Paleomagnetism of the Crocker Formation, Northwest Borneo: Implications for Late Cenozoic Tectonics

Paleomagnetism of the Crocker Formation, northwest Borneo: Implications for late Cenozoic tectonics

Andrew B. Cullen1,2, M.S. Zechmeister3,4, R.D. Elmore3, and S.J. Pannalal3 1Shell International Exploration and Production Company, 100 Hoekstade, Rijswijk, Netherlands 2Chesapeake Energy Corporation, 6100 N. Western Avenue, Oklahoma City, Oklahoma 73118, USA 3ConocoPhillips School of Geology and Geophysics, 100 E. Boyd Street, Norman, Oklahoma 73019, USA 4Shell Exploration and Production Company, 150 North Dairy Ashford, Houston, Texas 77079, USA

ABSTRACT printed not only by differential clockwise pull-apart basin (Briais et al., 1993; Replumaz rotation of crustal blocks during opening of and Tapponnier, 2003); the amount of seafl oor Tectonic models for Borneo’s Cenozoic the South China Sea (32–23 Ma), but also spreading is approximately balanced by 600 km evolution differ in several aspects, par- locally by a younger (after 10 Ma) counter- of left-lateral displacement along the Ailao ticularly in the extent to which they include clockwise rotation. Shan–Red River fault zone. In the collision- paleomagnetic data suggestive of strong extrusion model, there is no Tertiary subduction counterclockwise rotation between 30 and INTRODUCTION under northwest Borneo, and mass is conserved 10 Ma. Key areas are undersampled. We by subduction in the Pacifi c Ocean. present the results of a paleomagnetic study The Cenozoic tectonic evolution of Southeast The subduction-collision model (Fig. 1C) of Eocene to Early Miocene sandstones Asia refl ects the complex interactions of rifting, features long-lived subduction (Eocene–Early from northwest Sabah, principally from the subduction, continental collision, and large- Miocene) beneath northwest Borneo during Crocker Formation. We obtained reliable scale continental strike-slip faulting. The island which an extensive amount of proto–South site means from 11 locations along a 250 km of Borneo is at the leading edge of several conti- China Sea oceanic crust is consumed. Subduc- northeast-southwest transect using thermal nental blocks that protrude from Southeast Asia tion terminates progressively (southwest to demagnetization to isolate characteristic as a wedge into the Indo-Australian and Phil- northeast) as blocks of continental crust (Luco- remanent magnetization (ChRM) directions. ippine Sea plates (Fig. 1A). There are two end nia, Dangerous Ground, and Reed Bank) collide The Crocker Formation sandstones are per- members of tectonic models for Borneo (Figs. with northwest Borneo and Palawan (Holloway, vasively remagnetized; pyrrhotite dominates 1B, 1C): collision-extrusion (Briais et al., 1993; 1982; Lee and Laver, 1995; Hall, 1996; Longley, the ChRM signal. Locations can be grouped Replumaz and Tapponnier, 2003) and subduction- 1997). In this model, there is less displacement into different domains on the basis of the rela- collision (Hamilton, 1979; Lee and Laver, 1995; along the Red River fault and because it largely tive sense of rotation about a vertical axis. Hall, 1996). These models differ in four princi- decoupled from extension in the South China Mean ChRM directions for seven locations pal aspects: (1) the mechanism responsible for Sea, CW rotation of Borneo is not required. The between Kota Kinabalu and Keningau (dec- rifting and seafl oor spreading in the South China subduction-collision model has several permu- lination, dec 12°–19°; inclination, inc –22°– Sea ca. 32–16 Ma (Briais et al., 1993); (2) the tations. The most widely cited reconstructions 23°) indicate minor clockwise rotation and timing and amount of displacement along the are those of Hall (1996, 2002); honoring Fuller modest tilting, whereas two locations near large intercontinental strike-slip faults such as et al.’s (1999) interpretation of regional paleo- Tenom (dec 321°–345°, inc –6°–24°) record the Red River fault (Leloupe et al., 1995; Searle, magnetic data, these reconstructions show an counterclockwise rotation and modest tilt- 2006); (3) the amount of proto–South China Sea acceleration in subduction rate driven by strong ing. Although we cannot precisely date the crust subducted beneath Borneo (Rangin et al., (~50°) counterclockwise (CCW) movement of age of remagnetization, the results of fold 1999; Lee and Laver, 1995; Hall, 2002; Cullen, Borneo as a rigid block between 30 and 10 Ma. tests from 4 locations, interpreted within the 2010); and (4) the magnitude and nature of the Murphy (1998) and Morley (2002) pointed regional structural framework, strongly indi- late Tertiary rotation of Borneo (Hall, 1996, out, however, that the lack of known regional cate that remagnetization occurred between 2002; Murphy, 1998). structures of suffi cient magnitude to accom- 35 and 15 Ma, the waning stages of the Sara- In the collision-extrusion model (Fig. 1B), modate such a large rotation poses a challenge wak orogeny to an early phase of the Sabah India’s collision with Asia progressively dis- to the interpretation of the paleomagnetic data. orogeny. Our results pose serious diffi culties places the Sundaland, Indochina, and South Hutchison (2010), drawing attention to the lack for current tectonic models in which Bor- China blocks to the southeast along intercon- of paleomagnetic data in key areas of Borneo, neo rotates 50° counterclockwise as a rigid tinental strike-slip faults (e.g., Mae Ping and suggested that the large oroclinal bend in Bor- block between 30 and 10 Ma. With respect to Red River faults). In this model, Borneo, south neo’s interior highlands is strong evidence that prior paleomagnetic studies, we suspect that Palawan, and north Palawan rotate clockwise Borneo did not deform as a single rigid block. an early episode of strong regional counter- (CW) ~25° along with the Indochina block There are two fundamental issues regarding clockwise rotation (before 35 Ma) was over- as the South China Sea opens as a large-scale the paleomagnetic evidence for the rotation of

Geosphere; October 2012; v. 8; no. 5; p. 1146–1169; doi:10.1130/GES00750.1; 16 ﬁ gures; 2 tables. Received 8 September 2011 ♦ Revision received 23 March 2012 ♦ Accepted 27 March 2012 ♦ Published online 18 September 2012

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E105 E110 E115 E120 A B1C

INDIA SCS SCB

N20 ICB

PSP ICB MB N15 BORNEO Collision - Extrusion EVF C1B SCS INDIA SCS RB NP N10

UBF ICB BAL SP SS RB DG DG LUC LB SLB SB BB Figure 2 N5 BRL CS LUC TB BORNEO TL Subduction - Collision

KB 0 BORNEO ADL IAP

500 km

Figure 1. (A) Regional tectonic and geological features by ETOPO2v2 (http://www.ngdc.noaa.gov/mgg/fl iers/01mgg04.htmlbathymetry) and Shuttle Radar Topography Mission digital elevation model (yellow and brown <1000 m elevation). Inset box shows outline of map of Figure 2B. Major tectonic elements: IAP—Indo-Australian plate, ICB—Indochina block, PSP—Philippine Sea plate, SCB—South China block, SLB—Sundaland block, CS—Celebes Sea, SS—Sulu Sea, SCS—South China Sea. (Fault patterns are from Morley, 2002; Pubel- lier et al., 2005; Fyhn et al., 2009; Cullen et al., 2010). Heavy lines with fi lled triangles mark subduction zones; dashed lines with fi lled triangles mark active deep-water thrust belts adjacent to Neogene basins (BB—Baram Basin, KB—Kutei Basin, SB—Sandakan Basin, TB—Tarakan Basin). Fault zones and older tectonic boundaries: ADL—Andag line, BAL—Balabac line, BRL—Baram line, EVF—East Vietnam fault zone, LL—Lupar line, TL—Tinjar line, MPFZ—Mae Ping fault zone, RRFZ Red River fault zone, THFZ—Tua Hoa fault zone, UBF—Ulugan Bay fault, SF—Sangkulirang fault zone. Outline of oceanic crust and seafl oor-spreading anomalies in SCS in dashed lines are from Barckhausen and Roeser (2004) and Hsu et al. (2004). DG—Dangerous Grounds, MB—Macclesfi eld Bank, RB—Reed Bank, SP—south Palawan, NP—north Palawan, LUC—Luconia. (B, C) Two end-member models for the region’s late Cenozoic tectonic evolution discussed in text (adapted from Cullen et al., 2010).

Borneo, remagnetization and sampling density. (136 sites from the Schwaner Mountains, South ~60° CCW rotation by the end of the Oligocene Although the regional paleomagnetic data com- Kalimantan, Central Kalimantan, and Sarawak) (Almasco et al., 2000) predates the proposed piled by Fuller et al. (1999) included strongly and Palawan (38 sites); only 9 sites are from CCW rotation of southern and central Borneo, rotated, weakly rotated, and nonrotated sites, Sabah. In interpreting the paleomagnetic record which implies the presence of a signifi cant right- their analysis excluded weakly rotated and from Borneo, Fuller et al. (1999, p. 21) stated, lateral shear zone between northern Sabah and nonrotated sites older than 10 Ma on the prem- “…fall back to an essentially rigid plate model the rest of Borneo. Although such a shear zone ise that those sites were remagnetized to the with much of Kalimantan, Sarawak and south- has not been identifi ed onshore, the 20 Ma and modern geomagnetic fi eld direction. The data ern Sabah participating in a rotation of about 15 Ma reconstructions by Hall (2002) show compiled by Fuller et al. (1999) are heavily 50° CCW between 30 and 10 Ma.” The nuance the development of a dextral transform fault weighted toward the southern part of Borneo of this statement is signifi cant; south Palawan’s offshore along the Balabac line (Milsom et al.,

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1997) between Sabah and Palawan (Fig. 1) that study of 40 sites at 14 locations in northwest ceous (Metcalf, 2011). Late Mesozoic continen- accommodates the movement of Palawan dur- Sabah that sampled Eocene to Early Miocene tal arc igneous rocks in the Schwaner Mountains ing of the opening of the South China Sea. The sandstones of the Crocker, West Crocker, Meli- of Kalimantan provide suitable paleomagnetic kinematics of the fault predict CW rotation of gan, and Kudat Formations (Fig. 2). data (Haile et al., 1977). Borneo’s interior high- Palawan rather than the CCW rotation of Bor- lands are dominated by the Rajang-Embaluh neo. In a study of peninsular Malaysia, Richter REGIONAL GEOLOGICAL supergroup, a thick succession of strongly et al. (1999) similarly concluded that, if the FRAMEWORK deformed Late Cretaceous to Paleogene deep- regional Southeast Asia paleomagnetic data are water clastic sediments lifted to present-day taken at face value, there must have been bound- Borneo’s geological framework consists elevations of >1 km. The depositional and struc- aries between CCW rotating blocks. of three fundamental parts (Fig. 3): southwest tural history of the Rajang-Embaluh supergroup Sabah occupies a critical position with respect Kalimantan, the interior highlands, and the sur- is widely interpreted in terms of an accretion- to constraining the Cenozoic rotational history of rounding coastal lowlands with offshore basins. ary prism (Hamilton, 1979; Rangin et al., 1990; Borneo, but is an undersampled area with respect Southwest Kalimantan, Indonesia, is a Paleo- Tongkul, 1997a; Bakar et al., 2007). Hutchison to the surrounding regions. We address that short- zoic cored fragment of Australian Gondwana (1996) and Moss (1998), however, suggested coming and report the results of a paleomagnetic that was accreted to Indochina during the Creta- that the younger units of this succession postdate

