Andreas Mulch

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.

Hydrogen Isotopes in Eocene River Gravels and Paleoelevation of the Sierra Nevada

We determine paleoelevation of the Sierra Nevada, California, by tracking the effect of topography on precipitation, as recorded in hydrogen isotopes of kaolinite exposed in gold-bearing river deposits from the Eocene Yuba River. The data, compared with the modern isotopic composition of precipitation, show that about 40 to 50 million years ago the Sierra Nevada stood tall (≥2200 meters), a result in conflict with proposed young surface uplift by tectonic and climatic forcing but consistent with the Sierra Nevada representing the edge of a pre-Eocene continental plateau.

Reconstructing paleoelevation in eroded orogens

Hydrogen isotope and 40Ar/39Ar geochronological data are presented from muscovite within a crustal-scale extensional detachment of the Shuswap Metamorphic Complex, North American Cordillera. The hydrogen isotope compositions (δDms) of precisely dated muscovite attain values as low as −156‰ in the detachment mylonite, whereas footwall quartzite has a δDms value of −81‰. The very low δDms values in the detachment are best explained by infiltration of meteoric water, with maximum δD values of −135‰ ± 3‰, during extensional unroofing of the orogen at 49.0–47.9 Ma. On the basis of the empirically determined relationship between elevation and isotopic composition of precipitation, the reconstructed early Eocene paleoelevations of the orogen are 4060 ± 250 m to 4320 ± 250 m, at least 1000 m higher than the highest present-day peaks. We propose that the isotopic composition of surface-derived waters in extensional detachments represents a newly recognized method to estimate maximum paleoelevations attained immediately preceding extensional orogenic collapse.

The Cenozoic climatic and topographic evolution of the western North American Cordillera

Herein we present oxygen isotope records from Cretaceous to Recent terrestrial sediments in the western North American Cordillera. The purpose of this analysis is to use oxygen isotope records to understand the coupled surface elevation and climate histories of this region through the Cenozoic. To do this we constructed δ18O maps of surface waters for time intervals that trace the development of topography of western North America. These maps are based on 4861 oxygen isotope analyses from both published (4478) and new (383) data. We determined the δ18O values of surface waters using paleotemperatures derived previously from floral assemblages and the appropriate isotope fractionation factors. These data suggest that in the late Cretaceous to early Eocene the Sevier hinterland formed a plateau of unknown height. Around 50 Ma, a topographic wave developed in British Columbia and eastern Washington that swept southward reaching northeastern Nevada at ∼40 to 38 Ma, and southern Nevada ∼23 Ma. This southward encroachment of an Eocene Plateau (SWEEP) caused reorganization of drainage patterns such that the intraforeland basins of Wyoming and Utah drainages extended deep within the Sevier hinterland as the wave swept southward. The landscape within the Sevier hinterland developed into a rugged and high mountain range with the hypsometric mean elevation of ∼4 km and relief of ∼1.5 km. This Eocene highland was bordered on the west by a high Sierra Nevada ramp and on the east by the intraforeland basins that captured water draining these growing highlands. The spatial and temporal evolution of this highland correlates with the timing of volcanism and extension. These observations support tectonic models that call for north to south removal of the Farallon slab or piecemeal removal of mantle lithosphere. The isotopic data show that prior to growth of this highland the North American Monsoon (NAM) penetrated much farther north in the Paleocene/Eocene than today. The combined effects of global cooling, increasing latitudinal temperature gradients, and the generation of the orographic barrier created by the growing north to south highland produced a southward migration of the NAM front. By the Oligocene the hydrologic regime that we observe today was in place. It has been modified since then as a result of Basin and Range extension and collapse of the highlands in the mid-Miocene. This collapse allowed the NAM to penetrate farther north into the Great Basin of Nevada and Utah.

