Ari Matmon

Temporally and spatially uniform rates of erosion in the southern Appalachian Great Smoky Mountains

We measured 1 0 Be in fluvial sediment samples (n = 27) from eight Great Smoky Mountain drainages (1-330 km 2 ). Results suggest spatially homogeneous sediment generation (on the 10 4 -10 5 yr time scale and >100 km 2 spatial scale) at 73 ′ 11 t km - 2 yr - 1 , equivalent to 27 ′ 4 m/m.y. of bedrock erosion. This rate is consistent with rates derived from fission-track, long-term sediment budget, and sediment yield data, all of which indicate that the Great Smoky Mountains and the southern Appalachians eroded during the Mesozoic and Cenozoic at ∼30 m/m.y. In contrast, unroofing rates during the Paleozoic orogenic events that formed the Appalachian Mountains were higher (≥10 2 m/m.y.). Erosion rates decreased after termination of tectonically driven uplift, enabling the survival of this ancient mountain belt with its deep crustal root as an isostatically maintained feature in the contemporary landscape.

Desert pavement-coated surfaces in extreme deserts present the longest-lived landforms on Earth

All exposed rocks on Earth’s surface experience erosion; the fastest rates are documented in rapidly uplifted monsoonal mountain ranges, and the slowest occur in extreme cold or warm deserts—millennial submeterscale erosion may be approached only in the latter. The oldest previously reported exposure ages are from boulders and clasts of resistant lithologies lying at the surface, and the slowest reported erosion rates are derived from bedrock outcrops or boulders that erode more slowly than their surroundings; thus, these oldest reported ages and slowest erosion rates relate to outstanding features in the landscape, while the surrounding landscape may erode faster and be younger. We present erosion rate and exposure age data from the Paran Plains, a typical environment in the Near East where vast abandoned alluvial surfaces (10 2 –10 4 km 2 ) are covered by well-developed desert pavements. These surfaces may experience erosion rates that are slower than those documented elsewhere on our planet and can retain their original geometry for more than 2 m.y. Major factors that reduce erosion converge in these regions: extreme hyperaridity, tectonic stability, fl at and horizontal surfaces (i.e., no relief), and effective surface armoring by a clast mosaic of highly resistant lithology. The 10 Be concentrations in amalgamated desert pavement chert clasts collected from abandoned alluvial surfaces in the southern Negev, Israel (representing the Sahara-Arabia Deserts), indicate simple exposure ages of 1.5–1.8 Ma or correspond to maximum erosion rates of 0.25–0.3 m m.y. –1 . The 36

Dating offset fans along the Mojave section of the San Andreas fault using cosmogenic 26Al and 10Be

Analysis of cosmogenic 1 0 Be and 2 6 Al in samples collected from exposed boulders (n = 20) and from buried sediment (n = 3) from offset fans along the San Andreas fault near Little Rock, California, yielded ages, ranging from 16 to 413 ka, which increase with dis-Lance from their source at the mouth of Little Rock Creek. In order to determine the age of the relatively younger fans, the erosion rate of the boulders and the cosmogenic nuclide inheritance from exposure prior to deposition in the fan were established. Cosmogenic nuclide inheritance values that range between 8.5 x 10 3 and 196 × 10 3 atoms 1 0 Be g - 1 quartz were determined by measuring the concentrations and ratios of 1 0 Be and 2 6 Al in boulders (n = 10) and fine sediment (n = 7) at the outlet of the present active stream. Boulder erosion rate, ranging between 17 and 160 mm k.y. - 1 , was estimated by measuring 1 0 Be and 2 6 Al concentrations in nearby bedrock outcrops (n = 8). Since the boulders on the fans represent the most resistant rocks in this environment, we used the lowest rate for the age calculations. Monte Carlo simulations were used to determine ages of 16 ′ 5 and 29 ′ 7 ka for the two younger fan surfaces. Older fans (older than 100 ka) were dated by analyzing 1 0 Be and 2 6 Al concentrations in buried sand samples. The ages of the three oldest fans range between 227 ′ 242 and 413 ′ 185 ka. Although fan age determinations are accompanied by large uncertainties, the results of this study show a clear trend of increasing fan ages with increasing distance from the source near Little Rock Creek and provide a long-term slip rate along this section of the San Andreas fault. Slip rate along the Mojave section of the San Andreas fault for the past 413 k.y. can be determined in several ways. The average slip rate calculated from the individual fan ages is 4.2 ′ 0.9 cm yr - 1 . A linear regression through the data points implies a slip rate of 3.7 ′ 1.0 cm yr - 1 . A most probable slip rate of 3.0 ′ 1.0 cm yr - 1 is determined by using a Χ 2 test. These rates suggest that the average slip along the Mojave section of the San Andreas fault has been relatively constant over this time period. The slip rate along the Mojave section of the San Andreas fault, determined in this study, agrees well with previous slip rate calculations for the Quaternary.