Age SERIES Nomenclature Events Ma KD SRU Meliau Orogeny 10 Belait Fm. not sampled Laya-Laya Fault DRU Kudat Fm. Miocene Sabah South China Sea BMU Kudat Fm. Orogeny N6.30- 20 Meligan sandstone BL ...... NX TS Te m b u r o n g K I x MK 30 .... x x x

Oligocene BS West Crocker KoK KK SOU Sarawak N6.00- Orogeny JS SW NE Tenom Fault TO LK 40 Crocker Eocene Formation Cr N5.30- Rajang- KG 50 Embaluh Group WCr KGV Sapalut SP KM 60 Formation Tbr-WCr facies change

Paleocene TN N

Cr 50 km 70 Basement chert-spillite ultramafics Mlg Tbr U. Cretaceous B E 115.30 E 116.00 E 116.30 E 117.00 A

Figure 2. (A) Stratigraphic column (modifi ed from Cullen, 2010; Lambiase et al., 2008). BMU—Base Miocene unconformity, DRU—deep regional unconformity, SOU—Sarawak orogeny unconformity, SRU—shallow regional unconformity. (B) Simplifi ed geological map of western Sabah (Tongkul, 1997b; Tate, 2001; Hutchison, 2005; Tongkul, 2006) showing locations and units sampled: Cr—Crocker Forma- tion (squares), KD—Kudat Formation (hexagon), Mlg—Meligan sandstone (circles), WCr—West Crocker (circles). Other abbreviations: KoK—city of Kota Kinabalu, KGV—Keningau Valley, MK—Mount Kinabalu pluton, Tbr—Temburong Formation, TO—Telupid ophiolite (ultramafi c rocks shown in dark gray), SP—Sook Plains, TBV—Tambunum Valley, TNV—Tenom Valley, BL—Belud (BL) location, TS—TS car wash location, KK—Kota Kinabalu location, BS—Bukit Sepanger, NX—Nexus, JS—Jalan Salaiman, LK—Lok Kawi, KG— Keningau Pass, TN—Tenom, KM—Kampong Mua, KGV—Keningau Valley (see text).

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110 E 112 E 114 E 116 E 118 E N B Outline Figure 3B NP 48 CW 10 N

W E N. Palawan DG SPSO UBF SP

SS 8 N S N

SPSO

63 CCW

W E X 6 N S. Palawan Sabah TO

S Brunei Sabah CS 4 N CS Kalimantan N

18 CCW v v v v v v N v

v v Fig. 4 v 2 N W E v KQ Sarawak v v NW Borneo v v v

v v v v W E

Kalimantan 0 Sabah S Fig. 5 N x x x x N Schwaner Mtns. x 15 CCW S 49 CCW x x x x x x x W E W E W Kalimantan C. E. Kalimantan

S 100 km A S

Figure 3. (A) Regional geological framework of Borneo (simplifi ed after Tate, 2001; Hutchison, 2005). Abbreviations as in Figures 1 and 2. SPSO, south Palawan Sabah ophiolite, is outlined in black dashes with ultramafi c outcrops shown in dark gray. Sintang igneous suite is in white triangles; Paleogene fl ysch deposits are in light green with fold belt trend in green dashes showing oroclinal bend in highland; Plio- cene–Pleistocene volcanic plateaus are in white with small v symbols; gray X marks—Mount Kinabalu pluton. Plots of previously published paleomagnetic results (Fuller et al., 1991, 1999; Lumadyo et al., 1993; Schmidtke et al., 1990) are grouped into domains (discussed in text). The pie diagram plots show the range of data shaded, with the average declinations shown with arrows. On the plot for Sabah, KQ denotes Kappa Quarry intrusion. Figures with photos refer to locations in Figure 3. CCW—counterclockwise; CW—clockwise; C.E.—central eastern. (B) The regional Bouguer gravity fi eld (Cullen et al., 2010); red corresponds to high Bouguer values, blue to low values.

subduction and record deposition in a marginal highlands (Hamilton, 1979; Hall and Nichols, northeast of Sabah, and on south Palawan (Fig. 3). ocean basin that was subsequently deformed 2002; Morley and Back, 2008). A regional Bouguer gravity high that extends during the Sarawak orogeny (discussed in the The igneous record of northern Borneo is between these outcrops is interpreted as a region- following). The coastal lowlands and offshore rather sparse. The oldest igneous rocks are a ally extensive late Mesozoic ophiolite complex areas of Borneo are occupied by Neogene fore- series of Mesozoic aged variably serpentinized (Cullen, 2010). Paleomagnetic data indicating land basins that contain as much as 10 km of ultramaﬁ c bodies and chert-spillite assemblages very strong CCW rotation of the ophiolites at largely shallow-marine successions that record (Hutchison, 1975) that crop out in north-central Telupid and south Palawan (Fuller et al., 1999; extensive denudation, 4–6 km, of the interior Sabah (near the village of Telupid), on islands Almasco et al., 2000) support that interpretation.

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The Sintang igneous suite in southern Sara- In the case of the Sabah orogeny, isostatically Miocene) unconformity identifi ed in onshore wak and central Kalimantan comprises Oligo- driven uplift related to delamination of the lower western Sabah (van Hattum et al., 2006). These cene–Miocene calc-alkaline stocks and dikes crust or slab detachment exerted a postcolli- older unconformities are a strong indication that that are an important element of the paleo- sional buoyant infl uence on the orogen (Hutchi- the Sabah orogeny commenced in the earliest magnetic record, in part because several of son et al., 2000; Morley and Back, 2008). Miocene. Whether the Sabah orogeny was ini- these intrusions have absolute age determina- tiated by collision of the Dangerous Grounds tions (Schmidtke et al., 1990; Lumadyo et al., Sarawak Orogeny with Borneo owing to CCW rotation of Borneo 1993; Fuller et al., 1999; Prouteau et al., 2001). (Hall, 2002) or by underthrusting of the Dan- In northwest Sabah, Mount Kinabalu is a Late The Sarawak orogeny is defi ned by a pro- gerous Grounds driven by rifting in the South Miocene (7.85–7.22 Ma) granitic pluton that found angular unconformity that truncates iso- China Sea is an issue addressed in this study. intrudes the Crocker Formation and older ultra- clinally folded members of the Rajang-Embaluh The Dangerous Grounds are composed of small mafi c rocks (Vogt and Flower, 1989; Cottam supergroup (Hutchison, 1996), which makes a islands and shallow reefs in the eastern South et al., 2010). A satellite stock related to Mount large oroclinal bend in the central highlands and China Sea that have formed on blocks of rifted Kinabalu is one of the paleomagnetic sites of then extends into Sabah; the upper member, the continental crust. Fuller et al. (1999). The central highlands are Crocker Formation, is the principal unit sam- capped locally by a dissected tableland of fl at- pled in this study. Hutchison (2010) interpreted Meliau Episode lying basalt fl ows that have whole-rock K-Ar this oroclinal bend as the result of indentation dates between 1.8 and 1.3 Ma (van de Weerd, owing to collision of the Luconia block during The Meliau orogeny refers to latest Miocene 1987). The paleomagnetic record from all 17 the Sarawak orogeny. The Sarawak orogeny deformation in southeast Sabah that is charac- sites at 3 different locations more than 30 km unconformity is diachronous, becoming pro- terized by sinistral transpression that resulted apart is excellent, and records nonrotated gressively younger to the northeast. In Kaliman- in minor localized inversion and reactivation of reversed directions and an average inclination tan, the unconformity is capped by the Nyaan older structures rather than regional uplift and similar to those of the present fi eld (Lumadyo Volcanics, dated as 48.6 Ma (Moss, 1998), orogenesis (Balaguru et al., 2003; Balaguru and et al., 1993). whereas in Sarawak, biostratigraphic data indi- Hall, 2008). Interpreted within the context of In the northwest Borneo region, Cenozoic cate that the limestone cover sequences are Late a phase of the Sabah orogeny, the Meliau epi- deformation has resulted in the development Eocene, ca. 38 Ma (Hutchison, 2005). Although sode corresponds to formation of the shallow of several regional angular unconformities (Bol outcrops exposing this unconformity were not regional unconformity, which is characterized and van Hoorn, 1980; Levell, 1987; Hutchison, identifi ed, Hutchison (1996) interpreted the by compressive wrench faulting (Levell, 1987). 1992) that have been interpreted as the products Sarawak orogeny unconformity to extend into In their study of the deep-water fold-and-thrust of two episodes of orogeny: the Eocene Sara- Sabah, where Late Eocene to Oligocene sand- belt of the Baram Basin, Hesse et al. (2009) wak orogeny and the Middle to Late Miocene stones that contain ultramafi c rock fragments concluded that an along-strike decrease between Sabah orogeny (Hutchison, 1996). Regional crop out in central Sabah near the Telupid total shortening and gravity-driven shortening unconformities mark the end of a series of ophiolite and in northwest Sabah, on the Kudat refl ects a southwest to northeast increase in events that occurred prior to the unconformity, Peninsula, have been interpreted as being above the amount of Pliocene to Holocene basement- not the events themselves; even a short orogeny a Late to Middle Eocene unconformity (Cullen, involved shortening, a conclusion that implies lasts ~20 m.y. (Dewey, 2005). Thus, although 2010; Rangin et al., 1990). In addition, Schluter young regional CCW rotation consistent with Late Miocene deformation and basin inver- et al. (1996) identifi ed a Late Eocene uncon- sinistral transpression. sion in southeast Sabah has been attributed to formity throughout the southern South China the Meliau orogeny (Balaguru et al., 2003; Sea where Oligocene limestones overlie Late PREVIOUS PALEOMAGNETIC Balaguru and Hall, 2008), the short-term aspect Eocene marine clastic rocks. Thus, the body of STUDIES of those events suggests that the Meliau orogeny evidence suggests that the Sarawak orogeny was is a continuation of the Sabah orogeny rather of greater regional extent and magnitude than its Since Fuller et al.’s (1999) review of the than a third orogeny. provincial name implies. paleomagnetism of the greater South China Tectonic models for Borneo’s two principal Sea region, no signifi cant regional studies have episodes of orogenesis envision a series of conti- Sabah Orogeny been published. Although Almasco et al. (2000) nental blocks progressively colliding with Sara- presented a thorough analysis of the paleomag- wak, then Sabah, and lastly Palawan (Longley, The Sabah orogeny was fi rst proposed by netic record of Palawan, major aspects of that 1997; Hall, 1996; Murphy, 1998). Two episodes Hutchison (1996) to account for uplift of the work were fi rst discussed by Fuller et al. (1999). of strong deformation are recognized within Crocker Formation onshore and the formation Within that discussion of previous work, we our study area (D1 folds are refolded by D2); offshore of the deep regional unconformity highlight uncertainties and problems, some of these episodes can be interpreted as the records (ca. 15 Ma) and shallow regional unconformity which were noted by Fuller et al. (1999). To of either separate pulses within the continuum of (ca. 9 Ma; Bol and van Hoorn, 1980; Levell, maintain continuity, we retain the same geo- one orogeny or as the products of two distinct 1987). These unconformities are progressively graphic domains and the criterion for site reli- orogenies. The expression of orogenesis on Bor- younger to the northwest and their development ability used by Fuller et al. (1999); reliable α α neo has been strongly infl uenced by its tropical is attributed to basinward tilting of a set of inde- sites have 95 < 20 ( 95 is the 95% confi dence climate; high erosionally driven denudation pendently deforming basement blocks (Levell , ellipse). Rather than displaying separate pie rates have prevented building of topographic 1987). An older (20–22 Ma) unconformity diagrams for strongly rotated, weakly rotated, relief suffi cient to trigger tectonic denudation by identifi ed in southeast Sabah (Balaguru and and nonrotated sites, the data from each domain extensional orogenic collapse and exposure of Nichols, 2004) is in the equivalent stratigraphic are plotted on a single diagram to highlight the a metamorphic core (Hall and Nichols, 2002). position as the top of the Crocker (base of the range of rotational variation (Fig. 3).

