A thrust-and-fold belt is located on the northern piedmont of Chinese Tianshan Mountain, and it is featured by rows of emerging fault-related folds with the west-east strike (Avouac
Sampling site at the eastern tip of Anjihai anticline and the lithology. a) Google Earth image of the Anjihai anticline. The black square marks the location of the strata for sampling. The inset is a topographic profile of the eastern tip Anjihai anticline with the sample location; photos shown in b) and c) are the close look of the sampling site and strata.
Luminescence dating technique is of potential to constrain the depositional age of these deformed strata. The post-Infrared Infrared Stimulated Luminescence (post-IRIRSL) signals of potassium feldspars (K-feldspar) are achieved either in two steps (pIRIR, Thomsen
It was reported that the small aliquot K-feldspar MET-pIRIR ages were overestimated by 50% for a Holocene fluvial sample from Northern Tianshan (Fu
In this study, we are aiming to employ the state-of-art K-feldspar pIRIR and MET-pIRIR dating procedures to constrain the depositional age of the tectonically deformed fluvial sandy bed at the bottom of the outcrop on the eastern tip of Anjihai anticline. It is the first attempt of absolute dating, which is critical for the earthquake hazard evaluation for the cities nearby. The single grain K-feldspar pIRIR and MAR-MET-pIRIR procedures will be employed. For the single grain procedure, both the pIRIR225 and pIRIR290 signals are explored, taking consideration of their different stability and bleachability (Smedley
Previous studies have shown that the shortening rates of Anjihai anticline ranging from 0.5 mm/yr to 1.12 mm/yr during late Pleistocene, with luminescence ages available for its western tip (Deng
The sample was prepared following the procedures of Aitken (1998). It was treated with HCl and H2O2 to remove the carbonates and organic matter, respectively, and then wet-sieved to separate grains in the size range of 125–180 μm. Subsequently, the K-feldspar grains (ρ < 2.58 g/cm3) were enriched by density separation with the solution of lithium polytungstate. They were then subjected to HF (10%) etching for 40 min to remove the alpha particle irradiated layer and repeatedly rinsed by 10% HCl and distilled water to remove the fluorides.
For single grain measurements, the K-feldspar grains were mounted into the single grain disc with hole diameter of 200 μm and a depth of 200 μm. The single grain luminescence measurements were performed on an automated Risø DA-20 TL/OSL reader with dual – laser single grain attachment. The IR laser (λ = 830 nm, Pmax = 500 mW/cm2) with 90% of its full power was employed for IR stimulation, and the photon multiplier tube (PMT) of model 9235QB15 with blue filter pack (BG3 and BG39) mounted in front was used for detecting the blue emission from the K-feldspar grain. The dose rate of the 90Sr/90Y β irradiation source attached on the reader is 0.1852 ± 0.0044 Gy/s, calibrated using the single grain disc.
For the MAR-MET-pIRIR measurements, the K-feldspar grains were mounted on steel-stainless disc with silicon spray in diameters of 8 mm and 1 mm for large aliquot and small aliquot, respectively. The IR LEDs (λ=870 nm, Pmax=145 mW/cm2) with 90% of its full power was employed for all IR stimulations, and the PMT (model 9107QB70) with blue filter pack (BG3 and BG39) mounted in front was used for detecting the blue emission of the K-feldspar aliquots. The dose rate of the 90Sr/90Y β irradiation source attached on the reader is 0.2073 ± 0.0018 Gy/s, calibrated using the steel-stainless disc.
The radioactivity of the sample was measured with ORTEC GEM70P4-95 gamma spectrometry equipped with a Ge detector. The internal potassium concentration of K-feldspar grains was taken as 12.5% ± 0.5%. The dose rate of the sample was calculated by using the DRAC online calculator (Durcan
The pIRIR225 (Buylaert
Single-grain pIRIR and multiple-aliquot MET-pIRIR measurements procedures.
