Jeremy K. Hourigan

Cenozoic multiple-phase tectonic evolution of the northern Tibetan Plateau: Constraints from sedimentary records from Qaidam basin, Hexi Corridor, and Subei basin, northwest China

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.

EVIDENCE OF MIOCENE CRUSTAL SHORTENING IN THE NORTH QILIAN SHAN FROM CENOZOIC STRATIGRAPHY OF THE WESTERN HEXI CORRIDOR, GANSU PROVINCE, CHINA

New sedimentologic, stratigraphic, and compositional data from the Paleogene-Neogene stratigraphic 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 the 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 the result of proximal orogenesis in the North Qilian Shan fold and thrust belt. Supporting evidence includes the appearance of strongly northeast-trending paleocurrents, indicating paleoflow away from the Qilian Shan, clast lithologies 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 the 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.

EOCENE ARC-CONTINENT COLLISION AND CRUSTAL CONSOLIDATION IN KAMCHATKA, RUSSIAN FAR EAST

The age and origin of high-grade metamorphic rocks of the Sredinnyi Range, Kamchatka have been the subject of a long and controversial debate. Based on geochronologic data and its association with a collage of accreted oceanic terranes, leading interpretations argue that the Sredinnyi Range metamorphic rocks represent an accreted Precambrian or Mesozoic microcontinent. In this contribution, we present new data that indicate that these metamorphic rocks were formed from the Cretaceous-Paleocene sedimentary margin of northeast Russia when it was overridden during Eocene obduction of the Olyutorsky arc, a far-travelled oceanic island arc. Our data include new mapping and structural observations along the northern and eastern flanks of the Sredinnyi Range, and SHRIMP zircon and monazite U-Th-Pb age data from 15 key samples. These new isotopic data demonstrate that paragneissic units were formed from sediments with depositional ages locally no older than Late Cretaceous to Paleocene. Furthermore, the statistical similarity of zircon U-Pb grain-ages from the Kamchatka Schist with very low-grade turbidite sandstones of the Ukelayat and Khozgon Groups indicate that metasediments of the Sredinnyi Range are upgraded stratigraphic equivalents of northeast Russian marginal strata. SHRIMP U-Pb ages of zircon overgrowths and metamorphic monazite extracted from migmatite and gneiss indicate that peak metamorphism occurred at 52 Ma, which is synchronous with the onset of the Olyutorsky arc-continent collision. Heating and cooling occurred rapidly, at rates approaching 80°C/m.y. Rapid heating is attributed to syn-collisional, subduction-related magmatism. Thermochronology and structural observations indicate that exhumation was due to a combination of ductile and brittle thinning of the crust. We speculate that this thinning was caused by diapiric ascent of a low-density low-viscosity continental material beneath a dense structural lid of the obducted island arc.

Tectonic and chronostratigraphic implications of new 40Ar/39Ar geochronology and geochemistry of the Arman and Maltan-Ola volcanic fields, Okhotsk-Chukotka volcanic belt, northeastern Russia

The Okhotsk-Chukotka volcanic belt is part of an extensive late Early to Late Cretaceous Andean-style magmatic arc that spans the entire eastern margin of the Asian continent. The belt itself stretches 3000 km from the Chukotka Peninsula to the Uda River and comprises ∼1.2 x 10 6 km 3 of volcanic rock over a 500,000 km 2 area. Despite its size and regional tectonic significance, the time span of magmatic activity is poorly constrained and the subject of significant debate, mostly in the Russian literature. In this paper, we provide new geochronologic control on the timing of inception and cessation of magmatism for the Arman and Maltan-Ola volcanic fields. These field localities were chosen because they are well studied, relatively accessible, and contain floral assemblages that have been used to correlate volcanic sequences at the regional scale. The majority of the volcanic sequence was emplaced between 85.5 ′ 1.3 Ma and 74.0 ′ 1.2 Ma, as shown by 17 new 4 0 Ar/ 3 9 Ar ages. The Coniacian-Santonian to Campanian age range indicated is 15 m.y. younger than the Albian to early Cenomanian age range given by a synthesis of floral stratigraphic, K-Ar, and Rb-Sr geochronologic data. The calc-alkaline part of the volcanic section spans an apparent age range of 85.5 ′ 1.3 Ma to 80.7 ′ 0.8 Ma. Capping basalts were emplaced between 77.5 ′ 1.1 Ma and 74.0 ′ 1.2 Ma and exhibit a within-plate geochemical signature, which we attribute to a temporally and geochemically distinct, possibly extension-related, phase of magmatism. The apparent northwestward migration of the arc front from the interior (seaward) zone (Taigonos Peninsula, Magadan batholith) in Albian-Cenomanian time to the Arman and Maltan-Ola volcanic fields in Coniacian-Santonian to Campanian time may be explained by shallowing of the subducting paleo-Pacific (Kula?) oceanic plate. The flat-lying nature of these volcanic rocks and the within-plate geochemical affinity of the capping basalt unit are inconsistent with prevailing tectonic models for the cessation of arc magmatism and formation of the Sea of Okhotsk which require the collision of a microcontinental block or oceanic plateau with the northeast Asian margin in the Late Cretaceous.

