Paleomagnetic evidence for the emplacement mechanism of an enigmatic boulder accumulation on a coastal cliff top in New South Wales: implications for the Australian Megatsunami Hypothesis     

D. N. Thomas  P. W. Schmidt 《Australian Journal of Earth Sciences》2018,65(4):503-515

Paleomagnetic sampling and measurement of a boulder accumulation on Little Beecroft Head on the Illawarra coastline of New South Wales was undertaken to evaluate potential emplacement mechanisms. This deposit is of central importance in the Australian Megatsunami Hypothesis (AMH) debate, but to date, there has been no unequivocal determination of its provenance. The most likely emplacement mechanisms are by slow collapse during denudation of overlying strata, storm wave overwash or a combination of these. Characteristic Remanent Magnetisation (ChRM) directions were obtained from 15 individual boulders and the in situ bedrock platform on which they currently rest. The in situ Permian bedrock has a normal polarity mean ChRM direction of D/I = 1.6°/–66.7° (α₉₅ = 5.2°; k = 33.9) that is statistically indistinguishable from the Present Earth Field direction at the site. The magnetisation is most likely due to Cenozoic/recent weathering, which is common in surficial rocks throughout the Sydney Basin. ChRM directions for the boulders are stable but scattered, although not random, and the mean boulder direction is indistinguishable in geographic (i.e. current in situ) coordinates, at the 5% significance level, from the mean direction of the in situ bedrock. Further statistical tests confirm that the scatter in the mean directions of the boulders and the in situ bedrock is different, at the 5% significance level, with the boulder mean being more scattered. At an individual boulder level, some blocks have mean ChRM directions that are statistically indistinguishable from the mean in situ rock ChRM direction, whereas others are distinguishable at the 5% significance level.

These results indicate that the boulders were magnetised prior to emplacement but were not moved far from their original positions during emplacement. The emplacement age is constrained to the last ca 780 000 years. These observations strongly support the hypothesis that the Little Beecroft Head boulder deposit was emplaced by a non-catastrophic mechanism, namely slow collapse during denudation of pre-existing cliff material or overtopping from severe storms, which occur regularly on the east coast of New South Wales. Even if a catastrophic wave were responsible, the results constrain the age of that event to be older than 780 000 years. Therefore, the results presented here are not supportive of the AMH as it currently stands. Further paleomagnetic work, on similar deposits along the Illawarra coastline and from elsewhere in Australia, is needed to evaluate the interpretations presented here.  相似文献   

Influence of structural position on fracture networks in the Torridon Group,Achnashellach fold and thrust belt,NW Scotland     

《Journal of Structural Geology》2015

In fold-and-thrust belts rocks undergo deformation as fold geometries evolve. Deformation may be accommodated by brittle fracturing, which can vary depending on structural position. We use 2D forward modelling and 3D restorations to determine strain distributions throughout folds of the Achnashellach Culmination, Moine Thrust Belt, NW Scotland. Fracture data is taken from the Torridon Group; a thick, coarse grained fluviatile sandstone deposited during the Proterozoic. Modelling infers a correlation between strain and simple curvature; we use simple curvature to infer how structural position and strain control fracture attribute variations in a fold and thrust belt.In high curvature regions, such as forelimbs, fracture intensities are high and fractures are short and oriented parallel to fold hinges. In low curvature regions fractures have variable intensities and are longer. Fracture orientations in these regions are scattered and vary over short distances. These variations do not relate to strain; data suggests lithology may influence fracturing. The strain history of fold structures also influences fracturing; structures with longer deformation histories exhibit consistent fracture attributes due to moderate-high strain during folding, despite present day low curvature. This is in contrast to younger folds with similar curvatures but shorter deformation histories. We suggest in high strain regions fracturing is influenced by structural controls, whereas in low strain regions lithology becomes more important in influencing fracturing.  相似文献   

Inverse trishear modeling of bedding dip data using Markov chain Monte Carlo methods     

《Journal of Structural Geology》2015

We present a method for fitting trishear models to surface profile data, by restoring bedding dip data and inverting for model parameters using a Markov chain Monte Carlo method. Trishear is a widely-used kinematic model for fault-propagation folds. It lacks an analytic solution, but a variety of data inversion techniques can be used to fit trishear models to data. Where the geometry of an entire folded bed is known, models can be tested by restoring the bed to its pre-folding orientation. When data include bedding attitudes, however, previous approaches have relied on computationally-intensive forward modeling. This paper presents an equation for the rate of change of dip in the trishear zone, which can be used to restore dips directly to their pre-folding values. The resulting error can be used to calculate a probability for each model, which allows solution by Markov chain Monte Carlo methods and inversion of datasets that combine dips and contact locations. These methods are tested using synthetic and real datasets. Results are used to approximate multimodal probability density functions and to estimate uncertainty in model parameters. The relative value of dips and contacts in constraining parameters and the effects of uncertainty in the data are investigated.  相似文献   

