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[Abstract The uplift and associated exhumation of the Tibetan Plateau has been widely considered a key control of Cenozoic global cooling. The south‐central parts of this plateau experienced rapid exhumation during the Cretaceous–Palaeocene periods. When and how the northern part was exhumed, however, remains controversial. The Hoh Xil Basin (HXB) is the largest late Cretaceous–Cenozoic sedimentary basin in the northern part, and it preserves the archives of the exhumation history. We present detrital apatite and zircon (U‐Th)/He data from late Cretaceous–Cenozoic sedimentary rocks of the western and eastern HXB. These data, combined with regional geological constraints and interpreted with inverse and forward model of sediment deposition and burial reheating, suggest that the occurrence of ca. 4–2.7 km and ca. 4–2.3 km of vertical exhumation initiated at ca. 30–25 Ma and 40–35 Ma in the eastern and western HXB respectively. The initial differential exhumation of the eastern HXB and the western HXB might be controlled by the oblique subduction of the Qaidam block beneath the HXB. The initial exhumation timing in the northern Tibetan Plateau is younger than that in the south‐central parts. This reveals an episodic exhumation of the Tibetan Plateau compared to models of synchronous Miocene exhumation of the entire plateau and the early Eocene exhumation of the northern Tibetan Plateau shortly after the India–Asia collision. One possible mechanism to account for outward growth is crustal shortening. A simple model of uplift and exhumation would predict a maximum of 0.8 km of surface uplift after upper crustal shortening during 30–27 Ma, which is insufficient to explain the high elevations currently observed. One way to increase elevation without changing exhumation rates and to decouple uplift from upper crustal shortening is through the combined effects of continental subduction, mantle lithosphere removal and magmatic inflation., We present detrital apatite and zircon (U‐Th)/He data from late Cretaceous–Cenozoic sedimentary rocks of the Hoh Xil Basin (HXB). These data, combined with regional geological constraints and interpreted with inverse and forward model of sediment deposition and burial reheating, suggest that ~4–2.7 km vertical exhumation initiated at 30 Ma in the eastern HXB. A simple model of uplift and exhumation would predict a maximum of 0.8 km of surface uplift after upper crustal shortening during 30–27 Ma, which is insufficient to explain the high elevations currently observed. One way to increase elevation without changing exhumation rates and to decouple uplift from upper crustal shortening is through the combined effects of continental subduction, mantle lithosphere removal and magmatic inflation. ]

Water isotopes are an important tool for reconstructing the amount of atmospheric lifting related to high topography in the geologic past. However, our capacity for meaningful interpretation requires understanding the climatic setting and isolating the influence of orography on water isotopes. Patagonia9s simple, steady climatology and location within the Southern Westerlies makes it an ideal setting for successful application of water isotopes to measuring topography through time. Here we use hydrated volcanic glass to construct a new record of the size of the Patagonian Andes during the Cenozoic. We also utilize a novel method for identifying the contribution of orography in regional climate records. Our results show that variation in the observed record can largely be explained by variations in climate. Thus we conclude that the mountain range has maintained a size similar to modern since at least Paleocene. This result is in agreement with geologic data, which constrain the bulk of the surface uplift of the Andes to the Cretaceous. The reconstruction of the Patagonian Andes, which grew in the Cretaceous and remained high through the Cenozoic, is markedly different from the widely held view of Miocene formation of this mountain range. In particular, the topography appears to remain stable during the northward propagation and collision of offshore spreading centers.

Early glaciations in the Patagonian Andes show that fast initial glacial erosion is transient and is not a steady condition., We report a mountain-scale record of erosion rates in the central Patagonian Andes from >10 million years (Ma) ago to present, which covers the transition from a fluvial to alpine glaciated landscape. Apatite (U-Th)/He ages of 72 granitic cobbles from alpine glacial deposits show slow erosion before ~6 Ma ago, followed by a two- to threefold increase in the spatially averaged erosion rate of the source region after the onset of alpine glaciations and a 15-fold increase in the top 25% of the distribution. This transition is followed by a pronounced decrease in erosion rates over the past ~3 Ma. We ascribe the pulse of fast erosion to local deepening and widening of valleys, which are characteristic features of alpine glaciated landscapes. The subsequent decline in local erosion rates may represent a return toward a balance between rock uplift and erosion.

