D. Bourles

Slip rates of the Karakorum fault, Ladakh, India, determined using cosmic ray exposure dating of debris flows and moraines

most recent major glacial advance occurred at 90 ± 15 ka. This is consistent with the inference of others that alpine glaciers in this region have not necessarily expanded in concert with Northern Hemisphere continental ice sheets. If features, including lateral moraines, that Liu inferred to have been offset 300–350 m by the Karakorum fault date from the same period, they too imply a slip rate of 3–4 mm yr � 1 . This slip rate is comparable to rates of extension across grabens within Tibet. With recent evidence that slip along the Altyn Tagh fault occurs at � 10 mm yr � 1 , our rate suggests that slip along the boundaries of Tibet is not significantly more rapid than extension within the plateau. Hence, plate tectonics, in the strictest sense, ought not be applied to Tibet, because Tibet does not behave as a rigid plate. INDEX TERMS: 1824 Hydrology: Geomorphology (1625); 1035 Geochemistry: Geochronology; 9320 Information Related to Geographic Region: Asia; 8107 Tectonophysics: Continental neotectonics; KEYWORDS: Karakorum Fault, slip rate, cosmic ray exposure age, beryllium 10

10Be and 9Be in marine sediments and their potential for dating

Using sequential leaching experiments we have measured the fraction of the cosmogenic isotope 10Be (half-life 1.5 My), and its stable isotope 9Be, co-extracted with the following phases in marine sediments: exchangeable, calcium carbonate, Fe and Mn oxyhydroxides, organic, opal and detrital. The extraction of the authigenic phases typically release ∼70% of the 10Be but only ∼40% of the 9Be. In order to test its potential for dating marine sediments, we have measured the authigeniccally associated 10Be/9Be in 17 surface sediment samples from different ocean basins. The observed spatial variability, which appears to be influenced by proximity to continental input of 9Be, precludes the general and precise applications of leached 10Be/9Be for dating. However, the relatively constant value observed in surface sediments over large areas of the Pacific, Indian and Southern Atlantic oceans shows considerable promise for approximate dating of open oceans sediments. In order to test temporal variability, a profile of the authigenically associated 10Be/9Be ratio has been measured in the independently dated Central Pacific core, RC12-65. The observed variation in 10Be/9Be over the past nine My is approximately twice that expected on the basis of 10Be decay alone. Several possible explanations for such behavior and their potential consequences for the use of leached 10Be/9Be for dating are discussed.

In situ produced 10Be measurements at great depths: Implications for production rates by fast muons

In situ cosmogenic 10Be values have been used to investigate a Brazilian quartz vein from the surface to a depth of 15 m. At depths greater than 1000 g/cm2, deep enough for neutron-induced reactions to be insignificant, there is only a slight decrease in 10Be concentration with increasing depth. Our results are consistent with deep production of 10Be by a mechanism, presumably induced by fast muons, with an attenuation length of 5300±950 g/cm2 and a contribution of 0.65±0.25% to the total surface production. Results are compared with values from the literature and implications of this re-evaluation are discussed.

Slip rates along active faults estimated with cosmic-ray–exposure dates: Application to the Bogd fault, Gobi-Altaï, Mongolia

Dating morphological features displaced along active faults presents a major difficulty in evaluation of slip rates. We used in-situ–produced 10 Be to calculate minimum ages for alluvial surfaces misaligned by movement along a major active fault in the Gobi-Altai (western Mongolia). The maximum slip rate of ≈1.2 mm/yr suggested by this method contrasts strongly with rates of ≈20 mm/yr that we estimated by correlation of alluvial deposition with warm humid periods associated with the last glacial termination estimated to have occurred about 12 ka in western Tibet. The 10 Be-based slip rate indicates that strong earthquakes can occur along faults with low slip rates and demonstrates the contribution of cosmic-ray–exposure dating in Quaternary tectonic analyses.

