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The Neoarchaean Belingwe greenstone belt of the Zimbabwe craton consists of a well-preserved succession of rocks of low metamorphic grade and low strain. We present new age constraints for the Ngezi Group of the belt and discuss them in the light of greenstone belt evolution. We propose crustal destabilization of the Zimbabwe proto-craton as a result of a global flare of mantle-derived magmatism at ~ 2.75 Ga. This gave rise to extension of a Mesoarchaean granitoid-greenstone terrain, rapid subsidence and, initially, deposition of a transgressive cover succession (Manjeri Formation). High degrees of stretching of proto-cratonic and mantle lithosphere provided accommodation space for the emplacement of several kilometres of submarine volcanic rocks on deeply submerged granitoid-greenstone basement, herein referred to as proto-cratonic flood basalts. Once extension and magmatism had ceased, chemical and clastic sedimentary rocks of the Cheshire Formation were deposited prior to 2.71 Ga as a result of, and subjected to, compressional deformation, and much earlier than previously assumed. [ABSTRACT FROM AUTHOR]

Abstract: The Mesoarchean Tartoq greenstone belt, southern West Greenland, consists of tectonically imbricated slices of metamorphosed basalt, gabbro, peridotite, and sedimentary rocks and is intruded by felsic rocks (now mylonites) [...]

The Litchfield pluton is a poorly exposed 7 [km.sup.2] composite alkalic intrusive complex that cuts previously deformed and metamorphosed Silurian turbidites in south-central Maine. The pluton includes a variety of alkaline syenites, including the type locality of 'litchfieldite,' a coarse-grained cancrinite-, sodalite-, and lepidomelane-bearing nepheline syenite first recognized over 150 years ago and common in many petrologic collections. A new U-Pb zircon age of 321 ± 2 Ma from the nepheline syenite is interpreted to represent the crystallization age of the plutonic complex. A new biotite [sup.40]Ar/[sup.39]Ar age of 239 ± 1 Ma from the syenite is similar to previously published mica ages from the surrounding country rocks and dates the time of regional cooling in the area below ~ 300°C. Whole-rock chemical analyses of rocks of the Litchfield pluton reveal silica-undersaturated alkaline compositions that are consistent with formation in a within-plate tectonic setting. The age and geochemical characteristics of the alkalic igneous rocks near Litchfield are consistent with a model that invokes the generation of a small volume of alkalic magma beneath south-central Maine during a period of Carboniferous transcurrent tectonism in the northern Appalachian orogen. Le pluton de Litchfield est un complexe intrusif alcalin composite de sept kilometres carres peu affleurant qui recoupe des turbidites siluriennes precedemment deformees et metamorphisees dans le centre-sud du Maine. Le pluton inclui diverses syenites alcalines, notamment la localite type de la << litchfieldite >>, une syenite nephelinique renfermant de la cancrinite, de la sodalite et de la lepidomelane a grain grossier identifiee pour la premiere fois il y a plus de 150 ans et repandue dans maintes collections petrologiques. Une nouvelle datation U-Pb sur zircon de 321 ± 2 Ma de la syenite nephelinique est interpretee comme une representation de lage de la cristallisation du complexe de roches plutoniques. Une nouvelle datation de la syenite par biotite et [sup.40]Ar/[sup.39]Ar de 239 ± 1 Ma est similaire aux ages du mica des roches encaissantes voisines publies auparavant et situe le moment du refroidissement regional dans le secteur au-dessous d'environ 300°C. Des analyses chimiques sur roche totale des roches du pluton de Litchfield revelent des compositions alcalines sous-saturees de silice correspondant a une formation dans un milieu tectonique intraplaque. L'age et les caracteristiques geochimiques des roches ignees alcalines pres de Litchfield sont conformes a un modele evoquant la production dun volume modeste de magma alcalin sous le centre-sud du Maine durant une periode dactivite tectonique de coulissage du Carbonifere dans le nord de l'orogene appalachien. [Traduit par la redaction], INTRODUCTION Since the mid-1800s (Jackson 1845; Clarke 1886), the small village of Litchfield in south-central Maine (Fig. 1) has been known as a collecting locality for specimens of cancrinite, sodalite, [...]

