Showing papers on "Terrane published in 1984"

Late Caledonian sinistral displacements in Britain: Implications for a three‐plate collision model

Abstract: In recent years, most geotectonic syntheses of the North Atlantic Caledonides have adopted a two plate configuration, with a roughly E-W closure direction between Laurentia and Baltica producing the N-S striking Scandinavian and East Greenland Caledonides and inducing dextral strike-slip along the NE-SW oriented British sector of the Iapetus suture. The North German-Polish Caledonides are a third arm to the Appalachian-N. Atlantic Caledonide orogen. Recently, Ziegler has proposed that the mid-Palaeozoic deformation belts of S. Britain - N. Germany-Poland and also of central Europe were produced by the northward impact of microcontinental fragments onto the southern margin of the already sutured continental mass of Laurentia-Baltica. Some ascribe the whole Appalachian orogen to a sequence of terrane accretion events in mid-Ordovician through mid-Devonian time. A N-S collision direction would induce sinistral shear across the British sector of the Iapetus suture. In this paper, we outline the evidence for sinistral displacements within the British Caledonides and for their age. We conclude that the “non-metamorphic Caledonides” were produced by the northerly accretion of a Cadomian terrane in late Silurian-early Devonian time, and that this event was entirely separate from the earlier Laurentia-Baltica collision which produced the post-Grampian thrust related “metamorphic Caledonides” of Scotland.

P-T paths from garnet zoning: A new technique for deciphering tectonic processes in crystalline terranes

Abstract: A procedure to calculate quantitative rock pressure-temperature ( P-T ) paths based on chemical zoning profiles in garnet, combined with mineral chemistry from the other minerals in the assemblage, has been applied to samples from diverse tectonic settings with the result that the calculated P-T path is shown to be a sensitive monitor of tectonic processes. Terranes where metamorphic recrystallization paths are controlled by uplift and erosion show heating during decompression in the “prograde” P-T path and cooling during decompression in the “retrograde” path. In nappe terranes where hot rocks are emplaced over cool rocks, the upper plate shows cooling during decompression, whereas the lower plate shows heating during compression. In nappe terranes where cool rocks are emplaced over hot, the lower plate shows cooling during compression. Calculation of P-T paths from mineral zoning should provide a powerful new tool for deciphering tectonic processes in crystalline terranes.

New K-Ar dates from basalts and the evolution of the southern Rio Grande rift

Abstract: In the southern Rio Grande rift, two extensional regimes of different origin (but transitional with each other through the Miocene) can be interpreted from structures and rocks formed within the past 28 to 29 m.y. The earlier regime, which began about 28 to 29 m.y. B.P., is characterized by emplacement of “basaltic andesite” flows with relatively high strontium isotope ratios; formation of broad, relatively deep, northwest-trending basins; and incipient uplift of some of the region9s fault-block mountains. This regime appears to have developed in a back-arc setting, perhaps behind a rapidly steepening slab and a westward-sweeping arc system. The younger episode seemingly represents a renewal or acceleration of block faulting and volcanism during the latest Miocene and Pliocene, 9 to 3 m.y. B.P., after a long transitional period during the early and mid-Miocene when volcanism was absent and tectonism was less vigorous. The latest Miocene-Pliocene episode produced the modern northerly-trending rift basins and uplifts, regional uplift of the rift 1 to 2 km above sea level, and renewal of volcanism, this time dominated by relatively primitive alkali-olivine basalt. New basalt dates reveal that in the southern rift, modern ranges and basins were almost fully developed and that near-modern drainage ways were established across uplifts into bolsons by about 5.0 m.y. B.P. An ancestral Rio Grande had extended itself southward into the southern rift by 3 to 4 m.y. B.P., and the river entrenched itself into its modern valley between 0.7 and 0.5 m.y. B.P. Horst-graben development of the southern Basin and Range province, as well as associated basaltic volcanism, swept progressively eastward from southeastern California in the past 20 m.y., culminating in formation of the Rio Grande rift and other fault-block terrane in west Texas, New Mexico, and northern Chihuahua in the latest Miocene and Pliocene. Late Quaternary Basin and Range fault scarps increase in density eastward, which also suggests that more easterly parts of the province are youngest. These relationships support a previous model of an eastward-expanding, slab-free triangle (related to growth of the San Andreas transform), through which mantle upwelling triggers eastward-younging patterns of tectonism, volcanism, and uplift and promotes lithospheric thinning and increased heat flow. Across most of the southern Basin and Range and Rio Grande rift, the horst-graben structures related to growth of this triangle are superimposed on somewhat older (late Oligocene-middle Miocene) extensional terrane that appears to have formed in a back-arc or arc setting.

