Tectonics and climate are the two competitive factors sculpturing landforms. Observations on the Earth surface are affected by signals from both tectonic and climatic agents. How to clarify these signals is a key issue. We categorize factors affecting mountain growth as horizontal (extension, compression, and transpression) and vertical (mantle upwelling and climate change) forces to evaluate the driving forces of accelerated exhumation in Taibai Mountain. Based on apatite (U-Th)/He thermochronology, we document two stages of accelerated exhumation at ca. 52-46 Ma and ca. 24-19 Ma from the age-elevation relationship, confirmed by 1D half-space modeling and QTQt inverse modeling. In the framework of paleostress, the two accelerated exhumation events occurred during transpressional periods in the early Eocene and late Oligocene-early Miocene in East China. These two events were triggered by the localized contractional deformation at the intersection of the North Qinling and Fengxian-Taibai faults: The opposite-direction shearing of these two faults was responsible for the former event with an unroofing magnitude of ~1 km; the same-direction shearing of the two faults resulted in the latter event with an unroofing magnitude of ~0.6 km. The far-field effects of both India-Eurasia collision and Pacific subduction drove the accelerated exhumation at ca. 52-46 Ma. The lateral extrusion of the Tibetan Plateau acted as the main driving force for the accelerated exhumation at ca. 24-19 Ma, which may have been slightly influenced by the intensified Asian summer monsoon in the early Miocene.

As some of the most spectacular features on the Earth surface, mountain ranges can serve as boundaries for climates, fauna, flora, hydrological catchments, tectonic plates, and even human civilizations. How and when mountains grow has been a key issue in the geological community for decades (e.g., [110]). What mountain growth actually reflects is the vertical movement of the crust, which can be interpreted from low-temperature thermochronology in light of the fact that the exposed materials illustrate the passage through a certain closure isotherm or thermal window.

The driving forces for mountain growth can be classified as tectonic and climatic (e.g., [4, 11]). However, the two effects have been considered a “chicken and egg” conundrum [4, 9, 12, 13], which complicates the deciphering of mountain growth. To simplify this issue, we categorize the factors triggering the mountain growth as horizontal and vertical forces so that not only spatiotemporal coupling with mountain growth but also relevant geological clues can be distinguished and elucidated. The horizontal forces encompass extensional, compressional, and transpressional deformation, and the driving forces manipulating the topography stem from intraplate or interplate horizontal movements. In an extensional setting, the coupling of footwall uplift and hanging wall subsidence develops in a horst and graben structure, exposing deep materials in the footwall (Figure 1(a); e.g., West US: [1416]). In a compressional context, thrust faulting pushes the hanging wall on top of the footwall, resulting in the exposure of deep materials in the hanging wall (Figure 1(b); e.g., [7, 17, 18]). Although strike-slip faulting generally results in lateral motions of fault blocks rather than vertical motions, some specific geometries along strike-slip faults, such as fault intersections, restraining bends, contractional oversteps, and terminations, tend to localize contraction and uplift (Figure 1(c); [1922]). Therefore, the transpression can also generate mountain growth, which has been documented in several strike-slip faults, such as the Denali Fault [23, 24], East Kunlun Fault [20, 22], and San Andreas Fault [19, 21, 2527]. The vertical forces seem complex, since both deep (mantle upwelling) and surface (climate) dynamics can produce what we see as positive relief. Mantle upwelling can push the lithosphere and bend it towards the surface, resulting in positive dynamic topography (Figure 1(d); [28, 29]), for instance, a mantle plume (Afar mantle plume: [30]), asthenospheric upwelling (western USA: [31]), and a subducted slab window (northern Antarctic Peninsula: [32]). The climate effect creates accelerated erosion induced by precipitation (e.g., [3335]) or glaciation (e.g., [3638]), resulting in the vertical motion through the adjustment of isostasy (Figure 1(e); [4, 39]). The horizontal and vertical forces cover most of the possible mechanisms for the mountain growth.

Taibai Mountain, the peak of the Qinling Mountains (3767 m above sea level), is an ideal place to evaluate almost all the horizontal and vertical forces. This exhumed granitoid batholith has acquired high relief, preserving a continuous record of exhumation in thermochronology, and the neighboring Weihe Basin records information about paleostress evolution, which can be used to evaluate the horizontal and vertical forces (Figures 1, 2 & 3; [4042]). Regionally, the loess-paleosol sequences in the Loess Plateau preserve valuable climate proxies (Figure 2(a)), from which information such as precipitation can constrain the erosion intensity. Therefore, we can use the exhumation combined with background information at Taibai Mountain as an example to investigate the mechanisms of mountain growth. Previous works found scattered Cenozoic apatite (U-Th)/He and fission track ages, which were interpreted as accelerated exhumation events at ~50 Ma, ~22 Ma, and~8 Ma triggered by the far-field effect of the India-Eurasia collision [43, 44] or the Pacific subduction [43]. Due to limited sample size [43], previous works on apatite (U-Th)/He thermochronology (AHe) were not as detailed as that on apatite fission track thermochronology (AFT) [43, 44]. Compared with AFT (closure temperature of ~110 °C; [45]), AHe has a lower closure temperature of ~70 °C [46, 47], which can record lower magnitude exhumation and cooling. Thus, detailed AHe work provides new constraints on the exhumation process in the Cenozoic. During the prolonged exhumation process, specific events may change the exhumation pattern. Two events have the potential to disturb the slow steady exhumation process in this region. First, the Weihe Basin experienced two time intervals with transpressional stress fields in the early Eocene and early Miocene [42]. The two transpressional events deviated from the long-lasting extensional setting [42], modifying the style of crustal deformation in the Weihe Basin and the neighboring Qinling Mountains. Second, in the Cenozoic, a large portion of East Asia experienced at least three episodes of climate change expressed as intensified summer monsoon or precipitation during 40-34 Ma [48], 23.5-18.5 Ma [49, 50], and 9.5-5.0 Ma [50]. The former two periods are related to the onset of the Asian monsoon as the climate transited from a prevailing arid/semi-arid climate, although this onset time remains controversial [48, 49, 51, 52]. We can evaluate the influence of intensified summer monsoon during these three periods on the exhumation process inferred from apatite (U-Th)/He thermochronology.

In this paper, we interpret the exhumation process by using AHe age-elevation profile combined with 1D eroding half-space and QTQT inverse modeling and find two stages of accelerated exhumation. Building upon previous paleostress and paleoclimate works, we decipher the mechanisms for accelerated exhumation by evaluating the horizontal and vertical forces. In this framework, we propose a model for how the exhumation process of an intraplate mountain responds to the tectonic activities due to plate convergence, such as the India-Eurasia collision, Pacific subduction, and eastward extrusion of the Tibetan Plateau.

