Barry P. Kohn

Worldwide acceleration of mountain erosion under a cooling climate

Climate influences the erosion processes acting at the Earth’s surface. However, the effect of cooling during the Late Cenozoic era, including the onset of Pliocene–Pleistocene Northern Hemisphere glaciation (about two to three million years ago), on global erosion rates remains unclear. The uncertainty arises mainly from a lack of consensus on the use of the sedimentary record as a proxy for erosion and the difficulty of isolating the respective contributions of tectonics and climate to erosion. Here we compile 18,000 bedrock thermochronometric ages from around the world and use a formal inversion procedure to estimate temporal and spatial variations in erosion rates. This allows for the quantification of erosion for the source areas that ultimately produce the sediment record on a timescale of millions of years. We find that mountain erosion rates have increased since about six million years ago and most rapidly since two million years ago. The increase of erosion rates is observed at all latitudes, but is most pronounced in glaciated mountain ranges, indicating that glacial processes played an important part. Because mountains represent a considerable fraction of the global production of sediments, our results imply an increase in sediment flux at a global scale that coincides closely with enhanced cooling during the Pliocene and Pleistocene epochs.

Thermal history and tectonic subsidence of the Bohai Basin, northern China: a Cenozoic rifted and local pull-apart basin

The Bohai Basin is a part of the larger Bohai Bay Basin, a Cenozoic rifted intraplate basin. Heat flow measurements show that the Bohai Basin is characterized by present-day heat flow varying between 53 and 74 mW/m 2 with a mean of 63 mW/m 2 . However, thermal history analyses derived from vitrinite reflectance (VR) and apatite fission track (AFT) data, indicate that tertiary cooling took place following a period of much higher paleo-heat flow (70–90 mW/m 2 ) prior to ∼25 million years to the present. Furthermore, tectonic subsidence analysis reveals that the Bohai Basin experienced episodic sub-rifting processes from the Eocene to the end of the Oligocene. The post-rift thermal subsidence was superimposed by intensified subsidence from the late Miocene (12 million years) in the Bozhong Depression and from the early Quaternary (2.4 million years) in the East Liao Bay, indicating a rejuvenation of sedimentation related likely to transpressional structures developed locally on strike–slip faults. The present results indicate that there is good agreement between the reconstructed thermal history and the timing of the tectonic subsidence stages within the Bohai Basin. The Bohai Basin is an early Cenozoic rifted and local pull-apart sub-basin of the larger Bohai Bay Basin.

Thermochronological analysis of the dynamics of the Southern Alps, New Zealand

The isotopic ages of material exposed close to the Alpine fault in the Southern Alps, New Zealand, vary in a consistent fashion along the orogen. For the K-Ar system, muscovite and biotite ages display two distinct spatial trends that meet in the area of Fox Glacier, where a strong relationship is observed between K-Ar age and altitude. Excess argon is present in samples between Mount Kinnaird and Haast Pass in the southwest of the Southern Alps, giving unrealistically old, inconsistent K-Ar ages across this region. Incorporation of excess argon appears to be highly variable, however, and minimization of its influence on dynamical interpretations of thermochronology is possible through considering only the youngest age from a given area. Such treatment indicates that excess argon contamination does not play an important role in the systematic K-Ar age variation identified along the Southern Alps. The observed age distribution is consistent with estimates of the modern rapid uplift and exhumation of the Southern Alps beginning ca. 5 Ma. K-Ar mica ages older than this south of the Copland Valley indicate that substantial late Cenozoic exhumation occurred along the Southern Alps prior to the development of the modern tectonic regime, but at rates lower than at present. Total exhumation during the current phase of tectonism in the Southern Alps decreases from the Whataroa River southward. North of Fox Glacier, the Southern Alps have attained a dynamical steady state, such that the observed distribution of exhumation will not change significantly with further deformation and denudation under current conditions.

Shaping the Australian crust over the last 300 million years: insights from fission track thermotectonic imaging and denudation studies of key terranes

