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Discussion of a Miocene collisional belt in north Borneo: uplift mechanism and isostatic adjustment quantified by thermochronology

Journal, Vol. 157, 2000, 783–793

¹ Department of Geological Sciences, University College London, London WC1E 6BT, UK (e-mail: j.milsom{at}ucl.ac.uk)
² Veritas GeoServices, Suite 530, 3701 Kirby Drive, Houston TX 77098, USA (e-mail: rob.holt{at}veritasdgc.com)
³ 10 Lorong 5/19A, 46000 Petaling Jaya, Selangor, Malaysia (e-mail: cshutchison{at}usa.net)
⁴ Dept. Geosciences, University of Texas at Dallas, TX 75083, USA (e-mail: bergman{at}utdallas.edu) and Dept. Geological Sciences, Southern Methodist Univ., Dallas, TX 75275-0395, USA (e-mail: scb@mail.smu.edu)
⁵ ARCO China, Shekou, Shaenzhen, Guandong Province, 518067, P.R. China (e-mail: dswauge{at}mail.arco.com)
⁶ Vastar Resources Inc., 15375 Memorial Drive, Houston, TX 77079, USA (e-mail: jgraves1{at}vastar.com)

Scientific editing by Alex Maltman.

	Introduction

Top Introduction Principles of whole-crust... The Crocker-Trusmadi root and... Ophiolitic rocks of eastern... Continental crust beneath Sabah Gravity constraints on crustal... Principles of whole crust... The Crocker-Trusmadi root and... Ophiolitic rocks of Eastern... Continental crust beneath Sabah Gravity constraints on crustal... References

 
John Milsom & Robert Holt write: Knowledge of the gravity field is vital to any study of isostasy. This was acknowledged by Hutchison et al. (2000) when they noted that the work of Holt (1998) placed constraints on their geological model for Sabah. However, they went on to ignore these constraints, arguing that the depth of the crustal root beneath the Western Cordillera might have been overestimated due to ‘the model assumption of an excessively thick ophiolitic basement’ and that the presence of thick crust beneath Sabah was ‘in conflict with the outcropping geology’.

The Hutchison et al. interpretations of the deep structure of Sabah were illustrated in a series of cross-sections in which some features (notably the topography) were amplified for the sake of clarity. There was, however, no suggestion that any features would be underemphasized, and the depth scales, although described as only ‘rough guides’, indicated normal 5–10 km thicknesses for oceanic crust. Therefore, and in the light of the comments quoted above, the thin crust shown throughout Sabah and the generally somewhat greater thickness of the extended continental crust of the Dangerous Grounds must be regarded as essential elements of the model. The Late Miocene cross-section (Fig. 1), showing the end of orogeny in Sabah, is a guide to the Hutchison et al. view of the present-day situation. Crust is less than 30 km thick beneath the Western Cordillera and less than 20 km thick elsewhere. We contend that these values are incompatible with the gravity field, and that it is no more acceptable to present a geological interpretation that ignores geophysical data than a geophysical interpretation that ignores geology. Some way must be found of reconciling the two.

View larger version (22K): [in this window] [in a new window]   Fig. 1. Sabah in the Late Miocene, from Hutchison et al. (2000, fig. 6b).

	Principles of whole-crust gravity modelling

Top Introduction Principles of whole-crust... The Crocker-Trusmadi root and... Ophiolitic rocks of eastern... Continental crust beneath Sabah Gravity constraints on crustal... Principles of whole crust... The Crocker-Trusmadi root and... Ophiolitic rocks of Eastern... Continental crust beneath Sabah Gravity constraints on crustal... References

View larger version (84K): [in this window] [in a new window]   Fig. 2. Bouguer gravity in eastern Sabah. Contour interval 10 mGal. Geology simplified after Heng (1985) and Hutchison et al. (2000). Heavy dots show gravity station locations and, in the Upper Segama, indicate forestry roads used in 1995. The term ‘HEA ophiolite’ denotes the area shown as ophiolite in fig. 3 of Hutchison et al. (2000), and includes true ophiolite, metamorphic rocks and the chert–spilite formation.

