Australian impact cratering record: Updates and recent discoveries

ABSTRACT There are currently 31 confirmed structures of impact origin in Australia. More than 49 additional structures have been proposed to have formed due to asteroid impact but await confirmation. Many discoveries have been made in Australia in the time since the last comprehensive review of the Australian impact cratering record was published in a peer-reviewed journal in 2005. These include further expanding the record of confirmed craters, and providing new insights into a variety of impact-related processes, such as shock deformation, phase transitions in accessory minerals, new impact age determinations, studies of oblique impacts, and more. This update is a review that focuses principally on summarizing discoveries made since 2005. Highlights since then include confirmation of five new Australian impact structures, identification of Earth’s oldest recognized impact structure, recognition of shock deformation in accessory minerals, discovery of the high-pressure phase reidite in Australia, determination of the links between impact craters and some ore deposits, and publication of the first generation of numerical hydrocode models for some Australian craters.

Tectonic Switch From Triassic Contraction to Jurassic-Cretaceous Extension in the Western Tarim Basin, Northwest China: New Insights Into the Evolution of the Paleo-Tethyan Orogenic Belt

We use stratigraphic, sedimentological, and borehole data and seismic profiles from the western Tarim Basin to document its Mesozoic tectonic evolution. A nearly 60-km-wide, Triassic fold-and-thrust belt along the southwestern margin of Tarim Basin is unconformably overlain by a Jurassic-Cretaceous sedimentary sequence along a regional angular unconformity. The Lower-Middle Jurassic strata consist mainly of an upward-fining sequence ranging from terrestrial conglomerates to turbidite deposits, which represent the products of an initial rift stage. Palaeocurrent analyses show that sediments for these rift deposits were derived from the paleo-Kunlun and paleo-Tienshan Mountains to the southwest and northern, respectively. The overlying Upper Jurassic-Cretaceous series consist of coarse-grained, alluvial fan to braided river deposits in the lower stratigraphic member, and lagoonal mudstones and marine carbonates in the upper member. These finer-grained rocks were deposited in a subsiding basin, indicating that a significant change and reorientation in the drainage system should have occurred within the basin during the Early Cretaceous. The western Tarim Basin evolved from a syn-rift stage to a post-rift stage during the Jurassic-Cretaceous. A post-orogenic stretch developed due to the evolution of the Paleo-Tethyan orogenic belt in Central Asia is a likely geodynamic mechanism for this major tectonic switch from a contractional episode in the Triassic to an extensional deformation phase in the Jurassic-Cretaceous.

Rediscovery of a major alpine thrust : the Helvetic Basal Decollement

<p>Since two centuries the European Alps are a natural laboratory to study continental lithosphere deformation during mountain building. Since the early studies, a constant question has been to evaluate the importance of vertical versus horizontal displacements in the building of reliefs. Whilst the occurrence of large thrust sheets, as initially proposed from field observations, are now well explained in the frame of plate tectonics, controversies still arise on the precise geometry, amount, and timing of major thrusting during the orogeny.</p><p>We present a new detailed 3D structural study of the cover/basement relationships in the Chamonix synclinorium in between the Mont-Blanc (MB) and Aiguilles Rouges (AR) ranges. These massifs are two of the main external basement ranges of the western Alps.  The study allows deciphering the area structural history: the Mesozoic sedimentary cover has been thrust at least 10km NW above the Helvetic Basal Décollement (HBD) before to be offset by late steep thrusts during exhumation in the Miocene.</p><p>Such interpretation fundamentally diverges from the classical view of the sedimentary cover of the Chamonix synclinorium being expulsed from a former graben during a single deformation phase and implies that a major thrust phase lasting ~10 Ma has been overlooked. Our observations show that the HBD was a major thrust system active between ~30 and ~20 Ma, possibly until 15 Ma, with a shortening of more than 10km in the south to 20km in the north. It extends below most of the subalpine ranges and emerges in front of the Bauges and within the Chartreuse and Vercors massifs, and was rooted east of the External Cristalline Massifs (Mont-Blanc and Belledonne). During the Miocene, the HBD was cut by steep reverse faults and uplifted above the basement culmination of the External Cristalline Massifs obscuring its continuity and precluding its recognition as a major structure even if it was previously described at several localities.</p>

Structural evolution of the Nekézseny Fault – a displaced segment of the Dinaric-ALCAPA contact zone in NE Hungary (Bükk and Uppony Hills)

