Bodo Bookhagen

Late Quaternary intensified monsoon phases control landscape evolution in the northwest Himalaya

The intensity of the Asian summer-monsoon circulation varies over decadal to millennial time scales and is reflected in changes in surface processes, terrestrial environments, and marine sedi- ment records. However, the mechanisms of long-lived (2-5 k.y.) intensified monsoon phases, the related changes in precipitation distribution, and their effect on landscape evolution and sedimen- tation rates are not yet well understood. The arid high-elevation sectors of the orogen correspond to a climatically sensitive zone that currently receives rain only during abnormal (i.e., strength- ened) monsoon seasons. Analogous to present-day rainfall anom- alies, enhanced precipitation during an intensified monsoon phase is expected to have penetrated far into these geomorphic threshold regions where hillslopes are close to the angle of failure. We as- sociate landslide triggering during intensified monsoon phases with enhanced precipitation, discharge, and sediment flux leading to an increase in pore-water pressure, lateral scouring of rivers, and ov- ersteepening of hillslopes, eventually resulting in failure of slopes and exceptionally large mass movements. Here we use lacustrine deposits related to spatially and temporally clustered large land- slides (.0.5 km 3 ) in the Sutlej Valley region of the northwest Him- alaya to calculate sedimentation rates and to infer rainfall patterns during late Pleistocene (29-24 ka) and Holocene (10-4 ka) inten- sified monsoon phases. Compared to present-day sediment-flux measurements, a fivefold increase in sediment-transport rates re- corded by sediments in landslide-dammed lakes characterized these episodes of high climatic variability. These changes thus em- phasize the pronounced imprint of millennial-scale climate change on surface processes and landscape evolution.

Climatic forcing of erosion, landscape, and tectonics in the Bhutan Himalayas

A fundamental objective in studies of climate-erosion-tectonics coupling is to document convincing correlation between observable indicators of these processes on the scale of a mountain range. The eastern Himalayas are a unique range to quantify the contribution of tectonics and climate to long-term erosion rates, because uniform and steady tectonics have persisted for several million years, while monsoonal precipitation patterns have varied in space and time. Specifically, the rise of the Shillong plateau, the only orographic barrier in the Himalayan foreland, has reduced the mean annual precipitation downwind in the eastern Bhutan Himalaya at the Miocene-Pliocene transition. Apatite fission-track (AFT) analyses of 45 bedrock samples from an E-W transect along Bhutan indicate faster long-term erosion rates outside of the rain shadow in the west (1.0‐1.8 mm/yr) than inside of it in the east (0.55‐0.85 mm/yr). Furthermore, an AFT vertical profile in the latter segment reveals a deceleration in erosion rates sometime after 5.9 Ma. In this drier segment of Bhutan, there are remnants of a relict landscape formed under a wetter climate that has not yet equilibrated to the present climatic conditions. Uplift and preservation of the paleolandscape are a result of a climate-induced decrease in erosion rates, rather than of an increase in rock uplift rate. This study documents not only a compelling spatial correlation between long-term erosion and precipitation rates, but also a climatically driven erosion-rate change on the scale of the eastern Himalayas, a change that, in turn, likely influences that region’s recent tectonic evolution.

Segmentation of megathrust rupture zones from fore-arc deformation patterns over hundreds to millions of years, Arauco peninsula, Chile

(1) This work explores the control of fore-arc structure on segmentation of megathrust earthquake ruptures using coastal geomorphic markers. The Arauco-Nahuelbuta region at the south-central Chile margin constitutes an anomalous fore-arc sector in terms of topography, geology, and exhumation, located within the overlap between the Concepcion and Valdivia megathrust segments. This boundary, however, is only based on � 500 years of historical records. We integrate deformed marine terraces dated by cosmogenic nuclides, syntectonic sediments, published fission track data, seismic reflection profiles, and microseismicity to analyze this earthquake boundary over 10 2 -10 6 years. Rapid exhumation of Nahuelbutas dome-like core started at 4 ± 1.2 Ma, coeval with inversion of the adjacent Arauco basin resulting in emergence of the Arauco peninsula. Here, similarities between topography, spatiotemporal trends in fission track ages, Pliocene-Pleistocene growth strata, and folded marine terraces suggest that margin-parallel shortening has dominated since Pliocene time. This shortening likely results from translation of a fore-arc sliver or microplate, decoupled from South America by an intra-arc strike-slip fault. Microplate collision against a buttress leads to localized uplift at Arauco accrued by deep-seated reverse faults, as well as incipient oroclinal bending. The extent of the Valdivia segment, which ruptured last in 1960 with an Mw 9.5 event, equals the inferred microplate. We propose that mechanical homogeneity of the fore-arc microplate delimits the Valdivia segment and that a marked discontinuity in the continental basement at Arauco acts as an inhomogeneous barrier controlling nucleation and propagation of 1960-type ruptures. As microplate-related deformation occurs since the Pliocene, we propose that this earthquake boundary and the extent of the Valdivia segment are spatially stable seismotectonic features at million year scale.

