Surface uplift, fluvial incision, and geodynamics of plateau evolution, from the western margin of the Central Andean plateau

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2008.Includes bibliographical references.The Colca-Majes and Cotahuasi-Ocona rivers in southwest Peru that cut through the western margin of the Andean plateau en route to the Pacific Ocean incised canyons over 3 km deep in response to late Cenozoic surface uplift. This latest uplift represents a fundamental shift in the style and magnitude of surface deformation that had been ongoing since at least late Cretaceous time, but only relatively recently created what represents the only major barrier to atmospheric circulation in the Southern Hemisphere. Studying canyon incision history as a proxy for surface uplift offers a promising route to understanding how climate and tectonics have interacted throughout the evolution of the Central Andean plateau. In this thesis, a combination of bedrock low-temperature thermochronology (apatite and zircon (UTh)/He techniques), 40Ar/39Ar dating of valley-filling volcanic flows, and three-dimensional thermal modeling using a modified version of Pecube were applied to investigate the incision history of the rivers. Results suggested between 2.6 and 3.0 km of incision occurred in the deepest reaches of the canyons starting at ca. 10 to 11 Ma and ending between 2.3 and 3.5 Ma. The onset of surface uplift that is likely to have driven incision probably did not precede incision by more than one million years, although incision may have continued long after surface uplift ceased. The magnitude and timing of uplift that we infer is in broad agreement with uplift documented along the western margin in northern Chile, in the interior of the Andean plateau, and the eastern margin. Additional work on the style of structural accommodation of this uplift provide important context for interpreting what potential geodynamic mechanisms may have been responsible for this latest tectonic activity. Structural mapping revealed an early history of predominantly reverse fault activity that probably generated the first significant topography in the Central Andes. This compressional period was succeeded by a phase of longwavelenth warping deformation and normal kinematic movement on high-angle faults.(cont.) This later activity appears to have continued through the latest phase of uplift documented through river incision history, hence was likely responsible for accommodating uplift. Given the broadly synchronous timing of late Cenozoic uplift across the Andean plateau and the style of structural accommodation documented on the western margin, two possible geodynamic mechanisms appear most feasible for generating this latest phase of uplift: lithospheric delamination, and redistribution of ductile material through mid- to lower-crustal flow. Our initial studies of coastal uplift suggest that each is likely to have played a role, although additional work is required to understand how important each may have been in generating uplift.by Taylor F. Schildgen.Ph.D

Out-of-sequence faulting of the Jwalamukhi Thrust, India

The southernmost thrust of the Himalayan orogenic wedge that separates the foreland from the orogen, the Main Frontal Thrust, is thought to accommodate most of the ongoing crustal shortening in the Sub-Himalaya. Steepened longitudinal river profile segments, terrace offsets, and back-tilted fluvial terraces within the Kangra reentrant of the NW Sub-Himalaya suggest Holocene activity of the Jwalamukhi Thrust (JMT) and other thrust faults that may be associated with strain partitioning along the toe of the Himalayan wedge. To assess the shortening accommodated by the JMT, we combine morphometric terrain analyses with in situ 10Be-based surface-exposure dating of the deformed terraces. Incision into upper Pleistocene sediments within the Kangra Basin created two late Pleistocene terrace levels (T1 and T2). Subsequent early Holocene aggradation shortly before ~10 ka was followed by episodic reincision, which created four cut-and-fill terrace levels, the oldest of which (T3) was formed at 10.1 ± 0.9 ka. A vertical offset of 44 ± 5 m of terrace T3 across the JMT indicates a shortening rate of 5.6 ± 0.8 to 7.5 ± 1.1 mm a−1 over the last ~10 ka. This result suggests that thrusting along the JMT accommodates 40–60% of the total Sub-Himalayan shortening in the Kangra reentrant over the Holocene. We speculate that this out-of-sequence shortening may have been triggered or at least enhanced by late Pleistocene and Holocene erosion of sediments from the Kangra Basin

Local high relief at the southern margin of the Andean plateau by 9 Ma: evidence from ignimbritic valley fills and river incision

A valley-filling ignimbrite re-exposed through subsequent river incision at the southern margin of the Andean (Puna) plateau preserves pristine geological evidence of pre-late Miocene paleotopography in the northwestern Argentine Andes. Our new 40Ar/39Ar dating of the Las Papas ignimbrites yields a plateau age of 9.24 ± 0.03 Ma, indicating valley relief and orographic-barrier conditions comparable to the present-day. A later infill of Plio- Pleistocene coarse conglomerates has been linked to wetter conditions, but resulted in no additional net incision of the Las Papas valley, considering that the base of the ignimbrite remains unexposed in the valley bottom. Our observations indicate that at least 550 m of local plateau margin relief (and likely >2 km) existed by 9 Ma at the southern Puna margin, which likely increased the efficiency of the orographic barrier to rainfall along the eastern and southeastern flanks of the Puna and caused aridity in the plateau interior.Fil: Montero Lopez, Maria Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Salta. Instituto de Bio y Geociencias del Noroeste Argentino; ArgentinaFil: Strecker, Manfred R.. Universitat Potsdam; AlemaniaFil: Schildgen, Taylor F.. Universitat Potsdam; AlemaniaFil: Hongn, Fernando Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Salta. Instituto de Bio y Geociencias del Noroeste Argentino; ArgentinaFil: Guzman, Silvina Raquel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Salta. Instituto de Bio y Geociencias del Noroeste Argentino; ArgentinaFil: Bookhagen, Bodo. Universitat Potsdam; AlemaniaFil: Sudo, Masafumi. Universitat Potsdam; Alemani

