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Mélange versus forearc contributions to sedimentation and uplift, during rapid denudation of a young Banda forearc-continent collisional belt

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Highlights

  • We provide the first provenance analysis of synorogenic sediments from Timor.

  • Reworked mélange dominates early sediments, and all mud sedimentation.

  • Coarse sediments were sourced from the resistant overthrust Banda Terrane rocks.

  • Exhumation of resistant rocks from under mélange enhanced rapid uplift.

  • Re-dated DSDP262 stratigraphy consistent with synchronous collision across Timor.

Abstract

New sedimentary geochemistry and petrographic analyses provide the most extensive sedimentary documentation yet of the rapid denudation of the young Timor orogen. The data from three basins including two widely-separated, well-dated sections of the Synorogenic Megasequence of Timor-Leste, and a re-dated DSDP 262, constrain the source and timing of detrital sediment flux during forearc-continent collision along the Timor sector of the Banda Arc. The exhumed synorogenic piggy-back basins formed above a mélange unit that developed at the expense of a weak stratigraphic horizon in the Mesozoic stratigraphy, and was exhumed to the sea floor in latest Messinian time. Following an interval of deep marine chalky marl sedimentation, an increasingly muddy sediment flux indicates that the island of Timor became emergent and shed sediment by 4.5 Ma. Comparison of exhumed sections with similar patterns in the DSDP262 chemistry suggests that the sediment source was probably located some 50–60 km distant from the basin, which is consistent with the Aileu region of Timor-Leste that shows an appropriate exhumation history. All sedimentation between 4.5 and 3.2 Ma was probably derived from a low-relief, rapidly eroding, and mudstone-dominated landscape with geochemical affinities to the Triassic-mudstone-derived synorogenic mélange. The mélange unit overlies and surrounds the Banda Terrane, and was presumably structurally emplaced by propagation of a decollement through the Triassic rocks during the collision. After 3.2 Ma, sedimentation was dominated by hard rock lithologies of the Banda Terrane, consisting of forearc cover and basement, the latter including elements of metamafic rocks and metapelites. This phase of sedimentation was accompanied by rapid uplift, which may have been partly driven by a transient imbalance between rock uplift and denudation as resistant lithologies emerged from below mélange-like mudstone. Previous work has suggested that the timing of collision in Timor-Leste and West Timor was substantially different. Our reevaluation of DSDP 262 facies migration history in the context of the re-dating presented here, favours a relatively synchronous onset of uplift in both halves of the island, but with different partitioning of strain between the foreland and hinterland in each half of the island.

Introduction

A diversity of processes including magmatism, deformation, mélange generation, uplift, erosion and associated unroofing accompany forearc or arc continent collision events. The relative importance of these processes changes during these collisional events and associated sedimentation is modulated by parameters such as the collisional kinematics, stratification and inherited structure, composition and rheology of both the upper and lower plates (e.g., Afonso and Zlotnik, 2011, Brown et al., 2011, Duffy et al., 2013, Harris, 2011, Harris et al., 1998), as well as by climatic factors such as weathering and orographic weather patterns. Dismembered remnants of syn-collisional sedimentary basins are rare, but where present they can elucidate the short-lived and dynamic interplay of these processes as oceanic subduction complexes give way to arc-continent collision (Bayona et al., 2011, Dewey and Mange, 2000, Guo et al., 2012, Ryan, 2008, Tate et al., 2014, Zhu et al., 2005). Carefully documented examples of the evolution of young, well-exposed and dated basins, with clear geodynamic context (e.g., Dorsey, 1988, Nagel et al., 2014) provide a critical resource against which to evaluate the ability of syn-collisional sediments to accurately reflect and record the signals that are routinely extracted from them. In particular, analysis of a young orogen might be expected to provide a unique archive of the rates at which processes of early orogenesis can occur, including the timing and drivers of various tectonic phases (De Smet et al., 1990), the lithological characteristics of the source areas (Floyd et al., 1990), the history of erosional unroofing (DeCelles et al., 1998) and the relatively undocumented importance of reworking of the shale-dominated mélanges that are a ubiquitous and persistent component of collisional orogenesis (Barber, 2013, Festa et al., 2010).

The island of Timor is a contractional orogen located within the zone of collision between the Wetar sector of the Banda Forearc and the Australian continental margin, and in a region that has been the focus of recent reevaluation of its collisional history. Since the 1990s, several studies have attributed Timor’s development to underthrusting of a promontory on Australia’s NW shelf but the subsequent style of deformation remains debated (Duffy et al., 2013, Harris, 1991, Keep and Haig, 2010, Snyder et al., 1996, Tate et al., 2015). DSDP 262 was drilled in the 1970s to core the Timor Trough sediments, about 800 m south of the trough axis. The analysis of this core provided some evidence for the tectonic history of the trough, but dating of the core was problematic and age models variable (Heirtzler et al., 1974, Johnston and Bowin, 1981, Veevers et al., 1978). Multiple lines of evidence including palynology, foraminiferal micropaleontology and biomagnetostratigraphy suggest that Timor began uplifting from near-CCD depths prior to 4.5 Ma (Aben et al., 2014, Haig and McCartain, 2007, Nguyen et al., 2013, Tate et al., 2014), during a collisional history that may have begun prior to 7 Ma (Keep and Haig, 2010, Tate et al., 2014). Now, only remnants of the uplifted forearc crust remain (Standley and Harris, 2009), and Australian-affinity rocks including a widespread tectonic mélange and broken formation crop out across much of the island (Barber et al., 1986, Harris et al., 1998). Synorogenic piggy-back basins accumulated deep-marine marls and turbiditic sandstones on the lower northern slope of the Timor Trough; exhumed examples containing up to 1 km of sediment are found distributed around Timor. Two of these basins, the Marobo and Viqueque basins (Fig. 1b), contain well-dated sections with strong paleo-topographic and paleo-bathymetric constraints (Aben et al., 2014, Nguyen et al., 2013, Quigley et al., 2012, Tate et al., 2014). However, their provenance has not been systematically evaluated.