From tectonically to erosionally controlled development of the Himalayan orogen

Whether variations in the spatial distribution of erosion influ- ence the location, style, and magnitude of deformation within the Himalayan orogen is a matter of debate. We report new 40 Ar/ 39 Ar white mica and apatite fission-track (AFT) ages that measure the vertical component of exhumation rates along an ;120-km-wide NE-SW transect spanning the greater Sutlej region of northwest India. The 40 Ar/ 39 Ar data indicate that first the High Himalayan Crystalline units cooled below their closing temperature during the early to middle Miocene. Subsequently, Lesser Himalayan Crys- talline nappes cooled rapidly, indicating southward propagation of the orogen during late Miocene to Pliocene time. The AFT data, in contrast, imply synchronous exhumation of a NE-SW-oriented ;80 3 40 km region spanning both crystalline nappes during the Pliocene-Quaternary. The locus of pronounced exhumation de- fined by the AFT data correlates with a region of high precipita- tion, discharge, and sediment flux rates during the Holocene. This correlation suggests that although tectonic processes exerted the dominant control on the denudation pattern before and until the middle Miocene; erosion may have been the most important factor since the Pliocene.

Analysis of spatial and temporal extreme monsoonal rainfall over South Asia using complex networks

We present a detailed analysis of summer monsoon rainfall over the Indian peninsular using nonlinear spatial correlations. This analysis is carried out employing the tools of complex networks and a measure of nonlinear correlation for point processes such as rainfall, called event synchronization. This study provides valuable insights into the spatial organization, scales, and structure of the 90th and 94th percentile rainfall events during the Indian summer monsoon (June–September). We furthermore analyse the influence of different critical synoptic atmospheric systems and the impact of the steep Himalayan topography on rainfall patterns. The presented method not only helps us in visualising the structure of the extreme-event rainfall fields, but also identifies the water vapor pathways and decadal-scale moisture sinks over the region. Furthermore a simple scheme based on complex networks is presented to decipher the spatial intricacies and temporal evolution of monsoonal rainfall patterns over the last 6 decades.

Coastal deformation and great subduction earthquakes, Isla Santa María, Chile (37°S)

Isla Santa Maria at the active margin of south-central Chile is the result of earthquake-related uplift and deformation in the forearc since at least late Pleistocene time. Field mapping, dating of key depositional horizons, and analysis of seismic-refl ection profi les reveal ongoing deformation in this sector of the Chilean forearc. The 30 km 2 island is located ~12 km above the interplate seismogenic zone and 75 km landward of the trench. It is situated near the southern termination of the Concepcion earthquake rupture segment, where Charles Darwin measured 3 m of coseismic uplift during a M > 8 megathrust earthquake in 1835. Permanent postearthquake deformation from this earthquake and an earlier event in 1751 is registered by emerged, landward-tilted abrasion surfaces. Uplift at ~2 m/k.y. and tilting at ~0.025°/k.y. of the island have been fairly constant throughout the late Quaternary and have resulted in emergence of the island above sea level ~31 k.y. ago. The island is composed of a late Pleistocene upper, tilted surface with two asymmetric tilt domains, and Holocene lowlands characterized by uplifted and tilted strandlines. Industry offshore seismic-refl ection profi les covering an area of ~1800 km 2 and crustal seismicity reveal active reversefault cored anticlines surrounding Isla Santa Maria; the principal fault apparently roots in the plate-interface thrust. These reverse faults in the upper plate result from inversion of late Cretaceous to early Pliocene normal faults and rift structure of the Arauco forearc basin. Positive inversion of these inherited structures started between 3.6 and 2.5 Ma and resulted in continuous shortening rates of ~0.8 mm/yr. The seismic-refl ection profi les show that the asymmetric tilt domains and progressive syntectonic sedimentation are linked to the position of the island in the forelimbs of two converging anticlines, whereas their backlimbs have been removed by cliff retreat. The 2 m uplift contour of the 1835 earthquake is parallel to the strike of active faults and antiforms in the Arauco-Concepcion region. The close relation among the asymmetric uplift and tilt of the island, modern deformation patterns, and reverse faults rooted in the plate interface suggests that slip on the plate interface thrust infl uences, localizes, and segments surface deformation during large interplate earthquakes. Furthermore, the link between positive inversion of pre-existing structures, uplift, and tilt patterns in the forearc emphasizes the importance of inherited structural fabrics in guiding plate-boundary deformation.