Crustal evolution in north-east and east Africa from model Nd ages

Abstract: The relationships between the various crustal provinces that comprise north-east and east Africa are far from clear and reflect diverse tectonic and magmatic processes which took place during the Proterozoic. Here we present the results of an Nd isotope study on the major rock units of the Pan-African (1,100–500 Myr BP) terrane. Charnockites from Jabel Uweinat, a basement inlier at the junction of Egypt, Libya and the Sudan, yield middle Archaean model Nd ages, whilst model ages of <1,200 Myr have been obtained in a belt from the Eastern Desert of Egypt to north-west Kenya. This, therefore, represents a westward extension of the juvenile Pan-African crust of the Arabian Shield (Fig. 1). The shield itself is characterized by comparatively rapid crustal growth1, probably in an accreting island arc environment2. Between the two areas, there is a marginal zone of Pan-African rocks with relatively low ɛNd(T) values (see Table 1) due to increased contributions from the pre-Pan African crust. Overall, the Pan-African rocks from north-east and east Africa and those from the Damara of Namibia3 exhibit a wide range of ɛNd(T) from +7.5 to −18.0 which reflects regional changes in tectonic style and is not readily reconciled with simple models for the evolution of average continental crust.

Pre-carboniferous history of the Midland Valley of Scotland

Abstract: The pre-Carboniferous Midland Valley of Scotland comprises three tectonic elements: an arc, a proximal fore-arc basin and a marginal basin. These tectonic elements have been juxtaposed by strike-slip and thrust faulting, both of which have effected a 300% reduction in the width of the orogenic belt.Rocks which span Arenig to Late Devonian or Early Carboniferous times and which are found S of the Highland Boundary fault have no clasts of certain Dalradian provenance despite substantial uplift of the Dalradian block at this time. This, combined with other evidence, suggests the Midland Valley to have been remote from this rapidly uplifting terrane. The Dalradian block, eroded down by c. 410 Ma was thrust southeastwards in Late Devonian–Early Carboniferous times. However, this thrust movement was minor, yielding little sediment, but it caused Dalradian rocks to cover the northern margin of the Midland Valley where (1) the source for part of the Old Red Sandstone rocks existed and (2) the faults along which the Midland Valley block was transported to dock against the Dalradian block are thought to be present. The existing Highland Boundary fault is therefore seen as a late Old Red Sandstone reverse fault which covered more significant older structures.

Middle to Late Jurassic Tectonic Evolution of the Klamath Mountains, California-Oregon

Abstract: The geochronology, stratigraphy, and spatial relationships of Middle and Late Jurassic terranes of the Klamath Mountains strongly suggest that they were formed in a single west-facing magmatic arc built upon older accreted terranes. A Middle Jurassic arc complex is represented by the volcanic rocks of the western Hayfork terrane and consanguineous dioritic to peridotitic plutons. New U/Pb zircon dates indicate that the Middle Jurassic plutonic belt was active from 159 to 174 Ma and is much more extensive than previously thought. This plutonic belt became inactive just as the 157 Ma Josephine ophiolite, which lies west and structurally below the Middle Jurassic arc, was generated. Late Jurassic volcanic and plutonic arc rocks (Rogue Formation and Chetco intrusive complex) lie outboard and structurally beneath the Josephine ophiolite; U/Pb and K/Ar age data indicate that this arc complex is coeval with the Josephine ophiolite. Both the Late Jurassic arc complex and the Josephine ophiolite are overlain by the “Galice Formation,” a Late Jurassic flysch sequence, and are intruded by 150 Ma dikes and sills. The following tectonic model is presented that accounts for the age and distribution of these terranes: a Middle Jurassic arc built on older accreted terranes undergoes rifting at 160 Ma, resulting in formation of a remnant arc/back-arc basin/island arc triad. This system collapsed during the Late Jurassic Nevadan Orogeny (150 Ma) and was strongly deformed and stacked into a series of east-dipping thrust sheets. Arc magmatism was active both before and after the Nevadan Orogeny, but virtually ceased at 140 Ma.

