Developing an internally consistent methodology for K-feldspar MAAD TL thermochronology

Luminescence thermochronology and thermometry can quantify recent changes in rock exhumation rates and rock surface temperatures, but these methods require accurate determination of several kinetic parameters. For K-feldspar thermoluminescence (TL) glow curves, which comprise overlapping signals of different thermal stability, it is challenging to develop measurements that capture these parameter values. Here, we present multiple-aliquot additive-dose (MAAD) TL dose response and fading measurements from bedrock-extracted K-feldspars. These measurements are compared with Monte Carlo simulations to identify best-fit values for recombination center density ($\rho$) and activation energy ($\Delta E$). This is done for each dataset separately, and then by combining dose-response and fading misfits to yield more precise $\rho$ and $\Delta E$ values consistent with both experiments. Finally, these values are used to estimate the characteristic dose ($D_0$) of samples. This approach produces kinetic parameter values consistent with comparable studies and results in expected fractional saturation differences between samples.

Kinematic partitioning in the Southern Andes (39° S–46° S) inferred from lineament analysis and reassessment of exhumation rates

The Southern Andes are often viewed as a classic example for kinematic partitioning of oblique plate convergence into components of continental margin-parallel strike-slip and transverse shortening. In this regard, the Liquiñe-Ofqui Fault Zone, one of Earth’s most prominent intra-arc deformation zones, is believed to be the most important crustal discontinuity in the Southern Andes taking up margin-parallel dextral strike-slip. Recent structural studies, however, are at odds with this simple concept of kinematic partitioning, due to the presence of margin-oblique and a number of other margin-parallel intra-arc deformation zones. However, knowledge on the extent of such zones in the Southern Andes is still limited. Here, we document traces of prominent structural discontinuities (lineaments) from the Southern Andes between 39° S and 46° S. In combination with compiled low-temperature thermochronology data and interpolation of respective exhumation rates, we revisit the issue of kinematic partitioning in the Southern Andes. Exhumation rates are maximal in the central parts of the orogen and discontinuity traces, trending predominantly N–S, WNW–ESE and NE–SW, are distributed across the entire width of the orogen. Notably, discontinuities coincide spatially with large gradients in Neogene exhumation rates and separate crustal domains characterized by uniform exhumation. Collectively, these relationships point to significant components of vertical displacement on these discontinuities, in addition to horizontal displacements known from published structural studies. Our results agree with previously documented Neogene shortening in the Southern Andes and indicate orogen-scale transpression with maximal vertical extrusion of rocks in the center of the transpression zone. The lineament and thermochronology data call into question the traditional view of kinematic partitioning in the Southern Andes, in which deformation is focused on the Liquiñe-Ofqui Fault Zone.

Exploiting Thermochronology to Quantify Exhumation Histories and Patterns of Uplift Along the Margins of Tibet

The utilization of thermal-chronological data to constrain mountain building processes exploits the links among rock uplift, exhumation, and cooling during orogenesis. Conceptually, periods of rapid uplift and associated denudation will lead to cooling of rocks as they approach Earth’s surface. The linkage between uplift and exhumation can be complex, but in practice exhumation is often assumed to directly track uplift. The reconstruction of temperature-time histories via thermochronologic systems provides a proxy method to relate the cooling of rock as it is exhumed toward the surface to orogenesis. For the rapid exhumation rates that can occur in active orogenic systems the thermal history will be complex as a result of heat advection, rates of propagation of thermal perturbations, and other processes that affect the cooling behavior. These effects become amplified as exhumation rates increase, and in regions experiencing exhumation rates greater than ∼0.2–0.3 mm/yr (0.2–0.3 km/Ma) simple assumptions of cooling through a constant geotherm will bias the subsequent interpretation. Here we explore, through a suite of generalized models, the impact of exhumation rate and duration on the resulting thermal history and apparent age results. We then apply lessons from these simple exhumation systems to data sets from the high-relief ranges along the eastern margin of the Tibetan Plateau to determine exhumation histories constrained by those data. The resulting exhumation histories provide constraints on the onset of Cenozoic exhumation, the subsequent pace of exhumation, and on the tectonic history of one of the major fault systems in the central Longmen Shan.

Implications of the ongoing rock uplift in NW Himalayan interiors

The Lesser Himalaya exposed in the Kishtwar Window (KW) of the Kashmir Himalaya exhibits rapid rock uplift and exhumation (∼3 mm yr−1) at least since the late Miocene. However, it has remained unclear if it is still actively deforming. Here, we combine new field, morphometric and structural analyses with dating of geomorphic markers to discuss the spatial pattern of deformation across the window. We found two steep stream segments, one at the core and the other along the western margin of the KW, which strongly suggest ongoing differential uplift and may possibly be linked to either crustal ramps on the Main Himalayan Thrust (MHT) or active surface-breaking faults. High bedrock incision rates (>3 mm yr−1) on Holocene–Pleistocene timescales are deduced from dated strath terraces along the deeply incised Chenab River valley. In contrast, farther downstream on the hanging wall of the MCT, fluvial bedrock incision rates are lower (<0.8 mm yr−1) and are in the range of long-term exhumation rates. Bedrock incision rates largely correlate with previously published thermochronologic data. In summary, our study highlights a structural and tectonic control on landscape evolution over millennial timescales in the Himalaya.

