Evaluation of the 40Ar/39Ar technique for kimberlite geochronology: Three case studies from Finland

Dalton, Hayden1, Giuliani, Andrea 1,2, Hergt, Janet1, Maas, Roland1, Matchan, Erin1, O’Brien, Hugh3, Phillips, Professor David1, Woodhead, Jon1

1School of Earth Sciences, The University of Melbourne, Parkville, Australia, 2Institute of Geochemistry and Petrology, Department of Earth Sciences, Zurich, Switzerland, 3Geological Survey of Finland, Espoo, Finland

Kimberlites are enigmatic, volcanic rocks of both great economic and scientific importance due to acting as the primary host-rock to diamonds and being the deepest-derived continental magmas on Earth. Despite this significance, there remains debate concerning the sources of kimberlites and what triggers mantle melting to form these rocks. Robust determination of the timing of kimberlite eruption is a crucial prerequisite if we are to unravel the presence of any spatiotemporal relationships between kimberlite emplacement and large-scale tectonic processes, super-continental cycles or mantle plumes.

Despite the benefit of the remarkably high precision achieved with modern 40Ar/39Ar analytical techniques, and the presence of K-bearing groundmass phlogopite in many kimberlites, this technique has seldom been applied to kimberlites and related rocks. Early utilisation of this method revealed issues related to the presence of extraneous argon in mica macrocrysts and phenocrysts which yields anomalously old or maximum emplacement ages. Nonetheless, apparently reliable age results have been obtained on magmatic mica from kimberlites and related rocks. In this study we compare new, precise 40Ar/39Ar ages with other independent age constraints (e.g., Rb/Sr, U/Pb) on three clusters of kimberlites and related rocks from Finland to rigorously assess the instances where 40Ar/39Ar dating produces older apparent ages.

Our results indicate that sample selection and groundmass mica (phlogopite or kinoshitalite) separation needs to be extremely judicious prior to analysis. Where fresh mica phenocrysts are available for 40Ar/39Ar analyses we recommend that plateau results are interpreted with caution. Age spectra which are entirely flat, such that an age plateau includes 100% of the gas are likely the most accurate and precise reflection of the emplacement age of a kimberlite. In contrast, aliquots that yield younger apparent ages for heating steps preceding the plateau may reflect argon recoil redistribution resulting in anomalously older high-temperature/plateau ages when compared with independent age constraints. In cases where such discordance exists, we recommend that total-gas ages give a better approximation of the emplacement age and one which agrees more closely with ages from other geochronometers.


Biography

Hayden Dalton is presenting on behalf of the greater AuScope Geochemistry Laboratory Network. Hayden is a PhD researcher in the School of Earth Sciences at the University of Melbourne, his research focuses on the geochronology and geochemistry of kimberlites.

Hydrous polymetamorphic crustal rocks in an eclogite-bearing terrane record post-peak recrystallisation during arc-continent collision

Brown, Dillon1, Hand, Martin1, Morrissey, Laura2,1

1Department of Earth Sciences, University of Adelaide, Adelaide, SA, Australia. 2Future Industries Institute, University of South Australia, Mawson Lakes, SA, Australia