Evolution of the southeastern Lachlan Fold Belt in Victoria   总被引：2，自引：2，他引：0  

C. E. Willman  A. H. M. VandenBerg  V. J. Morand 《Australian Journal of Earth Sciences》2013,60(2):271-289

The Benambra Terrane of southeastern Australia is the eastern, allochthonous portion of the Lachlan Fold Belt with a distinctive Early Silurian to Early Devonian history. Its magmatic, metamorphic, structural, tectonic and stratigraphic histories are different from the adjacent, autochthonous Whitelaw Terrane and record prolonged orogen‐parallel dextral displacement. Unlike the Whitelaw Terrane, parts of the proto‐Benambra Terrane were affected by extensive Early Silurian plutonism associated with high T/low P metamorphism. The orogen‐parallel movement (north‐south) is in addition to a stronger component of east‐west contraction. Three main orogenic pulses deformed the Victorian portion of the terrane. The earliest, the Benambran Orogeny, was the major cratonisation event in the Lachlan Fold Belt and caused amalgamation of the components that comprise the Benambra Terrane. It produced faults, tight folding and strong cleavage with both east‐west and north‐south components of compression. The Bindian (= Bowning) Orogeny, not seen in the Whitelaw Terrane, was the main period of southward tectonic transport in the Benambra Terrane. It was characterised by the development of large strike‐slip faults that controlled the distribution of second‐generation cleavage, acted as conduits for syntectonic granites and controlled the deformation of Upper Silurian sequences. Strike‐slip and thrust faults form complex linked systems that show kinematic indicators consistent with overall southward tectonic transport. A large transform fault is inferred to have accommodated approximately 600 km of dextral strike‐slip displacement between the Whitelaw and Benambra Terranes. The Benambran and Bindian Orogenies were each followed by periods of extension during which small to large basins formed and were filled by thick sequences of volcanics and sediments, partly or wholly marine. Some of the extension appears to have occurred along pre‐existing fractures. Silurian basins were inverted during the Bindian Orogeny and Early Devonian basins by the Tabberabberan Orogeny. In the Melbourne Zone, just west of the Benambra Terrane, sedimentation patterns in this interval, in particular the complete absence of material derived from the deforming Benambra Terrane, indicate that the two terranes were not juxtaposed until just before the Tabberabberan Orogeny. This orogeny marked the end of orogen‐parallel movement and brought about the amalgamation of the Whitelaw and Benambra Terranes along the Governor Fault. Upper Devonian continental sediments and volcanics form a cover sequence to the terranes and their structural zones and show that no significant rejuvenation of older structures occurred after the Middle Devonian.  相似文献   

Geochemical evidence from Lower Permian volcanic rocks of northeast New South Wales for asthenospheric upwelling following slab breakoff     

G. Caprarelli  E. C. Leitch 《Australian Journal of Earth Sciences》2013,60(1):151-166

Within the Daly River basin, Northern Territory, three erosion surfaces are described and their relationships to deep weathering are discussed. The Bradshaw surface is the highest and oldest surface recognized. It is of considerable perfection and forms main divides; it is associated with a deep lateritic profile with a strongly silicified horizon forming the lower part of the pallid zone and extending into rocks immediately beneath. The Maranboy surface now forms secondary divides, with related rock‐cut terraces, below the level of the Bradshaw surface. In most areas it was produced by the stripping of the upper, less silicified parts of the Bradshaw pallid zone. The Maranboy surface is associated with a lateritic weathering profile less deep than the Bradshaw profile and mainly developed in the Bradshaw weathering mantle. A younger erosion surface, the Tipperary surface, advanced by the removal of the Maranboy re‐weathered layer exposing the resistant Bradshaw silicified rock which commonly forms a base‐level of denudation. The Tipperary surface consists of broad plains, gently undulating terrain, and dissected headwater valley floors. It is relatively unweathered and carries depositional mantles which are attributed to climatically induced slope instability.