Paleomagnetic and geologic data indicating 2500 km of northward displacement for the Salinian and related terranes, California

Abstract: Results of a pair of paleomagnetic studies of sedimentary rocks in the Salinian terrane of westernmost California indicate a northward displacement of about 2500 km since Cretaceous time Stratigraphic relations suggest that the Salinian terrane became amalgamated with several others by the Late Cretaceous and all underwent the same northward translation This composite terrane (allochthon) became sutured to cratonal North America by early Tertiary time One paleomagnetic study involved Upper Cretaceous turbidites of the Pigeon Point Formation Fine-grained, thin-bedded turbidites yielded the most reliable results, whereas medium- to coarse-grained sandstones are not always stably magnetized Contorted layers resulting from soft sediment deformation usually gave directional results parallel to those of undeformed sedimentary layers, thereby implying a postdepositional remanence process There is no indication of inclination error or other processes affecting the fidelity of the remanent magnetization Paleomagnetic results from four separate locations in the Pigeon Point Formation gave positive fold tests and mean inclination values near 36° A mean geomagnetic latitude calculated from these results is 212°±53°, much more southerly than the value of 465°±23° calculated for Pigeon Point using paleomagnetic pole data from Late Cretaceous rocks from cratonic North America Declination values are not antipodal for normal and reversed polarity sites Analysis of depositional current directions from sandstone beds of the same outcrops shows that the difference is the result of Neogene rotations of small crustal blocks adjacent to the San Gregorio fault In a sequence of Paleocene turbidites that crop out at Point San Pedro, four sample localities yielded positive fold tests with mean inclination values near −40° Declination values, however, were spread over a range between 230° and 320° Comparison with depositional current directions from the same outcrops indicates clockwise rotations related to Neogene right slip on the Pilarcitos fault The mean geomagnetic latitude calculated for the Point San Pedro sediments is 245°±40° and is more southerly than the 433°±25° latitude expected for that location For the Salinian terrane and the west coast of cratonic North America, absolute rates of latitudinal motion were 85 cm/yr northward and 3 cm/yr southward, respectively

Miogeoclines and suspect terranes of the Caledonian–Appalachian Orogen: tectonic patterns in the North Atlantic region

Abstract: The Caledonian–Appalachian Orogen was formed by the closing of a Paleozoic Iapetus Ocean. The continental margins of Iapetus are identified in the deformed early Paleozoic miogeoclines of the Caledonian–Appalachian Orogen. Ophiolitic vestiges of Iapetus, its oceanic plateaus, microcontinents, and volcanic arcs are Caledonian–Appalachian suspect terranes. These were assembled in interior parts of the orogen and locally they were emplaced structurally upon the adjacent miogeoclines.The modern North Atlantic Ocean opened along an axis that traversed the Paleozoic orogen longitudinally. Its opening dispersed the elements of the Paleozoic orogen and led to the present arrangement of disjunct Paleozoic miogeoclines and suspect terranes throughout the North Atlantic borderlands.The western or North American margin of Iapetus is represented by the miogeoclines along the west flank of the North American Appalachians and Caledonides of east Greenland. A small North American miogeoclinal segment occurs in the Britis...

