Zircon Fission Track Research Articles

Geological framework and fission track dating of pseudotachylyte of the Atotsugawa Fault, Magawa area, central Japan

This paper describes newly discovered pseudotachylyte along the Atotsugawa Fault at the Magawa outcrop, where this fault divides Quaternary deposits in the SW from Triassic Hida granitic rocks to the NE. Within several meters of the fault surface, pseudotachylyte veins are found with a thickness of less than 10 cm, but are displaced by fault brecciation. Zircon fission track dating of pseudotachylyte samples yields ages of 48.6–50.2 Ma (sample AT-A), 55.1 Ma (AT-A'-1) and 60.9 Ma (AT-D-1); the latter is similar to the fission track ages of 56.1–60.1 Ma for granitic protoliths. The results of fission track length analyses in zircon suggest that pseudotachylytes (AT-A and AT-D-1) and protolith granite are mostly annealed. Consequently, the pseudotachylyte (AT-A) reached the highest temperature during 48.6–50.2 Ma, thereby resetting the fission track system totally in zircon during faulting. Another pseudotachylyte (AT-A'-1) and its wall rock granite contain shortened tracks within zircon grains suggesting partial annealing. The age distribution pattern of the former also contains decomposed age after the normality test (Shapiro–Wilk test) in which the major age yields 52.5 Ma. Accordingly, these pseudotachylytes yield a peak age of about 50 Ma, whereas the peak ages of one pseudotachylyte (AT-D-1) and the protolith Hida granitic rocks are about 60 Ma, representing the thermal effects not caused by frictional heating but by intrusions of Late Cretaceous to Paleogene granitoids that are probably concealed below the exposed Triassic Hida granitic rocks. Such thermal effects did not affect the K–Ar muscovite age (149 Ma) for the protolith granite because of the higher closure temperature of this system. Using the new geochronological data, we can elucidate the cooling history of the Hida granitic rocks, and constrain the timing of the main pulse of pseudotachylyte generation along the Atotsugawa Fault at about 50 Ma.

Detrital Zircon Fission-Track Thermochronology of the Present-Day Isère River Drainage System in the Western Alps: No Evidence for Increasing Erosion Rates at 5 Ma

The Isère River system drains parts of the Western Alps in south-eastern France. Zircon fission-track data of the Isère River and its tributaries show a range of apparent cooling ages from about 7 to 150 Ma. Zircons with Jurassic to early Tertiary cooling ages are derived from partially reset or non-reset sedimentary cover units of the internal and external Alps, while grains belonging to the minimum age fraction are derived from areas of active river incision in the external crystalline massifs or from the Penninic front. With the absence of major normal faults, upper crustal exhumation in the Western Alps is driven by erosion. First-order long-term exhumation rate estimates based on minimum ages are about 0.5–0.6 km/Myr for the fastest exhuming areas, while drainage basin average rates based on central ages are about 0.2–0.4 km/Myr. These rates are slower than published short-term erosion rates determined from detrital quartz 10Be analyses in the Pelvoux massif. While present-day erosion is faster than the long-term average exhumation rates, the Isère River drainage zircon fission-track data do not show evidence for increasing erosion rates at 5 Ma. Exhumation has not been sufficient in this area to expose rocks with <5 Ma cooling ages today. The increase in erosion may have happened only in glaciated areas between 1 and 2 Ma.

Fission track thermochronology evidence for multiple periods of mineralization in the Wulonggou Gold deposits, eastern Kunlun Mountains, Qinghai Province

It is a puzzle to determine metallogenetic ages in the world. This article uses zircon fission track (FT) dating to probe the mineralizing epochs of Wulonggou (五龙沟) gold ore deposits in eastern Kunlun (昆仑) Mountains. Total of six zircon FT ages have been obtained and can be divided into groups of 235–216 and 197–181 Ma, as well as 162 and 124 Ma, revealing multiple epochs of metallogenetic processes took place in Wulonggou area, mainly first two age groups. The mineralizing ages become lower from northeast Yanjingou (岩金沟) to southwest Hongqigou (红旗沟). The second epoch of 197–181 Ma is first achieved by this work. It is shown that the FT ages consist with other isotopic data and reflect different regional thermotectonic events.

