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The terranes composing the basement of the Tian Shan were originally sutured together during two collisions in Late Devonian-Early Carboniferous and Late Carboniferous-Early Permian time. Since then, the range has repeatedly been uplifted and structurally reactivated, apparently as a result of the collision of island arcs and continental blocks with the southern margin of Asia far to the south of the range. Evidence for these deformational episodes is recorded in the sedimentary histories of the Junggar and Tarim foreland basins to the north and south of the range and by the cooling and exhumation histories of rocks in the interior of the range. Reconnaissance apatite fission-track cooling ages from the Chinese part of the range cluster in three general time periods, latest Paleozoic, late Mesozoic, and late Cenozoic. Latest Paleozoic cooling is recorded at Aksu (east of Kalpin) on the southern flank of the range, at two areas in the central Tian Shan block along the Dushanzi-Kuqa Highway, and by detrital apatites at Kuqa that retain fission-track ages of their sediment source areas. Available 40 Ar/ 39 Ar cooling ages from the range also cluster within this time interval, with very few younger ages. These cooling ages may record exhumation and deformation caused by the second basement suturing collision between the Tarim-central Tian Shan composite block and the north Tian Shan. Apatite data from three areas record late Mesozoic cooling, at Kuqa on the southern flank of the range and at two areas in the central Tian Shan block. Sedimentary sections in the Junggar and Tarim foreland basins contain major unconformities, thick intervals of alluvial conglomerate, and increased subsidence rates between about 140 and 100 Ma. These data may reflect deformation and uplift induced by collision of the Lhasa block with the southern margin of Asia in latest Jurassic-Early Cretaceous time. Large Jurassic intermontane basins are preserved within the interior of the Tian Shan and in conjunction with the fission-track data suggest that the late Mesozoic Tian Shan was subdivided into a complex of generally east-west-trending, structurally controlled subranges and basins.Apatite data from five areas record major late Cenozoic cooling, at sites in the basin-vergent thrust belts on the northern and southern margins of the range, and along the north Tian Shan fault system in the interior of the range. The thrust belts

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To test existing models for the formation of the Amerasian Basin, detrital zircon suites from 12 samples of Triassic sandstone from the circum‐Arctic region were dated by laser ablation‐inductively coupled plasma‐mass spectrometry (ICP‐MS). The northern Verkhoyansk (NE Russia) has Permo‐Carboniferous (265–320 Ma) and Cambro‐Silurian (410–505 Ma) zircon populations derived via river systems from the active Baikal Mountain region along the southern Siberian craton. Chukotka, Wrangel Island (Russia), and the Lisburne Hills (western Alaska) also have Permo‐Carboniferous (280–330 Ma) and late Precambrian‐Silurian (420–580 Ma) zircons in addition to Permo‐Triassic (235–265 Ma), Devonian (340–390 Ma), and late Precambrian (1000–1300 Ma) zircons. These ages suggest at least partial derivation from the Taimyr, Siberian Trap, and/or east Urals regions of Arctic Russia. The northerly derived Ivishak Formation (Sadlerochit Mountains, Alaska) and Pat Bay Formation (Sverdrup Basin, Canada) are dominated by Cambrian–latest Precambrian (500–600 Ma) and 445–490 Ma zircons. Permo‐Carboniferous and Permo‐Triassic zircons are absent. The Bjorne Formation (Sverdrup Basin), derived from the south, differs from other samples studied with mostly 1130–1240 Ma and older Precambrian zircons in addition to 430–470 Ma zircons. The most popular plate tectonic model for the origin of the Amerasian Basin involves counterclockwise rotation of the Arctic Alaska–Chukotka microplate away from the Canadian Arctic margin. The detrital zircon data suggest that the Chukotka part of the microplate originated closer to the Taimyr and Verkhoyansk, east of the Polar Urals of Russia, and not from the Canadian Arctic.

