and san nicolas el salvador vms districts together with recently determined u pb ages from the cuale area demonstrate that vms deposits in the guerrero terrane range from latest middle jurassic to early early cretaceous callovian to valanginian in age the oldest ages were obtained from the cuale district total age range of 157 4 4 1 ma for three units and the youngest are from the tlanilpa azulaquez district total age range of 139 7 2 5 ma for two samples volcanogenic massive sulfide deposits in the central belt resemble those of the bimodal siliciclastic deposit type whereas those in the coastal belt are more similar to the bimodal felsic type published geochemical and radiogenic isotope data from the various subterranes suggest that vms deposits in the coastal belt and most of those in the central belt are hosted within juvenile to slightly evolved arc settings the san nicolas and el salvador deposits of the zacatecas subterrane are the only deposits that unequivocally formed in a back arc setting our new u pb dating results together with existing stratigraphic geochemical and radiogenic isotope data from throughout the guerrero terrane are consistent with the guerrero terrane having developed as a west facing continental margin arc that was built on mainly oceanic crust and overlying early mesozoic siliciclastic fans shed from the east along the western margin of nuclear mexico slab rollback in the middle jurassic led to the development of a rocas verdes type continental back arc inboard of the arc which is now preserved as the arperos basin in the central and southern guerrero terrane and the host rocks for the san nicolas and el salvador deposits farther to the north in southeastern zacatecas state we have determined pb isotope compositions of sulfides from vms deposits and several other styles of mineralization from throughout the guerrero terrane a compilation of these and previously published data shows that isotopic compositions from the various vms deposits fall in a relatively confined field that overlaps the average upper crustal growth curve this suggests that metals were mainly sourced from continentally derived sedimentary strata that are interlayered with or immediately underlie the host volcanic section and or from the host volcanic rocks themselves or subvolcanic intrusive equivalents that had assimilated significant quantities of upper crustal material either at the point of origin or during subsequent ascent and eruption lead isotope compositions from the francisco i madera deposit in zacatecas state indicate that most of the sulfides in this deposit are epigenetic rather than syngenetic in origin and are likely tertiary in age

duration of strike slip movements in large shear zones the red river belt china abstract sixty four size fractions of zircon monazite xenotime and titanite were dated by the u pb method to temporally and spatially constrain left lateral movements in the red river shear zone between the depths of 15 and 20 km the shearing led to the formation of the high grade metamorphic gneisses that form the bulk of the 4200 m high diancang shan and 3100 m high ailao shan mountain ranges the omnipresence of melt layers and pockets in these gneisses shows that temperatures associated with tectonometamorphism lie at and above the solidus of continental crust and deformation patterns are very homogeneous over the entire length of the belt north of the gneisses subvolcanic bodies intrude unmetamorphosed cover rocks in particular eocene red beds along the projected trend of the red river shear zone for a leucogranitic layer in the diancang shan gneisses an age of 24 2 0 2 ma 2 is obtained for monazite and 22 4 0 2 ma is obtained for xenotime whereas zircon formed through two growth stages at 24 23 ma these ages record solidification of the anatectic melts over a period of at least 1 8 0 4 ma during which left lateral shear was continuously active in the ailao shan gneisses zircon and titanite ages are 26 3 0 3 and 26 1 0 2 ma for two monzonitic bodies and 24 1 0 2 and 22 4 0 2 ma for two pegmatitic layers that occur within the monzonitic intrusions these results show that zircon monazite titanite and probably also xenotime are closed systems for u and pb under melting conditions 650 c therefore the ages date mineral formation in the melt rather than cooling below certain closure temperatures a significantly older age of 35 0 0 1 ma is obtained for a monzonitic subvolcanic intrusion north of the diancang shan gneisses because these intrusions follow the trend of the shear zone they most likely represent the surface expression of early activity along that zone indicating that strike slip movement had already occurred in latest eocene time the total set of u pb ages shows that magmatic and metamorphic activity occurred from 35 to 20 ma strengthening the idea that the red river shear zone absorbed most of the intraplate movements required to compensate the opening of the south china sea between 32 and 16 ma the existence of monzonitic syenitic intrusions along the red river zone implies that melting affected source rocks other than the diancang shan and ailao shan gneiss protoliths potential sources to produce such rocks lie in the deep crust possibly including melts from the mantle consequently the red river shear zone must be a deep rooted structure along which melts migrated to the surface moreover the contemporaneity of shear motion and magma generation suggests that melting occurred in direct relation to strike slip activity

hairpin river loops and slip sense inversion on southeast asian strike slip faults in the golden triangle region of southeast asia northern thailand laos and burma southern yunnan the mekong salween and neighboring rivers show hairpin geometries where they cross active strike slip faults restoration of young left lateral offsets of these rivers leaves residual right lateral bends of many kilometers we interpret these hairpins as evidence of late cenozoic slip sense inversion on these faults about 5 to 20 ma near the red river fault stress field and slip sense inversion occurred ca 5 ma this implies that the present course of these large rivers has existed for at least several million years pliocene quaternary slip rates possibly on the order of 1 mm yr are inferred on each of the strike slip faults of the golden triangle

