Adam M. Hudson

Long-term east-west asymmetry in monsoon rainfall on the Tibetan Plateau

Variations in the strength and duration of the Asian Monsoon affect more than half the population of the planet, and there is intense interest in where, and how much, rainfall amounts will change regionally in response to warmer climate conditions. The modern monsoon region is divided into distinct subsystems, so the response may be regionally heterogeneous. Northern Hemisphere monsoon systems intensified during the early to mid-Holocene, increasing summer rainfall and creating wetter conditions across much of eastern Asia. In this study we use the area encompassed by high shorelines of early Holocene paleolakes in Tibet to reconstruct paleorainfall (and hence paleomonsoon) patterns during this very wet period. We found that the early Holocene relative paleolake expansions of 130 closed-basin lake systems in the central Tibetan Plateau display a strong east-west gradient. Paleolake areas expanded by approximately fourfold in the western plateau, compared to approximately twofold expansion in the eastern region. This early Holocene pattern mirrors the modern west-east climate division on the plateau: rainfall in the west is closely tied to the Indian summer monsoon (ISM) subsystem, and in the east is closely tied to a probable mix of ISM and East Asian summer monsoon (EASM) subsystems. Our results suggest that these modern climate divisions are an enduring feature of the plateau and that ISM rainfall increased much more than EASM rainfall in response to the same insolation forcing.

High late Miocene-Pliocene elevation of the Zhada Basin, southwestern Tibetan Plateau, from carbonate clumped isotope thermometry

The timing and pattern of Tibetan Pla- teau rise provide a critical test of possible mechanisms for the development and sup- port of high topography, yet views range widely on the history of surface uplift to modern elevations of ~4.5 km. To address this issue we present clumped isotope ther- mometry data from two well-studied ba- sins in central and southwestern Tibet, for which previous carbonate δ 18 O data have been used to reconstruct high paleoeleva- tions from late Oligocene to Pliocene time. Clumped isotope thermometry uses mea- surements of the 13 C- 18 O bond ordering in carbonates to constrain the temperature (T(Δ47)) and δ 18 O value of the water from which the carbonate grew. These data can be used to infer paleoelevation by exploiting the systematic decrease of surface tempera- ture and the δ 18 O-based paleoaltimetry and with paleontological and isotopic data indicating the presence of cold- adapted mammals living in a cold, high-ele- vation climate. We suggest that late Neogene elevation loss across the Zhada Basin catch- ment probably related to local expression of east-west extension across much of the south- ern Tibetan Plateau at this time.

Tectonic and structural control of fluvial channel morphology in metamorphic core complexes: The example of the Catalina-Rincon core complex, Arizona

Fluvial channels in metamorphic core complexes are preferentially oriented parallel and perpendicular to the direction of tectonic extension. This pattern has been variably attributed to such causes as tectonic tilting during extension, channel elongation by slip along the range-bounding detachment fault, and the exploitation of extension-related joint sets during channel incision. In this paper we use field measurements, digital elevation model analyses, and numerical modeling to test hypotheses for the tectonic and structural control of fluvial channels in metamorphic core complexes, using the Catalina-Rincon core complex in southern Arizona, USA, as a type example. Field measurements and aerial photographic analyses indicate that channels of all sizes exploit steeply dipping joint sets during fluvial incision. As a consequence, channels become preferentially aligned along those joint sets. First and second Strahler-order channels preferentially exploit a joint set oriented perpendicular to the extension direction, while higher-order channels preferentially exploit a joint set oriented parallel to the extension direction. While these observations support the joint-exploitation hypothesis for structural control of drainage architecture, numerical modeling indicates that the spatial distribution of rock uplift during the initial phase of extension plays a crucial role by determining which joint set is preferentially exploited by channels of which Strahler orders. Numerical models indicate that higher-order channels exploit the joint set that is most closely aligned with the direction of initial tectonic tilting, even if that tilting is active for only a short period of time following the initiation of uplift. We conclude that the drainage architecture in the Catalina-Rincon core complex is the result of a combination of joint exploitation and tectonic tilting mechanisms. Structure also plays an important role in controlling the longitudinal profiles of channels in metamorphic core complexes. Channels in the Catalina-Rincon core complex are characterized by structurally controlled knickpoints with a wide distribution of heights and spacings. Field observations indicate that the occurrence of structurally controlled knickpoints and the resulting variability in longitudinal profile form is related to spatial variations in joint density. Numerical models that incorporate spatial variations in joint density using a stochastic bedrock erodibility coefficient are capable of reproducing the statistical properties of longitudinal profiles in the Catalina-Rincon core complex, including the power spectrum of longitudinal profiles and the frequency size distribution of structurally controlled knickpoints. The results of this study illustrate the important roles played by both jointing and the spatial distribution of rock uplift on the geomorphic evolution of metamorphic core complexes. More broadly, the study provides a recipe for how to incorporate joint-related structural controls into landscape evolution models.

Impact of the North American monsoon on isotope paleoaltimeters: Implications for the paleoaltimetry of the American southwest

Paleoaltimetric studies have characterized in detail the relationship between carbonate oxygen isotope ratios (δ18Oc) and elevation in orogens with simple, single-moisture-source hydrological systems, and applied this relationship to ancient continental carbonates to provide constraints on their past elevation. However, mixing of different atmospheric moisture sources in low-elevation orogens should affect δ18Oc values, but this effect has not yet been confirmed unequivocally. In the American Southwest, summer monsoonal moisture, sourced in the Equatorial Pacific and the Gulf of Mexico, and winter moisture, sourced in the East Pacific, both contribute to annual rainfall. We present stable isotope results from Quaternary carbonates within the American Southwest to characterize the regional δ18Oc-elevation relationship. We then provide stable isotope results from local Eocene carbonates to reconstruct late Laramide paleoelevations. The Quaternary δ18Oc-elevation relationship in the American Southwest is not as straightforward as in more simple hydrological systems. δ18Oc changes with altitude are non-linear, scattered, and display an apparent isotopic lapse rate inversion above 1200 m of elevation. We speculate that decreasing surface temperatures at high altitudes limit the duration of carbonate growth to the summer months, biasing δ18Oc values toward higher values typical of the summer monsoon and leading to lapse rate inversion. δ18Oc-elevation relationships based on modern water isotope data or distillation models predict paleoelevations that range up to as much as 2 km higher than the modern elevations of 2000 to 2400 m for our late Eocene sites located at the southern edge of the Colorado Plateau. By contrast, our δ18Oc-elevation relationship for the American Southwest yields lower paleoelevation estimates. These alternate estimates nonetheless suggest that significant elevation (at least ∼1 km) had already been attained by the Eocene, but are also compatible with < 1 km of uplift by post-Laramide mechanisms. Our results show the limitations of standard δ18Oc-elevation models in complex hydrological systems and suggest that similar mechanisms may have led to summer-biased paleoaltimetry estimates for the initial stages of other orogenies —in the American Southwest and elsewhere.