timing of arc construction and metamorphism in the slate creek complex northern sierra nevada california late paleozoic to early mesozoic rocks in the sierra nevada and klamath mountains western united states preserve a record of lateral growth of continental crust by incorporation of ophiolites volcanic arcs and associated sedimentary rocks deciphering the timing of arc construction and metamorphism is critical for elucidating tectonic and thermal evolution during this type of crustal growth but is difficult in complex accretionary terranes in this study we address the timing of arc construction and regional metamorphism in the slate creek complex a volcano plutonic terrane in the central part of the northern sierra nevada new 40 ar 39 ar ages from relict volcanic hornblende demonstrate that the youngest volcanic unit in the slate creek complex is ca 170 ma at least 30 m y younger than previous estimates of ca 200 ma for metaplutonic rocks in the lower part of the complex consequently the slate creek complex is polygenetic with two distinct episodes of arc magmatism the distribution of mineral assemblages and textures indicates that greenschist and epidote amphibolite facies metamorphism is younger than 170 ma and is associated with crosscutting plutons that yield cooling ages of 150 ma and younger amphiboles from foliated rocks in the slate creek complex yield plateau ages of 156 1 0 6 and 152 0 0 7 ma that date cooling subsequent to or dynamic recrystallization during the metamorphic event age and lithologic similarities suggest that the lower parts of the slate creek complex correlate with plutons ca 200 ma and the volcanic cover sequence of the rattlesnake creek terrane in the klamath mountains the uppermost volcanic unit correlates with the ca 170 ma western hayfork terrane that structurally overlies the rattlesnake creek terrane metamorphism in the slate creek complex correlates broadly with a similar late metamorphic event in the klamath mountains

composite devonian island arc batholith in the northern sierra nevada california the bowman lake batholith intrudes the pre upper devonian shoo fly complex in the northern sierra nevada the eastern margin of the batholith extends to within 1 5 km of the base of a thick sequence of paleozoic island arc rocks that rests unconformably on the shoo fly complex hypabyssal silicic intrusions associated with the batholith penetrate the upper devonian sierra buttes formation the lowest volcanic unit of the arc sequence but do not extend to higher stratigraphic levels the intrusions in the sierra buttes formation possess marginal peperites and show other evidence of their injection into wet unconsolidated sediments on the basis of these field relations the batholith is interpreted to represent a subvolcanic magma chamber emplaced concurrently with deposition of sierra buttes arc rocks the batholith is composite and consists of trondhjemite granodiorite biotite granite and hornblende tonalite formed from discrete batches of magma injected into a common plutonic chamber mingling of tonalitic and trondhjemitic magmas produced abundant rounded inclusions of tonalite dispersed within trondhjemite and led to hybridization of the two magmas by fine scale intermixing zircons from the trondhjemitic granodioritic and granitic phases of the batholith and an associated sierra buttes rhyolite sill show complex u pb isotopic systematics a multistage history is suggested involving the incorporation of earliest paleozoic and or proterozoic zircons during magma generation or ascent in devonian time and one or more stages of disturbance in mesozoic cenozoic time consideration of these different discordance mechanisms results in a model u pb igneous age for the batholith of 364 to 385 ma middle to late devonian on the basis of the field relations a late devonian age for the batholith is adopted zircon characteristics and isotopic systematics suggest that petrogenesis of the granite involved mixing of trondhjemitic and or granodioritic magmas with partial melt from a separate large ion lithophile enriched source

formation and emplacement of the josephine ophiolite and the nevadan orogeny in the klamath mountains california oregon u pb zircon and 40ar 39ar geochronology cordilleran ophiolites typically occur as basement for accreted terranes in the klamath mountains ophiolitic terranes were progressively accreted by underthrusting beneath north america the josephine ophiolite is the youngest of the klamath ophiolites and forms the basement for a thick late jurassic flysch sequence galice formation this ophiolite flysch terrane forms an east dipping thrust sheet sandwiched between older rocks of the klamath mountains above and a coeval plutonic volcanic arc complex below the outcrop pattern of the roof orleans thrust indicates a minimum displacement of 40 km and geophysical studies suggest 110 km of displacement the basal madstone cabin thrust is associated with an amphibolitic sole and has a minimum displacement of 12 km a rapid sequence of events from ophiolite generation to thrust emplacement has been determined using 40 ar 39 ar and pb u geochronology ophiolite generation occurred at 162 164 ma a thin hemipelagic sequence was deposited from 162 to 157 ma and flysch deposition took place between 157 and 150 ma tight age constraints on thrusting and low grade metamorphism associated with ophiolite emplacement nevadan orogeny are provided by abundant calc alkaline dikes and plutons ranging in age from 151 to 139 ma deformation and metamorphism related to the nevadan orogeny appears to have extended from 155 to 135 ma most of the crustal shortening took place by thrusting constrained to have occurred from 155 to 150 ma on both the roof and basal thrusts minimum rates of displacement are 2 4 and 3 6 mm year for the basal and roof thrusts respectively but correlations with coeval thrusts yield rates of 8 4 and 22 mm year within the range of plate velocities the high displacement rates and synchronous movement along the basal and roof thrusts suggest that the ophiolite may have behaved as a microplate situated between western north america and an active arc from 155 to 150 ma a steep thermal gradient was present in the josephine galice thrust sheet from 155 to 150 ma with amphibolite facies conditions developed along the basal thrust after accretion of the ophiolite by underthrusting the ophiolite and overlying flysch underwent low grade dynamothermal regional metamorphism from 150 to 135 ma the upper age limit is tightly constrained by a 135 ma k feldspar cooling age syntectonic plutons as young as 139 ma and a lower cretaceous angular unconformity very rapid exhumation is indicated by the late valanginian to hauterivian age 130 ma of the unconformably overlying strata suggesting unroofing by extensional tectonics

stratigraphy of upper paleozoic volcanic rocks and regional unconformities in part of the northern sierra terrane california upper paleozoic volcanic rocks in the southern part of the taylorsville strike belt are unconformably truncated at the north fork of the american river by a lens of upper triassic conodont bearing limestone and lower and middle jurassic graywacke slate flysch deposits of the sailor canyon formation the volcanic sequence consists of submarine felsic pyroclastic debris flows and tuffaceous slate of the late devonian sierra buttes formation andesitic turbidites of the taylor formation a lower tuff siltstone member and an upper chert member of the peale formation and fine grained tuff of the reeve formation newly discovered radiolarians date the chert member of the peale as latest late mississippian and or early pennsylvanian shelly fossil fragments found 50 m above the chert member date the lower part of the reeve in this area as late early permian these rocks formed in the distal part of the submarine volcanic apron presumably in an island arc setting and were deposited unconformably on quartz rich clastic rocks and tectonically interlayered chert of the pre late devonian shoo fly complex the peale marks a period of quiescence in the late paleozoic volcanism that culminated in deep water chert deposition during latest mississippian to the middle pennsylvanian the peale reeve contact is a regional hiatus probably nondepositional in the south and becoming erosional to the north that spans most or all of the late pennsylvanian and early permian post late early permian to pre late triassic deformation produced northeast trending folds and a southeast trending fault in the late paleozoic volcanic rocks that are truncated nearly orthogonally by the late triassic unconformity and northwest trending folds in the jurassic sailor canyon formation

