RESEARCH Delineating the Exmouth Mantle Plume

RESEARCH

Delineating the Exmouth mantle plume (NW Australia) from denudation and magmatic addition estimates

Max Rohrman* 202 GLENWOOD DRIVE, HOUSTON, TEXAS 77007, USA

ABSTRACT

Volcanic margins are a class of large igneous provinces (LIPs) characterized by rifting-derived basaltic magmatism. This is commonly attributed to extension-related lithospheric thinning, generating decompression melting. Another mechanism influencing magmatism on volcanic margins is mantle plume–induced lithospheric thinning. Unfortunately, it is difficult to differentiate between these mechanisms because they seem to take place almost contemporaneously. Whereas rifted volcanic margins produce linear denudation and magmatic addition patterns, mantle plumes or active upwellings would generate more subcircular domal patterns. Here, I use magmatic addition and denudation patterns to discriminate between these scenarios in a data set from the volcanic margin offshore NW Australia. Seismic and well data results suggest the presence of a domal component that is used to delineate the Late Jurassic Exmouth mantle plume. This upwelling was centered on a highly extended and subsided continental fragment bounded by the present-day subsea Sonne and Sonja Ridges and includes the Cuvier margin and Cape Range fracture zone. The region is characterized by ~2.6 km of denudation and ~500 m of tectonic uplift, with erosion products acting as source material for the Early Cretaceous Lower Barrow delta. Denudation analysis indicates that only ~40% of the seismically detected magmatic underplate is melt related, with the effective underplate ~4 km thick near the locus of uplift and decreasing in the outer regions. Tectonic subsidence analysis, seismic stratigraphy, and plate reconstruction suggest that the plume-induced domal uplift preceded magmatism and breakup. Plume activity was followed by a westward-propagating hotspot track, possibly terminating in Greater India (present Tibet).

LITHOSPHERE; v. 7; no. 5; p. 589–600 | Published online 17 June 2015 doi:10.1130/L445.1

INTRODUCTION bilities (e.g., Moore et al., 1999; Ribe, 2004; plexes and flood basalt extrusions (Ridley and Agrusta et al., 2013; Brune et al., 2013). Richards, 2010). Volcanic margins are rifted margins char- One of the defining tests for the presence of Rifted margin uplift at magmatic rifts is acterized by massive igneous activity. These a mantle plume or active upwelling has been commonly characterized by linear uplift par- features have been commonly explained as a the evaluation of domal surface uplift predicted alleling the rifted margin (e.g., Menzies et result of extension and decompression melting by theoretical models (e.g., Farnetani and al., 2002), as a result of decompression melt- over a thermal mantle anomaly of 100–150 °C Richards, 1994). However, recently, it has been ing and subsequent magmatic addition at the (e.g., White and McKenzie, 1989; White et al., proposed that the uplift pattern might be more breakup margin. Furthermore, rift flank uplift 2008; Rooney et al., 2011), which is generally complex and show higher-frequency uplift su- is also influenced by the flexural rigidity or the explained as plume induced (e.g., Courtillot et perimposed on a lower-frequency pattern as a equivalent elastic thickness of the lithosphere. al., 1999; Montelli et al., 2004). Other explana- result of the stress state and lithospheric struc- Part of the flank uplift is transient, as a result tions include small-scale convection or state an ture (e.g., Burov and Cloetingh, 2009; Burov of the thermal anomaly or plume temporarily absence of significant thermal anomalies due and Gerya, 2014). Moreover, it has been sug- elevating the rift margin above sea level and to fertile mantle (e.g., Korenaga et al., 2002; gested that there might be no uplift at all for a allowing initial flood basalts to flow downhill, Anderson, 2005). Crucial factors in modern thermo-chemical plume (Sobolev et al., 2011). before subsiding and forming seaward-dipping popular volcanic rifting models are the pre- Nevertheless, domal uplift is still character- reflector series (e.g., White et al., 2008). These rift history and timing of the thermal anomaly istic of many large igneous provinces (LIPs), features are often difficult to unravel in outcrop (e.g., Bown and White, 1995; Armitage et al., as indicated by abundant geological and geo- studies where large flood basalt outpourings 2010). These models assume that lithospheric physical data (e.g., Saunders et al., 2007). Two have obscured earlier vertical motions and ero- thinning can only be achieved by extension. types of uplift are generated by a plume; the sion events. Alternatively, later erosion might Traditionally, actively upwelling mantle or first is transient uplift due to an upwelling ther- have stripped away any evidence of earlier up- plume material was thought to flatten below mal anomaly. The second is permanent uplift, lift phases. However, better horizontal and ver- stiff lithosphere (e.g., Farnetani and Richards, owing to plume-induced partial melting of the tical resolution can be obtained on magmatic 1994). However, recent studies suggest that lithosphere, generating a magmatic underplate rifted margins from offshore seismic profiles lithosphere can be eroded by plume-generated or high-velocity body at lower-crustal levels tied in with hydrocarbon exploration wells. convective currents or gravitational insta- (e.g., Tiley et al., 2004). Moreover, underplat- The NW Australian offshore area has long ing of high-density mantle melts can lead to been recognized as a volcanic margin (e.g., *Present address: Murphy Oil Corp, 9805 Katy Free- fractionation and shallower magmatic activity, White and McKenzie, 1989; Coffin and El- way #G200, Houston, Texas 77024, USA. evidenced by regional dolerite intrusion com- dholm, 1994) characterized by minor postrift