Radiometric dating of the Earlier Stone Age sequence in Excavation I at Wonderwerk Cave, South Africa: preliminary results

We present here the results of 44 paleomagnetic measurements, and single cosmogenic burial and optically stimulated luminescence ages for the Earlier Stone Age deposits from Wonderwerk Cave, Northern Cape, South Africa. The resulting paleomagnetic sequence: N>R>N>R>N constrains the Earlier Stone Age strata in this part of the site to between approximately 0.78-1.96 Ma. A single cosmogenic date of approximately 2.0 Ma from the base of the section offers some corroboration for the paleomagnetic sequence. Preliminary results indicate that the small lithic assemblage from the basal stratum may contain an Oldowan facies. This is overlain by several strata containing Acheulean industries. The preliminary radiometric dates reported here place the onset of the Acheulean at this site to approximately 1.6 Ma, which is roughly contemporaneous with that of East Africa.

Denali fault slip rates and Holocene–late Pleistocene kinematics of central Alaska

The Denali fault is the principal intracontinental strike-slip fault accommodating deformation of interior Alaska associated with the Yakutat plate convergence. We obtained the first quantitative late Pleistocene–Holocene slip rates on the Denali fault system from dating offset geomorphic features. Analysis of cosmogenic 10Be concentrations in boulders ( n = 27) and sediment ( n = 13) collected at seven sites, offset 25–170 m by the Denali and Totschunda faults, gives average ages that range from 2.4 ± 0.3 ka to 17.0 ± 1.8 ka. These offsets and ages yield late Pleistocene– Holocene average slip rates of 9.4 ± 1.6, 12.1 ± 1.7, and 8.4 ± 2.2 mm/yr−1 along the western, central, and eastern Denali fault, respectively, and 6.0 ± 1.2 mm/yr−1 along the Totschunda fault. Our results suggest a westward decrease in the mean Pleistocene– Holocene slip rate. This westward decrease likely results from partitioning of slip from the Denali fault system to thrust faults to the north and west.

CLIMATE IN THE DRY CENTRAL ANDES OVER GEOLOGIC, MILLENNIAL, AND INTERANNUAL TIMESCALES

Abstract Over the last eight years, we have developed several paleoenvironmental records from a broad geographic region spanning the Altiplano in Bolivia (18°S–22°S) and continuing south along the western Andean flank to ca. 26°S. These records include: cosmogenic nuclide concentrations in surface deposits, dated nitrate paleosoils, lake levels, groundwater levels from wetland deposits, and plant macrofossils from urine-encrusted rodent middens. Arid environments are often uniquely sensitive to climate perturbations, and there is evidence of significant changes in precipitation on the western flank of the central Andes and the adjacent Altiplano. In contrast, the Atacama Desert of northern Chile is hyperarid over many millions of years. This uniquely prolonged arid climate requires the isolation of the Atacama from the Amazon Basin, a situation that has existed for more than 10 million years and that resulted from the uplift of the Andes and/or formation of the Altiplano plateau. New evidence from multiple terrestrial cosmogenic nuclides, however, suggests that overall aridity is occasionally punctuated by rare rainfall events that likely originate from the Pacific. East of the hyperarid zone, climate history from multiple proxies reveals alternating wet and dry intervals where changes in precipitation originating from the Atlantic may exceed 50%. An analysis of Pleistocene climate records across the region allows reconstruction of the spatial and temporal components of climate change. These Pleistocene wet events span the modern transition between two modes of interannual precipitation variability, and regional climate history for the Central Andean Pluvial Event (CAPE; ca. 18–8 ka) points toward similar drivers of modern interannual and past millennial-scale climate variability. The north-northeast mode of climate variability is linked to El Niño–Southern Oscillation (ENSO) variability, and the southeast mode is linked to aridity in the Chaco region of Argentina.