Exhumation History of the Gangdese Batholith, Southern Tibetan Plateau: Evidence from Apatite and Zircon (U-Th)/He Thermochronology

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 coincident with both the proposed capture of the Yarlung Zangbo gorge by a foreland draining catchment and the intensification of the Asian monsoon, as well as normal faulting. Hence, the latest stage of exhumation might be attributed to the incision of the Yarlung Zangbo gorge, the activity of a north-south fault, or both.

Detrital zircon U–Pb reconnaissance of the Franciscan subduction complex in northwestern California

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 <32 Ma. (5) King Range terrane: dominated by IB and SNB zircons; parts 16–14 Ma based on microfossils. Overall, some Franciscan units are younger than previously thought, making them more compatible with models for the growth of subduction complexes by progressive accretion. From ca. 118 to 70 Ma, Franciscan sediments were sourced mainly from the nearby Sierra Nevada region and were isolated from southwestern US and Mexican sources. From 53 to 49 Ma, the Franciscan was sourced from both Idaho and the Sierra Nevada. By 37–32 Ma, input from Idaho had ceased. The influx from Idaho probably reflects major tectonism in Idaho, Oregon, and Washington, plus development of a through-going Princeton River to California, rather than radical changes in the subduction system at the Franciscan trench itself.

A reappraisal of the early slip history of the San Andreas fault, central California, USA

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.

Assembling the world’s type shallow subduction complex: Detrital zircon geochronologic constraints on the origin of the Nacimiento block, central California Coast Ranges

Temporal and spatial patterns in the architecture of the Franciscan Complex provide valuable insights into the subduction processes through which such patterns arise. The Nacimiento Franciscan belt is an allochthonous sliver of subduction assemblages in the central California Coast Ranges displaced either: (1) from southern California by >300 km of Neogene dextral slip along the San Andreas fault system or (2) from central California to southern California and back again, by >500 km of Late Cretaceous–Paleocene sinistral slip along the Sur-Nacimiento fault followed by San Andreas–related motion. New U-Pb detrital zircon data from 20 (meta)clastic samples indicate that the Nacimiento Franciscan section was assembled between ca. 95 and 80 Ma. Abundant Cretaceous (particularly Late Cretaceous) and diminishing amounts of Jurassic and Proterozoic zircon grains point to a southern California origin for Nacimiento Franciscan protoliths, precluding significant sinistral strike-slip along the Sur-Nacimiento fault. Furthermore, the suite of detrital zircon ages reported here bears a strong resemblance to new and existing data from subduction complexes in southern California that were emplaced during Laramide shallow subduction (i.e., Sierra de Salinas, Portal Ridge, Quartz Hill, Rand, San Emigdio, and Tehachapi schists). Hence, the Nacimiento Franciscan is distinct from Franciscan rocks in central and northern California and more likely represents an outboard element of the Late Cretaceous southern California low-angle subduction system. Upon restoring the Nacimiento block to its Late Cretaceous position, an inboard-younging trend is apparent in the composite Nacimiento–southern California schist belt, suggesting that progressively younger accretionary materials were underplated farther inboard by tectonic erosion. We posit that arc and forearc elements absent from southern California were removed by a combination of physical and tectonic erosion attending shallow subduction, interleaved in the subduction complex, and recycled into the mantle. Steepening of the Laramide slab was marked by a phase of crustal extension in the overriding plate. During this phase, the Sur-Nacimiento fault likely functioned as a segment of a low-angle normal fault system spanning the southern Sierra Nevada batholith to the Nacimiento accretionary system.

Four Cordilleran paleorivers that connected Sevier thrust zones in Idaho to depocenters in California, Washington, Wyoming, and, indirectly, Alaska

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.

Effects of inherited cores and magmatic overgrowths on zircon (U-Th)/He ages and age-eU trends from Greater Himalayan sequence rocks, Mount Everest region, Tibet

Previous constraints on the timing and rate of exhumation of the footwall of the South Tibetan detachment system (STDS) north of Mount Everest suggest rapid Miocene cooling from ∼ 700°C to 120°C between ∼14–17 Ma. However, 25 new single grain zircon He ages from leucogranites intruding Greater Himalayan Sequence rocks in the footwall of the STDS are between 9.9 and 15 Ma, with weighted mean ages between 10 and 12 Ma. Zircon grains exhibit a positive correlation between age and effective uranium (eU). Laser ablation zircon U-Pb geochronology, detailed SEM observations, and laser ablation depth-profiling of these zircons reveal low-eU 0.5–2.5 Ga inherited cores overgrown by high-eU 17–22 Ma rims. This intragranular zonation produces ages as much as 32% too young when a standard alpha-ejection correction assuming uniform eU distribution is applied. Modeling of the effects of varying rim thickness and rim eU concentration on the bulk grain eU and alpha-ejection correction suggests that zonation also exerts the primary control on the form of the age-eU correlation observed. Application of grain-specific zonation-dependent age corrections to our data yields zircon He ages between 14 and 17 Ma, in agreement with AFT and 40Ar/39Ar ages. Growth of magmatic rims followed by cooling to < 120°C within 1–6 million years supports rapid tectonic exhumation associated with slip along the STDS in the Miocene. This study highlights the importance of characterizing parent nuclide zonation in zircon He studies which seek to understand the timing of exhumation along exhumed crustal blocks.