geologic and geophysical evidence for the influence of deep crustal structures on paleozoic tectonics and the alignment of world class gold deposits north central nevada usa abstract geologic data concur with geophysical and isotopic data that suggest the presence of deep crustal fault zones along the battle mountain eureka bme trend and elsewhere in nevada the fault zones may have originated during proterozoic rifting of the continent and were likely substantially reactivated and modified during paleozoic tectonism five distinct paleozoic structural and stratigraphic domains are defined that demonstrate the complexity of paleozoic tectonic events and also lead to hypotheses about ways in which the margin could have been modified the current locations of these domains adjacent to the geophysically and isotopically defined indicators of the buried continent edge corroborate their interactions with the continental margin during the tertiary preexisting crustal fault zones were intersected and reopened during episodes of extension and served as the conduits for deep sourced gold rich fluids which were disseminated into paleozoic slope facies sedimentary rocks forming sediment hosted carlin type and other deposits multiple factors including the locations of these deep seated structures the original configuration of the lower paleozoic continental margin of nevada and its subsequent reactivation during the paleozoic all were fundamental controls on the location of younger mineral deposits a clearer understanding of the original configuration of the margin and of the effects of subsequent paleozoic and mesozoic tectonic events on the margin would provide insight into the locations of these and other prospective mineral belts

paleogeographic and paleotectonic setting of laramide sedimentary basins in the central rocky mountain region in the rocky mountain region between central montana and central new mexico sedimentologically isolated nonmarine basins were produced by basement deformation during the laramide orogeny within the area formerly occupied by a broad late cretaceous foreland basin in which laterally continuous marine facies had accumulated previously laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere laramide basins include perimeter basins along the cratonic periphery of the arcuate laramide province axial basins along a north south intramontane trend and ponded basins located farther west closer to the overthrust belt twelve specific stratigraphic and sedimentologic criteria for the onset duration and termination of laramide deformation allow the chronology of basin development to be inferred independently for each basin maastrichtian initiation of laramide deformation was approximately synchronous throughout the laramide province but termination of laramide deformation was systematically diachronous from north to south between early and late eocene time widespread eocene erosion surfaces truncate syntectonic laramide sequences and are overlain by largely volcanic and volcaniclastic post laramide strata of eocene age in the north and oligocene age in the south fluvial depositional systems draining toward the great plains were dominant in perimeter and axial basins but ponded basins were occupied at times by large lakes that served as regional sediment traps paleocene drainages from ponded basins also led eastward toward the continental interior but partly interconnected lakes that developed within several ponded basins by mid eocene time were either closed hydrologically or else they drained westward into the tyee paleoriver of the pacific northwest

late permian middle jurassic lithospheric thinning in peru and bolivia and its bearing on andean age tectonics abstract integrated studies and revisions of sedimentary basins and associated magmatism in peru and bolivia 8 22 s show that this part of western gondwana underwent rifting during the late permian middle jurassic interval rifting started in central peru in the late permian and propagated southwards into bolivia until the liassic dogger along an axis that coincides with the present eastern cordillera southwest of this region lithospheric thinning developed in the early jurassic and culminated in the middle jurassic producing considerable subsidence in the arequipa basin of southern peru this 110 ma long interval of lithospheric thinning ended 160 ma with the onset of malm earliest cretaceous partial rift inversion in the eastern cordillera area the lithospheric heterogeneities inherited from these processes are likely to have largely influenced the distribution and features of younger compressional and or transpressional deformations in particular the altiplano plateau corresponds to a paleotectonic domain of normal lithospheric thickness that was bounded by two elongated areas underlain by thinned lithosphere the high eastern cordillera of peru and bolivia results from late oligocene neogene intense inversion of the easternmost thinned area

large scale fractures related to inception of the yellowstone hotspot during middle miocene time western north america was subject to flood basalt volcanism dike swarm injection and broad scale fracturing and folding of the crust we propose a simple model to account for these events and for a regional pattern of geologic and geophysical features aeromagnetic maps reveal some of the most important elements of this pattern which are several narrow arcuate anomalies here referred to as the northern nevada rifts these rifts extend hundreds of kilometers across nevada and are likely caused by highly magnetic middle miocene mafic dikes with the aid of filtering techniques the anomalies can be traced into oregon together with other geologic features such as fold axes dike swarms and faults they produce a spoke like pattern fanning over 2208 of arc that converges toward a point near the oregon idaho border lat 448n a possible cause for this pattern is a point source of stress at the base of the crust related to the formation of the yellowstone hotspot the spoke like pattern however does not persist at large distances from the emerging hotspot several hundred kilometers to the south the northern nevada rifts deviate significantly 308 from a radial trend we show that a simple model imposing a point source of stress at the base of the crust and a regional stress field aligned with the presumed middle miocene stress direction fits the observed fracture pattern it thus accounts for both the radial pattern present near the nascent hotspot and the far field pattern due to regional stresses

structural setting style and timing of vein hosted gold mineralization at the pogo deposit east central alaska gold bearing veins within the liese zone of the pogo deposit display a two stage evolutionary history that records temporal variation in kinematics fluid chemistry and temperature several stacked shallow northwest dipping shear veins are developed at pogo and collectively comprise the liese zone veins consist of 1 early narrow biotite bearing shear veins 2 white quartz veins with pyrite arsenopyrite bands referred to as main stage quartz veins that have sericite fe mg carbonate alteration envelopes and which exploit the early shear veins and 3 extension veins that form as steeper offshoots from the main stage veins the presence and orientation of oblique fabrics developed in the older biotite bearing shear veins are indicative of top to the south displacement under ductile to semi brittle conditions at higher temperatures in contrast the orientation of the extension veins and local sigmoidal shapes indicate a component of top to the northwest normal displacement on the main stage veins in their present orientation and brittle to semi brittle conditions of formation dolomite sericite alteration surrounding main stage veins may represent late to post mineral hydrothermal fluid exploitation of vein margins during ongoing normal displacement along vein systems all types of veining overprint 107 106 ma post metamorphic granitic dykes molybdenite in main stage quartz assemblages has returned re os ages of 104 2 1 1 ma significantly older than 96 to 91 ma 40ar 39ar ages obtained from vein alteration assemblages that may reflect thermal resetting during post mineral fault related hydrothermal activity magmatism and or retrograde cooling of the lithologic sequence unlike typical mesothermal shear vein hosted gold systems pogo is temporally and tectonically separated from metamorphic deformation events and has a comparable kinematic and geometric architecture to cretaceous plutonic gold deposits in the region we interpret the deposit to have formed during a regional cretaceous extensional event during multi stage exploitation of extensional fault surfaces by hydrothermal fluid from a cooling magmatic source

