Ron Harris

The Permian of Timor: stratigraphy, palaeontology and palaeogeography

The Permian of Timor in the Lesser Sunda Islands has attracted the attention of palaeontologists since the middle of the nineteenth century because of the richness, diversity and excellent state of preservation of its fauna. These abundant fossil data have been compiled and updated for the present account. The Permian rocks of Timor were deposited on the northern margin of Australia. At the present time the northern margin of Australia, in the region of Timor, is involved in a continent–arc collision, where Australia is colliding with the Banda Arcs. As a result of this collision, Permian rocks of the Australian margin have been disrupted by folding and faulting with the generation of mud-matrix melange, and uplifted to form part of the island of Timor. Due to this tectonic disruption, it has proved difficult to establish a reliable stratigraphy for the Permian units on Timor, especially as the classic fossil collections were obtained largely from the melange or purchased from the local people, and do not have adequate stratigraphic control. Detailed systematic, structural, stratigraphic and sedimentological studies since the 1960s have provided a firmer stratigraphic and palaeogeographic background for reconsideration of the significance of the classic fossil collections. Permian rocks on Timor belong either to a volcanic-carbonate sequence (Maubisse Formation), or to a clastic sequence (Atahoc and Cribas formations) in which volcanics are less prominent. The Permian sequences were deposited on Australian continental basement which was undergoing extension with spasmodic volcanic activity. Carbonates of the Maubisse Formation were deposited on horst blocks and volcanic edifices, while clastic sediments of the Atahoc and Cribas formations were deposited in grabens. The clastic sediments are predominantly fine-grained, derived from a distant siliciclastic source, and are interbedded with sediments derived from the volcanics and carbonates of adjacent horst blocks. Bottom conditions in the graben were often anoxic. In the present account, events on Timor during the Permian are related to the regional tectonic context, with the northward movement of Australia leading to the amelioration of the climate from sub-glacial to sub-tropical, together with the separation of crustal blocks from the northern Australian margin to form the Meso-Tethys.

Thermal history of Australian passive margin cover sequences accreted to Timor during Late Neogene arc–continent collision, Indonesia

Abstract Paleotemperature indicators and apatite fission track analysis of Australian continental margin cover sequences accreted to the active Banda arc–continent collision indicate little to no heating during rapid late Neogene uplift and exhumation. Thermal maturation patterns of vitrinite reflectance, conodont alteration and illite crystallinity show that peak paleotemperatures (PPT) increase with stratigraphic and structural burial. The highest PPT is found in the northern hinterland of the accretionary wedge, which was beneath progressively thicker parts of the upper plate towards the north. Major discontinuities in the pattern of PPT are associated with the position of major thrust ramps such as those forming the Ramelau/Kekneno Arch (RKA). PPT for Upper Triassic to Neogene strata south of the RKA are 60–80°C, which are similar to, and in many cases lower than, correlative and age equivalent units drilled on the NW Australian Shelf. Permian to Lower Triassic sedimentary strata thrust over younger units within and north of the RKA have PPT of 100–220°C. Thrust sheets accreted beneath the upper plate have PPT approximately 90°C higher than those frontally accreted. Metamorphism of the northernmost units of these sequences yield PPT of >300°C. Thrust stacking yields an inverted thermal profile of PPT decreasing discontinuously downward and to the south (towards the foreland). The timing of PPT is constrained by apatite fission track ages from mostly Triassic continental margin cover sequences. Ages of Upper Triassic units are primarily coeval with deposition and show little evidence of thermal annealing, whereas those of Lower Triassic units are almost completely annealed and range from 1.8±0.5–19.2±9.7 Ma. The clustering of apatite fission track ages into two distinct groups indicates that the upper boundary of the partial annealing zone has remained for some time at a Triassic stratigraphic interval in the slope and rise of the NW Australian continental margin. The position of this zone on the present shelf is higher in the stratigraphic column due to the greater thickness of post-breakup shelf facies units. Thrust stacking of rise, slope and shelf units produces an inverted vertical profile of increasing apatite fission track age with depth. Lack of any long confined track lengths in apatite from all of the units requires rapid and recent exhumation of the thrust stack, which is coincident with rapid phases of Pliocene–Pleistocene exhumation documented throughout Timor. These data preclude pre-Late Miocene tectonic burial or pre-Pliocene exhumation of the NW Australian continental margin.

