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Auckland’s buried faults and their influence on its
geology
John R. Stewart
GSNZ Conference
26 November 2015
Western
Geophysical
zone
Motivation and Study
Area
• North Island active tectonic environment – hazards associated with
earthquakes and volcanoes
• Significant gravity contrast between:
1. Taupo Volcanic Zone (TVZ) and eastern accretionary margin
2. Curved North Island segment west of active Hauraki Rift
• Why a contrast and how stress accommodated west of a zone of active
back-arc rifting?
Satellite
Bouguer
gravity
Central Auckland
Auckland
Auckland a good candidate to for studying the
tectonics west of the TVZ because:
1. Recent magmatism (<0.6 Ka) Auckland
Volcanic Field (AVL) with a suite of recent
publications adding constraints to its
development
2. Good geological and geophysical datasets
available in this area
PROBLEM:
Structural fingerprint largely obscured by the
volcanic field, urban footprint and recent cover
sediments.
Integration of many datasets into GIS crucial for interpretation where:
• Geological data is disparate
• Geology under investigation is buried by cover sediments or rocks (recent volcanic rocks)
• Geological relationships are obscured or blurred by anthropogenic modification
…e.g. Auckland City
Geoscience interpretation methodology
Dat
aset
s
Geographic
Bathymetry
DEM (15m pixel)
Topographic
geomorphology
Drainage
Land use
Satellite
Aster/LandSat
Google Earth
Geological
Faults
Geology
Existing maps and interpretations
Drillholes
GERM database
DEVORA sampling
Structure
Bedding, fault measurements etc Geochemistry
Geophysical
Aeromagnetics
Gravity
Earthquakes Geonet database
Seismic reflection
Building a fault framework (1) Looking at major faults using DEM/Bathymetry
• Auckland has some active faults --
observable in eastern Auckland -> recent
displacement of Jurassic greywacke
basement blocks
• Displacement decreases towards the
north…difficult to trace beneath Miocene
Waitemata Group
-> suggests a change in accommodation of
displacement
• less focussed?
• Spread across different structures?
• Waitemata Basin detached from
basement?
• Previous studies have demonstrated the
presence of major terrane-parallel faults in
Central Auckland using drillhole and
elevation data (e.g. Kenney et al., 2012)
and relationships to the volcanic field
using statistical methods (e.g. von Vey
and Nemeth, 2009; Bebbington and
Cronin, 2010), but no unified model exists
• A more complete understanding of the
relationships between faults, fault
networks, volcanoes and tectonics can
allow improved hazard assessment and
analysis
No clear Structural
Fingerprint
Drury Fault
Papakura Fault
Wairoa Fault Auckland DEM (LINZ) and 20m Bathymetry (NIWA)
Building a fault framework (2) Multi-scale integration with DEM and Google Satellite interpretations
Google Earth fault interpretations in Auckland:
• Form surface mapping of Miocene Waitemata Group
turbidites coastal exposures and shallow submarine reef
areas
• Extrapolation of faults into geomorphology
• Faults typically only have small displacements (m’s)
1. Coastal Satellite interpretation
2. Fault extrapolation with
geomorphology
3. Up-scaled interpretation of structure using DEM
Building a fault framework (3) Fault pattern
2. Fault extrapolation with
geomorphology
• Contrasts in dominant interpreted fault
trend
• Major faults in SE Auckland continue
towards isthmus where preservation at
surface becomes fragmented
• No major NW-trends in isthmus faults
movement in central Auckland (isthmus),
that post-date the <250Ka Auckland
Volcanic Field
• Strong NNW-trend on North Shore and
SE Auckland
• Preference for NE-trending structures in
Auckland Isthmus
• SE Auckland combination of NNW- and
N-S with overprinting NE-trending
NORTH
SHORE
ISTHMUS
SE
AUCKLAND
Gravity Data (1) Interpolated gravity stations and processed imagery
NW-trending positive anomaly interpreted
as buried ultramafic in suture zone that
corresponds with Dun Mountain Ophiolite
Belt (Eccles et al., 2005; Williams et al., 2006)
Significant NE-trending features
correspond with changes in regional
basement trends (25° CCW from regional
trend
25°
Slope of the Bouguer Anomaly calculated
and imaged in QGIS to enhance gradients
within the data and assist interpretation of
faults
Easy for human eye to interpret
lineaments -> geology usually non-
linear…
Faults affecting gravity:
• Difficult to interpret without help of
surface fault framework
• Assumption that where gravity
approximates trends of fault networks at
the surface there is strong likelihood for
major structure at depth
• Can make qualitative assessments of
kinematics, e.g. NW-down or dextral
The resultant fault interpretation is a “best
fit” of the surface framework to the gravity
data
Gravity Data (2) Integrating fault framework to interpret basement faults
Fault interpretation (1) Integration of surface fault framework and gravity data
Continuity and density of various
fault orientations can:
• be used to aid in isolation of
structures or zones that have
highest displacement potential
• Show whether the fault or the
near-surface geology is younger
stratigraphic units
• Assess areas that may have high
densities of a particular fault
trend
• Everything helps when
attempting to establish a
kinematic model
• Few faults parallel main gravity
trends in central Auckland
(obscured by young volcanics or
contrasting strain accommodation
in Waitemata cover sequence?)