Ultrahigh- and high-pressure terranes are identified based on the occurrence of mafic eclogite-facies mineral assemblages, which effectively record burial and subduction metamorphic conditions. In such terranes however, mafic mineral assemblages are volumetrically minor compared to the continental rocks that host them, which typically preserve anhydrous quartzofeldspathic amphibolite-facies mineral assemblages. Continued debate centres around two hypotheses accounting for the petrology of the continental rocks: (1) the protoliths to the continental rocks did not respond to burial and subduction, and (2) the continental rocks developed high-pressure mineral assemblages which were subsequently overprinted during exhumation. However, less is known about continental rocks that preserve hydrous amphibolite-facies assemblages and schistose fabrics, which are also documented in high-pressure rock systems. In western Tasmania, south-east Australia, eclogite-facies mafic boudins with previously constrained subduction metamorphic conditions of 18–21.5 kbar and 650–700 °C are hosted by weakly foliated to mylonitic metapelitic continental rocks preserving hydrous amphibolite-facies assemblages dominated by siliceous muscovite, quartz, and garnet. Weakly foliated metapelites preserve relict kyanite and two possible textural generations of garnet, moderately foliated metapelites record evidence of partial melting, and mylonitic metapelites contain sillimanite within the rock fabric. Monazite LA–ICP–MS U–Pb geochronology documents two instances of monazite growth in the metapelites: a possible Mesoproterozoic growth event at c. 1385 Ma and a younger, yet poorly constrained growth event in the Cambrian. Rutile LA–ICP–MS U–Pb geochronology more precisely constrains Cambrian-aged metamorphism in the metapelites between 520–505 Ma. Monazite-garnet petrochronology reveals that Mesoproterozoic-aged monazite formed in a system with little or no influence of garnet whereas Cambrian-aged monazite formed in the presence of garnet during subduction. Metamorphic mineral equilibrium modelling of Cambrian subduction indicates that the weakly foliated metapelites best approximate peak metamorphism, recording metamorphic conditions of 13–17 kbar and 600–720 °C. Migmatitic metapelites record lower pressure conditions of 8–13 kbar and 660–740 °C and mylonitic metapelites equilibrated at 3.5–7 kbar and 590–680 °C. We infer that the amphibolite-facies metapelites record different stages of Cambrian-aged exhumation and, unlike their mafic counterparts, do not record burial or subduction. We attribute the inferred eradication of peak subduction mineral assemblages in the metapelites to the influence of fluid and localised deformation.


Biography

Dillon is a PhD student from the University of Adelaide working under the themes of metamorphic petrology, geochemistry, and petrochronology. His research focuses on understanding the geodynamic character and tectonism associated with the Cambro-Ordovician East Gondwana margin.

Sub-solidus replacement of rapakivi textures during high-temperature potassium metasomatism of the Mannum Granite

Forster, Marnie1,2, Goswami, Naina1,2, Lister, Gordon1, Reid, Anthony3

1Research School of Earth Sciences, Australian National University, Canberra, Australia, 2MinEx CRC, Canberra, Australia, 3Geological Survey of South Australia, Adelaide, Australia

The Mannum Granite is a high-temperature A-type granitoid emplaced during the late stages of the Delamerian Orogeny in South Australia. It has attracted considerable research interest over past decades because it is a porphyritic A-type granite that displays well-developed rapakivi structures and has been linked to the process of magma-mingling above deep-seated mafic igneous bodies. It contains many mafic enclaves of varied sizes and textures including potassium feldspar and quartz porphyroblasts with disseminated sulphides. The alkali feldspar cores of the rapakivi textures are lobate and overgrow the plagioclase rims, suggesting alkali feldspars are younger.

We conducted 40Ar/39Ar geochronology in conjunction with ultra-high-vacuum (UHV) step heating 39Ar diffusion experiments on K-feldspar and biotite from the Mannum granite. Conjoint inversion of data from the UHV 39Ar diffusion experiments using the Wunderkind program applied to data from the K-feldspar experiment demonstrates the presence of highly retentive core domains capable of retaining radiogenic argon even to temperatures in excess of 600°C. Inversion of the geochronology data using the MacArgon program produces a temperature-time history that suggests that the K-feldspar in the core of the rapakivi texture formed as the result of sub-solidus solid state replacement and/or metasomatism at ~443 Ma, and then cooled rapidly at ~440 Ma. Since biotite spectra which define a plateau age at ~472 Ma are relatively undisturbed, the temperature excursion associated with the metasomatic event would have to have been limited in its magnitude (<~500°C) and duration (<< 1 Ma). 

The biotite age (~473 Ma) is close to the previously determined early Ordovician crystallisation ages for the granites in this belt. In addition, the biotite appears to be highly retentive, with closure temperature for cooling at 20°C/Ma determined as the result of the UHV 39Ar diffusion experiment at ~485°C. These are thus exceptionally retentive biotites, which correlate with the fact that they are iron-rich and fluorine-rich, and fluorine-rich biotite typically exhibits higher argon retentivity. Overall this short/sharp thermal pulse may have occurred in consequence of fluid movement associated with the Old Teal Flat Shear Zone to the east, which contains fabrics dated in this study at ~445 Ma.

