Trending Belt Research Articles

Age and extent of the Barents and Kara ice sheets in Northern Russia

The youngest ice marginal zone between the White Sea and the Ural mountains is the W‐E trending belt of moraines called the Varsh‐Indiga‐Markhida‐Harbei‐Halmer‐Sopkay, here called the Markhida line. Glacial elements show that it was deposited by the Kara Ice Sheet, and in the west, by the Barents Ice Sheet. The Markhida moraine overlies Eemian marine sediments, and is therefore of Weichselian age. Distal to the moraine are Eemian marine sediments and three Palaeolithic sites with many C‐14 dates in the range 16–37 ka not covered by till, proving that it represents the maximum ice sheet extension during the Weichselian. The Late Weichselian ice limit of M. G. Grosswald is about 400 km (near the Urals more than 700 km) too far south. Shorelines of ice dammed Lake Komi, probably dammed by the ice sheet ending at the Markhida line, predate 37 ka. We conclude that the Markhida line is of Middle/Early Weichselian age, implying that no ice sheet reached this part of Northern Russia during the Late Weichselian. This age is supported by a series of C‐14 and OSL dates inside the Markhida line all of >45 ka. Two moraine loops protrude south of the Markhida line; the Laya‐Adzva and Rogavaya moraines. These moraines are covered by Lake Komi sediments, and many C‐14 dates on mammoth bones inside the moraines are 26–37 ka. The morphology indicates that the moraines are of Weichselian age, but a Saalian age cannot be excluded. No post‐glacial emerged marine shorelines are found along the Barents Sea coast north of the Markhida line.

Marginal formations of the last Kara and Barents ice sheets in northern European Russia

Glacial landforms in northern Russia, from the Timan Ridge in the west to the east of the Urals, have been mapped by aerial photographs and satellite images supported by field observations. An east‐west trending belt of fresh hummock‐and‐lake glaciokarst landscapes has been traced to the north of 67°N. The southern boundary of these landscapes is called the Markhida Line, which is interpreted as a nearly synchronous limit of the last ice sheet that affected this region. The hummocky landscapes are subdivided into three types according to the stage of postglacial modification: Markhida, Harbei and Halmer. The Halmer landscape on the Uralian piedmont in the east is the freshest, whereas the westernmost Markhida landscape is more eroded. The west‐ east gradient in morphology is considered to be a result of the time‐transgressive melting of stagnant glacier ice and of the underlying permafrost. The pattern of ice‐pushed ridges and other directional features reflects a dominant ice flow direction from the Kara Sea shelf. Traces of ice movement from the central Barents Sea are only discernible in the Pechora River left bank area west of 50°E. In the Polar Urals the horseshoe‐shaped end moraines at altitudes of up to 560 m a.s.l. reflect ice movement up‐valley from the Kara Ice Sheet, indicating the absence of a contemporaneous ice dome in the mountains. The Markhida moraines, superimposed onto the Eemian strata, represent the maximum ice sheet extent in the western part of the Pechora Basin during the Weichselian. The Markhida Line truncates the huge arcs of the Laya‐Adzva and Rogovaya ice‐pushed ridges protruding to the south. The latter moraines therefore reflect an older ice advance, probably also of Weichselian age. Still farther south, fluvially dissected morainic plateaus without lakes are of pre‐Eemian age, because they plunge northwards under marine Eemian sediments. Shorelines of the large ice‐dammed Lake Komi, identified between 90 and 110 m a.s.l. in the areas south of the Markhida Line, are radiocarbon dated to be older than 45 ka. The shorelines, incised into the Laya‐Adzva moraines, morphologically interfinger with the Markhida moraines, indicating that the last ice advance onto the Russian mainland reached the Markhida Line during the Middle or Early Weichselian, before 45 ka ago.