The rise and growth of Tibet

It is not difficult to be impressed by the grandeur of high mountainous regions, but it is difficult to reconstruct how the elevation of such regions evolved. A study of the Tibetan plateau does just that.A high old time in TibetThe Himalayan mountains are testament to the massive forces involved when continents collide, and the elevation history of the adjoining Tibetan plateau provides an extended record of the event. An analysis of palaeo-altitude at the centre of the Tibetan Plateau, using oxygen-isotope based measurements of carbonates, more than doubles the period of known existence of the central region of the plateau, to 35 million years. The surface elevation of Tibet has been more than 4 kilometres for all of that time. The new data are consistent with models that explain plateau uplift as a consequence of crustal thickening, rather than mantle thickening and convective removal

Cenozoic migration of topography in the North American Cordillera

Continental topography is the result of complex interactions among mantle convection, continental dynamics, and climatic and erosional processes. Therefore, topographic evolution of mountain belts and continental interiors reflects directly upon the coupling between mantle and surface processes. It has recently been proposed that the modern topography of western North America is partly controlled by the removal of the subducting Farallon plate and replacement of lithospheric mantle by hot asthenosphere, creating surface uplift of the Colorado Plateau, the southwestern United States, and northern Mexico, while concomitant subsidence characterizes the central United States. How the topography of the Cenozoic North American Cordillera evolved in the past is largely unknown, yet currently debated tectonic models each have a predictable topographic response. Here we examine Cenozoic surface uplift patterns of western North America based on a record of ∼3000 stable isotope proxy data. This data set is consistent with Eocene north to south surface uplift in the Cordillera, culminating in the assembly of an Eocene–Oligocene highland 3–4 km in elevation. The diachronous record of surface uplift and associated magmatism further supports tectonic models calling for the convective removal of mantle lithosphere or removal of the Farallon slab by buckling along an east-west axis. The Eocene–Oligocene development of rainout patterns similar to present-day patterns along the flanks of the Cordilleran orogen is therefore unlikely to be the result of late Mesozoic crustal thickening and associated development of an Andean-style Altiplano.

Recrystallization or cooling ages: in situ UV-laser 40Ar/39Ar geochronology of muscovite in mylonitic rocks

The intra-grain 40Ar/39Ar age distributions of muscovite from greenschist-facies mylonite zones are shown to be sensors of metamorphic history. Textures, compositions and in situ UV-laser ablation 40Ar/39Ar data from two extensional shear zones demonstrate that cooling ages can be distinguished from neo- or recrystallization ages. Deformed muscovite porphyroclasts and recrystallized shear band muscovite from the Pogallo Shear Zone (Ivrea Zone, southern Alps) reveal variable intra-grain 40Ar/39Ar ages with internal age variations of more than 65 Ma. The wide range of 40Ar/39Ar ages within compositionally homogeneous grains is consistent with diffusion-dominated argon loss controlled by observable intra-grain microstructures. In contrast, intra-grain 40Ar/39Ar ages of muscovite fish from Proterozoic mylonite of the Porsgrunn–Kristiansand Shear Zone (Southern Norway) display only minor age dispersion, and age variations are correlated with changes in muscovite composition. The sympathetic age–composition correlations in different muscovite microstructures directly relate to sequential neo- and recrystallization of muscovite and ultimately provide insight into the protracted recrystallization history during extensional deformation along the Porsgrunn–Kristiansand Shear Zone.

A Miocene to Pleistocene climate and elevation record of the Sierra Nevada (California)

Orographic precipitation of Pacific-sourced moisture creates a rain shadow across the central part of the Sierra Nevada (California) that contrasts with the southern part of the range, where seasonal monsoonal precipitation sourced to the south obscures this rain shadow effect. Orographic rainout systematically lowers the hydrogen isotope composition of precipitation (δDppt) and therefore δDppt reflects a measure of the magnitude of the rain shadow. Hydrogen isotope compositions of volcanic glass (δDglass) hydrated at the earths surface provide a unique opportunity to track the elevation and precipitation history of the Sierra Nevada and adjacent Basin and Range Province. Analysis of 67 well dated volcanic glass samples from widespread volcanic ash-fall deposits located from the Pacific coast to the Basin and Range Province demonstrates that between 0.6 and 12.1 Ma the hydrogen isotope compositions of meteoric water displayed a large (>40‰) decrease from the windward to the leeward side of the central Sierra Nevada, consistent with the existence of a rain shadow of modern magnitude over that time. Evidence for a Miocene-to-recent rain shadow of constant magnitude and systematic changes in the longitudinal climate and precipitation patterns strongly suggest that the modern first-order topographic elements of the Sierra Nevada characterized the landscape over at least the last 12 million years.