Located on the north flank of the Qinling Mountains (Figure 2), Taibai Mountain, the peak of Qinling Mountains (3767 m above sea level, with a relief of ~3 km), is an exposed granitoid batholith composed of three plutons with ages of 431±2 Ma, 214±2Ma, and 130±1 Ma by zircon U-Pb dating [53]. We consider this batholith as a proxy for Cenozoic exhumation of the middle section of the Qinling Mountains, which is part of the Qinling-Dabie orogenic belt formed by the collision of North China and Yangtze Cratons in the Triassic (Figure 2(a); [5456]). From north to south, the Shangdan suture zone and Mianlue suture zone, separate the orogen into a northern part consisting of the North China Craton (North Qinling), a middle part consisting of the Qinling Micro-plate, and a southern part consisting of the northern part of Yangtze Craton (Figure 2(a); [56, 57]). The Shangdan suture zone resulted from the collision between the North China Craton and Qinling Micro-plate in the Devonian [56, 57]. The Mianlue suture zones resulted from the collision between the Qinling Micro-plate and Yangtze Craton in the Triassic, representing the final amalgamation of the North China and Yangtze cratons [56, 57]. After the Triassic, this orogenic belt has undergone a post-collisional mountain building process characterized by large-scale deformation and pluton intrusion into the Archean crystalline basement, Precambrian metamorphic rocks, and Paleozoic-Mesozoic sedimentary rocks [57, 58]. Cenozoic sediments occur in intermountain basins and neighboring grabens (Figure 2(a)).

Situated ~200 km east to the northeast margin of the Tibetan Plateau, the exhumation of Taibai Mountain was probably influenced by the far-field effect of India-Eurasia collision, resulting in the uplift of the Tibetan Plateau, which experienced intraplate thickening and thrusting, and material extrusion to the east and west in the Cenozoic (Figure 2(a); [59, 60]). The northeastern margin of the Tibetan Plateau has undergone a history of compressional exhumation from the interior to the margin, where significant rapid exhumation occurred at ~50 Ma, ~23 Ma, ~13 Ma, and ~8 Ma (e.g., [17, 18, 22, 6167]). Triggered by the uplift and extrusion of the Tibetan Plateau, the Ordos Block (a subunit of the North China Craton) underwent counterclockwise rotation and formed basins on its margins [68], including the Weihe Basin, whose subsidence initiated in the Eocene [42]. Bounded by the North Qinling Fault, the Qinling Mountains and Weihe Basin constitute a horst and graben structure (Figures 2 and 3). The basin experienced subsidence and sedimentation with lacustrine and alluvial deposits covered by loess-paleosol sequences (Figure 3; State Seismological [69]) in response to the uplift of the Qinling Mountains in the Cenozoic.

As the master fault between the Qinling Mountains and the Weihe Basin (Figure 2), dipping 60-80°N, the North Qinling Fault has experienced several episodes of faulting in the Cenozoic [42]. They are as follows: right-lateral faulting during ca. 52-46 Ma, normal faulting during ca. 46-24 Ma, left-lateral faulting during ca. 24-15 Ma, normal faulting during ca. 15-10 Ma, and normal faulting with left-lateral slip component during ca. 3.5 Ma to the present [42]. North of the North Qinling Fault, at present, the Yuxia-Tieluzi Fault is also active as a sinistral strike-slip fault with a horizontal slip rate of 1.25-3.75 mm/yr extending through the East Qinling Mountains (Figure 2(a); [70]). Bounded by the Longxian-Mazhao Fault on the east and the North Qinling Fault on the south (Figure 2(b)), the Baoji Depression was once integrated with Taibai Mountain, or the Qinling Mountains, and experienced uplift and erosion. Based on the age of the Bahe Formation (11-7 Ma: [71]), this integration ended in the late Miocene, when the Bahe Formation started being deposited directly on the Precambrian crystalline rocks of the Baoji Depression, and this depression was then integrated with the Weihe Basin (Figures 2(b) and 3(a); [72, 73]). The integration of the basement of the Baoji Depression with the Qinling Mountains suggests that the segment of North Qinling Fault separating the Baoji Depression and Qinling Mountains did not generate large relief between the two areas. A peneplain (Figure 2(b); Paomaliang) at elevations of 3300-3500 m located west of the peak [74] developed during this integration, which ended in the late Miocene (11-7 Ma: [71]). After subsidence of the Baoji Depression, the preserved peneplain was tilted 7° to the south [74]. Connecting the North Qinling and East Kunlun faults (Figure 2(a)), the Fengxian-Taibai Fault on the western side of Taibai Mountain initiated after the early Mesozoic [75, 76]. In the Cenozoic, the Fengxian-Taibai Fault has mainly acted as a sinistral strike-slip fault (Figure 2; [75, 7779]). Its Cenozoic kinematics did not always reflect the paleostress field in East China, because the Fengxian-Taibai Fault mainly accommodated the shortening deformation in the northeast Tibetan Plateau [75, 7779], while Cenozoic tectonics of East China was impacted by the Tibetan-Plateau extrusion and/or Pacific subduction (e.g., [42, 68, 78]).

The Qinling Mountains experienced erosion and acted as the sediment provenance for the Weihe Basin (Figure 3(a)) providing important clues for the exhumation of the neighboring mountains. For example, the initiation of sediments is a proxy for the onset of normal faulting, and the sedimentary facies provide depositional environment and climatic information. The Weihe Basin holds ~6.5-km-thick sediments underlain by Precambrian crystalline basement. A stratigraphic column can be compiled from outcrops in the southern basin (Figure 3(b)). The strata from Eocene to Miocene are fluvial-lacustrine sediments. In the Quaternary, loess-paleosol sequences act as the dominant strata. The Lantian Formation (late Miocene to Pliocene) holds red clay. Currently, the oldest magnetostratigraphy ages are reported to be 11-7 Ma in the Bahe Formation (Figure 3(b); [71]). The earlier formations lack absolute age constraints. Mammal fossils are one of the best choices to constrain continental sedimentary ages in the Cenozoic due to the relatively rapid evolution and adaptive radiation of mammals. At the base of the sedimentary sequence, the Honghe Formation was deposited in the late Eocene, no earlier than 47.6 Ma (Figure 3(b)), as indicated by the first appearance of Deperetella sp. in this formation [80], whose age was well constrained by magnetostratigraphy in the Irdinmanha Formation of the Erlian Basin [81]. During the late Oligocene, there was a hiatus between the Bailuyuan Formation and the Lengshuigou Formation. The Lengshuigou Formation has a larger area of alluvial and lacustrine facies than the Bailuyuan Formation [82], implying an intensified water supply in the early Miocene. The onset of deposition of the Lengshuigou Formation is inferred to have occurred during 23.0-18.2 Ma (Figure 3(b)), according to the coappearances of Palaeomeryx sp. and Stephanocemas sp. in this formation [80] and in the Chetougou Formation of the Xining Basin [83]. The latter has an age of 23.0-18.2 Ma constrained by magnetostratigraphy [83].