Glacial isostatic uplift of the European Alps

Following the last glacial maximum (LGM), the demise of continental ice sheets induced crustal rebound in tectonically stable regions of North America and Scandinavia that is still ongoing. Unlike the ice sheets, the Alpine ice cap developed in an orogen where the measured uplift is potentially attributed to tectonic shortening, lithospheric delamination and unloading due to deglaciation and erosion. Here we show that ∼90% of the geodetically measured rock uplift in the Alps can be explained by the Earth's viscoelastic response to LGM deglaciation. We modelled rock uplift by reconstructing the Alpine ice cap, while accounting for postglacial erosion, sediment deposition and spatial variations in lithospheric rigidity. Clusters of excessive uplift in the Rhône Valley and in the Eastern Alps delineate regions potentially affected by mantle processes, crustal heterogeneity and active tectonics. Our study shows that even small LGM ice caps can dominate present-day rock uplift in tectonically active regions

The growth of a mountain belt forced by base-level fall: Tectonics and surface processes during the evolution of the Alborz Mountains, N Iran

The idea that climatically modulated erosion may impact orogenic processes has challenged geoscientists for decades. Although modeling studies and physical calculations have provided a solid theoretical basis supporting this interaction, to date, field-based work has produced inconclusive results. The central-western Alborz Mountains in the northern sectors of the Arabia-Eurasia collision zone constitute a promising area to explore these potential feedbacks. This region is characterized by asymmetric precipitation superimposed on an orogen with a history of spatiotemporal changes in exhumation rates, deformation patterns, and prolonged, km-scale base-level changes. Our analysis suggests that despite the existence of a strong climatic gradient at least since 17.5 Ma, the early orogenic evolution (from ~ 36 to 9-6 Ma) was characterized by decoupled orographic precipitation and tectonics. In particular, faster exhumation and sedimentation along the more arid southern orogenic flank point to a north-directed accretionary flux and underthrusting of Central Iran. Conversely, from ~ 6 to 3 Ma, erosion rates along the northern orogenic flank became higher than those in the south, where they dropped to minimum values. This change occurred during a ~3-Myr-long, km-scale base-level lowering event in the Caspian Sea. We speculate that mass redistribution processes along the northern flank of the Alborz and presumably across all mountain belts adjacent to the South Caspian Basin and more stable areas of the Eurasian plate increased the sediment load in the basin and ultimately led to the underthusting of the Caspian Basin beneath the Alborz Mountains. This underthrusting in turn triggered a new phase of northward orogenic expansion, transformed the wetter northern flank into a new pro-wedge, and led to the establishment of apparent steady-state conditions along the northern orogenic flank (i.e., rock uplift equal to erosion rates). Conversely, the southern mountain front became the retro-wedge and experienced limited tectonic activity. These observations overall raise the possibility that mass-distribution processes during a pronounced erosion phase driven by base-level changes may have contributed to the inferred regional plate-tectonic reorganization of the northern Arabia-Eurasia collision during the last ~ 5 Ma.Keywords: rock uplift, knickpoints, base-level fall, surface processes, orogenic processes, erosio

Kinematic behavior of southern Alaska constrained by westward decreasing postglacial slip rates on the Denali Fault, Alaska

Long-term slip rates for the Denali Fault in southern Alaska are derived using ^(10)Be cosmogenic radionuclide (CRN) dating of offset glacial moraines at two sites. Correction of ^(10)Be CRN model ages for the effect of snow shielding uses historical, regional snow cover data scaled to the site altitudes. To integrate the time variation of snow cover, we included the relative changes in effective wetness over the last 11 ka, derived from lake-level records and δ^(18)O variations from Alaskan lakes. The moraine CRN model ages are normally distributed around an average of 12.1 ± 1.0 ka (n = 22, ± 1σ). The slip rate decreases westward from ~13 mm/a at 144°49′W to about 7 mm/a at 149°26′W. The data are consistent with a kinematic model in which southern Alaska translates northwestward at a rate of ~14 mm/a relative to a stable northern Alaska with no rotation. This suggests progressive slip partitioning between the Denali Fault and the active fold and thrust belt at the northern front of the Alaska range, with convergence rates increasing westward from ~4 mm/a to 11 mm/a between ~149°W and 145°W. As the two moraines sampled for this study were emplaced synchronously, our suggestion of a westward decrease in the slip rate of the Denali Fault relies largely upon the measured offsets at both sites, regardless of any potential systematic uncertainty in the CRN model ages

Measurement of the cross-section and charge asymmetry of WW bosons produced in proton-proton collisions at s=8\sqrt{s}=8 TeV with the ATLAS detector

This paper presents measurements of the $W^+ \rightarrow \mu^+\nu$ and $W^- \rightarrow \mu^-\nu$ cross-sections and the associated charge asymmetry as a function of the absolute pseudorapidity of the decay muon. The data were collected in proton--proton collisions at a centre-of-mass energy of 8 TeV with the ATLAS experiment at the LHC and correspond to a total integrated luminosity of 20.2~\mbox{fb^{-1}}. The precision of the cross-section measurements varies between 0.8% to 1.5% as a function of the pseudorapidity, excluding the 1.9% uncertainty on the integrated luminosity. The charge asymmetry is measured with an uncertainty between 0.002 and 0.003. The results are compared with predictions based on next-to-next-to-leading-order calculations with various parton distribution functions and have the sensitivity to discriminate between them.Comment: 38 pages in total, author list starting page 22, 5 figures, 4 tables, submitted to EPJC. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2017-13