Northern Terrane Research Articles

New 1:20,000 scale geological maps, synthesis and history of investigation of the Isua supracrustal belt and adjacent orthogneisses, southern West Greenland: A glimpse of Eoarchaean crust formation and orogeny

The Isua area in the Nuuk region, southern West Greenland contains the Eoarchaean Isua supracrustal belt (mostly amphibolites derived from basalts, with chemical sedimentary rocks, felsic rocks and ultramafic units) and orthogneisses with similar ages that flank it to the north and south. These rocks suffered polyphase metamorphism up to amphibolite facies conditions and strain is generally high, but nonetheless the Isua area has most of the world's occurrences of Eoarchaean rocks preserved in a low strain state. This means that structural, geochemical and isotopic studies of Isua area rocks can provide the least ambiguous data on the origin and tectonic evolution of Eoarchaean crust.New Isua colour digital geological maps at 1:20,000 scale (observed outcrop geology and interpretive solid geology) and a synthesis are presented here. These incorporate advances in structural observations, re-interpretations of the lithologies and the large amounts of zircon dating produced since publication in 1986 of the last geological map of the entire Isua area. Essential in the synthesis is that the Isua area contains a southern ca. 3800Ma terrane and a northern ca. 3700Ma terrane. Many of the rocks in the two terranes of different age are broadly similar in appearance, but have been distinguished by U–Pb zircon dating. The boundary between the terranes lies within the Isua supracrustal belt and is most likely at a highly tectonised unit of chert, BIF and carbonate rocks named the dividing sedimentary unit, that contains sparse 3940–3750Ma detrital zircons. In the ca. 3700Ma terrane to the north, no >3750Ma detrital zircons have been found.The Isua supracrustal belt part of the ca. 3700Ma northern terrane has three geochemically distinct sub-terranes dominated respectively by ≥3715Ma boninites, ≥3715Ma island arc tholeiites plus picrites and 3710–3700Ma andesitic–dacitic rocks or sediments derived from them. In the Eoarchaean, these different arc- and maybe fore-arc related rocks were imbricated and juxtaposed with the ca. 3800Ma terrane along the dividing sedimentary unit, which was the structural décollement. The ca. 3800Ma terrane is also interpreted as a complex arc assemblage, similar in origin to the 3700Ma one, but containing small amounts of older ≥3850Ma crust. The assembly of these different terranes occurred between 3690Ma (youngest rocks unique to the northern ca. 3700Ma terrane) and 3660Ma (oldest distinctive intrusions common to both terranes). Hence our synthesis shows that Isua crustal evolution was by several juvenile crust formation steps restricted to the 3800 or the 3700Ma arc-related terranes, followed by complex collisional processes with later tectonic partitioning. This resembles crustal evolution widely seen in other younger Archaean terranes, and points to a continuity of crust formation along convergent plate boundaries back to at least 3800Ma.

Palaeoproterozoic to Palaeozoic magmatic and metamorphic events in the Shackleton Range, East Antarctica: Constraints from zircon and monazite dating, and implications for the amalgamation of Gondwana

