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The origin and antiquity of the subdued topography of the Yukon-Tanana Upland (YTU), the physiographic province between the Denali and Tintina faults, are unresolved questions in the geologic history of interior Alaska and adjacent Yukon. We present apatite fission-track (AFT) results for 33 samples from the 2300 [km.sup.2] western Fortymile district in the YTU in Alaska and propose an exhumation model that is consistent with preservation of volcanic rocks in valleys that requires base level stability of several drainages since latest Cretaceous-Paleocene time. AFT thermochronology indicates widespread cooling below ~110°C at ~56-47 Ma (early Eocene) and ~44-36 Ma (middle Eocene). Samples with ~33-27, ~19, and ~10 Ma AFT ages, obtained near a major northeast-trending fault zone, apparently reflect hydrothermal fluid flow. Uplift and erosion following ~107 Ma magmatism exposed plutonic rocks to different extents in various crustal blocks by latest Cretaceous time. We interpret the Eocene AFT ages to suggest that higher elevations were eroded during the Paleogene subtropical climate of the subarctic, while base level remained essentially stable. Tertiary basins outboard of the YTU contain sediment that may account for the required >2 km of removed overburden that was not carried to the sea by the ancestral Yukon River system. We consider a climate driven explanation for the Eocene AFT ages to be most consistent with geologic constraints in concert with block faulting related to translation on the Denali and Tintina faults resulting from oblique subduction along the southern margin of Alaska. La question de l'origine et de l'age du relief adouci des hautes terres du Yukon-Tanana (HTYT), la province physiographique situee entre les failles de Denali et de Tintina, demeure sans reponse dans le contexte de l'histoire geologique de l'interieur de l'Alaska et du Yukon voisin. Nous presentons les resultats d'analyses des traces de fission dans l'apatite (TFA) pour 33 echantillons provenant du district de Fortymile occidental, dans les HTYT en Alaska, et proposons un modele d'exhumation qui concorde avec la preservation de roches volcaniques dans les vallees, ce qui necessite que les niveaux de base de plusieurs bassins versants soient demeures stables depuis la periode du Cretace terminal-Paleocene. La thermochronologie par TFA indique un refroidissement repandu sous les ~110°C a ~56-47 Ma (Eocene precoce) et ~44-36 Ma (Eocene moyen). Des echantillons presentant des ages TFA de ~33-27, ~19 et ~10 Ma, obtenus pres d'une faille majeure d'orientation nord- est, semblent refleter l'ecoulement de fluides hydrothermaux. Le soulevement et l'erosion faisant suite au magmatisme a ~107 Ma avaient, au Cretace terminal, expose a divers degres des roches plutoniques dans divers blocs crustaux. Nous en concluons que les ages TFA eocenes indiqueraient que les zones de plus haute elevation ont ete erodees durant la periode de climat subtropical de la region subarctique au Paleogene, alors que le niveau de base demeurait essentiellement stable. Des bassins d'age tertiaire distaux par rapport aux HTYT contiennent des sediments qui pourraient expliquer les plus de 2 km de materiaux sus- jacents retires qui n'ont pas ete transportes vers la mer par le reseau ancestral du fleuve Yukon. Nous estimons qu'une explication reliee au climat pour les ages TFA eocenes est ce qui correspond le mieux aux contraintes geologiques, de concert avec le morcellement par failles associe a la translation le long des failles de Denali et de Tintina resultant de la subduction oblique le long de la marge meridionale de l'Alaska. [Traduit par la Redaction], Introduction The Yukon-Tanana Upland (YTU) of east-central Alaska (Fig. 1) is made up of Paleozoic metasedimentary and metaigneous rocks that formed along the northwestern margin of Laurentia and were subsequently [...]

