An investigation of the magnetic fabrics and the paleomagnetism of the Ghost Rocks Formation, Kodiak Islands, Alaska

Recent tectonic models based on the hypothesized existence of the Resurrection plate between the Kula and Farallon plates have questioned the location(s) of trench-ridge-trench (TRT) triple junction(s) along the Northern Cordilleran margin during Paleocene to Eocene time. The Paleocene Ghost Rocks Formation, located in the Kodiak islands, Alaska (latitude ~57°N), consists of pillow lavas and hypabyssal sills interbedded with turbidites, and is interpreted to have formed in a trench slope or slope basin during the passage of a TRT triple junction. A previous paleomagnetic study (Plumley et al., 1983) on the volcanic flows of the Ghost Rocks Formation suggests these rocks formed at latitudes significantly south of their present-day locations, at a latitude of ~41ºN during Paleocene time. Tectonic models, based on the assumed existence of the Resurrection plate, reject the conclusions of Plumley et al.\u27s paleomagnetic study, and instead suggest that these rocks have been remagnetized. Our study revisited the Ghost Rocks Formation in an effort to resolve the disputed location of this TRT triple junction. The focus of this thesis is on magnetic fabrics and paleomagnetism of two localities within the Ghost Rocks Formation: Jap Bay and Alitak Bay. More than 300 oriented core samples were obtained primarily from sedimentary rocks in two coherent sections of Jap Bay, Unit A and Unit B; and over 500 oriented core samples were taken from the turbidites and volcanic flows of Alitak Bay. The anisotropy of magnetic susceptibility was used to study the magnetic fabrics of these rocks. The majority of the sedimentary rocks showed magnetic fabrics typical of weakly deformed sediments with magnetic foliations oriented parallel to bedding, and cryptic magnetic lineations oriented perpendicular to the shortening direction. However, sediments from Unit B of Jap Bay showed a large portion of magnetic lineations oriented approximately parallel to the direction of slip on bedding parallel faults, becoming more pronounced in fold hinges. Magnetic lineations oriented parallel to the slip direction are not typical of weakly deformed sediments. The volcanic samples from Alitak Bay contained magnetic fabrics that can qualitatively be defined as foliated, lineated, and scattered. The paleomagnetism of the majority of the sedimentary rocks were magnetically unstable. Those from Unit A however, exhibited good magnetic behavior but the high unblocking temperature components fail the fold test. The magnetic behavior of the volcanic flows from Alitak Bay was good. Results from a series of fold tests using various structural corrections yield inconclusive results. However, rotation tests show positive results. The rotation corrected directions from Alitak Bay and in-situ directions of Kiliuda Bay from Plumley et al. (1983) pass a regional fold test yielding a mean paleomagnetic direction for the Ghost Rocks Formation corresponding to a latitude of ~41º. However, the somewhat arbitrary nature of these rotation corrections and failed conglomerate tests suggest that remagnetization of the rocks at Alitak Bay is also a likely possibility

Lithologic Controls on Focused Erosion and Intraplate Earthquakes in the Eastern Tennessee Seismic Zone

We present a new geomorphic model for the intraplate eastern Tennessee seismic zone (ETSZ). Previous studies document that the Upper Tennessee drainage basin is in a transient state of adjustment to ~150 m of base level fall that occurred in the Late Miocene. Using quantitative geomorphology, we demonstrate that base level fall resulted in the erosion of ~3,500 km3 of highly erodibility rock in an ~70 km wide by ~350‐km‐long corridor in the Paleozoic fold‐thrust belt above the ETSZ. Models of modern incision rates show a NE‐SW trending swath of elevated erosion ~30 km southeast of the center of the ETSZ. Stress modeling shows that lithologically focused erosion has affected fault clamping stress on preexisting, favorably oriented faults. We argue that the lithologically controlled transient erosional response to base level fall in the Upper Tennessee basin has given rise to and is sustaining earthquake activity in the ETSZ