	The Crocker-Trusmadi root and ‘ophiolitic basement’

Top Introduction Principles of whole-crust... The Crocker-Trusmadi root and... Ophiolitic rocks of eastern... Continental crust beneath Sabah Gravity constraints on crustal... Principles of whole crust... The Crocker-Trusmadi root and... Ophiolitic rocks of Eastern... Continental crust beneath Sabah Gravity constraints on crustal... References

 
The Holt (1998) gravity profile SAB1 across the Crocker–Trusmadi Ranges was modelled using only crust, mantle, sea water and offshore sediments. Greater complexity is not justified since there are large probable errors in elevation and terrain corrections in the cordillera. It is, however, hard to understand why Hutchison et al. considered a deep root unacceptable. The mountains rise to more than 2400 m, and local isostatic compensation would therefore require a maximum Moho depth approaching 50 km. The surprising factor is the lack of gravitational evidence for a similar root beneath the much higher Mt Kinabalu (Milsom et al. 1997).

Despite the claim by Hutchison et al., the models of Holt (1998) included no ‘ophiolitic basement’, thick or otherwise.The crust was characterized as ‘continental’ on the cross-sections but, more cautiously, ’of continental density’ in the text. It is on the Hutchison et al. cross-sections that normal oceanic crust is labelled ‘ophiolite’. To object to this is not mere pedantry. It has long been recognized that many ophiolites are forearc rather than normal ocean crust (e.g. Pearce et al. 1984). They are, moreover, characteristically emplaced by thrusting, and even the largest examples (e.g. eastern Papua, Oman and eastern Sulawesi) overlie sialic metamorphics or sediments on thrusts that are in places shallow dipping or horizontal. We feel that the term ‘ophiolitic basement’ is inappropriate even on land, and is best avoided.

	Ophiolitic rocks of eastern Sabah

Top Introduction Principles of whole-crust... The Crocker-Trusmadi root and... Ophiolitic rocks of eastern... Continental crust beneath Sabah Gravity constraints on crustal... Principles of whole crust... The Crocker-Trusmadi root and... Ophiolitic rocks of Eastern... Continental crust beneath Sabah Gravity constraints on crustal... References

 
Hutchison et al. did not specify the inconsistency they see between outcrop data and gravity interpretation, but it seems likely to derive from their assumption that the widespread, but discontinuous, outcrops of ‘chert–spilite formation’ are basement. This assumption certainly conflicts with the geophysical data, but the outcrops do not. Heng (1985) showed chert–spilite formation, ophiolite and ‘crystalline basement’ cropping out in roughly equal proportions in the area of the Hutchison et al. ‘ophiolite’. Bouguer gravity is negative over roughly a third of this area, decreasing rapidly westwards from Darvel Bay (Fig. 2). Negative Bouguer gravity is hard to reconcile with the spilites as the upper parts of 10 km thick oceanic crust (Fig. 1). It is entirely compatible with their being faulted together with, and partly overriding, melange and volcaniclastics, ‘making delineation of the true melange from the dismembered and imbricated ophiolite complex rather arbitrary’ (Clennell 1991, p. 412). During the gravity survey, using additional roads (Fig. 2), we saw no reason to dispute this latter statement.

	Continental crust beneath Sabah

Top Introduction Principles of whole-crust... The Crocker-Trusmadi root and... Ophiolitic rocks of eastern... Continental crust beneath Sabah Gravity constraints on crustal... Principles of whole crust... The Crocker-Trusmadi root and... Ophiolitic rocks of Eastern... Continental crust beneath Sabah Gravity constraints on crustal... References

 
Even Hutchison et al. have to concede that some old continental crust exists beneath eastern Sabah. The evidence was summarized by Hutchison (1989, p. 54), who noted that ‘along the Litog Klikog Kiri, . . . the gabbroic part of the ophiolite is intruded by granodiorite and coarse biotite tonalite (Kirk, 1968). Some of the granitoids contain up to 2.25% K₂O and, therefore, are thought to have risen from a buried continental basement which underlies the ophiolite but has not been found outcropping’. Regarding ages, ‘biotite in the metamorphic aureole gave an Upper Jurassic K–Ar date of 160 Ma, while the associated tonalite gave 150 Ma. The oldest K–Ar date was from biotite extracted from the tonalite, 210 Ma or Early Jurassic (Leong 1971)’.

This summary presents obvious problems, not least in having an older tonalite intruding a younger ophiolite, but these were at least minimized by the subsequent claim that other similar intrusions were very low in potassium and therefore components of the ophiolite. Leong (1998), however, has disputed this, arguing that only one of the potassium values obtained from acid/intermediate rocks in the Upper Segama could be considered low (let alone ‘extremely low’) and that the supposedly spurious Jurassic K–Ar dates have been confirmed by all later determinations.

It is not our intention to become involved in arguments between geologists based on purely geological evidence, particularly as Hutchison et al. will soon publish new K–Ar and geochemical data. It is, however, fair to point out that not all geologists agree with their interpretation of the outcrops.