Microstructural analysis shows sericite and calcite precipitated along grain boundaries, fractures and exsolution lamellae. The presence of interpenetrating grain boundaries, sericite, calcite and secondary K-feldspar is attributed to the infiltration of an acidic hydrothermal agent hydrolysing primary K-feldspar to produce secondary sericite and quartz, further reacting with Ca-rich plagioclase and quartz to produce secondary K-feldspar phase(s) and calcite. Microstructures thus confirm the presence of a potassium-rich metasomatic event at Mannum leading to fluid-assisted in-situ replacement of K-feldspars with concomitant precipitation of secondary mineral phases.

Acknowledgements

The work has been supported by the Mineral Exploration Cooperative Research Centre whose activities are funded by the Australian Government’s Cooperative Research Centre Program. This is MinEx CRC presentation.


Biography

Naina Goswami is a second year PhD candidate at The Australian National University (ANU). She finished her Masters (advanced) from ANU in 2018 and her B.Sc (Hons) Chemistry from University of Delhi 2016. She specialises in 40Ar/39Ar geochronology and geochemistry. Currently she is undertaking her PhD at ANU working on a MinEx-CRC project based in South Australia.

Dating the timing of motion in major ductile shear zones

Forster, Marnie1, Lister, Gordon2

1Research School of Earth Sciences, Australian National University, Canberra, Australia; 2The Virtual Explorer

There is confusion in the argon geochronology world as to what allows movement in a ductile shear zone to be dated. Some assert that all that is necessary is to data mica and to obtain a ‘plateau‘. But this is not at all sufficient to make the argument. 40Ar/39Ar geochronology (like all radiometric dating techniques) does not have the ability to date movement. It is the microstructural modification of existing grains that must be dated, e.g., growth or regrowth during movement. Otherwise it may be that the fabric forming minerals are remnant: namely, relicts of an earlier formed mineralogy, and the ages obtained not at all relevant to the timing of movement in a later shear zone.

An ideal circumstance would be clear and unambiguous demonstration that growth of a particular mica had taken place immediately prior to or during the operation of a ductile shear zone, and that mineral separation (or laser spots) had focussed on those volumes during measurement. An example would be sudden growth of mica porphyroblasts that were then rotated and aligned in a developing shear zone fabric, as occurs in retrograde shear zones in the Cycladic Eclogite-Blueschist Belt. Movement has not been dated, but an estimate has been obtained as to the timing of growth during or immediately preceding movement. Another optimal circumstance would be if the operation of the ductile shear zone had shredded mica, progressively reducing its diffusion dimension. This behaviour leads to staircase spectra characteristic of fractal diffusion. Such age spectra appear to be able to allow the distinction of the start and the end of shear zone operation. In some cases, the age of relict mica microstructures is also evident, e.g., in mica from the Main Central Thrust of the Himalaya.

A more difficult circumstance occurs when age spectra from fabric forming minerals appear to be unrelated to the timing of movement, e.g., for mica from greenschist facies ductile shear zones in the Cap de Creus, Spain. We inverted data from 40Ar/39Ar geochronology step-heating experiments, using potassium feldspar, after conjoint inversion of data from simultaneous ultra-high-vacuum (UHV) 39Ar diffusion experiments. The resultant temperature-time curves imply that these mylonites formed in Eocene to Oligocene time, and therefore that they are not Variscan or Jurassic, as previously argued. Their tectonic significance is likely to be as right-lateral strike-slip shear zones formed in transfer faults accommodating roll-back of the Tethyan subduction zone as it dragged Sardinia and Corsica away from the Palaeo-European margin during opening of the Gulf of Lyon.

These examples suggest caution needs to be exerted in the dating of movement in ductile shear zones. Laser-step heating (or laser spot analysis) is not suited for this purpose, since this method provides no information in respect to Arrhenius data. In consequence the retentivity of relevant minerals in respect to argon diffusion cannot be assessed. In addition, laser methods do not produce consistent detail in age spectra. In contrast, robotic methods applied to resistance-furnace step-heating experiments offer a cheap, efficient, and reliable way to obtain the detailed age spectra that are necessary (in conjunction with Arrhenius data) to characterise the pattern of argon release.


Biography

Gordon Lister and Marnie Forster are structural geologists with a particular interest in the theory and practice of argon geochronology, in particular in the study of the dynamics of the evolution of orogenic architecture and its impact on metallogenesis.