A comprehensive set of new geochronological data from different parts of the Shackleton Range in East Antarctica, comprising U–Pb single zircon and Th–U–Pb single and multi-grain monazite data, combined with published results, reveal a complex tectono-thermal history of the Shackleton Range. Three distinct, spatially separated terranes or units with different magmatic and metamorphic history are now recognised: (i) the Southern Terrane (Unit I) contains detrital components as old as 2850 Ma, experienced magmatism between 1850 Ma and 1810 Ma and underwent a medium- to high-grade metamorphic event at 1710–1680 Ma and, locally, again at 510 Ma; (ii) the Eastern Terrane (Unit II) occurs in the easternmost part of the Shackleton Range and contains c. 1060 Ma old Grenvillian granitoids, which experienced metamorphism at c. 600 Ma; and (iii) the Northern Terrane (Unit III) is characterised by 530 Ma old granites and diorites, which are hosted within paragneisses as well as mafic and ultramafic rocks. All rocks of Unit III experienced upper amphibolite- to granulite-facies and, locally, eclogite-facies metamorphism at 510–500 Ma. The geologic features of Palaeoproterozoic tectonism in the Southern Terrane are very similar to those of the Australo-Antarctic Mawson Continent. This may indicate that the Mawson Continent extends across the East Antarctic Shield into the Shackleton Range. The 1060 Ma and 600 Ma events in the Eastern Terrane have not been documented for any part of the Shackleton Range before and are correlated with Grenvillian and Pan-African tectonism in Dronning Maud Land. By implication, this suggests that the Pan-African Mozambique/Maud Belt continues into the Shackleton Range. The associated suture is located in the easternmost Shackleton Range and is related to the amalgamation of the Indo-Antarctic plate with West Gondwana. This was followed by further collision of the combined Indo-Antarctic/West Gondwanan block with East Gondwana at approximately 510 Ma in the Northern Terrane. A suture related to this latter collision can be traced in the Northern Shackleton Range and may continue northwards to the Sør Rondane Mountains and the Lützow Holm Bay area. Our data support the model that East Antarctica finally assembled during the Pan-African orogeny, rather than during earlier Mesoproterozoic events.

Siberia, the wandering northern terrane, and its changing geography through the Palaeozoic

Siberia, the wandering northern terrane, and its changing geography through the Palaeozoic

Lateral constraint rather than escape along a terrane boundary near Skelleftehamn

Mylonitised greywackes with superposed pseudotachylites (fossil earthquakes) mark the eastern end of a W-E trending terrane boundary between the Skellefte supracrustal group of Svecokarelia to the north and the Bothnian gneisses of Svecofennia to the south. This region has been claimed to provide the clearest evidence that early Proterozoic plate tectonics (>1.85 Ga) involved the same processes as Phanerozoic continental suturing and convergence. Comparisons between the tectonic histories of greywackes in, and on either side, of this boundary demonstrate that the southern terrane already had major recumbent folds at upper amphibolite metamorphic facies before it met and converged on the northern terrane and eventually rose relative to it. These nappes were refolded by upright folds that are circumferential to a syn-Svecofennian granite south of the boundary but the first folds visible in the lower grade rocks to the north. The youngest Svecofennian structures in the region were kink bands that developed in the boundary while it was a steep seismic zone. These kink bands record lateral tectonic constraint rather than lateral escape. West of the study area, mylonites of the terrane boundary are intruded by sheets of post Svecofennian Skellefte granites and plutons of 1.75 Ga Revsund granites.

Pre‐Alpine Crust in the Apuseni Mountains, Romania: Insights from Sm‐Nd and U‐Pb Data

The Apuseni Mountains in Romania expose pre‐Alpine crustal basement rocks between the Carpathian and Dinaride segments of the Alpine Orogen. Because of a lack of direct constraints, the age and plate affinity of the basement rocks have been largely uncertain. New Sm‐Nd data for key lithotectonic assemblages and U‐Pb zircon data for crosscutting plutons are presented. They show the presence of Early Proterozoic crustal components and help constrain the age and nature of several contrasting pre‐Alpine crustal domains. Basement rocks of the Apuseni Mountains consist of a northern granite‐gneiss terrane (Someş) bounded by an amphibolite/granodiorite belt (Codru), a composite igneous belt overprinted by Alpine shearing (Highiş‐Biharia), and a southern carbonate‐lens mica schist‐gneiss terrane (Baia de Arieş). The northern terrane has yielded TDM model ages ranging from 1.90 to 1.76 Ga and ϵNd(0) values ranging from −15.5 to −12.5. The southern terrane has yielded TDM model ages of 1.82–1.69 Ga and ϵNd(0) values of −11.7 to −7.2. Sm‐Nd data from Paleozoic granitic intrusions are consistent with the mixing of mantle‐derived magma and Proterozoic crust. U‐Pb zircon dates indicate that plutons were emplaced at ∼278 Ma in the northern gneiss terrane, at ∼372 Ma in its amphibolite rim, and at ∼364 and ∼206 Ma in the southern terrane. Low‐strain pods in the Alpine shear zone separating the two gneissic terranes were derived from the Late Cambrian Biharia Igneous Complex and the Early Permian Highiş Igneous Complex. The Codru amphibolite belt and the Biharia Igneous Complex may represent a Paleozoic suture‐magmatic belt pair of the Variscan Orogen. The Highiş Igneous Complex and associated clastic assemblages may record late Variscan transcurrent tectonism. The Apuseni crustal fragment is inferred to be of European‐plate affinity, similar to the Carpathian crust, and its characteristics support the correlation of basement domains across what were thought to be Alpine and Variscan sutures.