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subsidence and relatively limited magmatic activity (Symonds et al., 1998), with a large Seismic profile Gascoyne COT Abyssal intrusive component sourced by an underlying plain 1 N high-velocity body (Rohrman, 2013). Theories SDR Galah Gascoyne on the origin of the NW Australian LIP (Fig. 1) Lower Barrow delta Volcanic margin 18°S have been hampered by poorly constrained spa- gabbro Province Exmouth sill complex 3 tial and temporal evidence. Legacy models for plateau the LIP propose either a loosely constrained flood basalt 4 km A ESP and OBS profiles mantle plume without a clear hotspot track, or a J1 rifting-related mechanism (e.g., Mihut and Mül- selected wells B 2 ODP ler, 1998; Karner and Driscoll, 1999; Mutter et Pilbara craton E1 NG1 20°S 766 I1 al., 1989; Hopper et al., 1992). However, due to V1 bathymetry extensive seismic-reflection coverage and vari- 4 ous exploration wells drilled by the hydrocarbon S1 industry, this region provides an excellent natu- Sonja A1 ral laboratory in which to study the relationships Ju1 Ridge 5 Cuvier O1 22°S among magmatism, uplift, sedimentation, and Quokka YE1 breakup. In this article, I focus on: (1) estimat- Sonne margin Rise Cuvier Pa1 ing the spatial extent and relative amount of up- Ridge SP1 Abyssal H1 lift from observations on denudation (erosion), P1 magmatic addition, and sedimentation, and plain (2) explaining the results with a simple isostatic Wallaby mass balance and a one-dimensional melting Plateau Bernier model. The findings suggest the presence of a platform WS 100 km mantle plume before margin breakup in the NW 108°E 112°E 115°E Australian region. Moreover, the results explain most of the currently available data. Figure 1. Map of Exmouth region. SDR—seaward-dipping reflector series, COT— continent-ocean transition, BSB—Barrow subbasin, PS—Peedamullah Shelf, CRFZ—Cape Range fracture zone, WZFZ—Wallaby Zenith fracture zone. Vinck-1 GEOLOGIC SETTING (V1), Eendracht-1 (E1), Investigator-1 (I1), Sirius-1 (S1), Herdsman-1 (H1), Pendock-1 (P1), Sandy Point-1 (SP1), Paterson-1 (Pa1), Onslow-1 (O1), Jurabi-1 (Ju1), Jupiter-1 The NW Australian margin (Fig. 1) consists (J1), Yardie East-1 (YE1), Anchor-1 (A1), and North Gorgon-1 (NG1) are the main of a number of basins and continental fragments exploration wells used in this article. Deep seismic lines are from the literature with that formed as a response to several extensional observed high-velocity body (seismic velocities ≥ 7 km/s; numbered 1–5): 1—Fomin episodes (e.g., Longley et al., 2002; Gibbons et et al. (2000); 2—Mutter et al. (1989); 3—Goncharov et al. (2006); 4—Lorenzo et al. al., 2012). The south is characterized by Paleo- (1991); 5—Hopper et al. (1992). ODP—Ocean Drilling Program site; OBS—ocean bottom seismometer; ESP—expanded spread profile; WS—Wallaby Saddle. zoic basins such as the Bernier Platform. Further to the north, there are the Mesozoic Exmouth subbasin, Barrow subbasin, and Exmouth Pla- teau, while to the west, there is the fragmented and poorly known Wallaby Plateau (Fig. 1; Say- tectono- global ers et al., 2002; Stagg et al., 2004). Age (Ma) stage magmatics sea level lithology formation

The main basin-forming event took place in Aptian rift NESB EP SESB the Late Permian to accommodate up to 7 km 100m

po st Muderong Shale of deltaic Mungaroo Formation (Fig. 2). In the 125 Barremian ------

p ------Hauterivian ------Exmouth subbasin and Barrow subbasin, Late ……………. Birdrong sandstone

Valanginian t ………… Early underplating ………… Lower Barrow gp Triassic and Middle Jurassic extension provided Cretaceous Berriasian brea ku …………

accommodation for a thick Jurassic section Tithonian atic --….. Dupuy sandstone 150 e uplif (Fig. 2), while on the Exmouth Plateau, exten- Late Kimmeridgian gm 0m ----- Dingo claystone Oxfordian ---- plum ma ----- sion was limited, and the Early to mid-Jurassic Callovian Bathonian ------Athol fm interval is a highly condensed section, bounded Mid Bajocian n …. Aalenian ------….…. at the top by the Callovian regional unconfor- 175 ------….…. Learmonth Toarcian …. delta Jurassic mity (Longley et al., 2002). tensio …... Pleinsbachian -- .. ex flood basalts,

The southern Exmouth subbasin records ex- Early Sinemurian s, ..-- .. Murat siltstone ke tensive Early Cretaceous erosion. However, the Hettangian

200 di Brigadier fm northern Exmouth subbasin, Barrow subbasin, Rhaetian -…. --- Late --- …. and southern Exmouth Plateau record deposi- Norian sills, --- ….--- Mungaroo fm Triassic …. tion of the south-sourced Berriasian–Valangin- …. sandstone shale ian Lower Barrow Group delta (Figs. 1 and 2), --- limestone indicating major erosion of a largely missing Figure 2. Simplified tectono-stratigraphic chart for the Exmouth region. NESB—North Exmouth source area. Lower Barrow sediments are sepa- subbasin, EP—Exmouth Plateau, SESB—South Exmouth subbasin.

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rated from the overlying Birdrong Group by the body (Fig. 3A; Rohrman, 2013). Mutter et al. Finally, the area experienced postrift sedi- Valanginian unconformity, marking continental (1989) and Lorenzo et al. (1991) first identified mentation and subsidence throughout the Cre- breakup between Australia and Greater India this high-velocity body in expanded spread pro- taceous and Tertiary, while the Exmouth Plateau along the Cuvier margin, which is linked to the file (ESP) seismic data below the Exmouth Pla- was affected by a Campanian inversion event Gascoyne margin by the Cape Range fracture teau, interpreted as underplated magmatic mate- (Bradshaw et al., 1998), followed by regional zone transform (Robb et al., 2005). rial at the base of the crust (Hopper et al., 1992) Neogene compression as a result of plate reor- Magmatic sills and dikes of tholeiitic affinity with seismic velocities of ~7 km/s. Overall, in- ganization (e.g., Longley et al., 2002). (Ludden and Dionne, 1992) intruded the Munga- trusive activity seems more intense near the in- roo and younger formations and were tradition- tersection of the Cape Range fracture zone and PREVIOUS MODELS FOR MAGMATISM ally assigned a Late Jurassic to Early Cretaceous Cuvier margin, as evidenced by large gabbroic AND DENUDATION age (Symonds et al., 1998), predating and syn- intrusions (Müller et al., 2002), dike-dominated chronous with breakup (Fig. 2). Sills and dikes complexes (Fig. 1), and anomalously thick sills Lorenzo et al. (1991) viewed the Cape Range have been sampled in only two wells, ODP 766 originating from the Cape Range fracture zone fracture zone as a leaky transform as a result of and YE-1 (Fig. 1), and most of the evidence on region. Features synchronous with breakup are a hot ocean spreading ridge juxtaposed with the presence of intrusives comes from seismic- the extrusion of seaward-dipping reflectors and stretched continental crust of the Exmouth Pla- reflection data. Recently, indirect dating of vent flood basalts from the Gascoyne margin, Cuvier teau, generating an ~10-km-thick underplated structures associated with sill intrusion on the margin rift axis, Wallaby Saddle, and in the Ga- wedge below the Exmouth Plateau and ~3 km Exmouth Plateau and Exmouth subbasin yielded lah Province (Fig. 1; Symonds et al., 1998; Rey of denudation. Lorenzo et al. (1991) argued that Late Jurassic ages (Rohrman, 2013; Magee et et al., 2008). Extrusives are relatively scarce in the lack of a hotspot trail is evidence for the ab- al., 2013, 2015), predating breakup, whereas the Exmouth region compared with other volca- sence of a mantle plume and therefore supported McClay et al. (2013) inferred Early Cretaceous nic margins, with many of the extrusive rocks in an interpretation of secondary convection (Hop- ages for sills at the Cuvier breakup margin. The the South Exmouth subbasin probably eroded, per et al., 1992). However, shallow intrusions in origin of the Exmouth Plateau intrusive sheets as observed by truncated sills on seismic data the Exmouth Plateau sourced by the underlying is most likely from the underlying high-velocity (Mihut and Müller, 1998). underplate are Late Jurassic (Rohrman, 2013)

SW NE PGS multiclient New Dawn 2D survey A 2

MuMungngaarroooo fm 4 (s) 6

TWT sisilllls hard 8 lloowewer ccrrusust

10 Currently being PSDM reprocessed as Westraliarali ACCESSACCESS 1100 km soft

NW E1(Eendracht 1) I1(Investigator 1) SE PGS multiclient New Dawn 2D survey COT 2 B

4 MMuungngararoooo fm sisilllls (s) 6 TWT 8 lolowewer ccrrusust

Currently being PSDM rerreprocessedeprocessed as Westraliar i ACCESS 1010 km

Figure 3. Seismic profiles A and B; for location see Figure 1; yellow—Lower Barrow Group, purple—underplating, blue-green horizon—intra-Triassic marker. Sills in the Triassic are evidenced as high-amplitude subhorizontal reflections. TWT—two-way traveltime; COT—continent-ocean transition. Seismic images are courtesy of PGS.