plutonism oblique subduction and continental growth an example from the mesozoic of california major episodes of mesozoic plutonism in california correlate with periods of oblique subduction and trench parallel transport of western california along intrabatholithic faults major episodes of plutonism occurred in the late jurassic during left oblique convergence and in the mid cretaceous during right oblique convergence in contrast a conspicuous lull in plutonism but continuation of volcanism in the earliest cretaceous coincides with a time when the north america farallon convergence vector although large in magnitude was oriented perpendicular to the trench this correlation suggests that plutonism is facilitated by strike slip faulting within the batholithic belt one explanation which helps to solve the plutonic room problem is that plutons are passively emplaced at releasing bends in the strike slip faults and volume is conserved by thrusting at the trench if this correlation is generally applicable then it implies that mid crustal plutonism is limited beneath areas in which the convergence vector is subperpendicular to the trench continental growth in such areas may occur dominantly by volcanism

strains and structures associated with a terrane bounding stretching fault the melones fault zone central sierra nevada california abstract an examination of the structures total strains and rock types across the melones fault zone indicates that in the central sierra nevada this fault separates and locally imbricates two distinct terranes east of the fault a widespread early jurassic cleavage displays west northwest to northwest strikes and is associated with moderate to high strain intensities these structures form in jura triassic metasedimentary rocks in part derived from a continental source a weak northwest striking cleavage and low strain intensities occur west of the fault in lower jurassic volcanic rocks and upper jurassic slates and sandstones the fault zone itself consists of steep east dipping ductile brittle shear zones or stretching faults along which the early jurassic structures are transposed parallel to the jura cretaceous and fault related structures large flattening type strains are common in the fault zone and may be the result of heterogeneous strains developed when the shear zones bend around resistant blocks or by volume losses total strains in the eastern hanging wall reflect the superposition of fault related on older strains therefore stretching lineations in this hanging wall do not reflect directions of fault motion stretching lineations in the footwall and fault zone kinematic indicators in the fault zone and a small eastward jump in metamorphic grade suggest an east over west or reverse sense of movement no evidence for significant strike slip or oblique slip motion was found removal of footwall strains indicates that the jura cretaceous shortening across the fault was responsible for both the transposition of older structures east of the fault and the rotation of the fault to its present steep dips thus the melones fault is a large ductile brittle shear zone along which the eastern hanging wall merced river terrane moved to the southwest over the western footwall eastern foothills terrane

fault controlled pluton emplacement in the sevier fold and thrust belt of southwest montana usa abstract problems associated with syncompressional pluton emplacement center on the need to make room for magma in environments where crustal shortening not extension occurs on a regional scale new structural data from the pioneer and boulder batholiths of southwest montana usa suggest emplacement at the top of frontal thrust ramps as composite tabular bodies at crustal depths between 1 and 10 km frontal thrust facilitated pluton emplacement was accommodated by 1 a magma feeder zone created along the ramp interface 2 providing releasing steps at ramp tops that serve as initial points of emplacement and subsequent pluton growth and 3 localizing antithetic back thrusts that assist in pluton ascent a model of magma emplacement is proposed that involves these elements this model for syntectonic ramp top emplacement of plutons helps explain how space is made for plutons within fold and thrust belts

special paper major gold deposits and belts of the north and south american cordillera distribution tectonomagmatic settings and metallogenic considerations a compilation of economically viable gold concentrations containing 10 moz in the north and south american cordillera reveals the existence of 22 discrete belts in addition to five major isolated deposits most formed over the last 150 m y the gold concentrations are attributed to eight widely recognized deposit types of which porphyry sediment hosted and high sulfidation epithermal are economically the most important individual gold belts are typically several tens to hundreds of kilometers long dominated by single deposit types and metallogenically active for relatively brief periods 5 20 m y many of the gold belts and major isolated deposits were generated under extensional or transtensional tectonic conditions in either arc or back arc settings nevertheless the two main high sulfidation epithermal gold belts were generated in thickening or already thickened crust during low angle subduction eight other gold belts or districts also accompanied compression or transpression with two of them the main orogenic gold belts occupying fore arc sites there is a strong suggestion that the preeminent cordilleran gold concentrations formed either during or immediately following prolonged contraction the major gold deposits and belts occur along the craton edge as well as in adjoining accreted terranes but almost all are of postaccretionary timing many of the gold belts and isolated deposits were localized by crustal scale faults or lineaments which may be either parallel or transverse to the cordilleran margin the gold concentrations accumulated during active subduction commonly in close spatial and temporal association with intermediate to felsic medium to high k calc alkaline igneous rocks by contrast the low sulfidation epithermal gold deposits accompany bimodal volcanic pulses of calc alkaline tholeiitic or alkaline affinity however a gold alkaline rock association is uncommon a genetic link between gold mineralization and coeval magmatism is widely accepted for most of the deposit types the exceptions being the sediment hosted and orogenic gold deposits notwithstanding the small cumulative extent of the gold concentrations relative to the entire cordilleran margin there is a marked tendency for two or more belts or isolated deposits of different ages and genetic types to occur in close proximity within relatively restricted arc including fore and back arc segments in the case of the western united states for example six belts and four isolated major deposits make up a particularly prominent cluster if fortuity is discounted this clustering or pairing of gold concentrations must imply a predisposition of certain arc segments to gold mineralization an analogous situation is evident for other metals particularly copper and tin the reason for the recurrent generation of major deposits and belts dominated by one or more metals remains uncertain although heterogeneously distributed metal preconcentrations favorable redox conditions or other parameters somewhere above the subducted slab between the mantle wedge and upper crust are widely contemplated possibilities elucidation of the reason s for this metallogenic inheritance at the scale of limited arc segments poses an important and challenging series of research questions as well as being critical to the planning of potentially successful greenfield exploration programs