Island Arc–Related, Back-Arc Basinal, and Oceanic-Island Components of the Bela Ophiolite-Mélange Complex, Pakistan

The Bela ophiolite-mélange complex (BOMC) of Pakistan is a late Cretaceous (∼66 Ma) suprasubductionzone composite ophiolite. Its abundant mafic volcanic and hypabyssal rocks are divisible into three compositionally distinct suites—an ensimatic volcanic island-arc (suite 1) dominated by low-Ti (Ti < 7200 ppm) tholeiites, a back-arc basinal assemblage (suite 2) containing high-Ti (>10,000 ppm Ti) rocks, and high-Ti oceanic-island alkaline basaltic rocks (suite 3). Compared to suites 1 and 2, suite 3 rocks are high in light rare-earth elements (LREE), Zr, Nb, and Zr/Y, and low in Cr, Zr/Nb, Y/Nb, and Ti/Nb. In Zr/Y ratios, suite 1 < suite 2 < suite 3. The geographic distribution of the three BOMC suites defines a northern (north of latitude 26°50' N) island-arc terrane characterized by island-arc tholeiites (IAT) and a southern basinal terrane containing back-arc basinal basalts (BABB). Alkaline basalts apparently represent oceanic seamounts, and occur at isolated sites in both terranes. Some oceanic-island basalts (OIB) are emplaced in younger sediments overlying the ophiolite. As exceptions to the general distribution, a few tectonic blocks of high-Ti basalt crop out in the northern terrane, and a few low-Ti basalts occur in the southern terrane. Representative massifs with a completely intact ophiolite stratigraphic sequence include the Lak Baran massif in the arc terrane and the Bora Jhal massif in the basinal terrane. The arc terrane contains more extensive chromite deposits than does the basinal terrane. Arc chromites possess higher Cr/(Cr+Al) than do chromites of the basinal terrane, indicative of more refractory mantle source rocks and higher degrees of partial melting. Nd-Sr isotopic data on 10 BOMC lithologies support their arc-related nature and derivation from an enriched source mantle.

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.

Thermochronological evidence for late Proterozoic (Vendian) cooling in southwest Wedel Jarlsberg Land, Spitsbergen

Two Proterozoic terranes with different metamorphic histories are distinguished from geological mapping in southwestern Wedel Jarlsberg Land: a northern greenschist facies terrane and a southern amphibolite facies terrane which has been overprinted by greenschist facies metamorphism. To better characterize the tectonothermal history of these terranes we have obtained new 40Ar/39Ar mineral dates from this area. A muscovite separate from the northern terrane yielded a Caledonian plateau age of 432±7 Ma. The southern terrane yielded significantly older 40Ar/39Ar ages with three muscovite plateau dates of 584±14 Ma, 575±15 Ma, and 459±9 Ma, a 484±5 Ma biotite plateau date, and a 616±17 Ma hornblende plateau date. The oldest thermochronological dates are over 300 Ma younger than the age of amphibolite facies metamorphism and therefore probably do not represent uplift-related cooling. Instead, the Vendian dates correlate well with a regionally widespread magmatic and metamorphic/thermal resetting event recognized within Caledonian complexes of northwestern Spitsbergen and Nordaustlandet. The apparent Ordovician dates are interpreted to represent partial resetting, suggesting that late Caledonian greenschist facies overprinting of the southern terrane was of variable intensity.