Transition from subduction to arc-continent collision: Geologic and neotectonic evolution of Savu Island, Indonesia

Field analyses of stratigraphy, structure, and tectonic geomorphology of Savu Island defi ne the age and provenance of accreted Australian continental margin sequences and overlying synorogenic cover, and the structure, kinematics, and uplift history of the transition from subduction to collision in the eastern Sunda-Banda arc. The results highlight the dominant infl uence of lower plate composition and structure in shaping Savu Island and initiating intraforearc shortening. Provenance and biostratigraphic analyses of rocks accreted to the edge of the Sunda-Banda forearc indicate that they mostly consist of Triassic to Cretaceous synrift and postrift successions of the Australian continental margin. These rocks are similar in composition and provenance to Gondwana sequence units found throughout the Banda arc to the east, such as the Triassic Babulu, Jurassic Wai Luli, and Cretaceous Nakfunu Formations of Timor. Previously unrecognized units of pillow basalt are found interlayered with Jurassic beds and incorporated into melange and mud diapirs. These basalt occurrences have major and trace element compositions similar to those of Indian Ocean mid-oceanic ridge basalt and are likely associated with Jurassic development of the Scott Plateau volcanic margin. Southdirected thrusting of these units via a duplex thrust system detached the Middle Triassic section of the underthrust Scott Plateau. The Savu thrust system consists of a series of active north-directed thrust faults found onshore and offshore the north coast of Savu. Thrust faults mapped onshore, penetrated by the Savu #1 well and imaged in vintage seismic refl ection profi les, offset the youngest deposits of Savu. The uplift history and deformation pattern associated with the Savu thrust is investigated at a variety of temporal scales. Foraminifera-rich synorogenic deposits indicate low average surface uplift rates until after 1.9 Ma ago, when pelagic chalk deposits were raised from depths of >2500 m to the surface in fewer than 1 Ma. Island emergence is well documented by uplifted coral terraces that encrust the highest ridges to 338 m elevation. U/Th analysis of uplifted coral yields ages of 122 ka near sea level, indicating slow uplift rates of 0.2 mm/a over the past 400 ka. Most synorogenic deposits are stripped from the south coast, exposing parts of the accretionary wedge. The deeply eroded nature of this part of the island, combined with its steep fi rst-order stream gradients, indicates that it underwent rapid rock uplift and exhumation in the past 1‐2 Ma. However, the south coast region is now subsiding, as evidenced by drowned streams and southtilted, submerging coral terraces. Streams draining north over the Savu thrust system show convex-upward patterns with gradients commonly associated with intermediate uplift rates. Flights of coral terraces also document growth of the island to the north above thrust-related folds. These results inform us that the transition from subduction to collision involves (1) strain partitioning away from the subduction zone into the upper plate, (2) forearc closure, (3) structure inherited from the lower plate, (4) initiation of a crustal suture zone, and (5) uplift and exhumation of the accretionary wedge.

Geodynamic patterns of ophiolites and marginal basins in the Indonesian and New Guinea regions

Abstract Analysis of spatial, temporal, geological and geochemical patterns of ophiolites in the Indonesian and New Guinea region indicates a strong correlation with marginal basin development and closure. The spatial distribution of ophiolites is mostly linked with marginal basin producing zones of oblique convergence and collision. Strain partitioning in these zones creates a series of ephemeral plate boundaries between several independently moving lithospheric blocks. Repeated disruption of the diffuse boundaries between the blocks by changes in plate motion and collision-induced mantle extrusion creates space that is rapidly filled by new ocean basins in the upper plate of subduction zones. Suprasubduction zone (SSZ) spreading of these basins is enhanced by episodic extrusion of asthenosphere escaping collisional suture zones. Various closure events and global plate motion changes are reflected in the temporal distribution of marginal basin and ophiolite ages. Most ophiolite slabs in the Indonesian and New Guinea region represent fragments of oceanic lithosphere with a subduction zone component, as indicated by the common refractory petrochemistry of the mantle sequence and occurrence of boninite. Age and compositional heterogeneity may indicate that some ophiolite bodies are composite terranes. Collisions with buoyant lithosphere transform parts of these ocean basins into ophiolites. The connection between ophiolites and marginal basins is strongest where parts of actively spreading SSZ basins are partially represented as ophiolites in collision zones.