Dome formation and extension in the Tethyan Himalaya, Leo Pargil, northwest India

Metamorphic dome complexes occur within the internal structures of the northern Himalaya and southern Tibet. Their origin, deformation, and fault displacement patterns are poorly constrained. We report new fi eld mapping, structural data, and cooling ages from the western fl ank of the Leo Pargil dome in the northwestern Himalaya in an attempt to characterize its post‐middle Miocene structural development. The western fl ank of the dome is characterized by shallow, west-dipping pervasive foliation and WNW-ESE mineral lineation. Shear-sense indicators demonstrate that it is affected by east-west normal faulting that facilitated exhumation of highgrade metamorphic rocks in a contractional setting. Sustained top-to-northwest normal faulting during exhumation is observed in a progressive transition from ductile to brittle deformation. Garnet and kyanite indicate that the Leo Pargil dome was exhumed from the mid-crust. 40 Ar/ 39 Ar mica and apatite fi ssion track (AFT) ages constrain cooling and exhumation pathways from 350 to 60 °C and suggest that the dome cooled in three stages since the middle Miocene. 40 Ar/ 39 Ar white mica ages of 16‐14 Ma suggest a fi rst phase of rapid cooling and provide minimum estimates for the onset of dome exhumation. AFT ages between 10 and 8 Ma suggest that ductile fault displacement had ceased by then, and AFT track-length data from high-elevation samples indicate that the rate of cooling had decreased signifi cantly. We interpret this to indicate decreased fault displacement along the Leo Pargil shear zone and possibly a transition to the Kaurik-Chango normal fault system between 10 and 6 Ma. AFT ages from lower elevations indicate accelerated cooling since the Pliocene that cannot be related to pure fault displacement, and therefore may refl ect more pronounced regionally distributed and erosion-driven exhumation.

Holocene monsoonal dynamics and fluvial terrace formation in the northwest Himalaya, India

Aluminum-26 and beryllium-10 surface exposure dating on cut-and-fill river-terrace surfaces from the lower Sutlej Valley (northwest Himalaya) documents the close link between Indian Summer Monsoon (ISM) oscillations and intervals of enhanced fluvial incision. During the early Holocene ISM optimum, precipitation was enhanced and reached far into the internal parts of the orogen. The amplified sediment flux from these usually dry but glaciated areas caused alluviation of downstream valleys up to 120 m above present grade at ca. 9.9 k.y. B.P. Terrace formation (i.e., incision) in the coarse deposits occurred during century-long weak ISM phases that resulted in reduced moisture availability and most likely in lower sediment flux. Here, we suggest that the lower sediment flux during weak ISM phases allowed rivers to incise episodically into the alluvial fill.

Dominance of tectonics over climate in himalayan denudation

Landscape denudation in actively deforming mountain ranges is controlled by a combination of rock uplift and surface runoff induced by precipitation. Whereas the relative contribution of these factors is important to our understanding of the evolution of orogenic topography, no consensus currently exists concerning their respective infl uences. To address this question, denudation rates at centennial to millennial time scales were deduced from 10Be concentrations in detrital sediments derived from 30 small basins (10-600 km2) in an ~200-km-wide region in central Nepal. Along a northward, strike-perpendicular transect, average denudation rates sharply increase from <0.5 mm/yr in the Lesser Himalayas to ~1 mm/yr when crossing the Physiographic Transition, and then accelerate to 2-3 mm/yr on the southern flank of the high peaks in the Greater Himalayas. Despite a more than five-fold increase in denudation rate between the southern and northern parts of this transect, the corresponding areas display similar precipitation rates. The primary parameter that presents a signifi cant co-variation with denudation is the long-term rockuplift rate that is interpreted to result from the ramp-fl at transition along the Main Himalayan Thrust. We propose that, in this rapidly uplifting mountain range, landscapes adjust quickly to changing climatic conditions, such that denudation is mainly limited by the rate at which material is pushed upward by tectonic processes and made available for removal by surface processes. In this particular context, variations in precipitation appear to have only a second-order role in modulating the denudation signal that is primarily set by the background rock-uplift rate.

Multiple landslide clusters record quaternary climate changes in the northwestern Argentine andes

The chronology of multiple landslide deposits and related lake sediments in the semi-arid eastern Argentine Cordillera suggests that major mass movements cluster in two time periods during the Quaternary, i.e. between 40 and 25 and after 5 14 C kyr BP. These clusters may correspond to the Minchin (maximum at around 28^27 14 C kyr BP) and Titicaca wet periods (after 3.9 14 C kyr BP). The more humid conditions apparently caused enhanced landsliding in this environment. In contrast, no landslide-related damming and associated lake sediments occurred during the Coipasa (11.5^10 14 C yr BP) and Tauca wet periods (14.5^11 14 C yr BP). The two clusters at 40^25 and after 5 14 C kyr BP may correspond to periods where the El Nin ‹ o^Southern Oscillation (ENSO) and Tropical Atlantic Sea Surface Temperature Variability (TAV) were active. This, however, was not the case during the Coipasa and Tauca wet periods. Lake-balance modelling of a landslide-dammed lake suggests a 10^15% increase in precipitation and a 3^ 4‡C decrease in temperature at V30 14 C kyr BP as compared to the present. In addition, time-series analysis reveals a strong ENSO and TAV during that time. The landslide clusters in northwestern Argentina are therefore best explained by periods of more humid and more variable climates. < 2003 Elsevier Science B.V. All rights reserved.