	Gravity constraints on crustal thickness in eastern Sabah

Top Introduction Principles of whole-crust... The Crocker-Trusmadi root and... Ophiolitic rocks of eastern... Continental crust beneath Sabah Gravity constraints on crustal... Principles of whole crust... The Crocker-Trusmadi root and... Ophiolitic rocks of Eastern... Continental crust beneath Sabah Gravity constraints on crustal... References

 
As alternatives to continental or island-arc crust, the low Bouguer values beneath parts of the chert–spilite formation could be due to very thick oceanic crust, to hot, low density, mantle or to thick sediments on oceanic crust. However, sufficiently thick oceanic crust is found only in the large igneous provinces above mantle plumes (Mooney et al. 1998), and a plume beneath Sabah has not so far been suggested by anyone who has worked in the area.

The Hutchison et al. crustal thickness of about 15 km where ‘Mesozoic ophiolitic basement’ is at sea level (Fig. 1) could be compatible with the gravity field only if the underlying mantle were very light. Mantle here might have been hot during Miocene rifting, but even active spreading ridges seldom reach sea level. Today’s cooler mantle is thus very unlikely to support subaerial oceanic crust. Moreover, the boundaries of any thermal anomaly would be gradational, but the low Bouguer gravity measured onshore changes, in only a few tens of kilometres, to gravity patterns characteristic of a normal ocean basin in the SE Sulu Sea.

Sedimentation on oceanic crust might, eventually, produce crust as thick as that of a normal continent, and correspondingly low Bouguer gravity. However, although there is a rough correlation between low Bouguer values and sediment outcrops, there are also low values in areas shown as ophiolite by Hutchison et al.. Even the high fields in Darvel Bay are much too low for in situ oceanic crust, but are comparable with fields over ophiolites which are demonstrably thrust-emplaced (e.g. Milsom 1973).

In contrast to the problems posed by the ‘oceanic’ solutions, there are few obstacles to interpretation in terms of originally thick continental or island-arc crust, rifted and thinned by southwards propagation of Sulu Sea spreading to provide space for melange and clastic sediments. Gravity alone cannot determine sediment or basement thickness in the basins, since similar fields could be produced by various combinations of these parameters with plausible crustal and sediment densities. What is important, in evaluating the geotectonic setting of Sabah, is the gravitational evidence for crust 30–40 km thick along the rift flanks.

14 August 2000

C. S. Hutchison, S. C. Bergman, D. A. Swauger & J. E. Graves reply: We thank Milsom & Holt for their comments and welcome the opportunity to expand upon aspects we were not able to adequately address. Our main purpose was to provide and interpret new thermochronological data relevant to the Cenozoic denudation history of the collisional mountain belt. We offered a geological context, a tectonic synthesis, and schematic cross-sections to illustrate our main ideas. Milsom & Holt are bothered by our lack of attention to gravity data and the nature of our cross-sections (figs 5 & 6). They were stated to be schematic cartoons in which ‘. . . The scale is only a rough guide and certain features are amplified for the sake of clarity’. Our proposed model is based on integrated geological and thermochronological data. The gravity data of Milsom & Holt lack the necessary independent seismic calibration and supporting crustal property data to adequately test our model. The Late Miocene section should not be used as the present-day situation, as Milsom & Holt have suggested. During the intense Late Miocene compressional tectonism, which led to the Crocker Ranges fold and thrust belt, crustal thickening of originally thinned continental crust of the southern South China Sea (c. 5–15 km thick) undoubtedly occurred, leading to a crust c. 10–30 km thick. Milsom & Holt state that their model was based on ‘. . . only crust, mantle, sea water and offshore sediments. Greater complexity is not justified since there are large probable errors in elevation and terrain corrections in the cordillera’. This approach ignores the very thick sedimentary rock sequences which are well known in onshore Sabah and have significantly lower densities than standard crust (see Table 1), and therefore allow for a thin dense basement. We now respond to each section of the Milsom & Holt criticisms.

