Carmala N. Garzione

Rise of the Andes

The surface uplift of mountain belts is generally assumed to reflect progressive shortening and crustal thickening, leading to their gradual rise. Recent studies of the Andes indicate that their elevation remained relatively stable for long periods (tens of millions of years), separated by rapid (1 to 4 million years) changes of 1.5 kilometers or more. Periodic punctuated surface uplift of mountain belts probably reflects the rapid removal of unstable, dense lower lithosphere after long-term thickening of the crust and lithospheric mantle.

Stratigraphy, structure, and tectonic evolution of the Himalayan fold‐thrust belt in western Nepal

Regional mapping, stratigraphic study, and 40Ar/39Ar geochronology provide the basis for an incremental restoration of the Himalayan fold-thrust belt in western Nepal. Tectonostratigraphic zonation developed in other regions of the Himalaya is applicable, with minor modifications, in western Nepal. From south to north the major structural features are (1) the Main Frontal thrust system, comprising the Main Frontal thrust and two to three thrust sheets of Neogene foreland basin deposits; (2) the Main Boundary thrust sheet, which consists of Proterozoic to early Miocene, Lesser Himalayan metasedimentary rocks; (3) the Ramgarh thrust sheet, composed of Paleoproterozoic low-grade metasedimentary rocks; (4) the Dadeldhura thrust sheet, which consists of medium-grade metamorphic rocks, Cambrian-Ordovician granite and granitic mylonite, and early Paleozoic Tethyan rocks; (5) the Lesser Himalayan duplex, which is a large composite antiformal stack and hinterland dipping duplex; and (6) the Main Central thrust zone, a broad ductile shear zone. The major structures formed in a general southward progression beginning with the Main Central thrust in late early Miocene time. Eocene-Oligocene thrusting in the Tibetan Himalaya, north of the study area, is inferred from the detrital unroofing record. On the basis of 40Ar/39Ar cooling ages and provenance data from synorogenic sediments, emplacement of the Dadeldhura thrust sheet took place in early Miocene time. The Ramgarh thrust sheet was emplaced between ∼15 and ∼10 Ma. The Lesser Himalayan duplex began to grow by ∼10 Ma, simultaneously folding the north limb of the Dadeldhura synform. The Main Boundary thrust became active in latest Miocene-Pliocene time; transport of its hanging wall rocks over an ∼8-km-high footwall ramp folded the south limb of the Dadeldhura synform. Thrusts in the Subhimalayan zone became active in Pliocene time. The minimum total shortening in this portion of the Himalayan fold-thrust belt since early Miocene time (excluding the Tibetan zone) is ∼418–493 km, the variation depending on the actual amounts of shortening accommodated by the Main Central and Dadeldhura thrusts. The rate of shortening ranges between 19 and 22 mm/yr for this period of time. When previous estimates of shortening in the Tibetan Himalaya are included, the minimum total amount of shortening in the foldthrust belt amounts to 628–667 km. This estimate neglects shortening accommodated by small-scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening.

The kinematic evolution of the Nepalese Himalaya interpreted from Nd isotopes

Neodymium (Nd) isotopes from the Himalayan fold-thrust belt and its associated foreland basin deposits are useful for distinguishing between Himalayan tectonostratigraphic zones and revealing the erosional unroofing history as controlled by the kinematic development of the orogen. Neodymium isotopic data from the Himalayan fold-thrust belt in Nepal (n=35) reveal that the Lesser Himalayan zone consistently has a more negative ϵNd(0) value than the Greater and Tibetan Himalayan zones. Our data show the average ϵNd(0) value in the Lesser Himalayan zone is −21.5, whereas the Greater and Tibetan Himalayan zones have an average ϵNd(0) value of −16. These consistently distinct values throughout Nepal enable the use of Nd isotopes as a technique for distinguishing between Lesser Himalayan zone and Greater Himalayan zone rock. The less negative ϵNd(0) values of the Greater Himalayan rocks support the idea that the Greater Himalayan zone is not Indian basement, but rather a terrane that accreted onto India during Early Paleozoic time. Neodymium isotopic data from Eocene through Pliocene foreland basin deposits (n=34) show that sediment provenance has been dominated by Greater and Tibetan Himalayan detritus across Nepal. The ϵNd(T) values in the synorogenic rocks in western and central Nepal generally show an up-section shift toward more negative values and record the progressive unroofing of the different tectonostratigraphic zones. At ∼10 Ma in Khutia Khola and ∼11 Ma in Surai Khola, a shift in ϵNd(T) values from −16 to −18 marks the erosional breaching of a large duplex in the northern part of the Lesser Himalayan zone. This shift is not seen in eastern Nepal, where the ϵNd(T) values remain close to −16 throughout Miocene time because there has been less erosional unroofing in this region.