The Cenozoic paleostress interpreted from fault analysis in the Qinling-Weihe region went through six stages [42]. They are as follows: (1) Late Cretaceous to Paleocene WNW-ESE extension (ca. 83-52 Ma); (2) early Eocene transpression with NNE–SSW contraction (ca. 52-45 Ma); (3) middle Eocene to early Oligocene NE-SW extension (ca. 45-24 Ma); (4) transpression with WNW–ESE contraction from late Oligocene to early Miocene (ca. 24-15 Ma); (5) late Miocene NE–SW extension (ca. 15-9 Ma); and (6) NNW–SSE extension from late Pliocene to Quaternary, and there are no structural data in the interval of ca. 9 Ma-3.5 Ma. The Cenozoic strata classified by mammal fauna and pollen flora in the Weihe Basin lack absolute dates before 11 Ma [71]. Dating the tectonic events in the paleostress evolution is based on the strata influenced by the fault sets and the geochronological data that constrain the important events in the tectonic evolution of East China [42]. We consider these dates acceptable, since the paleostress evolution or tectonic events were regionally continuous and comparable in the Weihe Basin, Hefei Basin, and even Bohai Basin in East China [42].

Exhumation processes can be recorded by AHe thermochronometry, whose closure temperature is ~70 °C, corresponding to depths of 2-3 km in the crust [46, 47, 84]. In a monotonic cooling pattern, AHe thermochronology has the ability to record several cooling events through its closure temperature (~70 °C) by using an age-elevation profile [15].

To obtain a continuous age-elevation profile, we collected 19 samples of fresh granitoid along a transect from the top of Taibai Mountain at 3710 m a.s.l. (above sea level) to its north foot at 756 m a.s.l., with sampling intervals of 200-300 m in elevation and smaller gaps at low elevations (Figures 2(b) and 3(a)). We collected the samples from two separate vertical transects ~10 km apart: The transect from Tangyu consists of 15 samples, and the other transect from Honghegu consists of 4 samples (Figure 2(b)). The lithologies at 1100-800 m a.s.l. in the Tangyu transect appear to be metasedimentary rocks, and do not host apatite. From the Honghegu section, we collected three samples at 1119 m, 992 m, and 905 m a.s.l. as supplements to the former section and another one higher at 1852 m a.s.l. to evaluate possible east-west tilting.

The analyses for AHe dating were performed at the State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences (details in the supporting information (available here)).

We report raw dates and corrected dates for individual grains (Table S1). Our AHe dates (corrected date hereafter) share similar minimum dates with those reported in previous works (147Sm analyzed; [43]) in the age-elevation relationship (Figure S1). This similarity implies negligible contribution of 147Sm-generated 4He in our dataset. The two previous AFT datasets show discrepancies, with age differences as large as ~20 Myr at similar elevations (Figure S1; [43, 44]). Evaluating these discrepancies is beyond the scope of this paper. We do not consider the two AFT works further.

In our dataset, there is date dispersion in some samples (Table S1; Figure S1). AHe date dispersion is a relatively common phenomenon (e.g., [17, 8588]), which can be caused by several factors, such as fracture, inclusion, incomplete grains, 4He implantation, radiation damage, grain size, and U-Th zonation.

We find some imperfect grains with anomalous dates (summarized in Figure S1). By visual inspection under microscope (Figure S2), we reject an incomplete grain (14TB-32-G3), grains with fractures (14TB-7-G1, 14TB-15-G4), and grains with visible inclusions (14TB-2-G1, 14TB-10-G4, 14TB-11-G3, 14TB-17-G3, 14TB-19-G4, 14TB-25-G4, 14TB-31-G1, 14TB-31-G5, and 14TB-33-G5). Fracturing releases bound helium, resulting in a younger date than the true value [89]. Inclusions introduce parentless helium into the analysis [46]; incomplete grains make the Ft factor smaller due to smaller or no rims for helium loss near the fracture surface [89]. These two factors lead to anomalously high date values. After rejecting the grains influenced by these factors, we still find date dispersions. Grubbs’ test [90, 91], a statistical method for outlier detection, was introduced to evaluate the reliability of individual dates and reject the remaining outliers. Outlier detection has been applied in previous studies of (U-Th)/He thermochronology to obtain reliable dating results (e.g., [85, 86, 92]). Applicable to small datasets, Grubbs’ test is suitable for detecting outliers from datasets following the normal distribution ([90, 91]; details in supporting information (available here)), which is usually followed by dating results with reasonable precision [93, 94]. We reject outliers detected by Grubbs’s test, i.e., 14TB-1-G1, 14TB-3-G1, 14TB-4-G1, 14TB-8-G4, 14TB-11-G4, 14TB-15-G2, 14TB-19-G3, and 14TB-31-G2. These rejected grains have dates too large to be included in the samples, possibly due to effects of invisible inclusions, radiation damage, U-Th zonation, and 4He implantation. Although U-Th zonation and 4He implantation are hard to evaluate in our dataset due to the lack of direct observations, the two factors do have possible impacts on the AHe dates, since the potential U-Th zonation in apatite crystals can deviate the calculation of Ft correction [87, 95], and the adherent high-U-Th phases, such as zircons, can implant extra 4He into apatite grains [96].

After rejecting the outliers by the methods above, some samples exhibit positive date-eU correlations, i.e., 14TB-1, 14TB-2, 14TB-10, 14TB-12, 14TB-14, 14TB-17, 14TB-19, 14TB-25, and 14TB-33 (Figure S3), indicating the effect of radiation damage on (U-Th)/He dates. Samples 14TB-17 and 14TB-19 have gentle slopes, and the smallest ranges of age and eU, respectively. The two samples have an age difference of ~2 Myr within an elevation interval of ~300 m. It is hard for radiation damage to influence the individual dates profoundly in ~2 Myr, suggesting a combination of other factors. For the other samples, radiation damage does affect the dates. The effective U concentration (eU = [U] +0.235[Th]) is the parameter of radiation damage [97], which accumulates in the crystal lattice due to the kinetic energy of heavy nuclides acquired during U-series decay [98]. The accumulated radiation damage acts as barrier for the diffusion of 4He, resulting in higher date values [87, 97]. This phenomenon can be amplified in the patterns of reheating and long residence in the partial retention zone [99]. The long residence pattern is responsible for the correlations in light of the fact that Taibai Mountain has been in a mode of monotonic cooling since the Cretaceous (Heberer et al. [43]).