Medicine Bow orogeny: Timing of deformation and model of crustal structure produced during continent-arc collision, ca. 1.78 Ga, southeastern Wyoming

A four-dimensional model for the evolution of the late-Paleoproterozoic Cheyenne belt arc-continent suture is presented based on available geologic mapping, structural analysis, geophysical constraints, geobarometry, geochronology, and isotopic data. All of the data are consistent with a southeast-dipping suture and deformation that lasted from 1.78 Ga to at least 1.76 Ga, and possibly as late as 1.74 Ga. Archean crustal components and/or detritus were subducted at least 30 to 70 km south of the trace of the suture. There is considerable variation in crustal structure and tectonic evolution along strike of the Cheyenne belt. Thick-skinned, intracratonic uplift along the Laramie Peak shear zone and synorogenic emplacement of the Horse Creek anorthosite complex occurred in the east, whereas low-grade metamorphism and late cataclastic thrust faulting occurred in the west. Some of this lateral variation may reflect the influence of preexisting crustal features such as high-angle normal faults and crustal heterogeneities related to ca. 2.0-Ga rifting, whereas other differences may reflect variations in collision geometry. The lithospheric architecture of this arc-continent suture created compositional and structural anisotropies that influenced later deformation and magmatism, such as the generation and emplacement of the 1.43-Ga Laramie anorthosite complex and location of Paleozoic diamond-bearing diatremes. In this paper, the term “Medicine Bow orogeny” is proposed to describe the ca. 1.78-Ga arc-continent collision that formed the Cheyenne belt suture, and the subsequent structural evolution of the orogenic zone, which may have continued to ca. 1.74 Ga. The orogenic belt trends from southeastern Wyoming to northeastern Nevada, a distance of ∼1900 km. This term is invoked to differentiate the tectonic history along the Cheyenne belt from both the Yavapai orogeny to the south, and Central Plains orogeny to the east, because these orogenies include younger rocks and younger deformation. The arc terrane involved in the Medicine Bow orogeny is probably 50 to 100 km wide, and it makes up the basement of northern Colorado and continues at least as far south as the Soda Creek-Fish Creek shear zone in north-central Colorado. Accretion during the Medicine Bow orogeny represents a substantial addition to the North American continent.

Archaean quartz arenites in the Canadian Shield: examples from the Superior and Churchill Provinces

Units of remarkably pure Archaean quartz arenite occur in the northwestern part of the Superior Province and in the northern terrane of the Western Churchill Province (Rae Province) of the Canadian Shield. In the Superior Province, silica-cemented quartz arenites of Archaean age are well preserved in several greenstone belts. The example from the Keeyask Lake sedimentary assemblage displays tabular–planar and trough cross-beds, ripple marks, reactivation surfaces and pebble lag deposits. In spite of penetrative deformation and greenschist-grade metamorphism, primary textures are extremely well preserved, showing framework grains to be very well rounded and sorted. The succession of Keeyask Lake quartz-arenite beds is overlain by siltstones containing small-scale stratiform, domal and columnar stromatolites. A shallow-marine environment of deposition is inferred. Detrital heavy minerals include pyrite, magnetite, zircon, tourmaline, apatite, sphene and topaz. In the northern part of the Western Churchill Province (Rae Province), Archaean quartz arenites occur in northeasterly trending belts where intense structural deformation has in most places obscured or obliterated primary textures and structures. This has led to speculation that some of these units are metachert or recrystallized vein quartz, but local preservation of primary textures and structures provides clear evidence of epiclastic origin. In the example described herein, quartz arenites of the Woodburn Lake Group display sparse occurrences of trough and tabular–planar cross-beds, channels, ripple marks and pebble lag deposits. Probable environments of deposition for these quartz arenites include fluvial systems and shallow-marine shelf settings. The occurrence of unequivocal quartz-arenite clasts in beds of intercalated conglomerate provides direct evidence of at least two episodes of accumulation of almost pure quartz sand. Thin sections and polished slabs reveal frameworks of clastic quartz grains with little to no matrix (now mainly muscovite), and rare detrital grains of accessory heavy minerals, predominantly zircon and opaque iron oxides. Pyrite and other sulphides have been introduced along fractures, but some intergranular sulphide grains may be of detrital origin. The principal source for the quartz arenites in both areas must have been quartz-rich granitoid rocks. Conditions of intense chemical weathering are indicated. The widespread occurrence of extremely mature quartz arenites throughout Archaean terranes of the Canadian Shield, and in other shields of the world, are suggestive of crustal stability during early Earth history. The association of quartz arenites and ultramafic rocks, uncharacteristic of younger terranes, is now recognized in many Archaean greenstone belts of the Canadian Shield.