Outcrops within the Eocene Domengine Formation (central California) provide an exceptional opportunity to observe the relationship between depositional facies and submarine mass failure. The Domengine Formation is interpreted as an assemblage of heterolithic, shallow- to marginal-marine deposits that record progradation and aggradation prior to regional flooding and local collapse of the shelf. Structural analysis of soft-sediment folds and reverse faults within the informally named New Idria mass-transport deposit (MTD) suggests transport toward the west to southwest, which is consistent with paleoflow measurements from cross bedding within the MTD. Depositional facies have a strong influence on the character of soft-sediment deformation (e.g., style and scale of folding and faulting), and major layer-­parallel detachment surfaces within and below the New Idria MTD occur where mudstone-dominated or heterolithic units underlie sandstone-dominated units. Conditions favorable to gravitational instability and eventual collapse of the shelf include (1) high sedimentation rates within a rapidly aggrading system; (2) loading of under-compacted, mechanically weak, fine-grained ­layers by overlying, denser sand bodies; and (3) earthquake seismicity and/or over-steepening of slopes within a tectonically active, convergent margin setting. This study provides a rare detailed glimpse into the collapse of a deltaic stratigraphic sequence, yielding insight into the linkage between marginal marine facies and depositional architecture and the propensity for submarine mass failure of continental shelves.

The Arctic Alaska terrane of northern Alaska and Yukon is one of several exotic terranes in the North American Cordillera with putative early Paleozoic connections to the northern Caledonian-Appalachian orogen. The U-Pb and 40 Ar/ 39 Ar isotopic data from detrital minerals in pre-Mississippian sedimentary units of the northeastern Brooks Range are presented here to investigate the consequences of Caledonian orogenesis on sediment dispersal trends and the paleogeography of northern Laurentia. Neoproterozoic–Cambrian siliciclastic rocks of the informal Firth River group and the Neruokpuk Formation were most likely deposited along a passive margin that sourced Archean and Paleoproterozoic basement rocks of the Canadian shield and reworked Mesoproterozoic and younger sedimentary units. These strata are overlain by a Lower Ordovician–Lower Devonian succession of fine-grained siliciclastic turbidites, herein referred to as the Clarence River group, which records a prominent shift in provenance most likely associated with the onset of the Caledonian-Appalachian orogeny in northeast Laurentia. U-Pb detrital zircon age populations of ca. 470–420 and 990–820 Ma, along with 40 Ar/ 39 Ar detrital muscovite ages of ca. 470–430 Ma, support provenance connections with the East Greenland Caledonides, Pearya, and Svalbard. Partially reset 40 Ar/ 39 Ar ages in these sedimentary successions are linked to low-grade metamorphism associated with the Early–Middle Devonian Romanzof orogeny, a poorly understood tectonic event in the Brooks Range that is possibly associated with the emplacement of an allochthonous oceanic assemblage, herein named the Whale Mountain allochthon.