Constraints on Cenozoic tectonics in the southwestern Longmen Shan from low-temperature thermochronology

The Longmen Shan, located at the boundary between the Tibetan Plateau and the Sichuan Basin, has received considerable attention following the 2008 Wenchuan earthquake. However, the tectonic history of the southwestern segment of the range has remained poorly constrained. We present zircon fission-track, zircon (U-Th)/He, and apatite (U-Th)/He data from the Baoxing region in the southwestern Longmen Shan that provide the first constraints on the cooling and exhumation history of the region. All of the measured ages are Cenozoic, and the data suggest that exhumation of the Baoxing region was ongoing by ca. 15 Ma. Zircon (U-Th)/He ages from several samples appear to be affected by radiation damage, suggesting that damage may be a concern even in samples with Cenozoic cooling ages. Samples were collected from two bodies of Precambrian crystalline rocks separated by the Wulong fault, and for all three thermochronometers, ages west of the Wulong fault are systematically younger than ages to the east, indicating that the fault has accommodated differential exhumation since 8–10 Ma. The regions east and west of the Wulong fault have experienced 7–13 km and at least 7–10 km of exhumation, respectively. The magnitude of exhumation in the southwestern Longmen Shan is similar to that reported in the central Longmen Shan, indicating consistency along strike. The thermochronology data also suggest that the Erwangmiao fault in the southwestern Longmen Shan is analogous to the Beichuan fault in the central Longmen Shan, and therefore may represent a source of seismic hazard.

한국 동남부 마이오세 분지 화산암과 기반암의 피션트랙 연대 재검토와 연대층서 고찰

피션트랙(fission-track: FT) 연대측정 초기단계의 부적합한 연대보정법에 기인한 오류 원인을 정밀진단하고, 중복시료에 의한 재실험과 제타보정법에 의해 최초 보고된 피션트랙(FT) 연대를 재정의한다. 재검토된 FT 저콘연대는 기반암인 유천층군 유문암질-데사이트질응회암의 생성연대를 후기 백악기부터 고제3기 초(<TEX>$78{\pm}4$</TEX> Ma부터 <TEX>$65{\pm}2$</TEX> Ma)로 재정의하며, 곡강동유문암질응회암을 전기 에오세(<TEX>$52.1{\pm}2.3$</TEX> Ma) 산물로 정의한다. 전기 마이오세 화산암의 경우, 효동리화산암류 상부 데사이트질응회암(<TEX>$21.6{\pm}1.4$</TEX> Ma)과 범곡리화산암류 최상부 데사이트 용암(<TEX>$21.3{\pm}2.0$</TEX> Ma)의 FT 저콘연대는 각각 어일분지 남부와 와읍분지 중앙부의 상부 범곡리층군의 연대층서를, 그리고 금오리데사이트질응회암(<TEX>$19.8{\pm}1.6$</TEX> Ma)의 FT 저콘연대는 장기분지 내 후기 데사이트질 화산활동 시기를 정의한다. 데사이트질암의 FT 저콘연대와 현무암질-안산암질암의 기존 연대자료(대부분 K-Ar 전암, 일부 Ar-Ar)를 기반으로 하여, 한국 동남부의 마이오세 분지의 화산암과 기반암의 층서대비에 길잡이가 될 수 있는 참고연대를 설정 제안한다. 관계화산암의 연대에 기반하여 분지충전 퇴적지층의 퇴적시기도 추정한다. 제안된 참고연대는 마이오세 분지의 복잡한 분지구조와 비교적 짧은 연대범위에도 불구하고 지질층서에 잘 부합된다. 범곡리층군은 어일-와읍분지에서 장기층군보다 하위인 것이 확실하나, 장기분지의 전기 장기층군과 상당부분 중첩 병립되므로, 두 층군을 획일적으로 선후관계로 정의할 수는 없다. 장기분지에서 장기층군 하나로 묶여있는 일련의 지층군은 약 20 Ma를 기준으로 전기(23-20 Ma)의 안산암질-데사이트질암과 후기(20-18 Ma)의 현무암질암으로 뚜렷하게 구분된다. Erroneous fission-track (FT) ages caused by an inappropriate calibration in the initial stage of FT dating were redefined by re-experiments and zeta calibration using duplicate samples. Revised FT zircon ages newly define the formation ages of Yucheon Group rhyolitic-dacitic tuffs as Late Cretaceous to Early Paleocene (<TEX>$78{\pm}4$</TEX> Ma to <TEX>$65{\pm}2$</TEX> Ma) and Gokgangdong rhyolitic tuff as Early Eocene (<TEX>$52.1{\pm}2.3$</TEX> Ma). In case of the Early Miocene volcanics, FT zircon ages from a dacitic tuff of the upper Hyodongri Volcanics (<TEX>$21.6{\pm}1.4$</TEX> Ma) and a dacitic lava of the uppermost Beomgokri Volcanics (<TEX>$21.3{\pm}2.0$</TEX> Ma) define chronostratigraphies of the upper Beomgokri Group, respectively in the southern Eoil Basin and in the Waeup Basin. A FT zircon age (<TEX>$19.8{\pm}1.6$</TEX> Ma) from the Geumori dacitic tuff defines the time of later dacitic eruption in the Janggi Basin. Based on FT zircon ages for dacitic rocks and previous age data (mostly K-Ar whole-rock, partly Ar-Ar) for basaltic-andesitic rocks, reference ages are recommended as guides for stratigraphic correlations of the Miocene volcanics and basements in SE Korea. The times of accumulation of basin-fill sediments are also deduced from ages of related volcanics. Recommended reference ages are well matched to the whole stratigraphic sequences despite complicated basin structures and a relative short time-span. The Beomgokri Group evidently predates the Janggi Group in the Eoil-Waeup basins, while it is placed at an overlapped time-level along with the earlier Janggi Group in the Janggi Basin. Therefore, the two groups cannot be uniformly defined in a sequential order. The Janggi Group of the Janggi Basin can be evidently subdivided by ca. 20 Ma-basis into two parts, i.e., the earlier (23-20 Ma) andesitic-dacitic and later (20-18 Ma) basaltic strata.