We do not observe any clear positive correlation in the date-grain size relationship, suggesting that variation in the diffusion domain [100] is not the primary factor influencing date dispersion. Zonation in eU can also disperse the individual dates, in that the oscillation of eU in the rim generates a large error for the Ft correction, when eU in the rim is different from the bulk eU in the grain [95]. Meanwhile, the eU zonation endows an apatite crystal with heterogeneous diffusion domains and spatially variable radiation damage, which is hard to determine precisely [87, 95]. The date dispersions in most samples may be caused by undetected 4He implantation, invisible micro-inclusions, eU zonation, and radiation damage. The U concentrations of most grains are greater than 5 ppm, implying that the impact of 147Sm concentration on dates can be neglected [101]. Sample 14TB-22 was probably influenced by this effect, since three grains have U concentrations less than 5 ppm.

In our dataset, only two dates are anomalously young, while the other eighteen dates are anomalously old, suggesting that the AHe dates are most likely positively skewed (Figure S1). Among the factors responsible for date dispersion, crystal fracturing results in the anomalously young dates due to the release of bound helium from the crystal; U-Th zonation can both contribute to the anomalously young and old dates; the other factors, including inclusions, 4He implantation, incomplete grains, and radiation damage, lead to the anomalously old AHe dates. To summarize, more factors can contribute to anomalously old dates, resulting in the relatively large quantity of the anomalously old dates expressed by the positive skew of AHe dataset (Figure S1).

After data evaluation, we report the arithmetic mean and standard deviation (Table S1). At least 3 individual grains contribute to the age calculation for each sample. We consider a standard deviation smaller than ~20% of the mean to be reliable. Most samples meet this criterion. Sample 14TB-1 has an age of 69.8±14.1Ma, and this error is 20% of the mean. This result is reasonable because of the date-eU correlation. Sample 14TB-22 has an age of 4.7±2.0 Ma, whose error is too large; meanwhile, the individual grains do not show obvious date-eU correlation. We do not consider this sample further.

The sample ages decrease from 69.8±14.1 Ma at the peak to 17.8±5.4 Ma at the base (Table S1), indicating monotonic cooling in this time interval. Sample 14TB-25 from the Honghegu transect is located on the best-fit line defined by samples from the Tangyu transect, suggesting that possible west-east tilting is negligible. Thus, we stack the Tangyu and Honghegu profiles to obtain a single continuous age-elevation profile (Figure 4).

The age-elevation relationship of low-temperature thermochronology is one of the most straightforward methods for interpreting exhumation (e.g., [17, 102, 103]). Linear-regressed time intervals between or separated by inflection points share common exhumation rates. This method implicitly assumes a steady state for the thermal structure [6]. However, as erosion and tectonic activities continue throughout the exhumation of mountains, thermal pulses may disturb the presumed steady state, that is, a high erosion rate would correspond to a high geothermal gradient (e.g., [104]). In addition, the closure temperature varies with cooling rate and is amplified in low-temperature thermochronometry [105]. Thus, after the interpretation of the age-elevation profile, it is necessary to apply thermal modeling that considers a varying thermal structure to confirm the proposed exhumation process. The 1D eroding half-space modeling helps address this issue by plotting age functions in the space of erosion rate and geothermal gradient [104]. The reliability of modeling results can be confirmed by the converging of age functions in an area or a point in the rate-gradient plot for each stage and the pulse of geothermal gradient during the relatively rapid stage [104]. The exhumation process interpreted from low-temperature thermochronology corresponds to the variation in cooling path of the thermochronometer [6]. To find the best-fit cooling history from AHe age-elevation relationship, we introduce QTQt inverse modeling [106], which helps corroborate the stages interpreted from age-elevation relationship and 1D half-space modeling. Although geothermal gradient increases with increased erosion rate (e.g., [104]), the geothermal gradient with a reasonable uncertainty used in QTQt modeling can cover this deviation. We consider modeling results reliable when the acceptance rates of time, temperature, and offset are all in the range of 0.1-0.7. To obtain a convincing interpretation of the exhumation process, we apply the age-elevation profile, half-space thermal modeling and QTQt inverse modeling.

5.1. Age-Elevation Relationship

Most samples are located within a horizontal distance shorter than 10 km (Figure 2(b)), implying negligible effects of relief change on the age-elevation relationship [107]. In the age-elevation plot, we classify the exhumation process of Taibai Mountain into five stages based on the variations in slope (Figure 4). They are ca. 70-51 Ma, ca. 51-47 Ma, ca. 47-24 Ma, ca. 24-20 Ma, and ca. 19 Ma to the present, corresponding to slow, increased, slow, increased, and slow exhumation rates, respectively.

The two stages from ca. 52-46 Ma to ca. 24-19 Ma yield exhumation rates of 0.20±0.09 km/Myr and 0.22±0.13km/Myr, respectively (Figure 4). We consider them interruptions of the relatively slow exhumation process, and the other three slow stages are exhumed partial retention zones. Although the planation on the peak is tilted 7° to the south, its influence on the exhumation rate is negligible.

As mentioned above, samples 14TB-1 and 14TB-2 suffer from radiation damage. They are the lower remnants of the first exhumed partial retention zone. The slow exhumation stage of ca. 46-24 Ma produces a relief of ~1 km and a retention time of ~23 Myr. Based on the loss-production model [84], the temperatures of 90% and 10% 4He remaining in the crystal are calculated by the CLOSURE software [108] to be 67 °C and 43 °C, respectively. The elevations at ca. 46 Ma and ca. 24 Ma act as the sites of the 90% and 10% retentions, respectively. Thus, we obtain a geothermal gradient of ~24 °C/km, a typical gradient in orogenic belts [109], suggesting the validity of this partial retention zone. The last stage has only one usable sample, 14TB-33. Both the time-of-flight rate (~0.07 km/Myr) and the slope regressed with 14TB-32 (0.05±0.06km/Myr) show low exhumation rates (Figure 4), consistent with long residence in the partial retention zone, indicated by a positive date-eU relationship. The time-of-flight rate after ~17.8 Ma shows a slow-rate stage (Figure 4), though some cooling events may exist.

5.2. Thermal Modeling

5.2.1. 1D Half-Space Modeling

We use 1D eroding half-space modeling to corroborate the exhumation stages interpreted from the age-elevation profile. The 1D eroding half-space model converts cooling ages to erosion rate by plotting ages as a function of erosion rate and geothermal gradient [104]. Given the cooling-rate dependence of closure temperature, this method captures the transient variation in geothermal gradient with erosion rate in rate-gradient plot through the convergence or crossing of age function lines. This convergent area or crossing point expresses the most plausible erosion rate for a specific exhumation stage.