[1] Structural, geophysical, and thermochronological data from the transition zone between the Sierra Nevada and the Basin and Range province at latitude $39°N suggest $100 km westward encroachment of Basin and Range extensional deformation since the middle Miocene. Extension, accommodated primarily by east dipping normal faults that bound west tilted, range-forming fault blocks, varies in magnitude from <2% in the interior of the Sierra Nevada crustal block to >150% in the Wassuk and Singatse Ranges to the east. Geological and apatite fission track data from exhumed upper crustal sections in the Wassuk and Singatse Ranges point to rapid footwall cooling related to large magnitude extension starting at $14 -15 Ma. Farther to the west, geological and thermochronological data indicate a younger period of extension in the previously unextended Pine Nut Mountains, the Carson Range, and the Tahoe-Truckee depression initiated between 10 Ma and 3 Ma, and incipient post-0.5 Ma faulting to the west of the Tahoe-Truckee area. These data imply the presence of an extensional breakaway zone between the Singatse Range and the Pine Nut Mountains at $14 -15 Ma, forming the boundary between the Sierra Nevada and Basin and Range at that time. In addition, fission track data imply a Miocene preextensional geothermal gradient of 27 ± 5°C km À1 in the central Wassuk Range and 20 ± 5°C km À1 in the Singatse Range, much higher than the estimated early Tertiary gradient of 10 ± 5°C km À1 for the Sierra Nevada batholith. This might point to a significant increase in geothermal gradients coupled with a likely decrease in crustal strength enabling the initiation of extensional faulting. Apatite fission track, geophysical, and geological constraints across the Sierra Nevada-Basin and Range transition zone indicate a twostage, coupled structural and thermal westward encroachment of the Basin and Range province into the Sierra Nevada since the middle Miocene.

[1] During subduction at the Franciscan trench beginning at 170-160 Ma and continuing to the present, marine sedimentary and lesser volcanic rocks have been underthrust, accreted, and metamorphosed to form the Franciscan accretionary wedge. The South Fork Mountain Schist (SFMS) forms the eastern margin and structural top of the wedge and so was apparently the first unit of substantial size to accrete into the Franciscan.

A regional synthesis of new and existing geologic and thermochronologic data document late Cretaceous -early Cenozoic regional erosion, Oligocene -Miocene volcanism, and subsequent late Miocene extension of the Basin and Range Province in northwestern Nevada and northeastern California. Across an ϳ220-kmwide region between the Santa Rosa and Warner Ranges, conformable sequences of 35 to 15 Ma volcanic rocks are cut by only a single generation of high-angle normal faults that accommodated ϳ23 km of total east-west extension (ϳ12%). Fission-track, (U-Th)/He, geologic, and structural data from the Pine Forest Range show that faulting there began at 11 to 12 Ma, progressed at a relatively constant rate until at least 3 Ma, and has continued until near the present time. Extension in the Santa Rosa Range to the east took place during the same interval, although the post-6 Ma part of this history is less well constrained. Less complete constraints from adjacent ranges permit a similar timing for faulting, and we infer that extensional faulting in northwestern Nevada began everywhere at 12 Ma and has continued up to the present. Faulting in the Warner Range in northeastern California can only be constrained to have begun between 14 and 3 Ma, but may represent westward migration of Basin and Range extension during the Pliocene. Compared to the many parts of the Basin and Range in central and southern Nevada, extension in northwestern Nevada began more recently, is of lesser total magnitude, and was accommodated entirely by high-angle normal faults. Fission-track data document Late Cretaceous unroofing of Cretaceous (115 -100 Ma) granitic basement rocks in northwestern Nevada, followed by a long period of relative tectonic quiescence that persisted through Oligocene and Miocene volcanism until the onset of Basin and Range extension at ϳ12 Ma. The low magnitude of extension (12%) and early Tertiary stability suggest that the modern ϳ31 km thick crust in northwestern Nevada was only slightly thicker (ϳ35 km) prior to extension at 12 Ma, and was no thicker than ϳ38 km in the Late Cretaceous. This stands in contrast to other parts of the Basin and Range, where the crust was thickened to at least 45 to 50 km by Cretaceous thrusting and subsequently thinned to ϳ30 km by large magnitude (>50%) extension.