Northward displacement and accretion of Wrangellia: New paleomagnetic evidence from Alaska

Abstract: Wrangellia is a vast terrane that accreted along the Pacific margin of North America from Oregon to central Alaska. Previous paleomagnetic studies of the Triassic volcanic rocks of Wrangellia in south-central Alaska and British Columbia have given paleolatitudes of 10°–17°, which are anomalously low in comparison to results from similar-aged rocks of nuclear North America. The discrepancy in paleolatitude implies substantial northward drift of the terrane relative to the craton. We have extended paleomagnetic sampling to the northern boundary of the terrane in the Mount Hayes and Healy quadrangles of the central Alaska Range. This collection of 46 Triassic lava flows yields a mean paleolatitude of 13.9° (α95 = 3.8°) the polarity of the data and hence the hemisphere of origin are not resolved with certainty. In contrast, Triassic paleomagnetic poles from the stable part of North America predict a paleolatitude of 42.1°N (α95 = 4.6°) for the region. The northernmost fragment of Wrangellia probably reached mainland Alaska in the middle Cretaceous, when Upper Jurassic and Lower Cretaceous flysch within the intervening basin underwent severe deformation. Tertiary transcurrent faulting has modified the positions of Wrangellian fragments in Alaska, but such displacements are probably minor, as indicated by paleomagnetic data from early Tertiary volcanic rocks that overlap the terrane.

Relationships between plate motions and Late Cretaceous to Paleogene magmatism in southwestern Alaska

Abstract: Four major Late Cretaceous to Paleogene magmatic belts have been recognized in southwestern Alaska, and tentative boundaries have been drawn for each The belts indicate minimum ages by which the tectonostratigraphic terranes they overprint must have been linked together, though there has been significant postmagmatic tectonism The Kuskokwim Mountains belt (73–60 my BP), the Alaska Range belt (74–55 m,y BP), and the Gulf of Alaska belt (62–57 my BP) all may have been related to a period of rapid north-northeastward motion of the KuIa plate from about 74 to 56 my BP The angle of convergence was low with respect to the present orientation of the belts, suggesting the possibility of postmagmatic rotation of the belts A higher angle of convergence may have existed with respect to the Alaska Range belt, a probable arc, if postmagmatic rotation resulted from 300–400 km of right-lateral displacement on the Denali fault The Gulf of Alaska belt is in an anomalous forearc position with respect to the Alaska Range belt, but probably formed by a convergence-related mechanism The Kuskokwim Mountains belt is in an apparent back arc position, but the mechanism of its origin is uncertain It may have been part of an unusually broad Alaska Range arc or, less likely, it may have been a separate arc Magmatic quiescence occurred throughout south-western Alaska during a period of plate reorganization from about 56 to 43 my BP, a period which began with extremely rapid northward motion of the KuIa plate Magmatism resumed when the present period of rather slow northwestward Pacific plate motion began at about 43 my BP The Aleutian arc probably formed at this time, and has been episodically active in about the same location ever since

Pre-Grenvillian and Grenvillian lithotectonic regions in eastern Labrador—correlations with the Sveconorwegian Orogenic Belt in Sweden

Abstract: The Trans-Labrador batholith, Groswater Bay Terrane, and Lake Melville Terrane are three major crustal segments located adjacent to or within the Grenville Province in eastern Labrador. Each crustal segment is a distinct lithotectonic entity displaying contrasts with each other in proportions of rock types, structural style, and metamorphic imprint. Together they indicate a unilateral polarity to the region, partly reflecting Grenvillian tectonism, which sliced the region into thrust-bound blocks.In all three crustal segments, an Archean or Aphebian gneissic basement is inferred onto or adjacent to which ca. 1900–1700 Ma supracrustal rocks were deposited. Deformation, metamorphism, and granitoid pluton emplacement were partly coeval with and partly postdated the supracrustal assemblages. In the north, tectonothermal effects can be assigned to Hudsonian–Ketilidian orogenesis but their peak was 50–100 Ma later farther south. Post-tectonic granitoid plutons and layered mafic intrusions were emplaced at about...

Pressures, Temperatures and Metamorphic Fluids Across an Unbroken Amphibolite Facies to Granulite Facies Transition in Southern Karnataka, India

Abstract: A sampling traverse has been made across the late Archaean regional amphibolite-facies to granulite-facies transition in southern Karnataka, India. The traverse extends from the Peninsular Gneiss-Closepet Granite terrane in the north, through the incipient charnockite localities near Kabbaldurga and southwards into the charnockite massifs of the Biligirirangan and Andhiyur Hills.

