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 1146 For permission to copy, contact [email protected] © 2012 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/5/1146/3342918/1146.pdf by guest on 30 September 2021 Paleomagnetism of the Crocker Formation, NW Borneo 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 MP 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 Sara wak) (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., Geosphere, October 2012 1147 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. 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.