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1.
The combined Rhone and Aare Glaciers presumably reached their last glacial maximum (LGM) extent on the Swiss Plateau prior to 24 ka. Two well-preserved, less extensive moraine stades, the Gurten and Bern Stade, document the last deglaciation of the Aare Valley, yet age constraints are very scarce. In order to establish a more robust chronology for the glacial/deglacial history of the Aare Valley, we applied 10Be surface exposure dating on eleven boulders from the Gurten and Bern Stade. Several exposure ages are of Holocene age and likely document post-depositional processes, including boulder toppling and quarrying. The remaining exposure ages, however yield oldest ages of 20.7 ± 2.2 ka for the Gurten Stade and 19.0 ± 2.0 ka for the Bern Stade. Our results are in good agreement with published chronologies from other sites in the Alps.  相似文献   

2.
Rockfall ages in tectonically active regions provide information regarding frequency and magnitude of earthquakes. In the hyper-arid environment of the Dead Sea fault (DSF), southern Israel, rockfalls are most probably triggered by earthquakes. We dated rockfalls along the western margin of the DSF using terrestrial cosmogenic nuclides (TCN). At each rockfall site, samples were collected from simultaneously exposed conjugate boulders and cliff surfaces. Such conjugate samples initially had identical pre-fall (“inherited”) TCN concentrations. After boulder detachment, these surfaces were dosed by different production rates due to differences in post-fall shielding and geometry. However, in our study area, pre-rockfall inheritance and post-rockfall production rates of TCN cannot be evaluated. Therefore, we developed a numerical approach and demonstrated a way to overcome the above-mentioned problems. This approach can be applied in other settings where rockfalls cannot be dated by simple exposure dating. Results suggest rockfall ages between 3.6 ± 0.8 and 4.7 ± 0.7 ka. OSL ages of sediment accumulated behind the boulders range between 0.6 ± 0.1 and 3.4 ± 1.4 ka and support the TCN results. Our ages agree with dated earthquakes determined in paleoseismic studies along the entire length of the DSF and support the observation of intensive earthquake activity around 4–5 ka.  相似文献   

3.
To develop a more precise understanding of Alpine glacier fluctuations during the Holocene, the glacier forefields of the Triftjegletscher and the Oberseegletscher east of Zermatt in the Valais Alps, Switzerland, were investigated. A multidisciplinary approach of detailed geological and geomorphological field mapping combined with 10Be exposure and radiocarbon dating was applied. A total of twelve samples of boulders and bedrock were taken from both Little Ice Age (LIA) landforms, as documented by the Dufour map published in 1862, and from landforms outside of the LIA. The resulting 10Be ages range between 12590 ± 350 a and 420 ± 170 a. A piece of wood found embedded in the Little Ice Age moraine gave radiocarbon ages that range between 293 cal years BP up to modern (356–63 cal years before 2013). Based on these results, four tentative steps of the Holocene evolution could be distinguished. An early Holocene stage, which documents the decay of the Egesen stadial glaciers when the first parts of the study area became ice free. This was followed by a phase with no evidence of glacier advance. Then in the late Holocene, the glaciers advanced (at least) twice. An advance around 1200 a, as shown by several moraine ages, coincides with the Göschenen II cold phase. A more extensive readvance occurred during the LIA as shown on the historical maps and underpinned by one 10Be exposure age and the radiocarbon age. This later advance destroyed or overprinted the earlier landforms in most parts of the area.  相似文献   