ChiProfiler: ChiProfiler version 1.0.0

This is the first release of ChiProfiler. It will be updated periodically. ChiProfiler is series of MATLAB functions that utilizes TopoToolbox to analyze river longitudinal profiles ChiProfiler is a series of Matlab functions that utilize TopoToolbox version 2 (Schwanghart and Scherler, 2014) to conduct river profile analysis using the chi or integral method (Perron and Royden, 2013). ChiProfiler also allows users to generate maps of river network metrics, such as the normalized steepness index (ksn) and the integral quantity chi (Wobus et al., 2006; Willett et al., 2014). ChiProfiler was developed by Sean Gallen and is shortly described and applied in Gallen and Wegmann (in press). Please cite Gallen and Wegmann (in press) if you use these codes for scientific research. All users need to do is download TopoToolbox (https://topotoolbox.wordpress.com/) and it is easy to run ChiProfiler in Matlab. See ChiProfiler on GitHub for more information. If you have any questions or comments please contact the author: Sean F. Gallen sean.gallen[at]erdw.ethz.ch References Gallen, S.F., Wegmann, K.W.: River profile response to normal fault growth and linkage: An example from the Hellenic forearc of south-central Crete, Greece, Earth Surf. Dynam., in press, http://www.earth-surf-dynam-discuss.net/esurf-2016-52/. Perron, J.T., Royden, L.: An integral approach to bedrock river profile analysis, Earth Surf. Processes Landforms, 38, 570-576, 2013. http://dx.doi.org/10.1002/esp.3302 Schwanghart, W., Scherler, D.: Short Communication: TopoToolbox 2 – MATLAB-based software for topographic analysis and modeling in Earth surface sciences, Earth Surf. Dynam., 2, 1-7, 2014. http://dx.doi.org/10.5194/esurf-2-1-2014 Willett, S.D., McCoy, S.W., Perron, J.T., Goren, L., Chen, C.-Y.: Dynamic Reorganization of River Basins, Science, 343, 2014. http://dx.doi.org/10.1126/science.1248765 Wobus, C., Whipple, K.X., Kirby, E., Snyder, N., Johnson, J., Spyropolou, K., Crosby, B., Sheehan, D.: Tectonics from topography: Procedures, promise, and pitfalls, Geological Society of America Special Papers, 398, 55-74, 2006. http://dx.doi.org/10.1130/2006.2398(04

Modelling the systematics of cosmogenic nuclide signals in fluvial sediments following extreme events

The effect of punctuated mass-wasting events on longer-term erosion rates is not fully understood, and yet it is key to quantifying sediment generation, source-to-sink dynamics, and landscape evolution in active orogens. The measurement of terrestrial cosmogenic nuclides (TCNs) in river sediments is a common method for determining basin-averaged erosion rates over centennial to millennial timescales and is often used to compare erosional processes between catchments. However, these comparisons often overlook the role of landsliding rates and their spatial distribution in the measurement and potential variability of TCN signals. While it is widely accepted that basin-scale perturbations should temporarily dilute TCN concentrations as landsliding mobilizes new, low-concentration material, the impact of the catastrophic release of hillslope sediment caused by a single event on TCN signatures has not yet been systematically investigated. In this modelling study, we use a catchment in central Nepal to build upon previous modelling efforts to consider how TCNs are recorded in landscapes with varying erosion rates, landsliding rates, and spatial distribution of landslides. We then use the 25 April 2015 M-w 7.8 Gorkha earthquake, Nepal, as a case study to investigate how perturbations like earthquakes are recorded in TCN time series and transferred to and ultimately archived in the sedimentary record. We find that the likelihood of a perturbation being measured by TCN dilution is based on a multitude of factors, including background erosion rates, long-term landsliding rates, and the connectivity of newly released material to the fluvial system. Especially in landscapes like the central Himalaya with high background erosion and landsliding rates, changes in detrital TCN concentrations are not a reliable indicator of an upstream perturbation, nor should we expect a clear dilution signal following a major event. Our modelling results emphasize that TCN dilution is not a universal characteristic of high-magnitude landslide-triggering events.ISSN:0197-9337ISSN:1096-983

Erosion and weathering in carbonate regions reveal climatic and tectonic drivers of carbonate landscape evolution

Carbonate rocks are highly reactive and can have higher ratios of chemical weathering to total denudation relative to most other rock types. Their chemical reactivity affects the first-order morphology of carbonate-dominated landscapes and their climate sensitivity to weathering. However, there have been few efforts to quantify the partitioning of denudation into mechanical erosion and chemical weathering in carbonate landscapes such that their sensitivity to changing climatic and tectonic conditions remains elusive. Here, we compile bedrock and catchment-averaged cosmogenic calcite-36Cl denudation rates and compare them to weathering rates derived from stream water chemistry from the same regions. Local bedrock denudation and weathering rates are comparable, ∼20-40mmka-1, whereas catchment-averaged denudation rates are ∼2.7 times higher. The discrepancy between bedrock and catchment-averaged denudation is 5 times lower compared to silicate-rich rocks, illustrating that elevated weathering rates make denudation more spatially uniform in carbonate-dominated landscapes. Catchment-averaged denudation rates correlate well with topographic relief and hillslope gradients, and moderate correlations with runoff can be explained by concurrent increases in weathering rates. Comparing denudation rates with weathering rates shows that mechanical erosion processes contribute ∼50% of denudation in southern France and ∼70% in Greece and Israel. Our results indicate that the partitioning between largely slope-independent chemical weathering and slope-dependent mechanical erosion varies based on climate and tectonics and impacts the landscape morphology. This leads us to propose a conceptual model whereby in humid, slowly uplifting regions, carbonates are associated with low-lying, flat topography because slope-independent chemical weathering dominates denudation. In contrast, in arid climates with rapid rock uplift rates, carbonate rocks form steep mountains that facilitate rapid, slope-dependent mechanical erosion required to compensate for inefficient chemical weathering and runoff loss to groundwater systems. This result suggests that carbonates represent an end member for interactions between climate, tectonics, and lithology.ISSN:2196-632XISSN:2196-631