The ages of the erosion surfaces and the possibility of climatic changes in the area are also briefly discussed.  相似文献   

Late Ordovician island‐arc volcanic rocks,northern Capertee Zone,Lachlan Fold Belt,New South Wales     

P. F. Carr  C. L. Fergusson  J. W. Pemberton  G. P. Colquhoun  S. I. Murray  J. Watkins 《Australian Journal of Earth Sciences》2013,60(3):319-330

The Cape Hoskins volcanoes form part of the Quaternary volcanic island arc that extends from Rabaul in the east to the Schouten Islands in the west, and they overlie the northerly dipping New Britain Benioff Zone. The products of the volcanoes range in composition from basalt to rhyolite, and are normative in quartz and hypersthene. They contain phenocrysts of plagioclase and subordinate augite, hypersthene, and in most samples iron‐titanium oxides; some samples also contain olivine or quartz or both, and some pumice contains hornblende and, rarely, biotite.

Chemical analyses of 29 volcanic rocks are presented; 22 were also analysed for 17 minor elements — Rb, Ba, Sr, Pb, Zn, Cu, Zr, Y, Ni, Co, Sc, Cr, V, Ga, B, U, and Th.

Chemically the rocks have many of the characteristics of the ‘island arc tholeiitic series’, but do not show a pronounced relative enrichment in iron and appear to be relatively enriched in Sr. Compared with volcanic rocks from the northern part of the Willaumez Peninsula, they are lower in K (but not Na), Ti, Rb, Ba, Zr, Pb, Th, Ni, and probably also V, Cu, and Zn: these differences are attributed to the greater depth of the Benioff Zone beneath the Willaumez Peninsula. The more basic of the Cape Hoskins rocks are similar in most respects to lavas of comparable composition from Ulawun volcano to the east.  相似文献   

Petrological,geochemical and geochronological evidence for a Neoproterozoic ocean basin recorded in the Marlborough terrane of the northern New England Fold Belt   总被引：1，自引：1，他引：0  

M. C. Bruce  Y. Niu  T. A. Harbort  R. J. Holcombe 《Australian Journal of Earth Sciences》2013,60(6):1053-1064

Petrological, geochemical and radiogenic isotopic data on ophiolitic‐type rocks from the Marlborough terrane, the largest (～700 km²) ultramafic‐mafic rock association in eastern Australia, argue strongly for a sea‐floor spreading centre origin. Chromium spinel from partially serpentinised mantle harzburgite record average Cr/(Cr + Al) = 0.4 with associated mafic rocks displaying depleted MORB‐like trace‐element characteristics. A Sm/Nd isochron defined by whole‐rock mafic samples yields a crystallisation age of 562 ± 22 Ma (2σ). These rocks are thus amongst the oldest rocks so far identified in the New England Fold Belt and suggest the presence of a late Neoproterozoic ocean basin to the east of the Tasman Line. The next oldest ultramafic rock association dated from the New England Fold Belt is ca530 Ma and is interpreted as backarc in origin. These data suggest that the New England Fold Belt may have developed on oceanic crust, following an oceanward migration of the subduction zone at ca540 Ma as recorded by deformation and metamorphism in the Anakie Inlier. Fragments of late Neoproterozoic oceanic lithosphere were accreted during progressive cratonisation of the east Australian margin.  相似文献   

Mafic rocks spatially associated with Devonian felsic intrusions of the southern Lachlan Fold Belt: A possible mantle contribution to crustal evolution processes     

A. Soesoo  I. Nicholls 《Australian Journal of Earth Sciences》2013,60(5):725-734

Devonian basaltic to andesitic dykes and compositionally similar plutons of the southern Lachlan Fold Belt are often temporally and spatially closely associated with large granitic complexes. Mafic intrusions play a major role in the transfer of heat into the continental crust, providing a thermal ‘engine’ which leads to crustal melting, and geochemical/isotopic evidence indicates that they contribute chemical constituents to the products of this melting. Studied mafic‐intermediate dykes in the southern Lachlan Fold Belt have tholeiitic to alkaline affinities and include groups with both high and low Ti and K. Several dyke generations may be associated with a single felsic complex. Primitive mantle‐normalised trace‐element abundance patterns with negative Nb and Ti anomalies for basaltic/andesitic and gabbroic/dioritic rocks as young as Early Devonian most resemble those of modern island arcs and suggest an influence of subduction on mantle magma sources. However, some Middle and Late Devonian mafic rocks are enriched in light rare‐earth elements and other incompatible elements, lack significant Nb anomalies, and confirm the change to continental‐rift extensional settings clearly indicated by Lachlan Fold Belt geology.  相似文献   

