Elsevier

Tectonophysics

Volumes 712–713, 21 August 2017, Pages 600-622
Tectonophysics

Rapid cooling and exhumation as a consequence of extension and crustal thinning: Inferences from the Late Miocene to Pliocene Palu Metamorphic Complex, Sulawesi, Indonesia

https://doi.org/10.1016/j.tecto.2017.06.025Get rights and content

Highlights

  • Metamorphic rocks of the PMC have Early to Late Pliocene 40Ar/39Ar cooling ages.

  • Intruded S-type granites have similar Pliocene U-Pb, 40Ar/39Ar and (U-Th)/He ages.

  • Both show very fast cooling rates, indicating rapid exhumation of the complex.

  • Contemporaneous magmatism indicates decompressional melting related to extension.

  • Rates may record true speed of episodic exhumation averaged in older orogens.

Abstract

Metamorphic complexes forming high mountains of 1.5–2 km in Western Sulawesi were previously considered to be Mesozoic or older basement of Gondwana crust. However, many of the metamorphic rocks are much younger than previously thought. Some have Eocene sedimentary protoliths. New geothermobarometric and geochronological data from metamorphic rocks of the Palu Metamorphic Complex (PMC) and associated granitoids provide information on the timing and mechanisms of Neogene metamorphism and contemporaneous rapid exhumation. The metamorphic rocks are strongly deformed and some were partially melted to form migmatites. Schists contain relict andalusite, cordierite, staurolite and Mn-rich garnet which are wrapped by a pervasive fabric. 40Ar/39Ar dating of biotite, white mica and amphibole from strongly deformed, mylonitic schists and recrystallised amphibolites reveals cooling occurred in the Early Pliocene (c. 5.3–4.8 Ma) in the northern part and during the Late Pliocene (c. 3.1–2.7 Ma) in the southern part of the PMC. U-Pb, 40Ar/39Ar and (U-Th)/He analyses of various minerals from PMC metamorphic and S-type magmatic rocks give very similar mid to Late Pliocene ages, indicating very fast cooling and rapid exhumation, and show the high speed at which tectonic processes, including magmatism, exhumation, and reworking into a sediment, must have occurred. The high rates could be unique to this area but we suggest they record the true speed of metamorphic complex exhumation in a very young orogenic belt. Rates in older orogens appear lower because they are averages measured over longer periods of time. Contemporaneous magmatism and deformation are interpreted as a consequence of decompressional melting due to extension and thinning of the crust, promoted by possible detachment faults and normal faulting at the major NW-trending Palu-Koro and Tambarana Faults. In contrast, I-type magmatic rocks, separated from the PMC by the Palu-Koro Fault, were exhumed from upper crustal levels by erosion at moderate rates.

Introduction

Sulawesi is situated in a tectonically active convergent region between the Eurasian, Indo-Australian and Philippine Sea plates (Fig. 1). Recent studies have identified a major role for extension in the island's tectonic evolution during the Neogene (Spakman and Hall, 2010, Hall, 2011, Hall, 2012, Camplin and Hall, 2014, Hennig et al., 2014, Pownall et al., 2014) leading to the formation of deep basins between the four elongated land arms. Therefore, this area is of special interest as it allows us to study rocks which have formed very recently in the geological history and enable characterisation of the tectonic processes that drove cooling and exhumation.

The Neck of Sulawesi is a narrow strip of land that connects mid Central Sulawesi with the western end of the North Arm (Fig. 2a, b). It belongs to the Western Sulawesi province (Sukamto, 1975, Hamilton, 1979, Hall and Wilson, 2000, van Leeuwen and Muhardjo, 2005) which is underlain by Australian-origin continental basement (Jablonski et al., 2007, van Leeuwen et al., 2007, Cottam et al., 2011, Pholbud et al., 2012, Hall and Sevastjanova, 2012) that rifted from the Australian margin of Gondwana in the Late Jurassic and was added to the Sundaland margin in the Late Cretaceous (Smyth et al., 2007, Hall et al., 2009, Metcalfe, 2009, Hall, 2011, Hall, 2012, Hall and Sevastjanova, 2012, Hennig et al., 2016). Extension in the Eocene opened the Celebes Sea north of the island and led to rifting of the Makassar Straits, separating western Sulawesi from east Borneo (Hamilton, 1979, Weissel, 1980, Situmorang, 1982, Hall, 1996). Opening of the Makassar Straits has been attributed to back-arc rifting as a response to subduction rollback in SE Sundaland (Guntoro, 1999, Satyana, 2015).

The Neck forms the western boundary of the semi-enclosed Gorontalo Bay. High elevations of c. 1 to 2.5 km are observed on land all around Gorontalo Bay and contrast with significant depths in the bay (Fig. 2b) where there are sediment thicknesses of up to 6 s TWT (c. 6–10 km) and c. 1.5–2 km water depth (Jablonski et al., 2007, Pholbud et al., 2012). Shallow to deep marine sediments are found in the central North Arm in the Middle to Upper Miocene Dolokapa Formation (Bachri et al., 1993, Hennig et al., 2014, Rudyawan, 2015) as well as to the south of Gorontalo Bay in the central part of the island in the Pliocene Puna Formation (Simandjuntak et al., 1997) (Fig. 2a), indicating that the northern part of the island was largely a shallow marine area from the Middle Miocene, indicated by widespread Middle Miocene carbonates exposed on land (Sukamto, 1973). Pinnacle reefs developed on top of carbonate platforms offshore mark subsidence associated with the onset of uplift in the area and indicate that mountain building must be a relatively young process in the tectonic history of the island (Pholbud et al., 2012, Pezzati et al., 2014). The youngest Plio- to Pleistocene clastic sediments of the Celebes Molasse in the southern Neck region (van Leeuwen & Muhardjo, 2005) unconformably overlie metamorphic rocks of the Palu Metamorphic Complex and contain zircons as young as 3 Ma which support a latest Pliocene to Pleistocene depositional age (Nugraha, 2016).