• Fault segments join to form a more
en-echelon framework, particularly
in the Auckland isthmus area
Fault interpretation (2) Comparison between surface faults and basement faults
Riedel Shear Model
P R
Reverse?
R’
Faulting and volcanism (1) Potential for structural control on volcanism
Positions of known vents and intersecting
buffered fault segments faults (buffer = 300m )
Age data from Bebbington and Cronin (2010). See also Lindsay et al. (2011)
• Fault positions show relationship to
groups of aligned vents
• Curved fault structure showing
three dominant trends:
1. NNE
2. NE
3. E-W (minor localised
through a central zone)
250-200 Ka
227-150 Ka
180-90 Ka
ca. 83 Ka
Northern tip of
South Auck. Volc.
Field
ca. 1100 Ka
Fau
lt A
zim
uth
Average Age (Ka)
0
20
40
60
80
100
120
140
160
180
0 50 100 150 200 250 300
Cones Only (mostly with lavaflows)
Tuff ring only
Tuff rings with cones
Faulting and volcanism (3) Spatio-temporal model for faulting and volcanism
26-21 Ka
ca. 55 Ka
0.6 Ka ?
37-36 Ka
32-28 Ka
36-32 Ka
ca. 130 Ka
Ca. 70 Ka
16-10 Ka
Erupted volumes from Kereszturi et al., 2013
y = 5E+07e-0.287x
10000
100000
1000000
10000000
100000000
1E+09
0 2 4 6 8 10 12 14
Cones only (most with lava flows)
Tuff ring only
Tuff rings with cones
Distance from Major Fault (km)
Eru
pte
d V
olu
me (
m3)
One Tree Hill
Fault pattern associated with the
youngest volcanoes fits into a
Riedel model under dextral shear:
• Discrete tensional NE-SW
structures facilitating magma
ascent in AVF; become large
normal faults SW of terrane
boundary
• R and R’ orientations well
developed with early volcanism
occurring on
• Clockwise rotation of stress
field at ca. 55Ka
Satellite
Gravity
Regional Fault Interpretation Active dextral shear system?
AVF AVF
NNW-SSE
extensional
zone
Hauraki
Rift and
sinistral
transfer
faulting
Pull-
apart
tilting
zone
• Vaughan Stagpoole (GNS) for access to the national gravity database
• Helen Williams (MMG) for use of gravity stations collected during MSc
• Jennifer Eccles (Uni Auckland) for high-res magnetic data over Auckland
• Previous workers focussed on increasing our understanding of Auckland’s
geology
• NIWA holds bathymetric data for the Hauraki Gulf
https://www.niwa.co.nz/our-science/oceans/bathymetry/download-the-data
• Rose Diagrams: Grohmann, C.H. and Campanha, G.A.C., 2010. OpenStereo:
open source, cross-platform software for structural geology analysis. Presented
at the AGU 2010 Fall Meeting, San Francisco, CA.
Acknowledgements