Parameters for modeling in the MATLAB script provided by Willett & Brandon [104] include the following. We estimate the mean elevation of Taibai Mountain as 2238 m, an average between the peak at 3767 m and the foot at 709 m. The samples’ elevations are normalized to this mean elevation (Figure 4). The surface temperature at the mean elevation is estimated to be 5 °C, according to a mean annual temperature of 6.4 °C at ~2000 m [110] and a lapse rate of air temperature of ~6 °C/km. The modern geothermal gradient is set as 20-30 °C/km based on thermal structure results of the neighboring Weihe Basin [111]. The thermal parameters of AHe thermochronometry include closure temperature of 67 °C at 10 °C/Myr, frequency factor (Ω) of 7.64×107s1 and equivalent activation energy (Ea) of 138 kJ/mol [112]. Based on the interpretation of the age-elevation profile, and to include the inflection point in the calculation, we set the original onset ages of each stage as 70 Ma, 52 Ma, 46 Ma, 24 Ma, and 19 Ma (Figure 5), the latter four of which are based on the break-in-slope ages of the age-elevation relationship. As the first stage has no constraints on this parameter, we set it as 70 Ma, larger than ~69 Ma of 14TB-1. This age difference has little impact on the estimated erosion rate. To get the erosion rate of a specific time interval, sample ages and onset age of each stage are reduced by the onset time of the next stage. Moreover, the elevations relative to the mean elevation are subtracted by the accumulated erosion of later stages through multiplying the mean value of erosion rate by the duration of each stage. Thus, we perform modeling from the last stage, and then to the older stage one by one. Except for the last stage, this incremental strategy overcomes the limits of a time-averaged erosion rate to the present solely by dividing thermochronologic age by its distance to closure isotherm. We do not consider the outlier (14TB-3) interpreted in the age-elevation profile because of negative values calculated for input, which are not appropriate in the modeling. These parameters for the modeling are provided in Table S2 following the recommendation from Flowers et al. [113].

From old to young, the five stages of modeling have erosion rates of 0.01-0.02 km/Myr, 0.13-0.41 km/Myr, 0.02-0.05 km/Myr, 0.1-0.3 km/Myr, and 0.02-0.06 km/Myr (Figure 5). The age ranges include the errors of erosion rates, which are tracked through the convergent area bounded by geothermal gradients. The modeling results exhibit obvious pulses in erosion rates during the two episodes at ca. 52-46 Ma and ca. 24-19 Ma (Figure 5), whose rates are one order of magnitude higher than those of the other three episodes (Figure 5). This variation in erosion rates between stages resembles and confirms the stage interpretation from the age-elevation profile (Figure 4). In addition, the two high-rate pulses acquire higher transient geothermal gradients than the neighboring stages (Figure 5), implying the capture of isotherm compression due to accelerated erosion. Stage 3 captures a transient geothermal gradient of 20.0-22.5 °C/km, smaller than ~24 °C/km interpreted from the age-elevation profile. This result is reasonable, as this small gap may be propagated from the selected parameters and the age uncertainty involved in the calculation.

5.2.2. QTQt Inverse Modeling

QTQt inverse thermal modeling simulates cooling ages and diffusion parameters of low-temperature thermochronology to find the best-fit thermal history by using the Markov chain Monte Carlo method [106]. Here, the single-grain AHe dates with 1σ uncertainty, sample elevation, U-Th concentrations, and diffusion domain size (equivalent sphere radius) are input for inverse modeling. We use the apatite radiation damage accumulation and annealing model (RDAAM; [99]) for the simulation. We do not input the bottom sample (14TB-22), since this sample has an age of 4.7±2.2Ma, which is probably affected by the unanalyzed 147Sm. 14TB-25 shares sample age and uncertainty with 14TB-12 (Table S1), and the two samples only have an elevation difference of ~100 m. Thus, we select 14TB-12, and do not use 14TB-25 for the modeling. 14TB-19 and 14TB-31 have the same issue. We choose 14TB-19 for the modeling. 14TB-3 is not included because of its obvious deviation from the regression line (Figure 4). These mentioned samples are excluded to get reliable modeling results. We input 115±5 Ma and 200±20°C as the initial constraints, according to K-feldspar ages (Tc=200±20°C) of ca. 117-111 Ma [43]. The atmospheric lapse rate and present-day surface temperature are input as 6 °C/km and 5 °C, respectively [110]. The geothermal gradient is fixed at 25±15°C/km, since the 1D half-space modeling captures gradients varying around 25 °C/km during the Cenozoic (Figure 5), and the modern gradients measured in the neighboring Weihe Basin ranges between 20 and 30 °C [111]. Furthermore, the general form of thermal history changes little with geothermal gradients in the range of 20-40 °C/km (supplementary materials in [85] (available here)). Following the reporting protocol of Flowers et al. [113], these parameters required for modeling are presented in Table S3.

To get reasonable modeling results, we simulate the samples with two strategies (Figure 6). In Strategy 1, we simulate the samples from the top to the bottom with 50000 burn-in and 200000 post-burn-in iterations. The acceptance rates of time, temperature, and offset are all in the range of 0.1-0.7, indicating acceptable result. However, this modeling result shows only one episode of accelerated cooling at ca. 52-47 Ma (Figure 6(b)), different from the interpretation of age-elevation relationship and 1D half-space modeling with two accelerated cooling events at ca. 52-46 Ma and ca. 24-19 Ma (Figures 4 and 5). In Strategy 2, we do not use samples with elevations higher than 2517 m. The samples used for the simulation are from Stages 3, 4, and 5 and the bottom sample of Stage 2 (14TB-10). The burn-in and post-burn-in iterations are fixed as 35000 and 140000, respectively. The acceptance rates of 0.1-0.7 suggest acceptable results. This modeling constrains two accelerated cooling events at ca. 53-46 Ma and ca. 26-20 Ma (Figure 6(d)). We do not consider the cooling near 0 Ma (Figure 6(d)), because the curving up and converging of the cooling path toward 0 Ma was a result of the different values of atmospheric lapse rate (6 °C/km) and geothermal gradient (25 °C/km) in the modeling. Currently, all samples from the elevation transection are exposed at the surface, and their temperature difference depends on atmospheric lapse rate instead of geothermal gradient. The latter parameter acts the dependence for the temperature difference, when there are samples underneath the surface.