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West Kalimantan Northwest Borneo listed as Oligocene–Miocene by Schmidtke et al. (1990). An estimate for the absolute age of Late Mesozoic igneous rocks in the The northwest Borneo domain (Figs. 3 and other sites can also be assigned on the basis of Schwaner Mountains yielded what Haile 4A) refers to the area between the Schwaner the composition of the intrusions sampled: calc- et al. (1977) considered “satisfactory paleo- Mountains and the coastal area around the city alkaline diorites (23.34 Ma) and adakitic micro- magnetic data” from 46 of 48 oriented block of Kuching (Sarawak, Malaysia), where shal- tonalites (11.2 Ma). In addition, red mudrocks samples. Haile et al. (1977), interpreting the low intrusions of the Sintang igneous suite yield of the Eocene Silantek Formation also show data as recording a primary magnetization excellent paleomagnetic data that record a range a range in CCW rotations (Schmidtke et al., acquired when the rocks originally cooled, of CCW rotated sites, as well as two weakly CW 1990). The Silantek Formation is considered determined a Late Cretaceous paleomagnetic rotated sites (Schmidtke et al., 1990; Fuller et al., to have been deposited ca. 40 Ma (Hutchison, pole for West Kalimantan. Although the dec- 1999). The average and maximum rotations for 2010), but may extend into the Early Oligo- linations and inclinations have signifi cant the entire data set are 18.9° CCW and 51° CCW, cene (Hutchison, 2005) and we assign an age scatter, the results indicate CCW rotation respectively. Using the absolute age determi- of 35 Ma, latest Eocene. A plot of age versus with a maximum and average of 78° and 49°, nations from the intrusions at four sites, Fuller rotation (inset, Fig. 4B) shows that the amount respectively (Fig. 3). Negligible latitudinal et al. (1999) interpreted these data as recording of CCW rotation increases with age in the north- differences between the present-day and Late progressively larger CCW rotations of a primary west Borneo domain. For the igneous rocks Cretaceous magnetic poles for West Kali- magnetization in increasingly older intrusions, with CCW rotation the rate is 3.62°/m.y. At the mantan and the Malay Peninsula led Haile 16.4 Ma to 25.8 Ma. Weakly CCW and CW equatorial latitudes of Borneo, this angular rate et al. (1977) to conclude that these areas have rotated sites were considered to be remagnetized implies plate motion of ~4 cm/yr. remained close to their present latitudes and to the present fi eld, but no analytical data were Several aspects of the data in the Kuching have behaved as a unit that rotated CCW ~50° published in support of that interpretation. area merit discussion within the context of tec- since the middle Cretaceous. These conclu- The intrusions near Kuching can be grouped tonic models incorporating 50° CCW of a rigid sions indicate that the Sundaland block has into two categories with respect to composition Borneo between 30 and 10 Ma. behaved as a unifi ed tectonic element during and age (Prouteau et al., 2001): calc-alkaline 1. The Kuching Airport and Kuching Quarry the Cenozoic, and imply that paleomagnetic diorites (22.3–25.8 Ma) and adakitic micro- sites (12.2 ± 0.2 Ma, Prouteau et al., 2001) record sites from this block should have inclinations tonalites (14.6–6.4 Ma). The dates reported by 2.7° CW and 1.1° CCW rotations, respectively. close to the present-day geomagnetic fi eld, Prouteau et al. (2001) are important because Fuller et al. (1999) interpreted these weakly unless they have undergone rotation about a absolute ages can be assigned to the Kuch- rotated sites as being remagnetized to the pres- horizontal axis. ing Quarry and Bukit Stabor sites, which were ent magnetic fi eld. However, these sites record

2 N Pulau Silak 23.7 Ma 40 Gunung Bush 22.3 Ma Kuching Airport B Kuching Quarry Tiram 21.9 Ma dec. 2.7 / inc –2.8 35 Gunung Serapi 12.2 ± 0.3 Ma dec. 359 / inc -2.1 25.8 ± 1.9 Ma 30 dec. 128 / inc –1.2 R² = 0.9935 Bukit Stabar 25 12.9 ± 0.3 Ma

dec. 352 / inc +4 Siburan 20 Sinik Age Ma 15 Gembah o Kuching Igneous dated o o Kuching Igneous esmated 10 o Siburan Kalimantan Igneous dated 17.2 ± 1.9 o 5 o o dec 338 / inc +15 Silantek mudstones Gunung Murong Linear (Kuching Igneous dated) 9.3 ± 0.2. Ma Penkurari 0 14.6 ± 0.4. Ma –60 –50 –40 –30 –20 –10 0 10 Gunung Plandok Gunung Serambu Silantek degrees rotation (negative is CCW) 6.4 ± 0.2. Ma 11.3 ± 0.3. Ma mudtones

Bau Rd 1 N dec 358 / inc +23 30 km Sarawak A 110 E Kalimantan 111 E

Figure 4. Sintang igneous rocks (ﬁ lled polygons) and paleomagnetic sites (open circles) of the Kuching area, northwest Borneo domain (Schmidtke et al., 1990; Prouteau et al., 2001). Inset to upper right plots age versus rotation: black diamonds have absolute age determinations, gray diamonds have ages estimated based of composition of igneous rocks (see text for discussion), squares are sites in Kalimantan (Fuller et al., 1999), gray circles are red beds from the Silantek Formation, and dashed line is regression through the four sites near Kuching having absolute age determinations. CCW—counterclockwise.

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much shallower inclinations (–2.1° and 2.8°) of 42° CCW) than the older Gunung Serapi cover a larger area than the northwest Borneo than that of the present fi eld inclination near site (51° CCW). This suggests that the Silantek domain (Figs. 3 and 5A). Flat-lying Pliocene– Kuching (–20°), which implies rotation about Formation has rotated differentially CCW with Pleistocene basalts yield reliable paleomagnetic horizontal axes. Because the Kuching Airport respect to Gunung Serapi. data from 17 sites at 3 different locations more and Quarry sites appear to be structurally dis- 6. In relation to the Siburan site near Kuching, than 30 km apart that record nonrotated, but turbed, their declination data can also be inter- the dated igneous sites from Kalimantan (Fuller reversed, declinations with an average inclina- preted as recording rotation to near the present et al., 1999) appear to introduce a short-term tion close to that of the present fi eld (Lumadyo fi eld as an alternative to remagnetization. reversal in the sense rotation between 16 and et al., 1993). These locations provide an impor- 2. The Airport and Kuching Quarry sites are 19 Ma (Fig. 4B). tant control point when considering the timing ~10 km from the Bukit Stabor site. If the former The northwest Borneo domain data clearly and sense of rotation in other areas. The Sintang sites are remagnetized, then remagnetization at convey a sense of CCW rotation. In detail, how- suite and older Eocene sites yield rotated and the Airport and Kuching Quarry sites is a local ever, the data suggest that the domain did not nonrotated sites. The average and maximum event that occurred within the past 10 m.y. behave as a single rigid block. The northwest rotations are 15° CCW and 60° CCW, respec- 3. Bukit Stabor (12. 9 ± 0.3 Ma; Prouteau et al., Borneo domain is along a northwest-southeast– tively (Lumadyo et al., 1993; Fuller et al., 2001) records 8° CCW rotation. If CCW rotation striking early Tertiary suture, known as the 1999; Moss et al., 1998). The overall sense and ceased ca. 10 Ma, then the Bukit Stabor site sug- Lupar line (Fig. 1), which represents a poten- amount of rotation are similar to those observed gests that rotation of Borneo ceased abruptly. tial complicating factor when interpreting this in the northwest Borneo domain. The large 4. Gunung Serapi, the oldest site, shows the domain’s paleomagnetic data. We suspect that range of rotations within and between locations most rotation (28.8 Ma, 51° CCW), but has a low some measured rotations in this domain refl ect having similar ages suggests that central and inclination (–1.2°) relative to that of its paleo- localized deformation related to reactivation of eastern Kalimantan did not rotate as a coherent latitude, which should be similar to its present- faults along this suture. Because the sampled rigid block (Fig. 5B). day latitude if the interpretation of the West intrusions are represented by only single sites, In the Telen-Malnyu area, four closely spaced Kalimantan data is correct. The inclination the hypothesis that some of the observed CCW sites from the Sintang intrusions have age at Gunung Serapi suggests rotation about an rotations are related to minor shear zones could determinations that cluster tightly around inclined axis, although Schmidtke et al. (1990) be tested by sampling multiple sites from indi- 23 Ma, yet show rotations that range from made no structural correction. We suggest that vidual intrusions. 16° to 60° CCW (Fuller et al., 1999; Moss at least part of the observed rotation can be et al., 1997). The observed range in rotations attributed to local deformation, as suggested for Central and East Kalimantan between these sites could be attributed to dif- the Kuching Quarry and Kuching Airport sites. fering degrees of remagnetization, as proposed 5. Sites from Late Eocene to Early Oligocene The central and eastern Kalimantan domain by Fuller et al. (1999). The Telen-Malnyu sites (ca. 35 Ma) red beds in the upper member of the is in the Indonesian highlands of Borneo. Paleo- are along the Bengalon fault zone, which is Silantek Formation are less rotated (maximum magnetic data collected by multiple workers described as a complex set of interconnecting

114E 116E 118E 40 B

Telen-Malnyu 2N 30

L. Bagun 20 Naga Ruan Age Ma

10 0 Nakan 0 –60 –50 –40 –30 –20 –10 0 10 Degrees rotation (negative is CCW) A 100 km

Figure 5. (A) Map showing paleomagnetic sites of the central Kalimantan domain (Lumadyo et al., 1993; Moss et al., 1997; Fuller et al., 1999). Location names refer to nearby villages: Nakan (white triangles), Telen-Malnyu (gray triangles), Long (L.) Bagun (black square), Nagu Ruan (white square); gray circles are sedimentary rocks. SFFZ—Sangkulirang fault; BFZ—Bengalon fault zone. (B) Plot of age versus rotation plot with symbols as in Figure 5A. CCW—counterclockwise.

