No abstract

The Barkol Tagh and Karlik Tagh ranges of the easternmost Tien Shan are a natural laboratory for studying the fault architecture of an active termination zone of a major intracontinental mountain range. Barkol and Karlik Tagh and lesser ranges to the north are bounded by active thrust faults that locally deform Quaternary sediments. Major thrusts in Karlik Tagh connect along strike to the east with the left-lateral GobiTien Shan Fault System in SW Mongolia. From a Mongolian perspective, Karlik Tagh represents a large restraining bend for this regional strike-slip fault system, and the entire system of thrusts and strike-slip faults in the Karlik Tagh region defines a horsetail splay fault geometry. Regionally, there appears to be a kinematic transition from thrust-dominated deformation in the central Tien Shan to left-lateral transpressional deformation in the easternmost Tien Shan. This transition correlates with a general eastward decrease in mountain belt width and average elevation and a change in the angular relationship between the NNE-directed maximum horizontal stress in the region and the pre-existing basement structural grain, which is northwesterly in the central Tien Shan (orthogonal to SH max ) but more east-west in the eastern Tien Shan (acute angular relationship with SH max ). Ar-Ar ages indicate that major range-bounding thrusts in Barkol and Karlik Tagh are latest Permian-Triassic ductile thrust zones that underwent brittle reactivation in the Late Cenozoic. It is estimated that the modern mountain ranges of the extreme easternmost Tien Shan could have been constructed by only 10-15 km of Late Cenozoic horizontal shortening.

40Ar/"Ar ages from white mica in rocks of the internal zone of the Brooks Range contractional orogen indicate that the Nanielik antifomal duplex developed at about 120Ma and was remobilized on its southern boundary at c. 108 Ma. Blueschist facies metamorphism accompanied development of the antiform. The timing of the blueschist facies event and creation of the antiform overlap the period of shallow-seated deformation in the foreland fold and thrust belt and sedimentation in the foreland basin of the Brooks Range. Blueschist facies P -T conditions may therefore characterize the thicker parts of orogenic wedges in some orogenic systems; ancient blueschists need not necessarily be interpreted as indicators of active subduction or continent-continent collision.Microprobe analysis using quantitative wavelength-dispersive and electron backscattered electron imaging methods was used to characterize the composition of white micas in the dated samples. None of the samples was compositionally homogeneous; many contained 2-3 populations of white mica, including both potassic and sodic varieties. Samples which had undergone (in sequence) amphibolite, albite-epidote amphibolite and blueschist facies metamorphic events retained muscovites relict of the amphihiite facies event. Samples that had undergone only the blueschist facies event also contained multiple populations of mica, some probably from detrital sources.

The Ruby terrane is an elongate fragment of continental crustal rocks that is structurally overlain by thrust slices of oceanic crust. Our results from the Kokrines Hills, in the southcentral part of the Ruby terrane, demonstrate that the low-angle schistose fabric formed under high-P /low-T conditions, at peak conditions of 10.8-13.2 kbar and 425-550" C, consistent with the rare Occurrence of glaucophane. White mica 40Ar/39Ar cooling ages from these blueschists indicate that the metamorphism occurred prior to 144 f 1 Ma. The blueschist facies assemblages are partially replaced by greenschist facies assemblages in the eastern Kokrines Hills. In contrast, in the central and western Kokrines Hills, upper amphibolite to lower granulite facies metamorphism associated with extensive late Early Cretaceous plutonism has completely overprinted any evidence of an earlier high-PIT metamorphic history. Deformation accompanying the plutonism produced recumbent isoclinal folds in the plutonic rocks and pelitic gneisses of the wallrock; decompression reactions in the pelitic gneisses suggest that the deformation Occurred during exhumation. Thermochronological data bracket the time of intrusion and cooling below 500" C between 118 f 3 and 109 f 1 Ma.Our data from the schists of the Ruby terrane support the general assumption of many authors that the Ruby terrane was subducted beneath an oceanic island arc. This tectonic history is similar t o that described for other large continental crustal blocks in northern and central Alaska, in the Brooks Range, Seward Peninsula and Yukon-Tanana Upland. The current orientation of the Ruby terrane at an oblique angle t o these other crustal blocks and to the Cordilleran trend is due to post-collisional tectonic processes that have greatly modified the original continental margin.

Mesothermal, gold-bearing quartz veins are widespread within allochthonous terranes of Alaska that are composed dominantly of greenschist-facies metasedimentary rocks. The most productive lode deposits are concentrated in south-central and southeastern Alaska; small and generally nonproductive gold-bearing veins occur upstream from major placer deposits in interior and northern Alaska. Oreforming fluids in all areas are consistent with derivation from metamorphic devolatilisation reactions, and a close temporal relationship exists between high-T tectonic deformation, igneous activity, and gold mineralization. Ore fluids were of consistently low salinity, CO2-rich, and had 6~SO values of 7%o-12%o and 6D values between -15%o and -35%o. Upper-crustal temperatures within the metamorphosed terranes reached at least 450-500 ~ before onset of significant gold-forming hydrothermal activity. Within interior and northern Alaska, latest Paleozoic through Early Cretaceous contractional deformation was characterised by obduction of oceanic crust, low-T/high-P metamorphism, and a lack of gold vein formation. Mid-Cretaceous veining occurred some 50-100 m.y. later, during a subsequent high-T metamorphic/magmatic event, possibly related to extension and uplift. In southern Alaska, gold deposits formed during latter stages of Tertiary, subduction-related, collisional orogenesis and were often temporally coeval with calc-alkaline magmatism.