In the central Klamath Mountains, the English Peak plutonic complex (EPC) invaded the faulted contact between the outboard Eastern Hayfork and inboard North Fork terranes of the Western Paleozoic and Triassic Belt (WTrPz). This calc-alkaline igneous complex is composed of two small, ~1–2-km-diame­ter, relatively mafic satellitic plutons peripheral to the younger, much larger, ~10–15-km-diameter English Peak zoned granitic pluton. The EPC magmas were mantle derived and reflect temporary residence and mixing at various depths in the overlying crust, with initial storage and modification near the Moho, and uppermost crustal emplacement at 5–10 km depths. Phase assemblages suggest pre-emplacement magma storage at a depth of ~20–25 km for the early satellitic plutons, versus ~15–20 km for samples from the larger zoned granitic pluton. We obtained zircon U-Pb geochronologic results (reported as internal and external weighted-mean 207 Pb-corrected 206 Pb/ 238 U ages, 95% confidence level) from seven samples in the complex via laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS). The 172.3 ± 2.0 [3.7] Ma Uncles Creek and 166.9 ± 1.6 [3.4] Ma Heiney Bar satellitic plutons range from gabbro–quartz diorite to granodiorite in bulk-rock composition. The main English Peak pluton consists of an early stage of gabbro-tonalite (three samples: 160.4 ± 1.1 [3.1] Ma, 158.1 ± 1.1 [3.1] Ma, and 158.0 ± 1.2 [3.1] Ma) and a late stage (two samples: 156.3 ± 1.3 [3.1] Ma and 155.3 ± 1.2 [3.0] Ma) passing inward from tonalite through granodiorite to a central zone of granite. The 172 Ma age of the Uncles Creek pluton makes it coeval with Middle Jurassic Western Hayfork arc magmatism. In contrast, Heiney Bar and the main English Peak igneous ages overlap some of the oldest and youngest components, respectively, of the Middle to Late Jurassic Wooley Creek plutonic suite. Study of this multiple-intrusion complex provides an illuminating example of the gradual intermediate-to-felsic modification of the upper crust in the central Klamath Mountains. Inherited zircon ages of ca. 172 Ma in two other EPC samples indicate potential Middle Jurassic crustal sources or contaminants. Geochronologic correlation of the EPC with geologic histories of other Klamath terranes provides fresh insights for understanding spatial and temporal elements of Middle to Late Jurassic arc magmatism in the Klamath Mountains sector of the Cordilleran margin. This igneous activity illuminates some petrotectonic processes whereby accreted ophiolitic basement terranes were modified and incorporated into the evolving Jurassic continental crust. It took place prior to the earliest Cretaceous onset of westward transport of the stack of Klamath allochthons relative to the active Jura-Cretaceous Sierran calc-alkaline arc.

Excimer laser technologies enable a rapid and effective approach to simultaneous U/Pb geochronology and (U–Th)/He thermochronology of a wide range of detrital accessory minerals. Here we describe the ‘laser ablation double dating’ (LADD) method and demonstrate its viability by applying it to zircon and titanite crystals from the well-characterized Fish Canyon tuff. We found that LADD dates for Fish Canyon zircon (206Pb/238U – 28.63 ± 0.11 Ma; (U–Th)/He – 28.38 ± 0.73 Ma) are statistically indistinguishable from those obtained through established, traditional methods of single-crystal dating. The same is true for Fish Canyon titanite LADD dates: 206Pb/238U – 28.08 ± 0.90 Ma; (U–Th)/He – 27.98 ± 0.86 Ma. As anticipated, given that LADD involves the analysis of smaller amounts of material than traditional methods, it yields dates with higher analytical uncertainty. However, this does not substantially reduce the utility of the results for most applications to detrital datasets. An important characteristic of LADD is that it encourages the chemical characterization of crystals by backscattered electron, cathodoluminescence, and/or Raman mapping prior to dating. In addition, by permitting the rapid and robust dating of crystals regardless of the degree of their abrasion during sedimentary transport, the method theoretically should yield dates that are more broadly representative of those of the entire population of detrital crystals in a natural sample.

Geologic mapping, supported by 40 Ar/ 39 Ar and U-Pb geochronology and geochemistry of sedimentary and volcanic rocks, reveals the details of the Cenozoic depositional and tectonic history of the eastern Pinon Range and central Huntington Valley in the north-central Basin and Range Province, Nevada (USA). Cretaceous to Miocene supracrustal successions were studied in detail in order to compare the geologic evolution of the upper crust near the Ruby Mountains-East Humboldt Range (RMEH) metamorphic core complex (MCC) with the magmatic, metamorphic, and deformational history of the deep crust in the developing MCC. During the well-documented Late Cretaceous–Oligocene history of partial melting and infrastructure development within the RMEH, surface deposits in Huntington Valley reflect general tectonic quiescence, with evidence for the development of the shallow Elko Basin, minor extension, and eruption of southward-younging ignimbrite flare-up volcanism. Thin, discontinuous successions of Cretaceous–early Cenozoic sedimentary strata were locally blanketed by rhyodacite ignimbrites, domes, and subvolcanic intrusions of the Robinson Mountain volcanic field between 38.5 and 36.8 Ma. This magmatic event represents the first local expression of Cenozoic volcanism linked to the ignimbrite flare-up, and its onset occurred slightly after a renewal of partial melting in the RMEH beginning ca. 42 Ma. The volcanic section was subsequently tilted ∼10°–15° west before ca. 33.9 ± 0.4 Ma. Although melting continued at depth in the RMEH until after 30 Ma, there was no eruption of volcanic rocks after Robinson Mountain volcanism. An additional ≥10°–15° of westward tilting occurred between 31.1 ± 0.3 Ma and ca. 24.4 Ma, as bracketed by the 31 Ma tuff of Hackwood Ranch (which was probably erupted from a distant caldera) and an angular unconformity beneath the overlying Miocene Humboldt Formation. Neither of these tilting events and unconformities appears to represent significant (>∼1 km each) extension, but they could be surface expressions of magmatism, metamorphism, and crustal flow at depth. The Humboldt Formation includes >2000 m of sediment deposited mostly between ca. 16 and 12 Ma, with deposition lasting until at least ca. 8.2 Ma. Humboldt Formation sediments thicken eastward, toward the west-dipping fault that bounds the RMEH, and are interpreted as a basin that developed in the hanging wall of this fault system. Motion on this normal fault system led to the exhumation of metamorphic and igneous rocks of the core complex ∼10 m.y. after the documented cessation of partial melting, high-temperature metamorphism, and intrusion of granitoids into the deep crust ca. 29 Ma. Metamorphic clasts and a detrital zircon signature thought to represent RMEH provenance are first detected in 14.2 Ma or younger sediments.