Cumulative right-lateral fault slip rate across the Zagros-Makran transfer zone: role of the Minab-Zendan fault system in accommodating Arabia-Eurasia convergence in southeast Iran

The Zendan-Minab zone is the transition zone between the Zagros collision to the west and Makran subduction to the east. It is also linked to the north with the Nayband-Gowk fault system that bounds the Lut Block to the east. The total convergence rate between Arabia and Eurasia is estimated to range between 23 and 35 mm yr-1 in a NNE-trending direction. The deformation through the Minab-Zendan system is accommodated within two fault systems, the western N160°E-trending Minab-Zendan fault system and the eastern north-south Sabzevaran-Jiroft fault system. The study area is characterized by a well-defined succession of Quaternary deposit levels. The age of these deposits was estimated by archaeological data, regional palaeoclimate correlations and constrained by additional in situ10Be dating in another paper in this study. These deposits exhibit offsets, both lateral and vertical, that are evaluated by satellite image analysis and GPS profiles. Thanks to offsets and ages the strike-slip rates associated with the Minab-Zendan and the Sabzevaran-Jiroft fault systems are calculated to be 5.1 +/- 1.3 or 6.6 +/- 1.5, and 6.2 +/- 0.7 mm yr-1, respectively. These results allow an evaluation of the velocity vector of the Musandam Peninsula (Oman) with respect to the Lut Block of 11.4 +/- 2.0 or 12.9 +/- 2.2 mm yr-1 in a N10 +/- 15°E direction, close to the GPS estimates. This study also constrains the in-plane slip rates for each fault. Previous works indicate that the Zagros accommodates only 10 mm yr-1 of shortening, while 10 mm yr-1 should be accommodated by the Alborz mountains in northern Iran. This last 10 mm yr-1 may be accommodated through the Nayband-Gowk system and the East Iranian ranges, implying that the two fault systems constituting the Zagros-Makran transfer zone have different geodynamic roles. The western Minab-Zendan fault system links the Makran and Zagros deforming zones, whereas the northwestern Jiroft-Sabzevaran fault system is transmitting the deformation to the Nayband-Gowk system and then to the Alborz ranges. The presence of another such strike-slip zone within the Makran seems to indicate that the accommodation zone between the Zagros and Makran is wide, of the order of 400 km. We interpret this deformation pattern that accompanies the genesis of the immature transform zone by a flexure of the slab under the Zagros-Makran transfer zone instead of a tear in the slab that may be expected to induce a sharper transition zone.

Late Pleistocene to Holocene slip rates for the Gurvan Bulag thrust fault (Gobi-Altay, Mongolia) estimated with 10Be dates

[1] We surveyed morphotectonic markers along the central part of the Gurvan Bulag thrust, a fault that ruptured with the Bogd fault during the Gobi-Altay earthquake (1957, M 8.3), to document climatic and tectonic processes along the fault for the late PleistoceneHolocene period. The markers were dated using 10 Be produced in situ. Two major periods of alluviation ended at 131 ± 20 and 16 ± 4.8 ka. These appear to be contemporaneous with global climatic changes at the terminations of marine isotope stages (MIS) 6 and 2. The vertical slip rates, determined from offset measurements and surfaces ages, are 0.14 ± 0.03 mm/yr over the late Pleistocene-Holocene and between 0.44 ± 0.11 and 1.05 ± 0.25 mm/yr since the end of the late Pleistocene. The higher of these slip rates for the last � 16 kyr is consistent with paleoseismic investigations along the fault [Prentice et al., 2002], and suggests that, at the end of late Pleistocene, the fault evolved from quiescence to having recurrence intervals of 4.0 ± 1.2 kyr for surface ruptures with � 4 m vertical offset (similar to that of 1957). The inferred recurrence interval is comparable to that of the Bogd fault (3.7 ± 1.3 kyr) suggesting that the two faults may have ruptured together also earlier during the last � 16 kyr. INDEX TERMS: 7221 Seismology: Paleoseismology; 1208 Geodesy and Gravity: Crustal movements—intraplate (8110); 1824 Hydrology: Geomorphology (1625); 7230 Seismology: Seismicity and seismotectonics; 8107 Tectonophysics: Continental neotectonics; KEYWORDS: Late Pleistocene, Holocene, thrust fault, slip rate, 10Be dating, Mongolia

Hydrothermal scavenging on the Juan de Fuca Ridge: 23OThxs, 10Be, and REEs in ridge-flank sediments

