High-precision U–Pb ion microprobe analyses provide new constraints on the emplacement and origin of the Kinabalu granite in Sabah, northern Borneo. The granite is a sheeted laccolith-like body comprising dyke-fed granitic units that young downwards, each emplaced beneath the previous sheet. Analyses of concentric growth zones in zircons indicate crystallization between 7.85 ± 0.08 and 7.22 ± 0.07 Ma, and show that the entire pluton was emplaced and crystallized within less than 800 ka. Several pulses of magmatism are recognized, each lasting for a maximum of 250 ka, and possibly as briefly as 30 ka. The oldest ages coincide with the highest elevations whereas the youngest ages are found at lower elevations around the edge of the body. Based on these new age data and field observations we identify the biotite granodiorite, hornblende granite and porphyritic facies as the Upper, Middle and Lower Units respectively. Inherited zircon ages indicate different protoliths for the Upper and Middle Units. The Upper Unit is derived from attenuated continental crust of the South China margin subducted beneath Sabah. The Middle Unit is sourced from melting of the crystalline basement in Sabah with little or no contribution from South China crust. Supplementary material: Full U–Pb ion microprobe analytical data, and modal and major element composition data are available at http://www.geolsoc.org.uk/SUP18385 .

Thermochronological data from the Kinabalu granite, emplaced between c. 7.2 and 7.8 Ma, provide a unique record of northern Borneo's exhumation during the Neogene. Biotite 40 Ar/ 39 Ar ages (c. 7.32-7.63 Ma) record rapid cooling of the granite in the Late Miocene as it equilibrated with ambient crustal temperatures. Zircon fission-track ages (c. 6.6-5.8 Ma) and apatite (U-Th-Sm)/He ages (central age c. 5.5 Ma) indicate rapid cooling during the Late Miocene-Early Pliocene. This cooling reflects exhumation of the granite, uplift and erosion bringing it closer to the Earth's surface. Thermochronological age versus elevation relationships suggest exhumation rates of more than 7 mm a −1 during the latest Miocene and Early Pliocene. Neither the emplacement of the Kinabalu granite nor its exhumation is related to the Sabah orogeny, which terminated in the Early Miocene. Instead, granite magmatism was caused by extension related to subduction rollback of the Sulu Arc, and Mio-Pliocene exhumation of the Kinabalu granite was driven either by lithospheric delamination or break-off of a subducted slab beneath Sabah. Plio-Pleistocene tectonism offshore and onshore northern Borneo reflects continuing large-scale gravity-driven tectonics in the region.

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While most aspects of subduction have been extensively studied, the process of subduction initiation lacks an observational foundation. The Macquarie Ridge complex (MRC) forms the Pacific-Australia plate boundary between New Zealand to the north and the Pacific-Australia-Antarctica triple junction to the south. The MRC consists of alternating troughs and rises and is characterized by a transitional tectonic environment in which subduction initiation presently occurs. There is a high seismicity level with 15 large earthquakes (M > 7) in this century. Our seismological investigation is centered on the largest event since 1943: the 25 MAY 1981 earthquake. Love, Rayleigh, and P waves are inverted to find: a faulting geometry of right-lateral strike-slip along the local trend of the Macquarie Ridge (N30~ a seismic moment of 5 x 1027 dyn cm (M w = 7.7); a double event rupture process with a fault length of less than 100 km to the southwest of the epicenter and a fault depth of less than 20 km. Three smaller thrust earthquakes occurred previous to the 1981 event along the 1981 rupture zone; their shallow-dipping thrust planes are virtually adjacent to the 1981 vertical fault plane. Oblique convergence in this region is thus accommodated by a dual rupture mode of several small thrust events and a large strike-slip event. Our study of other large MRC earthquakes, plus those of other investigators, produces focal mechanisms for 15 earthquakes distributed along the entire MRC; thrust and right-lateral strike-slip events are scattered throughout the MRC. Thus, all of the MRC is characterized by oblique convergence and the dual rupture mode. The "true" best-fit rotation pole for the Pacific-Australia motion is close to the Minster & Jordan RM2 pole for the Pacific-India motion. Southward migration of the rotation pole has caused the recent transition to oblique convergence in the northern MRC. We propose a subduction initiation process that is akin to crack propagation; the 1981 earthquake rupture area is identified as the "crack-tip" region that separates a disconnected mosaic of small thrust faults to the south from a horizontally continuous thrust interface to the north along the Puysegur trench. A different mechanism of subduction initiation occurs in the southernmost Hjort trench region at the triple junction. Newly created oceanic lithosphere has been subducted just to the north of the triple junction. The entire MRC is a "soft" plate boundary that must accommodate the plate motion mismatch between two major spreading centers (Antarctica-Australia and Pacific-Antarctica). The persistence of spreading motion at the two major spreading centers and the consequent evolution of the three-plate system cause the present-day oblique convergence and subduction initiation in the Macquarie Ridge complex.

While most aspects of subduction have been extensively studied, the process of subduction initiation lacks an observational foundation. The Macquarie Ridge complex (MRC) forms the Pacific-Australia plate boundary between New Zealand to the north and the Pacific-Australia-Antarctica triple junction to the south. The MRC consists of alternating troughs and rises and is characterized by a transitional tectonic environment in which subduction initiation presently occurs. There is a high seismicity level with 15 large earthquakes (M > 7) in this century. Our seismological investigation is centered on the largest event since 1943: the 25 MAY 1981 earthquake. Love, Rayleigh, and P waves are inverted to find: a faulting geometry of right-lateral strike-slip along the local trend of the Macquarie Ridge (N30~ a seismic moment of 5 x 1027 dyn cm (M w = 7.7); a double event rupture process with a fault length of less than 100 km to the southwest of the epicenter and a fault depth of less than 20 km. Three smaller thrust earthquakes occurred previous to the 1981 event along the 1981 rupture zone; their shallow-dipping thrust planes are virtually adjacent to the 1981 vertical fault plane. Oblique convergence in this region is thus accommodated by a dual rupture mode of several small thrust events and a large strike-slip event. Our study of other large MRC earthquakes, plus those of other investigators, produces focal mechanisms for 15 earthquakes distributed along the entire MRC; thrust and right-lateral strike-slip events are scattered throughout the MRC. Thus, all of the MRC is characterized by oblique convergence and the dual rupture mode. The "true" best-fit rotation pole for the Pacific-Australia motion is close to the Minster & Jordan RM2 pole for the Pacific-India motion. Southward migration of the rotation pole has caused the recent transition to oblique convergence in the northern MRC. We propose a subduction initiation process that is akin to crack propagation; the 1981 earthquake rupture area is identified as the "crack-tip" region that separates a disconnected mosaic of small thrust faults to the south from a horizontally continuous thrust interface to the north along the Puysegur trench. A different mechanism of subduction initiation occurs in the southernmost Hjort trench region at the triple junction. Newly created oceanic lithosphere has been subducted just to the north of the triple junction. The entire MRC is a "soft" plate boundary that must accommodate the plate motion mismatch between two major spreading centers (Antarctica-Australia and Pacific-Antarctica). The persistence of spreading motion at the two major spreading centers and the consequent evolution of the three-plate system cause the present-day oblique convergence and subduction initiation in the Macquarie Ridge complex.