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

Abstract

Tectonic models for Borneo's Cenozoic evolution differ in several aspects, particularly 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 pervasively remagnetized; pyrrhotite dominates the ChRM signal. Locations can be grouped into different domains on the basis of the relative sense of rotation about a vertical axis. Mean ChRM directions for seven locations between Kota Kinabalu and Keningau (declination, 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 tilting. Although we cannot precisely date the age of remagnetization, the results of fold tests from 4 locations, interpreted within the regional structural framework, strongly indicate that remagnetization occurred between 35 and 15 Ma, the waning stages of the Sarawak orogeny to an early phase of the Sabah orogeny. Our results pose serious diffi culties for current tectonic models in which Borneo 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 counterclockwise 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) counterclockwise rotation.

Cenozoic Rotation History of Borneo and Sundaland, SE Asia Revealed by Paleomagnetism, Seismic Tomography, and Kinematic Reconstruction

“…Paleomagnetic data reported by Cullen et al () from the Upper Eocene‐Lower Miocene Crocker Formation and the Lower Miocene Kudat Formation reveal CW as well as CCW rotations that may be local rotations related to complex deformation. Field observations (Cullen et al, ) show folded strata with plunging noncylindrical fold axes. In such cases, a more advanced tectonic correction is required (Pueyo et al, ).…”

“…In such cases, a more advanced tectonic correction is required (Pueyo et al, ). Furthermore, the data from Cullen et al () show a wide spread in inclinations from −84.5° to 48.7°, suggesting syndeformational remagnetization. Fuller et al () also identified that the Crocker Formation experienced remagnetization, as demonstrated by present‐day field overprints.…”

The Cretaceous and Cenozoic tectonic evolution of Southeast Asia

“…13b), and Borneo begins to rotate counterclockwise relative to Sundaland from ∼ 50 to 10 Ma due to oroclinal bending (Hutchison, 2010). Our preferred model invokes collision of the Semitau continental fragment to northern Borneo in the mid-Eocene to induce the ∼ 37 Ma Sarawak Orogeny (Hutchison, 2010) and a potential metamorphic-induced remagnetization event on Borneo at ∼ 35 Ma (Cullen et al, 2012). Northern Palawan accretes to South Palawan, leading to an ophiolite obduction episode and final closure of the proto-South China Sea with the arrival of the Dangerous Grounds and Reed Bank continental blocks by 17 Ma (Hutchison, 1996;Hutchison et al, 2000).…”

“…Coinciding with the end of sinistral motion along the Red River fault at ∼ 17 Ma (Leloup et al, 1995), spreading in the South China Sea ceased (Briais et al, 1993), likely due to the docking of the Dangerous Grounds-Reed Bank continental fragment onto northern Sundaland (Hutchison, 2004;Hutchison et al, 2000). Recent work by Cullen et al (2012) indicates a strong counterclockwise rotation of Borneo between 30 and 10 Ma, and they acknowledge that an earlier counterclockwise rotation was likely overprinted by a ∼ 35 Ma remagnetization event. The relative motion between Borneo and stable Eurasia is accommodated by the subduction of the proto-South China Sea, meaning that the CW rotation of Indochina resulting from extrusion tectonics is compatible with the large CCW rotation of Borneo resulting from oroclinal bending.…”

“…1). Although no seafloor spreading history has been preserved between the core of New Guinea and the accreted terranes, a number of Cretaceous and Eocene sedimentary units record oscillations between a convergent setting to an extensional passive margin, terminated by ophiolite obduction events (Cullen et al, 2012;Baldwin et al, 2012;Pigram et al, 1989). The youngest accretionary event is related to the arc-continent collision of the Finisterre-Torricelli terranes (Figs.…”