The assembly of Gondwana 800-550 Ma : Meert J.G. & Van der Voo R., Journal of Geodynamics, 1997, 23/3–4 (223–235)

Southeastward subduction of oceanic lithosphere in front of the rifted continental block of southern China under an emergent oceanic basement in the eastern part of Sabah controlled the development of an elongate basin trending NE-SW. This basin became the site for the deposition of Middle Eocene-Early Miocene sediments. Active opening of the South China Sea Basin in NW-SE and N-S directions during the Middle Oligocene-Middle Miocene caused further subduction and narrowing of the basin. As a result the Middle Eocene-Early Miocene sediments were compressed into a fold-thrust belt trending approximately NE-SW in the western part of Sabah and NW-SE in the northern and eastern part of Sabah. The subduction was accompanied by volcanic activity in the eastern part of Sabah during the Early-Middle Miocene. The deformed sedimentary pile and underlying oceanic basement were then subjected to a NW-SE extension related to the opening of the Sulu Sea Basin during the Early-Middle Miocene. This resulted in the development of extensive chaotic deposits in the eastern and central part of Sabah. This extension also controlled the development of several circular basins for the deposition of the thick, Early-Late Miocene sediments. Continued extension in this region resulted in further southeastward subduction in southeastern Sabah producing the Late Miocene-Quaternary volcanics.

New Gravity and Magnetic Anomaly Maps in the Taiwan-Luzon Region and Their Preliminary Interpretation

We have compiled new free-air gravity anomaly (FAA) and magnetic anomaly maps, shedding light on the tectonics in the Taiwan-Luzon region. To have a suitable datum level for both the available gravity and magnetic anomaly data, the set of data from an ACT cruise, conducted during May 27 to June 21, 1996, was chosen as a reference. Based on the cross-over error analysis, all the other data were adjusted accordingly. Some satellite-derived, airborne or land data were also added to the compilation to obtain better coverage. Several major new insights into the Taiwan-Luzon region are revealed by the new maps. (1) A prominent NE-SW trending belt of gravity and magnetic anomalies is present in the onshore and offshore areas of southwestern Taiwan. The Peikang High is located on this belt. (2) Located in the offshore region west of Taiwan and to the north of the belt described above, the Taishi Basin, in contrast, occupies a relatively low FAA area. It could be regarded as a flexural basin on account of the loading of a thrust-and-fold belt in western Taiwan. (3) A probable NW-SE trending old transform fault is well imaged off southwestern Taiwan, which separates the lithosphere (plate) of the South China Sea from a trapped piece of the Philippine Sea plate. (4) Located east of the Luzon Are, the Huatung Basin contains several E-W trending magnetic reversals and two N-S trending old fracture zones. The Huatung Basin is separated from the West Philippine Basin by the 123E Fracture Zone . Accordingly, the Gagua Ridge corresponds to a transverse ridge bounding the 123E Fracture Zone. (5) The Luzon Are is abnormally concave toward the Manila Trench and becomes wider toward the south. The internal deformation of the Luzon Are in terms of several NE-SW discontinuities could be related to its collision with eastern Taiwan, (6) Three previously proposed NW-SE trending strike-slip faults in the southern Okinawa Trough, west of 123...