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and older than the Hauterivian (magnetic anom- data as a basis for modeling efforts to establish ward aggrading sequence. Profile B displays a aly M10N) ocean spreading ridge in the Cuvier a spatial denudation pattern and discriminate thinning high-velocity body (underplate) toward Abyssal Plain (Robb et al., 2005), indicating that between rifting- or mantle plume–related uplift. the Gascoyne margin breakup axis, with seismic the Late Jurassic underplate must have another velocities between 7.1 and 7.8 km/s obtained origin. Karner and Driscoll (1999) proposed a SEISMIC DATA from ocean bottom seismometers (Fig. 1; Gon- four-phased extension and breakup model for charov et al., 2006) and a basinward arch where the Exmouth Plateau and Exmouth subbasin to Various two- and three-dimensional (2-D the underplate is thickest. The Lower Barrow explain the current basin fill and crustal config- and 3-D) seismic-reflection data sets were used Group thickens toward the east and shows a uration. Estimated pure shear stretching factors in this study. However, the primary data set was prominent inversion in the center of the profile for the Permian are around 1.4 on the Exmouth the 2D NWS07 seismic survey acquired by indicative of a later Campanian inversion event Plateau, decreasing eastward, while in the Ex- PGS in 2007 using a shot interval of 37.5 m, a (Bradshaw et al., 1998). mouth subbasin and Barrow subbasin, Late Tri- streamer length of 8 km, and a nominal fold of assic and Middle Jurassic extension with a pure 106. Total record length is 12 s. Figure 3 shows DENUDATION AND UPLIFT shear stretching factor of ~1.2–1.5 provided two perpendicular seismic lines on the Exmouth accommodation for a thick Jurassic section Plateau., across the Cape Range fracture zone Methodology (Fig. 2). Ongoing extension in the Callovian and transform margin (A) and Gascoyne passive Kimmeridgian decreased stretching to values margin (B), respectively. Profile A depicts a Here, I use an Airy isostatic approach to esti- below 1.2. Meanwhile, the Exmouth Plateau ex- magmatic underplate (purple) derived from ESP mate denudation and uplift. Denudation is taken perienced limited extension in the Late Jurassic and gravity data (Fig. 1; Mutter et al., 1989; Lo- to be equal to erosion and is estimated from (b ~1). Tithonian–Valanginian lower-crustal and renzo et al., 1991), estimated at ~10 km thick seismic-reflection and well data as well as back- mantle thinning factors on the Exmouth Plateau near the Cape Range fracture zone (Figs. 1 stripping exploration wells, and it is calculated are around 2.65–2.8, while upper-crustal exten- and 2) and thinning toward the northeast and as the amount of missing section relative to a sion is limited (<1.2). This discrepancy was sourcing the shallower sill complex (Rohrman, reference point (either seismic data and/or offset interpreted as a crustal detachment by Karner 2013). These intrusive sheets are visible as high- well data). Erosion is assumed to be subaerial. and Driscoll (1999). Closer to the Gascoyne amplitude (hard) reflectors cutting the Triassic Uplift is defined as tectonic uplift (e.g., Rowley margin, crustal stretching increases to ~7, while Mungaroo stratigraphy (Fig. 3). The blue-green and White, 1998). lower-crustal and mantle stretching are around marker is an intra-Triassic (intra-Mungaroo) re- 10, resulting in breakup (Driscoll and Karner, flector indicative of 2–3 km of denudation near Data 1998). Initial oceanic half-spreading rates for the Cape Range fracture zone, consistent with the Cuvier Abyssal Plain and Gascoyne Abys- previous estimates (Lorenzo et al., 1991). The Two profiles were used to obtain denudation sal Plain are ~35 mm/yr (Fig. 1; Gibbons et overlying Lower Barrow Group (yellow) shows estimates, the first, C–C′ (Figs. 4 and 5), starts at al., 2012). Karner and Driscoll’s (1999) model a distinctive prograding pattern near the Cape the Cape Range fracture zone, runs SW-NE, and predicts partial melting using ambient mantle Range fracture zone, developing into a basin- uses denudation data from Lorenzo et al. (1991)

temperatures (Tp ~1330 °C), generating an underplate thickness of ~7 km near the Gascoyne

breakup margin to 600 m over the central Ex- C C’ mouth Plateau during Tithonian–Valanginian 4.5 Figure 4. (Top diagram) extension. However, ocean bottom seismometer 2 Denudation vs. distance lines and potential field modeling (Goncharov (km) 2.5 from Cape Range fracture et al., 2006; Direen et al., 2008) suggest that the 1 zone (CRFZ) along C–C′ maximum underplate thickness is offset from Figure 9 location profile (filled dots). Denu- the COT (Continental Ocean Transition) toward 0.5 underplate (km) dation estimates are from 0 I1 e Lorenzo et al. (1991) and the central Exmouth Plateau and thins near the Denudation V1 J1 Gascoyne margin (Fig. 3B). additional seismic mea- -1 Figure 9 location surements relative to a Finally, Mihut and Müller (1998) and Mül- Effectiv mid-Triassic reflector; typi- ler et al. (2002) proposed a mantle plume of 50 100 150 200 250 cal error is ±100 m. Filled ~400 km diameter for the region, centered on Distance from CRFZ (km) dot with error bar (±200 m) the eroded Bernier Platform (Fig. 1), with a denotes denudation esti-

mate from wells (V1, I1, poorly constrained hotspot track moving north- D D’ 4.5 west along the Wallaby Zenith fracture zone ? J1). Stars denote effective underplate measurement. (Fig. 1). The Wallaby Plateau has been recog- 2 (km) (Bottom diagram) Denu- nized to consist mainly of continental crust, and SP1 2.5 dation vs. distance from only the northern part, known as the Quokka H1 the Cuvier margin (CAP— 1 Pa1 Rise (Fig. 1) is still considered to be igneous Cuvier Abyssal Plain) along (Symonds et al., 1998; Sayers et al., 2002); thus, 0.5 underplate (km) transect D–D′ for selected 0 e this theory no longer holds. The proposed plume Denudation wells H1, SP1, and Pa1, with error bar up to ±500 m is also located away from main magmatism on -1 (for location of profiles, see

the Exmouth Plateau, Galah Province, and Cape Effectiv Fig. 5). Range fracture zone. In the next section, I will 50 100 150 200 introduce seismic and well data and use these Distance from CAP (km)