the smartville intrusive complex sierra nevada california the core of a rifted volcanic arc the smartville complex is a jurassic volcanic and plutonic arc in the northwestern sierra nevada that was deformed during the late jurassic nevadan orogeny we interpret the smartville intrusive complex to have formed during the incipient rifting of an active volcanic arc this interpretation is supported by the close relationship between volcanism and plutonism and by the close association between sheeted dikes and the younger plutonic rocks further support is found in the similarities that exist between the smartville complex and modern arcs that developed on oceanic crust either at a continental margin or in the ocean basins smartville volcanic rocks consist of a lower unit of tholeiitic submarine flows and pillowed flows that grades upward into an upper unit of calc alkaline pyroclastic and volcaniclastic deposits intrusive rocks include older units of metamorphosed gabbro and massive diabase which are intruded by a unit of 100 mafic and felsic sheeted and unsheeted dikes biotite hornblende tonalite and granophyric hornblende tonalite plutons are coeval with the dike complex and both rock types occur locally as dikes within the dike unit continuously and reversely zoned gabbro diorite plutons are also coeval with the dike complex granodiorite plutons are the youngest intrusive unit and may be related to the sierra nevada batholith relative age relations suggest that the smartville complex formed in a single volcanic plutonic system of pre nevadan age the dike complex intrudes and is intruded by both the tonalite and zoned gabbro diorite plutons both the upper and lower volcanic units are intruded by all of the plutonic and hypabyssal units and were deformed prior to the intrusion of the sheeted dike complex clasts of plutonic and hypabyssal rocks resembling the younger intrusives however occur locally in some of the youngest volcaniclastic rocks suggesting that shallow plutonism and volcanism could be broadly coeval early nevadan thrust faults juxtapose volcanic and older plutonic rocks of the smartville complex and mesozoic chert argillite broken formation to the east the younger plutons and the dike complex in the smartville are deformed by steep late nevadan faults and do not intrude chert argillite broken formation the youngest granodiorite plutons in the area truncate nevadan faults intrude the chert argillite formation and appear to be unrelated to the smartville complex the smartville volcanic arc underwent pre nevadan intra arc extension the sheeted dike complex is the primary manifestation of the rifting event the elongate shapes of the younger plutons which are coeval with the dike complex reflect extensional control on their emplacement volcanic rocks in the western and southern smartville complex were deformed prior to the intrusion of the dike complex

accretionary tectonics of the north american cordillera continental geology stands on the threshold of a change that is likely to be as fundamental as plate tectonic theory was for marine geology ongoing seismic reflection investigations into the deep crustal structure of north america are verifying that orogenic zones are underlain by low angle faults of regional extent brown et al 1981 the growing body of regional field relations is likewise delineating numerous orogenic sutures that bound discrete crustal fragments paleomagnetic and paleobiogeographic studies are revealing major latitudinal shifts and rotations within and between suture bounded fragments particularly within the north american cordillera such interdisciplinary studies are leading to a consensus that the cordillera has been built by progressive tectonic addition of crustal fragments along the continent edge in mesozoic and early cenozoic time such crustal growth is referred to as accretionary tectonics in this paper we review some of the important concepts in accretionary tectonics discuss the nature of the materials accreted between central alaska and southern california in jurassic and cretaceous time and consider the general relations between cordilleran accretion and the movement of lithospheric plates the concept of continents growing by peripheral accretion through geologic time has long been a topic of great interest with the advent of plate tectonics a number of different mechanisms for crustal accretion have arisen along with mechanisms for crustal attrition accretion mechanisms include the growth of imbricated sedimentary prisms along inner trench walls slicing off of submarine topographic irregularities within subducting plates and collision of continents and volcanic arcs by ocean basin closure tectonic attrition mechanisms include rifting transform faulting and strike slip or underthrust removal of inner trench wall materials coincident with or in place of accretionary prism growth growth of intraorogenic ocean basins by seafloor spreading is an additional important mechanism for creating accretionary materials as well as displacing crustal fragments an important implication of plate kinematic theory is the likelihood for accretionary and attritionary mechanics to operate in series both in time and space along continental margins since attrition by nature leaves little material evidence of having operated one of the major problems confronting cordilleran geologists lies in the recognition of such attrition within the ancient record particularly when interspersed with accretionary events the spectrum of accretion and attrition mechanisms viewed at cm yr 1 plate transport rates over time scales of 100 m y leads one to suspect a highly mobile history for continental margin orogens the serial arrangement of subducting transform and rifting links along the modern cordillera plate juncture system and both serial and parallel arrangements in the western pacific systems show the complex interplay of such mechanisms through space similar arrangements overprinted through time are suggested by the rock assemblages and structural patterns within the cordillera which presently resemble a collage of crustal fragments davis et al 1978 recognition of the structural state of this collage geologic field mapping and geophysical investigations will bring about a new level of understanding in the growth of continental crust and the reading of stratigraphic records within the fragments and future palinspastic restorations will lead to a new level of understanding in paleogeography and earth history the first problem to be considered is the recognition of native north american crust from exotic fragments that have been accreted to its edge

structure and tectonics of the northern sierra nevada recent detailed mapping suggests a new working hypothesis for the structure of the northern sierra nevada we propose that pre cretaceous rocks are deformed by a series of eastward directed over thrusts modified by west directed folds and faults the highest and westernmost tectonic unit is the jurassic smartville complex structurally below it there are imbricate thrust slices of jurassic and older ophiolitic and oceanic sedimentary rocks in the central belt and of paleozoic to jurassic sedimentary and volcanic rocks in the eastern belt the paleozoic feather river peridotite separates the eastern and central belts and our hypothesis suggests that both of its margins may be important thrust faults the east directed faults and folds are deformed by northwest trending upright and west vergent folds and reverse faults that control the present outcrop pattern these later structures have so modified the earlier east directed structures that the latter have been recognized only recently an important key to understanding the structure has been the recognition of ophiolitic complexes that can be correlated across major faults and that contain a pseudostratigraphy useful in determining local structures the events recorded by both the east and west directed deformations occurred during callovian through kimmeridgian time and represent the nevadan orogeny our hypothesis of early east directed overthrusts followed by west directed back folding and faulting implies shortening and thickening of the crust during the nevadan orogeny and is consistent with the idea that the northern sierra nevada is the result of a crustal collisional process

mesozoic transpression transtension subduction and metallogenesis in northern and central california middle paleozoic to middle jurassic terrane assemblies in the klamaths and sierran foothills consist of mafic ultramafic complexes fine grained terrigenous strata derived from previously accreted continental margin belts sutured oceanic terranes reflect c 230 myr of margin parallel slip involving chiefly transtension and transpression quartzofeldspathic clastic rocks and blueschists eclogites are very rare little devolatilization occurred at magmagenic depths hence coeval hydrothermal ore deposits and granitoids are uncommon in contrast nearly head on cretaceous subduction of the farallon plate generated the massive klamath sierra nevada volcanic plutonic arc reflecting dewatering of the eastward descending oceanic lithosphere in the magmagenic zone immature great valley forearc and franciscan trench deposits shed from the arc record c 70 myr of rapid crustal growth au bearing solutions rising from magmagenic depths exsolved from plutons and expelled from heated wall rocks were mobilized attending arc construction precipitation of gold bearing quartz veins occurred where h2o co2 bearing fluids encountered major geochemical discontinuities in the wall rocks