Apatite fission track thermochronology is a well‐established tool for reconstructing the low‐temperature thermal and tectonic evolution of continental crust. The variation of fission track ages and distribution of fission track lengths are primarily controlled by cooling, which may be initiated by earth movements and consequent denudation at the Earths surface and/or by changes in the thermal regime. Using numerical forward‐modelling procedures these parameters can be matched with time‐temperature paths that enable thermal and tectonic processes to be mapped out in considerable detail. This study describes extensive Australian regional fission track datasets that have been modelled sequentially and inverted into time‐temperature solutions for visualisation as a series of time‐slice images depicting the cooling history of present‐day surface rocks during their passage through the upper crust. The data have also been combined with other datasets, including digital elevation and heat flow, to image the denudation history and the evolution of palaeotopography. These images provide an important new perspective on crustal processes and landscape evolution and show how important tectonic and denudation events over the last 300 million years can be visualised in time and space. The application of spatially integrated denudation‐rate chronology is also demonstrated for some key Australian terranes including the Lachlan and southern New England Orogens of southeastern Australia, Tasmania, the Gawler Craton, the Mt Isa Inlier, southwestern Australian crystalline terranes (including the Yilgarn Craton) and the Kimberley Block. This approach provides a readily accessible framework for quantifying the otherwise undetectable, timing and magnitude of long‐term crustal denudation in these terranes, for a part of the geological record previously largely unconstrained. Discrete episodes of enhanced denudation occurred principally in response to changes in drainage, base‐level changes and/or uplift/denudation related to far‐field effects resulting from intraplate stress or tectonism at plate margins. The tectonism was mainly associated with the history of continental breakup of the Gondwana Supercontinent from Late Palaeozoic time, although effects related to compression are also recorded in eastern Australia. The results also suggest that the magnitude of denudation of cratonic blocks has been significantly underestimated in previous studies, and that burial and exhumation are significant factors in the preservation of apparent ‘ancient’ features in the Australian landscape.

An empirical test of helium diffusion in apatite: borehole data from the Otway basin, Australia

We have analyzed helium ages of apatites from several boreholes in the Otway basin, Australia, to evaluate whether laboratory helium diffusivity can be accurately extrapolated to conditions relevant in nature. Downhole apatite helium ages define a broad swath of values from 78–71 Ma at the surface (15°C) to nearly zero at depths corresponding to ambient temperatures of ∼80°C. The width of the swath results from uncertainties in corrected borehole temperatures, differences in the thermal history experienced by the various boreholes, and possibly from slightly different helium diffusivities among the detrital apatite grains studied. In the eastern Otway basin, the shape and position of the helium age profile is in good agreement with predictions based on the extrapolation of laboratory diffusivity data for Durango apatite coupled with published thermal histories for this part of the basin. In contrast, helium ages are much younger than predicted in the western Otway basin. Based on measured ages from Otway sediments, which have been essentially isothermal over the last few million years, an empirical diffusivity (D/α^2) of 2×10^(−15) s^(−1) can be assigned to apatites residing at downhole temperatures of 67–97°C. This empirical diffusivity is consistent with laboratory diffusion measurements, demonstrating that such measurements are reasonably accurate and can be applied with confidence to natural geologic settings. Given this confirmation of the laboratory diffusivity data, the discrepancy between the observed and modeled helium age profiles in the western Otway basin suggests that these sediments recently experienced higher temperatures than presently supposed.

History of uplift of the crystalline basement of Sinai and its relation to opening of the Red Sea as revealed by fission track dating of apatites

Abstract Thirty fission track ages have been obtained for apatite concentrates from Precambrian crystalline basement rocks of the Sinai Peninsula. The principal results of the study indicate that: (1) Apatite fission track ages do not date times of petrogenesis. (2) Uplift of the basement amounted to at least 5 km and this amount probably equals or exceeds that in other areas bordering the Red Sea. (3) Uplift rates approximated 0.1–0.2 mm/yr. (4) The patter of apatite ages reveals non-uniform uplift between fault-bound blocks, with greater uplift parallel to the Gulf of Suez, an area of extensional tectonics, and less uplift parallel to the Gulf of Elat, an area dominated by strike slip movements. (5) Domal uplift commenced in Sinai about 26.6 ± 3m.y. ago and continued for most of the Miocene with at least 3 km of uplift post-dating 9 m.y. ago. The relationships between geological events associated with rifting and spreading are established, and a time table for the different phases in the framework of the Red Sea rift system is proposed.

Apatite fission-track thermochronology of the Sierras Pampeanas, central western Argentina: Implications for the mechanism of plateau uplift in the Andes

Over much of its length, the Andean orogen is characterized by a generally east-vergent geometry and a progressive eastward (cratonward) migration of individual arc-orogenic elements. A departure from this model occurs in the Sierras Pampeanas region of central western Argentina where a terrane of active basement uplifts is currently developing to the east of the main Cordillera. Apatite fission-track data from uplifted Precambrian and Phanerozoic basement rocks of the Sierras Pampeanas constrain the tectonic development of this terrane and indicate that deformation associated with exhumation may have propagated broadly westward since the late Miocene. Two pre-Andean cooling events—during the Carboniferous–Permian (ca. 300–280 Ma) and the early Jurassic–middle Jurassic (ca. 200–174 Ma)—have been identified. The onset of Andean deformation is represented by a cooling event during the late Paleocene–middle Eocene. This cooling was followed by a period of middle Miocene–late Miocene reheating, during foreland basin-style sedimentation. Exhumation, and possible westward migration of the exhumation “front” in the Sierras Pampeanas, commenced during the late Miocene-Pliocene to the east of the dominantly east-vergent Precordillera fold-and-thrust belt. The apparent convergence of deformation in these two terranes and the progressive closure of Miocene-Pliocene intermontane basins in the Sierras Pampeanas may reflect the early stages of Andean plateau uplift. The relative timing of plateau development along strike suggests that lateral thickening of the orogen is progressing southward at least from the latitude of central Bolivia (ca. 20°S). Furthermore, the time-space coincidence between basement uplift and flattening of the subducted slab beneath the Sierras Pampeanas suggests that a relationship exists between westward displacement of the terrane and the dynamics of plate interaction.

