Neogene rock uplift and erosion in northern Borneo: …searg.rhul.ac.uk/pubs/cottam_etal_2013 Kinabalu...Kinabalu granite, Mount Kinabalu Neogene rock uplift and erosion in northern

Journal of the Geological Society Online First

10.1144/jgs2011-130, first published July 16, 2013; doiJournal of the Geological Society

M. A. Cottam, R. Hall, C. Sperber, B. P. Kohn, M. A. Forster and G. E. Batt Kinabalu granite, Mount KinabaluNeogene rock uplift and erosion in northern Borneo: evidence from the

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1

Despite its proximity to several prolific oil-producing basins, the tectonic history of northern Borneo is still incompletely under-stood. The region is often suggested to have experienced signifi-cant uplift and erosion during the Cenozoic (e.g. Hall & Wilson 2000; Hutchison et al. 2000; Hall & Nichols 2002; Morley & Back 2008). However, despite several dating studies (e.g. Jacob-son 1970; Rangin et al. 1990; Hutchison et al. 2000; Swauger et al. 2000), the exact timing of exhumation remains poorly con-strained and the ultimate cause of uplift and erosion is not known. The onshore sedimentary record of the region’s exhumation is almost entirely missing; almost all the material removed is now offshore in the deep-water clastic wedge between the coast and the NW Borneo Trough (Fig. 1).

Thermochronology, the use of temperature-sensitive radio-metric dating methods to reconstruct the thermal histories of rocks, has been shown to be a valuable tool for investigating the exhumation histories of rocks from a wide variety of tectonic settings (e.g. Gleadow et al. 2002; Batt et al. 2004; Clark et al. 2005; Stockli 2005; Kirstein et al. 2006; Spotila et al. 2007; Metcalf et al. 2009; Glorie et al. 2010). In particular, the varia-tion of thermochronometric data with elevation can be used to constrain both the timing and rates of exhumation (e.g. Zeitler et al. 1982; House et al. 2001; Reiners et al. 2003). Such meth-ods are particularly pertinent where a region lacks critical struc-tures and sedimentary records to identify the timing, magnitude and causes of exhumation via other means.

The Kinabalu granite, situated in northern Borneo (Fig. 1), offers a unique opportunity to investigate the timing and rates of the region’s exhumation using thermochronology. The Kinabalu

granite is exposed over a wide range of elevation in the flanks and summit plateau of Mount Kinabalu (e.g. Jacobson 1970), a towering peak of over 4000 m elevation situated at the northern end of the Crocker Ranges, which mostly lie around 2000 m or below (Fig. 1). The excellent exposures of the glaciated granite at higher elevations allow the collection of thermochronometric data over a total elevation range of around 2000 m, which can be used to determine timing and rates of exhumation.

Previous thermochronological studies have suggested that the Kinabalu granite was emplaced as early as c. 14 Ma, and was exhumed from c. 10 Ma (e.g. Jacobson 1970; Rangin et al. 1990; Hutchison et al. 2000; Swauger et al. 2000). Exhumation was previously interpreted to reflect post-orogenic collapse follow-ing the end of the Early Miocene Sabah orogeny (Hutchison 1996), when the extended passive continental margin of South China collided with north Borneo (Hall & Wilson 2000; Hutchison et al. 2000) and subduction terminated. However, recent dating studies (Cottam et al. 2010) have shown that the age of the granite is between 8 and 7 Ma, bringing these previous interpretations into question.

We present new thermochronological analyses (40Ar/39Ar, fis-sion-track and (U–Th–Sm)/He) on samples of the Kinabalu gran-ite collected from a 2000 m elevation profile up the southern slopes of Mount Kinabalu. We combine the results of the three principal thermochronometers to derive a detailed thermal history for the Kinabalu granite, and utilize the variation of thermochro-nometric data with elevation to estimate rates of exhumation. Together these data provide a unique record of exhumation within northern Borneo during the Neogene.

Neogene rock uplift and erosion in northern Borneo: evidence from the Kinabalu granite, Mount Kinabalu

M. A. CoTTAM1,2*, R. HALL1, C. SPERBER1, B. P. KoHN3, M. A. FoRSTER4 & G. E. BATT5

1SE Asia Research Group, Department of Earth Sciences, Royal Holloway University of London, Egham TW20 0EX, UK2Present address: BP Exploration Operating Company Limited, Wellheads Avenue, Dyce, Aberdeen AB21 7PB, UK

3School of Earth Sciences, University of Melbourne, Melbourne, Vic. 3010, Australia4Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

5Centre for Exploration Targeting, School of Earth and Environment, University of Western Australia, Perth, WA 6009, Australia

*Corresponding author (e-mail: [email protected])

Abstract: Thermochronological data from the Kinabalu granite, emplaced between c. 7.2 and 7.8 Ma, provide a unique record of northern Borneo’s exhumation during the Neogene. Biotite 40Ar/39Ar ages (c. 7.32–7.63 Ma) record rapid cooling of the granite in the Late Miocene as it equilibrated with ambient crustal temperatures. Zircon fission-track ages (c. 6.6–5.8 Ma) and apatite (U–Th–Sm)/He ages (central age c. 5.5 Ma) indicate rapid cooling during the Late Miocene–Early Pliocene. This cooling reflects exhumation of the granite, uplift and erosion bringing it closer to the Earth’s surface. Thermochronological age versus elevation relationships suggest exhumation rates of more than 7 mm a−1 during the latest Miocene and Early Pliocene. Neither the emplacement of the Kinabalu granite nor its exhumation is related to the Sabah orogeny, which terminated in the Early Miocene. Instead, granite magmatism was caused by extension related to subduction rollback of the Sulu Arc, and Mio-Pliocene exhumation of the Kinabalu granite was driven either by lithospheric delamina-tion or break-off of a subducted slab beneath Sabah. Plio-Pleistocene tectonism offshore and onshore northern Borneo reflects continuing large-scale gravity-driven tectonics in the region.

