%0 Thesis
%A van den Hove, Jackson Cornelius
%D 2017
%T A multi-scale analysis of factors controlling the dynamics of basaltic volcanic fields: Newer Volcanics Province, Australia
%U https://bridges.monash.edu/articles/thesis/A_multi-scale_analysis_of_factors_controlling_the_dynamics_of_basaltic_volcanic_fields_Newer_Volcanics_Province_Australia/4768294
%R 10.4225/03/58fd37246f6f3
%2 https://bridges.monash.edu/ndownloader/files/7830457
%2 https://bridges.monash.edu/ndownloader/files/8232500
%2 https://bridges.monash.edu/ndownloader/files/8232503
%2 https://bridges.monash.edu/ndownloader/files/8232506
%2 https://bridges.monash.edu/ndownloader/files/8232509
%K Basaltic volcanic fields
%K Newer Volcanics Province
%K Spatial analysis
%K Point alignments
%K Eruptive flux
%K Lake Purrumbete Maar
%K Potential field modelling
%K Geophysics
%X Basaltic
Volcanic Fields (BVFs) occur on all continents, in all tectonic environments
and host a diverse range of small scale basaltic volcanoes, which include the
most numerous types of volcanic edifices on Earth. They are associated with
small, often monogenetic, eruptions with long periods of quiescence. BVF are
poorly studied compared with more obviously threatening, and high eruptive flux
volcanic systems (volcanic arcs, mid-ocean ridges, continental rifts, and
mantle plumes). The range of models used to explain what drives volcanism at
the dozens of BVFs studied globally, stems from the variety of tectonic
settings in which they occur. The complexity and inconsistency between the
various models accounts for the need for further research on how these fields
develop and the hazards they pose to society. BVFs that have been studied in
detail are predominantly geometrically smaller examples, with characteristics
that suggest relationships with local tectonic processes.
The Newer Volcanics Province (NVP) is an expansive Pliocene
to Recent intraplate basaltic plains province, located in south-eastern
Australia. The NVP has several aspects that make it an interesting and unique
BVF to study. It is not readily relatable to any tectonic processes expressed
at the surface, it occurs in a compressional lithospheric stress field setting,
and it is host to some of the world’s largest maar volcanoes. The NVP therefore
provides an ideal case study for larger end-member examples of both volcanoes
and BVFs. The NVP as a whole, and many of its volcanoes, have been the subject
of geochemical, deep geophysical imaging, physical volcanology, and limited age
dating studies over the past half century. Despite this, there are significant
gaps in understanding what controls on volcanism in a compressive stress
regime, and the formation of very large maar volcanoes. Potential field
modelling and spatial analysis methods are proven effective methods in studying
basaltic volcanoes and volcanic fields. Hence, they are used in this thesis to
investigate the aforementioned unique aspects of the NVP, with the implications
being relevant to cases of basaltic volcanoes and BVFs worldwide.
Lake Purrumbete Maar (LPM) is a ~50 ka yrs old, large maar
volcano with a crater that is up to 2,800 m in diameter. Despite its age, Lake
Purrumbete’s near circular crater is well preserved, having undergone only
minor erosion and is. It is one of a number of maar volcanoes of the NVP that
rank amongst the largest examples in the world. Forward and inverse potential
field modelling is used to constrain the subsurface structures related to the
maar to assist in determining the factors that control the formation of such a
large maar. Results show that LPM is the result of at least four coalesced
vents that have produced a large shallow bowl shaped diatreme system, and not a
deep conical feature. This is consistent with features typical of maars hosted
in unconsolidated sediments, which is suggested for LPM by the occurrence of
irregular marl lithic clasts with peperitic textures in the tephra ring
deposits. Geometry inversions of the magnetic data indicate that the vents
extend to a greater depth than inferred by accidental lithics present in the
volcanic deposits (<250 m). This supports recent work that shows
accidental lithics present in the volcanic successions likely provide an
underestimate of the maximum depth of explosive eruptions. The vents of LPM
show no discernible alignment, although previous authors suggest its
emplacement was likely influenced by pre-existing crustal structures which it
is known to overly.