Geologic and seismic velocity structure of the crust/mantle transition in the Bay of Islands Ophiolite Complex

Abstract: Geological investigations of the Bay of Islands Ophiolite Complex show that while a typical layered ophiolite suite is present, the thickness of major lithologic units is extremely variable from place to place. The composition and internal structure of map-scale (kilometers across) lithologic units as well as the contacts that bound them are laterally variable. Inferred velocity-depth functions of the crust/mantle (“Moho”) transition reconstructed for this terrane as oceanic lithosphere suggest an extremely complex internal seismic structure. The crust/mantle transition from mafic to ultramafic lithologies varies across the ophiolite from a sharp geologic and seismic velocity discontinuity to a complexly interlayered transition zone as much as 3 km thick. The mafic/ultramafic transition is characterized by laterally discontinuous lithologic units on the order of hundreds of meters to a few kilometers thick and up to several kilometers long. These units (megalenses) may be composed of lithologies with higher or lower seismic velocities than those of the surrounding units. Solid-state deformation has produced seismically anisotropic materials in the upper mantle and lower crustal units. Substantial relief and slopes of up to at least 12° over lateral distances of about 10 km occur on the top of the crust/mantle transition zone as well as other geologic unit contacts. Although these types of geological features are striking to the field geologist working in ophiolites and are significant to the understanding of crustal accretion processes, many of them occur on such a small scale that they might go undetected in different types of seismic experiments in contemporary oceanic lithosphere.

Early Palaeozoic metamorphic history of the Midland Valley, Southern Uplands–Longford-Down massif and the Lake District, British Isles

Abstract: A model for the early Palaeozoic metamorphic history of the Midland Valley and adjacent areas to the S in Scotland, England and Ireland is based on the results of new field mapping, thin section petrography, electron probe microanalysis, X-ray diffractometry, conodont and palynomorph colouration and graptolite reflectance measurement.The oldest metamorphic rocks of the Midland Valley of Scotland, excluding xenoliths in post-Silurian lavas, are possibly the blueschist occurrences in the melange unit of the Ballantrae complex. These may be tectonised remnants of (?)pre-Arenig ocean-floor subducted during closure of the Iapetus Ocean. In the early Ordovician, the melange terrane was dynamothermally metamorphosed during obduction of newly-formed ocean crust. The obduction process piled up a thick sequence of various ocean-floor types such that burial metamorphism in parts reached pumpellyite-actinolite facies; elsewhere prehnite-pumpellyite and zeolite facies was attained.Whilst the Midland Valley acted as an inter- or fore-arc basin during the Late Ordovician and Silurian and experienced burial metamorphism, an accretionary prism was formed to the S. Accretion, tectonic burial and metamorphism of ocean-floor and trench sediment was continuous in the Southern Uplands and the Longford-Down massif of Ireland through Late Ordovician to Late Silurian times. Rocks at the present-day surface vary from zeolite facies to prehnitepumpellyite facies. Silurian trench-slope basin sediments can be recognised in part by their lower grade of burial metamorphism. Greenschist facies rocks of the prism probably lie close to the surface.The Lake District island-arc terrane of Northern England has an early Ordovician history of burial metamorphism up to prehnite-pumpellyite facies. The Late Ordovician and Silurian metamorphic history is one of sedimentary burial complicated by tectonism and intrusion of granite plutons to a relatively high level. The Iapetus suture is marked by a weak contrast in metamorphic grade.

A terrane of 1,350- to 1,400-m.y.-old silicic volcanic and plutonic rocks in the buried Proterozoic of the mid-continent and in the Wet Mountains, Colorado