The Tectonic and Uplift History of the Kuruketage Area in the North-East Edge of the Tarim Basin, China: Constraints from Detrital Zircon and Apatite Fission Track Data

This study uses the application of zircon fission track (ZFT) and apatite fission track (AFT) thermochronometry technique to investigate the tectonic and uplift history of the Kuruketage area, north-east edge of the Tarim Basin. Based on measured ZFT, AFT and equivalent vitrinite reflectance measurements of samples in sedimentary rocks in Kuruketage area, the temperature time evolution history from early Paleozoic strata was modeled. The results show that the youngest peaks of ZFT at 371-392Ma and 328 - 305.7Ma record Hercynian tectonic and uplift event; the AFT peaks at 134.5 - 164Ma, 73 - 100Ma and 35.4Ma mainly represent the Late-Cretaceous tectonic and uplift event in Kuruketage area. The AFT thermal modeling results from the early Paleozoic strata indicate that the maximum paleo-temperature (at 140 215°C) experienced in late Silurian to early Devonian, and the strata temperature decreased to about 120°C before the Late-Cretaceous.

HYDROCARBON POTENTIAL IN JORDAN

The paper presents results from a range of petroleum exploration studies carried out in Jordan – some regional in approach, others focussed on the NE of the country – together with a review of existing literature and industry reports. The dominant active structural feature in Jordan is the NNW‐SSE striking Dead Sea Transform, characterised in southern Jordan by extensional splay fault arrays, pull‐apart basins and relay‐sag basins. Elsewhere in the country there are regional‐scale strike‐slip faults (ENE‐WSW, NNW‐SSE) with multiple phases of reactivation. Stratigraphic studies have identified potential reservoir intervals within the Lower Palaeozoic Salib, Umm Sahm, Disi and upper Dubaydib (Risha Sandstone Member) Formations, and also within the Triassic and Cretaceous intervals. Potential source rocks occur within the Lower Palaeozoic Burj, lower Hiswa and Dubaydib Formations and at basal and Upper Silurian levels, with limited additional potential in the Triassic and possibly Permian successions. Apatite fission track analysis (AFTA), zircon fission track analysis (ZFTA) and vitrinite reflectance (VR) studies of selected wells and outcrop samples indicate that the thermal history of eastern Jordan is dominated by the effects of two major periods of relatively deep burial, with subsequent exhumation phases beginning in the Late Palaeozoic and Late Cenozoic. An intermediate episode (Mesozoic) of deep burial and exhumation is possible. Significant variations in heat flow regimes and burial patterns occur in different parts of Jordan, probably controlled by deep structure and burial history. Over much of the north of the country, the Lower Palaeozoic succession is in the gas window and the underlying section is overmature. The main hydrocarbon generation and expulsion phase in this region was in the Late Palaeozoic and was terminated by “Hercynian” (Late Palaeozoic) uplift. Further south, the Silurian and Upper Ordovician sections are in the oil window, and deeper source intervals are mature for gas. The Triassic interval is a valid hydrocarbon play in the NW and NE of Jordan, with oil sourced from either Permian, Triassic or Cretaceous kitchens. Upper Cretaceous source rocks within the thicker Mesozoic sections in the Azraq Graben are in the oil window and have sourced the small‐scale Hamza oilfield. In addition to the known widespread Upper Cretaceous – Paleocene oil shale potential in Jordan, the Ordovician and Silurian sections have potential for shale gas in northern Jordan and for shale oil further south.