tectonic setting of the jurassic smartville and slate creek complexes northern sierra nevada california jurassic ophiolitic complexes in california and oregon are widely distributed outboard of a jurassic and older continental margin whether these outboard complexes were exotic and collided with the jurassic continental margin or whether they were simply part of a wide forearc to a jurassic continental magmatic arc has long been at issue we have studied two such complexes and crosscutting intrusions in the fault bounded central and western belts of the northern sierra nevada we report new and revised u pb zircon dates for the early jurassic and younger slate creek complex ca 203 171 ma in the central belt the late jurassic smartville complex 159 3 ma in the western belt and crosscutting intrusions ranging from middle jurassic to early cretaceous age the new ages imply that the central and western belts have been part of the same terrane since middle jurassic time and that the fault separating them has accommodated only minor displacements since ca 160 ma isotopic compositions of common pb indicate derivation from mantle sources but evidence for inheritance of pre cambrian zircon is common in plutons within the early and late jurassic complexes and in middle jurassic late jurassic and early cretaceous crosscutting intrusions although there are several potential sources for such contamination the ages of the inherited zircon are consistent with a north american origin this contamination would have been accomplished most plausibly near the north american margin

timescales of orogeny jurassic construction of the klamath mountains an electronic supplement of this material may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type exit to leave the system paper 94ycj2454 timescales of orogeny jurassic construction of the klamath mountains b r hacker m m donato c g barnes m o mcwilliams and w g ernst diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order classical interpretations of orogeny were based on relatively imprecise biostratigraphic and isotopic age determinations that necessitated grouping apparently related features that may in reality have been greatly diachronous isotopic age techniques now have the precision required to resolve the timing of orogenic events on a scale much smaller than that of entire mountain belts forty five new 40ar 39ar ages from the klamath mountains illuminate the deformation metamorphism magmatism and sedimentation involved in the jurassic construction of that orogen leading to a new level of understanding regarding how preserved orogenic features relate to ancient plate tectonic processes the new geochronologic relationships show that many jurassic units of the klamath mountains had 200 ma or older volcanoplutonic basement subsequent formation of a large 170 ma arc was followed by contractional collapse of the arc collision with a spreading ridge may have led to large scale nw se extension in the central and northern klamaths from 167 to 155 ma coincident with the crystallization of voluminous plutonic and volcanic suites marked cooling of a large region of the central klamath mountains to below 350 c at 150 ma may have occurred as the igneous belt was extinguished by subduction of colder material at deeper structural levels these data demonstrate that the klamath mountains and perhaps other similar orogens were constructed during areally and temporally variant episodes of contraction extension and magmatism that do not fit classical definitions of orogeny

paleogeographic significance of a permian sedimentary megamictite in the central belt of the northern sierra nevada the central belt of the northern sierra nevada at oroville lake california includes large outcrops of permian limestone as clasts surrounded by siliceous diamictite limestone ranges from pebbles to megaboulders as much as 100 m in size and five clasts contain wolf campian to leonardian corals and fusulinids the diamictite which encloses limestone clasts has a pebbly mud texture and a chaotic fabric paleozoic megafossils occur in the mud matrix of the diamictite and mixing of limestone clasts and diamictite was a permian event the diamictite and enclosed limestone megaboulders collectively termed a megamictite probably represent a base of slope sedimentary deposit limestone boulders and megaboulders are tabular stratigraphic fragments which detached from lithified carbonate escarpments and moved downslope by sliding along their flat surfaces lithologies of limestone clasts indicate derivation from a shallow carbonate platform corals and fusulinids within limestone clasts include endemic taxa found in the mccloud limestone of the eastern klamath mountains and they represent a permian biogeographic province distinct from those of western terranes of the klamath mountains and the north american craton a large carbonate platform in the eastern klamath area is proposed as the source of limestone clasts in the central belt at oroville lake

paleovalley morphology and fluvial system evolution of eocene oligocene sediments auriferous gravels northern sierra nevada california implications for climate tectonics and topography in the northern sierra nevada california eocene oligocene fluvial sediments the auriferous gravels rest unconformably in well defined basement incised paleovalleys providing a record of paleolandscape geomorphology and drainage system evolution paleovalley surface mapping shows that local topography and bedrock lithology controlled the location of broad terraced paleovalley segments which formed in the central and eastern metamorphic bedrock belts and subsequently became areas of significant fluvial deposition high gradient high energy deposits are present near the edges of these areas suggesting that tributaries and outlets resided in steeper narrow paleovalleys multiple strath terraces are common in broad paleovalleys indicating that incision events occurred both before deposition and during paleovalley widening followed by periods of aggradation broad paleovalley deposits contain predominantly braided stream facies deposited in an energetically variable system multiple upward fining cycles bounded by erosional surfaces are composed of alternating coarse grained channel belt and fine grained floodplain packages an overall upward fining succession suggests that the locus of fluvial deposition shifted and depositional energy waned over time as the system backfilled successive broad paleovalleys paleochannel slope estimates of 0 004 0 055 were obtained based on average grain sizes of coarse channel fills anomalously thick up to 11 m laterally continuous 1 3 km floodplain units containing kaolinite rich clay and oxisol intervals record high fine grained sediment supply driven by high rates of chemical weathering in the warm eocene climate based on sedimentary characteristics combined with floral and detrital zircon data braided stream deposition within the basin occurred from the late early eocene through late eocene and aggradation primarily occurred in broad lower gradient reaches behind a major bedrock knickpoint created in the late mesozoic early cenozoic possibly due to asymmetric batholith unroofing the diachronous localized nature of paleovalley incision and braided stream aggradation within the northern sierra nevada invalidates previous estimates of the timing and amount of range uplift based on paleochannel reconstructions and gradients both internal and external drivers influenced the evolution of the fluvial system through 1 the pattern of basin filling as the locus of deposition migrated east to west 2 the warm eocene climate which increased chemical weathering and contributed a high sediment supply and 3 changes in base level due to eustasy and or laramide tectonism this record of laramide tectonics in the sierra nevada and basin and range is important for the timing of sierra nevada uplift for tectonic reconstructions of western north america and for studies of landscape evolution in tectonically active orogens