Abstract: Much of the Precambrian basement of the mid-continent region of the United States is underlain by silicic volcanic rocks and related silicic epizonal granite bodies. In the St. Francois Mountains of southeastern Missouri and in the buried basement of Illinois, Indiana, and eastern Ohio, these rocks mainly yield ages of 1,450 to 1,480 m.y. Previously published U-Pb ages from zircons showed that rhyolite and granite in the subsurface of southwestern Missouri, southern Kansas, and parts of Oklahoma were formed 1,350 to 1,400 m.y. ago. New U-Pb age data from zircons presented here show that this terrane makes up much of the basement of eastern and central Oklahoma, where it includes both rhyolite and granite, and extends into the Texas Panhandle, where the Panhandle Rhyolite Terrane, which was thought to be ∼1,200 m.y. old, yields zircon U-Pb ages of 1,350 to 1,400 m.y. Zircons from the San Isabel batholith of the southern Wet Mountains, Colorado, also yield U-Pb ages of 1,360 m.y. and are thus coeval with the granite and rhyolite of the southern mid-continent region. The San Isabel batholith intruded crust formed at least 1,700 m.y. ago. A possible model is that the granite and rhyolite of the mid-continent are essentially a veneer emplaced upon and within an older, possibly 1,700-m.y.-old crust; the mesozonal San Isabel batholith is a body formed during this igneous activity that is exposed in contact with older crust because of later uplift in the southern Rocky Mountains. The tectonic setting of the emplacement of these rocks is obscure but appears to be continent-marginal, because only younger rocks are known to the south.

Seismic-reflection study of the Precambrian crust of central Minnesota

Abstract: A COCORP (Consortium for Continental Reflection Profiling) seismic-reflection survey across the boundary zone between the late Archean Superior Province granite-greenstone terrane and older Archean gneiss and granulite terrane of the Minnesota River Valley shows numerous north-to-northwest–dipping reflection events throughout the crust. A particularly prominent and continuous series of such events projects to the surface near the trace of a fault zone that had been previously identified as the boundary between these two terranes, within the Great Lakes tectonic zone. The moderate dips of these events (about 30°), their correlation with presumed faults near the surface, their continuity over tens of kilometres, and the persistence of these and other reflection zones throughout the crust suggest that these events correspond to fault zones, probably thrusts. They may have originated during late Archean collision between the Superior Province crust and older continental crust to the south. Reflections are sparser and more concordant in the gneiss and granulite terrane than in the Superior Province. There are several prominent reflection groups with gentle northerly dips in the middle and upper crust of the granulite and gneiss terrane. Their distribution appears to correlate with the sources of aeromagnetic and gravity anomalies that are part of a belt that extends west to central South Dakota and that apparently reveal the position of the postulated Archean suture. The axes of deposition and deformation of Proterozoic metasedimentary rocks are approximately parallel to and apparently coincide with the boundary between two Archean terranes farther east in Minnesota, Wisconsin, and Michigan, but Proterozoic (Penokean) deformation has not been distinguished either in the study area or in the seismic sections. The Minnesota COCORP survey has demonstrated the capability of this seismic-reflection method to reveal important but previously unsuspected structures in complexly deformed Archean terranes. This has provided structural evidence that the separate subprovinces of the North American Archean crust might have been assembled by subduction-related collision.

Newly discovered youngest Cambrian or oldest Ordovician fossils from the Robertson Bay terrane (formerly Precambrian), northern Victoria Land, Antarctica

Abstract: Several species of fossils that indicate an age almost precisely at the Cambrian-Ordovician boundary were collected from a limestone olistolith found in turbidite sandstone and shale from Handler Ridge (lat 72°30.5′S, long 167°03′E), an area previously mapped as late Precambrian Robertson Bay Group. The rocks enclosing the olistolith are therefore of this age or younger, not Precambrian, and their presence in this position has important implications to the early Paleozoic history of the Transantarctic Mountains.