Upper Cretaceous sandstones from 17 localities from California to southeastern Alaska (United States) contain unexpectedly large populations of detrital zircons with Proterozoic U-Pb ages, with age peaks at 1800–1650 and 1380 Ma. These peaks are indicative of a sediment source region in the southern part of the Proterozoic Belt Supergoup basin in central Idaho, which hosts 1800–1650 Ma detrital zircons and which was intruded by rift-related 1380 Ma bimodal plutons and sills. Belt rocks were strongly uplifted and eroded during Late Cretaceous Sevier shortening and fed four paleoriver systems. The Lemhi Pass–Hawley Creek river system flowed east and sourced the Beaverhead-Harebell-Pinyon nonmarine megafan in the Cordilleran foreland basin. The Kione River flowed southwest to northern California, where it sourced a very large, ca. 82–80 Ma, ∼600-m-thick delta and submarine fan complex within the northern Great Valley forearc basin. Considerable Kione detritus also transited the forearc basin to reach the Franciscan trench, sourcing a pulse of deposition and subduction accretion in central California and even part of southern California. The Swakane River flowed northwest out of Idaho into Washington, sourcing the protolith for the high-grade Swakane gneiss. More speculatively, a Yakutat River may have flowed northwest and deposited Yakutat strata in a trench off Washington or British Columbia, before those rocks were translated north to southeastern Alaska. Recognition of a major source area in central Idaho for zircons with an uncommon age of 1380 Ma helps constrain the ca. 85–65 Ma paleogeography and paleotectonics of major sectors of the North American convergent margin orogen.

Mid-Palaeozoic assembly models for the Arctic Alaska–Chukotka microplate predict the presence of cryptic crustal sutures, the exact locations and deformational histories of which have not been identified in the field. This study presents data on the provenance of polydeformed and metamorphosed strata in the southern Brooks Range Schist Belt and Central Belt of presumed Proterozoic–Devonian depositional age, as well as for the structurally overlying strata, to help elucidate terrane boundaries within the Arctic Alaska–Chukotka microplate and to add new constraints to the palaeogeographical evolution of its constituent parts. The protoliths identified support correlations with metasedimentary strata in the Ruby terrane and Seward Peninsula and suggest a (peri-) Baltican origin in late Neoproterozoic–early Palaeozoic time. Proximity to Laurentia is only evident in what are inferred to be post-early Devonian age strata. By contrast, the North Slope and Apoon terranes originated proximal to Laurentia. The mid-Palaeozoic boundary between these (peri-) Baltican and (peri-) Laurentian terranes once lay between rocks of the Schist/Central belts and those of the Apoon terrane, but is obscured by severe Mesozoic–Cenozoic deformation. Whether this boundary represents a convergent or transform suture, when exactly it formed and how it relates to broader Caledonian convergence in the North Atlantic are still unresolved questions.

