OSL Surface Exposure Dating Of Glacial Boulders And Bedrock Fault Scarp

Optically stimulated luminescence dating of rock surfaces has attracted much interest over the past few years(e.g.Sohbati et al.,2011,2012a,2018;Lehmann et al.,2018;Liu et al.,2018)One of the most important tasks for optically stimulated luminescence dating of exposure rock surfaces is to establish residual luminesce depth profiles.As one of the two key coordinates of OSL depth profiles,depth is undoubtedly very important.We could cut the thickness of slices to?0.2 millimetres as the thinnest for now by using the wire cutting machine.Within the same depth in the rock core,we can get 2-3 times more slices(data points)than others to reduce uncertainties of the profiles greatlyIn the previous studies,the sensitivity-corrected natural IRSL signals(Ln/Tn)from rock slices were measured using IRSL or elevated temperature post-IR IRSL protocol However,the change in luminescence sensitivity between measurement of the natural signal and stimulation of the subsequent test doses(i.e.,the k value)would affect measured Ln/Tn values.However,variations in the shape of OSL depth profiles,disagree of dispersion of the saturated test dose sensitivity-corrected OSL signals among the slices can be greatly reduced by normalising the sensitivity-corrected Ln/Tn using one of the regenerative dose OSL signals(i.e.re-normalisation).Samples with different lithologies investigated in this study,we find that the OSL depth profiles have been corrected.These improvements are suitable for popularizing to rock surface luminescence dating for more accurate.Most studies have focused on burial rock surface dating and only a few exposure ages have been reported for rock surface luminescence dating(e.g.,Sohbati et al.,2012a;Freiesleben et al.,2015).Luminescence rock surface ages are rarely compared with dating methods other than luminescence dating of sediment(e.g.,Sohbati et al.,2012a).Four gneiss boulders on Holocene glaciofluvial fans offset by the Kongur normal fault in the Chinese Pamir Plateau were sampled to test the reliability and applicability of rock surface luminescence dating.The 10Be ages of the boulders ranged from 2.1 to 7.5 ka.Depth profiles of luminescence intensity were measured using a modified multi-elevated-temperature post-infrared infrared stimulated luminescence(MET-pIRIR)protocol,in which two consecutive stimulations with different power,15 and 131 mW/cm2,were performed for each temperature.The decay rate of the trapped charge at the surface (?) were calibrated using the 10Be age for each boulder sample.The resulting values for the parameter (?) were utilized to estimate the exposure time of other samples.Uncertainties in ? derived from fitting each MET-pIRIR signal depth profile,corresponding the parameter (?)t,and calculated ages were too large to allow precise comparisons to be made between vanished and unvarnished rock surfaces.The? was assumed to be the same for all MET-pIRIR signals,and so was fitted simultaneously to all profiles derived from a single sample;this significantly reduces the fitting uncertainties in the parameters ? and (?)t.The long-term relative bleaching rate of the IR50,MET-pIRIR110,MET-pIRIR170 and MET-pIRIR225 signals were evaluated by comparing the ratios of (?)t derived from fitting of luminescence-depth profile.The IR50 signal,MET-pIRIR110 and MET-pIRIR170 bleached tens,?5,and 2 times faster than the MET-pIRIR225 signals,respectively.These results highlight the potential and challenges in using rock surface luminescence methods as a viable dating tool for exposure ages.Understanding the partitioning of seismic and aseismic fault slip is critical for seismotectonic studies because it ultimately determines the seismic potential of faults.Distinguishing and estimating seismic and aseismic slip on geologic timescales(101-6 years)is challenging.Bedrock fault scarps provide arguably one of the best archives to help determine the nature and timing of past fault slip because they are associated with a particular fault,potentially record the whole slip history,and tend to remain unmodified because of their resistance to erosion.The use of bedrock fault scarps is limited because of the challenges of dating them.However,luminescence bleaching depth profiles of bedrock fault surfaces provide a newly developing method to constrain fault slip history.Modeling and application of luminescence bleaching depth profiles on a bedrock normal fault scarp along Langshan in north China help distinguish episodic seismic or progressive aseismic fault slip providing an important case study.Infra-red stimulated luminescence signals from rock slices exhibit increasingly deep bleaching profiles with height up the fault scarp.Four groups of luminescence depth profiles are evident at different heights up the fault,indicating three coseismic slips of 140,80,80 and 90 cm due to the most recent strong earthquakes and one slip by aseismic creep or seismic slip.The Langshan normal fault study shows that luminescence bleaching depth profile methods are relatively economical and quick to undertake;and are especially useful to reconstruct fault slip especially for events from few hundreds or thousand years oldA mathematical model describing degree of resetting of the luminescence-depth profiles of rock surfaces,exposure time and erosion rate is developed.Three methods for obtaining the actual exposure age and erosion rate of rock surface are explored:1)Limiting exposure age of samples only using the OSL signals;2)Estimating exposure age by using the maximum erosion rate of the same lithologic rocks in the same area;3)The actual exposure age and erosion rate of rocks are determined by limiting the exposure age obtained by other methods.In addition,we also try to obtain the true erosion rate and exposure age of rocks by the ratio of DHS of infrared luminescence signals at different temperatures.