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en echelon normal faults that form the bound- a reasonable paleomagnetic record for both Pala- GEOLOGICAL SUMMARY OF ary between the northern Kutei Basin and the wan blocks. Mesozoic sediments from the north UNITS SAMPLED Mangkalihat Peninsula (Cloke et al., 1999; Palawan block have declinations spread over Moss, 1998). Relatively steep inclinations at 40° CW rotation, but only minimal differences Crocker Formation the Telen-Malnyu sites (18°–45°) are consistent in their inclinations, and are interpreted to have with rotation from normal faulting. We consider been remagnetized prior to opening of the South The Crocker Formation is an assemblage that differential rotation owing to oblique slip China Sea (ca. 32 Ma) and to record differential of deep-water clastic rocks that includes the within the Bengalon fault zone is an equally CW rotations about vertical axes during rifting medium- to coarse-grained sandstones cored valid explanation for the observed range in the (Almasco et al., 2000). The Espina basalts on the for this study. We obtained a suffi cient number declinations at the Telen-Malnyu sites. south Palawan block record rotated 65° CCW and of cores from the Crocker Formation at 11 dif- In the Nakan area, lava fl ows in the Sintang have inclinations suggesting a paleolatitude near ferent locations to cover a wide geographic area suite and Eocene units were sampled at eight the present Celebes Sea (Almasco et al., 2000). (Fig. 2). At several additional locations along locations (Lumadyo et al., 1993); the lava fl ows The Celebes Sea also rotated CCW rotation in older road cuts (such as the Bukit Melinsung sec- in the Nakan area have been dated as 14 and Late Eocene and Early Oligocene time (Shibuya tion of Lambiase et al., 2008), the unit was too 22 Ma (van de Weerd et al., 1987), and we use et al., 1991); this led Almasco et al. (2000) to weathered and argillaceous to collect intact cores. the average of these two dates, 18 Ma, for the conclude that the Celebes Sea and south Palawan There are two contrasting models for the Nakan sites. Sites from the Nakan area data pass block shared a common rotational history. Fuller paleogeographic setting of the Crocker Forma- a regional fold test and display weak CW and et al.’s (1999) suggestion that, although they tion: (1) a basin-fl oor megafan of fi rst-cycle CCW rotations; magnetite is the primary mag- record different paleomagnetic results, the north sandstones derived from the Kalimantan domain netic carrier (Lumadyo et al., 1993). Lumadyo and south Palawan blocks have a common origin, (Crevello, 2001; van Hattum et al., 2006; Jack- et al. (1993) interpreted the data to record a non- is in agreement with Hinz and Schluter’s (1985) son et al., 2009), and (2) multiple, smaller, toe- rotated primary magnetization, whereas Fuller interpretation that Palawan region is underlain by of-slope fans derived largely from older mem- et al. (1999) suggested that the sites record per- stretched continental crust and that both the north bers of the Rajang-Embaluh Group (Lambiase vasive remagnetization. Eocene sandstones from and south Palawan blocks are parts of a microcon- et al., 2008; Cullen, 2010). The Crocker Forma- the Nakan area yield Cretaceous zircons (Moss tinent carried south by South China Sea seafl oor tion has a steep southeast regional dip, but is et al., 1998). Because the onset temperature for spreading. These blocks are linked by a cover deformed locally into large-scale folds. Oppos- annealing zircon (~200 °C) is lower than the sequence of Late Cretaceous to Eocene quartz- ing limbs of individual folds were cored at Curie temperature of magnetite (580 °C), remag- rich turbidites having granitic and acidic volcanic four locations (Bukit Sepanger, Kota Kinabalu netization of the Nakan area must be the result rock fragments that record a South China Sea [KK], Kinabalu Industrial [KI], and Tenom). of a diagenetic (chemical) event rather than a provenance (Suzuki et al., 2000). Therefore, both The Crocker Formation records two major epithermal event. Chemical remagnetization of Palawan blocks appear to share an early tectonic sodes of deformation (Fig. 6) that we attribute igneous rocks can be associated with low-grade history involving very strong CCW rotation fol- to the Sarawak and Sabah orogenies, ca. 40 and metamorphism (Ahmad et al., 2001; Yo-ichero lowed by CW rotation during rifting of the South 20 Ma, respectively. et al., 2000). Thin sections of the Nakan samples, China Sea. The age of the Crocker Formation is poorly however, show little to no alteration to support constrained. At the regional scale, Wilson (1964) the hypothesis of a diagenetic and/or epithermal Sabah assigned an Eocene–Early Miocene age for the event (Lumadyo et al., 1993). The fact the Nakan formation. Recent studies suggest that the out- sites pass a tilt test requires that remagnetization The paleomagnetic record for Sabah is based crops around Kota Kinabalu are Late Eocene predated folding, which is related to Early to on a preliminary study by Schmidtke et al. in age (Lambiase et al., 2008; Cullen, 2010). Middle Miocene inversion in the Kutai Basin (1985) that was included in syntheses by Fuller We note that the strong rotation recorded in (Cloke et al., 1999). Therefore, even if the Nakan et al. (1991, 1999). Although only fi ve loca- the Crocker mudstones has sense and magni- sites have been remagnetized, they should still tions have reliable data and augmenting analyti- tude similar to those recorded in older units at record a component of postfolding CCW rota- cal work was not published, several important Telupid, south Palawan, and the Celebes Sea. tion. Pending analytical evidence to the contrary, observations can be made (Fig. 3). A single site Assigning an Oligocene to Early Miocene age we believe that the Nakan sites represent folded from a Late Miocene satellite stock (Kappa for the red mudrocks of the Crocker Formation nonrotated primary magnetization. Quarry) of the Kinabalu pluton near the city of (ca. 25 Ma) implies rotation rate of 5.1°/m.y., Kota Kinabalu shows weak (11°) CCW rotation considerably faster that the rate calculated for Palawan and Celebes Sea (Fuller et al., 1999). Near the village of Telu- the northwest Borneo domain, 3.62°/m.y. Thus, pid in central Sabah, Cretaceous cherts from the paleomagnetic data are consistent with the Palawan, located north of Sabah and west of an ophiolitic assemblage record very strong Late Eocene age for the Crocker Formation in the main Philippine archipelago, consists of two (79°) CCW rotation; although not classifi ed as the area of Kota Kinabalu. blocks of contrasting geology juxtaposed across reliable, associated spillites also record strong the Ulugan Bay fault (Figs. 1 and 3). Late Paleo- (47°) CCW rotation. Along the coast near Kota Kudat Formation zoic and Mesozoic sedimentary rocks compose Kinabalu, three sites from marine red mudstones much of the north Palawan block, whereas the in the Crocker Formation record strong (average The Early Miocene Kudat Formation crops south Palawan block is dominated by the south 77°) CCW rotation. Although no specifi c data out on the Kudat Peninsula within a series of Palawan ophiolite of Late Cretaceous–Eocene were presented, Fuller et al. (1999) commented northwest-southeast–striking thrust sheets age (Raschka et al., 1985; Almasco et al., 2000). that the associated Crocker sandstones are (Tongkul, 1994, 2006). Near Kota Belud at Although only 38 of 147 paleomagnetic sites are mostly remagnetized close to the present fi eld, the southwest end of the peninsula, where the α rated reliable (e.g., 95 < 20°), those sites provide and that some sites have minor CW rotation. Crocker Formation has been thrust over

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/5/1146/3342918/1146.pdf by guest on 30 September 2021 Cullen et al. rmation of Maju record a record l syncline. 3 m 2 m f f D1 fold axes D1 fold D1 syncline axis D2-recumbent anticlinal fold anticlinal axis D2-recumbent Figure 6. Photographs of the Crocker Formation in the Kota Kinabalu area. (A) Maju East. (B) Bandar Sierra, 3 km east-southeast (A) Maju East. (B) Bandar Formation in the Kota Kinabalu area. 6. Photographs of the Crocker Figure defo At least two episodes of compressive point stratigraphically upward. White lines highlight bedding and yellow arrows East. folded (D2) to form an antiforma At Maju East, a D1 syncline is recumbently Formation. in the Crocker expressed (D1 and D2) are At Bandar Sierra, moderately plunging small folds (D1) have been cut by a younger thrust fault (D2). High-angle faults (f) may Sierra, moderately plunging small folds (D1) have been cut by a younger At Bandar third deformational event. B Maju East Location A

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the Kudat Formation , the structural grain road, traversing the Crocker Ranges, is domi- a regression at the top of the Temburong For- changes nearly 90°. Despite poor outcrop expo- nated by outcrops of Temburong mudrocks. mation related to the proto–Champion delta sures on the Kudat Peninsula, we cored four Exposures of the West Crocker Formation along (Sandal, 1996). sites, including opposing limbs of an anticline. the coastal highway are too deeply weathered to The site statistics are poor (Table 1). Although extract intact cores. The Kampong Mua loca- SAMPLING AND ANALYTICAL these data are included primarily for the pos- tion is our only data point for the West Crocker METHODS sible interest of others, we stress that these data Formation. indicate 45° CCW rotation of a postfolding Our sampling program comprises 42 sites characteristic remanent magnetization (ChRM) Meligan Sandstone from 15 locations that form a northeast-south- carried by pyrrhotite. west transect along the western coast of Sabah The Early Miocene Meligan sandstone (Fig. 2) that is favorably oriented to cross pos- West Crocker Formation crops out mainly in Brunei and Sarawak and sible northwest-southeast shear zones. Although consists of cross-stratifi ed, well-sorted, coarse the Crocker Formation was the principal unit The West Crocker Formation sandstones are to medium quartz arenites that record deposi- sampled (12 locations), the end points of this fi ne- to medium-grained arenites that represent tion in shoreface to shallow-marine environ- transect included locations from the Kudat For- the coarser grained lateral equivalents of the ments (Sandel, 1996; Hutchison, 2005). The mation (4 sites), the West Crocker Formation Temburong Formation shales (Wilson, 1964). Meligan sandstone is preferentially preserved (3 sites), and Meligan sandstone (2 sites). Most Although regional structural dip is steep to the in deeply eroded synclines and its contact with locations have multiple sites and each site con- southeast, the Temburong–West Crocker strata the Temburong is locally unconformable. The sists of ~8 oriented 2.5-cm-diameter drill cores. are complexly folded. The Sipitang-Tenom Meligan sandstone is considered to document At fi ve locations the exposures were suitable