Natural or regenerative dose Di | Natural or regenerative dose Di | Natural or regenerative dose Di | |
PH at 250°C for 60 s | PH at 320°C for 60 s | PH at 320°C for 60 s | |
IR laser @ 50°C for 1.2 s | IR laser @ 50°C for 1.2 s | IRLED @ 50°C for 100 s | |
IR laser @ 225°C for 1.2 s | IR laser @ 290°C for 1.2 s | IRLED @ 100°C for 100 s | |
IRLED @ 225°C for 100 s | IRLED @ 290°C for 100 s | IRLED @ 150°C for 100 s | |
Test dose | Test dose | IRLED @ 200°C for 100 s | |
PH at 250°C for 60 s | PH at 320°C for 60 s | IRLED @ 250°C for 100 s | |
IR laser @ 50°C for 1.2 s | IR laser @ 50°C for 1.2 s | IRLED @ 290°C for 100 s | |
IR laser @ 225°C for 1.2 s | IR laser @ 290°C for 1.2 s | Test dose | |
IRLED @ 290°C for 100 s | IRLED @ 325°C for 100 s | PH at 320°C for 60 s | |
Return to step1 | Return to step1 | IRLED @ 50°C for 100 s | |
IRLED @ 100°C for 100 s | |||
IRLED @ 150°C for 100 s | |||
IRLED @ 200°C for 100 s | |||
IRLED @ 250°C for 100 s | |||
IRLED @ 290°C for 100 s | |||
Return to step1 |
The MAR-MET-pIRIR procedure of Li
In this study, we employed three criteria for screening the data for analysis: the recycling ratio is between 0.9 and 1.1, the integrated test dose pIRIR intensity is larger than 100 and higher than the 3σ of the background. The central age model (CAM) and minimum age model (MAM), as well as the mean and median, were applied to De distributions determined by the single grain pIRIR and MAR-MET-pIRIR procedures, which were implemented in R package “Luminescence” (Dietze
Single grain pIRIR and MAR-MET-pIRIR procedures were employed to obtain the De distribution of the sample. For the single grain procedures, the pIRIR225 and pIRIR290 signals were measured following the procedures shown in
Typical luminescence decay curves and growth curves of the K-feldspar single grain pIRIR and MAR-MET-pIRIR signals. a) and b) are the decay curve and growth curve of single grain pIRIR225 signal, respectively; c) and d) are the decay curve and growth curve of single grain pIRIR290 signal, respectively; e) MAR-MET-pIRIR decay curves; f) typical growth curves of MAR-MET-pIRIR290 signals. Black line\squares show the growth curve with a test dose of 104 Gy. Red line and triangles show the growth curve with a test dose of 415 Gy.
The inheritance of thermally transferred signals to the signals of the test dose is always concerned in the pIRIR measurements, which may lead to inappropriate sensitivity correction (Nian
The frequency distribution of thermal transfer ratios for single grain pIRIR225 and pIRIR290 signals with different test doses. a) and b) pIRIR225 signal with a test dose of 31Gy and 93 Gy, respectively; c) and d) pIRIR290 signal with a test dose 31 Gy and 93 Gy, respectively.
The dependence of thermal transfer ratios on the test dose. The thermal transfer ratio is evaluated with a test dose of ∼103 Gy, ∼415 Gy and ∼622 Gy for MAR-MET-pIRIR signals, respectively.
With the preliminarily chosen test dose, the single grain pIRIR and MAR-MET-pIRIR procedures shown in
The dose recovery test is a way to investigate whether the initial sensitivity change is significant for employing IRSL, pIRIR and MET-pIRIR signals from single aliquot for De determination (Wallinga
The abanico plots of dose recovery results with a given dose of ∼556 Gy and test dose ∼93 Gy for single grain pIRIR225 and single grain pIRIR290 procedures. The solid line show the range between 0.9 and 1.1, while the dashed line show the range between 0.8 and 1.2.
The single grain pIRIR procedures were applied to four hundred K-feldspar grains for both signals, and the data screening criteria was the same as those adopted for dose recovery test. 170 and 116 out of 400 grains were accepted for analysis for pIRIR225 and pIRIR290 signal, respectively, and the corresponding De distributions are shown by Abanico plots in
The abanico plot of De distribution for single grain pIRIR225 and single grain pIRIR290 procedures with a test dose of 93 Gy. MAM De was calculated with sigmab of 0.13 and 0.36 for pIRIR225 and pIRIR290 signals, respectively.
The statistics of De values.
SG pIRIR225 | 93 | 269±12 | 445±13 | 468 | 456 | 36±2 | 170/400 |
SG pIRIR290 | 93 | 425±43 | 476±19 | 500 | 496 | 38±3 | 116/400 |
MAR-pIRIR2902 | 104 | 762±40 | 872±21 | 888 | 886 | 15±2 | 59/60 |
MAR-pIRIR2902 | 415 | 811±44 | 977±27 | 968 | 1007 | 17±2 | 56/58 |
Fifty-six 1 mm aliquots were measured with the MAR-MET-pIRIR procedure shown in
Kernel density estimate of MAR-MET-pIRIR290 De values. De distributions measured with test dose of ∼415 Gy and ∼104 Gy are shown in a) and b), respectively.