Exhumation of the southern Sierra Nevada–eastern Tehachapi Mountains constrained by low-temperature thermochronology: Implications for the initiation of the Garlock fault

New apatite and zircon fission-track and apatite (U-Th)/He data from nine samples collected on a north-south transect across the southern Sierra Nevada–eastern Tehachapi Mountains constrain the cooling and exhumation history over the past ∼70 m.y. The four northernmost samples yielded zircon and apatite fission-track ages of ca. 70 Ma, indicating rapid cooling from ∼250 °C to

Rift‐related origin of the Paleoproterozoic Kuncha Formation, and cooling history of the Kuncha nappe and Taplejung granites, eastern Nepal Lesser Himalaya: a multichronological approach

AbstractIn order to decipher the origin and tectonothermal history of the Kuncha nappe, we undertook a geological investigation in the Taplejung window in eastern Nepal, and carried out multichronological analyses of zircon, apatite, and mica of the Kuncha Formation and Taplejung granites. Three granite bodies that intrude into the Kuncha Formation show fission‐track (FT) ages of 6.2 to 4.8 Ma for zircon and 2.9 to 2.1 Ma for apatite, although the granites yielded zircon U–Pb ages of 1852 ± 24 Ma and 1877 ± 21 Ma, and muscovite 40Ar–39Ar ages of ca 1650 Ma. The U–Pb intercept age of detrital zircons from the Kuncha schist is 1888 ± 16 Ma, and this age represents a probable depositional age of the lower part of the Kuncha Formation. The FT ages of zircon and apatite from the same schist are found to be 5.4 and 2.5 Ma, respectively. No zircon younger than 1.6 Ga was identified from any of these samples. It means that the Kuncha nappe has never undergone thermal events after 1.6 Ga until the Himalayan orogeny in Miocene. The Kuncha Formation and overlying Kali Gandaki Supergroup as well as the Taplejung granites can be correlated with the Coronation Supergroup and the Hepburn intrusives in the Wopmay orogen, northwest Canada. They are interpreted to be deposits in the basins of continental rift system and subsequent passive‐margin settings. All the FT age data of zircon and apatite provide evidence that both the Kuncha nappe and overlying Crystalline nappe seem to have cooled laterally from the nappe front to the north. The FT ages from the front to central part of the nappe in Taplejung suggest that isotherm of 240°C, closure temperature of zircon FT, retreated toward north at about 10 mm/y during the middle to late Miocene and Pliocene.

The low-temperature thermochronological record of sedimentary rocks from the central Rovuma Basin (N Mozambique) — Constraints on provenance and thermal history

Fission track (FT) dating of detrital titanites, zircons and apatites combined with sandstone petrography from rocks of the Rovuma Basin was used to constrain the basin's provenance and its post depositional thermal history. A comparison of metamorphic basement and sedimentary titanite and zircon FT data indicates that erosion was localized along a zone which followed the margin of the Rovuma Basin and was the source for the late Jurassic to Cretaceous sandstones. Time–temperature models of apatite FT data show, that after the deposition, the sandstones were heated up to temperatures of ca. 60–110°C most likely due to a combination of intensified regional heat flux and burial heating caused by fast sedimentation in a transtensional pull-apart setting and intensified by regionally elevated heat flux. The utter western part of the basin was inverted between ca. 60 and 40Ma, concordant with the drop in the global eustatic sea level which led to a rearrangement of the source-to-sink system. Some reworked zircons were deposited in the Cenozoic strata in the eastern part of the basin.