nd sr pb systematics and age of the kings river ophiolite california implications for depleted mantle evolution sm nd whole rock and mineral data for the kings river ophiolite define two isochrons of 485 21 ma and 285 45 ma age with end 483 10 7 0 5 and end 285 9 9 1 1 respectively the 483 ma isochron is defined by samples of the main igneous construct samples from crosscutting diabase dikes and flaser gabbro sheets within the peridotite unit yield the 285 ma isochron the 483 ma data provide the first evidence of lower paleozoic oceanic crust in the sierran ophiolite belt new u pb analyses of zircons from a plagiogranite lying on the 483 ma sm nd isochron yield upper and lower intercepts with the concordia of 430 60 200 and 183 15 ma published zircon ages have underestimated the primary age of the ophiolite by 200 300 m y due to the effects of polymetamorphism the 483 ma samples have initial 87sr 86sr 0 7023 0 7030 206pb 204pb 17 14 17 82 207pb 204pb 15 37 15 52 208pb 204pb 36 80 37 38 the 285 ma samples have similar initial 87sr 86sr but more radiogenic pb the range in sr and pb compositions is probably due to introduction of radiogenic sr and pb during multiple post emplacement metamorphic events the high end low 87sr 86sr 206pb 204pb 207pb 204pb 208pb 204pb of the least disturbed samples are clearly diagnostic of a midocean ridge origin for the 483 ma portion of the ophiolite igneous activity at 285 ma is thought to have occurred in an arc or back arc setting or perhaps along a leaky transform the initial end 483 10 7 is indistinguishable from that of the similar age trinity peridotite jacobsen et al 1984 this value is the highest yet reported for the mesozoic or paleozoic depleted mantle and requires either a mantle source that was depleted 850 m y earlier than average or a source more highly depleted than average alternatively if such values were more typical of the early paleozoic mantle than is currently thought then there has been little evolution of the depleted mantle over the last 500 m y this requires that the modern mantle has been refluxed by material with low end such as continental crust

accretion of the franciscan complex attending jurassic cretaceous geotectonic development of northern and central california scattered igneous activity marks the triassic margin of california but an andean volcanic plutonic arc began to form in the sierra nevada white inyo range and klamath mountains at ca 170 ma during underflow of the oceanic lithosphere this calc alkaline belt supplied volcanogenic erosional debris to the mariposa galice proximal overlap sequence by ca 165 160 ma transpression also generated high pressure low temperature basaltic eclogites garnet blueschists and amphibolites along the middle and late jurassic convergent plate junction at ca 170 155 ma most of these returned surfaceward as tectonic blocks only during the mid cretaceous the high grade metamafic metamorphosed mafic blocks that formed during construction of the emergent arc and derived mariposa galice strata predate the onset of franciscan trench deposition near the end of jurassic time the klamath salient evidently migrated 200 km westward an earliest cretaceous westward stepout of the convergent junction apparently formed directly offshore from the klamath imbricate orogen but to the south preexisting oceanic crust capped lithosphere was trapped landward as the coast range ophiolite andean arc detritus began to accumulate on this ophiolitic basement within the great valley forearc and outboard franciscan oceanic trench at ca 145 140 ma because earliest cretaceous relatively continuous great valley strata were deposited on the stable north american plate protected from both surface and subcrustal erosion forearc terrigenous sedimentation also signaled coeval deposition in the outboard trench 10 25 m y after middle and late jurassic initiation of the continental margin calc alkaline arc voluminous sedimentation and the accretion of franciscan and san joaquin sections of great valley rocks took place during mid and late cretaceous time after a flare up in igneous arc activity at 125 120 ma the youngest sierran granites have ca 85 ma ages reflecting extinction of the magmagenic zone beneath northern and central california and presumably reflecting subhorizontal oceanic plate underflow attending the laramide orogeny feebly zeolitized upper cretaceous and lower tertiary franciscan coastal belt rocks derived from the now inactive arc underwent only low pressure burial metamorphism reflecting shallow offloading and or large scale transpression they were not appreciably subducted although coastal belt units are members of the franciscan lithotectonic assemblage their origin contrasts with the rest of the subduction complex

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

u pb sm nd and rb sr geochronological and isotopic study of northern sierra nevada ophiolitic assemblages california distinct ophiolitic assemblages occur as oceanic basement within three of the four regional tectonic belts of the northern sierra nevada new u pb zircon sm nd and rb sr data are presented for each assemblage providing critical geochronological and isotopic constraints on the petrogenesis and tectonic evolution of the ophiolitic and associated ensimatic assemblages ophiolitic assemblages include from west to east the smartville complex central belt and feather river belt the smartville complex represents an island arc volcanic plutonic sequence with a major late stage sheeted dike swarm the sm nd systems from a wide compositional spectrum of rocks record a 178 21 ma petrogenetic age and an nd t 9 2 0 6 zircon u pb systems on an uppermost dacite yield a 164 2 ma age and on a number of plagiogranite screens and dikes from the sheeted complex 162 1 ma ages the central and feather river belts are structurally complex polygenetic assemblages the u pb zircon and sm nd systems record major 205 ma and 315 ma petrogenetic events respectively both involving depleted mantle derived magmas such magmatism probably occurred in marginal basin transform systems developed within an older oceanic depleted mantle basement regime both sm nd and u pb zircon systems show local components of proterozoic sialic material the sialic contaminants were probably introduced into the system as craton derived detritus it is doubtful that any of the ophiolitic assemblages studied represent genetically related crust upper mantle sequences generated during the development of new oceanic lithosphere integration of the geochronological data with geological relations reveals a pattern of petrogenesis and tectonics whereby progressively younger ensimatic terranes were added to the continental margin through time by plate convergence and were ultimately welded into north american sial by a crosscutting batholithic belt this accretionary pattern is reflected in both the protolith ages and deformation metamorphic ages of each of the regional belts which progressively young westward crustal components of the accreted ensimatic terranes grew by mainly basaltic igneous activity within island arc marginal basin and leaky transform systems adjacent to the continent edge prior to final tectonic accretion such complexities are suggested to be typical of cordilleran type ophiolites and representative of the circum pacific erogenic style

a nd and sr isotopic study of the trinity peridotite implications for mantle evolution field evidence indicates that the trinity peridotite was partially melted during its rise as a part of the upwelling convecting mantle at a spreading center a sm nd mineral isochron for a plagioclase lherzolite yields an age t 427 32 ma and initial e nd 10 4 0 4 which is distinctly higher than that expected for typical depleted mantle at this time this age is interpreted as the time of crystallization of trapped melt in the plagioclase lherzolite p t field this time of crystallization probably represents the time when the massif was incorporated as a part of the oceanic lithosphere the sm nd model age of the plagioclase lherzolite total rock is t chur nd 3 4 ae this suggests that the trinity peridotite was derived from a mantle that was depleted rather early in earth history the peridotite contains many generations of pyroxenite dikes and some microgabbro dikes we report data for two dikes that clearly crosscut the main metamorphic fabric of the peridotite a microgabbro dike yields a sm nd mineral isochron age of t 435 21 ma and e nd 6 7 0 3 a pyroxenite dike yields an initial e nd 7 3 0 4 the initial e nd values for the pyroxenite and gabbro dikes are fairly similar to those for the depleted mantle at this time and are distinct from the lherzolite demonstrating that they are not genetically related rb sr data do not give any coherent pattern however some bounds can be put on initial sr values of e sr 21 for the plagioclase lherzolite and e sr 8 7 for the microgabbro dike it is plausible that the dikes represent cumulates left behind from island arc magmas that rose through the the oceanic lithosphere within the vicinity of a subduction zone major and trace elements and sm nd isotopic data indicate a multiple stage history for the trinity peridotite a small melt fraction was extracted from an undepleted source 3 4 ae or more ago to produce the proto lherzolite a large fraction of melt 12 to 23 was extracted from the proto lherzolite to produce the present rock the lherzolite was then crosscut by dikes from average depleted mantle 0 44 ae ago the data are compatible with the depleted mantle source being formed very early in earth history although most available data indicate that the depleted upper mantle has been relatively well stirred through time the trinity data suggest that very ancient nd isotopic values are preserved and thus chemical and physical heteorgeneities are sometimes preserved in the depleted source of mid ocean ridge basalts as well as the oceanic lithosphere which they intrude