<p>The Bükk and Uppony Hills (NE Hungary) are two adjacent structural units with correlations to the Northern Dinarides and Inner Western Carpathians (ALCAPA), respectively. These two units are separated by the Nekézseny Fault, which may therefore be considered as a presently displaced segment of the Dinaric-ALCAPA contact zone (Schmid et al. 2008). Along this contact zone, the Bükk-type Permo-Mesozoic formations are thrust over the Paleozoic and Senonian formations of the Uppony Unit (Schréter 1945). Despite of the Nekézseny Fault being a terrain boundary, its structural evolution has not been studied in details. Preliminary structural data suggested multiple faulting events between the latest Senonian and early Miocene (Fodor et al. 2005), however, the initial age of the contact zone has remained uncertain.</p><p>In this study a detailed structural analysis was carried out in order to understand the deformation geometry, kinematics and the timing of movements along the Nekézseny Fault. Our preliminary results show that the Nekézseny Fault developed in response to NW-SE shortening. Low-angle fractures within individual pebbles suggest an early (latest Cretaceous or early Paleogene) age for the NW-SE shortening, as pebble fracturing is limited to the early stage of diagenesis and requires soft or semi-consolidated fine-grained matrix.</p><p>The top-to-the-NW emplacement of the Bükk over the Uppony Unit was accompanied by the folding of the Senonian conglomerate in the footwall, where a large, almost isoclinal recumbent fold developed due to the estimated several km of displacement along the main contact zone. Despite of the similarity in the shortening directions, the top-to-the-NW shortening certainly post-dates the penetrative S-SE-vergent contractional structures present throughout the Bükk Hills, that are related to the latest Jurassic to Early Cretaceous nappe stacking and subsequent shortening (Csontos 1999). Microtectonic analysis of the Nekézseny Fault Zone proved that the main contact zone is a strongly distorted cataclastite zone, which suggests a late-stage low-temperature deformation. Similarly younger semi-ductile or low-temperature contractional structures (e.g. kink folds) were recognized in several parts of the Bükk Unit, all of which were dated tentatively to the late Cretaceous (Flórián-Szabó & Csontos 2002, Juhász 2020, McIntosh 2014, Koroknai et al. 2008, Scherman 2018). Our observations indicate that the top-to-the-NW displacement was much more extensive than previously thought and incorporated large part of the Bükk Unit. This shows that the top-to-the-NW displacement represents an important deformation phase, which should be integrated into the Mesozoic structural evolution of the Alpine-Dinaric area.</p><p>This study was supported by the research founds NKFIH OTKA 113013 and 134873, the ÚNKP-17-2 and ÚNKP-20-3 New National Excellence Program of the Ministry of Human Capacities.</p><p><strong>References:<br></strong>Csontos (1999): Bulletin of the Hungarian Geological Society 129, 4, 611-651.<br>Fodor et al. (2005): GeoLines 19: 141-161.<br>Juhász (2020): TDK thesis, ELTE, Budapest.<br>Koroknai et al. (2008): Journal of Structural Geology 30, 159-176.<br>McIntosh (2014): PhD thesis, University of Debrecen, Debrecen.<br>Scherman (2018): MSc thesis, ELTE, Budapest.<br>Schmid et al. (2008): Swiss Journal of Geosciences 101: 139-183.<br>Schréter (1945): Annual Report of the Geological Institute of Hungary, 1941-42: 197-237.</p>

Influence of inherited superposed folding on back-thrusting development: a case study from the Variscan foreland fold-and-thrust belt of Sardinia

<p>The Variscan foreland of SW-Sardinia consists of a Cambrian to lower Carboniferous succession polydeformed under very low-grade metamorphism. It is characterized by the following superposed structures: 1) E-W-trending upright folds; 2) N-S-trending inclined folds, penecontemporaneous with 3) W-ward fore-thrusts, and 4) E-ward back-thrusts.</p><p>A peculiar feature of this sector of the Variscan foreland is the widespread occurrence of back-thrusts, apparently more common than fore-thrusts, unlike the majority of foreland fold-and-thrust belts.</p><p>Our research focuses on the role played by the folded basement in limiting extensive fore-thrusts development and how fold shape and orientation, along with litho-stratigraphic heterogeneity, influenced the back-thrust geometry.</p><p>Generally, back-thrusts occur when the shortening can no longer be accommodated by fore-thrusts, usually because of buttressing induced by fore-thrust-related thickening and duplication of the stratigraphic succession. However, in the segment of Variscan foreland outcropping in SW-Sardinia, back-thrusting seems to be activated by a different mechanism. The inherited structural setting is characterized by two perpendicular generation of superposed folds that gave rise to a type 1 interference pattern with pluri-km-scale domes and basins. In particular, in the western sector of the foreland (i.e., the farthest from the nappe zone thrusted over the foreland) domes are made up of about 500 m thick lower Cambrian sandstone and limestones formations that may have acted as a buttress, hindering fore-thrusting propagation and facilitating extensive E-ward back-thrusting. This is corroborated by the large number of back-thrusts that crop out between the buttress and the nappe front.</p><p>In this area, back-thrusts affect the folded sedimentary succession that is progressively younger and weaker E-ward. As commonly accepted in thrust faults, ramps developed in the competent stratigraphic sequence, here made up of sandstones and limestones, and flats in weak stratigraphic horizons, here consisting of marly limestones and shales. As a result, in the study area the dip of back-thrusts decreases towards the nappes front, where the weaker lithologies have been overthrusted.</p><p>The back-thrusts’ surface is characterized by discontinuous antiforms and synforms that do not affect the underlaying succession; so, a later deformation phase that folded the back-thrusts can be ruled out. Therefore, the fault plane should have been deformed throughout the back-thrusts growth and development.</p><p>Interestingly, strictly relationships can be noticed between the fault plane geometry and the inherited structures in the footwall of the back-thrusts. Where the back-thrusts cut across upright limbs perpendicular to the back-thrust strike, the fault plane shows an antiformal shape; where the back-thrusts take place above the pre-existing synforms with the axis plunging towards the back-thrust dip, the fault plane takes the form of the underlying synforms. Instead, back-thrusts are uninfluenced by pre-existing folds where they cut either synforms with the axis that plunges opposite to the dip direction of the fault plane or antiforms, regardless the plunging direction of their axis.</p><p>To conclude, this research highlights the relevant role of the inherited structural setting on fold-and-thrust belt style and suggests that the strata attitude and the axes plunging directions of pre-existing folds could have a control in the back-thrust geometry.</p>