Fission track thermochronology of the Beni Bousera peridotite massif (Internal Rif, Morocco) and the exhumation of ultramafic rocks in the Gibraltar Arc

The Beni Bousera peridotite massif and its metamorphic surrounding rocks have been analyzed by the fission track (FT) method. The aim was to determine the cooling and uplift history of these mantle and associated crustal rocks after the last major metamorphic event that dates back to the Lower Miocene–Upper Oligocene time (∼22–24 Ma). The zircon FT analyses give an average cooling—i.e., below 320 °C—age of ∼19.5 Ma. In addition, the apatite FT data give an average cooling—i.e., below 110 °C—age of ∼15.5 Ma. Taking into account the thermal properties of the different thermochronological systems used in this work, we have estimated a rate of cooling close to 50 °C/Ma. This cooling rate constrains a denudation rate of about ∼2 mm year−1 from 20 to 15 Ma. These results are similar to those determined in the Ronda peridotite massif of the Betic Cordilleras documenting that some ultrabasic massifs of the internal zones of the two segments of the Gibraltar Arc have a similar evolution. However, Burdigalian sediments occur along the Betic segment (Alozaina area, western Betic segment) unconformably overlying peridotite. At this site, ultramafic rock was exposed to weathering at ages ranging from 20.43 to 15.97 Ma. Since the Beni Bousera peridotite was still at depth until 15.5 Ma, we infer that no simple age projection from massif to massif is possible along the Gibraltar Arc. Moreover, the confined fission track lengths data reveal that a light warming (∼100 °C) has reheated the massif during the Late Miocene before the Pliocene–Quaternary tectonic uplift.

Short-lived, fast erosional exhumation of the internal western Alps during the late early Oligocene: Constraints from geothermochronology of pro- and retro-side foreland basin sediments

Apatite and zircon fission-track analysis and single zircon fission-track–U/Pb double dating of Oligocene to Miocene pro- and retro-side foreland basin sedimentary rocks provide evidence for short-lived but fast erosional exhumation of at least 1.5–2 km/m.y. in the internal western Alps between ca. 30 and 28 Ma. This period of fast erosion is seen as a result of rapid surface uplift coupled with increasing orographic precipitation during this phase of orogenesis. Surface uplift may have been caused and sustained by different plate-tectonic processes such as a change in convergence direction, intermediate-depth slab breakoff, and emplacement of the Ivrea body during continental collision. The occurrence of contemporaneous volcanic activity on the pro-side of the western Alps on the subducting European plate between ca. 36 Ma and 30 Ma is seen in connection with slab rollback of the Apennine slab and upwelling of hot mantle material beneath the western Alps. Single zircon double dating shows that the exhumational signal in the detrital thermochronologic data is not compromised by volcanically derived zircons, as volcanic grains can be identified and removed from the zircon fission-track data set to obtain a pure exhumational signal. The signal of fast exhumation is observed in the zircon fission-track data of the pro-side foreland basin and in the apatite fission-track data and published 40Ar-39Ar data in the retro-side foreland basin. During late Oligocene times, erosion rates slowed down to rates similar to present-day erosion rates in the western Alps.

Late Eocene to Early Miocene Andean uplift inferred from detrital zircon fission track and U–Pb dating of Cenozoic forearc sediments (15–18°S)

Timing, amount, and mechanisms of uplift in the Central Andes have been a matter of debate in the last decade. Our study is based on the Cenozoic Moquegua Group deposited in the forearc basin between the Western Cordillera and the Coastal Cordillera in southern Peru from ∼50 to ∼4 Ma. The Moquegua Group consists mainly of mud-flat to fluvial siliciclastic sediments with upsection increasing grain size and volcanic intercalations. Detrital zircon U–Pb dating and fission track thermochronology allow us to refine previous sediment provenance models and to constrain the timing of Late Eocene to Early Miocene Andean uplift. Uplift-related provenance and facies changes started around 35 Ma and thus predate major voluminous ignimbrite eruptions that started at ∼25 by up to 10 Ma. Therefore magmatic addition to the crust cannot be an important driving factor for crustal thickening and uplift at Late Eocene to Early Oligocene time. Changes in subduction regime and the subducting plate geometry are suggested to control the formation of significant relief in the area of the future Western Cordillera which acts as an efficient large-scale drainage divide between Altiplano and forearc from at least 15.5 to 19°S already at ∼35 Ma. The model integrates the coincidence of (i) onset of provenance change no later than 35 Ma, (ii) drastic decrease in convergence rates at ∼40, (iii) a flat-subduction period at around ∼40 to ∼30 Ma leading to strong interplate coupling, and (iv) strong decrease in volcanic activity between 45 and 30 Ma.

