Tara N. Jonell

Controls on erosion in the western Tarim Basin: Implications for the uplift of northwest Tibet and the Pamir

We present here bulk sediment major element chemistry, Nd and Sr isotope ratios, and detrital apatite fission-track (AFT) and U-Pb zircon ages to characterize the provenance of the southwestern Taklimakan Desert (northwest China) and the three major rivers draining this region. We establish the spatial and temporal controls on erosion and sediment transport in the modern Tibetan rain shadow. The Hotan River drains the North Kunlun block and is characterized by zircon populations at 160–230 Ma and 370–520 Ma. The Yarkand River shares these grains with the Hotan, but also has a very prominent zircon population at 40–160 Ma, which is common in Karakoram basement, indicating heavy sediment flux from these ranges to that drainage. This implies a strong control on erosion by topographic steepness and precipitation mediated through glaciation. Our zircon data confirm earlier studies that indicated that the Taklimakan sand is derived from both the Kunlun and Pamir Mountains. AFT ages are younger in the Hotan River than in the Kashgar River, which drains the Pamir, and in both are younger than in the Transhimalaya and parts of the western edge of the Tibetan Plateau. Exhumation is estimated at ~1000 m/m.y in the North Kunlun and ~500 m/m.y. in the eastern Pamir, which have been exhuming more slowly than the western ranges in the recent past. Holocene aggradation terracing was dated using quartz optically stimulated luminescence methods and is mostly associated with times of fluctuating climate after 4 ka, with phases of valley filling dated at 2.6, 1.4, and 0.4 ka. The heights and volumes of the terraces show that sediment storage in the mountains is not a significant buffer to sediment transport, in contrast to the more monsoonal Indus system directly to the south. South of the Mazatag Ridge a significant eolian deposit accumulated ~500 yr ago, but this has been deflated in more recent times. Comparison of the modern river data with those previously measured from Cenozoic foreland sedimentary rocks shows that no sediment similar to that of the modern Yarkand River is seen in the geologic record, which is inferred to be younger than 11 Ma, and probably much less. Uplift of the North Kunlun had started by ca. 17 Ma, somewhat after that of the Pamir and Songpan Garze of northwestern Tibet, dated to before 24 Ma. Sediment from the Kunlun reached the foreland basin between 14 and 11 Ma. North Kunlun exhumation accelerated before 3.7 Ma, likely linked to faster rock uplift.

Onset of the Laramide orogeny and associated magmatism in southern New Mexico based on U-Pb geochronology

The Laramide orogeny is a classic yet controversial mountain-building event that resulted, in the southwest United States, in uplifts, sedimentation, and magmatism that can be used to constrain the onset of this event in the region and expand our knowledge of Late Cretaceous to Paleogene tectonism. The McRae Formation marks the onset of deposition in the Laramide Love Ranch Basin, which was located to the northeast of the west-northwest-trending coeval Rio Grande uplift in south-central New Mexico, but its age is not well constrained. A previously published late Maastrichtian age for the McRae Formation was based on the presence of dinosaur bones in the upper of two members of the formation. We obtained new U-Pb dates from one dacite clast and three ash-fall tuffs from the lower Jose Creek Member and from one ash-fall tuff from the lower part of the overlying Hall Lake Member of the McRae Formation. The clast yielded a date of 75.0 ± 1.1 Ma, whereas the ages of the tuffs, in ascending stratigraphic order, are 74.9 ± 0.7 Ma, 74.7 ± 0.6 Ma, 75.2 ± 1.3 Ma, and 73.2 ± 0.7 Ma. These dates indicate that the onset of Laramide deposition in the Love Ranch Basin must have occurred earlier, in late Campanian time, similar to deposition in the Laramide Ringbone Basin in southwestern New Mexico. In addition, U-Pb zircon dates of 75.7 ± 1.3 Ma and 75.0 ± 2.8 Ma were obtained on the Twin Peaks stock and on a dacite sill, respectively, in the Burro Mountains of southwestern New Mexico. These dates are similar to those of other Laramide arc magmatic centers in southern New Mexico, which have a limited range of ages from 75 to 70 Ma, including the Hidalgo Formation in the Little Hatchet Mountains, the Silver City-Pinos Altos region, and the Copper Flat porphyry system. These new and previously published dates indicate that during the onset of Laramide deformation in southwestern and south-central New Mexico, the angle of subduction of the Farallon plate may have been steep enough to allow partial melting of an asthenospheric wedge, resulting in arc magmatism far inboard of the trench.

Climatic and glacial impact on erosion patterns and sediment provenance in the Himalayan rain shadow, Zanskar River, NW India