Proterozoic sutures and terranes in the southeastern Baltic Shield interpreted from BABEL deep seismic data

A hitherto unknown terrane and its bounding sutures have been revealed by a combined study of normal-incidence and wide-angle seismic data along the BABEL profile in the Baltic Sea. This Intermediate Terrane is situated between a Northern Terrane of Svecofennian age and a Southwestern Terrane of Gothian age. It is delimited upwards by two low-angle and oppositely dipping sutures and occupies mainly middle and lower crustal levels with a width of ∼ 300 km at Moho level. The ∼ 1.86 Ga suture against the Northern Terrane is imaged by a prominent almost continuous NE-dipping crustal reflection from 3.5 to 14 s twt over 175 km. Where it downlaps on the Moho, sub-Moho velocities change from 8.2 to 7.8 km/s (±0.2) over less than 25 km. A relatively strong, NE-dipping normal-incidence and wide-angle reflection at 19–23 s twt indicates that the suture extends into the upper mantle. The pervasive NE-dipping reflection fabric of the Intermediate Terrane is interpreted as shear zones that developed during collision and possibly were reactivated by later events. High Poisson's ratios suggest a mafic composition or high fluid content. The ∼ 1.86 Ga collision was probably succeeded by continental break-up and removal of an unknown continent, except for the Intermediate Terrane. Subsequent formation of an east-dipping subduction zone further to the west led to the emplacement of 1.81-1.77-Ga-old granitoids in the southern part of the Transscandinavian Igneous Belt. The ∼ 1.65-1.60 Ga suture against the Southwestern Terrane is defined by a semi-continuous band of strong SW-dipping reflections between 3 and 8 s twt over 65 km, which are interpreted as a low-angle thrust zone along which Gothian crust overrode the Intermediate Terrane. The identification of three individual seismic terranes in the southeastern part of the Baltic Shield provides new evidence for Palaeoproterozoic plate tectonic processes.

The Kibaran of southern Africa: Tectonic evolution and metallogeny

The tectono-magmatic and metallogenic evolution of the ∼ 1100 Ma (Late Kibaran) high-grade Namaqua-Natal metamorphic belt is reviewed and compared with that of the Grenville Province and the Kibaran Belts of central and eastern Africa. In Namaqualand, 1300–1600 Ma supracrustal rocks are recognised, including arc-related sequences in the east and extensional foreland basin sequences in the west. These were laid down either on a heterogeneous basement of Eburnian (∼ 2000 Ma) age or on juvenile oceanic crust, prior to Late Kibaran deformation, metamorphism and multiple plutonic intrusion. The pre-Namaqua basement infrastructure has been recognised largely in the eastern and NW parts of the orogen, adjacent to areas of exposed Eburnian crust and probably only the Areachap Terrane in the east comprises juvenile Kibaran crust. In Natal, ∼1400 Ma, predominantly metavolcanic supracrustal gneisses were extensively intruded by voluminous plutonic batholiths during the ∼ 1050 Ma tectonothermal event. The northern terrane is interpreted as an ophiolitic assemblage, obducted to the NE onto the Kaapvaal Craton, whereas the southern domains are predominantly arc-related. No older basement is recognised and the entire belt comprises juvenile crustal material, none of which is apparently older than ∼ 1400 Ma. The kinematic evolution of the entire Namaqua-Natal Belt is comparable. In both areas, early SW- and NE- d directed thrusting and nappe emplacement during collision, were followed by pervasive steep transcurrent ductile shearing. In Natal. the later shearing event gave rise to major WNW-trending oblique sinistral transcurrent “steep belts”, whereas the same event in Namaqualand is manifested as major, subvertical, NW-trending dextral transcurrent shear zones. The difference in dominant sense of movement in the two regions can be attributed to changes in orientation of the adjacent Archaean craton margin, within a progressive, consistently oriented, NE-SW directed zone of plate convergence. In eastern Namaqualand and Namibia, the transcurrent shearing gave rise to a north-propagating stricke-slip rift system in which the volcano-sedimentary Koras-Sinclair-Ghansi sequences were deposited. Mineral cooling ages of ∼ 950 Ma, reported from throughout the belt, are considered to signal the end of the Kibaran cycle in southern Africa. The metallogenic evolution is discussed in the light of the tectono-magmatic model presented. Economically important stratiform volcanic and sedimentary exhalative base-metal occurrences are extensively associated with early supracrustal sequences throughout the belt. Other metal deposits and occurrences are predominantly related to the voluminous granitoid and more restricted metabasic suites, which characterise the younger rocks in both Natal and Namaqualand. The metallogenesis of the dominantly convergent-collisional Namaqua-Natal Belt is similar to that seen in the Grenville Province, but distinct from, and younger than, the lower grade, strike-slip basin-related Kibaran Belt of central and eastern Africa.

