Alex Pullen

Detrital zircon geochronology of pre-Tertiary strata in the Tibetan-Himalayan orogen

Detrital zircon data have recently become available from many different portions of the Tibetan-Himalayan orogen. This study uses 13,441 new or existing U-Pb ages of zircon crystals from strata in the Lesser Himalayan, Greater Himalayan, and Tethyan sequences in the Himalaya, the Lhasa, Qiangtang, and Nan Shan-Qilian Shan-Altun Shan terranes in Tibet, and platformal strata of the Tarim craton to constrain changes in provenance through time. These constraints provide information about the paleogeographic and tectonic evolution of the Tibet-Himalaya region during Neoproterozoic to Mesozoic time. First-order conclusions are as follows: (1) Most ages from these crustal fragments are <1.4 Ga, which suggests formation in accretionary orogens involving little pre-mid-Proterozoic cratonal material; (2) all fragments south of the Jinsa suture evolved along the northern margin of India as part of a circum-Gondwana convergent margin system; (3) these Gondwana-margin assemblages were blanketed by glaciogenic sediment during Carboniferous-Permian time; (4) terranes north of the Jinsa suture formed along the southern margin of the Tarim-North China craton; (5) the northern (Tarim-North China) terranes and Gondwana-margin assemblages may have been juxtaposed during mid-Paleozoic time, followed by rifting that formed the Paleo-Tethys and Meso-Tethys ocean basins; (6) the abundance of Permian-Triassic arc-derived detritus in the Lhasa and Qiangtang terranes is interpreted to record their northward migration across the Paleo- and Meso-Tethys ocean basins; and (7) the arrival of India juxtaposed the Tethyan assemblage on its northern margin against the Lhasa terrane, and is the latest in a long history of collisional tectonism. Copyright 2011 by the American Geophysical Union.

Triassic continental subduction in central Tibet and Mediterranean-style closure of the Paleo-Tethys Ocean

The Qiangtang metamorphic belt (QMB) in central Tibet is one of the largest and most recently documented high-pressure (HP) to near-ultrahigh-pressure (near-UHP) belts on Earth. Lu-Hf ages of eclogite- and blueschist-facies rocks within the QMB are 244–223 Ma, indistinguishable from the age of UHP metamorphism in the Qinling-Dabie orogen. Results of a U-Pb detrital zircon study suggest that protoliths of the QMB include upper Paleozoic Qiangtang continental margin strata and sandstones that were derived from a Paleozoic arc terrane that developed within the Paleo-Tethys Ocean to the north. We attribute QMB HP metamorphism to continental collision between the Qiangtang terrane and a Paleo-Tethys arc terrane. This collision, and the coeval South China–North China collision, may have slowed convergence between Laurasia and Gondwana-derived terranes and initiated Mediterranean-style rollback and backarc basin development within much of the remnant Paleo-Tethys Ocean realm.

Tibetan basement rocks near Amdo reveal “missing” Mesozoic tectonism along the Bangong suture, central Tibet

The U-Pb and 4 0 Ar/ 3 9 Ar studies of a unique exposure of crystalline basement along the Jurassic-Early Cretaceous Bangong suture of central Tibet reveal previously unrecognized records of Mesozoic metamorphism, magmatism, and exhumation. The basement includes Cambrian and older orthogneisses that underwent amphibolite facies metamorphism coeval with extensive granitoid emplacement at 185-170 Ma. The basement cooled to ∼300 °C by 165 Ma and was exhumed to upper crustal levels in the hanging wall of a south-directed thrust system during Early Cretaceous time. We attribute Jurassic metamorphism and magmatism to the development of a continental arc during Bangong Ocean subduction, and Early Cretaceous exhumation to northward continental underthrusting of the Lhasa terrane beneath the Qiangtang terrane. We speculate that a Jurassic arc extended regionally along the length of the Bangong suture, but in all other places in Tibet has been buried, either depositionally or structurally, beneath supracrustal assemblages.

The Gangdese retroarc thrust belt revealed

The Cretaceous–early Tertiary Gangdese arc in southern Tibet is generally attributed to the northward subduction of Neotethyan oceanic lithosphere prior to Indo-Asian collision. However, the history and tectonic significance of deformation and sedimentation in Tibet during this time interval have remained enigmatic. We show that contractional structures and clastic rocks near the city of Lhasa can be attributed to the development of a northward-propagating retroarc thrust belt that was active between 105 and 53 Ma. A kinematic model shows that the thrust belt could have accommodated >230 km (>55%) of N-S shortening. An episode of large magnitude (>160 km) and rapid (>8 mm/yr) shortening predated the onset of a magmatic flare-up ca. 69 Ma, which is linked to removal of overthickened mantle lithosphere. This tectonic history implies that southern Tibet underwent substantial crustal thickening and elevation gain prior to the Indo-Asian collision.