Gravity anomalies and deep structural controls at the Sabah-Palawan margin, South China Sea

Abstract The hydrocarbon-rich southeastern margin of the South China Sea is divided by NW-SE lineaments into a series of sharply contrasting segments distinguished by, among other things, abrupt changes in gravity patterns. The Sabah segment is bounded to the SW by the Tinjar or West Baram line and to the NE by the Balabac line at the southwest margin of the Sulu Sea. The most prominent gravitational feature of this segment is the strong freeair gravity low associated with the Sabah Trough which lies about 150 km offshore. Seismic reflection data suggest that loading by prograding sediments and gravity driven thrust sheets has depressed the extended continental crust of the South China Sea below its level of local isostatic compensation, producing the trough as a foreland basin in which sedimentation has failed to keep pace with subsidence. The load masses themselves, supported in part by the rigidity of the underlying crust and lithosphere, are above their levels of local compensation and deep Neogene sedimentary basins lie on the flanks of Bouguer and free-air gravity highs. Gravity values decrease across the Sabah coast so that the Crocker Ranges, including Mt Kinabalu, rest in rough isostatic equilibrium on presumably weaker lithosphere. The free-air gravity anomaly associated with the Sabah Trough is smaller than would have been predicted from the thickness of the water column, suggesting crustal thinning beneath the trough axis. This is not a characteristic of normal foreland basins and can therefore be assumed to predate basin formation. It can be concluded that the NE-SW trending belts of parallel gravity anomaly and geomorphology, of which the Sabah Trough is the most obvious, have been controlled by the pre-existing fabric of the crust and lithosphere of the South China Sea since they are discordant to the Palaeogene geological trends in Sabah. Reconstructions of the Tertiary history of the Sabah segment can be based on this assumption, which also suggests that sediments deposited in rift basins formed during the Palaeogene break-up of the South China margin were the source for much, if not all, of the hydrocarbon reserves of the area.

Timing of the North‐South China collision from new middle to late Mesozoic paleomagnetic data from the North China Block

We collected paleomagnetic samples from 43 sites of Middle and Upper Jurassic and Cretaceous rocks along a 250 km wide east‐west trending belt located north of the suture between the North and South China Blocks. In each case, laboratory treatment isolates dual polarity, antipodal magnetizations that are demonstrably pre‐folding. The paleomagnetic poles show very good coherence with coeval poles from other localities from the North China Block (NCB), suggesting the apparent polar wander path (APWP) from the NCB is not distorted by post‐Middle Jurassic local vertical axis block rotations. A comparison of the NCB APWP with that from the South China Block suggests that collision continued after the Early Jurassic and ended no later than the Late Jurassic. A cusp in the NCB APWP coincides with a marked angular unconformity that separates Middle Jurassic from Upper Jurassic and younger sediments. Together, these observations indicate that the scissor‐like collision and associated deformation ceased at the Middle to Late Jurassic boundary (∼159 Ma). The time of rapid (1°/Ma) relative angular velocity between the two plates (225 to 190 Ma) coincides with a peak in U‐Pb and Ar‐Ar dates obtained from metamorphic rocks in the suture. The combined Late Jurassic and Cretaceous paleomagnetic data from both blocks suggest that the united China plate traveled 2000 km north relative to Eurasia during the Late Jurassic before docking in the Cretaceous. This younger collision event is also marked by a cusp on the Cretaceous part of the Eurasian APWP.

Seismogenic zoning of southeast Australia

In the period from 1958 to 1996 the Australian National University seismological network has located more than 7600 earthquakes in southeastern Australia with a network concentrated between Canberra and Sydney. This catalogue of earthquakes provides a good database for the analysis of the temporal and spatial distribution of events in the region. For the area 37–33°S, 147–151°E, the catalogue is nearly complete for earthquakes with M > 2.0 and shows a characteristic logarithmic dependence of the number of earthquakes with magnitude. The slope of this logarithmic dependence b value for the entire region is 0.99 which differs significantly from that previously determined for the Dalton‐Gunning region (b = 0.81). The locations of the larger events lie on a northeast‐southwest trend, which is crossed by a zone with a relatively large number of smaller events. The major characteristics of the earthquakes in the region are determined by examining the patterns of the number of events and the total energy release in 1° x 1° cells and their interrelations with physiographic information and mapped surface faulting. Three major types of seismic behaviour can be recognised, but there is little apparent correlation with the geographical parameters. In particular, there is no obvious relation between most of the current activity and recognised geological faults. The combination of the cellular analysis with the detailed seismicity pattern suggests the presence of three main zones: (i) two offshore belts which parallel the coastline with many events close to the continental margin, (ii) a northwest‐southeast trending belt where the largest earthquakes are concentrated, which is interrupted by (iii) an approximately orthogonal zone of high earthquake activity which includes the Dalton‐Gunning region. The earthquake clusters in this zone may well be associated with continuing uplift of the Southern Tablelands.