Stratigraphic relations and U-Pb geochronology of the Upper Cretaceous upper McCoy Mountains Formation, southwestern Arizona

A previously unrecognized angular unconformity divides the Jurassic and Cretaceous McCoy Mountains Formation into a lower and an upper unit in the Dome Rock Mountains and Livingston Hills of western Arizona. The lower unit of the McCoy Mountains Formation consists of generally fine-grained quartzose and volcaniclastic strata that were deposited after cessation of Middle Jurassic explosive volcanism. The basal contact of the lower unit is disconformable in most places, but locally it has been interpreted to be gradational with the underlying silicic volcanic rocks. The upper unit is a fining-upward sequence of quartzo-feldspathic and arkosic conglomerate and sandstone that records uplift of a northern source terrane. A tuff in the lower part of the upper unit has a U-Pb crystallization age of 79 ± 2 Ma (Late Cretaceous). Rocks of the lower unit are deformed by pre-80 Ma thrust faults of the generally southward-vergent Maria fold and thrust belt, which bounds the outcrop belt of the McCoy Mountains Formation on the north. The upper unit is exposed only south of the fold and thrust belt. We interpret the intraformation unconformity in the McCoy Mountains Formation to have developed where rocks of the lower unit were deformed adjacent to the southern margin of the Maria fold and thrust belt. The upper unit of the formation is interpreted as a foreland-basin deposit that was shed southward from the actively rising and deforming fold and thrust belt. The apparent absence of an equivalent unconformity in the McCoy Mountains Formation in adjacent California is presumably a consequence of the observed westward divergence of the outcrop belt from the fold and thrust belt. Continued southward shortening deformed the entire formation under greenschist- and, locally, amphibolite-facies conditions soon after the upper unit was deposited. Tectonic burial beneath the north-vergent Mule Mountains thrust system in the latest Late Cretaceous (∼70 Ma) marked the end of Mesozoic contractile deformation in the area.

Kinematics and tectonothermal modeling—interpretation of heat flow observed on the Tibetan Plateau

Thirteen heat flow values along a N-S profile are employed as surface constraints on a tectonothermal model of the Tibetan Plateau. The modeling demonstrates 40 Ma of crust-mantle temperature-history, since the India-Eurasia collision. The main emphasis in this paper is put on the fact that a series of heat-generating and heat-absorbing processes such as enhancement of radiogenic heating, strain- and friction-heating, partial melting etc., were induced by tectonic deformations of the lithosphere. On this basis, equations defining terrane deformations in forms of shortening, thickening, extension, uplift and erosion were deduced using the principles of plate kinematics and 3-D mass balance. Then, these equations being coupled with each other are further used as systematically defined initial and boundary conditions for integrated tectonothermal simulation. The modeling results suggest heterogeneity in lithospheric thermal structure and sharp differences in tectonothermal evolution between the old northern terranes and the new southern ones. However, even in the warm terranes in the south, the scale of the tectonically induced heat-generating processes is smaller than usually expected.