Supplementary material: Full 40Ar/39Ar, fission-track and (U–Th–Sm)/He analytical data and 40Ar/39Ar age spectra plots can be found at www.geolsoc.org.uk/SUP18613.

research-articleResearch ArticleXXX10.1144/jgs2011-130M. A. Cottam et al.Neogene Uplift and Erosion in Northern Borneo2013



M. A. CoTTAM ET AL.2

Geological background

Country rocks

Northern Borneo has a basement of Mesozoic igneous and meta-morphic rocks overlain by a Cenozoic sedimentary cover. The base-ment includes basic igneous rocks, variably serpentinized peridotites and Triassic to Cretaceous rocks previously described as crystalline basement (Reinhard & Wenk 1951; Dhonau & Hutchison 1966; Koopmans 1967; Kirk 1968; Leong 1974). The latter resemble deformed ophiolitic rocks intruded by arc plutonic rocks and have been suggested to represent a Mesozoic intra-oceanic arc (Hall & Wilson 2000). The peridotites have been interpreted as part of a Cretaceous ophiolite (Hutchison 2005) emplaced in the Late Cretaceous or Early Paleogene (Newton-Smith 1967; omang & Barber 1996). Unusual peridotites exposed close to Mount Kinabalu have been interpreted to represent sub-continental mantle (Imai & ozawa 1991). The basement is in faulted contact with a cover sequence of predominantly deep-water turbidites and related depos-its assigned to the Eocene to Lower Miocene Trusmadi and Crocker Formations (Collenette 1965; van Hattum et al. 2006).

Basement and cover were folded and faulted during Eocene and oligocene deformation that was driven by the subduction of the proto-South China Sea beneath Borneo (Taylor & Hayes 1983; Rangin & Silver 1990; Tongkul 1991, 1994; Hall 1996, 2002; Hall & Wilson 2000; Hutchison et al. 2000). The attenuated South China continental margin collided with northern Borneo in the Early Miocene (Hall & Wilson 2000; Hutchison et al. 2000) resulting in

the Sabah orogeny (Hutchison 1996), which produced significant topography in the region (Hutchison et al. 2000). Much of the region is thought to have become emergent at this time, although most of present-day Sabah subsided below sea level in the late Early Miocene (Balaguru et al. 2003; Hall et al. 2008). Shelf edges off-shore of northern Sabah moved progressively outwards from the Middle Miocene onwards (e.g. Hazebroek & Tan 1993; Sandal 1996), and the region became emergent once more in the Late Miocene or Early Pliocene (Hall 2002; Balaguru et al. 2003; Tongkul & Chang 2003; Morley & Back 2008). The Pleistocene glacial tills of the Pinosuk Gravels (Collenette 1958) are the young-est sedimentary rocks known from the Kinabalu area.

Previous regional exhumation studies are based on a limited number of zircon and apatite fission-track analyses from the region’s sedimentary rocks (Hutchison et al. 2000; Swauger et al. 2000). Apatite fission-track (AFT) analyses from the central and western parts of Sabah suggest rapid uplift of this area in the Middle to Late Miocene, at rates of 0.5–0.7 km Ma−1 (Hutchison et al. 2000). AFT analyses from the eastern part of Sabah, and zircon fission-track (ZFT) analyses from across the region, yield oligo-Miocene and Cretaceous ages respectively, which are interpreted to reflect the provenance of the sedimentary rocks rather than their exhumation history (Hutchison et al. 2000; Swauger et al. 2000).

The Kinabalu granite

The Kinabalu granite forms Mount Kinabalu, the highest mountain (4100 m) between the Himalayas and New Guinea. The mountain has an unusual morphology, with steep flanks on three sides and a relatively flat summit plateau (between 3750 and 4100 m) that is cut in two by the 1800 m deep chasm of Low’s Gully. The granite has a roughly elliptical shape, elongated approximately NE–SW, with a major axis of c. 16 km and a minor axis c. 10 km. Petrographically it is a true granite (Reinhard & Wenk 1951; Kirk 1968; Kasama et al. 1970; Cottam et al. 2010), closely resembling Cordilleran granitoids (Frost et al. 2001). Amphibole geobarome-try (Vogt & Flower 1989) suggests emplacement at between 3 and 8 km. Dating of the granite based on whole-rock, hornblende and biotite K–Ar analyses (Jacobson 1970; Rangin et al. 1990; Bellon & Rangin 1991; Swauger et al. 2000) produced a wide range of ages (c. 13.7–1.3 Ma). Hutchison et al. (2000) interpreted the older ages (c. 13.7–10 Ma) to reflect cooling of the granite through tem-peratures of 500–300 °C during this interval. However, recent high-precision U–Pb secondary ion mass spectrometry (SIMS) dating (Cottam et al. 2010) has shown that the granite is in fact Late Miocene, requiring reconsideration of the older age estimates.

U–Pb ages suggest that the granite was emplaced and crystallized in less than 800 ka at c. 7.8 c. 7.2 Ma (Cottam et al. 2010). Three distinct lithological units are recognized based on mapping by Jacobson (1970) and U–Pb ages: a minor Upper Unit of biotite gran-odiorite, a volumetrically dominant Middle Unit of hornblende granite, and a Lower Unit of porphyritic hornblende granite (Cottam et al. 2010). The body is intruded by dykes of two broad types: abundant aplites and a range of microgranites including porphyritic and pyroxene-bearing varieties (Jacobson 1970). The oldest U–Pb crystallization ages come from samples of the Upper Unit collected at the highest elevations. The youngest U–Pb crystallization ages come from samples of the Lower Unit collected at the lowest eleva-tions. Based on this pattern of ages the granite is interpreted as a sheeted laccolith-like body, comprising at least three subhorizontal granitic sheets that young downwards, each emplaced as a dyke-fed sheet over a period of between 250 and 30 ka (Cottam et al. 2010).

Zircon inheritance patterns suggest that the Lower and Middle Units formed from melting of the northern Borneo ‘crystalline

S A B A H

S A R A W A K

MOUNTKINABALU

K A L I M A N TA N

B

PALAWAN

Fig. 1. Inset map shows the position of Borneo within SE Asia. Main map shows the position of Mount Kinabalu (black circle) at the northern end of the Central Borneo Mountains (dark grey, elevation >1000 m; white, elevation >1500 m). Fine black lines represent international boundaries between Malaysian Borneo (Sabah and Sarawak), Indonesian Borneo (Kalimantan) and Brunei (B).