Environmental geology problems in the Tyrrhenian coastal area of Santa Marinella, province of Rome, central Italy

The Tyrrhenian resort of S. Marinella (central Italy) is subjected to significant anthropogenic pressures during the summer vacation period, a common situation all along the Italian coast. Located 65 km NW of Rome on the southern slopes of the Tolfa Mountains, S. Marinella is built on a gently sloping, E–W trending belt which is cut by 14 N–S oriented ephemeral streams that discharge into the Tyrrhenian Sea. The low to medium permeability turbiditic sandstones which outcrop along this belt belong to the Late Cretaceous Pietraforte unit. Three environmental problems are addressed in this study. The first problem is related to the high water supply demand during the summer months which has forced local residents to dig a large number of wells. Extensive pumping from these wells has caused salt-water intrusion into the Pietraforte, thus compromising the domestic use of the groundwater. The second problem consists of the illegal dumping of urban solid waste, material that represents a hazard during significant rain events as well as a possible cause of groundwater contamination. The final issue addressed concerns the flooding potential of the 14 ephemeral streams that cross the inhabited area of S. Marinella, a risk which is highlighted by the disastrous flood which occurred on 2 October 1981 and during the period of the Roman Emperor Settimio Severo (205 A.D.). Some suggestions are proposed to mitigate and contain the effects of these problems.

Foreland deformation in the Central Adriatic and its bearing on the evolution of the Northern Apennines

Seismic profiles in the Central Adriatic show the presence of a WNW-ESE trending belt (Central Adriatic Deformation Belt, CADB) where broad folds of Quaternary age occur. Seismicity in the Adriatic foreland seems to be localised along the CADB which is interpreted as the result of foreland deformation linked to the Apennine fold-and-thrust belt and possibly due to the presence of an inherited structural discontinuity. Geological arguments indicate that the CADB lineament can continue underneath the Northern Apennines and might have affected its recent evolution, characterised by the rise of a linear orographic front.

Cretaceous Basaltic Terranes in Western Columbia: Elemental, Chronological and Sr-Nd Isotopic Constraints on Petrogenesis

Accreted terranes comprising Mid to Late Cretaceous picrites, basalts and dolerites occur in three north–south trending belts in western Colombia, in the Central Cordillera, Western Cordillera and along the Pacific coast. The geochemistry of these rocks is consistent with an oceanic plateau (plume-related) origin, and they most probably formed in the Pacific as part of the Caribbean oceanic plateau. These igneous rocks display small but significant inter-cordillera variations, being younger and more depleted in incompatible trace element ratios (and with more positive eNd values) to the west. The igneous rocks of the Pacific coast (Serrania de Baudo) are dated at 73–78 Ma (40Ar/39Ar), and those of the Western Cordillera at ∼90 Ma, whereas the volcanics of the Central Cordillera are believed to be older than 100 Ma. Most of the igneous rocks are basaltic, and it is suggested that they have fractionated from picritic primary magmas, generated by partial melting within a hot mantle plume. Variable and positive eNd values reveal that the plume must have been heterogeneous, originating from a mantle source with a long-term history of depletion. Partial melt modelling suggests that the composition of the basalts requires at least some input from a mantle source region containing garnet and that the extent of partial melting required to reproduce the composition of the erupted basalts is of the order of ∼20%. Mixing of melts from different depths, either in the mantle melting column or during fractionation in lithospheric magma chambers, can explain the relative homogeneity of basaltic lavas erupted to form this (and other) oceanic plateaux. The Caribbean–Colombian oceanic plateau may have formed at an oceanic spreading centre, and valuable comparisons can be made between Iceland and the Caribbean–Colombian plateau.