Why does Strategy 1 only get one accelerated cooling event at 52-47 Ma, and why can Strategy 2 can capture the accelerated cooling at ca. 25-20 Ma? The QTQt inverse thermal modeling translates elevation or depth into temperature according to their relationship with geothermal gradient [106]. Thus, in the monotonic cooling process, this method can capture one episode of accelerated cooling through a specific closure temperature or partial retention zone. However, our dataset includes two episodes of accelerated exhumation constrained by age-elevation relationship of AHe thermochronology (Figure 4). The accelerated stage at ca. 52-46 Ma from this relationship (Figure 4) shows ~1 km of unroofing, suggesting cooling through a temperature difference of ~25 °C, which is also the difference of the AHe partial retention zone (48-73 °C; Figure 6(b); [6, 108]). As such, the sensitive temperature threshold for AHe thermochronology has been totally occupied by the first accelerated stage, and there is no sensitive temperature interval for the second accelerated one. Therefore, Strategy 1 only captures one episode of accelerated cooling. In Strategy 2, 14TB-12 acts as the early/upper boundary of the slow stage, leading to the pulse at ca. 53-46 Ma in the modeling. Although the slow stage at ca. 46-24 Ma holds an unroofing magnitude of ~1 km (Figure 4), this does not impact the modeling, since we observe two accelerated cooling events (Figure 6(d)). The lower-elevation samples go through the AHe partial retention zone relatively rapidly at ca. 26-20 Ma (Figure 6(d)). This accelerated stage suggests ~0.6 km of denudation (Figure 4). This denudation magnitude is equivalent to a temperature difference of ~14 °C, smaller than the temperature difference of the partial retention zone. Thus, in Strategy 2, the AHe sensitive thermal window (48-73 °C; Figure 6(d); [6, 108]) was partially occupied by the accelerated stage at ca. 26-20 Ma, and there is sensitive temperature interval to record another accelerated cooling event. Consequently, two accelerated cooling events emerge here.

Both modeling results of Strategies 1 and 2 are reasonable. The first accelerated stages from both strategies overlap. We prefer ca. 52-46 Ma, which is constrained in detail from Strategy 1. The accelerated cooling events at ca. 52-46 Ma and ca. 26-20 Ma acquire cooling rates of ~10 °C/Myr and ~8 °C/Myr, respectively (Figure 6). These cooling rates are comparable to the exhumation rates generated from age-elevation relationship and 1D half-space modeling. The consistency of the three methods strongly supports our interpretations of accelerated exhumation stages in the early Eocene and early Miocene.

For reporting the exhumation stages, we prefer to use the inflection points interpreted in the age-elevation profile (ca. 52 Ma, ca. 46 Ma, ca. 24 Ma, and ca. 19 Ma), since they are simple, intuitive, and supported by thermal modeling. Moreover, the bounding ages or turning points from thermal modeling (Figures 4, 5, and 6) deviate from the inflection points by only ~1 Myr, less than 5% of the mean ages. Accounting for the ~20% standard deviation, the break-in-slope ages of the three systems are consistent. Therefore, the erosion and cooling rates from thermal modeling can be applied to stages interpreted from the age-elevation profile. In summary, from the age-elevation relationship, two stages of accelerated exhumation are depicted to be ca. 52-46 Ma and ca. 24-19 Ma, with the unroofing magnitudes of ~1 km and ~0.6 km, respectively. 1D-half-space modeling suggests exhumation rates of 0.13-0.41 km/Myr and 0.1-0.3 km/Myr, respectively. QTQt inverse modeling detects accelerated cooling during the two periods with cooling rates of ~10 °C/Myr and ~8 °C/Myr, respectively. The interpretations from the age-elevation relationship are assured by these two modeling methods.

Which mechanism triggered the accelerated exhumation? We evaluate the vertical and horizontal forces in light of the relevant clues, especially paleostress and paleoclimate.

6.1. Evaluation of the Vertical Forces

The vertical forces did not trigger the first accelerated exhumation (ca. 52-46 Ma) because of two facts. First, in most of China, there was no significant climate change in this period, when the prevailing arid/semiarid climate was dominated by the westerlies in most of East China [48, 49, 52]. Second, if mantle upwelling dominated the uplift, the orientation of the maximum principal axis would be vertical, and there would be regional uplift accompanied by exhumation similar to the Emeishan plume [114]. This is contrary to the horizontal orientations of maximum and minimum principal axes (σ1 and σ3) at this time (Figure 7; [42]). Therefore, we exclude the vertical forces for the first accelerated exhumation event. For the second accelerated exhumation at ca. 24-19 Ma, mantle upwelling under Taibai Mountain can also be excluded due to the horizontal orientations of σ1 and σ3 during this period (Figure 7; [42]). We evaluate the climate effect on the second accelerated exhumation in the next section.

6.2. Impact of Paleoclimate on the Exhumation

The impact of paleoclimate or climate change on the mountain growth mainly reflects isostatic rebound triggered by accelerated erosion [4] and can be recorded by low-temperature thermochronology (e.g., [9, 115117]). Temperate glaciation and precipitation are the two significant agents of climate influencing erosion or unroofing processes (e.g., [9, 3338, 118]).

6.2.1. Evaluation of Possible Glacier Erosion

Preserved glacial features on Taibai Mountain show the presence of Quaternary glaciers, which had disappeared by 15.1±1.0ka [119]. However, the temperatures in the Eocene and early Miocene were warmer with an arid/semi-arid climate before the Oligocene-Miocene transition and a continental monsoon climate since this transition [49, 52, 120, 121]. Because of high temperature and limited precipitation, it is unlikely that there were any glaciers on Taibai Mountain during the periods of accelerated exhumation at ca. 52-46 Ma and ca. 24-19 Ma. Thus, we exclude the glacier-induced erosion as the driving force for the two exhumation events.

6.2.2. Evaluation of the Accelerated Erosion Induced by Asian Summer Monsoon

The Qinling Mountains mark the southern limit of the Loess Plateau and a boundary between the northern temperate monsoon climate and the southern subtropical monsoon climate. The initiation of an Asian monsoon similar to that of today remains controversial. The deposits of red mudstone intercalated with gypsum beds in the Xining Basin and climate simulation suggest the existence of a monsoon climate in the Asian interior at 40-34 Ma driven by the coeval greenhouse conditions and a subsequent diminishment due to the global shift to icehouse conditions at ~34 Ma [48]. Another perspective favors the Oligocene-Miocene transition, when the climate changed from an arid/semiarid climate impacted by westerlies to a monsoon climate similar to the present climate over a large part of East Asia [49, 51]. This was inferred from climate proxies (loess-paleosol sequences, coalbeds, salt and gypsum deposits, and paleo-vegetation remnants) in the mainland China and the marine sediments in South China Sea [49, 51, 52, 122]. The onset of Asian monsoon in the Oligocene-Miocene transition was possibly forced by the growth of the Tibetan Plateau and the withdrawal of the Paratethys Sea in central Asia [49, 52, 122124]. Although the debate exists, three episodes at 40-34 Ma, 23.5-18.5 Ma, and 9.5-5 Ma can be interpreted as phases of intensified precipitation (Figure 7; [48, 125, 126]). Inferred from the red beds in the Xining Basin, the first event was regarded as the intensified rainfall in the monsoon-like pattern (Figure 7; [48]). Deduced from the red clay sequences at the Zhuanglang section, the last two events were interpreted as the intensified summer monsoon (Figure 7; [125, 126]).