The Yukon-Koyukuk Basin is a large depression that covers ∼118,000 km2 in western interior Alaska and is divided into two subbasins by a volcanic arc assemblage. Interpretations of the depositional setting of the northern Kobuk Koyukuk subbasin vary from a syncollisional forearc basin to a postorogenic successor basin formed by lithospheric extension. New results from sandstones and conglomerates collected from the Kobuk Koyukuk subbasin provide evidence for the timing of basin development, insight into the provenance of coarse siliciclastic sediments, and understanding of the nature of Cretaceous paleogeography and paleodrainage of Arctic Alaska. Early sedimentary rocks of the Kobuk Koyukuk subbasin contain abundant mafic to ultramafic volcanic and plutonic lithic fragments and mafic heavy minerals (e.g., spinel, clinopyroxene, and amphibole). They also contain abundant Middle Triassic to early Late Jurassic zircons (240−160 Ma; peak maximum ca. 200 Ma) that yield highly juvenile Hf isotopic compositions. Geochemistry of chromium spinels (Cr# = 0.17−0.86) suggests crystallization in an immature arc setting that likely developed over mid-ocean-ridge basalt−type crust. These early sediments originated from the mafic and ultramafic rocks of the Angayucham terrane, which was once much more extensive. These results suggest that the Angayucham terrane consists of an obducted Middle Triassic to early Late Jurassic oceanic arc complex that was coeval with oceanic- to continental-margin mafic arc magmatism in the Canadian Cordillera. Our generalized stratigraphy, along with U-Pb ages and Lu-Hf isotope analyses of zircons from sedimentary rocks of the Kobuk Koyukuk subbasin, reflects the tectonic and/or erosional unroofing of the adjacent southern Brooks Range and Ruby terrane. U-Pb ages of detrital zircons collected from the stratigraphically lowest mafic- to ultramafic-rich strata yield maximum depositional ages (107 Ma) that reflect initial erosion of the structurally highest Angayucham terrane and initiation of basin formation and deposition in the late Early Cretaceous. Continued uplift and erosion exposed structurally deeper metamorphic rocks, as revealed by incorporation of low-grade phyllites and eventually higher-grade metamorphic schistose lithic detritus and intermediate-composition (e.g., biotite) to metamorphic (e.g., chloritoid and xenotime) heavy mineral suites into the basin sediments. Differences in detrital zircon signatures between similar-age strata in the Colville foreland basin to the north of the Brooks Range and the Kobuk Koyukuk subbasin indicate that the sediments within the two basins were derived from two different sources, and the Brooks Range orogen acted as a drainage divide during late Early Cretaceous deposition.

This study was carried out to determine the initial age and time-span of interaction of two major crustal provinces of Asia: the Siberian craton and the Central Asian Orogenic Belt (CAOB). Single grain U–Pb ages of detrital zircon suites from the Neoproterozoic passive margin strata of the Siberian craton were used to determine the first appearance of extraregional source regions. Twelve Neoproterozoic clastic rocks from the Baikal-Patom margin of Siberia and one sample from the volcaniclastic Padrinsky Group that was deposited unconformably above accreted CAOB crust were analyzed. In total, 1152 grains were analyzed, 703 were used for provenance interpretation. Stratigraphically lower strata from the Siberian margin yield Archean – Paleoproterozoic detrital zircon ages, which are similar to, and probably derived from the Siberian Precambrian craton. A few extra-regional Mesoproterozoic grains are also present. The provenance shift happens in the upper portion of the section and is marked by the influx of extra-regional zircons. The youngest included zircons (610 Ma) constrain the age of deposition and provide evidence for the timing of initial interaction between CAOB and Siberia in this region. Neoproterozoic zircons also dominate the overlying sedimentary units of the CAOB, suggesting continuing convergence. Coeval volcaniclastic unit deposited across the accreted CAOB terrane has a similar U–Pb detrital zircon age distribution, strengthening the provenance links between the two sides of the orogen. Analysis of the local tectonics suggests that the beginning of accretion might have started even before the first appearance of Neoproterozoic zircon: during the development of a regional unconformity, capped by 635 Ma (?) “Snowball Earth” tillites of Dzhemkukan Formation. The absence of Neoproterozoic zircons in Dzhemkukan Formation is probably explained by the absence of development of an extremely thickened crust. Our data are in good agreement with data collected from other Neoproterozoic sedimentary basins of southern Siberian craton, including Cisbaikalia and Bodaibo Synclinorium.