The implications of muscovite sub-spectra in phengitic white mica on the theory and practice of argon geochronology

Forster, Marnie1, Lister, Prof. Gordon1

1Research School of Earth Sciences, Australian National University, Canberra, Australia

This study illustrates a new method for the quantitative determination of the timing of movement in ductile shear bands formed in mylonites, or in strongly stretched metamorphic tectonites. The method is of particular use where phengitic white mica is involved, since interlaying in this mineral is usually so fine as to preclude the application of laser methods. In any case, laser methods as they are currently applied, do not have the capability of extracting exact and detailed age-temperature spectra. Laser methods also fail to achieve the multitudinous steps of the age spectrum evident from our high-definition UHV diffusion experiments. Laser methods also lose all information in terms of the low-volume early release of argon that is essential for the recognition of sub-spectra. Computer modelling and simulation shows that such detail in the age spectrum is essential in terms of being able to accurately infer the timing and duration of metamorphic events,

Here we show that high-definition ultra-high-vacuum (UHV) 39Ar diffusion experiments using phengitic white mica are routinely able to extract muscovite sub-spectra in the first 10-30% of 39Ar gas release during 40Ar/39Ar geochronology. A critical factor is that the recognition of muscovite sub-spectra requires Arrhenius data in order to recognise the steps dominated by release of 39Ar from muscovite. In turn this requires precise measurement of temperature during each heating step. The muscovite sub-spectrum is distinct and separate to the main spectrum, which is itself dominated by mixing of gas released from phengite as well as muscovite. The muscovite sub-spectra allow consistent estimates of the timing of the formation of microstructural shear bands in various mylonites, as well as allowing quantitative estimates of temperature variation with time during the tectonic history of shear zones.

Our new data reveals hitherto unsuspected variation in the timing of exhumation of individual slices of the eclogite-blueschist belt, caused by Eocene and Miocene detachment-related shear zones. With excellent outcrop, the eclogite-blueschist belt exposed in the Cycladic archipelago in the Aegean Sea, Greece, offers a spectacular natural laboratory in which to decipher the structural geology of a highly extended orogenic belt and to ascertain the history of the different fabrics and microstructures that can be observed. Using phengitic white mica we demonstrate a robust correlation of age with microstructure, once again dispelling the myth that 40Ar/39Ar geochronology using this mineral, produces cooling ages alone. Previous work in the Cycladic eclogite-blueschist belt has incorrectly assumed that the diffusion parameters for phengitic white mica were the same as for muscovite. Arrhenius data suggest this is not the case, and that phengitic white mica is considerably more retentive of argon than muscovite. Previous workers have also erred in dismissing microstructural variation in age as an artefact, supposedly as the result of the incorporation of excess argon. This has led to inconsistencies in interpretation, because phengite is able to retain argon at temperatures that exceed those estimated using metamorphic mineral parageneses.

The argon system has been treated as a thermochronometer. However, we demonstrate a robust correlation between microstructure and age, down to the detail present in complex tectonic sequence diagrams produced during fabric and microstructural analysis of individual thin-sections. This points to new strategies being required in terms of the theory and practice of argon geochronology.


Biography

Dr Marnie Forster is following in the footsteps of Prof Ian McDougall running the ANU Argon Laboratory  with the assistance of Davood Vasegh and Agnes the argon robot.

High resolution 40Ar/39Ar geochronology in continental margin settings – the Aegean plate margin as a natural laboratory for subduction processes.

Wijbrans, Jan1, Uunk, Bertrum1, de Paz Alvarez,Manuel1, Huybens, Rosanne1, Brouwer, Fraukje1

1Department of Earth Sciences, Vrije Universiteit Amsterdam, the Netherlands

Time scales of tectonic and metamorphic processes in the greenschist-blueschist domain are essential to better understand the dynamics of accretionary wedges. However, such time scales are difficult to obtain because the number of geochronometers available to us is limited. For these geochronometers only partial setting, or resetting, of the isotopic clocks can be expected at temperatures reached in the blueschist and greenschist facies, whilst shear zone deformation may not cause full recrystallization. New, or more refined geochronological approaches, can thus shed additional light on the processes in accretionary wedges.

The Aegean subduction complex has emerged over the years as a key natural laboratory to study subduction processes. Our focus in recent years has been on the subduction related processes as experienced by the rocks of Syros and Sifnos islands. Here, we report (1) our efforts to further refine dating approaches:  40Ar/39Ar by dating of suites of single phengite crystals on an outcrop to section scale, and (2) our work exploring new approaches of dating of minerals free of lattice-bound potassium such as garnet, amphiboles and epidotes by dating the signal derived from the fluid inclusions by stepwise crushing.