4.
Only a few chronological constraints on Lateglacial and Early Holocene glacier variability in the westernmost Alps have hitherto been obtained. In this paper, moraines of two palaeoglaciers in the southern Écrins massif were mapped. The chronology of the stabilization of selected moraines was established through the use of 10Be cosmic ray exposure (CRE) dating. The equilibrium line altitude (ELA) during moraine deposition was reconstructed assuming an accumulation area ratio (AAR) of 0.67. Ten pre‐Little Ice Age (LIA) ice‐marginal positions of the Rougnoux palaeoglacier were identified and seven of these have been dated. The 10Be CRE age of a boulder on the lowermost sampled moraine indicates that the landform may have been first formed during a period of stable glaciers at around 16.2±1.7 ka (kiloyears before AD 2017) or that the sampled boulder experienced pre‐exposure to secondary cosmic radiation. The moraine was re‐occupied or, alternatively, shaped somewhat before 12.2±0.6 ka when the ELA was lowered by 230 m relative to the LIA ELA. At least six periods of stable ice margins occurred thereafter when the ELA was 220–160 m lower than during the LIA. The innermost dated moraine stabilized at or before 10.9±0.7 ka. Three 10Be CRE ages from a moraine of the Prelles palaeoglacier indicate a period of stationary ice margins at or before 10.9±0.6 ka when the ELA was lowered by 160 m with respect to the end of the LIA. The presented 10Be CRE ages are in good agreement with those of moraines that have been attributed to the Egesen stadial. Assuming unchanged precipitation, summer temperature in the southern Écrins massif at ~12 ka must have been at least 2 °C lower relative to the LIA.  相似文献   

5.
Cosmogenic nuclide surface exposure dating of boulders and erratics provides new constraints for a glacial chronology in the source area of the Urumqi River, Tian Shan, China. 10Be exposure ages of 15.0 ± 1.3–17.1 ± 1.5 ka from the Upper Wangfeng (UWF) moraines agree well with their previous relative age assignments to marine isotope stage (MIS) 2, but are younger than published AMS 14C and electron spin resonance (ESR) ages (from 22.8 ± 0.6 to 37.4 ka). This difference may result from variations in techniques, or could reflect the impact of surface erosion and sediment/snow cover on surface exposure dating. 10Be ages from the Lower Wangfeng (LWF) moraines (18.7 ± 1.8 and 16.2 ± 1.5 ka) are indistinguishable from the UWF exposure ages, but are significantly younger than previously reported thermoluminescence (TL) and ESR ages (37.7 ± 2.6–184.7 ± 18 ka). Either these two groups were formed during the same period (MIS 2) and there are problems with TL and ESR ages, or the moraines were of very different ages and the similar exposure ages result from different degrees of degradation. Erratics on rock steps and a drumlin along >8 km of the main glacial valley above the UWF have internally consistent and slightly decreasing 10Be exposure ages indicating glacier retreat >2.5 m a?1 after MIS 2 and before middle or late Holocene glacier re‐advances. This retreat rate is similar to rates observed from modern glaciers. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

6.
The tectonic evolution of the Mont Blanc range with regard to its cooling and exhumation history has been discussed and debated over many years and is still controversial. Recently, several low-temperature thermochronology studies have determined the cooling history of the massif in considerable detail and various tectonic models proposed to explain the young and fast exhumation signal established from these studies. Here we present detailed field data from the wider Mont Blanc area and assess possible exhumation processes in terms of these field constraints. Our observations indicate that none of the major faults or shear zones around the Mont Blanc massif (i.e. Mont Blanc shear zone, Mont Blanc back-thrust, Penninic thrust) was active in Late Neogene times and that young exhumation is therefore not controlled by movements along these structures. We demonstrate that the position of Mont Blanc in the bend of the western Alps plays an important role in its tectonic history and that simple 2D models are insufficient to explain its evolution. Interference between NW–SE compression and orogen-parallel extension along the Rhône-Simplon fault system resulted in a complex regional structural pattern, with strike-slip movements on both sides of the Mont Blanc massif. Young brittle faults are predominantly strike slip without significant vertical offset. The young (<2 Ma) rapid exhumation of Mont Blanc is more broadly distributed and cannot be directly linked to discrete faults bounding the massif. The mechanisms driving this recent accelerated exhumation must similarly be of broader scale.  相似文献   