The central Songpan–Ganzi belt, located on the eastern margin of the Tibetan Plateau, has a similar high elevation and low relief to parts of central Tibet. Thermochronological studies from the central Tibetan Plateau reveal a history of slow exhumation (rates −1 ) since 45Ma; however, the exhumation history of the central Songpan–Ganzi belt is unknown. To address this, we conducted an apatite and zircon (U–Th)/He thermochronology study of bedrock samples collected across the central Songpan–Ganzi belt and into central parts of the Tibetan Plateau. Zircon (U–Th)/He ages range from 54.2±7.5 to 146.5±10.0Ma and the majority of apatite (U–Th)/He ages fall between 74.7±19.0 and 35.7±9.4Ma. Thermal history models of these data show rapid cooling in the late Mesozoic and much slower cooling diagnostic of low rates of erosion throughout most of the Cenozoic. The late Mesozoic rapid cooling is consistent with the existence of significant topography and relief at least in some parts of the Songpan–Ganzi belt at that time. We find no evidence for a regional Miocene acceleration in erosion, although three samples from the headwaters of the Salween and Mekong rivers gave younger AHe ages between 15 and 16Ma that reflect an acceleration in river incision. Supplementary material: Laser ablation–inductively coupled plasma mass spectrometry zircon U–Pb data from central and eastern Tibet are available at www.geolsoc.org.uk/SUP18647.

The modern San Andreas fault system (California, United States) is widely considered to have formed in response to the initiation of Pacific–North American plate interaction ca. 27 Ma. Although there is general consensus on the magnitude and timing of Neogene displacement along the San Andreas system, its Paleogene history remains unresolved. In particular, ∼100 km of right-lateral offset between mid-Cretaceous plutonic rocks of the northern Salinian block and the western edge of Sierra Nevada basement remains unaccounted for after restoration of Neogene displacement along strike-slip faults of the San Andreas system. Our detrital zircon data invalidate a key Paleogene piercing point by demonstrating that displaced portions of the hypothesized Middle Eocene Butano–Point of Rocks submarine fan were never contiguous across the San Andreas fault. We instead show that the Eocene provenance characteristics exhibited by northern Salinian strata closely match those of the southern Sierra Nevada and northwestern Mojave Desert. This implies that the northern Salinian block was located at least 75–50 km farther south in Eocene time than previously recognized. Our data require (1) pre–23 Ma dextral slip along the San Andreas fault in central California, and/or (2) slip along a predecessor fault that formed prior to Pacific–North American plate interaction. This previously undocumented slip may indicate that significant Pacific–North American plate interaction propagated from the plate margin into the continental interior much earlier than conventionally believed. Alternatively, late Paleogene slip could predate the development of the modern plate boundary and represent inboard dextral strike-slip displacement along the eastern margin of the Salinian block, similar to the deformation that occurs today along the strike-slip Sumatra fault system.

To test previously suggested exhumation histories of the Gangdese Batholith in the central part of the Transhimalayan plutonic belt, we conducted paired apatite and zircon (U-Th)/He thermochronological investigations of the Yarlung Zangbo gorge in the central part of the batholith. Age-elevation relationships and multisystem thermochronometers showed three periods of accelerated exhumation (∼46–48, ∼22–18, and ∼11–8 Ma). Combining these data with previously published thermochronological ages and synthesizing these ages with regional geological events provides an entire exhumation history. The Cretaceous–Early Paleogene exhumation of the Gangdese Batholith was probably caused by both the northward subduction of the Neo-Tethys and the collision between the Lhasa and Qiangtang blocks. The Early Miocene rapid exhumation might be a response to shortening caused by the Gangdese Thrust or erosion driven by dynamic uplift following lithospheric delamination. In contrast, the Late Miocene exhumation is coi...