Predicting paleoelevation of Tibet and the Himalaya from δ18O vs. altitude gradients in meteoric water across the Nepal Himalaya

The δ18O value of meteoric water varies with elevation, providing a means to reconstruct paleoelevation if the δ18O values of paleowater are known. In this study, we determined the δ18O values of water (δ18Omw) from small tributaries along the Seti River and Kali Gandaki in the Nepal Himalaya. We found that δ18Omw values decrease with increasing altitude for both transects. δ18Omw vs. altitude along the Kali Gandaki in west-central Nepal fit a second order polynomial curve, consistent with increasing depletion of 18O with increasing elevation, as predicted by a Rayleigh-type fractionation process. This modern δ18Omw vs. altitude relationship can be used to constrain paleoelevation from the δ18O values of Miocene–Pliocene carbonate (δ18Oc) deposited in the Thakkhola graben in the southern Tibetan Plateau. Paleoelevations of 3800±480 m to 5900±350 are predicted for the older Tetang Formation and 4500±430 m to 6300±330 m for the younger Thakkhola Formation. These paleoelevation estimates suggest that by ∼11 Ma the southern Tibetan Plateau was at a similar elevation to modern.

Flexural subsidence by 29 Ma on the NE edge of Tibet from the magnetostratigraphy of Linxia Basin, China

Abstract This study provides a detailed magnetostratigraphic record of subsidence in the Linxia Basin, documenting a 27 Myr long sedimentary record from the northeastern edge of the Tibetan Plateau. Deposition in the Linxia Basin began at ∼29 Ma and continued nearly uninterruptedly until ∼1.7 Ma. Increasing rates of subsidence between 29 and 6 Ma in the Linxia Basin suggest deposition in the foredeep portion of a flexural basin and constrain the timing of shortening in the northeastern margin of the plateau to Late Oligocene–Late Miocene time. By Late Miocene–Early Pliocene time, a decrease in subsidence rates in the Linxia Basin associated with thrust faulting and a ∼10° clockwise rotation in the basin indicates that the deformation front of the Tibetan plateau had propagated into the currently deforming region northeast of the plateau.

Kinematic model for the Main Central thrust in Nepal

We present a kinematic model for the Himalayan thrust belt that satisfies structural and metamorphic data and explains recently reported late Miocene‐Pliocene geochronologic and thermochronologic ages from rocks in the Main Central thrust zone in central Nepal. At its current exposure level, the Main Central thrust juxtaposes a hanging-wall flat in Greater Himalayan rocks with a footwall flat in Lesser Himalayan rocks of the Ramgarh thrust sheet, which is the roof thrust of a large Lesser Himalayan duplex. Sequential emplacement of the Main Central (early Miocene) and Ramgarh (middle Miocene) thrust sheets was followed by insertion of thrust sheets within the Lesser Himalayan duplex and folding of the Main Central and Ramgarh thrusts during late Miocene‐ Pliocene time. Thorium-lead (Th-Pb) ages of monazite inclusions in garnets from central Nepal record the timing of coeval, progressive metamorphism of Lesser Himalayan rocks in the footwall of the Main Central thrust. Although this model does not rule out minor, late-stage reactivation of the Main Central thrust, major late Miocene reactivation is not required.

Uplift-driven climate change at 12 Ma: a long δ18O record from the NE margin of the Tibetan plateau

Carbonates from fluvial and lacustrine sediments were sampled from multiple measured sections in the Linxia basin of western China.Based on textural and mineralogical evidence, lacustrine carbonates are primary precipitates from lake water.A 29 million year record of the oxygen isotope composition of meteoric water is inferred from the N 18 O values of these carbonates.This inference is based on the most negative N 18 O values in the lake carbonates, which represent lake waters that have experienced the least evaporative enrichment.Carbonate N 18 O values, a proxy for rainfall N 18 O, are V310.5x throughout the interval of 29^12 Ma.At 12 Ma there is a shift to 39x, a value that remains into the Pliocene.This implies a major reorganization of atmospheric circulation patterns and a shift to more arid conditions at the NE margin of the Tibetan plateau with the post-12 Ma system similar to that of today.The 12 Ma event may represent the time at which the Tibetan plateau achieves sufficient elevation to block the penetration of moisture from the Indian Ocean or south Pacific into western China. The period of greatest aridity is from 9.6 to 8.2 Ma, a time interval which agrees well with other climate records. < 2003 Elsevier B.V. All rights reserved.

