Neotectonic deformation of northwestern Australia

Frontiers in Offshore Geotechnics II – Gourvenec & White (eds) © 2011 Taylor & Francis Group, London, ISBN 978-0-415-58480-7

Neotectonic deformation of northwestern Australia: Implications for oil and gas development J.V. Hengesh, K. Wyrwoll & B.B. Whitney The University of Western Australia, Perth, Australia

ABSTRACT: Although Western Australia is commonly viewed as a Stable Continental Region with low rates of earthquake activity, geological and geomorphological evidence indicates that active tectonic processes are occurring: (1) the north coast is accommodating crustal flexure due to the collision with the Banda Arc; (2) the central west coast exhibits evidence of active fold growth and reverse movement on reactivated normal faults; and (3) the Murchison region has clear evidence of Quaternary tectonic deformation, repeated surface rupturing events, and a record of two large magnitude historical earthquakes. The evidence of Neotectonic deformation in northwestern Australia indicates that a number of seismic sources are present and these sources have the potential to produce moderate to large magnitude earthquakes such as the Mw 7.1 Meeberrie event. Future seismic hazard assessments should implement refined seismic source models that treat distinct seismic sources and the epistemic uncertainty associated with those sources. These seismic sources should be considered in the selection and engineering of seabed infrastructure such as manifolds, flowlines, export pipelines, and anchorage systems, as well as onshore LNG plants and port facilities. 1

INTRODUCTION

Western Australia is commonly viewed as a “Stable Continental Region” (SCR). It is largely composed of Archean age terranes, such as the Yilgarn and Pilbara cratons, Proterozoic and Phanerozoic age basins such as the Kimberly, Canning, Carnarvon, and Eucla Basins, and intervening deformed belts such as the Albany Frazier, Capricorn, King Leopold, and Halls Creek orogens. There is no orogenesis (active mountain building) occurring in WesternAustralia (WA) and the landscape is severely weathered with deep regolith attesting to long term stability on a regional scale (Anand and Paine, 2002). However, the occurrence of large magnitude historical earthquakes such as the 1885 ML 6.6 Mt. Narryer, 1941 ML 7.1 Meeberrie, and 1967 ML 6.7 Meckering events, and geomorphic evidence of crustal deformation and fault scarps from previous earthquakes indicate that parts of WA are being actively deformed. EPRI (1994) established criteria to define SCRs, which include: (1) evidence for no tectonic activity younger than early Cretaceous (∼100 million years before present {Ma}); (2) no deformed forelands or orogenic belts younger than Cretaceous (∼65 Ma); (3) no anorogenic intrusions younger than Cretaceous; and (4), no rifting or significant extension younger than Paleogene (∼35 Ma). SCRs are divided into domains composed of extended and non-extended crust, and domains underlain by extended crust are further divided into continental margins and failed rifts. According to EPRI (1994), SCRs that are underlain by extended crust have greater seismogenic potential than

those underlain by non-extended crust.Although Western Australia broadly meets these criteria, regional tectonic warping as evidenced by coastal submergence in the north, folding, the presence of numerous fault scarps, and the occurrence of several large magnitude earthquakes suggest that tectonic deformation is occurring across parts of WA. Active crustal dynamic processes occurring in SCR’s remain an enigma within the earth sciences. Understanding these processes is important for characterizing the location, severity, and frequency of earthquake occurrence, and assessing potential triggers for submarine landslides, liquefaction, and site response for structural design of both offshore and onshore facilities. 2

The northern margin of the Australian Plate is involved in a complex collision with the Sunda and Philippine Sea plates (Figure 1). Relative motion of the Australian, Sunda, and Philippine Sea plates is constrained from repeated geodetic surveys that use Global Positioning System (GPS) satellites to measure the precise positions of survey points located throughout a region. The repeated surveys provide direct measurements of the rates and directions of motion of points on different tectonic plates. Reoccupation of GPS sites in the Pacific, Australia, Indonesia, and Southeast Asia between 1991 and 2003 indicate that: (1) theAustralian Plate is moving along an azimuth of 015◦ and is converging with the Sunda Plate/Banda Arc at a rate of

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REGIONAL TECTONIC SETTING

Figure 1. Regional tectonic setting showing major tectonic structures and relative motion vectors of tectonic plates.