We synthesized data from geologic maps, wells, seismic-reflection profiles, potential-field interpretations, and low-temperature thermochronology to refine our understanding of late Cenozoic extension and shortening in the Salinian block of the central California Coast Ranges. Data from the La Panza Range and southern Salinas Basin document early to middle Miocene extension, followed by Pliocene and younger shortening after a period of little deformation in the late Miocene. Extension took place on high-angle normal faults that accommodated ∼2% strain at the scale of the ∼50-km-wide Salinian block (oriented perpendicular to the San Andreas fault). Shortening was accommodated by new reverse faults, reactivation of older normal faults, and strike-slip faulting that resulted in a map-view change in the width of the Salinian block. The overall magnitude of shortening was ∼10% strain, roughly 4–5 times greater than the amount of extension. The timing and magnitude of deformation in our study area are comparable to that documented in other Salinian block basins, and we suggest that the entire block deformed in a similar manner over a similar time span. The timing and relative magnitude of extension and shortening may be understood in the context of central Coast Range tectonic boundary conditions linked to rotation of the western Transverse Ranges at the south end of the Salinian block. Older models for Coast Range shortening based on balanced fault-bend fold-style cross sections are a poor approximation of Salinian block deformation, and may lead to mechanically improbable fault geometries that overestimate the amount of shortening.

An integrated research of sedimentology, stratigraphy, and provenance analysis on eleven sedimentary sections from the Qaidam basin, Hexi Corridor, and Subei basin representing ∼36 km Cenozoic strata provides a detailed record of the northern Tibetan Plateau growth since the early Eocene. Sections are divided into three groups based on age, geological similarities, and geographical locations. Group One includes three early Eocene–late Miocene sections from the northern Qaidam basin; Group Two contains four sections along the Altyn Tagh fault (ATF) which preserve a complete stratigraphic record from the Oligocene to late Miocene; Group Three contains the four youngest sections investigated from the northeastern Qaidam basin and Hexi Corridor which preserve a record since the middle Miocene to Quaternary. Together, the sections reveal a multiple-phase tectonic history of the northern Tibetan Plateau. The punctuated history can be divided into four phases. (1) The Eocene Lulehe Formation from the northern Qaidam basin is interpreted as a synorogenic conglomerate deposited by high-gradient depositional systems. Strong unimodal paleocurrent towards the southwest, coarse lithology and distinct, recognizable clast types constrain the sediment source within the North Qaidam and South Qilian terranes, indicating activity on inferred thrust faults within the North Qaidam and South Qilian terranes in response to the initial India-Eurasia collision. The activity on these thrust faults continued through the late Eocene. (2) The early Oligocene conglomerate from Group Two formed in response to the sinistral transpression related to motion on the ATF, suggesting inception of substantial slip on the ATF in the early Oligocene in order to accommodate the continuing indentation of India into Eurasia. (3) Oligocene–early Miocene fine-grained fluvio-lacustrine sediments from Groups One and Two formed as the result of development of internal drainage systems in the Qaidam basin in response to the large-amplitude slip motion on the ATF. Paleocurrents collected from the Oligocene–early Miocene strata of Group One are northwest-directed, pointing towards the ATF, consistent with the pre-existing subsurface data showing the shift of depocenter from along North Qaidam and South Qilian terranes toward the ATF. (4) All eleven sections preserve a post-early Miocene upward-coarsening sequence, consistent with the extensive crustal shortening and topographic growth across the northern Tibetan Plateau.

The Okhotsk terrane, located east of the South Verkhoyansk sector of the Verkhoyansk fold-and-thrust belt, has Archean crystalline basement and Riphean to Early Paleozoic sedimentary cover similar to that of the adjacent the North Asian craton. However, 2.6 Ga biotite orthogneisses of the Upper Maya uplift of the Okhotsk terrane yielded Early Devonian 40Ar/39Ar cooling ages, evidence of a Mid-Paleozoic metamorphic event not previously known. These gneisses are also intruded by 375±2 Ma (Late Devonian) calc-alkaline granodiorite plutons that we interpret as part of a continental margin volcanic arc. Therefore, Late Devonian rifting, which affected the entire eastern margin of North Asia separating the Okhotsk terrane from the North Asian craton, was probably a back-arc event. Our limited 40Ar/39Ar data from the South Verkhoyansk metamorphic belt suggests that low grade metamorphism and deformation started in the Late Jurassic due to accretion of the Okhotsk terrane to the North Asia margin along the Bilyakchan fault. Shortening and ductile strain continued in the core of the South Verkhoyansk metamorphic belt until about 120 Ma due to paleo-Pacific subduction along the Uda-Murgal continental margin arc.