Qaidam Basin and northern Tibetan Plateau as dust sources for the Chinese Loess Plateau and paleoclimatic implications

The Chinese Loess Plateau of central Asia is composed of interbedded loess and paleosol layers, deposited during glacial and interglacial cycles, respectively, during the past ∼2.5 m.y. Understanding the provenance of loess is fundamental to reconstructing wind patterns during Quaternary glacial periods. We determined and compared U-Pb ages on zircon crystals from Loess Plateau strata and potential source areas. The results indicate that the loess was largely derived from the Qaidam Basin and the northern Tibetan Plateau to the west, both of which exhibit spatially extensive geomorphic landforms indicative of past (interpreted as pre-Holocene) wind erosion and/or deflation by westerly winds. This challenges the current paradigm that the loess of the Chinese Loess Plateau was largely sourced from deserts located to the northwest, as observed in the modern interglacial climate. We propose that during glacial periods, the mean annual positions of the polar jet streams were shifted equatorward, resulting in more southerly tracks for dust-generating storms and suppression of the East Asian monsoon by inhibiting the subtropical jet from shifting northward across the Tibetan Plateau.

East-west extension and Miocene environmental change in the southern Tibetan plateau: Thakkhola graben, central Nepal

East-west extensional basins are distributed across the southern half of the Tibetan plateau at an elevation of ∼4 km. These basins have generated much interest because of their potential implications for the regional tectonics and force distribution in the plateau. This study documents the sedimentology of the Miocene–Pliocene Thakkhola graben fill in order to reconstruct basin evolution and paleoenvironment. Analysis of depositional systems, paleodrainage patterns, and conglomerate clast provenance of the >1-km-thick graben fill sets limits on the timing of activity of the basin-bounding faults and the development of southward axial drainage in the basin. During the deposition of the oldest basin fill (Tetang Formation, ca. 11–9.6 Ma), probably in a restricted basin, minor motion occurred on the basin-bounding fault systems. An angular unconformity separates the Tetang and overlying Thakkhola Formations, where this contact can be observed in the southern part of the basin. Southward axial drainage was established by ca. 7 Ma with the onset of deposition of the Thakkhola Formation. Several episodes of damming of this drainage system are recorded by widespread lacustrine deposits in the southern part of the basin. Facies distribution and the progressive rotation of strata in the Thakkhola Formation indicate that the Dangardzong fault on the western edge of the basin was active at this time, and drainage ponding may have been related to displacement on normal faults associated with the South Tibetan detachment system to the south of Thakkhola graben. Contrasts between deposits of the Tetang and Thakkhola Formations provide evidence for environmental change in the basin. In the Tetang Formation, the abundance of lacustrine facies, the pollen record, and the absence of paleosol carbonate suggest that conditions were more humid than during subsequent deposition of the Thakkhola Formation. Environmental change in the Thakkhola graben coincided with environmental change observed in the Siwalik foreland basin sequence, Arabian Sea, and Bay of Bengal at ca. 8–7 Ma. Although this climate change event has been previously attributed to intensification of the Asian monsoon in response to uplift of the Tibetan plateau, paleoaltimetry data indicate that this region had already attained a high elevation by ca. 11 Ma. Thus, the Thakkhola graben stratigraphic record suggests that uplift of the southern Tibetan plateau and the onset of the Asian monsoon as inferred from paleoclimatic indicators were not directly related in a simple way.

Carbonate oxygen isotope paleoaltimetry: evaluating the effect of diagenesis on paleoelevation estimates for the Tibetan plateau

Carbonate oxygen isotope paleoaltimetry is based on analysis of the dO value of carbonate precipitated from surface water. Deciphering the diagenetic history is important for establishing whether particular carbonates are accurate recorders of paleosurface waters, which reflect paleoelevation. This study provides examples from southern, east–central, and northeastern Tibet of approaches aimed at evaluating the diagenetic history of lacustrine micrites and pedogenic carbonates. The most desirable technique for avoiding erroneous interpretations related to diagenetic overprinting is to analyze carbonates that are known to be primary, such as aragonitic shell material. In rocks that do not contain shell material, we have compared lacustrine micrites and pedogenic carbonates to diagenetic carbonate phases to determine the effects of diagenesis on the isotopic composition of primary carbonates. Where the potential effects of diagenesis are subtle or ambiguous, we have evaluated the fidelity of the carbonate record from systematic trends in C and O isotopes that agree with other interpretations of paleoenvironment, such as high frequency covariance in C and O that corresponds with changes in the Mg concentration of carbonates. Using these strategies, we have determined that diagenesis has not affected the isotopic composition of carbonates in the Late Miocene–Pliocene Thakkhola graben in southern Tibet and the Oligocene to Pliocene Linxia basin in northeastern Tibet. In the case of Paleogene basins in east–central Tibet, however, ambiguity in data precludes the determination of diagenetic effects. D 2004 Elsevier B.V. All rights reserved.