Cenozoic Oblique Collision of South American and Caribbean Plates: New Evidence in the Coastal Cordillera of Venezuela and Trinidad

The hinterland of the Caribbean Mts. orogen in Trinidad and Venezuela contains schist and gneiss whole protoliths are wholly or partly of continental provenance. The hinterland lies between the foreland thrust belt and terranes. The terranes are alien to continental South America (SA) and may have proto-Caribbean or Caribbean plate origins. The hinterland rocks were widely thought to come from sediments and granitoids of Mesozoic protolithic ages and to be of Cretaceous metamorphic age. Such rocks are now know to be of at least two or more types, as follows: (1) low grade, protoliths of pre-Mesozoic basement and shelfal cover of uncertain age range, inboard locus, Oligocene to mid-Miocene metamorphic ages younging eastward (Caracas, Paria, and Northern Range belts), and (2) higher grade including high P/T, varies protoliths of uncertain age range, Cretaceous and ( )early Paleogene metamorphic ages (Tacagua, Araya, Margarita). The geometry, protoliths, structures, and metamorphic ages of type 1 parautochthoneity and an origin as a thickened wedge of crust-cored passive margin cover. The wedge grew by accretion between about 35 and 20 Ma during oblique transport toward the foreland. The diachroneity of metamorphism implies, as does the timing of foreland deformation, that the wedge evolved in amore » right-oblique collision between northern SA and terranes moving wholly or partly with the Caribbean plate since the Eocene. Type 2 rocks probably came with the terranes and are products of convergent zone tectonics, either in the proto-Caribbean plate. The hinterland boundaries are brittle thrusts that are out of sequence and imply progressive contraction from mid-Cenozoic to the present.« less

HEAT FLOW ON THE QINGHAI-XIZANG(TIBETAN) PLATEAU AND THE MODELING OF THE TECTONO-THERMAL EVOLUTION

Thirteen observed heat flow values in combination with relevant geological and geophysical information are employed in the current paper to conduct a model study by means of direct inversion. The modeling demonstrates the tectono-thermal evolution of the Tibetan Plateau during the last 40 Ma since the continent-continent collision. The authors stress the fact that the tectonic deformations of terranes are usually the inducing factors for the deep-seated thermal activities in the crust and upper mantle. On this basis a series of kinematic equations of 3-D deformations of terranes in forms of shortening-thickening-uplifting-erosion-mass sliding were deduced using the principle of plate kinematics. These equations are further used as systematically defined initial and boundary conditions for simulating the integrated processes of tectono-thermal evolution, The results of the model study suggest that there exist sharp differences in the tectono-thermal evolution between the old northern terranes and the new southern ones. However, even in the warm terranes in the south, the scale of the deep-seated thermal processes is lower than that uxnally expected.

Evidence for late Ordovician amalgamation of volcanogenic terranes in the Iapetus suture zone, eastern Ireland

ABSTRACTA major transcurrent fault in the zone of the Iapetus suture in eastern Ireland separates Ordovician (pre-Ashgill) terranes. The stratigraphy of each terrane belongs to a dismembered volcanic arc system: the northern terrane is characterised by acid plinian eruptions and derivative sediments which are displaced relative to the andesitic southern terrane volcanism. Each was a separate palaeoenvironment with its own lithostratigraphical character and faunal elements which were juxtaposed across the fault. However, the late Ashgill to Silurian sediments in both terranes form part of a regional overstep sequence which links across the suture zone, such that the palaeogeographical contrasts were eliminated by the Silurian. The inference is that the detached terranes were gradually amalgamated by late Ordovician transtensional movements. This occurred when regional scale subduction-related volcanism had ended. Final assembly by early Devonian sinistral transpressive movements juxtaposed a northern terrane, akin to the Lake District/SE Ireland calcalkaline volcanic province, with a southern terrane in the tholeiitic province of eastern Ireland. As distinct from a singular fault trace, the Iapetus suture is regarded as a 100 km wide zone of anastomosing late Caledonian transcurrent faults whose precursors were active during late Ordovician (i.e. Taconic) terrane amalgamation.