The time overlap between the second accelerated exhumation of Taibai Mountain (ca. 24-ca. 19 Ma) and the monsoon intensification at 23.5-18.5 Ma suggests the impact of precipitation on erosion process (Figure 7). This also overlaps with the onset of sedimentation of the Lengshuigou Formation inferred to have occurred in the time interval of 23.0-18.2 Ma. The intensified precipitation carried by the summer monsoon accelerated the erosion on the neighboring mountains, which was recorded by sediments in the Weihe Basin. The sediments of the Bailuyuan Formation (Oligocene) and the Lengshuigou Formation (early Miocene) can be classified as alluvial fan, fluvial, and lacustrine sediments (Figure 8; [82]). Comparing the sediment distributions in the two formations, we find that the areas of both lacustrine and alluvial fan deposits in the Lengshuigou Formation are larger than those in the Bailuyuan Formation (Figure 8). Meanwhile, fluvial deposits in the Lengshuigou Formation have a smaller distribution area than those in the Bailuyuan Formation (Figure 8). The extended lacustrine deposits suggest the enhancement of precipitation, supplied by the intensified summer monsoon. The more extensive alluvial fan deposits in the south (Figure 8) indicate coeval enhanced sediment yields, whose provenance was the Qinling Mountains. These enlarged areas of lacustrine and alluvial fan deposits resulted in the reduced areas of fluvial deposits.

Also on the boundary between the Qinling Mountain and Weihe Basin, Huashan Mountain does not record accelerated exhumation at ca. 24-19 Ma, which was interpreted as a period of slow exhumation from low-temperature thermochronology [44, 127]. Since the climate change occurred globally or over a large region [48, 49, 52, 122], accelerated erosion due to the intensified summer monsoon should be experienced by the whole Qinling Mountains (Figure 2(a)). However, the thermochronological signals from Taibai and Huashan mountains in the early Miocene are different. This suggests that accelerated climate erosion was not the sole cause of the increase in exhumation rate inferred from low-temperature thermochronology. In the age-elevation relationship, the possible variation in cooling/exhumation rate will be indistinguishable, when the samples are collected within a short elevation/depth interval of <200 m [87]. Thus, in Taibai Mountain, the climate-induced erosion would not exceed an unroofing magnitude of ~200 m, which only accounted for a minor portion of the unroofing magnitude of ~600 m during the accelerated exhumation stage at ca. 24-19 Ma. Slightly influenced by climate erosion, this event should be mainly controlled by other mechanisms.

6.3. Accelerated Exhumation under the Conditions of Paleostress

The exhumed block, Taibai Mountain, is currently undergoing exhumation in the footwall of a normal fault (Figures 2 and 3). However, the North Qinling Fault has a complex kinematic history. Paleostress evolution has the ability to resolve this issue and evaluate the aforementioned horizontal forces.

Paleostress evolution can be utilized to decipher the possible kinematics of faults and the probable orientation of driving forces relevant to mountain growth in a certain time interval. By comparing exhumation stages with the paleostress evolution, we find that the two accelerated exhumation stages occurred during the two stages with transpressional stress fields (Figure 7). The first accelerated exhumation event (ca. 52-46 Ma) overlaps with the Eocene transpressional stress field (ca. 52-45 Ma; [42]). The second accelerated exhumation event (ca. 24-19 Ma) shares a similar onset time with the Oligocene-Miocene transpressional stress field (ca. 24-15 Ma; [42]), despite a shorter duration for the accelerated exhumation. To decipher the mechanisms for these two stages of accelerated exhumation, we discuss the local fault kinematics and the tectonic activities in the framework of paleostress.

6.3.1. Accelerated Exhumation due to Localized Contractional Deformation in the Fault Intersection

In the transpressional context, lateral motion on strike-slip faults results in compressional deformation in specific geometries, such as restraining fault intersections, restraining bends, contractional oversteps, and terminations [19, 21]. Focused deformation can be manifested as accelerated uplift and exhumation in these geometries [1922]. Taibai Mountain is located at the intersection of the North Qinling and Fengxian-Taibai faults (Figure 2(b)). During the transpressional periods, strike-slip motions of these two faults concentrated compression in this fault intersection, resulting in the accelerated uplift and exhumation in Taibai Mountain (Figure 9).

The North Qinling Fault accommodated dextral and sinistral strike-slip motions for the transpressional periods of ca. 52-45 Ma and ca. 24-15 Ma, respectively [42]. As the boundary between the East and West Qinling Mountains, the Fengxian-Taibai Fault mainly acted as a sinistral strike-slip fault in the Cenozoic, accommodating N-S compressional deformation and eastward extrusion of the Tibetan Plateau [75, 78]. During ca. 52-45 Ma, coeval motion of the sinistral strike-slip Fengxian-Taibai Fault and dextral strike-slip North Qinling Fault resulted in compression at their intersection, the area of Taibai Mountain, leading to localized uplift and exhumation in this corner (Figure 9(a)). For ca. 24-15 Ma, both the Fengxian-Taibai and North Qinling faults were sinistral strike slip, resulting in localized contractional deformation and vertical-axis rotation in the area of Taibai Mountain, which resulted in the accelerated exhumation at ca. 24-19 Ma (Figure 9(b)). This period of deformation corresponded to the onset of eastward lateral extrusion of the Tibetan Plateau in the early Miocene [42, 60, 78, 128130]. The duration of this exhumation event at ca. 24-19 Ma was shorter than that of the transpression at ca. 24-15 Ma. This was probably because the vertical-axis rotation accommodated part of the deformation and the strength of the rotation was probably relatively large at the initial period. Thus, the accelerated exhumation (ca. 24-19 Ma) occurred at the beginning of this transpressional stage (ca. 24-15 Ma).