Dating complete 100+ m sections by single grain phengite dating has the added benefit that all available lithologies in the section contribute to a histogram or PD- plot. Where previously a multigrain average from a single rock specimen was obtained, now is revealed that different units record an age range often as much as 10 Myrs wide, the oldest ages recording crystallization and the younger resetting following crystallization. Dating fluid inclusions by stepwise crushing allows the identification of multiple fluid reservoirs that contribute to the obtained signal sequentially. Typically, the first crushing steps reveal a reservoir that contains large amounts of excess 40Ar, whereas following exhaustion of this reservoir other distinctly different sources of argon are revealed. Age signals thus obtained are interpreted as documenting periods of increased fluid mobility during and following mineral crystallisation.

Combination of these two approaches provides new insights into the tectonic processes during deep subduction and subsequent exhumation to shallower depths.


Biography

Jan Wijbrans is currently professor of Argon Geochronometry at the Vrije Universiteit Amsterdam. Jan was an ANU postgraduate research scholar at the Research School of Earth Sciences, ANU from 1981 until 1985, when he worked under the guidance of prof. Ian McDougall on timescales of metamorphic processes using the 40Ar/39Ar method.

40Ar/39Ar geochronology of syn-kinematic phengite reveals the tectonic history of underthrusted European crust (W Alps): a synthesis

Rolland, Yann1

1EDYTEM, Université de Savoie Mont Blanc, La Bourget du Lac, France

The major problem of 40Ar/39Ar geochronology applied to deformation is often to find sufficiently large crystals that grew in a clear relationship to the deformation context, and that have been preserved in the following tectonic history.

The dating of white mica syn-kinematic fibres (phengite) in complement to their PT conditions (obtained by combined mineralogical mapping for instance) allows the dating of deformation stages at a depth constrained by PT calculations and along certain geothermal conditions. Kinematics of related shear zones provide the polarity of tectonic motions. The integration of such data into the tectonic framework of a geodynamic zone allow deciphering along-belt variations, which relate to strain propagation and changes in the stress field through time and space.

The External Crystalline Massifs (ECM) of the Alps provide a nice case example of a crustal domain in which this technique was successfully applied to the dating of brittle-ductile motions at mid-crustal levels (10-15 km). The Western Alps are a curved mountain belt that accommodated tectonic motions related to subduction dynamics in the Mediterranean domain after a brief period of continent-continent collision of the Apulian microblock with Eurasia. Subsequently, the timing and the along-belt variations of deformations has been progressive through time.

In this presentation, I propose a synthesis of analyses conducted at the External Alps scale to investigate the timing of shear zone deformation since the onset of Eocene-Oligocene collision. The data include 40Ar/39Ar stepwise dating of synkinematic phengites, the associated shear zone kinematics and thermobarometric constraints on the depth and temperature of deformation. The advent of new dating approaches, based on U-Pb dating of syn-kinematic allanite, of hydrothermal vein monazite, and more recently, of calcite from veins and fault gouges, do confirm the timing of deformation suggested by the obtained 40Ar/39Ar ages on micas.

These dating constraints are used to discuss the timing of deformation of underthrusted European crust related to changes in tectonic modes during the Alps’ tectonic history. These changes are related, at the larger scale, to the evolution of the Mediterranean domain, i.e., to the rapid underthrusting of the external part of European plate, dated in the ECM at 35-32 Ma, followed by the onset a mainly dextral strike-slip context. This strike-slip context is controlled by the anticlockwise rotation of Apulia, notably driven by the Apulian roll-back.


Biography

Yann has worked on many orogen examples, starting from his PhD work on the Himalaya-Karakoram, to the Alps, Caucasus, Tien Shan… He develops approaches coupling geochronology and geodynamics. He has used the Ar/Ar chronometer applied to dating shear zone activity since his post-doc at the ANU in 2001.