TABLE 1. SITE MEANS α Lat Long Dec Inc 95 Tilt dec Tilt inc Tmax Site Formation (°N) (°E) Strike Dip (°) (°) In situ K (°) (°) (°C) N/N0 BL1 Crocker 6.2918 116.3588 50 90 NC NC NC NC NC NC NC 0 TS1 Crocker 6.1680 116.2920 59 70 37.5 16.6 11.1 126 55.4 43.4 325 3/6 JS1 Crocker 6.0381 116.1417 199 94 15.6 –0.4 19.3 17 19.6 –3.4 420 5/6 LK1 Crocker 5.8339 116.0433 5 71 19 13.9 14.8 40 22.9 –8.3 340 4/5 NX1 Crocker 6.1048 116.1341 55 85 10.9 13 7.9 59 68.3 44 340 7/8 NX2 Crocker 6.1150 116.1170 47 66 11.4 2.5 9.2 54.0 33.4 33.3 380 6/6 KI1 Crocker 6.0829 116.1600 35 100 16.7 5 16.4 23 11.6 3 420 4/6 KI2 Crocker 6.0805 116.1576 237 88 0.4 –5.7 8.9 106 64.3 –56.4 460 4/6 KI3 Crocker 6.0805 116.1576 237 88 31.2 –0.6 13.9 44 56.7 –25.8 375 4/6 BS1 Crocker 6.0670 116.1549 35 58 3.5 13.9 15.5 25 30.5 33.9 320 5/8 BS2 Crocker 6.0670 116.1549 35 58 9.7 5.5 14.3 42 25.9 24.3 325 4/7 BS3 Crocker 6.0670 116.1549 27 66 19.4 9.2 10.7 75 32.4 10.6 350 4/7 BS4 Crocker 6.0670 116.1549 30 58 7.4 17.6 8.7 825 34.1 28.1 310 2/5 BS5 Crocker 6.0670 116.1549 30 58 8.8 –2.8 15.4 37 16 16.3 350 4/4 BS6 Crocker 6.0670 116.1549 30 58 3.1 2.6 15.3 37 17.3 24 320 4/4 BS7 Crocker 6.0667 116.1563 31 86 11.8 18.4 18.4 26 49.1 19.5 330 4/5 BS8 Crocker 6.0667 116.1563 31 86 3.2 16.3 11.5 65 47.3 27.8 325 4/5 BS9 Crocker 6.0667 116.1563 46 129 24 –9.5 18.9 17 52.5 23 325 4/6 BS10 Crocker 6.0667 116.1563 44 133 12.8 –9.1 12.5 55 59.4 28.8 280 5/7 BS11 Crocker 6.0667 116.1563 45 124 30 –11.4 15.4 26 43.7 18.7 325 4/6 ME1 Crocker 6.0454 116.0937 75 130 257 –32.8 12.1 104 280.2 21.8 325 4/6 ME2 Crocker 6.0454 116.0937 75 135 246 –23.7 0.5 990 278.1 10.5 345 4/6 KK1 Crocker 6.0498 116.1613 270 58 15.5 –17.2 13.5 345 47.1 –69.6 450 2/4 KK2 Crocker 6.0498 116.1613 265 71 359.9 –22.2 39.5 42 119.9 –84.5 340 2/4 KK3 Crocker 6.0496 116.1616 19 71 9.7 –6.2 34.4 –3 10.1 6.7 400 2/4 KK4 Crocker 6.0496 116.1616 19 71 0.4 13 26.8 22 25.9 21.5 400 4/4 KG1 Crocker 5.4381 116.1061 25 45 19.2 13.1 16.3 18 30.3 13.3 420 6/8 KG2 Crocker 5.4381 116.1061 25 45 6.2 22.3 16 24 28.8 28.6 420 5/8 KGV Crocker 5.3210 116.1386 323 27 333.7 18.2 16.2 239 340.8 11.4 400 5/6 TN1 Crocker 4.9940 115.8699 305 22 327.6 24 11.9 444 334.5 14.2 340 2/6 TN2 Crocker 4.9940 115.8699 305 22 345.7 0 15.9 61 343.6 –14.1 330 3/6 TN3 Crocker 5.0039 115.8439 65 43 352.6 8.5 45.1 9 1.9 48.7 325 3/5 TN4 Crocker 5.0039 115.8439 65 43 321 –6.2 13.3 87 321.1 2.5 420 3/7 KM1 West Crocker 4.9809 115.6985 255 6 NC NC NC NC NC NC NC 0/3 KM2 West Crocker 4.9992 115.6782 165 84 NC NC NC NC NC NC NC 0.3 KM3 West Crocker 4.9992 115.6782 0 55 NC NC NC NC NC NC NC 0*/3 SP1 Meligan 5.0027 115.5441 70 35 NC NC NC NC NC NC NC 0/2 SP2 Meligan 5.0389 115.5206 200 85 NC NC NC NC NC NC NC 0/3 KD1 Kudat 7.0386 116.7491 323 29 297.1 39 21.5 19 323.7 45.6 440 4/8 KD2 Kudat 6.9062 116.7251 100 70 359.2 4.2 8.3 87 334.9 71 300 5/9 KD3 Kudat 6.9157 116.7624 285 65 341.5 –4.7 20.7 36 322.7 –45.9 300 3/7 KD4 Kudat 6.8629 116.6999 220 64 292.9 34.1 36.6 12 294 –27.7 450 3/6 PGMF KK 5.5736 116.0408 1.255 –4.35 Note: All sites are sandstones. Dips >90° are overturned beds. NC—No characteristic remanent magnetization signal. PGMF—Present-day geomagnetic magnetic field (100 yr average). Calculated using National Oceanic and Atmospheric Administration National Geophysical Data Center web site (www.ngdc.noaa.gov/). Dec— α declination; Inc—inclination; K—a measure of grouping; 95 is the 95% cone of confidence. Tilt dec and Tilt inc are values after structural correction. Tmax—the maximum temperature that a linear component could be identified on the orthogonal plots. N/N0—the number of specimens with direction versus the number of demagnetized specimens. TN—Tenom, KK—Kota Kinabalu.

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to confi dently sample opposed anticlinal limbs netized in 35 steps to 700 °C; 36 additional maximum angular deviation of <15° using Fisher to conduct tilt tests. To minimize the effects of specimens were demagnetized using alternating (1953) statistics. For sites from the limbs of sin- tropical weathering and oxidation, our sampling fi eld (AF) techniques in 12 steps to 100 mT. The gle folds, the site means from both limbs were program targeted fresh exposures from new AF demagnetized samples tended to have non- subjected to a fold test following the method of housing and road construction. linear decay of NRM, making it diffi cult to iso- Enkin (2003). This fold test method determines Subsets of cores from each site were cut late any stable remanent magnetization; there- the timing of ChRM acquisition relative fold- into standard paleomagnetic specimens (2.5 × fore only thermally demagnetized specimens ing and potentially provides critical constraints 2.3 cm) and their natural remanent magnetiza- were subsequently analyzed. When subjected to for the relative timing of deformation. Sites are tions (NRM) was measured using a 3-axis 2G thermal demagnetization, more than 98% of the deemed to pass a fold test if the site means Enterprises DC-SQUID cryogenic magnetom- specimens from the Crocker and Kudat Forma- are more tightly clustered after correcting for the eter housed in a magnetically shielded space at tions showed linear decay of NRM (Fig. 7), and structural tilt; they fail a fold test if the site means the University of Oklahoma. To isolate possible their ChRM was isolated using the least squares are better grouped in situ and become more scat- primary and secondary NRM components, our method of Kirschvink (1980). tered as the result of structural correction. Based specimens were subjected to stepwise demagne- Site mean ChRMs were obtained by averaging on the outcome of the fold test, a location ChRM tization: 174 specimens were thermally demag- individual core specimen mean ChRMs with a direction was obtained by averaging.

W, Up W, Up

NRM 200°C NRM 280°C 420°C 300°C S, S N, N S, S N, N 420°C 0.1 mA/m 280°C 0.1 mA/m 200°C 340°C NRM 300°C NRM 200°C A B NX1 TN4 E, Down E, Down W, Up

NRM W, Up

400°C 340°C 0.1 mA/m NRM S, S N, N S, S N, N 340°C NRM 0.1 mA/m C NRM KM2

E, Down E, Down D SP1

Figure 7. Orthogonal vector plots of thermally demagnetized specimens from the Crocker Formation. (A) Nexus (NX1). (B) Tenom (TN4). (C) West Crocker Formation (Kampong Mua, KM2). (D) Meligan sandstone (Sipitang location specimen-1, SP1). A and B show maximum laboratory unblocking temperatures of 340 °C and 420 °C for isolation of the characteristic remanent magnetization (ChRM), suggesting that pyrrhotite and magnetite are the magnetic remanence carriers, respectively. C and D show absence of a ChRM signal. White symbols—vertical component; black symbols—horizontal component; NRM—natural remanent magnetization.

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Following the convention used by Fuller et al. The TS car wash location (TS), a fresh quarry Keningau and Tenom locations (Fig. 2). The α (1999), only those site means with 95 < 20° are 30 km north of Kota Kinabalu, yielded a single maximum unblocking temperatures for speci- deemed reliable. Dispersion caused by paleo- site. Stable ChRM directions with maximum mens from sites JS1 (420 °C) and LK1 (340 °C) secular variation of the geomagnetic fi eld is an unblocking temperatures of 325 °C suggest indicate that magnetite and pyrrhotite are the issue that should be addressed when assessing that pyrrhotite is the magnetic remanence car- respective chief magnetic remanence carriers if a declination could due to tectonic rotation. rier (Table 1). The in situ location mean has dec (Table 1). The Jalan Salaiman in situ location Secular variation is at a minimum at low lati- 37.5° and inc 16.6°; respective tilt-corrected val- mean has dec 15.6° and inc –0.4°; respective tudes and multiple sites were collected over a ues are 55.5° and 43.4° (Fig. 8A). tilt-corrected values are 19.6° and –3.4° (Fig. large area to average the secular variation. In Jalan Salaiman (JS) and Lok Kawi (LK) are 8B). The Lok Kawi in situ location mean has addition, if the rocks contain secondary magne- single site locations; Jalan Salaiman is within dec 19.9° and inc 13.9°; respective tilt-corrected tization, remagnetization processes commonly Kota Kinabalu and Lok Kawi is 15 km south values are 22.9° and –8.3° (Fig. 8C). take a long enough time to average secular of Kota Kinabalu and occupies an intermedi- The Nexus (NX) location is the westernmost variations. If the ChRM is primary and the age ate geographic position relative to the southern of four locations (NX, KI, Bukit Sepanger [BS], of the rock unit is known, then the calculated directions give an estimate of the paleolatitude at acquisition. If the ChRM is secondary, then N N the age of remagnetization is critical for interpreting the structural history of that unit. Selected core specimens (8) representative of α = 11.1 the different rock formations were subjected to 95 α95 = 11.1 saturation isothermal remanent magnetization (SIRM) tests, determining the dominant mag- WE netic minerals in each formation. An isothermal remanent magnetization (IRM) acquisition TS TS in situ tilt corrected curve for each specimen was obtained by pulse Specimen Mean ChRMs Specimen Mean ChRMs magnetization in 26 steps to 2500 mT. Because magnetic minerals (e.g., magnetite, pyrrhotite, A and hematite) acquire IRM at different rates and S S saturate over a range of specifi c fi eld intensities, N N IRM acquisition curves provide information on the presence and relative amounts of the magnetic minerals in a specimen (Kruiver et al., α = 19.3 α95 = 19.3 95 2001; Heslop et al., 2002, 2004). Analysis of the IRM acquisition curves was conducted using the IRMUNMIX 2.2 software program (Heslop W W et al., 2002) and cumulative log-Gaussian (CLG)

analysis was performed using the IRM-CLG 1.0 JS JS (Kruiver et al., 2001) to model the different coer- in situ tilt corrected Specimen Mean ChRMs civity contributions (Heslop et al., 2004). Specimen Mean ChRMs

LOCATION RESULTS B N N The site statistics are tabulated in Table 1. The majority of our locations are within 10 km of α = 14.8 α = 14.8 Kota Kinabalu, where the averaged magnetic 95 95 fi eld for the past 110 years is declination (dec) 1.25°E, inclination (inc) –4.35° (Table 1). As previously noted, fl at-lying Pliocene–Pleisto- W W cene basalts in central Kalimantan have similar small (reversed) declinations and low negative LK LK inclinations. Therefore, in discussion of our in situ tilt corrected Specimen Mean ChRMs Specimen Mean ChRMs data, the present-day magnetic fi eld is used the reference fi eld. C S S Crocker Formation Figure 8. Stereoplots showing in situ and tilt-corrected specimen The Belud (BL) location, a weathered road means and location mean characteristic remanent magnetizations α cut 50 km north of Kota Kinabalu, carried no (ChRMs) and 95 values. (A) TS car wash. (B) Jalan Salaiman (JS). stable remanence. This was the only such nega- (C) Lok Kawi (LK). Filled circles—positive inclinations; open tive result from the Crocker Formation. circles—negative inclinations; square—location mean.