Abstract We have investigated the geochemistry of a hydrothermally enriched sediment core recovered from the western flank of the N.Cleft Segment, Juan de Fuca Ridge, ∼8km west of the “MegaPlume” area previously identified near 45°N. The core contains varying biogenic, lithogenic, and hydrothermal components, as reflected in CaCO3, Al, and Fe contents, respectively. Horizons of pronounced hydrothermal input, in core-top sediments and at depth, exhibit increased concentrations of Fe, Cu, Zn, and Pb and shifts in Pb isotopic compositions toward nonradiogenic (MORB/hydrothermal) values. REE concentrations co-vary with hydrothermal Fe down-core, and shale-normalised REE distribution patterns exhibit both negative Ce-anomalies and positive Eu-anomalies, indicative of input from plume-particle fall-out. Unsupported 230Thxs activities down-core are consistent with continuous slow sediment accumulation rates of 0.54 cm/ky for ∼200 ky since the deposition of the deeper Fe-rich horizon. 10Be(0) and 9Be isotope concentrations also co-vary with hydrothermal Fe down-core and exhibit 10 Be (o) 9 Be ratios which approach that of Pacific Ocean deep water, indicative of a seawater-scavenging source. 10 Be (0) 230 OTh xs (0) ratios throughout most of Core GC88-6 are greater than mean Pacific Ocean values, indicating that hydrothermal scavenging can lead to significant net removal of dissolved 10Be into ridge-flank sediments.

Transpressional tectonics and stream terraces of the Gobi‐Altay, Mongolia

We studied the patterns, rates and evolution of fluvial terraces and fault system during the building process of an intracontinental transpressional mountain in the Gobi-Altay (Mongolia). By analyzing incisions and offsets of fluvial terraces and alluvial fans, we show that the massif has grown by outward migration of thrust faults through time. On the northern flank, the present bounding thrust fault began its activity ~600 ka ago, while a more internal sub-parallel fault was still active until ~200-100 ka. Vertical offset of an alluvial fan abandoned ~100 ka ago allows an estimate of 0.1 mm/yr Upper Pleistocene - Holocene uplift rate. The morphology of the catchment-piedmont system strongly suggests a periodical formation of the alluvial surfaces, controlled by the climatic pulses, at the beginning of the wet interglacial periods. The abandonment of the alluvial terraces lags by several thousand years the abandonment of the alluvial fans, showing a diachronous incision propagating upstream. The incision rate deduced from the different elevations of straths exceeds of one order of magnitude the rock uplift rate. This excess is mostly due to ongoing drainage network growth at the core of the massif, and incision due to alluvial apron entrenchment near the outlet. This implies that fluvial response is mainly controlled by drainage growth, interaction with piedmont and cyclic climatic variations, rather than by rock uplift.

Early Holocene climate recorded in geomorphological features in Western Tibet

Cosmic ray exposure ages for formation of perched alluvial terraces and for abandonment of an alluvial/debrisflow fan on opposite sides of the Tangtse Valley (the outflow at the northwest end of Lake Panggong, which is in the Karakorum Range of Western Tibet) provide evidence of a humid period at V11.5 to V7 ka. This is consistent with other regional records and supports a controversial chronology for the sedimentary record from Lake Panggong. Fan abandonment appears to have occurred at V11.5 ka as the climate presumably became more humid in response to initiation of enhanced monsoonal circulation, consistent with previously reported onset of humid conditions in a sedimentary record from the easternmost basin of the lake. In contrast, the terraces did not form until about 7 ka with downcutting of the transverse valley by overflow from Lake Panggong. This lag can be explained in light of the bathymetry of Lake Panggong; the modern lake consists of five basins separated by shallow sills, and outflow through the Tangtse Valley could not occur until the water level was substantially above its present level. The easternmost basin receives the inflow of the major rivers feeding the lake, making its chemistry highly sensitive to changes in precipitation. However, sustained wet conditions are required to fill the basins to the west to the sill depth necessary for overflow through the Tangtse Valley and resultant downcutting and terrace formation. ; 2003 Elsevier B.V. All rights reserved.

Application of in situ-produced cosmogenic 10Be and 26Al to the study of lateritic soil development in tropical forest: theory and examples from Cameroon and Gabon

Abstract Depth profiles of in situ-produced cosmogenic nuclides, including 10 Be ( T 1/2 =1.5×10 6 years) and 26 Al ( T 1/2 0.73×10 6 years), in the upper few meters of the Earths crust may be used to study surficial processes, quantifying denudation and burial rates and elucidating mechanisms involved in landform evolution and soil formations. In this paper, we discuss the fundamentals of the method and apply it to two lateritic sequences located in African tropical forests.