Variable rates of Late Quaternary surface uplift along the Banda Arc-Australian plate collision zone, eastern Indonesia

Abstract Radiometrically dated emergent coral terraces from southeastern Indonesia provide estimates of differential vertical strain in the Banda Arc-continent collision complex. At Semau island, two samples from the lowest emergent reef (5–7 m) yield 230Th dates that correspond to the 5a (c. 83 ka) sea-level highstand and a low surface uplift rate of 0.2–0.3 m per 1000 years. At Rote island, samples from the lowest emergent reef (c. 1–2 m) on both north and south sides of the island yield late Holocene ages and an average short-term uplift rate of c. 1–1.5 m per 1000 years. Similarity of ages from different samples on the north coast of Rote suggests possible coseismic emergence. Survey data from nine emergent reefs and marine notches up to 170 m in altitude on the south side of Rote indicate that uplift rates may have been c. 1–1.5 m per 1000 years for c. 120 000–130 000 years. Combined with previous studies, these results indicate that late Quaternary surface uplift rates vary an order of magnitude along the strike of the Banda orogen. Vertical displacement rates are greatest in young parts of the orogen where the shelf-slope break recently has been underthrust beneath the orogenic wedge, as at Rote, and in older parts of the orogen where retroarc thrust faulting occurs, as at Alor island.

Australia going down under: Quantifying continental subduction during arc-continent accretion in Timor-Leste

Models of arc-continent accretion often assume that the period of subduction of continental lithosphere before plate boundary reorganization is fairly short lived, yet the timescale of this period is poorly constrained by observations in the geologic record. The island of Timor is the uplifted accretionary complex resulting from the active collision of the Banda volcanic arc with the Australian continental margin. The exposure of underplated and exhumed Australian strata on Timor allows for the characterization of the structural history of accretion of uppermost Australian crust and the quantification of subduction of its original continental lithospheric underpinnings. New structural mapping in East Timor (Timor-Leste) reveals that duplexing of a 2-km-thick package of Australian continental strata has built the majority of the structural elevation of the Timor orogen. Coupling new structural observations with previous thermochronology results reveals the sequence of deformation within the orogen, the presence of subsurface duplexing below the hinterland slate belt, and motion along a foreland subsurface thrust ramp. Construction of balanced cross sections allows for the quantification of the amount of shortening in the orogen, and from that, the length of the subducted Australian continental lithosphere. Two balanced cross sections in East Timor reveal 326–362 km of shortening and that 215–229 km of Australian continental lithosphere have been subducted below the Banda forearc. These results highlight the fact that considerable amounts of continental lithosphere can be subducted while accreting only a thin section of uppermost crust. Continental subduction may have been favorable at Timor because of fast subduction rates, old oceanic crust at the consumed Australian margin, and subduction of some length of transitional crust. These results provide quantitative constraints for future numerical modeling of the geodynamics of continental subduction and arc-continent collision.

New interpretation of the Franciscan mélange at San Simeon coast, California: tectonic intrusion into an accretionary prism

Many concepts and interpretations on the formation of the Franciscan mélange have been proposed on the basis of exposures at San Simeon, California. In this paper, we show the distribution of chaotic rocks, their internal structures and textures, and the interrelationship between the chaotic rocks and the surrounding sandstones (turbidites). Mélange components, particularly blueschists, oceanic rocks, including greenstone, pillow lava, bedded chert, limestone, sandstone, and conglomerate, have all been brecciated by retrograde deformation. The Cambria Slab, long interpreted as a trench slope basin, is also strongly deformed by fluidization, brecciation, isoclinal folding, and thrusting, leading us to a new interpretation that turbiditic rocks (including the Cambria Slab) represent trench deposits rather than slope basin sediments. These rocks form an accretionary prism above mélanges that were diapirically emplaced into these rocks first along sinistral-thrust faults, and then along dextral-normal faults. Riedel shear systems are observed in several orders of scale in both stages. Although the exhumation of the blueschist blocks is still controversial, the common extensional fractures and brecciation in most of the blocks in the mélanges and further mixture of various lithologies into one block with mélange muddy matrix indicate that once deeply buried blocks were exhumed from considerable depths to the accretionary prism body, before being diapirically intruded with their host mélange along thrust and normal faults, during which retrograde deformation occurred together with retrograde metamorphism. Recent similar examples of high-pressure rock exhumation have been documented along the Sofugan Tectonic Line in the Izu forearc areas, in the Mineoka belt in the Boso Peninsula, and as part of accretionary prism development in the Nankai and Sagami troughs of Japan. These modern analogues provide actively forming examples of the lithological and deformational features that characterize the Franciscan mélange processes.