7.
A large number of the landslide dams located on the major rivers at the southeastern margin of the Tibetan Plateau have been previously identified through remote sensing analysis and field investigations. The Xuelongnang paleolake was one of the lakes formed by these landslide dams in the upper Jinsha River, where the association of a relict landslide dam, lacustrine sediment, and outburst sediment is well preserved. This preservation provides an opportunity to better understand the formation, evolution, and longevity of a large landslide-dammed lake in the upper Jinsha River. It was inferred that the Xuelongnang dammed lake may have been formed by an earthquake-induced paleoavalanche. The surface area of the lake at its peak was estimated at 7.0?×?106 m2, and the corresponding volume was approximately 3.1?×?108 m3. Two outburst flood events were determined to have occurred during the life span of the lake. Based on the 18 ages obtained from optically stimulated luminescence (OSL) and carbon-14 (14C) dating combined with stratigraphic sequences observed in the field, the paleolandslide-dammed lake was formed at approximately 2.1 ka and subsequently breached locally. The dammed lake was sustained for a period of some 900 years based on the chronological constraining. This study confirms that a major landslide-dammed lake can be sustained for at least hundreds of years and breached by several dam breaks in multiple periods, which contributed to the preservation of the knickpoints at millennial scale along the major rivers in the study area.  相似文献   

8.
The impact of the Laurentide Ice Sheet (LIS) deglaciation on Northern Hemisphere early Holocene climate can be evaluated only once a detailed chronology of ice history and sea‐level change is established. Foxe Peninsula is ideally situated on the northern boundary of Hudson Strait, and preserves a chronostratigraphy that provides important glaciological insights regarding changes in ice‐sheet position and relative sea level before and after the 8.2 ka cooling event. We utilized a combination of radiocarbon ages, adjusted with a new locally derived ΔR, and terrestrial in‐situ cosmogenic nuclide (TCN) exposure ages to develop a chronology for early‐Holocene events in the northern Hudson Strait. A marine limit at 192 m a.s.l., dated at 8.1–7.9 cal. ka BP, provides the timing of deglaciation following the 8.2 ka event, confirming that ice persisted at least north of Hudson Bay until then. A moraine complex and esker morphosequence, the Foxe Moraine, relates to glaciomarine outwash deltas and beaches at 160 m a.s.l., and is tightly dated at 7.6 cal. ka BP with a combination of shell dates and exposure ages on boulders. The final rapid collapse of Foxe Peninsula ice occurred by 7.1–6.9 cal. ka BP (radiocarbon dates and TCN depth profile age on an outwash delta), which supports the hypothesis that LIS melting contributed to the contemporaneous global sea‐level rise known as the Catastrophic Rise Event 3 (CRE‐3).  相似文献   

9.
In situ terrestrial cosmogenic nuclide (TCN) exposure dating using 10Be is one of the most successful techniques used to determine the ages of Quaternary deposits and yields data that enable the reconstruction of the Quaternary glacial history of the Tibetan Plateau and the surrounding mountain ranges. Statistical analysis of TCN 10Be exposure ages, helps to reconstruct the history of glacial fluctuations and past climate changes on the Tibetan Plateau, differences in the timing of glacier advances among different regions. However, different versions of the Cosmic‐Ray‐prOduced NUclide Systematics on Earth (CRONUS‐Earth) online calculator, which calculates and corrects the TCN ages of Quaternary glacial landforms, yield different results. For convenience in establishing contrasts among regions, in this paper, we recalculate 1848 10Be exposure ages from the Tibetan Plateau that were published from 1999 to 2017 using version 2.3 of the CRONUS‐Earth calculator. We also compare the results obtained for 1594 10Be exposure ages using different versions (2.2, 2.3 and 3.0) of the CRONUS‐Earth calculator. The results are as follows. (1) Approximately 97% of the exposure ages are less than 200 ka. A probability density curve of the exposure ages suggests that greater numbers of oscillations emerge during the Holocene, and the peaks correspond to the Little Ice Age, the 8.2 ka and 9.3 ka cold events; the main peak covers the period between 12 and 18 ka. (2) In most areas, the newer versions of the calculator produce older 10Be exposure ages. When different versions of the CRONUS‐Earth calculator are used, approximately 29% of the 10Be exposure ages display maximum differences greater than 10 ka, and the maximum age difference for a single sample is 181.1 ka.  相似文献   