The timescale of the aseismic to seismic deformation in a cooling pluton: 40Ar-39Ar ages of the solid-state deformation in the Adamello (Southern Italian Alps)

<p>The northern Adamello is crosscut by ductile shear zones and pseudotachylyte-bearing faults, both compatible with the same stress field, with ductile shear zones crosscut by brittle faults. These relations are coherent with the re-equilibration of the pluton-related thermal anomaly to temperatures typical of the base of the seismogenic continental crust (T = 250 – 300°). Our new <sup>40</sup>Ar-<sup>39</sup>Ar ages help to constrain the absolute age and duration of each deformation phase.</p><p>Samples included wall-rock biotite, bulk ultramylonites and pseudotchylytes. Before stepwise heating <sup>40</sup>Ar-<sup>39</sup>Ar measurements, samples were characterized by microstructural, geochemical and petrological analyses.</p><p>The wall-rock biotite is 33.4±0.1 Ma old, independently of grainsize. Mylonites feature complex age spectra between 28-31 Ma, including biotite and altered feldspar. Four pseudotachylyte matrices are clustered around 30-31.5 Ma, and two samples have 25-26 Ma ages.</p><p>Ductile shearing active 2 Ma after wall-rock emplacement indicates either low strain rates, or a long-lasting thermal anomaly, which might be due to high emplacement depth, and/or the progressive assemblage of adjacent plutons through small magma pulses. Seismogenic faulting overlaps with mylonitization around 31 Ma; younger pseudotachylyte ages may be due to late-stage reactivation.</p>

Shallow deformation kinematic history, a new insight from carbonates U-Pb direct dating by LA-ICP-MS imaging technique.

<p>This project aims to refine direct dating of carbonates by the U-Pb system, using a new LA-ICP-MS imaging technique that incorporates complementary element and textural analysis information. The direct dating of carbonates in deep time has been considered desirable for decades (e.g. Jahn and Cuvellier, 1994) given their ubiquity in the Earth system, and carbonates are a key phase for dating geological processes such as brittle-ductile deformation in carbonate successions. This new method facilitates detailed (on the scale of tens of microns) mapping of U-Pb isotope and element distributions (cf Drost et al., 2018), and is here applied to carbonate vein dating to constrain local and regional histories of deformation or fluid activity.</p><p>In this presentation we focus on a sample from the Carboniferous North Dublin Basin, Ireland. The basin has been affected by deformation that led to tight chevron folds and kinematically-linked dextral en-echelon vein sets. Additionally bedding-parallel veins and  slickenfibres are common. The deformation has been conventionally assumed to be of Variscan age, and some Variscan U-Pb ages are recorded in this study. However many calcites analysed yield late Eocene ages, a deformation phase that is hitherto undetected on the Irish mainland. Our data indicate repeated fault slip over a peroid of at least c. 4 my during late Eocene times and, thus, demonstrate the ability of the LA-ICP-MS imaging approach to not only unravel complex polyphase deformation histories in carbonates but also to resolve processes on fine temporal and spatial scales.</p><p> </p><p> </p><p>DROST, K., CHEW, D., PETRUS, J. A., SCHOLZE, F., WOODHEAD, J. D., SCHNEIDER, J. W. & HARPER, D. A. T. 2018. An Image Mapping Approach to U-Pb LA-ICP-MS Carbonate Dating and Applications to Direct Dating of Carbonate Sedimentation. Geochemistry, Geophysics, Geosystems, 19<strong>,</strong> 4631-4648.</p><p>JAHN, B.-M. & CUVELLIER, H. 1994. Pb-Pb and U-Pb geochronology of carbonate rocks: an assessment. Chemical Geology, 115<strong>,</strong> 125-151.</p>