We report the results of new mapping, whole-rock major, minor, and trace-element geochemistry, and petrography for metaigneous rocks from the Mount Veta area in the westernmost part of the allochthonous Yukon-Tanana terrane (YTT) in east-central Alaska. These rocks include tonalitic myloite gneiss and mafic metaigneous rocks from the Chicken metamorphic complex and the Nasina and Fortymile River assemblages. Whole-rock trace-element data from the tonalitic gneiss, whose igneous protolith was dated by SHRIMP U-Pb zircon geochronology at 332.6 ± 5.6 Ma, indicate derivation from tholeiitic arc basalt. Whole-rock analyses of the mafic rocks suggest that greenschist-facies rocks from the Chicken metamorphic complex, a mafic metavolcanic rock from the Nasina assemblage, and an amphibolite from the Fortymile River assemblage formed as island-arc tholeiite in a back-arc setting; another Nasina assemblage greenschist has MORB geochemical characteristics, and another mafic metaigneous rock from the Fortymile River assemblage has geochemical characteristics of calc-alkaline basalt. Our geochemical results imply derivation in an arc and back-arc spreading region within the allochthonous YTT crustal fragment, as previously proposed for correlative units in other parts of the terrane. We also describe the petrography and geochemistry of a newly discovered tectonic lens of Alpine- type metaharzburgite. The metaharzburgite is interpreted to be a sliver of lithospheric mantle from beneath the Seventymile ocean basin or from sub-continental mantle lithosphere of the allochthonous YTT or the western margin of Laurentia that was tectonically emplaced within crustal rocks during closure of the Seventymile ocean basin and subsequently displaced and fragmented by faults. Nous presentons les resultats d'une nouvelle cartographie, de geochimie sur la roche entiere, des elements majeurs, mineurs et traces, et de petrographie de roches meta-ignees du secteur du mont Veta dans la partie la plus a l'ouest du terrane allochtone Yukon-Tanana (YTT) dans le centre-est de l'Alaska. Ces roches comprennent des gneiss tonalitiques etmylonitiques et des roches mafiques meta-ignees du complexe metamorphique Chicken et des assemblages Nasina et Fortymile River. Les donnees provenant des elements en trace de la roche entiere du gneiss tonalitique, dont le protolithe igne a ete date par geochronologie la microsonde ionique a haute resolution et a haut niveau de sensibilite (SHRIMP) U-Pb sur zircon a 332,6 ± 5,6 Ma, indiquant une provenance d'un arc basaltique tholeiitique. Des analyses de la roche entiere des roches mafiques suggerent que les roches au facies des schistes verts du complexe metamorphique Chicken, une roche metavolcanique mafique de l'assemblage Nasina et une amphibole de l'assemblage Fortymile River, se soient formees en tant que tholeiite d'arc insulaire dans un contexte d'arriere-arc. Un autre schiste vert de l'assemblage Nasina possede des caracteristiques geochimiques de basalte de dorsale medio-oceanique (MORB) et une autre rochemafiquemeta-ignee de l'assemblage Fortymile River possede des caracteristiques geochimiques d'un basalte calco-alcalin. Nos resultats geochimiques indiquent une derivation dans une region d'arc et d'arriere-arc s'etalant a l'interieur du fragment de croute du terrane allochtone YTT, tel que propose anterieurement pour des unites correlatives dans d'autres parties du terrane. Nous decrivons aussi la petrographie et la geochimie d'une lentille tectonique, recemment decouverte, de metaharzburgite de type alpin. La metaharzburgite est interpretee comme etant un copeau de manteau lithospherique provenant de sous le bassin oceanique de Seventymile ou d'un manteau lithospherique subcontinental du terrane allochtone YTT ou de la bordure ouest de la Laurentie, un craton qui a etemis en place dans les roches de la croute durant la fermeture du bassin oceanique de Seventymile et par la suite deplace et fragmente par des failles. [Traduit par la Redaction], Introduction The allochthonous Yukon-Tanana terrane (YTT) makes up a large, closepin-shaped province of pericratonic, regionally metamorphosed and deformed rocks that extends from east-central Alaska southeastward into Yukon and British Columbia, [...]