Bitterroot and Anaconda Core Complexes: Cretaceous Ductile Flow and Eocene Detachment Faulting in the Northern U.S. Rocky Mountains Defined by Ar/Ar Thermochronology

Foster, David1

1Department of Geological Sciences, University of Florida, Gainesville, United States

The Bitterroot and Anaconda metamorphic core complexes of western Montana and central Idaho, U.S.A. were exhumed by Eocene extensional detachment faulting between about 53 and 38 Ma.  The rocks in the lower plates of these core complexes include highly sheared Proterozoic to Palaeozoic metasediments, Cretaceous granitoids, and Eocene felsic plutonic rocks. Early exhumation of the core rocks occurred after crust of in the Montana hinterland was dramatically thickened (forming the “Montanaplano”) between about 130 and 80 Ma. By Late Cretaceous time the middle crust of the orogenic pile was plastically deforming, resulting in large-scale nappes and shear zones. Voluminous intermediate to felsic igneous rocks intruded the shear zones and formed sheet-like plutons at thrust ramps.  Plastic flow led to ductile thinning of the middle crust coincident with out-wedging in the Montana fold and thrust belt. Rocks exposed in these two core complexes experienced about 5-10 km of exhumation in the Late Cretaceous on the basis of metamorphic assemblages, reconstruction of sections, and thermochronology of upper plate rocks. Eocene extension began in the Northern Rockies about 53 Ma and resulted in linked extensional detachments and magmatism in both core complexes. The Bitterroot complex underwent as much as 15 km of additional exhumation along a ductile to brittle detachment system that initiated with amphibolite facies mylonite. The Anaconda complex underwent about 10-12 km in the deepest part and records green-schist facies mylonite overprinted by transitional brittle-ductile fabrics, and brittle deformation.  Ar/Ar mica cooling ages from both detachments decrease from west to east and constrain the rates of fault slip and unroofing. Exhumation of the middle crustal rocks was at least a two-stage process with significant Cretaceous-Paleogene thinning by ductile flow, followed by unroofing beneath the east-rooted detachment systems. Thermochronology data indicate that both detachment systems developed as composite structures with shallower crustal levels and less exhumation in the west rooting to about 10-km deeper to the east. The hanging wall of the Anaconda complex includes the less-deformed Cretaceous Boulder batholith and related volcanic rocks, while hanging wall rocks of the Bitterroot complex, in the Sapphire Range, record Cretaceous exhumation and more limited Eocene unroofing.


Biography

Professor David A. Foster is Chair of the Department of Geological Sciences at the University of Florida.  His research is focussed on applications of geochronology and thermochronology to Precambrian to Recent tectonics.

Timescale of events around the Cretaceous-Paleogene Boundary: Links between the Chicxulub impact, Deccan volcanism, and the Cretaceous-Paleogene mass extinction

Sprain, Courtney J.1, Renne, Paul R.2,3, Clemens, William A.3, Wilson, Gregory P.4, Self, Steve3, Vanderkluysen, Loyc5, Pande, Kanchan6, Fendley, Isabel7, and Mittal, Tushar8 

1University of Florida, Gainesville, United States, 2Berkeley Geochronology Center, Berkeley, USA, 3University of California-Berkeley, Berkeley, USA, 4University of Washington, Seattle, USA, 5Drexel University, Philadelphia, USA, 6Indian Institute of Technology, Mumbai, India, 7University of Oxford, Oxford, United Kingdom, 8Massachusetts Institute of Technology, Cambridge, USA

The Cretaceous-Paleogene boundary (KPB) mass extinction is one of the most important biotic turnover events in Earth history. This event is important to study for several reasons, the most relevant being its implications on our understanding of the effects of abrupt climate change. Although the temporal coincidence between the Chicxulub crater and the KPB has strongly implicated the impact as the main player in the mass extinction, the eruption of the Deccan Traps (DT) cannot be dismissed as a possible contributor. The timing of DT eruptions spans the KPB and, furthermore, the onset of DT volcanism roughly coincides with Late Cretaceous records of environmental change. Both the Chicxulub impact and DT volcanism have similar environmental forcing mechanisms, albeit acting on different timescales. Until recently, insufficient geochronology has made it difficult to tease apart effects from either agent. 