At a larger scale, this study has undertaken spatial analysis
of the location of volcanoes of the NVP to assess if crustal structures or
tectonic processes have influenced their locations. Before undertaking
alignment analysis on a point-set of NVP volcanoes, the capability of several
reproducible alignment identification methods (Hough transform, random sample
consensus (RANSAC), three-point regression, and two-point azimuth methods) were
tested and compared against one another to determine their suitability. This
was done using synthetic point-sets that have enforced alignments and clustered
point distributions aimed at replicating the distribution of BVFs. The Hough
transform method was the most robust method for identifying enforced alignment
trends within the point-sets. The three-point regression method also was
effective in identifying significant alignment trends, but produces a high
percentage of coincidental alignments in clustered point-sets. The RANSAC
method adopted from the field of image analysis was the least effective method
tested. It was unable to consistently reproduce enforced alignment trends in
clustered point-sets. The two-point azimuth method is ideal for validating
alignment trends identified by the other methods that identify and define the
locations of individual significant alignments.
The Hough transform and two-point azimuth methods were used
along with cluster analysis methods to interrogate the distribution of NVP
eruption centres (the term “eruption centre” is used in this thesis to refer to
small monogenetic volcanoes that primarily form from a single eruption event)
(Boyce, 2013) and a point-set of coeval vents (An eruption centre may have a
one or more vents/eruption points that form during the single eruption event.
Two or more vents related to the same eruption centre are termed “coeval vents”
throughout this thesis) (Tibaldi, 1995).. Results show that alignments between
vents are primarily oriented parallel with σHmax, whilst the alignment trends
between eruption centres are preferentially aligned with pre-existing faults
and predominantly along fault trends oriented near parallel with σHmax
throughout most of the NVP. It is suggested that pre-existing faults could play
an important role in preventing dikes from stalling and forming sills where
BVFs such as the NVP are hosted in a compressive tectonic setting. Like most
BVFs the NVP has a clustered distribution of eruption centres, whilst
individual clusters are observed with more random to uniform distributions. The
distribution of vents the NVP shows a good correlation with areas of thin
lithosphere. The thickness of the lithosphere is likely a major factor
controlling the location of volcanism in conjunction with edge-driven
convection asthenospheric upwelling. The shear-driven upwelling of the
asthenosphere into zones of thin/extended lithosphere is used to explain
volcanism in other cases worldwide including; the Pannonian Basin (Central
Europe), and volcanism along the Rio Grande rift (North America).
Controls on volcanism at sixteen BVFs including the NVP are
discussed, based on a comparison of their size, volume and eruptive flux.
First, measurement of the dimensions of the NVP was undertaken by mapping and
amending current geological maps of extrusive volcanic deposits using
high-resolution aeromagnetic data, and modelling volume using deposit thickness
data from >1472 boreholes. Results show the NVP is large (23,100±530
km2) voluminous (680–900 km3 DRE) and high-flux (0.15–0.2 km3/ka) example
relative to comparable low-flux IBVFs (0.0001 – 0.1 km3/ka). All the BVFs used
for comparison have eruptive fluxes an order of magnitude or more less than
examples of plume related volcanoes (Kilauea) and BVFs (Eastern Snake River
Plains). Most lower flux BVFs also show no systematic age migration pattern in
volcanism suggestive of a fixed mantle plume, and those with detailed
geochronology and volume data often show a correlation between their eruptive
flux and the rate of local tectonic processes. It is suggested that the NVP and
most low- and high-flux BVFs are the result of upwelling occurring in the
asthenosphere, related to tectonic processes; without requiring additional
thermal input from a deep mantle source, as is inferred for several cases.
Considering a control on volcanism by tectonic processes, the range of eruptive
flux of tectonically controlled BVFs is related to variations in the rate of
the effecting tectonic process, mantle composition, and the size of the mantle
source zone where melt generation and accumulation is taking place.
%I Monash University