Metamorphic rocks in central Tibet: Lateral variations and implications for crustal structure

Insights about lateral variations in the age, composition, and structure of the central Tibetan crust are provided by geologic investigations of metamorphic rocks in the Qiangtang terrane. Previous studies have shown that a tectonic melange of Triassic age with blueschist- and eclogite-bearing blocks within a greenschist-facies matrix is exposed over an E-W distance of ∼600 km in the central Qiangtang terrane. New mapping shows that the melange extends over a N-S distance of ∼150 km, nearly to the trace of the early Mesozoic Jinsha suture in the north. The melange, exposed structurally beneath Upper Paleozoic to Mesozoic strata in the footwalls of early Mesozoic normal faults, is composed mostly of Paleozoic metasedimentary and crystalline rocks. These findings support the hypothesis that a large part of the central and northern Qiangtang terrane crust is composed of supracrustal rocks. The Duguer Range,

What happens when n= 1000? Creating large-n geochronological datasets with LA-ICP-MS for geologic investigations

The direct age dating of individual mineral components in sedimentary rocks through the analysis of radiogenic parent and daughter isotopes has been routinely applied to better understand sediment provenance and dispersal patterns for several decades. Time, labor, and financial cost—sadly, not scientific inquiry—are typically the determining factors in the number of analyses run for a sedimentary rock sample during provenance investigations. The number of observations reported for detrital zircon provenance investigations using secondary ion mass spectrometers SIMS and laser-ablation inductively-coupled-plasma mass-spectrometers LA-ICP-MS typically range from n = 60–120. In this range, minor, but commonly geological relevant, age components are commonly not identified from the sample aliquot. In addition, the relative proportions of zircon ages from within an age component are typically unreliable for intersample comparisons because the relative proportions of ages from aliquots of n = 60–120 may poorly reflect the ‘true’ proportions of ages from a sample. This study investigates the practicality and usefulness of generating large-n (n = 300–1000) datasets. A LA-MC-ICP-MS and LA-SC-ICP-MS were used to generate four n ≈ 1000 datasets. We show that precision large-n U–Pb detrital zircon datasets can be created using LA-ICP-MS with total sample-run analysis times that are on par with more traditional studies. At best, most provenance investigations based on n = 60–100 have been statistically limited to identifying principle age components. The statistical robustness on n = 1000 datasets not only significantly increase the probability that exotic or low abundance age components (i.e., f < 0.05) are identified in detrital samples, but it allows for the quantitative comparisons between relatively high abundance age components in samples. This potentially transformative outcome of large-n has the potential to stimulate new avenues of research in sedimentology and tectonics.

Eolian cannibalism: Reworked loess and fluvial sediment as the main sources of the Chinese Loess Plateau

The loess and paleosol sequences of the Chinese Loess Plateau are composed of Quaternary dust, the origin of which has been the subject of considerable debate. Some recent U-Pb geochronological studies of eolian zircons have proposed the existence of two major wind pathways: from the north and northwest, through the Badan Jaran, Tengger, and Mu Us Deserts during interglacials, and from the west, through the Qaidam Basin during glacials. Others have emphasized the importance of Yellow River sediment supply in the Chinese Loess Plateau sediment budget. However, tracking dust source regions through U-Pb dating in a statistically robust manner is particularly complex given the similar age peaks in the age probability distributions of potential source regions in Central Asia. This paper presents 2410 new U-Pb ages of detrital zircons from wind-eroded strata, Quaternary eolian deposits, and modern river sands in central China in order to increase the robustness and the spatial resolution of zircon age distributions in dust source regions. We then propose a new mixture modeling technique to statistically address the contribution of these different sources to the Chinese Loess Plateau sedimentary budget. Our contribution estimates indicate that eolian supply to the Chinese Loess Plateau is dominated (60%–70%) by reworking of Yellow River sediment. Moreover, evidence of Qaidam Basin–sourced zircons (15%–20%) in both loess (glacial) and paleosol (interglacial) layers corroborates the existence of an erosive wind pathway through the Qaidam Basin during glacials and implies that a substantial portion of the interglacial dust is recycled from older glacial loess. We propose that sediment reworking of Yellow River sediment and older loess deposits by wind on the Chinese Loess Plateau homogenized the eolian zircon populations toward a glacial provenance due to higher (2–20 times) dust accumulation rates during glacials. These findings suggest that the Chinese Loess Plateau has evolved as a more dynamic landform than previous thought, where wind deflation, fluvial input, lateral transport, and accumulation of sediment are equally important. These internal reworking effects would then significantly bias the paleoclimatic interpretations based on eolian dust properties of the Chinese Loess Plateau.