Onshore evidence suggests that a plate is missing from published reconstructions of the northeastern Pacific Ocean in Paleocene--Eocene time. The Resurrection plate, named for the Resurrection Peninsula ophiolite near Seward, Alaska, was located east of the Kula plate and north of the Farallon plate. We interpret coeval near-trench magmatism in southern Alaska and the Cascadia margin as evidence for two slab windows associated with trench-ridge-trench (TRT) triple junctions, which formed the western and southern boundaries of the Resurrection plate. In Alaska, the Sanak-Baranof belt of near-trench intrusions records a west-to-east migration, from 61 to 50 Ma, of the northern TRT triple junction along a 2100-km-long section of coastline. In Oregon, Washington, and southern Vancouver Island, voluminous basaltic volcanism of the Siletz River Volcanics, Crescent Formation, and Metchosin Volcanics occurred between ca. 66 and 48 Ma. Lack of a clear age progression of magmatism along the Cascadia margin suggests that this southern triple junction did not migrate significantly. Synchronous near-trench magmatism from southeastern Alaska to Puget Sound at ca. 50 Ma documents the middle Eocene subduction of a spreading center, the crest of which was subparallel to the margin. We interpret this ca. 50 Ma event as recording the subduction-zone consumption of the last of the Resurrection plate. The existence and subsequent subduction of the Resurrection plate explains (1) northward terrane transport along the southeastern Alaska-British Columbia margin between 70 and 50 Ma, synchronous with an eastward-migrating triple junction in southern Alaska; (2) rapid uplift and voluminous magmatism in the Coast Mountains of British Columbia prior to 50 Ma related to subduction of buoyant, young oceanic crust of the Resurrection plate; (3) cessation of Coast Mountains magmatism at ca. 50 Ma due to cessation of subduction, (4) primitive mafic magmatism in the Coast Mountains and Cascade Range just after 50 Ma, related to slab-window magmatism, (5) birth of the Queen Charlotte transform margin at ca. 50 Ma, (6) extensional exhumation of high-grade metamorphic terranes and development of core complexes in British Columbia, Idaho, and Washington, and extensional collapse of the Cordilleran foreland fold-and-thrust belt in Alberta, Montana, and Idaho after 50 Ma related to initiation of the transform margin, (7) enigmatic 53-45 Ma magmatism associated with extension from Montana to the Yukon Territory as related to slab breakup and the formation of a slab window, (8) right-lateral margin-parallel strike-slip faulting in southern and western Alaska during Late Cretaceous and Paleocene time, which cannot be explained by Farallon convergence vectors, and (9) simultaneous changes in Pacific-Farallon and Pacific-Kula plate motions concurrent with demise of the Kula-Resurrection Ridge. Keywords: tectonics, Eocene, Kula plate, Farallon plate, North America, magmatism.

Many accretionary prisms are dominated by different structural belts, melange and relatively coherent flysch terrains. Controls on flysch and melange accretion are not well understood. New geologic mapping results on the southern Kenai Peninsula are reported and a structural model to explain the dramatic alterations in accretion style from melange of the McHugh complex to coherent turbidites of the Valdez Group is proposed.

The Central Maine Basin is the largest expanse of deep-marine, Upper Ordovician to Devonian metasedimentary rocks in the New England Appalachians, and is a key to the tectonics of the Acadian Orogeny. Detrital zircon ages are reported from two groups of strata: (1) the Quimby, Rangeley, Perry Mountain and Smalls Falls Formations, which were derived from inboard, northwesterly sources and are supposedly older; and (2) the Madrid, Carrabassett and Littleton Formations, which were derived from outboard, easterly sources and are supposedly younger. Deep-water deposition prevailed throughout, with the provenance shift inferred to mark the onset of foredeep deposition and orogeny. The detrital zircon age distribution of a composite of the inboard-derived units shows maxima at 988 and 429 Ma; a composite from the outboard-derived units shows maxima at 1324, 1141, 957, 628, and 437 Ma. The inboard-derived units have a greater proportion of zircons between 450 and 400 Ma. Three samples from the inboard-derived group have youngest age maxima that are significantly younger than the nominal depositional ages. The outboard-derived group does not share this problem. These results are consistent with the hypothesised provenance shift, but they signal potential problems with the established stratigraphy, structure, and (or) regional mapping. Shallow-marine deposits of the Silurian to Devonian Ripogenus Formation, from northwest of the Central Maine Basin, yielded detrital zircons featuring a single age maximum at 441 Ma. These zircons were likely derived from a nearby magmatic arc now concealed by younger strata. Detrital zircons from the Tarratine Formation, part of the Acadian foreland-basin succession in this strike belt, shows age maxima at 1615, 980 and 429 Ma. These results are consistent with three episodes of zircon recycling beginning with the deposition of inboard-derived strata of the Central Maine Basin, which were shed from post-Taconic highlands located to the northwest. Next, southeasterly parts of this succession were deformed in the Acadian orogeny, shedding detritus towards the northwest into what remained of the basin. Finally, by Pragian time, all strata in the Central Maine Basin had been deformed and detritus from this new source accumulated as the Tarratine Formation in a new incarnation of the foreland basin. Silurian-Devonian strata from the Central Maine Basin have similar detrital zircon age distributions to coeval rocks from the Arctic Alaska and Farewell terranes of Alaska and the Northwestern terrane of Svalbard. We suggest that these strata were derived from different segments of the 6500-km-long Appalachian-Caledonide orogen. [ABSTRACT FROM AUTHOR]