NEoGENE UPLIFT AND ERoSIoN IN NoRTHERN BoRNEo 3

basement’ (Cottam et al. 2010). Least-squares modelling demon-strates that the Upper Unit biotite granodiorite cannot be derived from the hornblende granite of the Middle Unit by closed-system fractional crystallization alone (Vogt & Flower 1989; Cottam et al. 2010). Zircon inheritance patterns show that there is also a compo-nent derived from melting of the attenuated continental crust of the South China margin thrust beneath Sabah (Cottam et al. 2010).

Methods

A total of 45 thermochronological analyses (10 40Ar/39Ar; six fis-sion-track; 29 (U–Th–Sm)/He) were performed on 32 granite sam-ples (Fig. 2, Table 1). Twenty-nine of these samples form a near-continuous elevation profile up the southern flank of Mount Kinabalu (Fig. 2). The samples in this profile may be further subdi-vided into three zones based on geology and topography: samples from dykes intruding country rock on the mountain’s lower slopes (<3000 m); samples of Kinabalu granite collected from the moun-tain’s upper slopes (3000–3800 m); samples of Kinabalu granite collected form the mountain’s relatively flat summit plateau (>3800 m). We combine the results of all three thermochronome-ters (40Ar/39Ar; fission track; (U–Th–Sm)/He) to obtain the most complete exhumation history possible for the Kinabalu granite. In particular, we utilize the variation of thermochronometric data with elevation to estimate rates of exhumation (e.g. Zeitler et al. 1982; House et al. 2001; Reiners et al. 2003). All samples were crushed, graded using disposable nylon cloth sieves in a brass collar, and separated using conventional electromagnetic and heavy liquid

techniques. High-purity mineral separates were handpicked from the resulting 100–250 µm fraction, thus any contamination in anal-ysis is assumed to be intra-grain contaminants.

40Ar/39Ar analyses

Ten mineral separates (nine biotite; one hornblende) were analysed in the Argon Laboratory of the Research School of Earth Sciences, Australian National University using the furnace step-heating tech-nique, and following procedures described by McDougall & Brown (2006). Samples were irradiated for 20 MWh in position 5C of the McMaster Nuclear Reactor (McMaster University, Canada), using Sanidine 92-176 from Fish Canyon Tuff (K–Ar reference age 28.10 ± 0.04 Ma) as the Fluence Monitor (Spell & McDougall 2003). Ages were calculated using the 40K abundances and decay constants of Steiger & Jäger (1977). Uncertainties in isotopic ratios and ages are quoted at the ±1σ level. Weighted mean ages and uncertainties were calculated using the eArgon software (written by G. Lister; <http://rses.anu.edu.au/tectonics/programs/>), using the method of asymp-totes and limits (Forster & Lister 2004) to characterize the apparent age spectra (Table 1). Steps showing anomalous outgassing during experi-mental heating were not included in weighted mean age calculations.

Zircon fission-track analyses

Six samples were analysed at the School of Earth Sciences, University of Melbourne. Zircon separates were mounted in FEP Teflon discs, ground, polished to an optical finish and etched in a eutectic

Timpohon

Poring

Mesilau

Kundasang

NationalPark HQ

116.6o E116.5o E

6.1o N

6.0o N

see inset map

*

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*

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7069

6833

7166

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Middle Unit; hornblende granite

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Upper Unit; biotite granite

Crystalline basement

Fault

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*AHe dated

Sample ID(pre�x CS-0**)

ZFT dated

Ar-Ar dated

Geology Samples74

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25 55

Fig. 2. (a) Simplified geological map of the Mount Kinabalu region (modified from Jacobson 1970), showing the locations of the 33 samples analysed in this study. Grey box shows area enlarged in (b); symbols illustrate the types of thermochronological analyses undertaken.




Tab

le 1

. Sum

mar

y of

U–P

b SI

MS,

Ar/

Ar

and

fiss

ion-

trac

k th

erm

ochr

onol

ogic

al d

at fr

om th

e K

inab

alu

gran

ite

Sam

ple

no

.L

itho

logy

1L

ocat

ion2

Lon

gitu

de

(°E

)3L

atit

ude

(°N

)3E

leva

tion

(m

)3U

–Pb

SIM

S4

40A

r/39

Ar5

Fis

sion

tr

ack6

(U–T

h)/

He7

Pha

seA

ge

(Ma)

(±1σ

)P

hase

Age

(M

a)(±

1σ)

Pha

seA

ge

(Ma)

(±1σ

)M

ean

trac

k le

ngth

(m

m)

± S

E

(mm

)P

hase

Age

(M

a)(±

1σ)

CS

-021

Upp

er U

nit

Wes

tern

Pla

teau

116.

551

6.07

838

27Z

irco

n7.

850.

08B

ioti

te7.

540.

04A

pati

te5.

980.

42C

S-0

23U

pper

Uni

tW

este

rn P

late

au11

6.55

46.

076

3938

Bio

tite

7.54

0.05

Zir

con

6.6

0.3

10.4

70.

10A

pati

te5.

620.

39C

S-0

36U

pper

Uni

tW

este

rn P

late

au11

6.55

06.

079

3812

Bio

tite

7.54

0.05

Apa

tite

6.00

1.90

CS

-018

Mid

dle

Uni

tW

este

rn P

late

au11

6.56

76.

065

4096

Zir

con

7.64

0.11

Apa

tite

5.63

0.40

CS

-020

Mid

dle

Uni

tW

este

rn P

late

au11

6.56

26.

068

3847

Zir

con

7.69

0.07

Apa

tite

6.12

0.43

CS

-059

Mid

dle

Uni

tW

este

rn P

late

au11

6.56

06.

071

3980

Zir

con

7.58

0.07

Bio

tite

7.50

0.05

Apa

tite

5.26

0.46

CS

-013

Mid

dle

Uni

tW

este

rn P

late

au11

6.56

36.

066

3692

Zir

con

6.5

0.3

10.0

40.

38A

pati

te5.

850.

41C

S-0

11M

iddl

e U

nit

Cen

tral

Sad

dle

116.

567

6.06

634

80B

ioti

te7.

420.

05Z

irco

n5.

80.

310

.15

0.13

Apa

tite

5.45

0.39

CS

-070

Mid

dle

Uni

tE

aste

rn P

late

au11

6.58

36.