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A B underplate thickness Late Jurassic/Early from seismic velocities Cretaceous denudation underplating extent underplating extent ? at emplacement at emplacement 18°S ? 18°S time ? time ? C’ sediment transport directions J1 E1 Lower Barrow delta E1 J1 ODP I1 20°S I1 20°S 766 NG1 V1 NG1 V1 S1 S1 C Ju1 A1 Ju1 A1 O1 O1 YE1 ? D 22°S 22°S SP1 SP1 Pa1 H1 Pa1 D’ H1 P1 P1

4 km 4 km ? ? 108°E 112°E 100 km 108°E 112°E 100 km

Figure 5. (A) Underplating thickness from expanded spread profile and ocean bottom seismometer seismic velocities, interpolated and hand-contoured from profiles in Figure 1. (B) Denudation estimates hand contoured from well and seismic-reflection data as well as calculated from effective underplate thickness using Equation 1, away from wells. Figure shows depositional pattern for Lower Barrow Group (e.g., Longley et al., 2002) and sediment source from provenance (Lewis and Sircombe, 2013). ODP—Ocean Drilling Program site. For explanation of symbols, see Figure 1.

and seismic-reflection data calibrated to offset more, these wells have large sections of Upper denudation patterns along the Gascoyne margin well data (Fig. 3) up to ~60 km from the Cape Jurassic Dingo Formation (Fig. 2) missing. Ero- with small flexural rigidity differences are more Range fracture zone, while denudation at larger sional products were deposited as the sand-rich likely generated by rifting owing to its better- distances from the Cape Range fracture zone Lower Barrow Group (well I1). However, since defined rift-parallel nature and underplating dis- is based on backstripped exploration wells V1 the Dingo Formation consists of mainly clay- tribution (Fig. 5). (Vinck-1), I1 (Investigator-1), and J1 (Jupiter-1; stones, the origin of most of the Lower Barrow Cloetingh et al., 1992). Relatively little denuda- Group must have been elsewhere, most likely Model and Application tion seems to have taken place at distances from to the southwest of the Cape Range fracture 60 km to 200 km from the Cape Range fracture zone, as evidenced from Figure 3A, depicting To relate denudation measurements from zone (V1 to J1; Fig. 4), which is typical for large prograding deltaic sequences from the south in wells and seismic data to mantle plume–in- tracts of the Exmouth Plateau. Errors from seis- the Lower Barrow Group. Denudation at H1 is duced magmatic underplating (e.g., Rowley and mic-reflection data are assumed to be ±100 m between 500 m to 1.5 km, while at YE-1, it is White, 1998; Tiley et al., 2004) and lithospheric within 60 km of the Cape Range fracture zone, ~1 km, decreasing to ~500 m at Ju1 (Fig. 5B). thinning, we need to quantify denudation (D). since most denudation has taken place on the This is consistent with the study of Müller et I assume lithospheric thinning and magma in- Triassic Upper Mungaroo Formation, the stra- al. (2002), who quoted ~1.5 km of denudation trusion are taking place instantaneously and are tigraphy of which is well defined from seismic between SP1 and Pa1 (Fig. 5B). Overall, the de- in isostatic equilibrium with mid-oceanic ridges stratigraphy and well data. However, these er- nudation pattern defined in Figure 5B follows an (Fig. 7). All of the underplate is emplaced at the ror estimates are up to ±500 m when there are ellipsoid or domal shape, with maximum denu- Moho in Figure 7, but in reality, a smaller part large eroded regions, and there are no suitable dation around 2.6 km at the Cuvier margin and is distributed through the crust and upper mantle reference sections. This applies to profile D–D′, Cape Range fracture zone and about one third as sills, although this does not influence the cal- where denudation is not well defined, mainly of the dome missing owing to plate tectonics. culations. Assuming Airy isostasy and keeping owing to poorer-quality seismic data and rapidly Moreover, the intersection between the Cape the top of the lithosphere at the reference level changing geology, so we have to rely on well Range fracture zone and Cuvier margin also ap- before and after underplating, denudation D is data. Figure 6 shows a well correlation panel pears to have the highest amount of magmatic defined as: from the Exmouth Plateau into the Exmouth intrusions. However, there could be a compo- XL()ρ−mxρ+ραmm∆−TP()ρ−ρw subbasin. The section is flattened on top Jurassic nent of rifting-related uplift along the Cuvier D = , (1) (top gray zone) and suggests significant erosion margin due to potentially large differences in ()ρ−msρ

at the Jurassic-Cretaceous boundary in the Ex- flexural rigidity between the margin and the where rm is the density of the lithospheric 3 mouth subbasin (wells H1, YE1, Ju1). Further- cratonic hinterland. Magmatic intrusion and mantle (3300 kg/m ), rw is the density of wa-

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Exmouth Plateau D ρw W ρs Exmouth Sub basin ρ South s ρc t Exmouth Sub basin c ρc ρx X

a ρ 0km ρm m

1km Birdrong sst ρm (1–αΔT) L Barrow base

ρa ρa 2km

H1 I1 Exmouth plateau 3km I1 A B Figure 7. Mass balance cartoon for isostatic calculations. (A) Column of Ju1 BSB lithosphere with thickness a in equi- 4km YE1 librium with mid-oceanic ridge and underlain by asthenosphere with Ju1 density r , where W is water layer H1 Pa1 a (H2O) with density rw, tc is the crustal P1 thickness subdivided into sediments

5km YE1 with density rs and crystalline crust

with density rc. (B) Lithosphere with Figure 6. Well correlation section from Exmouth subbasin to Exmouth Plateau, flattened on the denudation D as a result of underplat- Callovian unconformity. Jurassic rocks are in gray; sst—sandstone. Well P1 (Pendock 1) is on the ing (X) and hotter than normal mantle upthrown rift flank of the Exmouth subbasin, known as the Bernier Platform, and contains only L. With L, a layer of lithosphere with Paleozoic rocks below the Callovian-Valanginian unconformity. BSB—Barrow subbasin. density rm replaced by hotter litho-

sphere with density rm (1 – aDT ). X is

the underplate with density rx. 3 ter (1000 kg/m ), rs is the density of sediments ness of ~20 km, confirmed by seismic data 3 (2600 kg/m ), rx is the density of underplated (Symonds et al., 1998). Since X is dependent material (3000 kg/m3), L is the thickness of on L, I will use an empirical one-dimensional ing the melt thickness (McKenzie and Bickle, lithosphere replaced by hotter plume-derived melting model for anhydrous peridotite (e.g., 1988). Figure 8 depicts the modeled relation-

mantle, a is the thermal expansion coefficient McKenzie and Bickle, 1988; Katz et al., 2003) ship between X and L for various Tp. Further- -5 –1 (3 × 10 °C ), DT is the temperature differ- to solve this problem. The melting function is more, it is assumed that elevated Tp ≥ 1380 °C

ence between normal and hot mantle (°C), X defined as the intersection of Tp and the soli- (DT ≥ 50 °C) is indicative of a mantle plume is the thickness of the underplate (meters), and dus, as well as subsequent melt fraction curves (e.g., White and McKenzie, 1989), based on an P is paleo–water depth just before denudation (McKenzie and Bickle, 1988). Since the melt ambient mantle temperature of 1330 °C. occurs (meters). Present-day topography is be- fraction, solidus, and liquidus are a function of To calibrate the model, I use profile C–C′ low sea level and therefore does not influence depth, we can integrate the melt function over (Figs. 4 and 5), since the Exmouth Plateau has Equation 1. X is taken as being dependent on lithospheric thickness L (Fig. 7) to obtain the reasonably well-documented underplate thick-