67 to 75 mm/yr relative to a fixed Eurasian reference frame (Figure 1) (Bock et al., 2003; Nugroho et al., 2009); (2) the Philippine Sea plate is moving northwestward at a rate of approximately 110 mm/yr relative to Eurasia (DeMets, et al., 1994); and (3) the Sunda Plate is moving east-northeastward at a rate of about 7 to 11 mm/yr relative to Eurasia (Simons et al., 2007). The northern boundary of the Australian Plate follows the Sunda Arc subduction zone and the Banda Arc collision zone (Figure 1). The Australian Plate consists of two main parts including Australian continental crust and oceanic crust of the Indian Ocean, and the differences in crustal type control the nature of processes occurring along the plate boundary. Subduction, in the past, has occurred along the entire Sunda and Banda plate boundary, but now is limited to that part of the plate boundary where oceanic crust of the Indian Ocean is colliding with continental crust of the Sunda Plate (McCaffrey and Nabelek, 1984). Here, the thinner, denser oceanic crust is subducted northward beneath the thicker, less dense continental crust. Subduction extends from the Andaman Islands in the northwest to approximately 120–121◦ east longitude near the island of Flores (Audrey-Charles, 1975; Karig et al., 1987). East of this location, the oceanic crust of the Indian Ocean already has been consumed and subduction of oceanic lithosphere has ceased (Silver et al., 1983; Genrich et al., 1996). The plate boundary from Flores to East Timor is now characterized by collision of the Australian continental crust with fragments of the former island arc (e.g. Sumba, Rote, and Timor islands) and accretion of those fragments to the Australian Plate. The main deformation front now occurs along the northern side of the island arc on a system of south-dipping north-verging reverse faults that are referred to as the Bali-Flores and Wetar thrusts (McCaffrey and Nabelek, 1984). The formation of a south dipping thrust system on the north side of the island arc and similarities in motion vectors for both Australia and Timor (Genrich et al., 1996), indicates that the former island arc is being accreted to the

Australian Plate and the subduction zone has reversed polarity (compared to the Sunda Arc to the west). The collision of the Australian continental crust along the southern Banda Arc has caused profound changes in the style of deformation including cessation of north-directed oceanic subduction, accretion of the former island arc, and reversal of subduction polarity along the Flores and Wetar thrusts (east of 120◦ east). The collision of the Australian continental crust with the South Banda Arc also is causing warping and deformation of northern and northwestern Australia. Therefore, although the main active plate boundary lies on the north side of the Banda Arc, northern Australia is responding to the effects of the collision. We speculate that the transition from oceanic subduction to continental collision at approximately 120◦ east longitude is generating stresses in the Australian crust that are a source of strain energy for earthquakes observed in northern WA. Furthermore, although the main deformation front now lies on the north side of Timor, structures along the Timor trough may still be active earthquake sources. 3 TECTONIC DEFORMATION IN A STABLE CONTINENTAL REGION 3.1 Tectonic flexure Northeast directed convergence of the Australian Plate with the Banda Arc is causing downward flexure of the continental lithosphere in response to the collision. The flexure is most evident along the Kimberly and has formed an anomalously wide continental shelf and sinuous shoreline morphology consistent with crustal subsidence and coastal submergence. The continental shelf, shown on Figure 2, is over 500-km wide (at the position of East Timor) and decreases to less than 100-km wide near Exmouth (Cape Range). From Exmouth southward along the West Australian coast the continental shelf is typically between 40- and 100-km wide.


Figure 2. Digital elevation model of the northwest Australian continental shelf. Note wide shelf between Australia and Timor and narrowing of the shelf to the southwest. Elevation data from Geosceince Australia (2002).

Figure 3. Sea-level prediction diagram showing correlation between surface elevations and sea-level stages. From top to bottom, geomorphic surface elevations are −25 m, −40 m, −50 m, −70 m, −85 m, and −125 m. Elevation from Van Andel & Veevers, 1965. Sea-level curve from Lambeck and Chappell, 2001. Upper and lower curves indicate uncertainty.

The anomalously wide shelf in the north is inferred to be the result of marine erosion during progressive tectonic subsidence. Van Andel & Veevers (1965) recognized a series of submergent shoals, shelfs and terraces and postulated that these could have formed through a combination of tectonic subsidence and sealevel fluctuations. However, at the time, the theory of plate tectonics had not yet emerged, data on sea-level fluctuations were sparse, and thus it was not possible to document the nature and rate of deformation. Van Andel & Veevers (1965) recognized six main submergent surfaces at elevations of −120 to 140 m (at the shelf break), −85 m, −70 m, −50 m, and −40 m (along the continental shelf), and −25 m at a series of submergent atolls on the shelf break (Karmt Shoals) (Figure 2).The geomorphic surface elevations are plotted on a sea-level prediction plot (Figure 3), which is tentatively used to correlate the geomorphic surface