10.
Relict rock glaciers have considerable potential for contributing to palaeoclimatic reconstruction, but this potential is often undermined by lack of dating control and problems of interpretation. Here we reinvestigate and date four proposed ‘rock glaciers’ in the Cairngorm Mountains and show that the morphology of only one of these appears consistent with that of a true rock glacier produced by creep of underlying ice or ice‐rich sediment. All four features comprise rockslide or rock avalanche runout debris, and the possibility that all four represent unmodified runout accumulations cannot be discounted. Surface exposure dating of the four debris accumulations using cosmogenic 10Be produced uncertainty‐weighted mean ages of 15.4 ± 0.8 ka, 16.2 ± 1.0 ka, 12.1 ± 0.6 ka and 12.7 ± 0.8 ka. All four ages imply emplacement under cold stadial conditions, two prior to the Windermere Interstade of ca. 14.5–12.9 cal. ka BP and two during the Loch Lomond Stade of ca. 12.9–11.5 cal. ka BP. The above ages indicate that paraglacial rock‐slope failure on granite rockwalls occurred within a few millennia after deglaciation. The mean exposure ages obtained for runout debris at two sites – Strath Nethy (16.2 ± 1.0 ka) and Lairig Ghru (15.4 ± 0.8 ka) – are consistent with basal radiocarbon ages from Loch Etteridge, 22 km to the southwest (mean = 15.6 ± 0.3 cal. ka BP) and imply widespread deglaciation of the Cairngorms and adjacent valleys before 15 ka and possibly 16 ka. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

11.
Sized aggregates of glasses (47–84 wt% SiO2) were fused from igneous-derived cohesive fault rock and igneous rock, and step-heated from ~400 to >1,200 °C to obtain their 39Ar diffusion properties (average E=33,400 cal mol?1; D o=4.63×10?3 cm2 s?1). At T<~1,000 °C, glasses containing <~69 wt% SiO2 and abundant network-forming cations (Ca, Fe, Mg) reveal moderate to strong non-linear increases in D and E, reflecting structural modifications as the solid transitions to melt. Extrapolation of these Arrhenius properties down to typical geologic T-t conditions could result in a 1.5 log10 unit underestimation in the diffusion rate of Ar in similar materials. Numerical simulations based upon the diffusion results caution that some common geologic glasses will likely yield 40Ar/39Ar cooling ages rather than formation ages. However, if cooling rates are sufficiently high, ambient temperatures are sufficiently low (e.g., <65–175 °C), and coarse particles (e.g., radius (r) >~1 mm) are analyzed, glasses with compositions similar to ours may preserve their formation ages.  相似文献   

12.
Matthews, J. A. & Winkler, S. 2010: Schmidt‐hammer exposure‐age dating (SHD): application to early Holocene moraines and a reappraisal of the reliability of terrestrial cosmogenic‐nuclide dating (TCND) at Austanbotnbreen, Jotunheimen, Norway. Boreas, 10.1111/j.1502‐3885.2010.00178.x. ISSN 0300‐9483. Schmidt‐hammer exposure‐age dating (SHD) and terrestrial cosmogenic‐nuclide dating (TCND) are complementary techniques that can be used for mutual testing. SHD is low‐cost but requires local control points of known age and may be affected by local geological variation and other environmental factors that influence weathering rates. TCND is vulnerable to the occurrence of anomalous boulders, other geomorphological uncertainties and the effects of snow‐shielding at high altitudes. Both techniques are sensitive to post‐depositional disturbances if other than solid bedrock is sampled. SHD was applied to two moraine ridges beyond the Little Ice Age limit of Austanbotnbreen in the Hurrungane massif, southern Norway. Independent regional and experimental local age‐calibration curves were used to reappraise previous TCND results. Neither the two boulder surfaces nor their proximal bedrock surfaces could be differentiated statistically in terms of SHD exposure ages or their mean R‐values (±95% confidence intervals), which ranged from 40.73±1.72 to 43.34±0.69. The best of the independent regional‐calibration curves produced SHD exposure ages of 9413±723 and 9304±602 years, which are consistent with moraine formation early (c. 10.2 ka) and late (c. 9.7 ka) within the late‐Preboreal Erdalen Event. The current precision of SHD, as reflected in 95% confidence intervals of ±500–900 years, enables rejection of a Finse Event (c. 8.2 ka) age for either moraine. Results are consistent with a retracted Austanbotnbreen between the Erdalen Event and the Little Ice Age, and a modified model of Neoglaciation.  相似文献   