Quantitative resolution of the debate over antiquity of the central Australian landscape: implications for the tectonic and geomorphic stability of cratonic interiors

We report the first measure of long- (∼100 Myr) and short- (∼1 Myr) term denudation rates from key geologically stable landforms in the Davenport Range, central Australia. These landforms have previously been assigned a Cambrian age, which arguably places them amongst the oldest persistent landforms on the continent, if not on Earth. Our results from combined apatite fission track thermochronology and in situ cosmogenic radionuclide analyses using ^(10)Be and ^(26)Al show that while average exhumation rates are low, the denudation history for this cratonic region is incompatible with extreme, sub-aerial longevity and long-term tectonic and geomorphic stability. Our revised model for the landscape evolution of this region is consistent with one of maximum burial prior to and during the Mesozoic, followed by a phase of kilometre-scale exhumation that was largely complete by the beginning of the Cainozoic. We suggest that a similar process of burial and exhumation has probably been responsible for the sub-aerial preservation of seemingly ancient landforms elsewhere in Australia.

Peneplain formation in southern Tibet predates the India-Asia collision and plateau uplift

The uplift history of Tibet is crucial for understanding the geodynamic and paleoclimatologic evolution of Asia; however, it remains controversial whether Tibet attained its high elevation before or after India collided with Asia ∼50 m.y. ago. Here we use thermochronologic and cosmogenic nuclide data from a large bedrock peneplain in southern Tibet to shed light on the timing of the uplift. The studied peneplain, which was carved into Cretaceous granitoids and Jurassic metasediments, is located in the northern Lhasa block at an altitude of ∼5300 m. Thermal modeling based on (U-Th)/He ages of apatite and zircon, and apatite fission track data, indicate cooling and exhumation of the granitoids between ca. 70 and ca. 55 Ma, followed by a rapid decline in exhumation rate from ∼300 m/m.y. to ∼10 m/m.y. between ca. 55 and ca. 48 Ma. Since then, the peneplain has been a rather stable geomorphic feature, as indicated by low local and catchment-wide erosion rates of 6–11 m/m.y. and 11–16 m/m.y., respectively, which were derived from cosmogenic 10 Be concentrations in bedrock, grus, and stream sediment. The prolonged phase of erosion and planation that ended ca. 50 Ma removed 3–6 km of rock from the peneplain region, likely accomplished by laterally migrating rivers. The lack of equivalent sediments in the northern Lhasa block and the presence of a regional unconformity in the southern Lhasa block indicate that the rivers delivered this material to the ocean. This implies that erosion and peneplanation proceeded at low elevation until India9s collision with Asia induced crustal thickening, surface uplift, and long-term preservation of the peneplain.

Fission-track analysis of basement apatites at the western margin of the Gulf of Suez rift, Egypt: evidence for synchroneity of uplift and subsidence

Fifty-six apatite fission-track ages and 52 horizontal confined track-length measurements are reported from Precambrian crystalline rocks along the western margin of the Gulf of Suez, Egypt. Ages fall in the range of ca. 11–385 m.y. and older ages often occur within very close geographic proximity to younger ones, indicating non-uniform uplift. The wide range in ages is accompanied by a systematic variation in the distribution of horizontal confined fission track lengths. On the basis of apatite fission track ages and their length distributions, data fall into three distinct groups. Group I: ages ranging from 43 to 385 m.y. Length distributions are all positively skewed and with decreasing age become progressively broader with shorter mean track length. Group II: ages ranging from 23 to 31 m.y. Length distributions are negatively skewed with either a distinct tail or a small peak of short tracks. Group III: ages ranging from 11 to 20.5 m.y. Length distributions are al unimodal, narrow, negatively skewed and have the longest mean lengths among samples studied. Apatite ages from groups I and II are interpreted as “mixed ages” as a result of cooling during uplift from different levels within the apatite partial track annealing zone. Ages from Group III are interpreted as “cooling ages” due to uplift from the apatite total track annealing zone with minor partial annealing. Correcting the ages of the two oldest samples in this group for track-length reduction yields ages of21 ± 2.2and23 ± 1.5m.y. It is proposed that the onset of rift-flank uplift in the Gulf of Suez—northern Red Sea area occurred between 21 and 23 m.y. ago. Fission-track analysis in combination with subsidence data from the Gulf of Suez basin, indicate that commencement of basement uplift postdate the start of rifting and is interpreted as evidence for passive rifting at the Gulf of Suez. Furthermore, this uplift is contemporaneous with, and is directly related to, the process of extension and subsidence at the Gulf of Suez.