097

3527

Zir

con

7.44

0.09

Apa

tite

6.40

1.80

CS

-072

Mid

dle

Uni

tE

aste

rn P

late

au11

6.58

36.

074

4007

Zir

con

7.46

0.08

Bio

tite

7.49

0.03

Apa

tite

5.53

0.39

CS

-055

Mid

dle

Uni

tS

outh

east

mar

gin

116.

594

6.06

023

52H

ornb

lend

e7.

480.

12

CS

-079

Mid

dle

Uni

tN

orth

east

mar

gin

116.

579

6.07

439

75B

ioti

te7.

630.

03

CS

-033

Mid

dle

Uni

tE

aste

rn P

late

au11

6.57

86.

088

3732

Zir

con

6.3

0.3

10.3

60.

10A

pati

te5.

130.

44C

S-0

27L

ower

Uni

tS

outh

ern

mar

gin

116.

565

6.05

630

96Z

irco

n7.

320.

09B

ioti

te7.

320.

08Z

irco

n6.

20.

210

.37

0.09

Apa

tite

4.71

0.33

CS

-025

Low

er U

nit

Sou

thw

est m

argi

n11

6.52

36.

076

3938

Bio

tite

7.41

0.11

C

S-0

06D

yke

Cou

ntry

roc

k S

outh

116.

560

6.04

627

25Z

irco

n6.

30.

310

.28

0.22

Apa

tite

5.74

0.40

(U–T

h–S

m)/

He

anal

yses

are

als

o li

sted

whe

re a

ppli

cabl

e.1 A

fter

Cot

tam

et a

l. (2

010)

.2 G

eogr

aphi

cal l

ocat

ion

on M

ount

Kin

abal

u (s

ee F

ig. 2

).3 L

ongi

tude

and

lati

tude

dat

a (d

ecim

al d

egre

es);

ele

vati

on d

ata

base

d on

pos

itio

n w

ithi

n a

deta

iled

dig

ital

ele

vati

on m

odel

.4 F

rom

Cot

tam

et a

l. (2

010)

; mea

n ag

e of

all

mag

mat

ic a

naly

ses.

5 Wei

ghte

d m

ean

age.

6 Cen

tral

age

(se

e F

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).7 W

eigh

ted

mea

n ag

e (s

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ig.5

).




KoH–NaoH melt at 220 °C (Gleadow et al. 1976). Neutron irradia-tions were carried out in the well-thermalized (high thermal/fast neu-tron flux ratio) X-7 facility of the HIFAR reactor (now decommissioned) at Lucas Heights, Australia. FT ages were meas-ured using the external detector method with Brazil Ruby muscovite used to record induced tracks (Gleadow 1981). Thermal neutron flu-ence was monitored by measuring the track density in muscovite attached to the Corning-1 glass standard. Ages were calculated using the zeta calibration method, following procedures described by Hurford & Green (1983). A factor of 0.5 was used to correct for the difference between track registration geometries of zircon internal surfaces and the external muscovite detector (Gleadow & Lovering 1977). Errors were calculated using the ‘conventional method’ of Green (1981) and are expressed as one standard deviation (Table 1).

Apatite (U–Th–Sm)/He analyses

Ninety-five age determinations from 29 samples were analysed at the School of Earth Sciences, University of Melbourne. Samples were routinely analysed in triplicate, with additional analyses (up to a

maximum of six) undertaken where intra-sample variation exceeded analytical error. Analyses typically comprised between one and three grains (95% of analyses), but up to seven grains, depending on gas yield. Grains were selected following the routine described by Farley (2002) and mounted in platinum capsules. He extraction was carried out using a solid-state diode laser with 820 nm wavelength and fibre-optic coupling following established laboratory protocols (House et al. 2000). Following degassing, grains were dissolved in HNo3 and U–Th–Sm was measured by inductively coupled plasma mass spectrometry (ICP-MS) using a second-generation Varian quadru-pole system. Analyses were calibrated using the reference material BIVo-1, and Mud Tank apatite and international rock standard BCR-2. Durango apatite (McDowell et al. 2005) was routinely ana-lysed with each batch of samples and served as further check on accuracy and precision. All ages were calculated and corrected for α-emission (FT correction) following the approach of Farley et al. (1996). Analytical uncertainties are conservatively assessed to be c. 6.2% (±1σ) incorporating grain-size measurements (estimated uncer-tainty of c. 5 µm), gas analysis and ICP-MS uncertainties, as well as an α-correction-related constituent, but not those related to possible U and Th zonation. Accuracy and precision of U, Th and Sm content range up to 2% (at ±2σ), but are typically better than 1%.

Results

40Ar/39Ar dating40Ar/39Ar weighted mean ages (WMAs) for the 10 biotite and horn-blende samples range from 7.63 ± 0.03 to 7.32 ± 0.08 Ma (Table 1). Three samples from the Upper Unit give identical WMAs of c. 7.54 Ma. Five samples from the Middle Unit range in age between 7.63 ± 0.03 and 7.42 ± 0.05 Ma. Two samples from the Lower Unit give the youngest weighted mean ages of 7.41 ± 0.11 and 7.32 ± 0.08 Ma. All 40Ar/39Ar WMAs are either younger than or within uncertainty of zircon U–Pb SIMS ages from the equivalent intrusive unit of the pluton (Table 1). The oldest consistent 40Ar/39Ar WMAs are from the Upper Unit, and the youngest 40Ar/39Ar WMA from the Lower Unit. This pattern of ages provides qualitative support for the suggestion that the granite was assem-bled as a number of discrete sheets (as opposed to a diapiric plu-ton), each of which was intruded beneath an overlying layer (Cottam et al. 2010).