L and on mantle potential temperature Tp. Fur- instantaneous frozen melt thickness. Further- nesses derived from several ESP and ocean thermore, I assume that flexural rigidity of the more, a lid of lithosphere with thickness a – L bottom seismometer data (e.g., Mutter et al., lithosphere is low owing to previous pre-Juras- (Fig. 7) is placed on top, to reduce the melting 1989; Lorenzo et al., 1991; Fomin et al., 2000; sic extension events, approximating isostatic column. This static model yields very simi- Figs. 1, 3, and 5A), although there is a wide conditions. Lithospheric thickness during the lar results to a pure shear melting model, like range in seismic vintages. However, using mid-Jurassic must have been ~100 km, and that of White and McKenzie (1989) when L is these in Equation 1 and assuming no contribu- this serves as the isostatic compensation level. between 0 and 50 km (equivalent to b = 1–2). tion from lithospheric thinning yields denuda- The Jurassic lithospheric thickness is estimated However, the pure shear melting model thins tion values that are too high compared to seis- from an originally 120-km-thick lithosphere in both crust and mantle lithosphere, whereas mic- and exploration well–derived denudation. the Permian, stretched by 1.4 (Driscoll and the present model only thins the mantle litho- Using the observed values of D from C–C′ Karner, 1998), taking into account a 60 m.y. sphere, similar to depth-dependent stretching. (Fig. 4), it is possible to calculate X and L. A rea- thermal time constant (~50% of thermal re- The effect of this difference is slightly lower sonable fit is obtained whenD T = 100–150 °C

equilibration; Turcotte and Schubert, 2002). melt thicknesses for the pure shear model, as a and maximum Tp = 1430–1480 °C; lower Tp This calculation yields a Jurassic crustal thick- result of the shape of the geotherms influenc- values would require higher lithospheric thin-

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stretching mode l Middleton, 2002) up to 250 km away from tectonic subsidence history to J1. Tectonic uplift β = 2.65 - 2.8 the Cape Range fracture zone in a subcircular (U) at the Cape Range fracture zone is estimated pattern and mimicking the underplating extent from the amount of eroded Triassic section from

(Fig. 5). A comparison of the X/L relationship Figure 4 (D = 2.6 km), calculated back into the obtained from Figure 8 with measured denuda- tectonic subsidence curve (U = 545 m) using

tion (Figs. 4 and 5), using Equation 1, suggests the relation from Chadwick (1985): U = D(rm 10 that the true or effective underplate thicknesses – rs)/rm (Fig. 9). Timing of earliest plume uplift are ~40% of the seismically measured value. is from recognition of the Callovian unconfor- This implies that measured seismic underplate mity, followed by intrusive magmatism in the

thickness (X) in km (high-velocity body) thicknesses do not consist Tithonian (Rohrman, 2013) on the Exmouth of 100% crystallized melt and, instead, are a Plateau. Anomalous subsidence after the plume mix of intruded melt and lower-crustal host rock event (Fig. 9) can be explained by the flow of 5 CRFZ (White et al., 2008), since seismic velocities for hot plume material toward the breakup axes

Underplate lower crust and underplate overlap (Mooney et during extension (e.g., Sleep, 1997; Buck, 2004;

me al., 1998). High-velocity body thicknesses be- Armitage et al., 2009). Following plume exit lu p model low oceanic crust (Fig. 5A) were not consid- from the region, cooling further contributes to ered in the analysis, since they are most likely thermal subsidence. J1 affected by later extension and decompression- 0 50 100 related magmatism. REGIONAL SIGNIFICANCE AND Lithospheric thinning (L) in km The denudation curve in profile C–C′ (Fig. 4) HOTSPOT TRACK was calculated using Equation 1 with a conver- Figure 8. Lithospheric thinning vs. underplate thickness, using parameterization and poten- sion factor for X of 40% and decreases from Although the Cape Range fracture zone–Cu- tial temperatures from McKenzie and Bickle ~4 km near the Cape Range fracture zone to vier margin area suffered maximum denudation, (1988; P—peridotite). Potential temperatures 440 m at J1. L is estimated around 30–40 km thinning, and underplating, most of it must have for mantle melting containing 50% eclogite (E; (Cape Range fracture zone) to 10-15 km (J1). taken place on a continental fragment that filled mid-ocean-ridge basalt–peridotite mix) from Late Jurassic paleo–water depths are presumed the current Cuvier Abyssal Plain gap, before Spandler et al. (2008) serve as a relative high to drop from 0 near the Cape Range fracture zone breakup. Where did this fragment go? One solu- case and are shown for comparison only. Gray to 100 m over the Exmouth Plateau. We can now tion is that this sliver drifted away with Greater box is lithospheric thinning (b) estimated for the Exmouth Plateau during the Late Jurassic–Early use the denudation/underplate relationship from India and was subsequently integrated in the Hi- Cretaceous (Karner and Driscoll, 1999). Stippled profile C–C′ and apply it to profile D–D′, where malayan orogeny. However, Gibbons et al. (2012) horizontal lines are Cape Range fracture zone underplate thicknesses are missing (Fig. 5A), us- proposed that the Sonja Ridge and Sonne Ridge (CRFZ) and J1 locations (see Figs. 1 and 4) with ing Equation 1 (Fig. 4). Measured denudation in region (Fig. 1) are continental crust fragments effective underplate thickness from denudation the west of D–D′ suggests a calculated effective that rifted away from the Cuvier margin. To test analysis using Equation 1 and range of possible underplate of 2.5 km and L = 17.5–23.3 km, but if the Sonne and Sonja Ridge fragments could be mantle thinning solutions, dependent on DT. For explanation, see text. higher values are possible closer to the COT the missing pieces that were originally located to (Fig. 4). Thus, I propose that this region is close the south of the Cape Range fracture zone, I use to where a plume/upwelling hit the lithosphere, the present-day Exmouth and Wallaby Plateau consistent with the magmatic record. crustal thicknesses as an analog (Symonds et al., ning estimates and lateral temperature gradients Figure 9 depicts tectonic subsidence versus 1998). This suggests that the Sonja-Sonne Ridge between the Cape Range fracture zone and J1 time for two locations in Figure 4 (Cape Range fragment’s initial Late Jurassic crustal thickness location, while lower L values would require a fracture zone and J1). While J1 is constrained must have been similar at ~15–20 km, as a result