Regional progressive high‐pressure metamorphism, Seward Peninsula, Alaska

Abstract: Blueschist-facies rocks on the Seward Peninsula constitute a structurally coherent terrane measuring at least 100 × 150 km. Radiometric age data indicate that high-pressure metamorphism probably occurred in Jurassic rather than in Palaeozoic or Precambrian time, as previously suggested. Protolith sediments (Nome Group) are of intracontinental basin or continental margin type, and of lower Palaeozoic and possibly late Precambrian age, thus predating the high pressure metamorphism by more than 200 m.y. Blueschist-facies mineral assemblages were developed in almost all lithologies of the Nome Group, and are best preserved in FeTi-rich metabasites (glaucophane + almandine + epidote) and pelites (glaucophane + chloritoid + phengite). A lawsonite–crossite subfacies was developed in possible Nome Group rocks on the east flank of the Darby Mountains. Albite–epidote–amphibolite facies assemblages characterize Nome Group rocks in the southwestern part of the Peninsula. Metamorphism in the central zone of the terrane passed from early lawsonitic to subsequent epidote–almandine–glaucophane schist subfacies with the local development (east of the Nome River) of eclogitic assemblages. The high pressure metamorphic minerals were synkinematic with the development of mesoscopic-scale intrafolial isoclinal folds and a flattening foliation of consistent orientation. Initiation of uplift probably corresponded to the growth of barroisite rims on earlier sodic and actinolitic amphiboles, and partial post-kinematic greenschist facies replacements record later stages of decompression. Ophiolites and melange are not associated with the Seward Peninsula blueschists. The high-pressure metamorphism was caused by tectonic loading of a continental plate by an allochthon of indeterminate origin. The PT conditions of high pressure metamorphism were approximately 9–11 kbar, 400–450°C, thus falling between the PT paths of the Shuksan and Franciscan terranes.

Californian blueschists, subduction, and the significance of tectonostratigraphic terranes

Abstract: Glaucophane and related schists are present as tectonic fragments in ophiolitic suture zones and as discrete lithotectonic belts along the accreted Mesozoic/Tertiary Californian margin. Occurrences include parts of the Klamath Mountains, the western Sierran Foothills, the Coast Ranges, faulted marginal segments of the Mojave Desert, the Transverse Ranges, and the southern California borderland. These high-pressure, low-temperature blueschist assemblages reflect the thermal regime of subduction-zone environments. Considerable underflow accompanied drifting and the assembly of far-traveled tectonostratigraphic terranes, as documented by sea-floor magnetic anomaly patterns and age relationships of the oceanic crust-capped lithosphere: the eastern limbs of paleo-Pacific plates (especially the Farallon-Cocos), have been extensively or completely overridden by the westward-encroaching North American plate—7000 km since Early Cretaceous time and nearly 10 000 km since Jurassic time. Subduction is attested to by remnant high-pressure mineral assemblages scattered throughout California; by construction of related, roughly contemporaneous calc-alkaline volcanic-plutonic belts and forearc basin deposits; and by the stranding of ophiolitic complexes. Although substantial northward drift transported exotic oceanic and continental materials to the growing Californian crust and caused extensive dislocation of the post-Paleozoic continental margin, much of the plate motion evidently involved a large component of convergence and eastward underflow. Terrane shuffling has complicated the picture, but the dominant mechanism of continental growth at the Californian margin during Mesozoic and Paleogene time was subduction.

The development of the SW Pacific Margin of Gondwana: Correlations between the Rangitata and New England Orogens

Abstract: Prior to formation of the Tasman Sea, the Late Paleozoic-Mesozoic Rangitata Orogen of New Zealand and New Caledonia abutted the Paleozoic New England Orogen of eastern Australia. Comparison of the record of Permian-Cretaceous igneous and deformational events from the two orogens suggests that their tectonic evolution was interrelated and is a consequence of convergent plate interaction along the southwest Pacific margin of Gondwana. The following relations are proposed: (1) termination of arc volcanism and widespread sedimentation in New England, together with the onset of regional deformation and crustal anatexis were synchronous with the commencement of volcanism and sedimentation within the Rangitata Orogen; (2) Early Permian andesitic volcanism in eastern New England represents an along-strike extension of the Brook Street terrane of New Zealand; (3) Late Permian regional deformation in New England coincides with both a break in subduction-related igneous activity in the New England and Rangitata Orogens and a shift in the locus of this activity; (4) Late Permian-Triassic calc-alkaline igneous activity in New England correlates with a phase of relatively continuous accumulation of pyroclastic material in the forearc basin of the Rangitata Orogen; (5) cessation of plutonism in New England corresponds with commencement of formation of the Esk Head Melange in New Zealand and the probable commencement of juxtaposition of the Te Anau and Alpine Assemblage; (6) Late Cretaceous epizonal plutons intruded into the New England Orogen are similar in character and age to those emplaced during the final phases of Rangitata orogenesis, and both appear to mark initial stages of rifting associated with formation of the Tasman Sea. The generation of Permian and Triassic igneous activity in eastern New England by convergent plate interaction results, on present reconstructions of the Gondwana margin, in an excessively wide arc-trench gap succession, for the position of the trench is constrained to east of New Caledonia from the Permian onward. This suggests there may have been some rearrangement of tectonic elements along the margin resulting in widening of the Lord Howe Rise between Australia and New Caledonia in the period between termination of igneous activity in New England and formation of the Tasman Sea in the Late Cretaceous.