Erosion is a key step in the destruction and recycling of the continental crust, yet its primary drivers continue to be debated. The relative balance between climatic and solid Earth forces in determining erosion patterns and rates, and in turn orogenic architecture, is unresolved. The monsoon-dominated frontal Himalaya is a classic example of how surface processes may drive focused denudation and potentially control structural evolution. We investigate whether there is a clear relationship between climate and erosion in the drier Himalayan rain shadow on the periphery of the Tibetan Plateau, where a coupled climate-erosion relationship is less clear. We present a new integrated data set combining bulk petrography, geomorphometric analysis, detrital U-Pb zircon geochronology, and bulk Nd and Sr isotope geochemistry from modern river sediments that provides constraints on spatial patterns of sediment production and transport in the Zanskar River. Zanskar River sands are dominated by Greater Himalayan detritus sourced from the glaciated Stod River catchment, which represents only 13% of the total basin area. Prevalent zircon peaks from Cambrian-Ordovician (440-500 Ma) and Mississippian- Permian (245-380 Ma) units indicate more abundant pre-Himalayan granitoids in the northwest Indian Himalaya than in the central and eastern Himalaya. Erosion from the widely exposed Tethyan Himalaya, however, appears modest. Spatial patterns of erosion do not correlate with highest channel steep-ness. Our data demonstrate that Zanskar differs from the monsoon-soaked frontal Himalaya and the arid, extremely slow-eroding Tibetan orogenic interior in that focused erosion and sediment production are driven by glaciers. Subsequent remobilization of glacially derived sediments is likely controlled by monsoonal rainfall, and we suggest sediment reworking plays an important role. These data support a strong climatic control on modern orogenic erosion in the Himalayan rain shadow on the periphery of the Tibetan Plateau.

Distribution and source of organic matter in surface sediment from the muddy deposit along the Zhejiang coast, East China Sea

To constrain organic matter compositions and origins, elemental (TOC, TN, C/N) and stable carbon (δ13C) and nitrogen isotope (δ15N) compositions are measured for surface sediments collected from muddy deposit along the Zhejiang coast, East China Sea. The results showed that the TOC, TN, C/N, δ13C, and δ15N were 0.19-0.67%, 0.03-0.09%, 6.76-9.22, -23.43 to -20.26‰, and 3.93-5.27‰, respectively. The δ13C values showed that the mixing inputs of terrigenous and marine organic matter generally dominated sedimentary organic matter in the west part, and the sedimentary organic matters were mainly influenced by the marine organic matter in the east part of the study area. A stable carbon isotope two end member mixing model estimates ~38% terrestrial -derived and ~62% marine-derived inputs to sedimentary organic matter. Microbial mineralization strongly controls δ15N values, and therefore cannot be used to identify the provenance of organic matter for the Zhenjiang coast.

THE LIFECYCLE OF THE EOCENE-OLIGOCENE SCHOOLHOUSE MOUNTAIN CALDERA, MOGOLLON-DATIL VOLCANIC FIELD, SOUTHWEST NEW MEXICO

Megabreccia deposits in calderas are diagnostic of a collapse event where hot ash and gasses interact with precaldera rocks and basement rocks. The late-Eocene Schoolhouse Mountain caldera (SMC) in the Mogollon-Datil volcanic field (MDVF) of southwest New Mexico is a relatively unstudied caldera, where little is known regarding the caldera stratigraphy and the timespan of magmatism. We undertook field mapping, petrology, and 40Ar/39Ar geochronology to develop a model for the evolution of the caldera and its relationship to MDVF volcanism. Volcanism began with the pre-collapse dacite lava flows and dikes of the Saddlerock Canyon sequence, dated at 35.2 ± 0.2 Ma (U-Pb zircon). The top of this lowermost sequence has interlayered dacite flows and rhyolite tuffs overlain by lithic-rich rhyolite tuffs, flows, and breccias at the base of the overlying Kerr Canyon sequence. Tuffs are separated by volcaniclastic sandstone or flow-banded rhyolite. Clasts within breccia include rhyolite with abundant phenocrysts and pumice, Cretaceous sandstone, and sparse Proterozoic granite. Clasts range from up to 15 cm in the lithic breccia, and up to 4 m in the megabreccia. Total thickness of the Kerr Canyon sequence is ~800–2000 m but may be thicker in the central area and tapering to the edges, suggesting infilling of a topographic low. Several clasts in the breccia have apparent injection of rhyolite into the clasts in a wispy, flame-like structure, discolored thermally altered rinds, and vesiculation of the rhyolite near the clasts from degassing. Thus, we suggest this is a collapse breccia associated with an eruption, rather than a lahar deposit consisting of cold, reworked volcanic material. Sanidine crystals from two boulder clasts and the rhyolite matrix were dated using 40Ar/39Ar single-crystal laser fusion. The clasts yielded identical ages of 35.34±0.03 Ma and the matrix had ages of 35.17±0.11 Ma and 35.35±0.05 Ma. Some clasts have abundant quartz and sanidine phenocrysts similar to the Kneeling Nun tuff, which is similar in age (McIntosh et al., 1992), raising the possibility that these were derived from this unit. Two tuffs at similar stratigraphic levels in this sequence had 40Ar/39Ar biotite dates of 34.71±0.14 Ma and 34.33±0.08 Ma. This may indicate that the Kerr Canyon sequence was erupting at 34.7–34.3 Ma and that the breccia contains clasts with “inherited” sanidine that were not reset during emplacement. The higher units in this system have an uncertain relationship with respect to the history of the SMC. The next highest unit, Mangas Creek, consists of quartz-latitic tuffs and andesite lava flows, volcaniclastic sandstones, and volcanic breccias up to ~2000 m. Overlying this is the McCauley Ranch tuff, with a sanidine 40Ar/39Ar date of 33.99±0.04 Ma, and the Cherokee Canyon tuff (600 m), which has a sanidine 40Ar/39Ar date of 33.84±0.02 Ma. Thus, the eruptive history of this caldera ranges from at least as old as 34.71 Ma to 33.84 Ma, or approximately 1.1 m.y.