higher Tp or the addition of eclogite. Using the by well analysis using standard backstripping of earlier extension episodes. Denudation analy- 50/50 mid-ocean-ridge basalt (MORB)–peri- techniques (e.g., Cloetingh et al., 1992), the sis from profile C–C′ (Fig. 4) suggests ~3 km dotite mix solidus (Fig. 8; e.g., Spandler et al., Cape Range fracture zone location lacks a well denudation for the Sonja-Sonne Ridge fragment, 2008), mimicking addition of 50% eclogite to but has similar Triassic seismic stratigraphy and consisting of Triassic Mungaroo Formation and peridotite mantle, it is possible to make some crude estimates about melt thicknesses, since experimental data on the MORB-peridotite mix liquidus are not well constrained (e.g., Perter- plume activity (km) mann and Hirschmann, 2003). However, the Figure 9. Tectonic subsid- 1 ence vs. time for Jupiter-1 lowest DT would still be ~75 °C. The high Tp uplift (U) is consistent with elevated temperatures inferred (J1) well and pseudowell from Early Cretaceous tholeiitic sill geochem- at the Cape Range frac- 2 timing of uplift CRFZ ture zone (CRFZ), showing

istry at Ocean Drilling Program (ODP) Site subsidence effect of transient and per- 766 (Ludden and Dionne, 1992) (Fig. 1) and breakup J1 manent uplift. Note that elevated heat flow from paleothermal indica- 3 there is tectonic uplift at J1

ctonic Permanent uplift due to underplating tors in various Exmouth Plateau and Exmouth as well, but this amount is subbasin wells (J1, I1, Ju1) suggesting a heat Te very small (<50 m). 200 150 100 50 0 flow increase from ~55 mW/m2 to ~70 mW/m2 at the Jurassic-Cretaceous boundary (He and Age (Ma)

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Early Cretaceous mafic rocks, and requires an Jurassic (Ghori et al., 2005) and deflecting the main sill complexes are sourced by the under- effective underplate of ~4 km for the Sonja- upwelling toward the thinner lithosphere of plate closer to the center of the plume. Sonne Ridge fragments (Fig. 1) and 30–40 km the Exmouth region where partial melting took Having established the initial plume or up- of lithospheric thinning. Palynology and zir- place. This is consistent with the recognition of welling location, we can now concentrate on con provenance studies of the Lower Barrow an large low-shear-velocity province edge be- the surface expression of plume activity us- Group (Fig. 2) on the Exmouth Plateau suggest low Western Australia at 160 Ma, assumed to ing the plate model of Gibbons et al. (2012) mainly erosion products from the Mungaroo be the birthplace of mantle plumes (Torsvik et (Fig. 10). This model has the Australian con- Formation (Fig. 5B; Exon and Buffler, 1992; al., 2010). Underplating, magmatism, and denu- tinent relatively stable during the Mesozoic. Lewis and Sircombe, 2013) from a southern dation were focused on the intersection of the Ocean spreading of normal-thickness oceanic direction, consistent with major denudation of Cuvier margin and Cape Range fracture zone crust at the Cuvier and Gascoyne margins com- the Sonja-Sonne Ridge fragments, while Lower (Fig. 1), characterized by a remnant positive menced in the Hauterivian (Robb et al., 2005; Barrow Group shale analysis points to an ad- Bouguer gravity anomaly (Rey et al., 2008). On Rey et al., 2008). At this point, the expression ditional mafic volcanic component (Fig. 1; impact, the plume head spread below the rest of plume activity or hotspot track left the area Exon and Buffler, 1992). Further evidence for of the Exmouth region (Fig. 9A), generating in a relative western direction (Fig. 10) to arrive a southern source area comes from conglom- thinned lithosphere, underplating, elevated heat at the igneous Quokka Rise (Figs. 1 and 10B; erates in the Lower Barrow Group, sampled flow, and shallow intrusives. The limit of under- Sayers et al., 2002), reflecting eastward move- by wells in the northern part of the Exmouth plating from ESP and ocean bottom seismom- ment of the Australian continent and assuming subbasin, indicating a proximal source in the eter data is depicted in Figure 5 and is based, a stationary plume. Finally, the hotspot moved south. Sediment mass balance calculations apart from aforementioned data, on the presence to the Zenith Plateau, just before Quokka Rise for the eroded region (~70,000 km3) compare of a thermal spike at the Jurassic-Cretaceous and Zenith Plateau were separated by rifting ca, 3 with the Lower Barrow delta (~90,000 km ), boundary from vitrinite reflectance/Tmax mod- 120 Ma (Fig. 10C; Gibbons, et al., 2012), hav- although approximately a fifth of the delta eling (He and Middleton, 2002). While wells ing been active for 30–45 m.y. There is a pos- was derived from Greater India or Antarctica within the underplate extent region (Fig. 5; e.g., sibility that the hotspot could have moved to the (Fig. 1; Lewis and Sircombe, 2013). I1, J1, Ju1, H1) do have this modeled heat spike, Greater India plate at that time; evidence exists Subsequent stretching of the lithosphere wells outside this area (e.g., NG1, A1) do not. in Tibet, where the Comei magmatic province with a stretching factor of 2–3 would thin the This upwelling event was separate from later records mafic sills and dikes dated between 150 Sonja-Sonne Ridge fragment crust to 5–10 km, rift-related magmatism, as evidenced by an and 130 Ma in strongly folded Late Triassic to resembling oceanic crustal thickness, but with- offset in the magnetic and seismic signature of Cretaceous sedimentary rocks (Zhu et al., 2008, out coherent magnetic striping (Ebinger and the underplate from rifting-related magmatism 2009). Interestingly, the recent plate reconstruc- Casey, 2001). This also explains the similar (Goncharov et al., 2006; Direen et al., 2008), as tions of Gibbons et al. (2012) place the Comei present-day water depth of the underplated well as the postulated Late Jurassic age of the province very close to the NW Australian mar- Sonja-Sonne Ridge fragment compared with underplate (Rohrman, 2013). It is likely that the gin (Fig. 10), suggesting that the Comei prov- normal-thickness Cuvier Abyssal Plain oceanic region with hot thin asthenosphere exceeds that ince could be a missing piece of the Exmouth crust (Sayers et al., 2002; Stagg et al., 2004). depicted for underplating, although the evidence mantle plume. I argue that the timing for plume impact was is sparse. I speculate that the small sill complex between ca. 165 and 136 Ma, causing mag- in the northeast of the Exmouth Plateau, outside DISCUSSION matism and uplift (Figs. 2 and 9). The origin the underplated region (Fig. 1), consists of low- of the plume or active upwelling can possibly Ti tholeiites or alkaline basalts, directly sourced The model and data integration presented be tracked down from below the Pilbara cra- from the hot thin asthenosphere, similar to the here delineate a Late Jurassic mantle plume in ton (Fig. 1), undergoing an uplift phase in the Afar hotspot (Beccaluva et al., 2009), while the the greater Exmouth region, feeding underplate

~165-136 Ma 130 Ma 120 Ma magnetic GB striping Tethys GB Tethys EP GB EP Ocean EP Ocean QR ZP Comei QR WP WP Greater WP Comei Greater India Greater WS Comei WS India A India B C

Figure 10. (A) Plume main activity period. Dashed line indicates plume conduit, while stippled outline denotes minimum plume head, based on high-velocity body extent. Gray polygons indicate reconstructed sill complexes derived from plume-fed underplate (Fig. 1). Gray stippled outlines depict potential extension of sill complexes over Sonja Ridge–Sonne Ridge region. WZFZ—Wallaby Zenith fracture zone, GB—Gascoyne block, EP—Exmouth Plateau, WP—Wallaby Plateau, WS—Wallaby Saddle, ZP—Zenith Plateau, filled black area—Comei large igneous province; other abbreviations, as in Figure 1. (B) Westward-progressing hotspot track and location at Quokka Rise (QR). (C) Continuing hotspot track and hotspot at Zenith Plateau location.