The Topsails igneous terrane, Western Newfoundland: evidence for magma mixing

Abstract: The Topsails igneous terrane of Western Newfoundland contains a diverse suite of igneous rocks, but consists mainly of Silurian alkaline to peralkaline granites and rhyolites. The terrane exhibits evidence for the coexistence of mafic and salic magmas in the form of composite dykes and flows, sinuous, boudined mafic dykes cutting granites and net vein complexes. Field data and major and trace element chemical data suggest that these magmas mixed to produce limited volumes of more or less homogeneous hydrids.

Basin Evolution During Change from Convergent to Transform Continental Margin in Central California

Abstract: Miocene nonmarine and shallow marine strata exposed east of San Francisco Bay record a change from convergent-margin tectonics to transform margin tectonics. During the middle Miocene, the East Bay area occupied the oceanward side of a shelved forearc basin that was progressively incorporated in the evolving San Andreas strike-slip orogene. Patterns of deposition in the broad forearc basin were relatively simple: andesitic arc-derived detritus was transported the full width of the forearc basin from the Sierras to the East Bay area. In contrast, the wrench-tectonic regime produced complex patterns of sedimentation displaying greater local variability. On the basis of stratigraphic data, we infer that the west-facing slope of the forearc basin in the East Bay area was reve sed about 13 Ma with uplift of the area between the eventual traces of the San Andreas and Hayward faults on the site of the present bay. A fluvial clastic wedge was shed eastward into the East Bay area from this uplifted terrane of Mesozoic subduction complex and forearc basin rocks. Initial rupturing along the Hayward fault trend followed the uplift at about 10 Ma. Loci of basaltic volcanism (10-7 Ma) along these fractures interfinger with the clastic wedge. A similar pattern of uplift and drainage reversal apparently presaged the onset of wrenching along the nearby Calaveras trend from 8-6 Ma. Expansion of the strike-slip orogene segmented the outer forearc basin into local basins, some characterized by episodic lacustrine deposition and probable internal drainage. By the end of the M ocene, Sierran arc volcanism waned at the latitude of San Francisco Bay, and arc-derived volcaniclastics were fully supplanted by recycled Coast Range-derived detritus in the East Bay area. Certain of these Coast Range sediment sources, particularly blueschist-bearing Franciscan terranes, permit an estimate of 7-27 km (4-17 mi) of total right slip on the Hayward fault.

The Wathaman batholith: largest known Precambrian pluton

Abstract: The Early Proterozoic Wathaman batholith, in northern Saskatchewan and Manitoba, is a 900 km long, megacrystic granite–granodiorite intrusion that straddles the junction between ensialic miogeoclinal and probably ensimatic eugeoclinal–island-arc terranes of the "Trans-Hudson Orogen," of the western Churchill Province. Although the largest Precambrian batholith known, it is, apart from marginal complexities, remarkably homogeneous throughout and, unlike comparably sized and situated Phanerozoic batholiths, shows no evidence of multiple intrusion, nor does it have comagmatic early mafic phases. However, it may be considered as just one phase of a larger batholithic belt that also includes numerous smaller plutons. Taken as a whole the composite batholithic belt is similar in many aspects to Mesozoic Pacific rim batholithic belts, and like them probably was emplaced during plate collision.The batholith is affected by pervasive internal deformation, is bounded on the northwest by major blastomylonite zones, a...