1146 Paleomagnetism of the Crocker Formation, northwest Borneo: Implications for late Cenozoic tectonics Andrew B. Cullen 1,2 , M.S. Zechmeister 3,4 , R.D. Elmore 3 , and S.J. Pannalal 3 1 Shell International Exploration and Production Company, 100 Hoekstade, Rijswijk, Netherlands 2 Chesapeake Energy Corporation, 6100 N. Western Avenue, Oklahoma City, Oklahoma 73118, USA 3 ConocoPhillips School of Geology and Geophysics, 100 E. Boyd Street, Norman, Oklahoma 73019, USA 4 Shell Exploration and Production Company, 150 North Dairy Ashford, Houston, Texas 77079, USA ABSTRACT Tectonic models for Borneo’s Cenozoic evolution differ in several aspects, par- ticularly in the extent to which they include paleomagnetic data suggestive of strong counterclockwise rotation between 30 and 10 Ma. Key areas are undersampled. We present the results of a paleomagnetic study of Eocene to Early Miocene sandstones from northwest Sabah, principally from the Crocker Formation. We obtained reliable site means from 11 locations along a 250 km northeast-southwest transect using thermal demagnetization to isolate characteristic remanent magnetization (ChRM) directions. The Crocker Formation sandstones are per- vasively remagnetized; pyrrhotite dominates the ChRM signal. Locations can be grouped into different domains on the basis of the rela- tive sense of rotation about a vertical axis. Mean ChRM directions for seven locations between Kota Kinabalu and Keningau (dec- lination, dec 12°–19°; inclination, inc –22°– 23°) indicate minor clockwise rotation and modest tilting, whereas two locations near Tenom (dec 321°–345°, inc –6°–24°) record counterclockwise rotation and modest tilt- ing. Although we cannot precisely date the age of remagnetization, the results of fold tests from 4 locations, interpreted within the regional structural framework, strongly indi- cate that remagnetization occurred between 35 and 15 Ma, the waning stages of the Sara- wak orogeny to an early phase of the Sabah orogeny. Our results pose serious difficulties for current tectonic models in which Bor- neo rotates 50° counterclockwise as a rigid block between 30 and 10 Ma. With respect to prior paleomagnetic studies, we suspect that an early episode of strong regional counter- clockwise rotation (before 35 Ma) was over- printed not only by differential clockwise rotation of crustal blocks during opening of the South China Sea (32–23 Ma), but also locally by a younger (after 10 Ma) counter- clockwise rotation. INTRODUCTION The Cenozoic tectonic evolution of Southeast Asia reflects the complex interactions of rifting, subduction, continental collision, and large- scale continental strike-slip faulting. The island of Borneo is at the leading edge of several conti- nental blocks that protrude from Southeast Asia as a wedge into the Indo-Australian and Phil- ippine Sea plates (Fig. 1A). There are two end members of tectonic models for Borneo (Figs. 1B, 1C): collision-extrusion (Briais et al., 1993; Replumaz and Tapponnier, 2003) and subduction- collision (Hamilton, 1979; Lee and Laver, 1995; Hall, 1996). These models differ in four princi- pal aspects: (1) the mechanism responsible for rifting and seafloor spreading in the South China Sea ca. 32–16 Ma (Briais et al., 1993); (2) the timing and amount of displacement along the large intercontinental strike-slip faults such as the Red River fault (Leloupe et al., 1995; Searle, 2006); (3) the amount of proto–South China Sea crust subducted beneath Borneo (Rangin et al., 1999; Lee and Laver, 1995; Hall, 2002; Cullen, 2010); and (4) the magnitude and nature of the late Tertiary rotation of Borneo (Hall, 1996, 2002; Murphy, 1998). In the collision-extrusion model (Fig. 1B), India’s collision with Asia progressively dis- places the Sundaland, Indochina, and South China blocks to the southeast along intercon- tinental strike-slip faults (e.g., Mae Ping and Red River faults). In this model, Borneo, south Palawan, and north Palawan rotate clockwise (CW) ~25° along with the Indochina block as the South China Sea opens as a large-scale pull-apart basin (Briais et al., 1993; Replumaz and Tapponnier, 2003); the amount of seafloor spreading is approximately balanced by 600 km of left-lateral displacement along the Ailao Shan–Red River fault zone. In the collision- extrusion model, there is no Tertiary subduction under northwest Borneo, and mass is conserved by subduction in the Pacific Ocean. The subduction-collision model (Fig. 1C) features long-lived subduction (Eocene–Early Miocene) beneath northwest Borneo during which an extensive amount of proto–South China Sea oceanic crust is consumed. Subduc- tion terminates progressively (southwest to northeast) as blocks of continental crust (Luco- nia, Dangerous Ground, and Reed Bank) collide with northwest Borneo and Palawan (Holloway, 1982; Lee and Laver, 1995; Hall, 1996; Longley, 1997). In this model, there is less displacement along the Red River fault and because it largely decoupled from extension in the South China Sea, CW rotation of Borneo is not required. The subduction-collision model has several permu- tations. The most widely cited reconstructions are those of Hall (1996, 2002); honoring Fuller et al.’s (1999) interpretation of regional paleo- magnetic data, these reconstructions show an acceleration in subduction rate driven by strong (~50°) counterclockwise (CCW) movement of Borneo as a rigid block between 30 and 10 Ma. Murphy (1998) and Morley (2002) pointed out, however, that the lack of known regional structures of sufficient magnitude to accom- modate such a large rotation poses a challenge to the interpretation of the paleomagnetic data. Hutchison (2010), drawing attention to the lack of paleomagnetic data in key areas of Borneo, suggested that the large oroclinal bend in Bor- neo’s interior highlands is strong evidence that Borneo did not deform as a single rigid block. There are two fundamental issues regarding the paleomagnetic evidence for the rotation of For permission to copy, contact editing@geosociety.org © 2012 Geological Society of America Geosphere; October 2012; v. 8; no. 5; p. 1146–1169; doi:10.1130/GES00750.1; 16 figures; 2 tables. Received 8 September 2011 ♦ Revision received 23 March 2012 ♦ Accepted 27 March 2012 ♦ Published online 18 September 2012 Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/5/1146/3342918/1146.pdf by guest on 08 June 2020