lode and placer gold composition in the klondike district yukon territory canada implications for the nature and genesis of klondike placer and lode gold deposits we have determined the major and trace element composition of approximately 2 700 gold particles from 21 gold bearing mesothermal quartz veins and 35 placer gold samples from the klondike district in western yukon territory measured au ag cu and hg contents were used to define a characteristic geochemical signature or fingerprint for each of the vein samples these signatures were then compared with the various compositional populations that we have distinguished within each of the placer samples preliminary conclusions derived from the study include 1 placer gold in both recent stream deposits and in the pliocene to pleistocene white channel gravels is detrital in origin 2 the placer gold is mainly if not entirely derived from mesothermal quartz veins 3 all lode sources for the placer gold have not yet been located and 4 gold composition data can be used to identify the lode signature for placer gold and if it has not been removed by erosion help locate and link placer gold to its specific lode source

subduction arc reversal and the origin of porphyry copper gold deposits in island arcs the tertiary quaternary porphyry copper gold deposits of the southwest pacific rim particularly in luzon mainland papua new guinea bougainville guadalcanal and fiji mostly formed after a reversal of arc polarity where this reversal has not occurred e g new zealand and japan porphyry copper gold deposits are absent or scarce hence older porphyry copper gold deposits may be useful clues to tectonic history enrichment of magmas in gold may have resulted from two stage melting the tectonic setting of porphyry copper gold deposits differs from that inferred for volcanic hosted massive sulfide deposits

western united states extension how the west was widened abstract cenozoic extension in the western united states presents a complex interrelation of extension volcanism and plate boundary tectonics that defeats simple notions of active or passive rifting forces driving extension can originate at plate boundaries through basal traction basal normal forces or from buoyancy forces internal to the crust and lithospheric mantle the latter two are most responsible for driving extension where it is observed in the basin and range the complex evolution of the northern basin and range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust in contrast crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 ma to drive extension in the southern basin and range the central basin and range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western united states

from andesitic volcanism to the formation of a porphyry cu au mineralizing magma chamber the farallon negro volcanic complex northwestern argentina abstract the geological relations and geochemical composition of extrusive and subvolcanic intrusive rocks of the upper miocene farallon negro volcanic complex nw argentina were studied to reconstruct the evolution of a deeply eroded andesitic volcano over most its life history of some 3 5 my this paper is part of an integrated study of the processes that generate magmatic hydrothermal fluids responsible for the formation of porphyry cu au deposits here we present new data on bulk rock chemical and nd sr and pb isotopic compositions of unaltered volcanic and subvolcanic intrusive rocks for which geological relations and ages are constrained by field based stratigraphy and extensive ar ar geochronology the combined results indicate that voluminous volcanism started at ca 9 7 ma with the eruption of basaltic andesite to andesite lava flows and pyroclastic deposits this basic to intermediate volcanism persisted until 7 5 ma and was followed at 7 35 ma by the eruption of dacites as the last recorded major extrusive event the oldest andesitic intrusions emplaced at 9 0 ma were followed by hypabyssal intrusions of basaltic andesite andesite and dacite composition which intruded the volcanic sequence between 8 3 and 6 1 ma intrusions at 6 8 0 2 ma led to the formation of over 800 mt of porphyry cu au ore at bajo de la alumbrera it was formed by a large flux of magmatic hydrothermal fluids channeled through several phases of fractured dacitic porphyry which together with immediately following barren andesitic intrusions make up a composite stock after this magmatic hydrothermal event only minor rhyodacitic to rhyolitic intrusions were emplaced until magmatic activity ceased at 6 1 ma geochemical data for the magmatic rocks indicate that the evolution of the system was dominated by mixing of a mafic magma with a felsic magma that contained a component of crustal material as indicated by linear compositional correlations between compatible and incompatible elements initial strontium isotopic ratios vary between 0 7047 and 0 7118 while 143nd 144nd range from 0 5127 to 0 5124 and both isotopic variations correlate with each other and with sio2 content of the rocks after 8 5 ma a continuous increase in the silica content of intrusive rocks suggests the gradual formation of a magma chamber a substantial magma reservoir 7 km3 must have been established by the time of porphyry cu au mineralization to allow rapid release of a large volume of ore forming magmatic brine as a result of wholesale crystallization and de volatilization of the magma chamber