origin and emplacement of ophiolites ever since their recognition ophiolites have been a source of controversy although all workers have agreed upon their importance a number of conflicting schools of thought have become unified in the interpretation of ophiolite complexes as fragments of oceanic crust and mantle formed at spreading centers an expanded version of the penrose ophiolite definition is proposed herein called the ophiolite association taking into account rocks which bear upon the origin and significance of ophiolite complexes an ideal ophiolite association includes the following units from bottom to top 1 a crystalline basement and shallow water sedimentary sequence 2 a tectonic unit of thrust slices of continental margin and rise and abyssal sediments and or melange 3 a metamorphic unit as much as a few hundred meters thick generally with higher grade rocks over lower grade ones 4 ultramafic tectonite unit composed dominantly of multiply deformed peridotite dunite and minor chromitite 5 a cumulate complex ultramafic at the base grading to mafic or intermediate at the top 6 a noncumulus unit of varitextured gabbro and minor trondhjemite 7 a sheeted dike complex 8 an extrusive section of massive and pillowed flows and intercalated sediments 9 an abyssal or bathyal sediment sequence which may include radiolarian chert red pelagic limestone metalliferous sediments breccias and or pyroclastic deposits and 10 postemplacement deposits of laterite reef limestone or shallow marine or subaerial sediments this expanded ophiolite association allows greater precision in inferring original origin and emplacement some ophiolite complexes particularly in the tethys herein designated tethyan type exhibit units 1 2 3 and 4 whereas others particularly in the cordilleran belt herein termed cordilleran type lack units 1 2 3 and in places 4 the emplacement of the former probably occurred during collision of a passive margin with a subduction zone or incipient subduction zone emplacement of the cordilleran type generally is not clear cut the nature of the spreading center which produced a given ophiolite is indicated more clearly by the presence or absence of units 5 9 than by chemistry or mineralogy some ophiolite complexes lack complete sections and may have formed along transform faults others contain volcanic pyroclastic deposits instead of pelagic sedimentary sequence and probably formed within or near an arc several ophiolite complexes may have originated by conversion of a transform fault into a subduction zone such conversion may result from normal plate evolution or from major plate reorganization major times of ophiolite emplacements appear to correspond with global plate reorganizations

hot and deep rock record of subduction initiation and exhumation of high temperature high pressure metamorphic rocks feather river ultramafic belt california abstract studies of a 10 to 300 m thick unit of high grade metamorphic rock external schists that crops out along the western border of the feather river ultramafic belt frb northern california yield new insights into subduction initiation and ophiolite emplacement processes the high temperature t foliation of the external schists dip moderately to steeply eastward beneath the ultramafic rocks of the frb a 150 km long slab of suboceanic upper mantle and the high t fabric shows a tops to the west frb side up sense of shear the structurally highest external schists record peak metamorphic conditions of 650 760 c at 1 3 2 2 gpa in contrast sheeted dikes of the devil s gate ophiolite that overlie the ultramafic rocks yield metamorphic conditions of 710 730 c at about 0 3 0 7 gpa a km scale lens of amphibolite within ultramafic rocks yields somewhat lower pressures than the structurally highest external schist as do the structurally lower rocks within the external schists significant exhumation of the external schists relative to the structurally overlying ophiolitic rocks occurred along at least two major zones and the most significant exhumation was accommodated at least 1 5 km structurally above the ultramafic external schist contact based on available geochronology intraoceanic subduction may have initiated at approximately 240 ma and exposure of the external schist occurred prior to the deposition of rocks in the structurally highest part of the calaveras complex minimum 177 ma a subduction complex that structurally underlies the external schists high t metamorphism of the devil s gate ophiolite may have resulted from partial failed ridge subduction

isotopic constraints on the origin of appalachian mafic complexes isotopic analyses of modern oceanic basalts and ophiolites have shown that both modern and ancient oceanic crust have a characteristic nd and sr isotopic signature indicative of derivation from a depleted mantle reservoir it also appears that the nd isotopic system is not appreciably disturbed by metamorphism these isotopic characteristics have been extended to the pt sal kings kaweah and josephine ophiolites of california we have used these characteristics in an attempt to identify pieces of proto atlantic oceanic crust among the mafic and ultramafic rocks of the appalachians sm nd mineral isochrons for the baltimore mafic complex maryland bmc yield an age of 490 20 my which we interpret as the igneous crystallization age bmc whole rock samples do not define isochrons and have initial isotopic compositions of 6 4 nd t 2 2 51 sr t ll5 nd t and sr t are anti correlated this is not the signature of depleted mantle and oceanic crust hut is similar to old continental crust we propose that the bmc is a mafic continental intrusion possibly subduction related which was contaminated with old continental crust during emplacement whole rock samples from the thetford mines complex qe tmc do not define isochrons and have 1 5 nd t 4 2 2 6 sr t 114 these data do not in any way reflect the signature of normal oceanic crust these results are in contrast with geologic relationships that show the tmc to have the characteristics of an ophiolite complex the tmc is chemically and isotopically similar to a class of other ophiolites that have affinities to modern honinites the chunky gal amphibolite north carolina lake chatuge complex north carolina and hazen s notch amphibolite vermont were found to have a depleted mantle signature with 5 nd t 8 and may he fragments of oceanic crust the webster addie body north carolina has nd t 1 sr t 30 and is not isotopically similar to oceanic crust or the other north carolina mafic bodies analyzed from these isotopic results we infer that appalachian mafic rocks have diverse origins some are continental intrusives bmc others are probably fragments of oceanic crust vermont and north carolina amphiholites future models for the development of the appalachians must allow for these various origins the possibility that some onhiolites are not normal oceanic crust but have an origin in a partially continental setting or as anomalous oceanic crust may require further attention

age relationships and depositional environments of paleozoic strata northern sierra nevada california upper paleozoic volcanic rocks in the northern sierra nevada rest unconformably on the lower paleozoic shoo fly complex late ordovician conodonts recovered from an exotic limestone block in shoo fly melange have north american affinities the complex may have traveled little from its original location magmatism may have begun as early as early devonian and persisted into early mississippian time the volcanic rocks constitute a longitudinal section through a relatively immature volcanic arc dominated by tholeiitic magmas two lithologically distinct members of the overlying peale formation are given formation status the predominantly volcanogenic lower member is renamed the keddie ridge formation the name peale formation is retained for the upper chert member two new late mississippian early pennsylvanian radiolarian localities in the peale chert at the north end of the paleozoic outcrop belt correlate well with a previously described assemblage at the south end and thus rule out time transgressive deposition early permian fusilinids in a chert pebble conglomerate only 10 m upsection from an early pennsylvanian radiolarian locality tightly constrain a regional depositional hiatus the peale chert displays common soft sediment deformation slump structures intraformational breccias and abrupt termination of thick sequences along strike locally the chert is intruded by tholeiitic basalt these features suggest deposition in small tectonically active basins possibly in an extensional back arc regime the overlying permian sequence of arc tholeiites is truncated by mesozoic strata in most areas but appears to grade into triassic shale and limestone in the extreme northwest