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and KK) that traverse two folds 15 km north of 10A). The location comprises 3 sites with carrier(s), Crevello (2001) reported that the red α Kota Kinabalu (Fig. 9A). The NX specimens acceptable 95 values. Unblocking temperatures mudrock facies is hematitic. We suspect that show a linear decay indicating a single compo- ranged from 460 to 375 °C, suggesting that hematite is the dominant magnetic mineral and nent of magnetization having a northern ChRM pyrrhotite and magnetite are the magnetic rema- the ChRM in the red mudrocks represents a pre- (Fig. 7A); maximum temperature (Tmax) and nence carriers (Table 1). The KI in situ location folding, possibly primary, signal. IRM data indicate that pyrrhotite is the domi- mean has dec 16.1° and inc –0.4°; respective The Bukit Sepanger (BS) location consists nant ChRM carrier (Tables 1 and 2). The NX tilt-corrected values are 53.2° and –23.3° (Fig. of 11 sites where more than 500 m of continu- α sites have 95 < 10° (Table 1). The in situ site 9C). The in situ location mean has a signifi - ous stratigraphic succession is exposed (Fig. α means have dec 10.9° and 11.2°, and inc 13° cantly lower 95, indicating that ChRM acquisi- 10A). All sites have stable ChRM directions. and 2.5°. The tilt-corrected site means have dec tion postdates folding at this location. The red The tight cluster of unblocking temperatures 68.3° and 33.4° and inc 44° and 33.3° (Table 1). mudrocks exposed in the overturned limb of about a mean of 323 °C (Table 1) indicates that α The 95 of the in situ location mean is signifi - the KI anticline (Fig. 9A) represent the same pyrrhotite is the magnetic remanence carrier. cantly lower than that for the tilt-corrected loca- Crocker Formation lithology that records 77° The respective in situ and tilt-corrected Bukit tion mean, showing that the ChRM was acquired CCW rotation (Fuller et al., 1999). Although Sepanger location mean ChRM directions are after tilting (Fig. 9B). we were unable to recover intact core speci- dec 12.8°, inc 4.6° and dec 35.5°, inc 17.2°. α The KI location is an anticlinal fold with a mens of the red mudrocks at the KI location and The 95 of the in situ location mean, 8.0, is sig- steeply dipping overturned eastern limb (Fig. could not determine chief magnetic remanence nifi cantly lower than that for the tilt-corrected

Unit with red mudrocks BS 1-12 KK 1-4 NX 3,4 KI 2, 3 NX 1,2, KI 1,21 KI 3,4 Ocean

Nexus Member Maju Member

NW 5 2km km SE

N N NN

α α 95 = 23.9 95 = 68.0 α α 95 = 25.3 95 = 64.7

W E W E

NX NX KI KI in situ tilt corrected in situ tilt corrected Site Mean ChRMs Site Mean ChRMs Site Mean ChRMs Site Mean ChRMs B C

SS SS

Figure 9. (A) Cross section through the NX-KI-BS-KK (see Fig. 2 for abbreviations) locations showing KI and KK anticlines used for fold tests; no vertical exaggeration. Inset photo shows overturned red mudrocks that record strong counterclockwise rotation. (B) NX in situ and α tilt-corrected specimen mean and location mean characteristic remanent magnetizations (ChRMs) and 95 values. (C) KI values.

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TABLE 2. MINERAL MAGNETIC DATA of the Crocker Range (Fig. 2). This ridge, as Contribution Magnetic well as a similar feature between Keningau and Formation Site SIRM B DP (%) mineral 1/2 Tambunum, is interesting from a regional struc- Crocker BS4 1.30 45.7 0.34 57.0 Magnetite 0.74 56.2 0.12 33.0 Pyrrhotite tural perspective because the northwest strike 0.22 562.3 0.34 10.0 Hematite of the Crocker Formation is nearly orthogonal NX2 1.00 38.2 0.23 39.5 Magnetite the main Crocker Range and the Keningau Pass 1.30 67.6 0.16 43.7 Pyrrhotite 0.29 939.7 0.37 11.3 Hematite location. The results from the Keningau Valley KK1 0.31 27.4 0.42 24.3 Magnetite location are reliable, and maximum unblocking 0.94 63.5 0.23 72.7 Pyrrhotite temperatures (400 °C) indicate that magnetite 0.04 929.0 0.18 3.0 Hematite KG1 4.73 46.7 0.31 67.6 Magnetite and/or pyrrhotite are magnetic remanence car- 2.30 127.4 0.42 32.4 Pyrrhotite riers (Table 1). TN1 1.20 41.5 0.28 45.8 Magnetite The in situ location mean has dec 337.5° 1.20 68.9 0.20 46.2 Pyrrhotite and inc 18.2°; respective tilt-corrected values 0.20 549.5 0.59 8.0 Hematite West Crocker are 340.8° and 11.4° (Table 1; Fig. 12B). In the Temburong KM1 1.10 55.0 0.42 6.3 Pyrrhotite absence of multiple sites for a structural correc- 16.00 1905.5 0.23 93.7 Hematite tion, we prefer interpreting the CCW rotation Meligan SP1 1.10 55.0 0.31 100.0 Pyrrhotite at Keningau Valley as recording post-ChRM acquisition deformation similar to our other Kudat KD4 1.40 57.7 0.23 1.9 Pyrrhotite locations. 70.0 891.3 0.38 98.1 Hematite The Tenom (TN) location consists of four –3 Note: SIRM—saturation isothermal remanent magnetization in 10 A/m; B1/2 is the mean remanence coercivity (in mT); DP is the half-width of the distribution (in mT). These mineral sites near the crest of the Tenom Pass in a road magnetic parameters were obtained from measured IRM acquisition curves, following Kruiver et al. recently cut through the Crocker Range between (2001). Site abbreviations are those used in text. Sipitang and Tenom. Sites TN1 and TN2 are from east-dipping limb and sites TN3 and TN4 are from the steeply west dipping forelimb of location mean, 14.6 (Fig. 10B), which indicates small thrusts observed at the location (Fig. 10C) an asymmetric anticline, which is locally over- that the ChRM was acquired after tilting. Dur- to result from footwall deformation beneath the turned (Fig. 13A). Of the four Tenom sites, three ing progressive untilting, optimal grouping for younger thrust fault. are deemed reliable. IRM data and maximum the location mean occurred at 50% untilting, The Kota Kinabalu (KK) location consists of unblocking temperatures ranging from 310 to which suggests that ChRM acquisition was four sites from opposed limbs at the crest of a 420 °C indicate that pyrrhotite and/or magnetite α late synfolding to postfolding. At the southeast small upright anticline. The 95 values for the are the magnetic remanence carriers (Tables 1 end of this location, the Crocker Formation is site means from this location are poor relative to and 2). The respective in situ and tilt-corrected locally overturned, forming a small kink fold the other locations along the NX-KK transect, Tenom location mean ChRM directions are (Fig. 10C). Sites BS7–BS11 are from a single a circumstance we attribute to a combination dec 331.4°, inc 5.9 and dec 333.1°, inc 0.9. The α bed in both the upright and overturned limbs of of localized crestal faulting and late-stage fl uid 95 of the in situ location mean is signifi cantly that kink fold. The ChRMs from the overturned fl ow, as indicated by mild hematite staining lower than that for the tilt-corrected location limb (BS9–BS11) have negative inclinations, (Fig. 11A). For the only site deemed reliable, mean (Fig. 13B). A fold test using the 3 reliable whereas sites from the upright limb (BS7–BS9) KK1, the in situ and tilt-corrected site mean Tenom sites gave the best grouping of data at have positive inclination. Because the sites are ChRM directions are dec 15.5°, inc –17.2 and 32.1% ± 14.0% untilting, indicating that mag- within a single bed, the simplest explanation dec 47.1°, inc 69.6° (Table 1). We note that netization was acquired late synfolding to post- α for these opposed inclinations is that the kink 95 for in situ location mean is lower than that folding. fold postdates ChRM acquisition. Restoring the for the tilt-corrected location mean (Fig. 11B), overturned limb to the 86° southeast dip it had which is consistent with the postfolding ChRM West Crocker Formation prior to development of kink fold results in posi- acquisition from other limbs of the NX-KI- tive inclinations for all 5 sites (Fig. 10C), which BS-KK folds. The Kampong Mua (KM) location consists indicates that the kink fold postdates ChRM The Keningau Pass (KG) location consists of of three sites from a compact anticline along acquisition. two closely spaced sites from southeast-dipping the Sipitang-Tenom road (Figs. 2 and 13A). To test our kink-fold hypothesis, we recal- outcrops exposed during construction of the Although the outcrop exposure was fresh and culated the in situ location mean for the Bukit Papar-Keningau road over the Crocker Range competent to core, none of the specimens ana- Sepanger location after making a structural (Fig. 2). The Keningau Pass site means are reli- lyzed gave stable magnetic remanence when correction for the kink fold, restoring it to it a able, and unblocking temperatures of ~420 °C thermally demagnetized (Fig. 7C). The IRM prekink dip of 86° southeast. This test reduced suggest that magnetite is the dominant magnetic data indicate that hematite is present (Table 2). α the 95 value from 8.6 to 5.9 (Fig. 10D) and sup- remanence carrier (Table 1). The in situ site These results contrast with the Crocker Forma- ports the hypothesis that kink folding postdates means have dec 19.2° and 6.2° and inc 13.1° tion, which is dominated by pyrrhotite and mag- ChRM acquisition. Establishing the age of the and 22.3°. The tilt-corrected site means have netite in its magnetic mineralogy. Considering kink fold offers a way place a lower (younger) dec 30.3° and 28.8° and inc 13.3° and 28.6° the fresh nature of this road cut location, the lack age limit for timing ChRM acquisition. The (Table 1; Fig. 12A). of a preferred magnetic orientation suggests Bukit Sepanger sites are within the footwall of The Keningau Valley (KGV) location con- that the hematite refl ects primary deposition a thrust fault that postdates tilting (Lambiase sists of a single site from a ridge separating the as a heavy mineral rather than in situ oxidation et al., 2008). We interpret the kink fold and Keningau and Tenom Valleys on the eastern side and/or weathering.