New sedimentologic, stratigraphie, and compositional data from the Paleogene-Neogene stratigraphie succession exposed in the northwest Hexi Corridor and within the North Qilian Shan, provide evidence to suggest that crustal shortening in the North Qilian Shan fold-thrust belt initiated during die Miocene. The section is composed of four lithostratigraphic units: Oligocene-Miocene fine- to coarse-grained Unit 1, Miocene conglomeratic Unit 2, and Pliocene-Pleistocene conglomeratic Units 3 and 4. Unit 3 lies in angular unconformity over both Units 1 and 2, and Unit 4 contains a progressive unconformity. The onset of conglomerate deposition at the base of Unit 2 suggests an increase in depositional energy, which we interpret as me result of proximal orogenesis in the North Qilian Shan fold and tiirust belt. Supporting evidence includes the appearance of strongly northeast-trending paleocurrents, indicat-ing paleoflow away from the Qilian Shan, clast lidiologies that match sources in the North Qilian Shan, and sandstone with detrital framework modes that indicate a recycled orogen source. In contrast, Unit 1 contains paleocurrent indicators that are variable but generally trend northward and sandstone and clast compositions which are more diagnostic of a continental block source. Detrital zircon age determinations from Unit 1 are also not consistent with a source in the North Qilian Shan; rather, they suggest a provenance in hinterland regions within the South Qilian Shan and North Qaidam terranes. In sum, these results are all consistent with initiation of proximal uplift of the North Qilian Shan during deposition of the gradational transition from Unit 1 to Unit 2, demonstrating shortening in the Qilian Shan before die late Miocene. This comprehensive study tightens our understanding of when far-field stress related to the India-Eurasia continent-continent collision reached the northeastern edge of the Tibetan Plateau.

In northwestern California, the Franciscan subduction complex has been subdivided into seven major tectonostratigraphic units. We report U-Pb ages of ≈2400 detrital zircon grains from 26 sandstone samples from 5 of these units. Here, we tabulate each unit’s interpreted predominant sediment source areas and depositional age range, ordered from the oldest to the youngest unit. (1) Yolla Bolly terrane: nearby Sierra Nevada batholith (SNB); ca. 118 to 98 Ma. Rare fossils had indicated that this unit was mostly 151–137 Ma, but it is mostly much younger. (2) Central Belt: SNB; ca. 103 to 53 Ma (but poorly constrained), again mostly younger than previously thought. (3) Yager terrane: distant Idaho batholith (IB); ca. 52 to 50 Ma. Much of the Yager’s detritus was shed during major core complex extension and erosion in Idaho that started 53 Ma. An Eocene Princeton River–Princeton submarine canyon system transported this detritus to the Great Valley forearc basin and thence to the Franciscan trench. (4) Coastal terrane: mostly IB, ±SNB, ±nearby Cascade arc, ±Nevada Cenozoic ignimbrite belt; 52 to

Geochronology and thermochronology on detrital material provides unique constraints on sedimentary provenance, depositional ages, and orogenic evolution of source terrains. In this paper we describe a method and case-studies of measurement of both U/Pb and (U-Th)/He ages on single crystals of zircon that improves the robustness of constraints in each of these areas by establishing both formation and cooling ages of single detrital grains. Typically these ages correspond to crystallization and exhumation or eruption ages, and their combination can be used to more confidently resolve candidate source terrains, establish maximum depositional ages, and constrain the thermal histories of orogenic source regions. U/Pb dating is accomplished by laser-ablation ICP-MS in a small pit on the exterior of the crystal, and He dates are then determined on the bulk grain by conventional laser-heating and dissolution techniques. We present examples from Mesozoic aeolian sandstones, both modern and Paleogene fluvial sediments, and active margin turbidite assemblages from the Cascadia and Kamchatka margins. Important results include the fact that detritus from ancient orogens may dominate sediments thousands of kilometers away, crustal melting and exhumation appear to be spatially-temporally decoupled in at least two orogens, and first-cycle volcanic zircons older than depositional age are surprisingly rare in most settings except in the continental interior. In the case of the Kamchatkan, and possibly Olympic, turbidites, zircon He ages are partially reset. We present a method for estimating the extent of resetting of each grain and the thermal history of the sample, based on coupled (U-Th)/(He-Pb) age patterns among all the grains.

DOI = 10.3126/hjs.v5i7.1309 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.121-122