The Tertiary Eurekan orogen of northernmost North America differs from a standard fold-and-thrust belt in several respects. It lacks a metamorphic-plutonic hinterland and wedge-shaped profile; instead, 2-km-high mountains of southeast Ellesmere Island face the frontal trust, whereas halotectonic polygons, gentle warps, and subdued topography extend form Axel Heiberg Island in the center of the system back to the Sverdurp Rim at its rear. Clastic deposits are not located in a single flexural foredeep but are distributed in topographic lows amid several thrust sheets. The age of the stratigraphy, amount of displacement, and intensity of strain all increase cratonward, and the system's width-to-length ratio is anomalously high. The orogen is attributed to Greenland's pivotal movement relative to North America, which formed a braided Cenozoic plate boundary in Ellesmere and Axel Heiberg Islands, not a single transform in Nares Strait as previously proposed. The three major strands of this braided system are the Parrish Glacier, Vesle Fiord, and Stolz thrusts. They die out toward a structural pole of rotation in the south, whereas to the north, movement is accommodated by a dextral transpression zone extending from Lake Hazen to northern Greenland. Field studies on Ellesmere Island combined with a regional synthesis of previous work show that Eurekan deformation style is indicative of pivotal tectonism, that the contraction necessary to accommodate Greenland's displacement is similar in magnitude to that documented by Eurekan structures, and that the ages of continental structures are compatible with the paleomagnetic record of seafloor spreading in Labrador Sea and Baffin Bay. Previously proposed sinistral strike-slip displacement is not confirmed; rather, eastvergent thrusts and dextral wrench faults typify the system. We propose that the Eurekan orogeny marked a change from tip propagation to pivotal tectonism as the North Atlantic rift system penetrated the entire width of the Laurasian continent.

We report detrital zircon ages from modem sands from rivers and beaches of New England. Sand samples were collected from the Connecticut, Thames, Pawcatuck, Blackstone, Merrimack, Saco, Presumpscot, and Androscoggin-Kennebec watersheds, and from Cape Cod. About 1200 detrital zircons were analyzed by LAICPMS; 984 of these passed strict filters for concordance. A composite plot of the 984 detrital zircon ages from sands and a new plot of 230 igneous ages from the literature show similar age distributions. On the basis of both datasets, the times of peak magmatism in the New England Appalachians, in round numbers, are at 580-610, 440-450, 420, 405, 370-380, 180-200, and 125 Ma. Most zircons from individual watersheds can be attributed to dated igneous rocks in the same watershed. A few zircons are probably exotic and can be interpreted as having been transported into the sampled watershed by ice, and a handful of other zircons likely represent undated plutons within the sampled watershed. The correspondence between the detrital and igneous datasets underscores the proxy value of detrital zircons from river sands in regional geologic reconnaissance--even in glaciated regions. Most detrital zircons from the modem passive margin of New England are Phanerozoic in age, reflecting various Appalachian sources; the age distribution of zircons older than 700 Ma closely resembles the age distribution of detrital zircons from the next-older passive margin that flanked this part of Laurentia after the breakup of Rodinia, that is, the Cambrian passive margin of the Appalachians. [ABSTRACT FROM AUTHOR]