Geosphere, October 2012 1159

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/5/1146/3342918/1146.pdf by guest on 30 September 2021 Cullen et al. +18.4 –9.5 +16.3 –9.1 –11.4 +9.4 +9.2 +15.4 Inclination SE SE small ocation mean nclinations of Overturned limb on (Fig.10C) fold secondary 11 10 7 1 m 9 8 values after the overturned limb was restored to its pre-kink to its pre-kink the overturned limb was restored values after 95 α NW C 50 m E = 5.9 95 α N S BS BS in situ = 14.6 tilt-corrected 95 α Site Mean ChRMs in the fold (sites 9–11) after correcting for kink = 8.0 95 = 8.0 α 95 α BS in situ S NN NW southeast dip. the overturned limb. (D) Stereo plot for the BS location ChRM mean and plot for the overturned limb. (D) Stereo white circles show sites BS7–BS11. Posted to the right are inclinations; values with solid arrows are the post-kink corrected i the post-kink corrected are inclinations; values with solid arrows Posted to the right are show sites BS7–BS11. white circles characteristic remanent magnetizations (ChRMs) of the BS location. (C) Outcrop photo of overturned kink fold at Bukit Sepanger; magnetizations (ChRMs) of the BS location. (C) Outcrop characteristic remanent Figure 10. (A) Outcrop photo of Bukit Sepanger (BS) section. (B) Stereoplots showing in situ and tilt-corrected site mean and l showing in situ and tilt-corrected (BS) section. (B) Stereoplots photo of Bukit Sepanger 10. (A) Outcrop Figure BS SS N A in situ Site Mean ChRMs Site Mean ChRMs D B W W

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NW SE

α α = 180.0 95 = 58.2 95

W + + E

KK KK In situ tilt corrected Site Mean ChRMs Site Mean ChRMs B S S

Figure 11. (A) Outcrop photo of crest of Kota Kinabalu (KK) location anticline; circles are site locations. (B) In situ and tilt-corrected speci- α men and location means, characteristic remanent magnetizations (ChRMs), and 95 values for the KK anticline location.

Meligan Sandstone DISCUSSION drawn from our observations, which if answered should allow more rigorous tectonic models to The Sipitang (SP1, SP2) locations do not This study represents the most comprehen- be advanced. carry stable magnetic remanence when ther- sive paleomagnetic sampling program under- mally demagnetized (Fig. 7D). The IRM sig- taken in Sabah to date; 11 of 15 locations along Pervasive Remagnetization of the nal from the Meligan sandstones shows that a 250 km northeast-southwest transect have Crocker Sandstones pyrrhotite is present (Table 2); we interpret as reliable site means. Our results present us with a primary accessory heavy mineral. At the SP2 four straightforward observations. We believe, Locations having reliable ChRM site means location, the Meligan was so tightly cemented however, that the paleomagnetic database is recorded declinations and inclinations that are that it was necessary to collect an oriented block not yet suffi cient to draw tightly constrained distinctly different from the present-day geo- sample for coring with a drill press in the labora- conclusions integrated within a regional tec- magnetic fi eld (Table 1). For example, the mean tory; this suggests that it has not been affected tonic framework. Therefore, we direct our dis- ChRM direction derived from the 19 reliable by surfi cial oxidation. cussion toward framing testable hypotheses sites around Kota Kinabalu (dec 13.39°, inc

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N N 3.22°) records a relative CW rotation with a positive rather than negative inclination. Our data demonstrate that the Crocker Formation sandstones record an older remagnetization event, rather than carrying the signal of the α95 = 16.3 α95 = 16.0 present-day fi eld as was suggested by Fuller et al. (1999; commenting on their unpublished data). The widespread geographic coverage of + E E our locations suggests that remagnetization was a regional event. Apatite fi ssion track and KG 1,2 KGV vitrinite refl ectance data indicate that maximum In situ In situ burial temperatures for the Crocker Formation site mean ChRMs Specimen means were ~150–200 °C (Hutchison et al., 2000; Anuar et al., 2003). In thin section, the Crocker Formation sandstones lack quartz overgrowths

A B and sutured grain contacts that would indicate S S relatively deep and hot maximum burial condi- Figure 12. Stereoplots showing in situ and tilt-corrected specimen mean characteristic tions. These paleotemperature indicators sug- α gest that the burial temperatures never exceeded remanent magnetizations (ChRMs) and 95 values. (A) Keningau Pass (KG) location. (B) Keningau Valley (KGV) location. the Curie temperature for either pyrrhotite or

WNW TN3-4 TN-1 ESW Tenom SP1 Fault SP2 WCR Tmbr MLG KM 1-3

Cr Cr

A 10 km VE 2X

α95 = 22.9 α95 = 38.0

W E

TN TN in situ tilt corrected Site Mean ChRMs (α95 < 20) Site Mean ChRMs (α95 < 20)

B S S

Figure 13. (A) Cross section from Sipitang (SP) to Tenom (TN) (modiﬁ ed from Wilson, 1964; Cullen, 2010) through the SP, Kampong Mua (KM), and TN locations. (B) Stereoplots show in situ and tilt-corrected site mean characteristic remanent magnetizations (ChRMs) and α 95 values for the TN location. Disharmonic folding in the Temburong Formation is schematically depicted, but is in accordance with ﬁ eld observations.

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magnetite. Crocker sandstones showed a single red mudrocks preserves an older, probably neo domain). Remagnetization of the Crocker linear decay trend during thermal demagnetiza- primary, magnetization that was not reset by Formation sandstones generally postdates tion, indicating a single event (Fig. 7). expelled foreland basin fl uids owing to the folding, although tilt tests from the Tenom and The SIRM data from the Crocker sandstones impermeable nature of the Crocker Formation Bukit Sepanger locations suggest remagnetiza- show that both pyrrhotite and magnetite are mudrocks. If so, the amount of CCW rotation tion could be a late synfolding. Working with present (Table 2) and the range in Tmax (280– in the mudrocks is even greater than reported a relatively small, but extremely well exposed 450 °C) suggests that both phases may carry the when corrected for the younger CW rotation outcrop at Bukit Sepanger, Lambiase et al. ChRM (Table 1). In considering these observa- recorded in the Crocker sandstones. The strong (2008) documented two episodes of folding. tions in relation to time-temperature remag- CCW rotations observed in Eocene and older The earliest episode, D1, records syndeposi- netization curves for magnetite and pyrrhotite units in south Palawan, the Celebes Sea, and tional folding in an active deep-water fold-thrust (Dunlop et al., 2000), we favor interpreting the central Sabah (Fig. 3) lead us to interpret the belt during generation of the large-scale folds ChRM in the Crocker Formation sandstones, paleomagnetic data from the Crocker Formation of the NX-KK cross section (Fig. 9A). During particularly for pyrrhotite, as a chemical (dia- as indirectly supportive of an Eocene age for the the second episode of deformation (D2), the genetic) rather than thermal event. A number of Crocker Formation. D1 folds were recumbently. The east-dipping chemical mechanisms have been proposed for limb of our Bukit Sepanger sites has been inter- the origin of the ChRMs that reside in pyrrhotite No ChRM Signal in the West Crocker preted as being in the footwall of a younger (D2) and magnetite in sedimentary rocks; pyrrho- and Meligan Sandstones thrust fault (Lambiase et al., 2008). Strong D2 tite mechanisms include authigenesis caused deformation cutting across earlier D1 structures by preexisting magnetite reacting with pyrite In contrast to the Crocker Formation, nei- is also observed at Bandar Sierra, 3.5 km east during burial metamorphism under reducing ther the West Crocker nor Meligan sandstones of Bukit Sepanger (Fig. 6B). In Cullen (2010), conditions (Gillett, 2003), oxidation of pyrite yielded a ChRM signal, even though the IRM this earlier phase of deformation was attributed (Salmon et al., 1988), and thermochemical data indicate that hematite and pyrrhotite, to the Late Eocene to Early Oligocene Sara- sulfate reduction (Peirce et al., 1998). These respectively, are present. The Kampong Mua wak orogeny. The structural relations at Bukit mechanisms can be related to such processes and Sipitang samples are from fresh outcrops Sepanger offer the possibility for establishing as migration of hydrocarbons (Machel and Bur- that lacked the argillaceous weathering profi le the relative timing of remagnetization, if the ton, 1991), diagenesis of gas hydrates (Housen that characterized the Belud location, the only ChRM signal was acquired prior to the second and Musgrave, 1996), and release of pore fl uids other location that lacked a ChRM signal. We episode of deformation and if the age of the sec- (Urbat et al., 2000). Late-stage remagnetization tentatively interpret the West Crocker and Meli- ond deformation can be established. by the diagenetic formation of sulfi de-bearing gan sites as never having acquired a ChRM sig- As shown earlier, restoration of the over- minerals, such as pyrrhotite, is documented nal, rather than as having acquired such a signal turned limb of the kink fold at Bukit Sepanger on the island of Sakhalin and may be related and subsequently having it obliterated by later improved the pre-tilt location mean (Fig. 10D) to migration of hydrocarbons (Weaver et al., events such as weathering. We acknowledge and restored the negative inclinations (BS9– 2002). Magnetite ChRMs can be explained that, owing to the limited number of sites in BS11) back to the positive inclinations that char- by burial diagenetic mechanisms such as these units, this interpretation should be tested acterize the site (Fig. 10C). This suggests that maturation of organic matter (Banerjee et al., with additional sampling and analyses. development of the kink fold postdates ChRM 1997; Blumstein et al., 2004) and clay diagen- The supposition that the Meligan and West acquisition. During our reconnaissance program, esis (Katz et al., 2000), or by fl uid migration Crocker Formations were not affected by the block samples collected from overturned beds events (Elmore et al., 1999, 2001). In light of same fl uids that pervasively remagnetized the at 2 sites from Maju East, 500 m east of Bukit the orogenic setting of northwest Borneo, we Crocker Formation is consistent in treating these Sepanger, yielded reliable site means (Table 1; attribute acquisition of the ChRM signal in the units as a tectonostratigraphic element that Fig. 14B) that we initially considered to record Crocker Formation sandstones as the product of is distinctly different from the Crocker Forma- extremely strong CCW rotation, similar to the large-scale fl uid fl ow in a foreland basin, as has tion (Fig. 2). From this viewpoint, the Crocker Crocker Formation red mudrocks. Subsequent been documented in similar settings elsewhere Formation was thrust over the foreland basin to our full fi eld program, the Maju East site was (e.g., Elmore et al., 2001; Enkin, 2003). Our sediments of the Sarawak orogeny (i.e., West further cleared for additional housing, exposing site mean declinations are all northward with Crocker and Temburong Formations) along an antiformal syncline that records large-scale shallow inclinations from the Crocker Forma- such faults as the Tenom fault during the Sabah recumbent refolding of the Crocker Formation tion, indicating that they record normal polarity orogeny (Figs. 2 and 13A; Cullen, 2010). (Fig. 6A). This new exposure led us to reconsider magnetization. Because the late Cenozoic is a Because the Sabah orogeny does not appear to our original interpretation at the Maju East loca- time of rapidly alternating normal and reversed have produced the pervasive remagnetization tion in terms of refolding during D2. Although polarity states, we consider crystallization of recorded in the Crocker Formation, we suggest we do not know the precise orientation of the D2 the phases carrying the ChRM to have occurred that remagnetization of the Crocker Formation fold axis, and therefore cannot correctly restore in a relatively short interval, 1–2 m.y. records basin-wide fl uid expulsion related to the the lower limb of the recumbent fold at the Maju It is remarkable that the red mudrocks of Sarawak orogeny. East site, several lines of evidence suggest that the Crocker Formation in the Kota Kinabalu the D2 fold axis was subhorizontal. First, out- area show strong CCW rotation (Fuller et al., Age of ChRM Acquisition crop patterns of long linear ridges and the gentle 1991), whereas the mean paleomagnetic direc- plunge of fold axes (our observations, and tion derived for the sandstones around Kota We treat the Crocker Formation as Late map from Crevello, 2001) suggest that tilting Kinabalu (JS, LK, NX, KI, BS, and KK in Eocene in age (albeit with a range of uncer- is minor. Second, because southwest to north- Fig. 2) shows weak CW rotation. As a work- tainty, 35 ± 5 Ma), similar to the age of the red east paleofl ow directions (reviewed by Hutchi- ing hypothesis we propose that the ChRM in beds in the Silantek Formation (northwest Bor- son, 2005) are nearly orthogonal to regional

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D2 correction assuming ME 1-2 horizontal axis

500 cm

In Situ ME 1-2

20 cm 1 m

50 cm

20 cm

Figure 14. (A) Outcrop photo of Maju East location showing large load casts on overturned limb of antiformal syncline. (B) Stereoplot shows in situ mean characteristic remanent magnetizations (ChRMs) with a notional structural correction for D2 deformation assuming a horizontal fold axis. (C) Horizontal to gently plunging ﬂ ute marks and load casts in Crocker Formation sandstones.