To better understand the effects of both the Chicxulub impact and the DT in the KPB crises, we developed a high-precision chronologic framework that outlines the temporal sequence of biotic and climatic changes, and proposed perturbations, around the KPB using 40Ar/39Ar geochronology and paleomagnetism. This work was primarily conducted in two areas: the Hell Creek region of NE Montana, USA and the Deccan Traps, India. The Hell Creek region is one of the best-studied terrestrial KPB sites in the world. We developed a high-precision chronostratigraphic framework for fluvial sediments within the Hell Creek, using 40Ar/39Ar dating, magnetostratigraphy, and chemical fingerprinting. This work constrained the timing of terrestrial faunal decline and recovery while calibrating North American Land Mammal Ages biostratigraphy. The coupling of our magnetostratigraphic sections and high-precision 40Ar/39Ar ages further allowed for calibration of the circum-KPB polarity chron (C29r) at unprecedented precision, enabling correlation of our record to other KPB records around the globe. To better understand the role of the DT in the KPB extinction, we developed high precision 40Ar/39Ar ages for >20 lavas ranging the entire DT stratigraphy.

Tying all of this work together, we are able to determine: 1) the decline in terrestrial faunas began between 400 ka and 150 ka pre-KPB, 2) terrestrial disaster faunas are constrained to the first ~25 ka of the Paleogene, and recovery occurred gradually over the next 850 ka, 3) over 90% of the DT volume was erupted in < 1 Ma, with 50-75% emplaced post-KPB, 4) the onset of volcanism is approximately coincident with the onset of pre-KPB warming, but despite this 5) pre-KPB records of climate change coincide temporally with the eruption of the smallest DT phases, suggesting that if the DT are the source of pre-KPB climate change, the release of climate-modifying gases cannot be directly related to eruptive volume as previously assumed. Overall, our new work highlights the close temporal relationship between the Chicxulub impact, Deccan volcanism, and the KPB. But more work is needed, specifically addressing Deccan volatile release and eruption tempo, in order to fully understand the impact of the DT on the Earth system and its role in the mass extinction.


Biography

Courtney Sprain is an Assistant Professor at the University of Florida. She received her Ph.D. in Earth and Planetary Science in 2017 from the University of California, Berkeley and her B.S. degrees in Geology and Geophysics from the University of Minnesota in 2012. She specializes in 40Ar/39Ar Geochronology and Paleomagnetism

The Role of Isostasy in the Evolution and Structural Styles of Fold and Thrust Belts

Ibrahim, Youseph1, Rey, A/ Prof. Patrice1

1University Of Sydney, Sydney, Australia

Fold and thrust belts (FTB) are highly deformed regions that form as the crust accommodates shortening. The evolution of FTB’s records the dynamic interplay between crustal and surface
processes, in conjunction with the rocks’ intrinsic properties. The stacking of thrust sheets and mass transfer of sediment during orogenesis imposes a load on the lower crust and the mantle underneath, inducing isostatic adjustment and a flexural response, which may also contribute to the overall architecture of FTB’s. The tempo at which a fold and thrust belt forms is a consequence of plate kinematics. The tempo of the isostatic response, however, is reliant on the rheology of the mantle and the elastic thickness of the crust. Here, we focus on the role isostasy plays in controlling structural style in FTB’s. We run two-dimensional, coupled thermal and mechanical, numerical experiments using the Underworld framework to explore the interplay between the rate of compression and the rate of isostasy on the structural evolution of FTB’s.

The numerical model runs in a cartesian domain by solving the conservation of energy, mass, and momentum equations. The numerical domain is 42 km wide and 16 km tall, with a grid resolution of 80 m. From top to bottom, the model consists of ‘sticky air’, 4 km of sediment that alternates in competence at 500 m intervals, a 3 km thick basement, and a virtual basal layer, which allows us to implement a local ‘psuedo-isostasy’ boundary condition. Models are run with varying compressional velocities and isostatic rates.

Our suite of models demonstrates the relationship between tectonic and isostatic rates. When the tectonic rate is greater than the isostatic rate, subsidence or flexure is post-tectonic mainly, and
therefore isostasy is unlikely to play a role in the development of the FTB, however, it may modify its architecture post-loading. Alternatively, when the tectonic rate is slower than or equal to the isostatic rate, subsidence will keep pace with tectonic loading. In this scenario, isostasy plays an important role in the development of FTB’s, influencing the topographic elevation generated, the outward extent of the FTB, and thrust fault angles.


Biography

Youseph is a first year Ph.D. student at the University of Sydney studying the evolution and structural styles of fold and thrust belts in Central Australia and Papua New Guinea.

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