elevations to sea-level high and low-stands. Figure 3 shows a strong correlation of surface elevations to the sea-level curve and using the maximum ages predicts minimum subsidence rates of 0.22 to 0.25 mm/yr for the shelf area south of the Timor trough (Table 1 and Figure 4). The hinge line of this flexure lies south of Broome, where coastal morphology changes from wave erosional (stable) to sinuous (drowning), and last interglacial (Marine Isotope Stage 5e) shoreline deposits (circa 120–130 ka) are present to the south and appear to be absent to the north. If the last interglacial deposits are now submerged the minimum coastal subsidence rate in the vicinity of Broome would be approximately 0.05 mm/yr. This indicates that the tectonic flexure associated with Australia’s collision with the Banda Arc diminishes from north to south and reaches zero north of the Cape Range where coastal uplift is observed and Last interglacial marine units are at their expected heights (Kendrick et al., 1991). The tectonic flexure documented by subsidence of geomorphic surfaces indicates that the continental lithosphere of northwestern Australia is being actively deformed due to the collision with the Banda Arc. This deformation will cause strain in the crust, and therefore the area of tectonic flexure may have greater seismogenic potential than areas not undergoing flexure.

3.2

The 320-km long section of coast from the Cape Range to Cape Cuvier exhibits evidence of Neogene uplift and folding.The folding and uplift have strong regional expression, giving rise to the Cape Range, Cape Rough and Giralia Ranges, as well as the Lake MacCleod basin. Each of these ranges is mapped as an anticlinal


Folding and faulting in the Cape Range

Table 1.

Correlation of geomorphic surface elevations and ages.

OIS stage

Sea-level (m)

Surface elevation (m)

Subsidence (m)

Age min. (ybp)

Age max. (ybp)

5e 5d 5b 3 3 2

3 −16 −30 −57 −75 −118

−25 −40 −50 −70 −85 −125

28 24 20 13 10 7

118000 106000 82000 54000 31000 19000

123000 108000 92000 60000 43000 28000

OIS = Oxygen Isotope Stage; ybp = years before present; m = metre

Figure 4. Subsidence rate diagram showing net subsidence and age values used in the rate calculation, and linear regression.

fold with intervening west-dipping reverse reactivated normal faults (Myers and Hawking, 1998). Evidence of uplift is spatially associated with the anticlinal structures. A series of at least four marine terraces occur along the west coast of the 120-km long N-NE trending Cape Range anticline. The presence of these elevated shoreline deposits indicates long-term emergence of the coast and active fold growth. The growth of the Cape Range anticline implies crustal shortening and movement along associated thrust faults. Coastal uplift also is expressed in the anomalous height of Pleistocene marine deposits exposed in coastal sections west of Lake MacLeod. Uplift of this section of coast appears associated with anticlinal structures, which has prevented a number of rivers from reaching the coast, and resulted in formation of the Lake MacLeod evaporate basin. In the Lake McLeod area, folding of Miocene and younger sedimentary deposits has formed the Gnargoo Range and diverted the Lyndon and Minilya rivers around the resulting anticlines (Figure 5). Development of these supercedent streams may indicate Quaternary uplift rates in excess of incision rates and can provide constraints on rates of fold growth and associated fault slip rates.

Figure 5. Neogene folding at Lake MacCleod. Black lines are fold axes. Streams flow around the noses of folds (supercedent) indicating fold growth controlled stream position. Modified from GSWA, 1985.

The crustal shortening and fold growth is likely associated with reverse movement along former extensional structures and is more pronounced in this region than elsewhere in the Carnarvon Basin. The geomorphic evidence for reactivation of former extensional structures indicates that these are potential seismogenic sources that should be explicitly considered in seismic hazard assessments. These structures follow the continental shelf break and cross much of the Exmouth Plateau (Myers and Hawking, 1998) and therefore may be near-field sources of ground shaking for offshore facilities. 3.3 Surface faulting and historical earthquakes The Mt. Narryer fault zone in central WA is located near the northwest margin of the Yilgarn craton and the eastern margin of the Southern Carnarvon Basin


Figure 6. Digital terrain model (30 m digital elevation data reproduced by permission of the Western Australian Land Information Authority, 2010) showing oblique view to northeast along the Mt. Narryer fault scarp at the Roderick River. Arrows point to fault scarp and line shows position of topographic profile on Figure 7.

Figure 7. Topographic profile across the Mt. Narryer fault showing offset of alluvial valley surface (2009 data reproduced by permission of Western Australian Land Information Authority, 2010).