Cenozoic thermo-tectonic evolution of the northeastern Pamir revealed by zircon and apatite fission-track thermochronology

The northeastern Pamir is a key location to explore Asian intracontinental tectonic processes during the Cenozoic. New zircon fission-track (ZFT) data show a 20- to 50-km-wide region of partially reset ages on the northeastern margin of the Pamir salient, interpreted as an exhumed and tilted partial annealing zone (PAZ). Widespread ZFT age peaks at ~50Ma within the ZFT PAZ likely date early motion of the Kashgar–Yecheng transfer system (KYTS), but suggest this fault system was narrower in the Early Cenozoic than it is today. Apatite fission-track (AFT) ages of ~10–6Ma, combined with field observations across the KYTS, hint at an episode of strong thrusting-related exhumational cooling, which indicates that the modern fault system probably formed at this time. To the southwest of the KYTS, the combination of new fission-track and existing thermochronology data allows establishing temperature-time trajectories that present diachronous rapid cooling from ~450 to 120°C in the Sares (>13–10Ma), Muztagata (~10–7Ma) and Kongur Shan (~3–1Ma) domes. Rapid cooling in the eastern Sares and southern Muztagata massifs is driven by doming, as supported by kinematic analyses of the Shen-ti fault. Successive rapid cooling of these massifs confirms eastward propagation of doming processes, shortly postdating magma emplacement at ~11Ma. We propose that the synchronicity of regional tectonism, magmatism and metamorphism implies that strong crustal thickening and contraction occurred beneath the northeastern Pamir during the Middle–Late Miocene, possibly associated with initial collision between the Pamir and Tian Shan.

Focused Pliocene–Quaternary exhumation of the Eastern Pamir domes, western China

The Kongur Shan and Muztagata domes are the most prominent topographic features in the Eastern Pamir indenter corner. In spite of several previous bedrock thermochronology studies, the extent, processes and mechanisms of dome exhumation remain unclear. We present 1158 detrital zircon fission-track (DZFT) ages from modern glacier and river sediments along the domes to better constrain the spatial distribution of exhumation. Our thermochronologic dataset from the Chakragil massif shows a sharp increase in the proportion of DZFT ages >10Ma from basins that drain the peak region to those further to the north, which suggests that the Kalagile fault forms the northern boundary of the rapidly exhuming dome structure. A cluster of Middle Miocene DZFT ages in these samples documents pre-doming cooling, possibly associated with motion on the Main Pamir thrust and/or Oytag fault. Large fractions of DZFT grain ages <10Ma in samples from catchments east of the divide likely originate from the Chakragil, Kongur Shan and Muztagata massifs, implying focused exhumation consistent with DZFT age distributions in samples from catchments draining the western flank of the range and bedrock geo- and thermochronologic data from the core of the domes. Furthermore, the majority of the samples contain two distinct DZFT age groups at ∼6–4Ma and ∼3–1Ma respectively. These ages are synchronous with major fluvial sedimentation phases at the outlets of the rivers draining northeastward into the Tarim Basin and suggest rapid Pliocene–Quaternary exhumation of the domes. We interpret the ∼6–4Ma age group to record the onset of extension along the entire Kongur Shan fault; its timing with respect to compressional deformation supports a model of gravitational collapse of over-thickened crust. The fraction of ∼3–1Ma DZFT ages is larger than what would be inferred from the bedrock data alone. We interpret this to reflect recent focused denudation in response to glaciation and fluvial incision of the domes and their possible feedback with extensional faulting.

Thermal history of the Maramureş area (Northern Romania) constrained by zircon fission track analysis: Cretaceous metamorphism and Late Cretaceous to Paleocene exhumation

Thermal history of the Maramureş area (Northern Romania) constrained by zircon fission track analysis: Cretaceous metamorphism and Late Cretaceous to Paleocene exhumation

Evidence of extensional metamorphism associated to Cretaceous rifting of the North-Maghrebian passive margin: The Tanger-Ketama Unit (External Rif, northern Morocco)