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and shallower intrusive sheets and influencing of plagioclase and clinopyroxene (cumulate tism. However, Armitage et al. (2010) stressed subsequent continental breakup. The novel ap- gabbro; Ludden and Dionne, 1992), consistent the importance of rift history, stating that prior proach in this isostatic assessment is that plume- with the underplating theory (Farnetani et al., extension is crucial for the amount of generated

derived hot mantle with a Tp of 100-150 °C 1996), though a normal oceanic source can- magmatism along the margins. NW Australia is, above ambient mantle was directly linked to not be completely ruled out (Ishiwatari, 1992). however, complex and fragmented, making it lithospheric thinning, subsequent melting, Other dredge samples from the region are highly difficult to verify this theory. and underplate generation, resulting in the ob- weathered (Sayers et al., 2002) and unsuitable Figure 11 depicts a cartoon of the evolu- served denudation. The plume interpretation is for detailed geochemistry interpretation. tion of the Exmouth region along a N-S profile

consistent with recently obtained Tp ~1560 °C Mantle plumes are sometimes linked to driv- crossing the Exmouth Plateau and terminating from olivines found in 128 Ma picrites of the ing continental breakup through the viscous nor- on the Bernier Platform, using concepts out- Comei region (Fig. 10; Xia et al., 2014)), sug- mal stress of a mantle diapir initiating breakup lined in this paper (e.g., Fig. 2), starting with gesting even higher excess mantle temperatures (e.g., Hill et al., 1992). However, dynamic mod- the preplume extensional setting in the Early– (160–270 °C), although this is dependent on the els tend to overestimate viscous normal stress Middle Jurassic (A), followed by plume activ- choice of ambient mantle temperatures (Putirka, and subsequent uplift (e.g., Farnetani and Rich- ity, uplift, underplating, and sill intrusion in the 2005). An alternative to the present model is em- ards, 1994), and plume-related faults are seldom Late Jurassic (B). This process leads to passive placement of the underplate laterally like a giant observed, suggesting normal stresses are rela- flow of the hot asthenospheric mantle to regions sill from above a plume conduit. In this case, tively minor. Highly eroded Precambrian LIPs where active extension takes place (e.g., Sleep, all the denudation at J1 is due to the sill, ~1.5 display radial dike swarms attributed to plume 1997), with faults being weakened by intruding times thicker than in the plume head case, since activity (Ernst et al., 2005), but these are most magma, leading to renewed or continuing mag- the lithospheric thinning component in Equa- likely fracture related and consist of fractionated matism and breakup in the Valanginian (C). By tion 1 is zero. However this model seems less tholeiitic dolerite, suggesting an underplate ori- that time, the plume (now a hotspot) has left the likely, because seismic-reflection data below gin (Cox, 1980). A decade ago, Burov and Guil- area, and extension-related flood basalts (future the Exmouth Plateau suggest evidence for the lou-Frottier (2005) proposed a dynamic mantle seaward-dipping reflectors) are being generated high-velocity body to be sourced by presumed plume model using a more realistic rheology for while the asthenosphere is still anomalously ultramafic sills/dikes (Rohrman, 2013) derived the lithosphere, implying that the uplift pattern hot during the Valanginian. This is evidenced from the thinned lithosphere. Fertile mantle and might be more complex and mimic features that by linear sill complexes close to the Gascoyne delamination (Elkins Tanton and Hager, 2000) are commonly associated with plate tectonics. and Cuvier passive margin, possibly related to were used by Sobolev et al. (2011) to explain However, if the plumes are relatively weak, hot breakup (McClay et al., 2013). After the hot the Siberian flood basalts. However, in the case convective features, then the associated magma- plume material has thermally decayed, regional of the Exmouth region, there is no evidence for tism and circulation of hot volatiles are more tilting generates seaward-dipping reflectors, fol- such an extensive melting event from the geo- likely to generate fault weaknesses and perhaps lowed by normal ocean spreading commencing logical record. Furthermore, eclogite would décollements (Heinson et al., 2005), which are in the Hauterivian (D). reduce denudation, owing to its high density. typical of the Exmouth Plateau. Hence, plumes Although the present model successfully ex- Thus, it seems unlikely that significant amounts could enable breakup, instead of driving it (e.g., plains currently available data, there are alter- of eclogite melting (>15%) were involved in the Buck, 2004; Armitage et al., 2009; Brune et al., natives. Essentially, the proposed model is very Exmouth mantle plume, even when considering 2013). These observations are confirmed in this similar to White and McKenzie’s (1989) origi- higher values for the thermal expansion coef- study and seem consistent with other volcanic nal model, without a crustal thinning compo- ficient a( ) (Schutt and Lesher, 2006). Limited margins, such as the Deccan Traps and North nent. It can be argued that the configuration and sampled magmatism at ODP Site 766 pro- Atlantic (Saunders et al., 2007), where the se- denudation pattern at the start of rifting (Fig. 9) vides evidence for low-pressure fractionation quence of events is more obscured by magma- can be explained by simply placing a 100-km-

basalt flows Gascoyne Cuvier North South extension extension rifting induced extended sills/dikes underplating sdrs surface proto-CRFZ lithosphere sdrs CRFZ crust H2O underplate partial cooling Convective removal base lithosphere melting Flow of hot convective Ocean of lithosphere Ocean plume material spreading spreading Upwelling plume A. Early-Mid Jurassic B. Late Jurassic C. Valanginian D. Hauterivian

Figure 11. Cartoon of timing of events along N-S profile (Fig. 1). (A) Preplume tectonic setting, with extended crust as a response to Early Triassic and mid-Jurassic extension. CRFZ—Cape Range fracture zone. (B) Impact of thermal plume generating lithospheric thinning, uplift, underplating, and shallow intrusive units in the Late Jurassic and onset of renewed extension at the Gascoyne and Cuvier margins. (C) Plume exit to the west (out of plane), with hot plume material flowing toward extended regions, generating decompression melting and volcanic margin magmatism (e.g., Galah Province, underplating, basalt flows [future seaward-dipping reflectors]) and breakup in the Valanginian. (D) Cooling of hot mantle to ambient conditions, subsidence, formation of seaward-dipping reflector series (sdrs), followed by initiation of normal ocean spreading in the Hauterivian.