age origin and significance of an unconformity that predates island arc volcanism in the northern sierra nevada the paleogeographic position of rocks of the northern sierra nevada relative to north america has long been the topic of debate a detailed study of the lakes basin region of the northern sierra nevada substantiates that the shoo fly complex of early paleozoic age was folded and unconformably overlain by an island arc sequence in late paleozoic time microfossils recovered during the study represent the first in situ fauna ever collected from the shoo fly complex and provide an ordovician silurian lower limit on the early deformation late jurassic nevadan deformation strongly affected all rocks in the region regional considerations suggest that mid paleozoic deformation may have been widespread in the sierra nevada and could indicate an early phase of an eastward prograding suture that ultimately resulted in emplacement of the roberts mountain allochthon of the great basin

geochemistry and tectonic setting of the ophiolitic ingalls complex north cascades washington implications for correlations of jurassic cordilleran ophiolites abstract this study synthesizes new and existing chemical data to interpret the tectonic setting of the late jurassic ingalls complex of the northwest cascades washington and to compare it with other cordilleran ophiolites of similar age mafic rocks of the ingalls complex represent a spectrum of magma types including normal type and enriched mid ocean ridge basalt within plate basalt and island arc basalt based on mafic magma chemistry compositions of cr spinels in associated peridotite and field relations the ingalls complex is a suprasubduction zone ophiolite formed in a back arc basin cut by an oceanic fracture zone regional tectonic relations also support this hypothesis our synthesis of available chronologic geochemical and stratigraphic data for all jurassic cordilleran ophiolites shows that the ingalls complex has similarities and differences with each the most compelling likeness is between the ingalls complex and josephine ophiolite both of which exhibit a wide range of magmatic af

pleistocene glaciations in chukotka russia moraine mapping using satellite synthetic aperture radar sar imagery abstract using satellite synthetic aperture radar sar imagery we are able to identify many glacial features on the russian landscape that were previously unobservable by scientists outside russian institutes observations from sar imagery provide information on glacial extent moraine morphology and non sedimentary glacial features such as valley morphology and flow direction moraine sequences in several areas of chukotka record multiple pleistocene glacial events moraine position along with several morphologic characteristics such as relative degradation and surface slope can be determined from sar imagery and used to estimate relative age of glacial features by conducting comparative studies on moraines of known age in alaska we are able to estimate the ages of moraines in russia and develop a framework for the correlation of glacial history across bering strait our study of these images suggests that the last major glaciation late wisconsin was much less extensive than older pleistocene events whose marks remain on the landscape beyond the younger moraines

significance of mid mesozoic peridotitic to dioritic intrusive complexes klamath mountains western sierra nevada california a mid mesozoic plutonic suite ranging from ultramafic gabbroic rocks to dioritic rocks and commonly intruded by younger granitoids is widespread in the klamath mountains western sierra nevada california the ultramafic gabbroic rocks are clinopyroxene rich and commonly vary from wehrlite to olivine hornblende clinopyroxenite and melagabbro associated dioritic rocks include biotite two pyroxene diorite monzodiorite and the granitic rocks typically range from tonalite to granodiorite these plutonic complexes are coextensive and broadly coeval with a suite of weakly metamorphosed volcanic rocks ranging in composition from basalt to basaltic andesite to andesite the volcanic rocks are chiefly volcaniclastic and are characterized by several distinct phenocryst assemblages clinopyroxene plagioclase olivine or hornblende the plutonic complexes and the associated volcanic rocks are restricted to terranes that lack continental sialic crust and that at least locally contain juvenile ophiolitic crust generated adjacent to the locus of magmatism thus their petrogenesis may be related to recurrent fracturing and magmatism within a rifted ensimatic arc

time relations and structural stratigraphic patterns in ophiolite accretion west central klamath mountains california new geochronological data and published structural and stratigraphic data show that two distinctly different ophiolitic assemblages formed in general proximity to one another at nearly the same time and were subsequently imbricated along a regional thrust zone the josephine ophiolite constitutes a complete oceanic crust and upper mantle sequence which lies within the western jurassic belt of the klamath province within the study area the josephine ophiolite was formed by seafloor spreading at about 157 m y before present it was immediately covered by a thin pelagic and hemipelagic sequence which grades into a thick flysch sequence both of which comprise the galice formation the galice flysch was derived from volcanic arc and uplifted continental margin orogenic assemblages a major nonvolcanic source for the galice flysch appears to have been the western paleozoic and triassic belt of the klamath province exposed to the east proximal volcanic arc activity migrated to the site of the josephine galice section by 151 m y and is represented by numerous dikes and sills which intrude the ophiolite and galice formation the preston peak ophiolite is a polygenetic assemblage consisting of 1 a pre mid jurassic tectonitic peridotite amphibolite substrate which represents disrupted and unroofed basement of the western paleozoic and triassic belt and 2 an upper mafic complex which was intruded through and constructed above the tectonite substrate at about 160 m y the mafic complex consists primarily of diabase hypabyssal rocks that are overlain by diabase clast breccia and hemipelagic deposits a major arc plutonic complex was emplaced into the preston peak ophiolite in at least two pulses at 153 and 149 m y major phases of this complex consist of wehrlite gabbro pyroxene diorite and hornblende diorite the josephine ophiolite is interpreted as the remnants of interarc basin crust the preston peak ophiolite is interpreted as either a primitive remnant arc complex or a rift edge facies for the josephine interarc basin the galice formation represents a submarine fan complex that was built on juvenile crust of the josephine basin floor during the time interval of 153 to 149 m y the locus or arc magmatism migrated to an area which included the interarc basin floor and the remnant arc or basin edge the basin shortly thereafter closed by convergent tectonics during the nevadan orogeny resulting in the imbrication of the josephine and preston peak ophiolites and their superimposed arc assemblages the transition from seafloor spreading generation of josephine ophiolite to its tectonic accretion by convergence and basin closure occurred within 5 to 10 m y the process of rifting and ophiolite formation in series with convergence and ophiolite accretion is considered an important mechanism for generating and displacing allocthonous terranes in the klamath mountains sierra nevada region and perhaps throughout the western cordillera