Geologic mapping and fracture studies have documented the fundamental patterns of joints and faults in the thick sequence of rhyolite tufts at Yucca Mountain, Nevada, the proposed site of an underground repository for high-level radioactive waste. The largest structures are north-striking, block-bounding normal faults (with a subordinate left-lateral component) that divide the mountain into numerous 1-4-km-wide panels of gently east-dipping strata. Blockbounding faults, which underwent Quaternary movement as well as earlier Neogene movement, are linked by dominantly northwest-striking relay faults, especially in the more extended southern part of Yucca Mountain. Intrablock faults are commonly short and discontinuous, except those on the more intensely deformed margins of the blocks. Lithologic properties of the local tuff stratigraphy strongly control the mesoscale fracture network, and locally the fracture network has a strong influence on the nature of intrablock faulting. The least faulted part of Yucca Mountain is the north-central part, the site of the proposed repository. Although bounded by complex normal-fault systems, the 4-km-wide central block contains only sparse intrablock faults. Locally intense jointing appears to be strata-bound. The complexity of deformation and the magnitude of extension increase in all directions away from the proposed repository volume, especially in the southern part of the mountain where the intensity of deformation and the amount of vertical-axis rotation increase markedly. Block-bounding faults were active at Yucca Mountain during and after eruption of the 12.8-12.7 Ma Paintbrush Group, and significant motion on these faults postdated the 11.6 Ma Rainier Mesa Tuff. Diminished fault activity continued into Quaternary time. Roughly half of the stratal tilting in the site area occurred after 11.6 Ma, probably synchronous with the main pulse of vertical-axis rotation, which occurred between 11.6 and 11.45 Ma. Studies of sequential formation of tectonic joints, in the context of regional paleostress studies, indicate that north- and north-west-striking joint sets formed coevally with the main faulting episode during regional east-northeast-west-southwest extension and that a prominent northeast-striking joint set formed later, probably after 9 Ma. These structural analyses contribute to the understanding of several important issues at Yucca Mountain, including potential hydrologic pathways, seismic hazards, and fault-displacement hazards. Keywords: structural geology, Yucca Mountain, Nevada, faults, joints.

The Mesozoic magmatic history of the North American margin records the evolution from a more segmented assemblage of parautochthonous and allochthonous terranes to the more cohesive northern Cordilleran orogenic belt. We characterize the setting of magmatism, tectonism, and epigenetic mineralization in the western Fortymile mining district, east-central Alaska, where parautochthonous and allochthonous Paleozoic tectonic assemblages are juxtaposed, using sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon geochronology, whole-rock geochemistry, and feldspar Pb isotopes of Mesozoic intrusions and spatially associated mineral prospects. New SHRIMP U-Pb zircon ages and published U-Pb and 40Ar/39Ar ages indicate four episodes of plutonism in the western Fortymile district: Late Triassic (216-208 Ma), Early Jurassic (199-181 Ma), mid-Cretaceous (112-94 Ma), and Late Cretaceous (70-66 Ma). All age groups have calc-alkalic arc compositions that became more evolved through time. Pb isotope compositions of feldspars from Late Triassic, Early Jurassic, and Late Cretaceous igneous rocks similarly became more radiogenic with time and are consistent with the magmas being mantle derived but extensively contaminated by upper crustal components with evolving Pb isotopic compositions. Feldspar Pb isotopes from mid-Cretaceous rocks have isotopic ratios that indicate magma derivation from upper crustal sources, probably thickened mid-Paleozoic basement. The origin of the mantle component in Late Cretaceous granitoids suggested by Pb isotopic ratios is uncertain, but we propose that it reflects asthenospheric upwelling following slab breakoff and sinking of an inactive inner subduction zone that delivered the previously accreted Wrangellia composite terrane to the North American continental margin, after the outer Farallon subduction zone was established. Epigenetic Pb-Zn-Ag ± Cu prospects in the western Fortymile district are spatially associated with splays of the northeast-trending Kechumstuk sinistral- normal fault zone and with ca. 68-66 Ma felsic intrusions and dikes. The similarity between Pb isotope compositions of feldspars from the Late Cretaceous igneous bodies and sulfides from the epithermal prospects suggests a Late Cretaceous age for most of the mineralization. Fluid flow along the faults undoubtedly played a major role in mineralization. We interpret displacement on the northeast-trending faults to be a far-field effect of dextral translation along Late Cretaceous plate-scale boundaries and faults that were roughly parallel to the subsequently developed Denali and Tintina fault systems, which currently bound the region. [ABSTRACT FROM AUTHOR]