Zircon fission-track dating

ZFT analyses yield Late Miocene central ages that form a tight cluster between 6.6 ± 0.3 and 5.8 ± 0.3 Ma (Table 1; Figs 3 and 4), with a weighted mean age of 6.3 ± 0.4 Ma (95% confidence level). Central ages are significantly younger than equivalent zircon U–Pb SIMS ages (see Cottam et al. 2010) and 40Ar/39Ar cooling ages (this study), and are consistent with progressive cooling of the pluton below a ZFT closure temperature (c. 205 ± 18°C; Bernet 2009) rel-evant to the cooling rate estimated for our samples. Zircon mean track lengths are characteristically long for all samples: between 10.47 ± 0.10 and 10.04 ± 0.38 µm, similar to typical unannealed track lengths in zircon of c. 10–12 µm (e.g. Hasebe et al. 1994). Plotted against elevation at the 1σ uncertainty level, ZFT central ages from the main sampling transect define a steep, age–elevation relationship (AER) that is essentially age invariant (Fig. 4).

Apatite fission-track dating

AFT analyses were undertaken in this study with the intention of comparison with the AFT results previously reported from the Kinabalu granite (Swauger et al. 2000). However, apatite crystals

2500

2750

3000

3250

3500

2500

2250

22502000

17501500

1250

3750

3500

32503000

3000

3250

2750

3500

3750

3250

3000

2750

2500

2000

1750

345.15 ± 0.44

206.12 ± 0.43

366.00 ± 1.90

215.98 ± 0.42

235.62 ± 0.39

6.6 ± 0.3

185.63 ± 0.40

606.40 ± 0.56

595.26 ± 0.46

144.39 ± 0.38

635.22 ± 0.37

725.53 ± 0.39

664.68 ± 0.33

706.40 ± 1.80

695.50 ± 1.10

335.13 ± 0.44

6.3 ± 0.3

685.30 ± 1.70

715.67 ± 0.41

284.63 ± 0.32

135.85 ± 0.416.5 ± 0.3

09a4.17 ± 037

274.71 ±0.336.2 ± 0.2

495.00 ± 1.60

09b5.50 ± 0.39

065.74 ± 0.40

6.3 ± 0.304

5.27 ± 0.37

315.40 ± 0.46

655.62 ± 0.39

115.45 ± 0.395.8 ± 0.3

104.79 ± 0.42

235.62 ± 0.39

6.3 ± 0.3

Sample IDAHe age ±1σZFT age ±1σ

Summit plateauUpper slopesLower slopes

Fig. 3. Digital elevation model (contours in metres) with location of samples analysed in this study annotated with zircon fission-track central ages and apatite (U–Th–Sm)/He weighted mean ages. Symbols correspond to those used in Figures 4 and 5.




from all the samples studied were plagued by abundant crystallo-graphic dislocations, which proved impossible to confidently dis-tinguish from genuine fission tracks. Although a small number of AFT ages were produced (Sperber 2009), they were anomalously old with extremely large errors and these ages are not presented here. Previously published AFT studies (e.g. Swauger et al. 2000)

included no descriptions and reported few or no supporting ana-lytical data, and it is unclear if similar problems were encountered. The undetected presence of crystallographic dislocations, and their erroneous inclusion in fission-track counting, would result in anomalous AFT ages and would explain why the AFT ages of Swauger et al. (2000) are older than zircon U–Pb ages (Cottam et al. 2010) from the granite.

Apatite (U–Th–Sm)/He dating

of the 95 apatite (U–Th–Sm)/He (AHe) age determinations per-formed on the 29 samples analysed, 17 age determinations yielded ages that are clearly older than the age of intrusion (and 40Ar–39Ar biotite and zircon FT ages) or widely dispersed (exceeding analyti-cal error at the ±2σ level) from other results for the same sample. These outliers have been removed from consideration in the fol-lowing discussion. Several reasons for the seemingly anomalous and wide dispersion of AHe ages have been proposed, including actinide-enriched micro-inclusions, crystal size variation, U–Th zoning, α-radiation damage, He implantation from external sources and breakage of crystals (e.g. Fitzgerald et al. 2006; Shuster et al. 2006; Spiegel et al. 2009; Brown et al. 2011; Ault & Flowers 2012). These are not discussed in any detail here, but may in part also account for some of the slight intra-sample AHe age variation observed.

For the majority of the 29 samples triplicate age determinations yield ages that are within analytical error and are considered to rep-resent a single age population. WMAs for such samples (Table 1, Fig. 5) were calculated using the Isoplot/Ex 2.29 program of Ludwig (2001). These 29 AHe analyses yield latest Miocene to Early Pliocene WMAs that range from 6.40 ± 1.80 to 4.17 ± 0.37 Ma (errors at ±1σ). All AHe WMAs are younger than, and therefore consistent with, ZFT ages from equivalent samples. Plotted at the 2σ uncertainty level AHe WMAs from the summit plateau, upper slopes and lower slopes display are indistinguishable, defining a steep age–elevation relationship (AER) that is essentially age invariant over the 2000 m sampled (Fig. 5). The U–Th–Sm–He apatite compositions for all 78 analyses are plotted on a ternary diagram (Fig. 6). A central age of 5.54 ± 0.08 Ma (1SE error) is cal-culated using the protocol described by Vermeesch (2008).

Interpretation

Late Miocene emplacement and crystallization of the Kinabalu granite40Ar/39Ar WMAs show that the granite cooled rapidly beneath 40Ar/39Ar biotite closure temperatures (c. 325 °C, e.g. Harrison et al. 1985; McDougall & Harrison 1999) following its emplacement, and has not subsequently been reheated above these temperatures. We interpret this rapid cooling to reflect crystallization of the magma as it equilibrated with ambient crustal temperatures. Therefore biotite 40Ar/39Ar closure temperatures (c. 325 °C) place an upper limit on the ambient crustal temperatures (and by extrapolation, depth) at which the granite was emplaced. Assuming a geothermal gradient of c. 25 °C km−1, and a surface temperature of c. 25 °C (not unreasonable for a location that has been at equatorial latitudes during the entire Cenozoic), this would suggest a maximum depth of emplacement of c. 12 km. Younger, Mio-Pliocene, ZFT and AHe ages suggest that cooling did not proceed beneath the closure temperatures of these systems (c. 205 °C for ZFT, e.g. Bernet 2009; c. 70 °C for AHe, e.g. Ehlers & Farley 2003), providing an estimation of the minimum tem-perature and depth of emplacement. Again, assuming a geothermal gradient of c. 25° C km−1, and a surface temperature of c. 25 °C, this

2000

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5.00 5.50 6.506.00 7.00

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ghte

d av

erag

e ag

e 6.