salt water infiltration in two artificial sea inlets in the belgian dune area in the dune area of the westhoek nature reserve situated in the western belgian coastal plain two artificial tidal inlets were made aiming to enhance biodiversity the infiltration of salt water in these tidal inlets was carefully monitored because a fresh water lens is present in the phreatic dune aquifer this forms an important source of fresh water which is for instance exploited by a water company the infiltration was monitored over a period of two years by means of electromagnetic borehole measurements em39 and by measurements of fresh water heads and temperature using a large number of observation wells em39 observations point to aquifer heterogeneity as a determining factor in the movement of the salt infiltration water it is shown that part of the infiltration water moves further in the dunes instead of towards the sea on the long term run possibility exists that salt water enters the extraction s capture zone this issue needs further monitoring and study fresh water head and temperature data illustrate that the main period of infiltration is confined to spring tide when large amounts of salt water enter the tidal inlets

the role of mantle delamination in widespread late cretaceous extension and magmatism in the cordilleran orogen western united states extension during plate convergence and mountain building is widely recognized yet the causes of synconvergent extension remain controversial here we propose that delamination of lithospheric mantle aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating incursion of fl uids and partial melting explains many enigmatic yet prevalent aspects of the metamorphic magmatic and kinematic history of the sevier laramide orogen of the western united states during the late cretaceous extension heating anatexis magmatism and perhaps rock uplift were widespread during a restricted time interval in the late cretaceous 75 67 ma along the axis of maximum crustal thickening within the mojave sector of the sevier orogen and to a lesser extent within the interior of the idaho utah wyoming sector to the north similar processes may have been active in the peninsular range sierran western mojave and salinian segments of the mesozoic cordilleran arc these processes are viewed as predictable consequences of the thermal rheological and dynamic state of the overlying crust following delamination of mantle lithosphere beneath isostatically compensated mountain belts the proposed delamination would have occurred immediately prior to eastward propagation of lowangle subduction of the farallon plate during the inception of the laramide orogeny following delamination extension and anatexis of the north american crust were aided locally by egress of slab derived fl uids from the low angle farallon slab we suggest that lithospheric delamination may have aided in the shallowing of the slab to achieve lowangle subduction geometry delamination has been proposed to be common in areas of thickened continental lithosphere in the terminal phase or late in orogenesis the late cretaceous delamination event proposed here for the sevier laramide orogen occurred during protracted plate convergence and was synchronous with and followed by continued shortening in the external part of the orogen

reconnaissance k ar geochronology of the maricunga gold silver belt northern chile the maricunga metallogenic belt in northern chile is defined by the presence of both intrusion related porphyry type and volcanic hosted high sulfidation acid sulfate epithermal deposits containing gold and or silver the nine principal mineralized centers in the belt including the la coipa la pepa marte and lobo deposits were dated radiometrically using the k ar method hypogene alunite from advanced argillic assemblages was dated from eight of the deposits the 18 new radiometric ages permit subdivision of the maricunga belt into two overlapping longitudinal subbelts the western of early miocene including latest oligocene age 25 20 ma and the eastern of middle miocene age 14 12 5 ma this spatial and temporal subdivision of the alteration and mineralization accords well with that defined previously for volcanic rocks in the region and is substantiated further by regional structural and volcanic relationships the migration of the magmatic and metallogenic activity from the western to the eastern subbelt may be attributed to shallowing of the subduction zone which was initiated at 18 to 16 ma the western and eastern maricunga subbelts are broadly correlative with two discrete volcanic alteration events in the el indio precious metal belt 300 km to the south the maricunga subbelts are also integral parts of an eastward younging sequence of gold bearing magmatic metallogenic subbelts of paleocene to late miocene age which span an east west distance of 380 km in this transect 26 degrees 28 degrees s of the central andes

genetic links among fluid cycling vein formation regional deformation and plutonism in the juneau gold belt southeastern alaska gold bearing quartz vein systems in the juneau gold belt formed within a 160 km long by 5 to 8 km wide zone along the western margin of the coast mountains alaska vein systems are spatially associated with shear zones adjacent to terrane bounding mid cretaceous thrust faults analysis of vein orientations and sense of shear data define a stress configuration with greatest and least principal axes oriented subhorizontally with northeast southwest trends and subverticaly respectively this local stress configuration is compatible with the far field plate configuration during eocene time isotopic ages of vein formation indicate that fluid cycling occurred between 56 5 and 52 8 ma and are consistent with a genetic link between veining and a change in plate motion in early eocene time veining was also synchronous with the latter stages of rapid exhumation and voluminous plutonism immediately inboard of the gold belt we propose a model in which interacting tectonic events facilitated fault valve action and vein development along now exhumed shear zones

active faulting and tectonics of burma and surrounding regions this study of the active tectonics of burma and surrounding regions is based mostly on an interpretation of landsat imagery and on fault plane solutions for shallow and intermediate earthquakes fold axes in the indoburman ranges and an east dipping inclined zone of intermediate depth earthquakes suggest that a slab of oceanic lithosphere was recently subducted to the east under the indoburman ranges fault plane solutions of shallow earthquakes however do not show underthrusting at the present time instead p axes are oriented roughly north south parallel not perpendicular to the fold axes in the ranges p axes for fault plane solutions of intermediate shocks also trend north south parallel to the strike of the subducted slab and may indicate that the hanging slab is being dragged north by india through the surrounding asthenosphere the sagaing fault probably accommodates most of the right lateral slip of india past indochina but india s northward movement also causes internal deformation within burma thailand and yunnan differences between the tectonic fabric seen on the landsat images and the tectonic regime compatible with the fault plane solutions throughout the region imply major changes in the directions of relative motion in the orientations of strain and in the style of deformation in the late cenozoic presumably these changes reflect the transition from subduction to collision and strike slip motion as india penetrated into eurasia past burma large parts of burma and thailand may have rotated several tens of degrees clockwise in this process