Magmatic history and crustal genesis of western South America: Constraints from U-Pb ages and Hf isotopes of detrital zircons in modern rivers

Western South America provides an outstanding laboratory for studies of magmatism and crustal evolution because it contains Archean–Paleoproterozoic cratons that amalgamated during Neoproterozoic supercontinent assembly, as well as a long history of Andean magmatism that records crustal growth and reworking in an accretionary orogen. We have attempted to reconstruct the growth and evolution of western South America through U-Pb geochronologic and Hf isotopic analyses of detrital zircons from 59 samples of sand mainly from modern rivers. Results from 5524 new U-Pb ages and 1199 new Hf isotope determinations are reported. Our data are integrated with previously published geochronologic and Hf isotopic information, yielding a combined record that includes >42,000 ages and >1900 Hf isotope analyses. These large data sets yield five main conclusions: (1) South America has an age distribution that is similar to most other continents, presumably reflecting processes of crustal generation and/or preservation related to the supercontinent cycle, with age maxima at 2.2–1.8 Ga, 1.6–0.9 Ga, 700–400 Ma, and 360–200 Ma; (2) <200 Ma magmatism in western South America has age maxima at ca. 183, 166, 149, 125, 110, 88, 65, 35, 21, and 4 Ma (with significant north-south and east-west variations), yielding an average cyclicity of ∼33 m.y.; (3) for the past 200 m.y., no correlation exists between magmatism and the velocity of convergence between central South America and Pacific oceanic plates, the age of the downgoing plate, or the absolute motion of South America; (4) Hf isotopes record reworking of older crustal materials during most time periods, with incorporation of juvenile crust at ca. 1.6–1.0 Ga, 500–300 Ma, and ca. 175–35 Ma; and (5) the Hf isotopic signature of <200 Ma magmatism is apparently controlled by the generation of evolved crust during crustal thickening and eastward arc migration, versus juvenile magmas during extensional tectonism and westward and/or outboard migration of arc magmatism.

Resilience of the Asian atmospheric circulation shown by Paleogene dust provenance

The onset of modern central Asian atmospheric circulation is traditionally linked to the interplay of surface uplift of the Mongolian and Tibetan-Himalayan orogens, retreat of the Paratethys sea from central Asia and Cenozoic global cooling. Although the role of these players has not yet been unravelled, the vast dust deposits of central China support the presence of arid conditions and modern atmospheric pathways for the last 25 million years (Myr). Here, we present provenance data from older (42–33 Myr) dust deposits, at a time when the Tibetan Plateau was less developed, the Paratethys sea still present in central Asia and atmospheric pCO2 much higher. Our results show that dust sources and near-surface atmospheric circulation have changed little since at least 42 Myr. Our findings indicate that the locus of central Asian high pressures and concurrent aridity is a resilient feature only modulated by mountain building, global cooling and sea retreat.

Influence of pre-Andean crustal structure on Cenozoic thrust belt kinematics and shortening magnitude: Northwestern Argentina

The retroarc fold-and-thrust belt of the Central Andes exhibits major along-strike variations in its pre-Cenozoic tectonic configuration. These variations have been proposed to explain the considerable southward decrease in the observed magnitude of Cenozoic shortening. Regional mapping, a cross section, and U-Pb and (U-Th)/He age dating of apatite and zircon presented here build upon the preexisting geological framework for the region. At the latitude of the regional transect (24–25°S), results demonstrate that the thrust belt propagated in an overall eastward direction in three distinct pulses during Cenozoic time. Each eastward jump in the deformation front was apparently followed by local westward deformation migration, likely reflecting a subcritically tapered orogenic wedge. The first eastward jump was at ca. 40 Ma, when deformation and exhumation were restricted to the western margin of the Eastern Cordillera and eastern margin of the Puna Plateau. At 12–10 Ma, the thrust front jumped ∼75 km toward the east to bypass the central portion of a horst block of the Cretaceous Salta rift system, followed by initiation of new faults in a subsystem that propagated toward the west into this preexisting structural high. During Pliocene time, deformation again migrated >100 km eastward to a Cretaceous synrift depocenter in the Santa Barbara Ranges. The sporadic foreland-ward propagation documented here may be common in basement-involved thrust systems where inherited weaknesses due to previous crustal deformation are preferentially reactivated during later shortening. The minimum estimate for the magnitude of shortening at this latitude is ∼142 km, which is moderate in magnitude compared to the 250–350 km of shortening accommodated in the retroarc thrust belt of southern Bolivia to the north. This work supports previous hypotheses that the magnitude of shortening decreases significantly along strike away from a maximum in southern Bolivia, largely as a result of the distribution of pre-Cenozoic basins that are able to accommodate a large magnitude of thin-skinned shortening. A major implication is that variations in the pre-orogenic upper-crustal architecture can influence the behavior of the continental lithosphere during later orogenesis, a result that challenges geodynamic models that neglect upper-plate heterogeneities.