contrasting early and late mesozoic petrotectonic evolution of northern california devonian middle jurassic terrane assemblies in the klamath mountains and sierra nevada foothills consist chiefly of ophiolite chert argillite sequences mafic ultramafic complexes are oceanic whereas associated fine grained deep water terrigenous sediments were derived mainly from adjacent previously docked klamath sierran terranes coeval calc alkaline arc rocks are volumetrically rare geologic and petrochemical relations suggest a rifted arc origin for klamath mafic metavolcanic units inter layered with distal turbidites in the 170 200 ma north fork terrane detrital zircon u pb ages indicate that the clastic debris had a regional eastern klamath source the eastern hayfork cherty melange contains ophiolitic scraps and distinctive olistostromal sandstone blocks evidently derived from the nearby eastern klamath antelope mountain quartzite the seaward 200 ma rattlesnake creek terrane is an ophiolitic melange with north fork petrotectonic affinities the north fork eastern hayfork rattlesnake creek amalgam correlates with the calaveras complex and the outboard jura triassic arc belt in the sierran foothills geochemical bulk rock and zircon u pb age data support interpretation of the 200 ma jura triassic arc as an adjacent offshore mafic belt overlying a 300 ma ophiolitic basement these oceanic complexes were sutured against the central metamorphic belt eastern klamath feather river northern sierra terrane backstop before deposition and deformation of the outboard upper jurassic galice and mariposa formations klamath sierran terrane assemblies reflect 230 m y of transpression transtension involving only minor episodes of subduction producing ubiquitous ophiolite chert argillite lithologies and rare felsic arc rocks in contrast the late jurassic to largely cretaceous klamath sierra nevada quartzo feldspathic volcanic plutonic arc attests to massive calc alkaline magmatism attending a strong eastward component of underflow by the farallon plate the coeval galice mariposa formations followed by the cretaceous great valley forearc and franciscan trench deposits are first cycle felsic debris shed mainly from the klamath sierran arc these units record 70 m y of rapid sialic crustal growth attending major periods of approximately margin normal convergence this profound transition in northern california included devonian middle jurassic rifting drifting and stranding of ophiolite chert argillite terranes along an adjacent curvilinear continental margin then nearly head on cretaceous subduction that resulted in massive calc alkaline igneous activity the erosion of which generated the felsic great valley group forearc basin and franciscan complex trench clastic sedimentary units

arc related sodic hydrothermal alteration in the western united states sodium rich hydrothermal alteration is widely developed in permian to jurassic arc igneous rocks of the western united states but it is rare in younger rocks na rich alteration reflects a paleogeographic control on development two varieties naca and na can be defined by mineralogic and compositional changes both are common in permian to jurassic arc plutonic and volcanic rocks and are accompanied by fe and cu zn mineralization geologic relations and mineralogic stable isotopic and fluid inclusion data indicate that the fluids responsible for sodium rich alteration were isotopically heavy moderately to highly saline fluids of marine formation and or meteoric origin with or without a magmatic component the transition away from widespread na ca rich alteration in the late mesozoic corresponds to and is consistent with the change from an early mesozoic marine and or arid rifted arc setting to a fully emergent continental arc environment in the cretaceous

radioisotopic and biostratigraphic age relations in the coast range ophiolite northern california implications for the tectonic evolution of the western cordillera the coast range ophiolite cro in northern california includes two distinct remnants the elder creek ophiolite is a classic suprasubduction zone ophiolite with three sequential plutonic suites layered gabbro wehrlite pyroxenite quartz diorite a mafi c to felsic dike complex and mafi c felsic volcanic rocks the entire suite is cut by late mid oceanic ridge basalt morb dikes and overlain by ophiolitic breccia the stonyford volcanic complex sfvc comprises three volcanic series with intercalated chert horizons that form a submarine volcano enclosed in sheared serpentinite structurally below this seamount are melange blocks of cro similar to elder creek u pb zircon ages from plagiogranite and quartz diorites at elder creek range in age from 165 ma to 172 ma u pb zircon ages obtained from cro melange blocks below the sfvc are similar 166 172 ma 40 ar 39 ar ages of alkali basalt glass in the upper sfvc are all younger at 164 ma radiolarians extracted from chert lenses intercalated with basalt in the sfvc indicate that the sedimentary strata range in age from bathonian unitary association zone 6 6 of baumgartner et al 1995a near the base of the complex to late callovian to early kimmeridgian unitary association zones 8 10 in the upper part the sfvc sedimentary record preserves evidence of a major faunal change wherein relatively small sized polytaxic radiolarian faunas were replaced by very robust oligotaxic nassellarian dominated faunas that included praeparvicingula spp we suggest that cro formation began after the early middle jurassic 172 180 ma collision of an exotic or fringing arc with north america and initiation of a new or reconfi gured east dipping subduction zone the data show that the cro formed prior to the late jurassic nevadan orogeny probably by rapid forearc extension above a nascent subduction zone we infer that cro spreading ended with the collision of an oceanic spreading center ca 164 ma coincident with the oldest high grade blocks in the structurally underlying franciscan assemblage we further suggest that the classic nevadan orogeny represents a response to spreading center collision with shallow subduction of young lithosphere causing the initial compressional deformation and with a subsequent change in north american plate motion to rapid northward drift j2 cusp causing sinistral transpression and transtension in the sierra foothills these data are not consistent with models for late jurassic arc collision in the sierra foothills or a back arc origin for the cro

polygenetic ophiolite belt of the california sierra nevada geochronological and tectonostratigraphic development the assumption that ophiolite sequences are generated at essentially one point in geologic time by the process of sea floor spreading is critical for modern concepts in the tectonics of ophiolites and for topics dealing with their structure and petrology however this assumption has only been verified in a few locations by an integrated geochronological and structural stratigraphic approach many ophiolite sections are reconstructed from structurally disrupted sequences with the idealized ocean floor model in mind such reconstructions are prone to error without adequate age control on each of the reconstructed fragments this is a significant problem in structurally complex regions where more than one generation of ophiolite may be present in this paper new pb u zircon ages are presented for key locations along a 375 km segment of the western sierra nevada ophiolite belt these age data are combined with structural stratigraphic observations and published ages and significant tectonic implications for the ophiolite belt emerge three different ophiolitic assemblages are recognized with igneous ages of about 300 200 and 160 m y b p rocks of the 300 m y assemblage are in a completely disrupted array of metamorphic tectonite slabs and serpentinite matrix melange fragments of upper paleozoic seamounts occur in association with the ophiolitic melange and together these assemblages constitute the basement framework for the western sierra pb u and k ar isotopic systematics are complex within this framework and indicate a polymetamorphic history systematics in the 200 and 160 m y assemblages are less complex and give tighter igneous age constraints rocks of the 200 m y assemblage are in a semi intact state with only local tectonite and melange zones rocks of the 160 m y assemblage are intact but nevertheless deformed both the 200 and 160 m y assemblages have equivalent age basinal volcanic sedimentary sequences that lie unconformably above the ophiolitic melange basement in each case the basinal sequences locally extend conformably into the upper stratigraphic levels of the age equivalent ophiolite sections these relations along with vestiges of intrusive contacts between the edges of both younger ophiolites and the melange basement indicate that the younger ophiolites underwent igneous formation in proximity to the melange basement the sierran ophiolite belt is considered to have formed by a multistage process initiated by the early mesozoic tectonic accretion of upper paleozoic sea floor in general proximity to the ancient continental margin regional metamorphism and ophiolitic melange resulted this accretionary nucleus became the basement of jurassic age primitive volcanic arc terranes which underwent rifting episodes during the production of the 200 and 160 m y ophiolites the rifting episodes resulted in the formation of sedimentary basins which were the depositional sites of volcanic sedimentary sequences non volcanic sources for the basinal sedimentary rocks include the melange basement and continental margin terranes contact zones between pre existing basement and the juvenile ophiolitic sequences created during the rifting episodes consist of dynamothermal metamorphic aureoles protoclastic deformation zones and cross cutting dikes such edge zone assemblages are in most localities obscurred or destroyed by superimposed deformations resulting from convergent and perhaps transform motions along basin edges both the 200 and 160 m y basins were destroyed by compressional orogenic episodes shortly after their formational episodes destruction of young ophiolite floored basins may be a common course of events when small oceanic type plates are generated along continental margin environments such tectonic settings are ideal for the emplacement of young ophiolite sheets

