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