13.
Rock falls in the Mont Blanc Massif in 2007 and 2008   总被引：1，自引：1，他引：0  
Due to a lack of systematic observations, the intensity and volume of rock falls and rock avalanches in high mountain areas are still poorly known. Nevertheless, these phenomena could have burly consequences. To document present rock falls, a network of observers (guides, mountaineers, and hut wardens) was initiated in the Mont Blanc Massif in 2005 and became fully operational in 2007. This article presents data on the 66 rock falls (100 m3?≤?V?≤?50,000 m3) documented in 2007 (n?=?41) and 2008 (n?=?25). Most of the starting zones are located in warm permafrost areas, which are most sensitive to warming, and only four rock falls are clearly out of permafrost area. Different elements support permafrost degradation as one of the main triggering factors of present rock falls in high mountain areas.  相似文献   

14.
The northeastern portion of the Mont Blanc massif in western Switzerland is predominantly comprised of the granitic rocks of the Mont Blanc intrusive suite and the Mont Blanc basement gneisses. Within these metamorphic rocks are a variety of sub-economic Fe skarns. The mineral assemblages and fluid inclusions from these rocks have been used to derive age, pressure, temperature and fluid composition constraints for two Variscan events. Metamorphic hornblendes within the assemblages from the basement amphibolites and iron skarns have been dated using 40Ar/39Ar, and indicate that these metamorphic events have a minimum age of approximately 334 Ma. Garnet-hornblende-plagioclase thermobarometry and stable isotope data obtained from the basement amphibolites are consistent with metamorphic temperatures in the range 515 to 580 °C, and pressures ranging from 5 to 8 kbar. Garnet-hornblende-magnetite thermobarometry and fluid inclusion studies indicate that the iron skarns formed at slightly lower temperatures, ranging from 400 to 500 °C in the presence of saline fluids at formational pressures similar to those experienced by the basement amphibolites. Late Paleozoic minimum uplift rates and geothermal gradients calculated using these data and the presence of Ladinien ichnofossils are on the order of 0.32 mm/year and 20 °C/km respectively. These uplift rates and geothermal gradients differ from those obtained from the neighbouring Aiguilles Rouges massif and indicate that these two massifs experienced different metamorphic conditions during the Carboniferous and Permian periods. During the early to late Carboniferous period the relative depths of the two massifs were reversed with the Aiguilles Rouges being initially unroofed at a much greater rate than the Mont Blanc, but experiencing relatively slower uplift rates near the termination of the Variscan orogeny. Received: 23 April 1996 / Accepted: 27 August 1996  相似文献   

15.
Rock avalanches are common in the Mont Blanc massif, which is bordered by valleys with large resident and tourist populations and important highways. This paper combines historical data with detailed geomorphological mapping, stratigraphic observation, and absolute and relative dating, to interpret several deposits resulting from rock avalanching onto glaciers.Nineteen rock falls and rock avalanches are described, ranging in volume from 10,000 m3 to 10 × 106 m3. They occurred between 2500 BP and AD 2007 at six sites. The events at three sites (Miage and Drus Glaciers, and Tour des Grandes Jorasses) are characterised by short travel distances; those at Brenva, Triolet, and Frébouge Glaciers exhibit excessive travel distances.Interactions between rock avalanches and glaciers are of four types: (i) rock-avalanche triggering, where glacial and paraglacial controls include debuttressing of rockwalls due to glacier thinning and retreat, oversteepening of rock slopes by glacial erosion, and effects of glaciers on permafrost; (ii) rock-avalanche mobility, in which mobility and travel distance are modified by channelling of rock-avalanche debris by moraines and valleys, incorporation of ice and snow (often >50% for large events), and irregularities on the glacier surface; (iii) deposit sedimentology, where melting of incorporated ice transforms the final deposit by reducing its thickness typically to <5 m, and debris of variable thicknesses is juxtaposed in a hummocky deposit with chaotic piles of angular rock debris; and (iv) glacier dynamics where insulating debris deposited upon a glacier produces a debris-covered glacier of different dynamics, and high elevated scars can favour the formation of small glaciers.  相似文献   