mafic injections in situ hybridization and crystal accumulation in the pyramid peak granite california located in the northern sierra nevada the middle jurassic pyramid peak granite and associated dioritic rocks display superbly exposed interlayered sheets of mafic felsic and hybrid rocks the mafic hybrid layers show a consistent asymmetry sharply chilled on one side and gradational through hybrids to granite on the other and dip to the southwest load casts and silicic pipes along the chilled mafic northeastern margins of the sheets indicate that depositional way up is to the southwest and that the adjacent mount tallac roof pendant was the original floor of the intrusion orientations of the pipes indicate that the layers were tilted after deposition and solidification we interpret the sheets to be intramagmatic flows that formed by injections of mafic magma into a silicic magma chamber followed by flow across the crystal rich base of that chamber the granitic and hybrid rocks interlayered with the mafic sheets represent material that accumulated on the chamber floor between episodes of mafic replenishment most contamination of the mafic sheets probably occurred because of flow front instabilities generated as the mafic magma entered and spread across the chamber floor the upward gradation from diorite to granite with decreasing abundance and size of mafic enclaves can best be explained by convective stirring at the upper boundaries of the dioritic replenishments the pyramid peak granite was probably initiated as a thin concordant sheet intruding into gently dipping jurassic supracrustal rocks the pluton was constructed incrementally with many replenishments of felsic and mafic magma and dominantly solidified by accumulation of crystals on the floor of the chamber the present steep dips of the layers suggest that downward warping of the chamber floor may have been a factor in accommodating the incrementally constructed pluton

tectonic significance of post accretion rifting of a mesozoic island arc terrane in the northern sierra nevada california this paper presents a new interpretation for the tectonic evolution of the smartville complex that forms the western belt of the foothills metamorphic belt in the northern sierra nevada california new geological petrological and geochronological information suggest that the smartville complex may form part of an early jurassic island arc terrane that was probably built on a pre existing oceanic basement inferred oceanic basement is represented by multiply deformed and metamorphosed ophiolitic rocks that crop out in the central belt north and east of the smartville complex that are intruded by numerous lower jurassic dikes of an incipient island arc character lower jurassic volcanic and plutonic rocks of the slate creek and possibly the lake combie complexes in the central belt are interpreted as fragments of the early jurassic arc terrane accretion of the ensimatic arc terrane into the continental margin of north america may have occurred along east vergent thrusting around middle jurassic time and

petrotectonic evolution and melt modeling of the penon blanco arc central sierra nevada foothills california the penon blanco arc of the jurassic triassic arc belt in central california is composed of the jasper point formation penon blanco formation and coeval don pedro intrusive suite all exposed in the core of the cotton creek anticline the jasper point formation consists of 900 m of massive to pillowed lavas and up to 50 m of depositionally overlying chert and transitional basalts it passes upward into the penon blanco formation which is made up of 700 m of crystal lithic basaltic tuff 1 3 5 km of augite rich volcaniclastic rocks and up to 3 5 km of massive to brecciated flows of augite phyric basalt the penon blanco formation is paraconformably overlain by the oxfordian kimmeridgian mariposa formation which provides a minimum age of juxtaposition for the penon blanco arc against the inboard calaveras complex new geochemical data from the penon blanco arc show that the two volcanic suites are geochemically distinct jasper point basalts are tholeiitic and are characterized by high large ion lithophile element lile abundances moderate high field strength element hfse and heavy rare earth element hree abundances and low ti v ratios penon blanco basalts are calc alkaline have higher lile abundances lower hfse and hree abundances and lower ti v ratios geochemical modeling of melt sources indicates that both units formed by melting of a depleted spinel bearing mantle source at 30 60 km depth by low to moderate amounts of partial melting 3 5 for jasper point and 5 7 5 for penon blanco the geochemical modeling and field data suggest that the jasper point basalts are similar to normal mid ocean ridge basalt n morb and were associated with forearc rifting while the penon blanco basalts represent the transition to arc volcanism and head on subduction this model is consistent with interpretations for a single ensimatic arc on the jurassic margin of north america

isotopic age constraints on middle paleozoic deformation in the northern sierra nevada california allochthons of the lower paleozoic shoo fly complex in the northern sierra nevada were assembled and internally deformed prior to formation of a devonian permian island arc sequence u pb data on zircons indicate ages of 423 5 15 ma for a submarine tuff within the uppermost thrust slice of the shoo fly complex and 378 5 10 ma for a granitic intrusion that may be cogenetic with the lower part of the arc sequence these data indicate late early silurian shoo fly deposition and proximity to active volcanism as well as late middle devonian initiation of arc related magmatism

structure and tectonics of an early mesozoic oceanic basement in the northern sierra nevada metamorphic belt california evidence for transform faulting and ensimatic arc evolution the n s trending structural grain of the sierra nevada metamorphic belt bends sharply to an e w orientation along the big bend fault zone at the northern end of the smartville complex multiply deformed and metamorphosed early mesozoic and older rocks of the jarbo gap ophiolite crop out within and north of the big bend fault zone the jarbo gap ophiolite includes serpentinized harzburgite tectonite dunite metagabbro amphibolite and metavolcanic metasedimentary rocks which are commonly separated from each other by steeply dipping and along strike discontinuous fault contacts locally serpentinite occurs as diapiric intrusions and or fault bounded blocks in metavolcanic rocks or is stratigraphically overlain by metavolcanic and metasedimentary rocks metasedimentary rocks include hemipelagic sandstones and mafic talus breccias composed of ophiolitic detritus amphibolite and metagabbro display multiple deformation features associated with mineral assemblages indicating retrograde metamorphism from amphibolite to greenschist facies raser gabbro within the metagabbro unit shows mylonitic bands and shear zones and is intruded by a leucodiorite which has a u pb zircon age of 204 2 ma the internal structure and rock types of the jarbo gap ophiolite characterize a heterogeneous oceanic crust that is consistent with an oceanic fracture zone setting the jarbo gap ophiolite is intruded by 200 ma dikes that have a geochemical composition similar to high magnesium andesites or boninitic lavas of arc volcanoes these intrusions are coeval with calc alkaline plutonic and hypabyssal rocks of the smartville and slate creek complexes and are interpreted as feeders for an incipient early jurassic arc both complexes include 200 ma plutonic and volcanic rocks that are intruded by 160 ma dikes and plutons these relations suggest that the jarbo gap ophiolite may be part of the oceanic basement of a jurassic island arc terrane which includes the smartville and slate creek complexes the e w orientation of the big bend fault zone is interpreted to be a manifestation of a regional tectonic discontinuity separating the northern sierra nevada and the klamath mountains provinces regional reconstructions and correlations and the paleomagnetic data from both regions suggest that this discontinuity is either a sinistral transform boundary between the two provinces or the northern termination of the sierra nevada province which was probably translated northward for nearly 500 km in and or prior to early cretaceous time