3 M

a ±

0.4

(95%

con

�den

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terv

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ZFT central ages (±1σ)

Upper slopesSummit plateau

Lower slopes

Fig. 4. Age v. elevation plot for zircon fission-track (ZFT) central ages from Mount Kinabalu plotted at the 1σ uncertainty level. (Note that elevation scale begins at 2000 m.) Symbols correspond to those used in Figures 3 and 5.




would suggest a minimum depth of emplacement of c. 7 km. These qualitative estimates suggest the granite was emplaced at a depth of between 12 and 7 km, and are similar to previous emplacement depth estimates based on amphibole thermobarometry (between 8 and 4 km; Vogt & Flower 1989). More precise estimation of emplace-ment depth remains difficult, because of the lack of suitable thermo-barometers, and because magmatism is likely to have caused some local perturbation of the regional geothermal structure.

Latest Miocene to Pliocene cooling through ZFT and AHe closure temperatures

ZFT central ages record cooling of the Kinabalu granite to below the ZFT closure temperature of c. 205 °C (e.g. Bernet 2009) during the latest Miocene (ZFT mean age 6.3 ± 0.4 Ma). The steep, age-invariant nature of the ZFT AER and the long unannealed ZFT lengths observed are both consistent with extremely rapid cooling of the granite through the ZFT closure temperature window. Samples with paired 40Ar/39Ar and ZFT ages (Table 1) illustrate a minimum of c. 120 °C of cooling in c. 1 myr. AHe ages record cool-ing of the granite to below the AHe closure temperature of c. 70 °C (e.g. Ehlers & Farley 2003) during the latest Miocene and Early Pliocene (AHe central age 5.54 ± 0.08 Ma). The steep, age-invariant nature of the AHe AER is consistent with extremely rapid cooling of the granite through the AHe closure temperature window. Samples with paired ZFT and AHe ages suggest that the Kinabalu granite continued to cool at minimum rates of c. 120 °C Ma−1 into the Early Pliocene.

Following common assumptions (e.g. Zeitler et al. 1982; Reiners & Brandon 2006; Metcalf et al. 2009; Glorie et al. 2010), we inter-pret these latest Miocene to Early Pliocene ZFT and AHe ages (ZFT mean age c. 6.3 Ma; AHe central age c. 5.54 Ma) to reflect rapid cooling owing to exhumation, regional uplift and erosion bringing the granite closer to the Earth’s surface. The steep AERs observed for both our ZFT and AHe data suggest that the granite was exhumed extremely rapidly, the slope of the AER being pro-portional to the exhumation rate. Estimates based on ZFT and AHe

ages and closure temperatures indicate exhumation rates of the order of several kilometres per million years. ZFT and AHe ages (ZFT mean age c. 6.3 Ma; AHe central age c. 5.54 Ma) and closure temperatures (c. 205 °C and c. 70° C respectively), indicate 135 °C of cooling in 0.76 myr. Assuming a geothermal gradient of c. 25° C km−1, this would equate to exhumation of 5.4 km in 0.76 myr, at an averaged rate of c. 7.1 km Ma−1.

However, the calculation of exhumation rates’ AERs is poten-tially complicated by both the advection of heat and the topographic deflection of near-surface isotherms, which may be warped upwards, mimicking the topography in a dampened fashion (e.g. Kohn et al. 1984; Stüwe et al. 1994; Mancktelow & Grasemann 1997; House et al. 1998; Stüwe & Hintermüller 2000; Braun 2002; Ehlers & Farley 2003; Reiners et al. 2003). The effects of topo-graphic deflection are most significant for low-temperature ther-mochronometers and long topographic wavelengths (< c. 100 °C and typically > c. 5 km; e.g. Stüwe et al. 1994; Reiners et al. 2003). Although ZFT ages are unaffected by topographic deflection owing to their higher closure temperature (>100 °C), AHe age data may be. If this is not considered, variations in AHe closure temperature depths between samples may lead to overestimation of ‘true’ exhu-mation rates (e.g. Stüwe et al. 1994; House et al. 1998; Reiners et al. 2003; Metcalf et al. 2009). As such, the exhumation rates calculated here, uncorrected for advection and topographic deflec-tion, reflect the extreme upper end of the likely range.

Discussion

Differences between new and previous dating

The thermochronometric dates reported in this study are notably younger than those previously reported using comparable tech-niques. For example, the 40Ar/39Ar ages reported here (c. 7.63–7.32 Ma) are younger than the majority of those previously reported based on K–Ar dating of similar rocks (c. 13.7–1.3 Ma: Jacobson 1970; Rangin et al. 1990; Hutchison et al. 2000; Swauger et al. 2000). The majority of the previously published K–Ar ages are

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2.50 3.503.00 4.00 5.00 6.00 7.00 8.004.50 5.50 6.50 7.50 8.50 10.501.500.500 9.509.00 10.00

Age (Ma)

1.00 2.00

AHe weighted mean ages (±2σ)

Upper slopesSummit plateau

Lower slopes

Fig. 5. Age v. elevation plot for apatite (U–Th–Sm)/He (AHe) weighted mean ages from Mount Kinabalu plotted at the 2σ uncertainty level. (Note that elevation scale begins at 2000 m.) Symbols correspond to those used in Figures 3 and 4.




older than the Late Miocene (c. 7.8–7.2 Ma) crystallization age of the pluton and must be reinterpreted in terms of subsequent disrup-tion of argon systematics. In contrast, the new 40Ar/39Ar ages pre-sented here are entirely consistent with the progressive cooling of a Late Miocene pluton. Likewise, we consider that many of the pre-viously reported AFT ages (c. 8.1–6.7 Ma; Swauger et al. 2000; Hutchison 2005) must also be reinterpreted. AFT ages, correspond-ing to lower closure temperatures, would be expected to post-date ZFT ages from comparable samples. However, the previously reported AFT ages frequently predate the ZFT ages presented in this study. These anomalously old AFT ages were published with few or no supporting data, or accompanying information such as track length measurement, and it is unclear if problems were encountered with crystallographic dislocations (as found in this study). Such dislocations, and their confusion with genuine fission tracks, could lead to overestimation of a sample’s true age and might explain the anomalously old AFT ages reported.