	Principles of whole crust gravity modelling

Top Introduction Principles of whole-crust... The Crocker-Trusmadi root and... Ophiolitic rocks of eastern... Continental crust beneath Sabah Gravity constraints on crustal... Principles of whole crust... The Crocker-Trusmadi root and... Ophiolitic rocks of Eastern... Continental crust beneath Sabah Gravity constraints on crustal... References

	The Crocker–Trusmadi root and ‘ophiolitic basement’

Top Introduction Principles of whole-crust... The Crocker-Trusmadi root and... Ophiolitic rocks of eastern... Continental crust beneath Sabah Gravity constraints on crustal... Principles of whole crust... The Crocker-Trusmadi root and... Ophiolitic rocks of Eastern... Continental crust beneath Sabah Gravity constraints on crustal... References

 
We apologize for erroneously stating (p. 790) that Holt (1998) used a ‘. . . model assumption of an excessively thick ophiolitic basement’. In fact he assumed normal continental basement for much of Sabah, an assumption with which we strongly disagree. Based on extensive regional geological mapping, most of central, northern and eastern Sabah is underlain by a mixed assemblage of fragments of Mesozoic ophiolitic rocks, of variable metamorphic grade (the higher grades have been misleadingly referred to as the ‘Crystalline Basement’), with sparsely exposed granitic basement of possible continental character, which has yielded a maximum K–Ar age of 210 Ma (Leong 1998). We use the term ‘ophiolite’ in the traditional lithological sense without any genetic connotation and do not equate it with ‘oceanic crust’, despite conventional interpretations as detailed by Coleman (1977) and Gass & Smewing (1981). ‘Ophiolite’ is a complex rock assemblage of mafic and ultramafic intrusive and extrusive rocks associated with variably sheared serpentinites, sheeted dykes, pillow lavas and cherts, initially described by Steinmann (1906; see Jackson 1997). We believe that many Mesozoic and Cenozoic obducted ophiolite belts of SE Asia are fragments of fore-arc or back-arc lithosphere. Chemically, the Sabah ophiolite more closely resembles island arc lithosphere than MORB (Omang & Barber 1996). Furthermore, all known mineral deposits in Sabah are of island-arc type (porphyry copper, eipthermal, chromite and Cyprus-type). There is a total absence of mineral deposits requiring continental crust basement.

The Kinabalu batholith and satellite stocks intrude Mesozoic ophiolitic rocks and overlying Trusmadi and Crocker Formation turbidites. This complex situation of low density sandstone (c. 2.5 g cc^–1) and biotite–hornblende granitoid (c. 2.6–2.7 g cc^–1), juxtaposed against high density mafic and ultramafic rocks (c. 2.9 g cc^–1) with an unknown contact geometry, makes gravity modelling highly tenuous, especially in the absence of accurate and precise elevation and terrain corrections.

The regional low Bouguer gravity field of much of Sabah is an inescapable fact, yet its origin is open to discussion. We offer two alternatives: (1) the underlying upper mantle is heterogeneous and possesses anomalously low density; (2) crustal densities are heterogeneous and anomalously low. The labeling of portions of our cross-sections as ‘ophiolite’ was ambiguous and simply reflected the dominant lithology shown on the cartoon sections. We should have labeled them as ‘mafic/ultramafic crust’ to remove potential confusion with ‘oceanic crust’. We labeled the South China Sea crust as ‘ophiolite’, in a broad sense to include fore-arc and back-arc mafic/ultramafic lithosphere, or even arc material itself, accreted to the margin of Sundaland during the late Cretaceous and early Cenozoic.

	Ophiolitic rocks of Eastern Sabah

Top Introduction Principles of whole-crust... The Crocker-Trusmadi root and... Ophiolitic rocks of eastern... Continental crust beneath Sabah Gravity constraints on crustal... Principles of whole crust... The Crocker-Trusmadi root and... Ophiolitic rocks of Eastern... Continental crust beneath Sabah Gravity constraints on crustal... References

 
We chose not to discuss the regional gravity data in detail because of the lack of independent geophysical data. We agree that the Darvel Bay ophiolite closely coincides with a Bouguer gravity high as recognized by Beattie (1986). Its presence near sea level reflects the underlying mantle and crustal properties, which are unknown.

	Continental crust beneath Sabah

Top Introduction Principles of whole-crust... The Crocker-Trusmadi root and... Ophiolitic rocks of eastern... Continental crust beneath Sabah Gravity constraints on crustal... Principles of whole crust... The Crocker-Trusmadi root and... Ophiolitic rocks of Eastern... Continental crust beneath Sabah Gravity constraints on crustal... References

 
We agree with Leong (1998) that a complex basement composed of both ophiolitic (mafic–ultramafic) and granitic crust underlies much of Sabah. The granites of Ulu Segama with Jurassic to Cretaceous mica and feldspar K–Ar ages (Swauger et al. 1995) are evidence for local continental basement. However, these sparse granites represent very small volumes. The vast majority of the mapped Crystalline Basement is mafic to ultramafic, including metabasalts, gabbros, amphibolites, and serpentinites. The Cenozoic mélanges, widely distributed in central and eastern Sabah, exclusively contain mixed mafic–ultramafic, chert, and Cenozoic sandstone blocks, and lack any granitic, gneissic, or otherwise ‘continental’ clasts (Clennell 1992).