The abiotic and biotic drivers of rapid diversification in Andean bellflowers (Campanulaceae)

Summary The tropical Andes of South America, the worlds richest biodiversity hotspot, are home to many rapid radiations. While geological, climatic, and ecological processes collectively explain such radiations, their relative contributions are seldom examined within a single clade. We explore the contribution of these factors by applying a series of diversification models that incorporate mountain building, climate change, and trait evolution to the first dated phylogeny of Andean bellflowers (Campanulaceae: Lobelioideae). Our framework is novel for its direct incorporation of geological data on Andean uplift into a macroevolutionary model. We show that speciation and extinction are differentially influenced by abiotic factors: speciation rates rose concurrently with Andean elevation, while extinction rates decreased during global cooling. Pollination syndrome and fruit type, both biotic traits known to facilitate mutualisms, played an additional role in driving diversification. These abiotic and biotic factors resulted in one of the fastest radiations reported to date: the centropogonids, whose 550 species arose in the last 5 million yr. Our study represents a significant advance in our understanding of plant evolution in Andean cloud forests. It further highlights the power of combining phylogenetic and Earth science models to explore the interplay of geology, climate, and ecology in generating the worlds biodiversity.

Isotope transport and exchange within metamorphic core complexes

Field observations from the Shuswap metamorphic core complex in British Columbia indicate that meteoric fluids were focused along a sub-horizontal shear zone at a depth of at least 7 km. Fluid-rock interactions associated with this flow system resulted in oxygen isotope depletion of mylonitic rocks up to 4 permil in a region less than 900m wide. Dating of the recrystallized shear zone fabric and deformation-assisted fluid flow indicates that this paleo-fluid flow system was relatively short lived, (<1 Ma). Here we present idealized numerical representations of a metamorphic core complex system to assess the hydrologic and thermal controls on fluid-rock isotopic exchange. Our analysis focuses on understanding the relative importance of fault versus matrix controlled fluid flow, reactive-surface area, crustal permeability structure, and isotopic composition of the recharging fluids. The analysis permits us to bracket the possible permeability and surface area conditions that are consistent with field observations. We conclude that downward fluid flow along brittle fault systems and isotope exchange patterns could only be produced by a fracture flow dominated system. We found the fault permeability had to be greater than 10−16 m2 but less than or equal to 10−15 m2. Upper plate crystalline rocks adjacent to the fault zone had to have a permeability less than 10−17 m2. The above findings are valid assuming a lateral water table gradient of 5 percent, a shear zone surface area of 3.0×10−4 m2/mole, crustal rock surface area of 1.0×10−5 m2/mole, total duration of flow of 200,000 years, and a basal heat flux of 90 mW/m2. Fault zone surface areas are much too small to be consistent with pervasive grain boundary fluid-rock isotope interactions. Rather, the best fit surface areas were consistent with a fracture spacing of 0.25 m for the shear/fault zones and a 5 m spacing for surrounding upper and lower plate rocks. We found that fracture aperture widths of about 0.02 mm for the fault/shear zone units and 0.002 mm for the surrounding upper and lower plate rocks were consistent with the permeability values obtained from our generic modeling exercise. Imposing a more strongly 18O-depleted oxygen isotope composition for the meteoric recharge was directly reflected in lower computed δ18O rocks. However, the effects were non-unique and to some degree, masked by the large oxygen reservoir within the crustal rocks. Computed rock isotopic values consistent with field observations could have been produced with either heavier δ18O fluids in the recharge area over a longer period of infiltration or lighter δ18O fluid compositions in the recharge region over shorter periods of time.