The distribution pattern of diagenetic conditions to very low-grade metamorphism in the eastern Rif has been determined based on a study of clay-mineral assemblages and illite crystallinity of Mesozoic metapelites. Low-grade conditions were reached in marbles and also in the Beni-Malek serpentinites, as suggested by the mineral assemblages present in the marbles and antigorite growth in serpentinites. Previous thermochronological data are based on i) 40Ar/39Ar in amphiboles from greenschists, ii) K/Ar in white micas from metasandstones, and iii) fission tracks in apatites and zircons from metasandstones. These data indicate a Late Cretaceous age (80 Ma) for the very low- to low-grade metamorphism. We propose an evolutionary model for the Tanger-Ketama Unit consisting of a Lower Cretaceous sequence deposited in half-graben basins over an exhumed serpentinized mantle in a setting similar to the West Galician non-volcanic margin. The sediments underwent diagenesis to very low-grade metamorphism under relatively high heat flow in this extensional setting. Miocene contractional deformation of the Tanger-Ketama Unit resulted in a penetrative crenulation cleavage associated to asymmetric inclined folds. This crenulation developed, mostly by solution-transfer processes, without significant mineral growth. Miocene metamorphism reset the apatite fission-tracks, but metamorphic conditions were not high enough to reset either the K/Ar ages or the zircon fission tracks.

Mesozoic and Cenozoic Cooling History of the Qiangtang Block, Northern Tibet, China: New Constraints from Apatite and Zircon Fission Track Data

Mesozoic and Cenozoic Cooling History of the Qiangtang Block, Northern Tibet, China: New Constraints from Apatite and Zircon Fission Track Data

Erosional history of the Prydz Bay sector of East Antarctica from detrital apatite and zircon geo‐ and thermochronology multidating

Approximately 98% of East Antarctica is covered by the East Antarctic Ice Sheet (EAIS), which has covered parts of the continent since the early Oligocene (34 Ma) and obscures evidence about the region's tectonic and erosional history. To better constrain the subglacial record, we analyzed geo‐ and thermochronologic dates of Oligocene‐Quaternary sediments from Prydz Bay, which drains ∼16% of the EAIS. We used multidating techniques, measuring U‐Pb, fission track, and (U‐Th)/He dates on apatite and zircon grains and40Ar/39Ar dates on hornblende grains to determine crystallization and cooling ages. Apatite and zircon U‐Pb dates and hornblende40Ar/39Ar dates are dominantly ∼500 Ma, recording Pan‐African metamorphism and magmatism. Zircon fission track dates record cooling at ∼250–300 Ma and ∼120 Ma from Permian‐Triassic (300‐201 Ma) rifting and Cretaceous (120 Ma) magmatic resetting. Mean apatite fission track dates decrease from ∼280‐210 Ma in early Oligocene samples, with lag times decreasing from ∼250‐180 My, indicating increasing erosion rates. Miocene‐Quaternary (10.7‐0 Ma) samples show a smaller range from ∼180 to ∼150 Ma. Youngest measured apatite He ages also decrease from ∼100 Ma to ∼25 Ma in Oligocene‐Miocene samples. These results indicate increasing erosion rates (&lt;0.02 km/My to &gt;0.2 km/My) in catchments draining to Prydz Bay in the early Oligocene, with slower erosion since the late Miocene. This erosion was likely achieved by glacial incision into pre‐existing valleys, reaching depths of ∼2.8–3.0 km by the late Miocene. This is consistent with EAIS models showing a transition to less erosive, cold‐based conditions following the mid‐Miocene climatic optimum.

Thermochronological dating of brittle structures in basement rocks: A case study from the onshore passive margin of SW Sri Lanka

The brittle structural inventory of southern and southwestern Sri Lanka has been studied by kinematic, mineralogical and thermochronological techniques. Thermochronological analyses of faults comprise apatite and zircon fission track (FT) data and apatite (U‐Th‐Sm)/He ages from fault planes and undisturbed host rocks, and range between ∼65 and ∼230 Ma. The ages of both settings are undistinguishable for topographic altitudes below ∼100 m, while fault planes from higher elevations are significantly younger than the corresponding host rocks. Thermal history modeling and qualitative interpretation of the thermochronological data identify at least five episodes of thermal overprint associated with faulting activity occurring at 159 ± 18, 144 ± 14, 120 ± 10, 94 ± 8, and 70 ± 10 Ma. The kinematic, mineralogical and thermochronological data collectively show that Sri Lanka was subjected to major N‐S oriented extension subsequent to the Gondwana breakup. The resulting brittle structures exert primary control on the island's geomorphology, especially on the Southern Escarpment of the Sri Lankan Highlands.