Cretaceous Basaltic Terranes in Western Colombia: Elemental, Chronologicaland Sr-Nd Isotopic Constraints on Petrogenesis

Accreted terranes comprising Mid to Late Cretaceous picrites, basalts and dolerites occur in three north–south trending belts in western Colombia, in the Central Cordillera, Western Cordillera and along the Pacific coast. The geochemistry of these rocks is consistent with an oceanic plateau (plume-related) origin, and they most probably formed in the Pacific as part of the Caribbean oceanic plateau. These igneous rocks display small but significant inter-cordillera variations, being younger and more depleted in incompatible trace element ratios (and with more positive εNd values) to the west. The igneous rocks of the Pacific coast (Serranía de Baudó) are dated at 73–78 Ma (40Ar/39Ar), and those of the Western Cordillera at ∼90 Ma, whereas the volcanics of the Central Cordillera are believed to be older than 100 Ma. Most of the igneous rocks are basaltic, and it is suggested that they have fractionated from picritic primary magmas, generated by partial melting within a hot mantle plume. Variable and positive εNd values reveal that the plume must have been heterogeneous, originating from a mantle source with a long-term history of depletion. Partial melt modelling suggests that the composition of the basalts requires at least some input from a mantle source region containing garnet and that the extent of partial melting required to reproduce the composition of the erupted basalts is of the order of ∼20%. Mixing of melts from different depths, either in the mantle melting column or during fractionation in lithospheric magma chambers, can explain the relative homogeneity of basaltic lavas erupted to form this (and other) oceanic plateaux. The Caribbean–Colombian oceanic plateau may have formed at an oceanic spreading centre, and valuable comparisons can be made between Iceland and the Caribbean–Colombian plateau.

Flow stratigraphy of selected sections of the Rajmahal basalts, eastern India

The Rajmahal basalts form a north-south trending belt extending over 4100 km 2 along the eastern margin of the Indian Shield. The basalts form part of a widespread magmatic episode which coincided with continental break-up between India and Australia-Antarctica during the Late Jurassic-Early Cretaceous. In the sections examined, between 2 and 15 flows have been encountered, ranging in composition from tholeiitic basalt to dacite (pitchstone). The flows vary in thickness from < 1 m to > 70 m. Significant volcanic features observed include vent sites, marked by volcanic breccia, welded agglomerate, tuff and lapilli-tuff (including volcanic bombs). The tuffs occur in widely scattered areas, and are more abundant than hitherto recognised. These rocks were probably erupted during the early stages of volcanic activity, following extensive differentiation in sub-volcanic magma chambers. Several characteristics of sediments interbedded with the lavas suggest a humid, sub-tropical lacustrine depositional environment during eruption of the lower part of the volcanic sequence, and a temperate climate during emplacement of the upper flows. Quiescent eruption of the Rajmahal basalts took place largely through fissures sited at the faulted margin of the Indian Shield. This magmatic activity is interpreted as resulting from extension of the lithosphere above a mantle plume.

Mesozoic and Cenozoic plutonic evolution of SE Asia: evidence from Sumatra, Indonesia

Abstract New K/Ar mineral ages from the Barisan Mountains of southern Sumatra suggest four main periods of plutonic activity: Miocene-Pliocene (20–5 Ma), Early Eocene (60–50 Ma), Mid-Late Cretaceous (117–80 Ma) and Jurassic-Early Cretaceous (203–130 Ma). These and all other published ages from exposed plutons in western Sumatra indicate a further period of plutonic activity in the Permian (287–256 Ma). They also suggest either that Early Mesozoic activity began in the Late Triassic, or that there were two distinct magmatic cycles, one in the Late Triassic to Early Jurassic (220–190 Ma) and one extending from the Mid-Jurassic to Early Cretaceous (170–130 Ma). In addition, poorly controlled ages from eastern Sumatra indicate that the important Triassic to Early Jurassic (240–195 Ma) tin-belt magmatism of the peninsular Malaysia Main Range Province extends into that area. Preliminary geochemical studies on the Mesozoic granitoids of the Barisan Range of southern Sumatra confirm that they are calc-alkaline, I-type, metaluminous, subduction-related volcanic arc granites (VAG). They broadly correspond to the southerly extension of a combination of the Central Valley and Western Granite Provinces of Thailand and Burma, and underline the fact that there has been a history of subduction-related magmatism along the southwestern edge of Sundaland since earliest Mesozoic times. The plutonic suites are crudely arranged in subparallel, locally overlapping, NW-SE trending belts, focused along deep-seated faults that have acted as magmatic conduits. It is proposed as a preliminary model that breaks in plutonic activity broadly correspond to changes in approach angle and/or rate of subduction, and that in some instances at least they relate to periods of collision and accretion of allochthonous material (terranes, slivers or blocks) of both oceanic and continental character. At least two such events seem to have occurred during the Mesozoic-Cenozoic tectono-plutonic evolution of Sumatra. One in the early Middle Cretaceous reflects collision and accretion of the oceanic Woyla terranes, and one in the latest Cretaceous is possibly related to collision of a continental sliver/block, the West Sumatra terrane, to the Sundaland margin.