River incision on volcanoes: Case study of La Réunion, Mauritius and Rodrigues islands, Indian Ocean

International audienc

River incision on volcanoes: Case study of La Réunion, Mauritius and Rodrigues islands, Indian Ocean

International audienc

River incision on volcanoes: Case study of La Réunion, Mauritius and Rodrigues islands, Indian Ocean

International audienc

Pleistocene terrace formation, Quaternary rock uplift rates and geodynamics of the Hellenic Subduction Zone revealed from dating of paleoshorelines on Crete, Greece

Quaternary paleoshorelines are common landforms on the island of Crete, a forearc high above the Hellenic Subduction Zone. These geomorphic markers are useful on Crete and elsewhere in determining coastal uplift rates, the identification of active geologic structures, and to constrain geodynamic models and seismic hazards. Controversy exists in the literature regarding the formation mechanisms and age of late Pleistocene paleoshorelines on Crete that has led to competing models of the uplift history, tectonic evolution, and seismic hazards of the Hellenic forearc. We present new mapping and results from luminescence and radiocarbon geochronology of paleoshoreline deposits that constrain the spatial and temporal pattern of rock uplift around the Cretan coastline. Existing and new radiocarbon data are variable and show no obvious age-elevation trends within individual terrace sequences. By contrast, nearly all luminescence ages, some from shorelines dated with radiocarbon, show positive age-elevation trends and range from 60–220 ka suggesting that all dated paleoshorelines are beyond the limits of radiocarbon. We propose that the inconsistencies between the different geochronological methods are the result of secondary contamination of young carbonate, possibly from meteoric waters, that bias radiocarbon in Cretan Pleistocene marine fossils. Most luminescence ages closely correlate with the timing of mid-to-late Pleistocene relative sea level highstands, consistent with stratigraphic observations. Calculated coastal uplift rates using a Monte-Carlo error analysis range from ∼0–1.2 mm/yr; the lowest uplift rates are found along the northern and eastern coasts of the island, while the most rapid are focused along the southern and western coasts where active normal faults are observed offsetting paleoshoreline sequences. Based on this new data, we favor a tectonic model where slip along upper crustal normal faults acts to locally augment a steady regional signal of uplift along the south and west coast, interpreted to result from the deep underplating of rock at the base of the subduction wedge beneath Crete. Arcward of the contact between the upper plate Moho and the top of the subducting slab, crustal thinning will occur in the orogenic wedge resulting in subsidence along the north coast of Crete

Timing of exotic, far-traveled boulder emplacement and paleo-outburst flooding in the central Himalayas

Large boulders, ca. 10 m in diameter or more, commonly linger in Himalayan river channels. In many cases, their lithology is consistent with source areas located more than 10 km upstream, suggesting long transport distances. The mechanisms and timing of “exotic” boulder emplacement are poorly constrained, but their presence hints at processes that are relevant for landscape evolution and geohazard assessments in mountainous regions. We surveyed river reaches of the Trishuli and Sunkoshi, two trans-Himalayan rivers in central Nepal, to improve our understanding of the processes responsible for exotic boulder transport and the timing of emplacement. Boulder size and channel hydraulic geometry were used to constrain paleo-flood discharge assuming turbulent, Newtonian fluid flow conditions, and boulder exposure ages were determined using cosmogenic nuclide exposure dating. Modeled discharges required for boulder transport of ca. 103 to 105 m3 s−1 exceed typical monsoonal floods in these river reaches. Exposure ages range between ca. 1.5 and 13.5 ka with a clustering of ages around 4.5 and 5.5 ka in both studied valleys. This later period is coeval with a broader weakening of the Indian summer monsoon and glacial retreat after the Early Holocene Climatic Optimum (EHCO), suggesting glacial lake outburst floods (GLOFs) as a possible cause for boulder transport. We, therefore, propose that exceptional outburst events in the central Himalayan range could be modulated by climate and occur in the wake of transitions to drier climates leading to glacier retreat rather than during wetter periods. Furthermore, the old ages and prolonged preservation of these large boulders in or near the active channels shows that these infrequent events have long-lasting consequences on valley bottoms and channel morphology. Overall, this study sheds light on the possible coupling between large and infrequent events and bedrock incision patterns in Himalayan rivers with broader implications for landscape evolution