16.
Quaternary glaciation of Mount Everest   总被引：1，自引：0，他引：1  
The Quaternary glacial history of the Rongbuk valley on the northern slopes of Mount Everest is examined using field mapping, geomorphic and sedimentological methods, and optically stimulated luminescence (OSL) and 10Be terrestrial cosmogenic nuclide (TCN) dating. Six major sets of moraines are present representing significant glacier advances or still-stands. These date to >330 ka (Tingri moraine), >41 ka (Dzakar moraine), 24–27 ka (Jilong moraine), 14–17 ka (Rongbuk moraine), 8–2 ka (Samdupo moraines) and ～1.6 ka (Xarlungnama moraine), and each is assigned to a distinct glacial stage named after the moraine. The Samdupo glacial stage is subdivided into Samdupo I (6.8–7.7 ka) and Samdupo II (～2.4 ka). Comparison with OSL and TCN defined ages on moraines on the southern slopes of Mount Everest in the Khumbu Himal show that glaciations across the Everest massif were broadly synchronous. However, unlike the Khumbu Himal, no early Holocene glacier advance is recognized in the Rongbuk valley. This suggests that the Khumbu Himal may have received increased monsoon precipitation in the early Holocene to help increase positive glacier mass balances, while the Rongbuk valley was too sheltered to receive monsoon moisture during this time and glaciers could not advance. Comparison of equilibrium-line altitude depressions for glacial stages across Mount Everest reveals asymmetric patterns of glacier retreat that likely reflects greater glacier sensitivity to climate change on the northern slopes, possibly due to precipitation starvation.  相似文献   

17.
We reconstructed a chronology of glaciation spanning from the Late Pleistocene through the late Holocene for Fish Lake valley in the north‐eastern Alaska Range using 10Be surface exposure dating and lichenometry. After it attained its maximum late Wisconsin extent, the Fish Lake valley glacier began to retreat ca. 16.5 ka, and then experienced a readvance or standstill at 11.6 ± 0.3 ka. Evidence of the earliest Holocene glacial activity in the valley is a moraine immediately in front of Little Ice Age (LIA) moraines and is dated to 3.3–3.0 ka. A subsequent advance culminated at ca. AD 610–900 and several LIA moraine crests date to AD 1290, 1640, 1860 and 1910. Our results indicate that 10Be dating from high‐elevation sites can be used to help constrain late Holocene glacial histories in Alaska, even when other dating techniques are unavailable. Close agreement between 10Be and lichenometric ages reveal that 10Be ages on late Holocene moraines may be as accurate as other dating methods. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

18.
On 12 September AD 1717, a rock volume larger than 10 million m3 collapsed onto the Triolet Glacier, mobilized a mass composed of ice and sediment and travelled more than 7 km downvalley in the upper Ferret Valley, Mont Blanc Massif (Italy). This rock avalanche destroyed two small settlements, causing seven casualties and loss of livestock. No detailed maps were made at the time. Later investigators attributed accumulations of granitic boulders and irregular ridges on the upper valley floor to either glacial deposition, or the AD 1717 rock avalanche, or a complex mixture of glacial deposition, earlier rock avalanche and AD 1717 rock avalanche origin. In this study, we present cosmogenic 10Be exposure ages from nine boulders in the extensive chaotic boulder deposit with irregular ridges, two from Holocene glacier‐free areas, and one from a Little Ice Age moraine. Exposure ages between 330 ± 23 and 483 ± 123 a from eight of nine boulders from the chaotic deposit indicate that at least seven were deposited by the AD 1717 rock avalanche. The other three boulders yielded 10Be exposure ages of 10 900 ± 400, 9700 ± 400 and 244 ± 97 a, respectively. Our results are in good agreement with the existing chronology from dendrochronology and lichenometry, and radiocarbon analysis of wood samples, but not with older 14C ages from a peat bog in the upper part of the valley. Based on the new age control, the rock avalanche deposits cover the whole bottom of the upper Ferret valley. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