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thick hot asthenosphere layer at the base of (e.g., van Hinte, 1978), which can account for High-velocity body thicknesses were used as the lithosphere, tapering toward the edge, with denudation errors of several hundred meters. input to calculate mantle thinning and subse- a similar configuration as in Figure 10 and an This makes it difficult to validate the proposed quent denudation analysis. Where high-velocity isostatic compensation level at 200 km. This model, when modeled denudation is equal or body data are absent, available denudation data eliminates the prerequisite of a hot upwelling less than the error estimate. However, by locat- were used to calculate high-velocity body thick- being able to thermally thin the lithosphere, but ing the region of plume impact from maximum ness and mantle thinning, filling in the gaps. It it would generate no underplating, unless fertile denudation and increased magmatism, these is- transpires that only ~40% of the high-velocity mantle (eclogite) is added (Fig. 8). However, the sues can be circumvented. Moreover, a recent body in the Exmouth region is directly related amount of added eclogite has to be very high assessment of paleo–water depths at deep-water to the upwelling event, with the remaining per- (up to 50%) to generate the underplate thick- margins (Crosby et al., 2011) indicated that un- centage consisting of preexisting lower crust. nesses required. This seems in contradiction certainties can be greatly reduced by sticking to Denudation is explained by a simple thermal with geochemical studies (Sobolev et al., 2007) a few rules: (1) using negligible bathymetry at model, linking plume-induced lithospheric thin- that quote eclogite percentages of <20% for the the start of rifting, (2) using geometry of seis- ning with melting and underplate formation.

Siberian LIP and selected ocean island basalts. mic reflectors, such as clinoforms, indicative The Tp of the plume is estimated at 75–150 °C Thus, it seems that this scenario is less plausible of potential changes in paleobathymetry, and above ambient mantle temperatures (~1330 °C) than the one originally proposed. (3) relating the predicted time-depth cooling of and does not require addition of eclogite, al- A rifting and decompression melting origin nearby oceanic crust to deep-water margins. though the presence of small amounts (<15%) (Nielsen and Hopper, 2004; Armitage et al., The Exmouth region has undergone multiple remains possible. The thermal plume eroded 2009) seems unlikely due to the denudation and additional denudation events since breakup, al- up to 30–40 km lithosphere of the prebreakup magmatic addition distribution pattern, with though their magnitudes have been limited. The Sonne Ridge–Sonja Ridge–Cape Range fracture higher denudation and magmatic thickening to- most important is the global Valanginian low- zone–Cuvier margin region and generated an ward the transfer zone margin (Cape Range frac- stand event (Fig. 2), coinciding with breakup ~4-km-thick effective underplate and ~500 m of ture zone), instead of the passive margin, where and generating a few hundred meters of erosion tectonic uplift. Meanwhile, distant regions like relatively minor denudation and magmatic addi- and subsequent redeposition (Ghori et al., 2005) the Exmouth Plateau and Wallaby Plateau only tion are observed. The underplated region along postdating plume-related uplift (Fig. 2). This underwent 10–15 km of lithospheric thinning by the Cape Range fracture zone (Lorenzo et al., event is identified off the Peedamullah Shelf the plume head, with effective underplating less 1991) could be described as part of transform (PS) in O1, an area unaffected by underplat- than 500 m thick and denudation less than a few margin generation (e.g., Robb et al., 2005; Gregg ing (Fig. 1). Another, Campanian uplift event hundred meters. Therefore, it might be difficult et al., 2009; Hebert and Montesi, 2011). How- inverted the Lower Barrow Group sediments to delineate a plume event below sedimentary ever, the scale and spatial extent of magmatism on the Exmouth Plateau, with minimal erosion basins when lithospheric thinning factors are would be more difficult to explain. Moreover, (Bradshaw et al., 1998). Finally, a more local- low (<20 km), due to error margins in measure- deposition of the Lower Barrow Group indicates ized uplift phase took place in the Neogene as ments equaling or exceeding modeled values. denudation of a much larger area than that de- a result of plate reorganization (e.g., Longley However, in this case, the region of highest defined by the Cape Range fracture zone and south et al., 2002). Taking these uncertainties into nudation and observed intrusive magmatism de- Exmouth subbasin. Depth-dependent stretching account, calculated plume-induced denudation lineates the plume. Widespread plume-induced (e.g., Karner and Driscoll, 1999) is theoretically values most likely represent maximum values. lithospheric thinning set the boundary condi- almost identical to the model presented here. tions for subsequent extension-related magma- Both models are characterized by mantle thin- CONCLUSIONS tism and breakup. After initial upwelling, the ning in the absence of, or with limited, upper- thermal plume can be traced as a hotspot to the crustal stretching. However, the Karner and This paper proposes that a mantle plume Quokka Rise and Zenith Plateau in the mid-Cre- Driscoll (1999) model predicts lower-mantle event took place in the Exmouth region in the taceous and possibly to Greater India. thinning factors (2.65) close to the Cape Range Late Jurassic, predating breakup and generat- fracture zone, increasing northeastward to 2.8, ing lithospheric thinning, magmatism, and tec- ACKNOWLEDGMENTS while underplate thicknesses decrease in the op- tonic uplift as delineated by isostatic denudation I would like to thank PGS for permission to use seismic images from the New Dawn two-dimensional seismic survey. posite direction, requiring a decrease in stretch- analysis. The locus of uplift was located below John Armitage and an anonymous reviewer are thanked for ing. Figure 8 depicts the stretching scenario. a now highly extended and subsided continen- useful and constructive reviews. Furthermore, I would like to In order to make this work, higher T near the tal fragment known as the Sonne Ridge–Sonja thank Lithosphere Editor Arlo B. Weil for editorial handling, p as well as various anonymous reviewers for thorough com- Cape Range fracture zone (+50 ° C hotter) is re- Ridge region, with erosional evidence recorded ments on previous versions of the manuscript.

quired, while at the J1 location, Tp needs to be in the sedimentary record of the Exmouth Pla- –50 °C cooler compared to ambient mantle. This teau as the Barremian to Valanginian Lower REFERENCES CITED is theoretically possible, if the age of the Cape Barrow delta. Elevated heat flow and extensive Agrusta, R., Arcay, D., Tommasi, A., Davaille, A., Ribe, N., and Range fracture zone underplate is similar to the intrusive magmatism further bear evidence to Gerya, T., 2013, Small scale convection in a plume fed low viscosity layer, beneath a moving plate: Geophysi- timing of seafloor spreading in the Cuvier Abys- the active upwelling event. A crucial factor in cal Journal International, v. 194, p. 591–610, doi:10.1093 sal Plain. However, sills and underplate are older this analysis is the recognition and spatial extent /gji/ggt128. than seafloor spreading. Hence, the stretching of a high-velocity body below continental crust Anderson, D., 2005, Large igneous provinces, delamination and fertile mantle: Elements, v. 1, p. 271–275, doi: scenario does not work, and the plume model from ESP and ocean bottom seismometer data, 10.2113/gselements .1.5.271. provides a better and more elegant solution. recognized as the source of shallow sills and Armitage, J.J., Henstock, T.J., Minshull, T.A., and Hopper, J.R., Some uncertainties remain. The main issues dike complexes observed in seismic-reflection 2009, Lithospheric controls on melt production during continental breakup at slow rates of extension: Applica- concern the large uncertainty in denudation es- data from the Exmouth Plateau, Exmouth sub- tion to the North Atlantic: Geochemistry Geophysics Geo- timates, up to ±500 m and paleo–water depths basin, and Wallaby Plateau (Rohrman, 2013). systems, v. 10, p. Q06018, doi:10.1029/2009GC002404.

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