Enhanced Quaternary exhumation in the Namche Barwa syntaxis, eastern Himalaya

The Namche Barwa syntaxis in the eastern Himalaya is rapidly evolving in terms of its tectonics and topography. Here we constrain the exhumation history of the Yigong River to the immediate north of the syntaxis across different time scales using a multidisciplinary approach. Our new thermochronometric data reveal an acceleration of exhumation rates since 2 Ma in the downstream of the Yigong. Cosmogenic nuclides and thermoluminescence thermochronometry analyses confirm persistent rapid exhumation in the lower Yigong over the Quaternary with further increased exhumation in the last 100 ka. Together with the analysis of the morphology of the Yigong River profile, we interpret that northward expansion of the syntaxis together with capture of the Yigong by the Yarlung Tsangpo River during this expansion is responsible for the exhumation history of the Yigong River in the Quaternary.

Crustal Thermal Structure and Exhumation Rates in the Southern Alps Near the Central Alpine Fault, New Zealand

© 2020. American Geophysical Union. All Rights Reserved. We investigate orogenic uplift rates and the thermal structure of the crust in the hanging wall of the Alpine Fault, New Zealand, using the hypocenters of 7,719 earthquakes that occurred in the central Southern Alps between late 2008 and early 2017, and previously published thermochronological data. We assume that the base of the seismogenic zone corresponds to a brittle-ductile transition at some fixed temperature, which we estimate by fitting the combined thermochronological data and distribution of seismicity using a multi-1-D approach. We find that exhumation rates vary from 1 to 8 mm/yr, with maximum values observed in the area of highest topography near Aoraki/Mount Cook, a finding consistent with previous geologic and geodetic analyses. We estimate the temperature of the brittle-ductile transition beneath the Southern Alps to be 410–430°C, which is higher than expected for Alpine Fault rocks whose bulk lithology is likely dominated by quartz. The high estimated temperatures at the base of the seismogenic zone likely reflect the unmodeled effects of high fluid pressures or strain rates.

Crustal Thermal Structure and Exhumation Rates in the Southern Alps Near the Central Alpine Fault, New Zealand

© 2020. American Geophysical Union. All Rights Reserved. We investigate orogenic uplift rates and the thermal structure of the crust in the hanging wall of the Alpine Fault, New Zealand, using the hypocenters of 7,719 earthquakes that occurred in the central Southern Alps between late 2008 and early 2017, and previously published thermochronological data. We assume that the base of the seismogenic zone corresponds to a brittle-ductile transition at some fixed temperature, which we estimate by fitting the combined thermochronological data and distribution of seismicity using a multi-1-D approach. We find that exhumation rates vary from 1 to 8 mm/yr, with maximum values observed in the area of highest topography near Aoraki/Mount Cook, a finding consistent with previous geologic and geodetic analyses. We estimate the temperature of the brittle-ductile transition beneath the Southern Alps to be 410–430°C, which is higher than expected for Alpine Fault rocks whose bulk lithology is likely dominated by quartz. The high estimated temperatures at the base of the seismogenic zone likely reflect the unmodeled effects of high fluid pressures or strain rates.

Insights of Spatiotemporal evolution of Yamuna Valley, Garhwal Himalaya: Derived from Fission track dating and Morphotectonic analysis

&lt;p&gt;Substantial set of recent documentation with sophisticated statistical and analog models have recognized dynamic interchange between subsurface crustal distortion and exogenic erosional processes as the root of geomorphic evolution of Himalaya. Low temperature thermochronology provides insights to enumerate nature and timing of tectonic course from extracted thermal records of vertical moving rock block over geological past. In present study, we used Apatite fission track technique to calculated exhumation rates of Yamuna valley, Garhwal Himalaya. AFT ages of Lesser Himalaya Sequence of Purola region various between 4.0 &amp;#177;0.8 myr to 9.5&amp;#177;0.6 myr. While AFT ages of LHS along Yamuna River varies form 2.3&amp;#177;0.5 myr to 5.6&amp;#177;0.6 myr and exhumation rates are 2.3-1.2 mm/yr. calculated age of Apatite sample near Main Central Thrust (MCT) is 2.3&amp;#177;0.5 myr which exhumed at the rate of 2.3 mm/yr. Exhumation rates of Purola region are 0.8-1.6 mm/yr.&lt;/p&gt;&lt;p&gt;To link the exhumation rates with present day morphology we used 2 methods; 1) Calculate morphotectonic parameters of Yamuna River valley; 2) compare our AFT ages and exhumation rates with early studies. Drainage pattern in the tectonically active zone is vigorously susceptible to mechanisms such as folding, faulting and basin tilting. Such deformation processes influence the phase of geomorphology, drainage pattern, river incision, elongation, asymmetry, and diversion. Mathematical quantification of drainage morphology elucidate spatio-temporal effect of tectonics. Morphotectonic parameters are stream length gradient index (SL), valley floor height to width ratio (Vf), asymmetry factor (Af), basin shape index (BS) and hypsometric integral (HI) extracted from SRTM DEM with resolution of 30m and are calculated in ArcGIS 10.3. These parameters further integrated to define a single Indaex of relative Active Tectonic (IRAT). Value of IRAT is very high in upper Yamunotri region and low to moderate in Purola region. The exhumation rates are further compared with erosion rates from early studies. Erosion rates derived from &lt;sup&gt;10&lt;/sup&gt;Be nuclides (Scherler et al 2014) show very slow erosion rate in Purola region (~ 0.13&amp;#177;0.01 mm/yr) while for Yamunotri region higher erosion rate (&gt;4.9 mm/yr) is recorded. These erosion rates are attributed to subsurface geometry of MCT.&lt;/p&gt;&lt;p&gt;All three approaches together construct an evolution record of study area over geological past. &amp;#160;Exhumation history of Apatite and erosion rates from early studies conclude Yamuna river valley, specifically upper region of valley is very active while Purola region is less active. Morphotectonic parameters harmoniously present similar picture. These combined study point toward relegate control of climate and dominance of ongoing sub-surficial deformation along MCT in Yamuna River valley on geological time scale.&lt;/p&gt;