19.
2010年青海玉树Ms 7.1级大地震引发了一系列次生地质灾害,其中地震落石是除地震滑坡外沿断裂带及其邻侧最常见的现象。对玉树震区落石的调查发现,该区多处存在非常典型的多期地震落石分布现象,指示该区地震落石的发育与其他古地震现象类似,具有多期性和一定的原地复发性。实地调查表明,该区地震落石分布的主要特征为:多集中发育在活动断裂带附近的陡峭基岩斜坡下方,分布零散,且滚动较远,并常与古地震滑坡相伴生。初步获得的8个地震落石钙膜U系测年结果分布在距今6030±300a BP、4720±210a BP、3530±490~3560±280a BP、2010±160a BP、1090±70a BP、760±20a BP和230±20a BP年龄段,与该区古地震探槽和滑坡反映的地震事件比较吻合,进一步揭示玉树断裂带附近在全新世中晚期发生过多期可导致地表产生地震落石的事件。同时也说明,地震落石及其钙膜测年是特别值得进一步探索的潜在古地震研究方法或途径。  相似文献   

20.
The Northern Zagros Suture Zone (NZSZ), formed as a result of the collision between Arabian and Sanandaj-Sirjan microplate, is considered as part of the Zagros orogenic belt. NZSZ is marked by two allochthonous thrust sheets in upward stacking order: lower and upper allochthon. The Bulfat complex is a part of the upper allochthon or “Ophiolite-bearing terrane” of Albian-Cenomenion age (97–105 Ma). Voluminous highly sheared serpentinites associated with ophiolites occur within this upper allochthon. In addition, the Gemo-Qandil Group is characterized by gabbroic to dioritic Bulfat intrusion with a crystallization age spanning from ~45 to ~?40 Ma, as well as extensive metapelites with contact to the Walash-Naupurdam metavolcanic rocks. Due to the deformation in the Sanandaj-Sirjan Zone along the eastern side of the Iraqi segment of NZSZ, the Gemo-Qandil Group was regionally metamorphosed during late Cretaceous (~?80 Ma). This tectono-compressional dynamics ultimately caused an oscillatory deformation against Arabian continental margin deposits as well. During these events, gabbro-diorite intrusion with high-grade contact metamorphic aureoles occurred near Bulfat. Thus, there is an overlap between regional and contact metamorphic conditions in the area. The earlier metamorphic characteristic can be seen only in places where the latter contact influence was insignificant. Generally, this can only observed at a distance of more than 2.5 km from the contact. According to petrographic details and field observations, the thermally metamorphosed metapelitic units of the metasediment have been completely assimilated, with only some streaks of biotite and relicts of initial foliation. They strongly resemble amphibolite-grade slices from the regional metamorphic rocks in the region. Metapelitic samples far from the intrusion give similar biotite cooling ages as the intrusive rocks. Thus, they may be affected by the same thermal event. 40Ar/39Ar dating of biotite in metapelite rocks of Bulfat by step-wise heating with laser gave average weighted isotopic ages of 34.78?±?0.06 Ma. This is interpreted as crystallization/recrystallization age of biotite possibly representing the time of cooling and uplift history of the Bulfat intrusion. Cooling and exhumation rates for the Bulfat gabbro-diorite rocks were estimated as ~?400 °C/Ma and ~?3.3 mm/year respectively. According to petrographic details, field observations and Ar/Ar dating concerning the contact metamorphism near Bulfat due to the gabbro-diorite intrusion, no significant deformation is visible during exhumation processes after the Paleogene tectono-thermal event, indicating that isotopic ages of 34.78?±?0.06 Ma could mark the timing of termination of the island arc activity in the Ophiolite-bearing terrane (upper allochthon).  相似文献   

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