The initiation of sedimentation in the Weihe Basin occurred at or after 47.6 Ma, inferred from the first appearance of the mammal fossil Deperetella sp. [80, 81] in the Honghe Formation overlying the Precambrian basement (Figure 3). This initiation marked the start of normal faulting between the Qinling Mountains and Weihe Basin, suggesting a shift from transpression to extension [42]. Almost simultaneously (ca. 47 Ma), the AHe thermochronology in Taibai Mountain exhibits a transition from relatively rapid exhumation to slow exhumation (Figure 4), which is contrary to the classic interpretation that the onset of normal faulting usually appears at the turning point from a low exhumation rate to a high rate [131]. The section of AHe thermochronology in Taibai Mountain is located >10 km far away from the Longxian-Mazhao Fault (Figure 2(b)), whose intersection with the North Qinling Fault can separate the latter fault into the western and eastern segments. The Longxian-Mazhao Fault and western segment of the North Qinling Fault act as the eastern and southern boundaries, respectively, for the Baoji Depression (Figure 2(b)). Deposition in this depression began with the Bahe Formation at 11-7 Ma overlying the Precambrian basement (Figure 3; [72, 73]), suggesting that the Baoji Depression merged into the Weihe Basin in the late Miocene. However, the Weihe Basin to the east of the Longxian-Mazhao fault had experienced subsidence and sedimentation since the late Eocene (ca. 47 Ma). These lines of evidence implies that the Longxian-Mazhao Fault and eastern segment of the North Qinling Fault started their normal faulting at ca. 47 Ma, acting as the basin bounding fault from the Eocene to the late Miocene, after which both the western and eastern segments of the North Qinling Fault acted as the boundary fault similar to that at present. Therefore, the initiation of normal faulting at ca. 47 Ma would be recorded in the Precambrian basement of the Baoji Depression rather than Taibai Mountain, because of the relatively long distance (>10 km) from the Longxian-Mazhao Fault experiencing obvious normal faulting to the AHe section of Taibai Mountain. In addition, subsidence in the Baoji Depression did not initiate until the late Miocene, implying the limited component of normal faulting along the western segment of the North Qinling Fault separating the basement of this depression and Taibai Mountain from the late Eocene to late Miocene.

6.3.2. Accelerated Exhumation in the Framework of Plate Tectonic Activities

Although Taibai Mountain is located in Central China, far from the active plate boundaries, the Cenozoic accelerated exhumation in Taibai Mountain and accompanied fault motions can reflect the driving forces from the perspective of plate tectonics. The far-field effect of Pacific subduction and the far-field effect of India-Eurasia collision in the Cenozoic are the options responsible for the local geological activities in the area of Taibai Mountain. How these plate tectonic activities would influence the accelerated exhumation in Taibai Mountain can provide insights into the geodynamic background for a specific exhumation event.

During the transpressional period of ca. 52-45 Ma, both the dextral strike-slip North Qinling Fault and sinistral strike-slip Fengxian-Taibai Fault fit the transpressional stress field in East China (Figures 7 and 9(a); [42]). The transpressional event at ca. 52-46 Ma in Taibai Mountain was also coeval with the rapid north-thrusting event in the West Qinling Mountains at ca. 50-45 Ma [17, 132] and the north-south shortening in the east Kunlun Shan from the Paleocene to early Eocene [22, 65]. These N-S compressional events in the northeast Tibetan Plateau resulted from the far-field effect of the India-Eurasia collision ([42, 59, 60, 133] and references therein). The NNE-SSW direction of the maximum principal axis (σ1) in East China suggests the impact of the N-S compression [42]. Of the two continuous paleostress stages in the Late Cretaceous-early Eocene (ca. 83-52 Ma) and the early Eocene (ca. 52-45 Ma), the minimum principal axis (σ3) remained in the WNW-ESE direction (Figure 7; [42]). The WNW-ESE striking faults in the Qinling Mountains together with the North Qinling Fault maintained a dextral strike-slip component from the Late Cretaceous to early Eocene (Figure 7; [42, 58]), with transtensional faulting during ca. 85-52 Ma and transpressional faulting during ca. 52-45 Ma [42]. Without N-S compression, this dextral component was a result of E-W extension due to the far-field effect of Pacific subduction beneath the Eurasia Plate in the Paleocene [42, 58, 134]. Transpressional faulting for these faults in the early Eocene resulted from the superposition of the far-field effects of India-Eurasia collision and Pacific subduction. In this framework, we ascribe the accelerated exhumation in Taibai Mountain at ca. 52-46 Ma to the far-field effects of both the India-Eurasia collision and Pacific subduction.

The transpressional stress field at ca. 24-15 Ma shared an onset time with the second accelerated exhumation ca. 24-19 Ma. During this event, both the North Qinling and Fengxian-Taibai faults acted as sinistral strike-slip faults (Figure 9(b); [42, 75, 78]). The transpression from the late Oligocene to early Miocene occurred not only along these two faults but also along the W-E to WNW-ESE striking faults in the Qinling Mountains, suggesting the widespread impact of the eastward lateral extrusion of the Tibetan Plateau [42, 60, 78, 128130]. The NE-SW striking Fengxian-Taibai Fault with the sinistral strike-slip component also suggests the profound impact of this lateral extrusion. Accommodating the sinistral strike-slip motions of the North Qinling and Fengxian-Taibai faults, the accelerated exhumation at ca. 24-19 Ma in Taibai Mountain was driven by the eastern lateral extrusion of the Tibetan Plateau.

Applying AHe thermochronology, we distinguish five stages of exhumation in a single continuous age-elevation profile with a relief of ~3 km from 69.8±14.1 Ma at the elevation peak to 17.8±5.3 Ma at the base in Taibai Mountain, in the northern flank of W-E striking Qinling Mountains. This five-stage exhumation process is confirmed by 1D half-space modeling and QTQT inverse modeling. Two stages are interpreted as accelerated exhumation at ca. 52-46 Ma and ca. 24-19 Ma with exhumation rates of 0.13-0.39 km/Myr and 0.11-0.24 km/Myr, respectively. They acted as interruptions of relatively slow exhumation or tectonic quiescence.

The two stages of accelerated exhumation were coeval with the periods of transpressional stress field in East China. Located at the intersection of the North Qinling and Fengxian-Taibai faults, these two accelerated exhumation events in Taibai Mountain were triggered by the localized contractional deformation in the fault intersection of the North Qinling and Fengxian-Taibai faults. The opposite-direction shearing of the two faults led to the accelerated exhumation event at ca. 52-46 Ma with an unroofing magnitude of ~1 km, whereas the same-direction shearing brought about the event at ca. 24-19 Ma with an unroofing magnitude of ~0.6 km. The far-field effects of both the India-Eurasia collision and Pacific subduction beneath the Eurasia plate provided driving forces for the accelerated exhumation at ca. 52-46 Ma. Lateral extrusion of the Tibetan Plateau drove the accelerated exhumation event at ca. 24-19 Ma, which may have also been slightly influenced by the increased erosion due to intensified summer monsoon in the early Miocene.

The data used to support the findings of this study are included within the supplementary materials.

The authors declare no conflict of interest regarding this manuscript.

Ms. Qian Guo is acknowledged for her assistance in apatite separation. This work was supported by the Basic Research Program for the Institute of Geology, China Earthquake Administration (No. IGCEA2004), the National Natural Science Foundation of China (No. 41930106), and the National Key R&D Program of China (No. 2018YFC1504101).

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