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shortening (southeast-northwest), the amount 6, 11, and 23 therein) show that the shallow opment of the deep regional unconformity and of plunge on folds can be estimated from the regional unconformity (ca. 8.2 Ma) is virtu- the waning stages of the Sarawak orogeny inclination of fl ute marks and load casts. Over ally undeformed, which puts a lower limit on (Figs. 2 and 15). the entire study area, including Maju East, these D2. In contrast, the section beneath the deep features plunge gently (Fig. 14C). Our inclina- regional unconformity is strongly folded. CW and CCW Rotations Observed tion data are also consistent with minimal post- Although the pre–deep regional unconformity ChRM tilting. Using a horizontal fold axis as a phase of the Sabah orogeny represents a good In contrast to the CCW rotations that domi- fi rst-order approximation for unfolding D2, the candidate for the relative timing D2 (between nate West Kalimantan, northwest Borneo, cen- Maju East data restore to a pre-D2 CW rotation 22 and 15 Ma), we indicated here that the tral Kalimantan, south Palawan, and the older similar to that observed at Bukit Sepanger (Fig. Sabah orogeny did not appear to produce per- units in Sabah, both CW and CCW rotations are 14B), although with larger declinations and vasive remagnetization. Therefore, we must observed in the Crocker Formation (Fig. 15). inclinations. also consider D2 as a continuation of the Sara- The different polarities in rotations are not ran- We cannot put an absolute age on the tim- wak orogeny, a strong possibility in light of domly distributed, but rather can be grouped into ing of D2. It is observed at several locations late-synfolding ChRM acquisition indicated three discrete geographic domains (Fig. 16). In in the Kota Kinabalu area, suggesting that it at Tenom and Bukit Sepanger. The preponder- the Kota Kinabalu area, the Crocker Formation not a local event confi ned to Bukit Sepanger. ance of evidence suggests that remagnetization sandstones record an average of 12° CW rota- In the shallow offshore, immediately west of of the Crocker Formation sandstones occurred tion. It is interesting that a single site from a Late our study area, seismic data (Cullen, 2010, fi gs. between 15 and 35 Ma, that is, between devel- Miocene (8.2 Ma) granite near Kota Kinabalu

Crocker Sandstones

Crocker mudstones 80

Telupid chert

Kappa granite 70 NWB Igneous

NWB redbeds 60 CE Kalimantan

SW Kal 50 SP Age Ma N P 40

0 –100 –80 –60 –40 –20 0 20 40 60 degrees rotaon (CCW is negave)

Figure 15. Plot of rotation versus age for all data (this study; Fuller et al., 1991, 1999; Lumadyo et al., 1993; Schmidtke et al., 1990). Dashed vertical lines show uncertainty in the age of the Crocker Formation; open dashed green circles indicate age of sandstones with ﬁ lled green circles indicating our best estimate for the age of characteristic remanent magnetization acquisition. CCW—counterclockwise; CE—central eastern; SP—south Palawan; NP—north Palawan; NWB—northwest Borneo; SW Kal—southwest Kalimantan.

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KD 43 deg Site rotation

TS NX-KI-BS-KK No ChRM 38 12 BL JS LK 16 19 11 KG MK 13

KGV TO

TN 26 28

50 km

Figure 16. Vertical-axis rotation estimates derived from the locality means plotted on a digital elevation model. Arrows show sense of rotation. Smaller gray symbol shows clockwise rotation for single site at the Kappa Quarry discussed in text. ChRM—characteristic remanent magnetization. For abbreviations, see Figure 2.

shows 11° CCW rotation (Fuller et al., 1999); The data from the Kudat Peninsula, although pervasively remagnetized. Rock magnetic analy- this could be related to the Meliau episode of the of marginal quality, indicate fairly strong (46°) ses indicate the ChRM signal in the Crocker Sabah orogeny. If additional sampling confi rms CCW rotation that appears to postdate folding Formation sandstones resides in pyrrho tite and that Late Miocene CCW rotation occurred over of Oligocene to Early Miocene sandstones. magnetite. Fold tests at fi ve locations show that a larger area of the pluton, then it would imply An Early Miocene or younger age CCW rota- ChRM acquisition postdates folding. Sandstones that the Crocker sandstones in the Kota Kinabalu tion of the Kudat Peninsula is problematic not from the Temburong–West Crocker and Meli- domain may have actually undergone 23° CW only to the data around Kota Kinabalu, but also gan units lack stable remanence, suggesting that rotation, consistent with the extrusion-collision with respect to the data indicating that south these units had a different diagenetic history than model for opening the South China Sea. Near Palawan’s CCW rotation was concluded by the the Crocker Formation sandstones. the Keningau Pass area (KG1 and KG2), the Oligocene (Almasco et al., 2000). If accepted Placing constraints on the timing for ChRM Crocker Formation sandstones also show mild at face value, the data imply that these areas acquisition is problematic. A mean paleomag- CW rotation similar to that of the Kota Kinabalu moved independently at different times. netic direction using 6 locations in the Kota area, indicating a large domain of CW rotation. Kinabalu area indicates that the Crocker sand- South of the KG1 and KG2 locations, however, CONCLUSIONS stones record 12° CW rotation that postdates Crocker sandstones from the Tenom and Kenin- folding related to the Sarawak orogeny (D1). A gau Valley locations record a change in the polar- Reliable ChRM measurements from Ceno- second episode of deformation (D2), recorded ity of rotation, showing moderately strong CCW zoic sandstones of the Crocker and Kudat For- at Bukit Sepanger, Maju East, and Bandar rotation. How far this CCW rotated domain mations at 12 locations along a 250 km north- Sierra, appears to postdate ChRM acquisi- extends to the southwest is not known, due to east-southwest transect on the northwest side of tion. D2 deformation was suffi ciently intense lack of sampling; further work is needed. Sabah, Malaysia, show that these sandstones are to recumbently fold the D1 (pre-ChRM) folds.

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Placed within a regional geological context and the entire island of Borneo and south Palawan Bol, A.J., and van Hoorn, B., 1980, Structural styles western previous paleomagnetic studies, we suggest rotating CCW as a rigid block between 30 and Sabah offshore: Geological Society of Malaysia Bul- letin, v. 12, p. 1–16. the following sequence of events. An early epi- 10 Ma. Instead the data imply that strain has Briais, A., Patriat, P., and Tapponnier, P., 1993, Updated sode (before 35 Ma) of strong regional CCW been partitioned in a complex manner with interpretation of magnetic anomalies and sea fl oor spreading stages in the South China Sea: Implications rotation, recorded in the red mudrocks of the several overprinting episodes of deformation. for the Tertiary tectonics of Southeast Asia: Jour- Crocker Formation, ended with the Sarawak The present data, including regional geological nal of Geophysical Research, v. 98, p. 6299–6328, orogeny, during which the Crocker Formation mapping and age determinations, are not suf- doi:10.1029/92JB02280. Cloke, I.R., Moss, S.J., and Craig, J., 1999, Structural con- was strongly deformed (D1) and its sandstones fi cient to resolve the manner in which strain trols on the evolution of the Kutai Basin, East Kaliman- pervasively remagnetized during regional has been partitioned. However, the recovery of tan: Journal of Asian Earth Sciences, v. 17, p. 137–156, fl uid expulsion. Owing to their impermeable reliable paleomagnetic data over a wide area doi:10.1016/S0743-9547(98)00036-1. Cottam, M., Hall, R., Sperber, C., and Armstrong, R., 2010, nature, mudrocks of the Crocker Formation of Sabah should encourage workers to collect Pulsed emplacement of layered granite: New high- were not remagnetized during the Sarawak additional paleomagnetic data to help resolve precision age data from Mount Kinabalu, North Bor- neo: Geological Society of London Journal, v. 167, orogeny. A younger episode of deformation remaining structural, tectonic, and diagenetic p. 49–60, doi:10.1144/0016-76492009-028. (D2) that postdates ChRM acquisition in the questions. Crevello, P.D., 2001, The great Crocker submarine fan: A Crocker sandstones may represent an early (ca. world-class foredeep turbidite system: Proceedings, ACKNOWLEDGMENTS Indonesian Petroleum Association, 28th Annual Con- 15–22 Ma) phase of the Sabah orogeny or the vention, Jakarta, p. 378–407. waning late stages of the Sarawak orogeny (ca. We thank Ibrahim bin Amnan (Minerals and Geo- Cullen, A.B., 2010, Transverse segmentation of the Baram- 35 Ma). Therefore, we interpret that acquisi- science Department of Malaysia, Sabah) for approving Balabac Basin, NW Borneo: Refi ning the model of export permits for our samples. We also thank Calum Borneo’s tectonic evolution: Petroleum Geoscience, tion of the ChRM signal in the Crocker sand- v. 16, p. 3–29, doi:10.1144/1354-079309-828. Macdonald of Shell E&P for his role in approving the stones around the Kota Kinabalu area occurred Cullen, A.B., Reemst, P., Henstra, G., Gozzard, S., and Ray, funding for this study and for granting Cullen time A., 2010, Rifting of the South China Sea: New per- between 15 and 35 Ma. Limited data from the to lead in the fi eld work, and Chris Morley and three spectives: Petroleum Geoscience, v. 16, p. 273–282, West Crocker and Meligan units indicate that anonymous reviewers for their constructive criticisms doi:10.1144/1354-079309-908. the Sabah orogeny had little effect with respect and comments that enabled us to sharpen the focus of Dewey, J.F., 2005, Orogeny can be very short: National our work within the limitations of the data. Academy of Sciences Proceedings, v. 102, p. 15,286– to remagnetization. 15,293, doi:10.1073/pnas.0505516102. Paleomagnetic data from the Crocker and REFERENCES CITED Dunlop, D. J., Ozdemira, O., Clark, D. A., and Schmidt, P. 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