Resolving spatial heterogeneities in exhumation and surface uplift in Timor‐Leste: Constraints on deformation processes in young orogens

Although exhumation and surface uplift are important parameters in understanding orogenesis, the opportunity to measure both in close proximity is rare. In Timor-Leste (East Timor), deeply exhumed metamorphic rocks and piggyback deepwater synorogenic basins are only tens of kilometers apart, permitting direct relation of uplift and exhumation by comparing micropaleontology to thermochronology interpreted through one-dimensional thermal modeling. Foraminifera in two deepwater synorogenic basins suggest basin uplift from depths of 1–2 km to depths of 350–1000 m between 3.35 and 1.88 Ma. Thermochronologic sampling was conducted in the central mountain belt between these basins. Of four muscovite 40Ar/39Ar samples, one provides a reset age of 7.13 ± 0.25 Ma in the Aileu high-grade belt that suggests ~9–16 km of exhumation since that time. Eighteen zircon (U-Th)/He samples contain a group of reset ages in the Aileu Complex ranging from 4.4 to 1.5 Ma, which suggest exhumation rates of 1.0–3.1 mm/yr with 2.7–7.8 km of exhumation since these ages. Thirteen apatite (U-Th)/He ages in the Gondwana Sequence range from 5.5 to 1.4 Ma, suggesting 1–2 km of exhumation and defining a pattern of exhumation rates (ranging from 0.2 to 1.3 mm/yr) that positively correlates with average annual rainfall. Seven apatite fission track samples display varying degrees of partial resetting, with greatest resetting where apatite (U-Th)/He ages are youngest. Together, these data demonstrate extreme variability in surface uplift and exhumation over small spatial scales. We propose ongoing subsurface duplexing driven by subduction and underplating of Australian continental crust as the predominant driver for surface uplift and uplift-induced exhumation.

Waves of destruction in the East Indies: the Wichmann catalogue of earthquakes and tsunami in the Indonesian region from 1538 to 1877

Abstract The two volumes of Arthur Wichmanns Die Erdbeben Des Indischen Archipels [The Earthquakes of the Indian Archipelago] (1918 and 1922) document 61 regional earthquakes and 36 tsunamis between 1538 and 1877 in the Indonesian region. The largest and best documented are the events of 1770 and 1859 in the Molucca Sea region, of 1629, 1774 and 1852 in the Banda Sea region, the 1820 event in Makassar, the 1857 event in Dili, Timor, the 1815 event in Bali and Lombok, the events of 1699, 1771, 1780, 1815, 1848 and 1852 in Java, and the events of 1797, 1818, 1833 and 1861 in Sumatra. Most of these events caused damage over a broad region, and are associated with years of temporal and spatial clustering of earthquakes. The earthquakes left many cities in ‘rubble heaps’. Some events spawned tsunamis with run-up heights >15 m that swept many coastal villages away. 2004 marked the recurrence of some of these events in western Indonesia. However, there has not been a major shallow earthquake (M≥8) in Java and eastern Indonesia for the past 160 years. During this time of relative quiescence, enough tectonic strain energy has accumulated across several active faults to cause major earthquake and tsunami events, such as those documented in the historical records presented here. The disaster potential of these events is much greater now than in the past due to exponential growth in population and urbanization in areas destroyed by past events. Supplementary material: Translation of the catalogues into English, scanned PDFs of the original catalogues and geographical locations of most place names found in the catalogue (as a KMZ file) are available at https://dx.doi.org/10.6084/m9.figshare.c.2860405.v1

Rooted Brooks Range ophiolite: Implications for Cordilleran terranes: Comment and Reply

[Saltus et al. (2001)][1] present a two-dimensional model for rooted versus ophiolitic mafic and ultramafic rocks throughout the Cordillera based on a local geophysical anomaly in part of the southwest Brooks Range. Their model estimates a mafic and ultramafic rock body thickness of ∼8 km.