Neogene exhumation of the Kinabalu granite

The combined results of our thermochronometric analyses provide a detailed thermal history of the Kinabalu granite (Fig. 7), which suggests rapid cooling of the granite in the Late Miocene and Early Pliocene. This history is typical of many reported for exhumed

crustal rocks, with cooling following a characteristically asymp-totic curve (e.g. Fitzgerald et al. 1995, 2006; Metcalf et al. 2009; Glorie et al. 2010). Late Miocene 40Ar/39Ar ages (c. 7.63–7.32 Ma) reflect crystallization of the magma as it equilibrated with ambient crustal temperatures following its emplacement as a number of intrusive pulses between c. 7.8 and 7.2 Ma (Cottam et al. 2010). Latest Miocene to Early Pliocene ZFT and AHe ages (ZFT mean age c. 6.4 Ma; AHe central age c. 5.5 Ma) reflect rapid cooling of the Kinabalu granite owing to exhumation.

our calculated exhumation rates are greater than those previ-ously predicted for the Mio-Pliocene exhumation of the NW Borneo region using a mass-balancing approach (0.29 km Ma−1; Morley & Back 2008). However, allowing for a reduction owing to advection and topographic deflection, our rates are comparable with those reported from areas of active exhumation such as the Greater Himalaya (e.g. Searle et al. 1999; Kirstein et al. 2006), the Southern Alps of New Zealand (e.g. Tippett & Kamp 1993; Batt et al. 2000) and the European Alps (e.g. Bernet et al. 2001). Although not directly constrained by our new data, Figure 7 illus-trates that a more subdued rate of exhumation is implicit from the Early Pliocene to the present.

Neogene tectonics of northern Borneo

Exhumation of the Kinabalu granite has previously been attributed to the rapid uplift and erosion of northern Borneo during the latter stages of the Sabah orogeny, and has been commonly considered in terms of compression and shortening (e.g. Swauger et al. 2000; Hutchison et al. 2000). However, our new thermochronological ages demonstrate that most exhumation occurred during the latest Miocene and Early Pliocene, significantly post-dating the Sabah orogeny, which terminated in the Early Miocene (e.g. Hutchison 1996; Balaguru et al. 2003; Hall et al. 2008; Fig. 8). In addition,

Arithmetric mean = 5.47 Ma, s.e. = 70.02 ka, MSWD = 32.38Geometric mean = 5.44 Ma, s.e. = 70.77 ka, MSWD = 29.86Central age = 5.54 Ma, s.e. = 77.19 ka, MSWD = 126.8795% C.I. = [5.39 Ma, 5.69 Ma]

Arithmetric mean = 5.47 Ma, s.e. = 70.02 ka, MSWD = 32.38Geometric mean = 5.44 Ma, s.e. = 70.77 ka, MSWD = 29.86Central age = 5.54 Ma, s.e. = 77.19 ka, MSWD = 126.8795% C.I. = [5.39 Ma, 5.69 Ma]

157.66(He-7.59E-4)/9.66E-1

(U-3.15E-3)/9.66E-1 (Th-3.06E-2)/9.66E-1

291.32 [Sm] 1587.31

84 Ma

34 Ma

14 Ma

6 Ma

3 Ma894 ka

Fig. 6. U–Th–Sm–He compositions for 78 Kinabalu apatite analyses plotted on a ternary diagram. White ellipse is the weighted geometric mean composition, and the central AHe age of 5.54 ± 0.08 Ma (±1SE; note also the relatively restricted 95% confidence interval age range) is calculated from this value as described by Vermeesch (2008). The relatively large MSWD values should be noted; these reflect minor apatite chemical compositional variations between different phases of the intrusion. This, however, does not reflect on the underlying ages, which can be regarded as identical within the range of analytical uncertainties (see also Fig. 5). For the plot an uncertainty of 2% was estimated for U, Th and Sm determinations, and 1.5% for He measurements.

Age (Ma)

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ture

(oC

)

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zr U-Pb

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bt Ar

0 1 2 3 4 5 6 7 8 90

100

200

300

400

500

600

700

800

900

hb Ar

Fig. 7. Thermal history of the Kinabalu granite based on the thermochronological analyses presented in this study (40Ar/39Ar, ZFT, AHe) and zircon U–Pb SIMS ages from Cottam et al. (2010). Width and height of boxes reflect errors associated with age and temperature constraints respectively. Bold black ellipses show ZFT weighted mean age (6.4 ± 0.4 Ma) and AHe central age (5.54 ± 0.08 Ma) respectively.




there is little evidence of Mio-Pliocene contractional deformation on land in northern Borneo, coeval with either emplacement or exhumation of the Kinabalu granite. Compression and shortening related to the Sabah orogeny cannot therefore be the cause of regional uplift in the Middle to Late Miocene, or exhumation of the Kinabalu granite in the Mio-Pliocene (Fig. 8).

Extension is a possible explanation. Extension in Sabah has pre-viously been related to post-orogenic collapse following the termi-nation of proto-South China Sea subduction beneath northern Borneo in the late Early to Middle Miocene (e.g. Hutchison et al. 2000). However, such extension would be too early to account for Mio-Pliocene exhumation of Kinabalu (Fig. 8). Extension is also known to have affected the eastern lowlands of northern Borneo later in the Miocene (Hutchison 1992; Hutchison et al. 2000), and brought rocks of the Trusmadi and Crocker Formations into faulted contact during the Middle Miocene (Hutchison et al. 2000). This is plausibly related to extension in the nearby Sulu Sea marginal basin (Fig. 1), which began in the late Early Miocene (Nichols et al. 1990; Rangin & Silver 1990; Fig. 8) in response to subduction of the Celebes Sea beneath the Sulu Arc (e.g. Rangin 1989). Middle and Upper Miocene volcanic rocks (e.g. Kirk 1968; Rangin et al. 1990; Chiang 2002) of the Dent and Semporna peninsulas (Fig. 1)

are the products in south Sabah of this NW-directed subduction. Celebes Sea subduction rollback in the Early and Middle Miocene is suggested (Hall 2013) to have resulted in granite magmatism in Palawan at 14–13 Ma (Suggate et al. 2013) and possibly Kinabalu at 8–7 Ma (Cottam et al. 2010), with extension preceding magma-tism by c. 2–3 Ma. These episodes of extension could account for emergence and growth of a landmass in Sabah during the Middle and Late Miocene (e.g. Hall 2002, 2013; Balaguru et al. 2003; Tongkul & Chang 2003; Morley & Back 2008), creating a topo-graphic feature (Fig. 8) into which the Kinabalu granite was intruded. Uplift of northern Borneo during the Middle and Late Miocene is supported by several lines of evidence (Hall 2013) including the outward migration of offshore palaeo shelf edges from the Middle Miocene onwards (e.g. Hazebroek & Tan 1993; Sandal 1996). However, once again these events appear to be too old to account for Mio-Pliocene exhumation.