Dependence of MAR-MET-pIRIR De values on stimulation temperature. a) with a test dose of 415 Gy; b) with a test dose of 104 Gy.
MAR-MET-pIRIR De values determined with a test dose of 104 Gy are compared.
Although the K-feldspar pIRIR or MET-pIRIR signals are of potential to date the water-lain sediments beyond late-Pleistocene, challenges arise both internally and externally, which include (1) the inheritance of pIRIR signals due to thermal transfer effect (Nian
The effect of thermally transferred pIRIR and MET-pIRIR signals were suppressed by increasing the size of the test dose. The test dose was optimised to be ∼20% to ∼40% of the CAM De for the single grain pIRIR and MAR-MET-pIRIR procedures, respectively, which made the contribution of thermally transferred signals to the corresponding pIRIR and MET-pIRIR signals to be less than 15% (
Since the sample is water-lain, the potential incomplete bleaching evidenced by large OD of single grain pIRIR De value necessitates the implementation of the MAM. The rationale for implementing the MAM is that the scatter in single grain De values is mainly due to different bleaching extents of pIRIR signal of K-feldspar grains before deposition. However, the anomalous fading also varies among grains, and it is difficult to measure the fading rate on individual grain level. Therefore, there is a potential risk of biasing to grains with more fading rather than sufficient bleaching when implementing the MAM. As an alternative, the pIRIR sensitivity is a possible index for anomalous fading of K-feldspar grains (Smedley
Dependence of K-feldspar single grain De value on the pIRIR sensitivity (response to a test dose of 93 Gy) a) pIRIR225 and c) pIRIR290 signals. The solid line is linear fit and the dashed line is the 95% confidence band. The dependence of CAM De and OD on the lowest pIRIR sensitivity employed to screen the K-feldspar for analysis are shown in b) and d).
The De distributions determined by different procedures are shown in
Comparison of De values obtained by the single grain pIRIR procedures and MAR-MET-pIRIR290 procedures. Various statistical models were applied to the measured De values. The D0 and 2·D0 values are shown for MAR-MET-pIRIR290 procedures with test doses of 104 Gy and 415 Gy, respectively. The shaded region shows the whole range of De values measured with different procedures.
The characteristic saturation dose (Do) is of concern when an old sample is dated. The Do value of the MET-MAR-pIRIR290 signals are 644 ± 76 Gy and 603 ± 41 Gy with the test dose of 415 Gy and 104 Gy, respectively (
With consideration of various uncertainties associated with different measurement procedures, the MAM De of 811 ± 44 Gy determined by using the MAR-MET-pIRIR290 procedure with the test dose of 415 Gy is the most reliable for the fluvial silt of this study. The concentration of uranium, radium, thorium and potassium measured by using the gamma spectrometry equipped with Ge detector is 16.25 ± 4.62 Bq/kg, 9.90 ± 0.30 Bq/kg, 19.32 ± 0.30 Bq/kg and 593.33 ± 10.01 Bq/kg. The internal potassium content and water content are estimated to be 12.5 ± 0.5% and 10 ±5%, respectively. The resultant dose rate for the 125–180 μm K-feldspar grains is 2.85 ± 0.85 Gy/ka, and the burial age determined for this fluvial silt sample is 284 ± 86 ka. This age is much younger than the early Pleistocene epoch assigned to the fluvial strata before (Deng
In this study, multi-methods luminescence dating was applied to a fluvial silt sample taken from the sandy bed of the basal deformed fluvial strata at the eastern tip of Anjihai anticline on the northern piedmont of Tian Shan. The single grain pIRIR and MAR-MET-pIRIR procedures were optimised with consideration of the methodological uncertainties such as thermal transfer and initial sensitivity change. The central age model and minimum age model were applied to the De distribution determined by the single grain pIRIR225, single grain pIRIR290 and MAR-MET-pIRIR290 signals. The MAM De values are 11%–17% lower than the CAM De values except for the single grain pIRIR225 procedure. The MAM De values determined by the single grain pIRIR procedures are underestimated by more than 40% when compared with the MAM De of MAR-MET-pIRIR290 procedure, which might be attributed to the uncorrected fading of single grain pIRIR measurements. The MAR-MET-pIRIR procedure, which suffers less from the initial sensitivity change and enables the stable signal to be isolated, gives the more reliable De value for the sandy fluvial sample of this study. The MAM MAR-MET-pIRIR290 De of 811 ± 44 Gy results in a burial age of 284 ka, which is much younger than that inferred previously and may imply a more active folding of the eastern tip of Anjihai anticline.