Pre‐Tabberabberan deformation in eastern Tasmania: A southern extension of the Benambran Orogeny     

A. R. Reed 《Australian Journal of Earth Sciences》2013,60(6):785-796

Recumbent folding in eastern Tasmania affected turbidites containing Lower to Middle Ordovician (Bendigonian Be1 to Darriwilian Da3) fossils, but not stratigraphically overlying turbidites containing Silurian (Ludlow) graptolites, and is of a timing consistent with Ordovician to Silurian Benambran orogenesis on the Australian mainland. Two subsequent phases of upright folding post‐date deposition of turbidites containing Devonian plant fossils but pre‐date intrusion of Middle Devonian granitoids, and are of Tabberabberan age. A closely spaced disjunctive cleavage (S₂), associated with the first phase of Tabberabberan folding, everywhere cuts a slaty cleavage (S₁) associated with the earlier formed recumbent folds. However, refolding associated with development of S₂ is not always clear in outcrop and it is proposed that coincident tectonic vergence between the two events has resulted in reactivation of recumbent D₁ structures during the D₂ event. The transition to rocks not affected by recumbent folding coincides with a marked change in sedimentology from shale‐ to sand‐dominated successions. This contact does not outcrop but, from seismic data, appears to dip moderately to the east, and can only be explained as an unconformity. The current grouping of all pre‐Middle Devonian turbidites in eastern Tasmania into the one Mathinna Group is misleading in that the turbidite sequence can be subdivided into two distinct sedimentary packages separated by an orogenic event. It is proposed that the Mathinna Group be given supergroup status and existing formations placed into two new groups: an older Early to Middle Ordovician Tippogoree Group and a younger Silurian to Devonian Panama Group.  相似文献   

Lake Boga Granite,northwestern Victoria: mineralogy,geochemistry and geochronology     

S. J. Mills  W. D. Birch  R. Maas  D. Phillips  I. R. Plimer 《Australian Journal of Earth Sciences》2013,60(3):281-299

The Devonian Lake Boga Granite in northern Victoria, while almost entirely under thin Murray Basin cover, is one of the largest plutons in the western Lachlan Fold Belt. Its only exposure is a quarry penetrating the Cenozoic sediments. In the quarry, prominent pod pegmatites and miarolitic cavities suggest a high level of emplacement. The granite, a non-magnetic, fractionated S-type, contains a large range of accessory minerals, including primary uranium- and REE-bearing phosphates and oxides, and primary copper sulfides. Monazite-series minerals show an exceptionally wide range of compositions, from normal monazite-(Ce) through cheralite (Ca – Th-rich) to rare huttonitic monazite (Th-rich) and brabantite; U contents in monazite also vary widely (0 – 7.9 wt%). Primary low-Ca uraninites are well preserved and are unusual in having low Th/U and LREE. Late-stage cavity fluorapatite crystals up to several centimetres across show intricate elemental zoning patterns with extreme U gradients (<10 – 6900 ppm) in some crystals. New ⁴⁰Ar – ³⁹Ar ages for magmatic biotite, muscovite and K-rich feldspar average 365 ± 3 Ma, which approximates the emplacement age of the granite. This is supported by a 377 ± 12 Ma U – Th – Pb (CHIME) age for primary uraninite. New whole-rock geochemical data support earlier observations: the granite is strongly fractionated (SiO₂ 70.7 – 76.0 wt%; 4.2 – 0.6 wt% FeO_t) and peraluminous (ASI = 1.23 – 1.45), and has slightly elevated Na₂O and P₂O₅ (0.30 wt%) contents compared with other fractionated S-type granites from the Lachlan Fold Belt. Trace-element abundances are typical of fractionated granites, although U and Cu concentrations vary strongly and reach >60 and ≈1400 ppm, respectively. REE patterns also vary strongly, from LREE-enriched with moderate Eu depletion, to flat with strong Eu depletion. The flattest of the REE patterns, in samples with FeO_total < 1%, are characterised by M-type tetrad effects. These and other samples also show low (subcrustal average) and variable Zr/Hf (35 – 16) and Nb/Ta (8 – 4) ratios; these and other unusual elemental fractionations are related to changes in elemental partitioning during the late magmatic stage, when felsic peraluminous magma and high-temperature magmatic fluid coexisted.  相似文献