late paleozoic to jurassic silicic magmatism at the gondwana margin analogy to the middle proterozoic in north america a vast region of upper paleozoic to middle jurassic 300 150 ma silicic magmatic rocks that erupted inboard of the gondwana margin is a possible phanerozoic analogue to the extensive middle proterozoic 1500 1350 ma silicic magmatic province that underlies much of the southern mid continent of north america like the north american rocks the gondwana silicic magmas appear to be melts of crust that formed about 200 300 m y earlier in the north american case this older crust formed and was accreted to the continent during a major period of crustal formation 1700 1900 ma whereas in the gondwana case the crust that melted consisted mainly of magmatic are terranes accreted to the continental margin during the paleozoic in both cases basic to intermediate magmatic rocks are extremely rare and magmatism is less abundant in regions that contain older and previously melted crust the similarities between the north american and gondwana silicic rocks suggest that both suites formed in extensional settings where basaltic magmas ponded at the base of the preheated crust caused extensive crustal melting that inhibited upward passage of the basalts in both cases silicic volcanism occurred after major assembly of a supercontinent by subduction and accretion processes and before breakup of the supercontinent by analogy with the polar wander curves for gondwana the granite rhyolite provinces may have formed during a period of very slow motion of the supercontinents relative to the poles

surface ruptures of the mw 6 8 march 2011 tarlay earthquake eastern myanmar abstract field observations indicate the m w 6 8 tarlay myanmar earthquake of 24 march 2011 resulted from the rupture of a short section of the left lateral nam ma fault we document coseismic left lateral offsets that range from approximately 10 cm to about 2 m along a 19 km long section of the fault based upon both field investigations and the interpretation of high resolution satellite imagery comparison of these early field and later remote measurements suggests that most of the paddy field offsets became obscured within a year or two our measurements indicate the magnitude of sinistral offset decreases gradually eastward from the epicentral area to the rupture terminus within the tarlay basin along the southern edge of a 2 3 km wide transtensional stepover our measurements confirm that the earthquake resulted from rupture of a structurally distinct segment of the nam ma fault bounded on the west by the fault s terminus and on the east by the tarlay basin stepover if the 215 km nam ma fault were to rupture entirely in a single event the magnitude of the resulting earthquake would likely be about m w 7 7 online material field observations and photos after the 2011 tarlay earthquake table of offset and maps with location and offset

the tectonic and metallogenic framework of myanmar a tethyan mineral system myanmar is perhaps one of the world s most prospective but least explored minerals jurisdictions containing important known deposits of tin tungsten copper gold zinc lead nickel silver jade and gemstones a scarcity of recent geological mapping available in published form coupled with an unfavourable political climate has resulted in the fact that although characterized by several world class deposits the nation s mineral resource sector is underdeveloped as well as representing a potential new search space for a range of commodities many of myanmar s known existing mineral deposits remain highly prospective myanmar lies at a crucial geologic juncture immediately south of the eastern himalayan syntaxis however it remains geologically enigmatic its mesozoic recent geological history is dominated by several orogenic events representing the closing of the tethys ocean we present new zircon u pb age data related to several styles of mineralization within myanmar we outline a tectonic model for myanmar from the late cretaceous onwards and document nine major mineralization styles representing a range of commodities found within the country we propose a metallogenetic model that places the genesis of many of these metallotects within the framework of the subduction and suturing of neo tethys and the subsequent himalayan orogeny temporal overlap of favourable conditions for the formation of particular deposit types during orogenic progression permits the genesis of differing metallotects during the same orogenic event we suggest the evolution of these favourable conditions and resulting genesis of much of myanmar s mineral deposits represents a single evolving mineral system the subduction and suturing of neo tethys

jurassic epithermal au ag deposits of patagonia argentina abstract important precious metal deposits have been discovered during the last 10 years in the deseado massif region of patagonia argentina this region is a plateau consisting of middle to upper jurassic volcanic rocks that host fracture controlled epithermal au ag mineralization these mineral deposits represent low sulfidation type hydrothermal systems and formed following the main period of volcanism probably during the late jurassic period the presence of silica sinters and veins interpreted to be feeder structures indicates that the tops of some deposits have been preserved with erosion of younger sedimentary and volcanic rocks now exposing these systems these deposits represent diverse erosional levels ranging from sinter formed at the paleosurface to intermediate au ag rich quartz veins to base metal bearing au ag veins that represent deeper levels of the epithermal systems the best known deposits are in the cerro vanguardia district where vein systems occur over a 350 km 2 area and veins can be traced for more than 14 km geological mapping of the district has resulted in the definition of over 140 km of veins current reserves are over 3 2 million ounces au equivalent with grades of 10 g t au and 120 g t ag the geologic characteristics of the region and the presence of major au ag deposits provide targets for continuing exploration in the deseado massif