The high mountains on land are mainly formed by mid to lower crustal metamorphic rocks and granitoid intrusions. The timing and mechanisms of exhumation of these rocks are poorly known and investigated in this study by geothermobarometry, 40Ar/39Ar analysis and (U-Th)/He thermochronology.

Section snippets

Regional geology

Sulawesi is subdivided into five tectono-stratigraphic units (Fig. 2a) which are the Western Sulawesi magmatic belt, the Northern Sulawesi volcanic arc, the Central Sulawesi Metamorphic Belt, the East Sulawesi Ophiolite, and the micro-continental fragments of Banggai-Sula and Tukang Besi-Buton (Sukamto, 1975, Hamilton, 1979, Hall and Wilson, 2000, van Leeuwen and Muhardjo, 2005).

Methodology

Five metamorphic and 19 granitoid samples from fresh and mainly unaltered rocks were sampled from the Neck and mid Central Sulawesi in this study and analysed by geothermobarometry, 40Ar/39Ar and/or (U-Th)/He thermochronology.

Palu Metamorphic Complex

The Palu Metamorphic Complex is generally divided into a metapelite unit on the west side of the complex and a gneiss unit on the east side (Sukamto, 1973, van Leeuwen and Muhardjo, 2005). Rocks observed in the field showed the latter comprises mainly high-grade metamorphic rocks, including biotite granite-gneisses and biotite-amphibole granite-gneisses, and subordinate pyroxene gneisses, marbles and migmatites. Gneisses and marbles are recumbently folded with tight to isoclinal folds at

Structural analysis

Structural data is summarised on stereonets (Fig. 9a, b) and in a sketch box model (Fig. 9c).

SJH11_1214

Two different kinds of biotites were identified in this sample (Supplementary File 1). The first generation forms inclusions in garnet porphyroblasts (Biotite 1) and is assumed to be in equilibrium with the garnet, while the second generation of biotite represents flakes of the surrounding foliation (Biotite 2). A representative analysis of Biotite 1 has 2.11 wt% TiO2 and a Mg-number of 0.51. Biotite 2 has 2.65 and 2.73 wt% TiO2 and Mg-numbers of 0.48 and 0.49. The Ti-in-biotite geothermometer

Palu Metamorphic Complex

Amphibole, white mica and biotite were separated from biotite (− white mica) schists and an interbanded amphibolite of the PMC. The minerals analysed were separated from the dominant metamorphic fabric which has mainly overprinted older fabrics.

Apatite and zircon (U-Th)/He analysis

(U-Th)/He analysis was carried out on apatites of 16 granitoid samples (I- and S-type) from the Neck and mid Central Sulawesi (Table 1.1). Additionally, zircons were analysed from sample SJH03 (Table 1.2) to extend the cooling path.

Ages obtained from the apatites that have not been excluded from further discussion as indicated previously are Pliocene to Pleistocene and range between 1.7 ± 0.1 Ma and 3.7 ± 0.2 Ma. Their age-elevation relationship is summarised in Fig. 12a. The samples analysed show no

Metamorphism of the PMC

Metamorphic rocks were previously interpreted as Mesozoic or older basement rocks (Sukamto, 1973, Sopaheluwakan et al., 1995, Parkinson, 1998, Calvert and Hall, 2003, van Leeuwen and Muhardjo, 2005). Recent U-Pb zircon dating on schists of the PMC showed that some of these rocks have Eocene protoliths, identified from detrital zircons, and are therefore younger than previously thought (Hennig et al., 2016, van Leeuwen et al., 2016). Zircon U-Pb rim ages of 3.6 Ma (Hennig et al., 2016) from

Conclusions

  • (1).

    40Ar/39Ar ages from white mica, biotite and amphibole from schists and interbanded recrystallised amphibolites reveal cooling below ~ 570–370 °C during the Early Pliocene (c. 5.3–4.8 Ma) in the north, and Late Pliocene (c. 3.1–2.7 Ma) in the south.

  • (2).

    Cooling of these metamorphic minerals during Pliocene metamorphism is also recorded in zircon rims (c. 3.6 Ma) of gneisses to the east near Toboli, and in combination with 40Ar/39Ar ages, reveal high cooling rates of c. 320 + 105/− 80 °C/Ma for the PMC.

  • (3).

    The

Acknowledgements

This study was supported by the SE Asia Research Group which is funded by a consortium of oil companies. We thank Afif Saputra and Alfend Rudyawan for their help and assistance during fieldwork, as well as Mike Cottam, Ian Watkinson and Tim Breitfeld for helpful discussions. 40Ar/39Ar analysis was undertaken at ANU RSES Argon Facility. We also thank Abaz Alimanovic who performed the (U-Th)/He analysis, Andrew Beard for assisting with the mineral chemical analysis (EDS/WDS-SEM), and Yuntao Tian

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