The provenance of the feldspar-bearing Crocker Formation and younger sandstones indicates a granitic source character, which conflicts with the abundance of mafic and ultramafic basement rocks exposed throughout much of Sabah. Zircon fission-track age data for these sandstones which we presented indicate a Cretaceous granitic provenance, which lies outside of Borneo or was eroded during the Paleogene denudation event that produced the Crocker turbidites. We believe the provenance for Crocker turbidites was mainland Asia (e.g., Southern China), which was much more proximal to Borneo during the Palaeogene prior to extrusion of Indochina and subsequent offset of >500–700 km along the Ailao Shan–Red River Shear Zone through China and Vietnam (Leloup et al. 1995, 2000). The apparent contradiction of the exposed mafic crystalline basement and quartz-rich felsic Cenozoic sandstone provenance is but one of the many enigmas in Sabah geology. A useful analogue may be made with the thick quartz-rich Miocene turbidites, probably derived from erosion of the Crocker Ranges, which occur in the deep marginal basins of the southern Sulu and Celebes Seas (Hutchison 1996, pp. 4–9).

	Gravity constraints on crustal thickness in eastern Sabah

Top Introduction Principles of whole-crust... The Crocker-Trusmadi root and... Ophiolitic rocks of eastern... Continental crust beneath Sabah Gravity constraints on crustal... Principles of whole crust... The Crocker-Trusmadi root and... Ophiolitic rocks of Eastern... Continental crust beneath Sabah Gravity constraints on crustal... References

 
We showed the Mesozoic ophiolite basement fragment west of Tawau (our fig. 6) slightly thicker than that to the east, because of probable west-vergent thrust imbrication. However, the cross sections were not drawn to precise scale and are locally exaggerated for clarity. These ophiolite fragments may attain thicknesses of 10–20 km and we do not envisage them as normal 6 km thick ‘oceanic crust’ (cf. Mooney et al. 1998). We envisage these ophiolites as fore-arc or back-arc fragments of mafic and ultramafic lithosphere. Pliocene MORB-like tholeiitic basalt lavas in the Mostyn Estate (Kunak) indicate the underlying mantle was hot enough to melt. Hot springs are common in the region, attesting to high heat flow possibly related to shallow basaltic intrusions. Therefore, basic geological evidence exists for local mantle and crustal perturbations in temperature and density, perhaps suggesting a small plume or local mantle upwelling. An anomalous low-density mantle is consistent with the surface geological evidence.

Mount Kinabalu is an enigma in the regional tectonic history and should be emphasized. The Crocker Ranges and the Kinabalu massif may be in isostatic disequilibrium due to the youth of the orogenesis. Our new data tightly constrain the intrusion and cooling history between >700°C to <60°C to within the Late Miocene (13.7–7 Ma), whereas previous radiometric age data disparately ranged from 2 to 20 Ma (Jacobsen 1970; Vogt & Flower 1989). We interpret the available geochemical and petrologic data to indicate that the granitic melts originated by lithospheric melting, probably caused by collisional processes, perhaps involving slab-break off, rather than normal subduction-related processes. The intrusion and denudation of the Kinabalu massif coincided with the rapid cooling of the Crocker Ranges induced by compressional tectonics and denudation of over 3–4 km of overburden since 10–15 Ma.

In conclusion, we agree with several of the points raised by Milsom & Holt, regret several minor errors in our paper, and emphasize that our cross-sections are simply schematic cartoons to illustrate aspects of our proposed tectonic model. The Milsom & Holt model, which considered only one alternative that conflicts with a variety of geological data, is unlikely. Alternative gravity models considering reasonable geological assemblages should be evaluated using independently calibrated crustal thickness. We look forward to seeing the fruits of future geological and geophysical studies to constrain better the tectonic evolution of this enigmatic collisional belt, and to reconcile many of the apparently conflicting facts of Sabah geology.

18 September 2000

	References

Top Introduction Principles of whole-crust... The Crocker-Trusmadi root and... Ophiolitic rocks of eastern... Continental crust beneath Sabah Gravity constraints on crustal... Principles of whole crust... The Crocker-Trusmadi root and... Ophiolitic rocks of Eastern... Continental crust beneath Sabah Gravity constraints on crustal... References

 

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