Spatial and temporal relationships between mid‐Tertiary magmatism and extension in southwestern Arizona

Cenozoic magmatism in southwestern Arizona, which is within the Basin and Range tectonic province, occurred almost entirely between 15 and 25 Ma. Volcanic rocks typically consist, in ascending order, of (1) a thin sequence of mafic to intermediate lava flows, (2) voluminous felsic lava flows and pyroclastic rocks with minor to moderate amounts of intermediate to mafic lava flows, and (3) basalt and andesite. Volcanic rock sequences rest disconformably on pre‐Tertiary bedrock in most areas but locally overlie substantial coarse clastic debris that was deposited immediately before and during earliest magmatism. Prevolcanic clastic debris is interpreted as a consequence of local early normal faulting. In most regions, tilting related to extension began later and occurred during or after eruption of felsic volcanic rocks and before the end of younger mafic volcanism. Extension generally ended before about 17 Ma except in a northwest trending belt adjacent to the relatively unfaulted and topographically elevated Transition Zone tectonic province which is adjacent to the Colorado Plateau. Rapid cooling of metamorphic core complexes and tilting of young basalts and coarse clastic rocks continued in this belt until as recently as 11 Ma. Extension was extreme in this belt, whereas it was generally moderate to slight in other parts of southwestern Arizona. Large‐magnitude extension was not associated with areas of greatest igneous activity, and rapid cooling and exhumation of core complexes postdated local magmatism. These relationships are inconsistent with theories that relate genesis of metamorphic core complexes to magma intrusion in the upper crust. Except for young extension in this northwest trending belt, there are no apparent regional migration trends for either magmatism or extension within southwestern Arizona. Lack of substantial extension before magmatism and general lack of magmatism during youngest extension are inconsistent with the hypothesis that magmatism was the product of decompression melting during lithospheric extension. The long duration and large magnitude of extension adjacent to the Transition Zone tectonic province and within an area of earlier crustal thickening are consistent with the hypothesis that extension was driven by the gravitational potential energy of elevated land mass and crustal roots. Regional magmatic heating apparently weakened the lithosphere and triggered extension but did not control extension locally.

The stratigraphy, geochronology and paleophysiography of a Miocene fresh-water interarc basin, southern Chile

During Miocene time, the Andean region between 38° and 39°S was an area where active volcanoes, lakes and rivers formed under a temperate to cold humid climate. The volcanic products and sedimentary deposits which accumulated in lakes and rivers adjacent to the volcanoes constitute the Cura-Mallín Formation. The deep roots of the volcanic chain are represented by middle to late Miocene granitoids which formed a north-south trending belt that separated a marine fore-arc basin (Temuco basin) to the west from the continental sedimentary deposits of the Cura-Mallín Formation to the east. Twenty-three KAr dates ranging between 20 and 11 Ma, constrain the age of the Cura-Mallin Formation to the early to middle Miocene. The alternation of pyroclastic strata and lava flows in the Guapitrío Member of the Cura-Mallín Formation suggests the former existence of strato-volcanoes, whilst the presence of ignimbrites suggests caldera formation. A major lacustrine basin ( >100 km long) represented by some facies of the Rio Pedregoso Member of the Cura-Mallín Formation existed from at least 17.5 ± 0.6 to 13 ± 1.6 Ma. Lacustrine accumulations terminated during the progradation of deltas and infilling of volcanic material. The Cura-Mallín Formation lakes could have been formed, in part, by intra-arc extension, by the damming of rivers caused by volcanic products and/or by the filling of calderas. The occurrence of Gilbert-type delta complexes in the Rio Pedregoso Member may indicate a steep basin margin generated by faulting and/or volcanic accretion.