(Williams, 1979). The fault is approximately 120-km long and strikes in a northeast direction. Historical reports of earthquake strong ground shaking suggest that the fault zone may have produced the 1885 ML 6.6 Mt. Narryer earthquake (Clark, 2006). The epicenter for the 1941 ML 7.1 earthquakes also is located near the fault zone and so it is likely that the Mt. Narryer fault zone has produced two historical large magnitude earthquakes. The 1941 ML 7.1 Meeberrie earthquake is the largest earthquake to have been recorded in Australia. The Mt. Narryer fault zone includes at least four left-stepping en-echelon fault segments. From north to south the segment lengths are 11 km, 33 km, 40 km, and 35 km. The northern fault segments are expressed by strong vegetation alignments and fault scarps on the order of 1 to 1.5 m high (Clark, 2006).The linear nature and subvertical dip of the northern scarps suggests a significant strike slip component of motion. The two southern segments of the fault zone are expressed by a west-side up reverse sense of displacement and have formed east facing scarps across the Roderick and Sanford river alluvial valley deposits (Williams et al., 1983; Myers, 1997; Clark, 2006). Analysis of imagery and digital terrain models indicate that the scarps across the Sanford and Roderick rivers have

captured and diverted active stream flow, formed sag ponds, and impounded Lake Wooleen.The alluvial surfaces in both valleys are uplifted, warped and incised (Figures 6 and 7). Folding in the hanging wall of the fault zone has caused uplift and abandonment of the main river channel and incision of the river through the fold. Where the river cuts through the fold the channel pattern changes from braided to incised. Scarp heights of 3 to 8 m (Figure 7) suggest that the fault has experienced multiple surface rupturing events in Quaternary time. Preliminary analysis of drainage patterns and stream profiles west of the Mt. Narryer fault zone suggest that additional fault scarps may be present. 4

Although Western Australia is commonly viewed as a Stable Continental Region with low rates of earthquake activity, geological and geomorphological evidence indicates that active tectonic processes are occurring: (1) the north coast is accommodating crustal flexure due to the collision with the Banda Arc; (2) the central west coast exhibits evidence of active fold growth and movement on reactivated normal


IMPLICATIONS FOR OIL AND GAS DEVELOPMENT

faults; and (3) the Murchison region has clear evidence of Quaternary tectonic deformation, repeated surface rupturing events, and a record of two large magnitude historical earthquakes. The Mt. Narryer fault may be an analogy for the types of earthquakes that can occur on the reactivated normal faults along the Cape Range and Exmouth Plateau. In this case, a magnitude 7 or greater earthquake is a possible scenario for faults within 50 to 150 km of major offshore production facilities and onshore processing plants. Depending on the activity rates applied to these potential seismic sources, the incorporation of specific fault sources (in seismic source models for the region) could result in large contributions to the overall ground motion hazard at a site. The presence of active seismic sources also is an important consideration for evaluation of surface fault rupture hazards, selection of time histories used in site-specific site-response analysis, analysis of slope instability, and assessment of liquefaction potential. These seismic sources provide triggering mechanisms for instability and permanent ground deformation and should be considered in the selection and engineering of seabed infrastructure such as manifolds, flowlines, export pipelines, and anchorage systems, as well as onshore LNG plants and port facilities. REFERENCES Anand, R. R. and M. Paine, 2002. Regolith geology of the Yilgarn Craton, Western Australia: implications for exploration, Australian Journal of Earth Sciences 49, 3–162. Audley-Charles. M.G., 1975. The Sumba fracture – A major discontinuity between eastern and western Indonesia, Tectonophysics, 26, 213-228. Bock, Y., Prawirodirdjo, L., Genrich, J.F., Stevens, C.W., McCaffrey, R., Subarya, C., Puntodewo, S.S.O., and E. Calais, 2003. Crustal motion in Indonesia from Global Positioning System measurements. Journal of Geophysical Research 108 (B8), 2367. Clark, D. J., 2003. Reconnaissance of recent fault scarps in the Mt Narryer region, W. Australia, Minerals & Geohazards Div. Earthquake Hazard & Neotectonics Group, Canberra. DeMets, C., Gordon, R.G., Argus, D.F., and S. Stein, 1994. Effect of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions. Geophysical Research Letters 21, 2191–2194. Genrich, J.F., Bock, Y., McCaffrey, R., Calais, E., Stevens, C.W., and C. Subarya, 1996. Accretion of the southern Banda arc to the Australian plate margin determined by

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