world class base and precious metal deposits a quantitative analysis over 62 percent of the 193 000 metric tons of gold discovered to date is located in four countries and more than 68 percent occurs in four types of mineral deposits about 55 percent of the 1 740 000 metric tons of silver found is in four countries and 45 percent is in four types of deposits fifty six percent of the 1 52 billion metric tons of discovered copper is from four countries and four types of deposits contain 88 percent of the total over 50 percent of both the 713 000 000 metric tons of zinc and 349 000 000 metric tons of lead discovered to date come from four countries and 70 percent of both metals occur in four types of deposits all discovered gold would fit in a cube with a height of 22 m silver in a 55 m cube copper in a 550 m cube zinc in a 460 m cube and lead in a 310 m cube at least 74 percent of gold silver zinc and lead is in deposits having average grades above the respective median grades and 44 percent of copper is in deposits with average grades above the median grade of all deposits lower grade deposits contain less total metal than higher grade deposits tonnage of mineralized rock is an even better predictor of contained metal with over 96 percent of each metal s total residing in deposits having greater than median size and between 47 and 79 percent of metal contained in the largest 10 percent of deposits world class deposits defined as the upper 10 percent of deposits in terms of contained metal account for over 86 percent of all gold 79 percent of silver 84 percent of copper 71 percent of zinc and 73 percent of lead these giant deposits contain at least 100 metric tons 3 2 moz gold 2 400 metric tons 77 moz silver 2 million metric tons copper 1 7 million metric tons zinc or 1 million metric tons lead mineral deposits occur rarely in the earth s crust and large ones are especially uncommon this analysis shows that only the unusually large deposits can significantly affect supply

production of jurassic rhyolite by anatexis of the lower crust of patagonia abstract the mid jurassic marifil and chon aike volcanic rocks of eastern patagonia are part of one of the largest silicic igneous provinces known rb sr geochronology indicates eruptive ages of 175 190 ma for the marifil complex mostly toarcian aalenian the majority of the rocks are isotopically uniform with initial 87sr 86sr 0 7067 0 0003 and endt 4 2 primary magmas of andesitic composition were generated by partial melting of mafic grenvillian lower crust indentified by depleted mantle model ages of 1150 1600 ma lower crustal pyroxene granulite xenoliths can be modelled as residual jurassic source although they may alternatively be co genetic cumulates the dacite rhyolite suite formed by crystal liquid fractionation processes from the primary andesites involving multistage crystallization and re melting during magma ascent with the later stages of evolution being explicable by fractional crystallization of plagioclase amphibole and accessory minerals the province represents large scale lower crustal reworking associated with the unique tectonic and igneous environment of gondwana break up

jurassic to holocene tectonics magmatism and metallogeny of northwestern mexico the present metallic distribution in northwestern mexico is the culmination of superposed magmatism tectonism erosion and burial over more than 150 m y detailed palinspastic reconstructions of preextensional configurations the first study of its kind for this region clarify the interplay among these features on the present distribution and character of mineralized geologic systems this new synthesis goes beyond previous metallogenic investigations of northwestern mexico by separating events into specific timing and structural relationships and by restoring the geology to its preextensional configuration metallogenic factors such as enrichment preservation and erosion play major roles in the present distributions and for the first time are related to the overall metallogenic framework of northwestern mexico the analysis concludes that modern metallogenic patterns are the result of the complex superposition and subsequent redistribution of geologic systems in a way that is related directly to the regional history rather than simply metallic belts or interpreted angle of a subducting slab three main extensional events in the oligocene holocene have been restored and the palinspastic distributions have been analyzed reconstructions reveal the following 1 mineralization events igneous centers and sedimentary sequences are continuous across the gulf of california and other areas with large amounts of extension 2 middle tertiary gold silver mineralization in baja california may be the western part of the sierra madre oc present address bhp billiton avenida america vespucio sur 100 8th floor las condes santiago chile e mail john mark g staude bhpbilliton com cidental metallogenic province thus expanding the previously recognized extent of this province s mineralization 3 late cretaceous early tertiary porphyry copper deposits and intrusive centers form a narrower belt than previously noted and are traceable for over 400 km with parts of the belt buried beneath the younger sierra madre occidental volcanic fields 4 interpreted alignments of older geologic features including lineaments of ore deposits are displaced in the reconstructions 5 sedimentary rock hosted gold deposits and low angle detachment gold systems are closely related and occur around core complexes by using structurally restored time slices it becomes clear that older deposit types tend to be those formed at greater depths and more proximal to intrusions whereas younger deposits formed at shallower depths are less eroded and are more commonly volcanic rock hosted these characteristics express themselves in the regional distribution of deposit types second mineralization of widely differing ages is spatially superposed commonly associated with coeval magmatic and tectonic events the structural and magmatic events together with paleodistribution of ore deposits define a new framework to interpret the metallogenic history of northwestern mexico

the chon aike province of patagonia and related rocks in west antarctica a silicic large igneous province the field occurrence age classification and geochemistry of the mesozoic volcanic rocks of patagonia and west antarctica are reviewed using published and new information dominated by rhyolitic ignimbrites which form a bimodal association with minor mafic and intermediate lavas these constitute one of the largest silicic igneous provinces known equivalent in size to many mafic lips diachronism is recognized between the early middle jurassic volcanism of eastern patagonia marifil and chon aike formations and the middle jurassic earliest cretaceous volcanism of the andean cordillera el quemado ibanez and tobifera formations this is accompanied by a change in geochemical characteristics from relatively high zr and nb types in the east to subalkaline arc related rocks in the west although the predominance of rhyolites remains a constant factor all of the associated mafic rocks are well fractionated compared to direct mantle derivatives petrogenetic models favour partial melting of immature lower crust as a result of the intrusion of basaltic magmas possibly with some hybridisation of the liquids and subsequent fractionation by crystal settling or solidification and remelting the formation of large amounts of intracrustal silicic melt acted as a density barrier against the further rise of mafic magmas which are thus rare in the province