Uranium‐lead zircon ages from the Median Tectonic Zone, New Zealand

Abstract The Median Tectonic Zone (MTZ) of New Zealand is a generally north trending belt of Mesozoic subduction‐related I‐type plutonic, volcanic, and sedimentary rocks in South Island and Stewart Island that separates Permian strata of the Eastern Province Brook Street Terrane from lower to mid‐Paleozoic Gondwana margin assemblages of the Western Province. High‐precision isotope dilution U/Pb ages of zircons from 30 rocks are reported. Pre‐digestion leaching of zircon in hydrofluoric acid yielded significantly more concordant residues by removing common Pb and dissolving more soluble high‐U domains that have been more affected by relatively recent Pb loss. The results show that MTZ magmatism ranges in age from at least Early Triassic to Early Cretaceous (247–131 Ma), with a pronounced gap in the Middle Jurassic. Triassic plutons tend to occur on the eastern side of the MTZ, and they intrude volcanic/sedimentary sequences of the MTZ in Nelson and eastern Fiordland. These sequences are in turn intruded by...

Spinel + Quartz Granites and Associated Metasedimentary Enclaves from the Taltson Magmatic Zone, Alberta, Canada: A View into the Root Zone of a High-Temperature, S-Type Granitic Batholith

The Taltson magmatic zone (TMZ) makes up the southern part of the early Proterozoic (ca. 1.9-2.0 Ga) Ta l t son-The lon orogen, a nor th-south trending belt that extends for more than 2000 km along strike from central Alberta to the Arctic Ocean (Hoffman, 1988; Ross et al., 1991). The TMZ is dominated by felsic plutonic rooks, in particular by peraluminous grantoids (Bostook et al., 1987; Goff et al., 1986). The present study focuses on the pe t ro logy of a spectacular outcropping of one such granitoid body and its associated enclaves. The rooks are unusual in that both the granites and the enclaves contain the high-temperature mineral assemblage, spinelquartz-cordierite-garnet+sillimanite. The aim of this study is to document the pressure-temperature conditions indicated by this assemblage and to assess the magmatic and metamorphic processes leading to its development. Descriptions of rooks from other parts of the TMZ (Bostock et al., 1987; Grover et al., 1993) suggest that processes documented at this outcrop may be representative of those that operated over much of the TMZ.

The Saxonian Granulites: a metamorphic core complex?

The brachyanticlinal structure of the Saxonian Granulite Complex was formed, or at least overprinted, by large-scale crustal extension, which is documented in an extreme reduction of the metamorphic profile and a pressure-temperature path with nearly isothermal decompression. the granulite is situated within a south-west-northeast trending belt characterized by basaltic volcanism and an intrabasinal, sediment-producing rise, both active in Frasnian times. These features are taken to reflect at least part of the extension which has exhumed the high grade rocks; however, Cambro-Ordovician rifting possibly initiated the process. The Frasnian palaeogeography and geophysical data suggest that the axis of the granulite dome extends at least 150 km towards the south-west. The overall setting strongly resembles the metamorphic core complexes of the western USA, but the exhumation of rocks preserving evidence of metamorphic pressures of 10–12 kbar requires much higher extensional strain, possibly due to the longer period of stretching. Early Carboniferous compression and minor post-collisional extension (Late Carboniferous to Permian) have not significantly modified the earlier geometry. U-Pb and Sm-Nd data ranging between 380 and 340 Ma are interpreted to record retrogression during uplift rather than peak metamorphism.