Pattern and tempo of great escarpment erosion

Extensive geological, geophysical, structural, and geochronological data suggest that the locations of great escarpments bordering passive margins are exceptionally stable and are probably determined by crustal structure. Together, the data do not support the established paradigm of ongoing, significant, and parallel escarpment retreat. Rather, thermochronologic data and sedimentary sequences in ocean basins suggest that initial, tectonically controlled rift escarpments undergo rapid and significant erosion only during the earliest stages of seafloor spreading. Development of stable passive margin escarpments follows this period of intense erosion. Escarpments increase in sinuosity as embayments retreat more rapidly than interfluves. Measurements of 24 escarpments suggest that sinuosity, and the rate at which it increases, depends upon the location of maximum uplift, the geometry of the preescarpment drainage system, and margin age. All data suggest that the location of passive margin escarpments does not change significantly over time.

Displacement history of a limestone normal fault scarp, northern Israel, from cosmogenic 36Cl

The abundance of cosmogenic 36 Cl, measured in 41 limestone samples from a 9 m high bedrock fault scarp, allows us to construct the 14 kyr fault displacement history of the Nahef East normal fault, northern Israel (300 m above sea level, N33° latitude). The Nahef East fault is one of a series of fault scarps located along the 700 m high Zurim Escarpment, a major geomorphic feature. Samples at the top of the scarp have the highest nuclide concentrations (79 x 10 4 atoms (g rock) -1 ); samples at the base have the lowest (11 x 10 4 atoms (g rock) -1 ), Using chemical data from the samples, Nahef East fault scarp geometry, and surface and subsurface production rates for the 36 Cl-producing reactions, we have constructed a numerical model that calculates 36 Cl accumulation on a scarp through time, given a series of unique displacement scenarios. The resulting model 36 Cl concentrations are compared to those measured in the scarp samples. Faulting histories that result in a good match between measured and modeled 36 Cl abundances show three distinct periods of fault activity during the past 14 kyr with over 6 vertical meters of motion occurring during a 3 kyr time period in the middle Holocene. Smaller amounts of displacement occurred before and after the period of most rapid motion. The episodic behavior of the Nahef East fault indicates that the average displacement rate of this fault system has varied through time.

Amplified erosion above waterfalls and oversteepened bedrock reaches

�� erosion rate at the upstream end of the flow acceleration zone above a waterfall, Fr is the Froude number at this setting, and n ranges between 0.5–1.7. This amplification expression suggests that erosion at the lip could be as much as 2–5 times higher relative to erosion at a normal setting with identical hydraulic geometry. Utilizing this erosion amplification expression in numerical simulations, we demonstrate its impact on reach-scale morphology above waterfalls. Amplified erosion at the lip of a waterfall can trigger the formation of an oversteepened reach whose length is longer than the flow acceleration zone, provided incision wave velocity (Vi) at the upstream edge of the flow acceleration zone is higher than the retreat velocity of the waterfall face. Such an oversteepened reach is expected to be more pronounced when Vi increases with increasing slope. The simulations also suggest that oversteepening can eventually lead to steady state gradients adjacent to a waterfall lip provided Vi decreases with increasing slope. Flow acceleration above waterfalls can thus account, at least partially, for prevalent oversteepened bedrock reaches above waterfalls. Using the cosmogenic isotope Cl-36, we demonstrate that incision wave velocity upstream of a waterfall at the Dead Sea western escarpment is probably high enough for freefall-induced oversteepening to be feasible.

Late Pliocene and Pleistocene reversal of drainage systems in northern Israel: tectonic implications

Abstract The arching of the Galilee, northern Israel, is associated with sediment loading in the Dead Sea Transform and Rift. During the Pleistocene, the arching caused the formation of the main North–South water divide in the region and the reversal of stream flow direction. A reconstruction of a main paleochannel which drained large areas in the eastern Galilee to the Mediterranean enabled the determination of age and amplitude of the arching. This reconstruction is based on topographic analysis of thirteen sites containing fluvial remnants in the Beit-Hakerem Valley. We demonstrate that the widespread normal faulting cannot explain the present-day drainage pattern. Dating of basalt clasts from ancient alluvial remnants along the Beit-Hakerem Valley provides a maximum age limit of 1.8 Ma to the paleochannel. The Pleistocene tectonism arched the Galilee by 200 m over a wavelength of 40–60 km. A comparison between arched and unarched segments of the rifts margins indicates that fluvial and slope processes on the rift escarpment cannot explain the location and shape of the main water divide. In the Galilee, tectonism is the only factor that controls the formation, location and shape of the main water divide.