Arctic Reconstruction from an Alaskan Viewpoint: ABSTRACT

Field, seismic, structural, and stratigraphic data were used to reconstruct the geologic history of the Arctic in 10-m.y. time slices from the present to mid-Jurassic - the initial opening of the Arctic Ocean. A basic assumption is that Lomonosov Ridge, Alpha Ridge, Mendeleyev Ridge, and Chukchi Plateau are all foundered continental plates. Opening of the Arctic occurs in two stages: Late Jurassic - Cretaceous for the Canada basin and Neogene for the Eurasian basin. Opening is facilitated by two subparallel transform shears - the Arctic (Kaltag-Porcupine) on the east and the Chukchi on the west. Deformation is essentially tensional on the Barents side of the Arctic and shear-compressional on the Alaska side. The development of Chutkoya, North Slope, Brooks Range, north-west Canada, Seward Peninsula, and central Alaska can be sequentially related to Arctic opening, modified by impingement on the northern terrane of allochthonous terranes arriving from the south - the Pacific plates of Tintina, Denali, Orca (Prince William-Chugach-Yakutat), Anadyr, Khatyrka, Kolyman, and other minor terranes. The North Slope of Alaska, a passive, rifted, subsided margin, is restored to line up with a similar margin on Alpha Ridge. Northeastern Alaska (the Romanzof Mountain area) lines up opposite the north end ofmore » the Sverdrup Rim, near Prince Patrick and Borden Islands.« less

The cheyenne belt: analysis of a proterozoic suture in Southern Wyoming

The Cheyenne belt of southeastern Wyoming is a major shear zone which separates Archean rocks of the Wyoming province to the north from 1800-1600 Ma old eugeoclinal gneisses to the south. Miogeoclinal rocks (2500-2000 Ma old) unconformably overlie Archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift-formed continental margin. Intrusive tholeiitic sills and dikes are interpreted as rift-related intrusions and a date of 2000 Ma on a felsic differentiate of these intrusions gives the approximate age of rifting. There are no known post-2000 Ma felsic intrusions north of the Cheyenne belt. Volcanogenic gneisses and abundant syntectonic calc-alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks. The volcanics are a bimodal basalt-rhyolite assemblage. Plutons include large gabbroic complexes and quartz diorite (1780 Ma), syntectonic granitoids (1730-1630 Ma) and post-tectonic anorthosite and granite (1400 Ma). There is no evidence for Archean crust south of the Cheyenne belt. Structural data (thrusts in the miogeoclinal rocks, vertical stretching lineations, and the same fold geometries north and south of the shear zone) suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane (island arc) over the northern terrane (craton and miogeocline), probably as a continuation of the south-dipping subduction which generated calc-alkaline plutons of the southern terrane. A metamorphic discontinuity across the shear zone, with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south, also suggests thrusting of the southern terrane (deeper crustal levels) over the northern terrane (shallower levels). The Cheyenne belt may be a deeply-eroded master decollement, perhaps analogous to a ramp in the master decollement in the southern Appalachians. This interpretation of the Cheyenne belt as a Proterozoic suture zone provides an explanation for the geologic, geochronologic, geophysical, metallogenic, and metamorphic discontinuities across the shear zone.

Supplementary Data for: Rb-Sr and U-Pb Geochronology and Distribution of Rock Types in the Precambrian Basement of Missouri and Kansas

Most of what we know about basement rocks in Kansas and Missouri is derived from cores and cuttings from deep drilling. These rocks may be divided into a northern terrane, underlain by rocks consisting of abundant granite commonly showing cataclastic textures and by metavolcanic to metasedimentary rocks; and a southern terrane, underlain almost exclusively by rhyolitic flows and ash-flow tuffs and epizonal granite plutons. The northern terrane is interrupted in central Kansas by mafic igneous rocks and flanking arkosic sedimentary rocks of the Central North American Rift System.