Diachronous exhumation of the Carpathians from low-temperature thermochronology

&lt;p&gt;The Carpathians fold-and-thrust belt results from oblique collision of ALCAPA and Tisza-Dacia plates with the eastern European margin. It formed during the Oligocene and Miocene, propagating laterally from NW to SE as clearly demonstrated by balanced-cross sections (Nakapelyukh et al., 2017; Castellucio et al., 2016; Merten et al., 2010). The coeval development of the foreland basin (Roure et al., 1993) is revealed by an axial transport system that prograded from NW to SE, ultimately supplying sediments to the Black Sea (de Leeuw et al., 2020). However, lacking a regional synthesis and integration of thermochronology data, lateral propagation of exhumation in the orogen has not been demonstrated yet.&lt;/p&gt;&lt;p&gt;&amp;#160;We reconstruct the exhumation history of the entire Carpathians from the Oligocene onwards and link it with the development of the Carpathians foreland basin (CFB) using a source-to-sink approach. We compiled more than 500 apatite and zircon fission-track and (U-Th)/He ages from the literature. This comprehensive database was separated by region (Western, Eastern, and South-Eastern Carpathians) and by tectonic domain (as defined in Schmid et al., 2008). This partitioning allows for the inversion of large datasets, reflects the tectonic complexity of the belt, and avoids spurious spatial correlations (Schildgen et al., 2018). The thermochronology data was inverted using Pecube (Braun et al., 2012) to constrain exhumation rates in a Bayesian approach. We thus obtain estimates of exhumation rates through time along the belt (with their uncertainty) and convert these into bulk &amp;#160;sediment fluxes over time, permitting tracking of sediment routing from the eroding belt to the CFB. Ultimately, these data will be used to unravel deeper geodynamics, including the possible effects of slab detachment on the evolution of the belt and its foreland basin.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Key words: Low-temperature thermochronology, Carpathians, exhumation, source to sink, Pecube inversions.&lt;/p&gt;

Middle Miocene rise of the Greater Himalaya establishing a new orographic barrier

&lt;p&gt;The Himalaya is the highest and steepest mountain range on Earth and an efficient north-south barrier for moisture-bearing winds. The close coupling of changes in topography, erosion rates, and uplift has previously been interpreted as an expression of a climatic control on tectonic deformation. Here, we present 17 new zircon U/Th-He (ZHe) bedrock-cooling ages from the Sutlej Valley that &amp;#8211; together with &gt;100 previously published mica &lt;sup&gt;40&lt;/sup&gt;Ar/&lt;sup&gt;39&lt;/sup&gt;Ar, zircon and apatite fission track ages &amp;#8211; allow us to constrain the crustal cooling and exhumation history over the last ~20 Myr. Using 1D-thermal modeling, we observe a rapid decrease in exhumation rates from &gt;1 km/Myr to &lt;0.5 km/Myr that initiated at ~17-15 Ma across the entire Greater and Tethyan Himalaya, as far north as the north-Himalayan Leo Pargil gneiss dome. This decrease is recognized both in the hanging and footwall of major Miocene shear zones and suggests that cooling is associated to surface erosion rather than to tectonic unroofing. We explain the middle Miocene deceleration of exhumation with major reorganization of Himalayan deformation and the onset of the growth of the Lesser Himalayan duplex. This resulted in accelerated uplift of the Greater Himalaya above a mid-crustal ramp, and thus forming a new efficient orographic barrier. The period of slow exhumation in the upper Sutlej Valley coincides with a period of internal drainage in the south-Tibetan Zada Basin further upstream, which we interpret to be a consequence of tectonic damming of the upper Sutlej River. External drainage of the Zada Basin was re-established ~1 Ma, when we observe exhumation rates in the upper Sutlej Valley to accelerate again. Our new finding document that the location of tectonic deformation processes control the first order spatial pattern of both climatic zones and erosion across the orogen.&lt;/p&gt;