The good age control on the Timor-Leste sections, and our new age data for the DSDP 262 site in the Timor Trough, together with the coarse grained nature of Timor-Leste’s synorogenic basins, provide a useful opportunity to evaluate their provenance, relate it to the published spatiotemporal patterns of exhumation, uplift and structural development (Tate et al., 2014, Tate et al., 2015), and examine the importance of mélange reworking in nascent collision zones. We address this opportunity by investigating the petrographic and geochemical stratigraphy of the exhumed carbonate, marl and siliciclastic coarsening-up succession, using the stratigraphic control of two of the basins in central and east Timor Island (Aben et al., 2014, Haig and McCartain, 2007, Tate et al., 2014) to identify evolutionary trends. We compare our data with the DSDP 262 drill site in the Timor Trough (Fig. 1), for which we present a refined age model, and studies from West Timor and islands to the west (Roosmawati and Harris, 2009). In this way, we provide a new spatio-temporal perspective on the geodynamic evolution of the Timor forearc-continent collision zone and important new insights into the importance of mélange reworking for mudstone provenance and episodic uplift.

Section snippets

Geology and pre-collisional stratigraphy of Timor

The Australian plate in the Timor Sea region presently travels NNE at ∼70 mm yr1 relative to the Sunda Shelf (Bock et al., 2003, Genrich et al., 1996, Koulali et al., 2016, Kreemer et al., 2000, Nugroho et al., 2009) (Fig. 1a). Subduction of old oceanic lithosphere north of what is known as the Banda Embayment of the Australian continental margin began around 12 Ma or shortly before and propagated eastward, resulting in development of the Banda Arc as an eastwards extension of the Sunda Arc (

Stratigraphy

The rapid Pliocene uplift and erosion of proto-Timor (Nguyen et al., 2013) was accompanied by deposition of the Synorogenic Megasequence of Timor-Leste, which is now well exposed in uplifted basins mostly on the south side of Timor (Audley-Charles, 1968, Haig et al., 2007) (Fig. 1). In general terms, the Synorogenic Megasequence comprises a gently deformed, coarsening upward succession that began depositing during the latest Miocene. The synorogenic nomenclature of Timor preferred in this

Previous work and relevant geochemical datasets

The provenance of the synorogenic basins of Timor has only been qualitatively evaluated and the sediments are generally assumed to be derived largely from the Banda Terrane forearc basement (Audley-Charles, 1967, Audley-Charles, 1968, Kenyon, 1974). A wealth of potentially useful geochemical datasets are emerging that allow this hypothesis to be refined. The geochemistry of igneous and metamorphic rocks of the Banda Terrane are reported by Harris (1992), Harris (Harris, 2006), Standley and

Methods

This study compares descriptions and geochemical data for conglomerate clasts, sandstones and mudstones derived from the synorogenic basins of Timor-Leste, with lithologies and relevant geochemical datasets from the various terranes.

Marobo basin

Tate et al. (2014) and Aben et al. (2014) dated a section of the Marobo Basin (Duffy et al., 2013) (Fig. 1b). The section location in the Cailaco River is shown in Supplementary Fig. S1 and the measured section in Fig. 3. The base of the section consists of less than 2 m of massive, creamy, moderately indurated foraminiferal limestone of the Batu Putih Formation that overlies the Synorogenic Mélange and incorporates mélange clasts in its lowest few centimeters. The basal limestone grades into >40

Marobo basin

The Marobo conglomerates are composed of dark grey sericite schist (30–63%), quartz (0.5–10%) and feldspar (4–22%), supplemented by small but regular amounts of intraformational sandstone (0–10%), quartzite (0–6%), calcareous schist (0–6%) and epidote schist (0–3%). Rare chloritoid schist clasts are present at the base of the section, along with a single occurrence of four igneous clasts in a sample from the base of the clastic part of the section (65 m - CR24a). The sericite schist clasts are

Transition from carbonate to terrigenous sedimentation – an uplift record

In all the studied basins across Timor, the base of dated sections occupies a narrow range of 60 kyr between 5.59 and 5.53 Ma, has a deep marine foraminiferal assemblage and overlies the Synorogenic Mélange (Haig and McCartain, 2007, Tate, 2014, Tate et al., 2014). Even though the 5.5–4.5 Ma basal sediments of the Marobo and Viqueque sections of Timor-Leste must have accumulated within 200 km from the arc, which was active between at least ∼7.1 Ma (Abbott and Chamalaun, 1981, Honthaas et al., 1998)

Conclusions

We have presented new data for the Synorogenic Megasequence from several basins including well-dated measured sections in central and eastern Timor (the Marobo and Viqueque basins respectively) and a re-dated DSDP 262. Our analysis indicates that:

  • (1)

    Arc magmatism did not contribute to the sediments accumulating on the forearc since >5.5 Ma.

  • (2)

    The geochemical evolution of DSDP 262 sediments records the emergence of modern land at a distance of ∼50 km.

  • (3)

    In the exhumed synorogenic basins, the DSDP

Acknowledgements

This research was primarily funded by a Royal Society of New Zealand Marsden Fund grant to MQ (Fast-start grant M1137). BD was supported by a New Zealand Tertiary Education Commission Top Achiever Scholarship. RH was supported by various NSF grants. DJJvH was supported by ERC – Netherlands Starting Grant 306810 (SINK) and NWO VIDI – Netherlands grant 864.11.004. The authors gratefully acknowledge the support of the University of Canterbury, where much of this research was conducted, and of our

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