Rb-Sr and U-Pb geochronology and distribution of rock types in the Precambrian basement of Missouri and Kansas

Most of what we know about basement rocks in Kansas and Missouri is derived from cores and cuttings from deep drilling. These rocks may be divided into a northern terrane, underlain by rocks consisting of abundant granite commonly showing cataclastic textures and by metavolcanic to metasedimentary rocks; and a southern terrane, underlain almost exclusively by rhyolitic flows and ash-flow tuffs and epizonal granite plutons. The northern terrane is interrupted in central Kansas by mafic igneous rocks and flanking arkosic sedimentary rocks of the Central North American Rift System. Data from 63 Rb-Sr whole-rock analyses yield ages ranging from 1,153 to 1,748 m.y., but these are considered to represent only the minimum ages of the rocks. Ages derived from U-Pb analyses of suites of cogenetic zircons from 22 rock samples indicate that some rocks of the northern terrane were formed 1,610 to 1,650 m.y. ago. These are apparently intruded by younger granite plutons formed 1,450–1,470 m.y. and 1,340–1,380 m.y. ago. Rocks of the southern terrane were formed 1,460–1,480 m.y. ago in the St. Francois Mountains terrane of southeastern Missouri and its buried equivalents, but these are about 1,380 m.y. old in southwestern Missouri and southern Kansas. Rocks of the Central North American Rift System in Kansas are assumed to be about 1,100 m.y. old by geophysical and drill-hole extension to their outcrop in the Lake Superior region, where they have been dated. Both the northern and southern terranes are notable for the great abundance of granitic rocks and the scarcity of intermediate to mafic igneous rocks. Quartzite is the most abundant metamorphic rock. Although the decrease in ages from north to south in the mid-continent region suggests their sequential accretion at the edges of a pre-existing continent, the rock assemblages are not consistent with convergent plate-margin suites of the Andean type. The great volumes of rhyolite and epizonal granite of the southern terrane may represent melting of thickened, somewhat older crust following accretion at the continental margin. Major-element chemical data are presented for basement rocks from Kansas, Oklahoma and Texas, and these are compared with the compositions of similar rocks from the St. Francois Mountains. Names and locations are given for wells from which samples were obtained.

Geology and Uranium Deposits Along Northeastern Margin of McDermitt Caldera Complex, Southern Malheur County, Oregon: ABSTRACT

The adjoining Aurora and Bretz uranium prospects are along the northeastern ring-fracture system of the Miocene McDermitt caldera. A series of block faults, constituting the ring-fracture system, divides the area into two contrasting terranes. The northern terrane, comprising the caldera wall and outflow facies, includes a series of mafic to silicic lavas and rhyolite ash-flow tuffs (Bretz Series). Rocks of the southern terrane (Aurora Series) represent infilling of the caldera after End_Page 770------------------------------ collapse. They include an enormously thickened pile of rhyolite ash-flow tuff, equivalent to the youngest Bretz Series ash-flows, intruded and overlain by rhyolite flow domes and lavas. These rhyolites are overlain by a sequence of intermediate lava flows, flow breccia, and pyroclastic breccia constuting the Aurora Lavas. Tuffaceous lacustrine sediment covers most of the southern terrane and much of the ring-fracture system. Hydrothermal alteration associated with mercury deposits at the old Bretz mine is clearly controlled by one of the ring faults, but uranium mineralization nearby is stratigraphically controlled and not directly associated with mercury. Uranium concentrations occur along several horizons, including (1) geologic contacts, unconformities, and redox boundaries in the Bretz Series; (2) a widespread horizon in the tuffaceous lake sediments; and (3) potentially commercial deposits along the flow boundaries and interflow breccias in the Aurora Lavas. The control of uranium by solution channelways along stratigraphic boundaries and lack of crosscutting veins, except in local areas of the Aurora Lavas, suggests that supergene mechanisms were important in ore formation. However, a combination of hydrothermal and supergene processes is favored to explain all the features observed, with the uranium in the lacustrine sediments precipitated from surficial hot springs active during mineralization. End_of_Article - Last_Page 771------------