ela icp ms u pb zircon geochronology of regional volcanism hosting the bajo de la alumbrera cu au deposit implications for porphyry related mineralization ela icp ms u pb zircon geochronology has been used to show that the porphyritic intrusions related to the formation of the bajo de la alumbrera porphyry cu au deposit nw argentina are cogenetic with stratigraphically well constrained volcanic and volcaniclastic rocks of the late miocene farallon negro volcanic complex zircon geochronology for intrusions in this deposit and the host volcanic sequence show that multiple mineralized porphyries were emplaced in a volcanic complex that developed over 1 5 million years volcanism occurred in a multi vent volcanic complex in a siliciclastic intermontane basin the complex evolved from early mafic intermediate effusive phases to a later silicic explosive phase associated with mafic intrusions zircons from the basal mafic intermediate lavas have ages that range from 8 46 0 14 to 7 94 0 27 ma regionally extensive silicic explosive volcanism occurred at 8 0 ma 8 05 0 13 and 7 96 0 11 ma which is co temporal with intrusion of the earliest mineralized porphyries at bajo de la alumbrera 8 02 0 14 and 7 98 0 14 ma regional uplift and erosion followed during which the magmatic hydrothermal system was probably unroofed shortly thereafter dacitic lava domes were extruded 7 95 0 17 ma and rhyolitic diatremes 7 79 0 13 ma deposited thick tuff blankets across the region emplacement of large intermediate composition stocks occurred at 7 37 0 22 ma shortly before renewed magmatism occurred at bajo de la alumbrera 7 10 0 07 ma the latest porphyry intrusive event is temporally associated with new ore bearing magmatic hydrothermal fluids other dacitic intrusions are associated with subeconomic deposits that formed synchronously with the mineralized porphyries at bajo de la alumbrera however their emplacement continued from 7 10 0 06 to 6 93 0 07 ma after the final intrusion at bajo de al alumbrera regional volcanism had ceased by 6 8 ma 6 92 0 07 ma

petrogenetic and metallogenetic responses to miocene slab flattening new constraints from the el indio pascua au ag cu belt chile argentina the eocene miocene volcanic and hypabyssal rocks of the el indio pascua au ag cu belt in the southern central andean flat slab region are medium high k calc alkaline arc suites ranging in composition from andesite to rhyolite a significant transition in magmatic trace element chemistry coinciding with a pronounced reduction in magma output occurred in the late middle miocene as documented by 40ar 39ar geochronology the upper eocene lower middle miocene rocks exhibit low sr y ratios 50 minor heavy ree fractionation with sm yb ratios not exceeding 3 5 and in some cases minor negative eu anomalies in contrast the largely dacitic rocks erupted after ca 13 ma are depleted in y 10 ppm have generally high but variable sr y ratios 30 200 exhibit moderate middle and heavy ree fractionation sm yb 3 7 5 9 and lack negative eu anomalies the latter features are characteristic of adakitic suites i e slab melts but the regional temporal and spatial distribution of arc magmatism precludes a major magma source in the downgoing slab this evolution is interpreted as reflecting a progressive increase in pressure and the availability of water in the lower crustal site of magma generation establishing both garnet and hornblende as major stable phases in the residuum the pressure in the lower crust increased in response to episodic crustal thickening related to the shallowing of the slab a process recorded by the incision of three regional pediplains over the period 17 6 ma elimination of the subarc asthenospheric mantle and much of the lithospheric mantle by ca 10 ma permitted direct incursion of slab derived highly oxidised metal and volatile rich supercritical fluids into the lower crust stimulating melting of mafic garnet amphibolitic and eclogitic assemblages

the northern nevada rift regional tectono magmatic relations and middle miocene stress direction as defined by the most recent aeromagnetic surveys the north northwest trending northern nevada rift zone extends for at least 500 km from southern nevada to the oregon nevada border at several places along the rift the magnetic anomaly is clearly related to north northwest trending dikes and flows that based on new radiometric dating erupted between 17 and 14 ma and probably during an even shorter time interval the tectonic significance of the rift is dramatized by its length its coincidence in time and space at its northern terminus with the oldest silicic caldera complex along the yellowstone hot spot trend and its parallelism with the subduction zone along the north american coast prior to the establishment of the san andreas fault the northern nevada rift is also equivalent in age trend and composition to feeder dikes that fed the main eruptive pulse 95 volumetrically off the columbia river flood basalts in northern oregon 15 5 16 5 ma because of these similarities both regions are considered to be part of an enormous lithospheric rift that propagated rapidly south southeast and north northwest respectively from a central mantle plume the site of the initial breaching of the north america plate by this plume is probably the mcdermitt volcanic center at the north end off the rift near the oregon nevada border the present north northwest trend of the rift and its internal elements such as dikes and lava filled grabens record the orientation of the arc normal extensional stress in this back arc region at the time of emplacement paleomagnetic evidence presented by others and interpreted to indicate block rotations at three sample localities is not consistent with either a rotation of dikes within the rift or with a regional rotation of the entire rift the present north northwest trend of the rift reflects the state of stress in the basin and range during middle miocene time and is consistent with stress indicators of similar age throughout the basin and range and rio grande rift provinces