An alternative is that deeper processes drove exhumation and uplift. There are two obvious possibilities. Lithosphere thickened during the Sabah orogeny beneath Kinabalu could have delaminated as suggested for the Sierra Nevada in California (Manley et al. 2000; Zandt et al. 2004), which would have triggered rapid uplift as the lithospheric anchor was removed (Hall et al. 2009). Alternatively,

Rapi

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* Oldest dated crustin the Sulu Sea(e.g. Nichols et al., 1990)

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Uplift and exhumation of northern Borneo

00

11.61

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Perio

d

Epoc

h

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(Ma)

Fig. 8. Synthesis of the Neogene tectonics of northern Borneo based on the new thermochronological data presented in this study and evidence from published studies.




rapid uplift could have been initiated by break-off of a subducted slab (e.g. van de Zedde & Wortel 2001) following termination of Celebes Sea subduction at the Sulu Arc. These suggestions are supported by the change in composition of the underlying mantle inferred from the major change in compositions of volcanic rocks (from calc-alkaline to ocean island type) erupted in south Sabah after c. 5 Ma (Chiang 2002; Macpherson et al. 2010; Fig. 8) and by the high-velocity anomaly in the upper mantle beneath Sabah at depths of 200–300 km observed in P-wave tomographic models (Bijwaard et al. 1998). Either process would be expected to lead to rapid uplift and exhuma-tion, potentially aided by deep crustal flow (Hall 2011).

Seismic data from offshore northern Borneo and Brunei show young extensional faults that cut sediments of post-Early Miocene age (e.g. Franke et al. 2008). There are also spectacular examples of very young mass transport complexes, coherent slump complexes and mega-slides (e.g. McGilvery & Cook 2003; Gee et al. 2007). offshore of Brunei, Hesse et al. (2009) have demonstrated that young exten-sion is balanced by coeval contraction, typical of a gravity-driven fold and thrust belt. The same is not true for the region offshore of Mount Kinabalu, where the observed contraction is greater than extension. Field observations of precipitous, lineated scarps dipping steeply to the south, and strong east–west lineations observed on satellite imagery, provide qualitative support for significant extensional fault-ing along the southern flanks of Mount Kinabalu. We suggest that this faulting reflects onshore extension that balances the contraction observed in fold and thrust belt system offshore of Mount Kinabalu. We suggest that Plio-Pleistocene movements in northern Borneo reflect deformation driven by large-scale gravitational collapse of the region following its rapid exhumation in the latest Miocene and Early Pliocene, as recorded in the Kinabalu granite.

Conclusions

The new data reported in this study record the cooling history of the Kinabalu granite. 40Ar/39Ar ages (c. 7.63–7.32 Ma) indicate rapid cooling from high temperatures in the Late Miocene as the granite equilibrated with ambient crustal temperatures. Based on an average geothermal gradient of 25 °C km−1 and the thermochronometric clo-sure temperatures of different mineral systems, the granite is esti-mated to have been emplaced at depths of between c. 12 and 7 km.

ZFT and AHe ages and AERs indicate a period of rapid cooling during the Late Miocene–Early Pliocene (Fig. 8) that reflects exhu-mation of the granite, uplift and erosion bringing it closer to the Earth’s surface. Calculations based on ZFT and AHe ages (c. 6.3 Ma and c. 5.54 Ma respectively) and closure temperatures (c. 205 °C and c. 70 °C respectively) suggest an exhumation rate of more than 7 km Ma−1 during the latest Miocene and Early Pliocene, greater than those previously predicted for northern Borneo as a whole using a mass-balancing approach.

The Mio-Pliocene emplacement and exhumation of the Kinabalu granite are unrelated to the Sabah orogeny, which terminated in the late Early Miocene (Fig. 8). Instead, we propose that granite mag-matism was caused by extension related to subduction rollback of the Sulu Arc, and Mio-Pliocene exhumation of the Kinabalu gran-ite was driven either by lithospheric delamination or by break-off of the Celebes slab that was subducted northwards beneath south Sabah and the Sulu Arc until the end of the Miocene. Plio-Pleistocene extensional faulting, slumping and sliding observed offshore of northern Borneo and Brunei and including onshore faulting of the Kinabalu granite reflect continuing large-scale grav-ity-driven tectonics in the region (Fig. 8).

This work was supported mainly by the SE Asia Research Group at Royal Holloway, funded by a consortium of oil companies. We thank Sabah Parks

for permission to work and sample in the Mount Kinabalu National Park, and F. Tongkul (Universiti Malaysia Sabah), J. Nais and M. Lakim (Sabah Parks) for support and assistance. The University of Melbourne thermochronology laboratory receives infrastructure support under the AuScope Program of NCRIS. Neutron irradiation costs for fission-track samples were covered by the Australian Institute of Nuclear Science and Engineering (AINSE). M.A.F. acknowledges the support of an Australian Research Fellowship provided by the Australian Research Council (ARC) associated with the Discovery Grant DP0877274, and additional support provided by the Research School of Earth Sciences, Australian National University. D. Phillips from the University of Melbourne is thanked for organizing the irradiation of argon samples at McMasters Reactor. A. Raza and A. Almimanovic (both at University of Melbourne) assisted with zircon fission-track and apatite (U–Th–Sm)/He dating respectively. D. Waltham provided valuable advice. The late C. Hutchison provided useful discussions over many years. We thank S. Back, P. Vermeesch and A. Carter for thorough and helpful reviews of the paper.

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Received 7 November 2011; revised typescript accepted 22 March 2013.Scientific editing by Andrew Carter.



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