Christopher L. Fergusson

CRETACEOUS–TERTIARY CONVERGENCE AND CONTINENTAL COLLISION, SANANDAJ–SIRJAN ZONE, WESTERN IRAN

Abstract The Sanandaj–Sirjan Zone contains the metamorphic core of the Zagros continental collision zone in western Iran. The zone has been subdivided into the following from southwest to northeast: an outer belt of imbricate thrust slices (radiolarite, Bisotun, ophiolite and marginal sub-zones, which consist of Mesozoic deep-marine sediments, shallow-marine carbonates, oceanic crust and volcanic arc, respectively) and an inner complexly deformed sub-zone (late Palaeozoic–Mesozoic passive margin succession). Rifting and sea-floor spreading of Tethys occurred in the Permian to Triassic but in the Sanandaj–Sirjan Zone extension-related successions are mainly of Late Triassic age. Subduction of Tethyan sea floor in the Late Jurassic to Cretaceous produced deformation, metamorphism and unconformities in the marginal and complexly deformed sub-zones. Deformation climaxed in the Late Cretaceous when a major southwest-vergent fold belt formed associated with greenschist facies metamorphism and post-dated by abundant Palaeogene granitic plutons. In the southwest of the zone a Late Cretaceous island arc—passive margin collision occurred with ophiolite emplacement onto the northern Arabian margin similar to that in Oman. Final closure of Tethys was not completed until the Miocene when Central Iran collided with the northeast Arabian margin.

Dextral transpression in Late Cretaceous continental collision, Sanandaj-Sirjan Zone, western Iran

The Sanandaj–Sirjan Zone of western Iran is a metamorphic belt (greenschist–amphibolite) that was uplifted during Late Cretaceous continental collision between the Afro-Arabian continent and the Iranian microcontinent. In the June area, 300 km southwest of Tehran, the Late Palaeozoic–Mesozoic succession was affected by two major episodes of deformation. The first deformation formed tight folds and axial plane schistosity. These are strongly overprinted by second deformation structures that formed during Late Cretaceous continental collision under dextral transpression. The convergence has a low obliquity and has significant deformation partitioning into two domains. (1) A widespread schist and marble domain with intensely folded and foliated rocks that are cut by thrusts and have an overall south-southwest vergence. (2) A domain with wide zones of mylonitic granite, amphibolite and less common calcite mylonite that are affected by a foliation with the same orientation as in rocks of the schist and marble domain. Rocks of this domain also contain an intense sub-horizontal stretching lineation and abundant shear-sense criteria indicating dextral shear. This contrasts with many zones of transpression where strike-slip shearing is taken up along discrete faults. A syn-D2 pluton (the Galeh–Doz pluton) has a major S-shaped bend within it, imparted during the dextral transpression.

New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190

Moore, G. F., Taira, A., Klaus, A., Becker, L., Boeckel, B., Cragg, B. A., Dean, A., Fergusson, C. L., Henry, P., Hirano, S., Hisamitsu, T. et al. (2001). New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190. Geochemistry, Geophysics, Geosystems, 2, Article No: 2001GC000166.

Interactions between deformation and fluids in the frontal thrust region of the NanTroSEIZE transect offshore the Kii Peninsula, Japan: Results from IODP Expedition 316 Sites C0006 and C0007

Integrated Ocean Drilling Program (IODP) Expedition 316 Sites C0006 and C0007 examined the deformation front of the Nankai accretionary prism offshore the Kii Peninsula, Japan. In the drilling area, the frontal thrust shows unusual behavior as compared to other regions of the Nankai Trough. Drilling results, integrated with observations from seismic reflection profiles, suggest that the frontal thrust has been active since ∼0.78–0.436 Ma and accommodated ∼13 to 34% of the estimated plate convergence during that time. The remainder has likely been distributed among out-of-sequence thrusts further landward and/or accommodated through diffuse shortening. Unlike results of previous drilling on the Nankai margin, porosity data provide no indication of undercompaction beneath thrust faults. Furthermore, pore water geochemistry data lack clear indicators of fluid flow from depth. These differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape. In turn, fluid pressures may affect deformation. Well-drained, sand-rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe. Recent resumption of normal frontal imbrication is inferred from seismic reflection data. Associated decollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region. This evidence consists of seafloor bathymetry, normal faults documented in cores, and low porosities in near surface sediments that suggest removal of overlying material. Overall, results provide insight into the complex interactions between incoming materials, deformation, and fluids in the frontal thrust region.

Convergence and intraplate deformation in the Lachlan Fold Belt of southeastern Australia

Abstract The Lachlan Fold Belt of southeastern Australia is dominated by a widespread oceanic association including Cambrian submarine mafic volcanics and an overlying widespread Ordovician quartz-rich turbidite and black shale succession with scattered mafic to andesitic volcanic centres. These rocks are overlain by, or in fault contact with, deep-marine to continental Silurian to Early Carboniferous successions with mafic to silicic volcanics and abundant plutonic rocks. Major deformation, achieved by thrusting and folding, affected the fold belt during the Silurian to Middle Devonian and the Early Carboniferous. Shortening estimates vary throughout the belt but the widespread Ordovician quartz turbidite succession usually has at least 60% shortening which implies an original width of the fold belt of about 1700 km. Strike-slip faulting was important on a local to regional scale, but large-scale strike-slip displacements appear unlikely. The large values of shortening are consistent with development of the belt in a convergent margin setting at least for the Silurian to Carboniferous. The nature of the lower crust of the Lachlan Fold Belt has always presented a problem of interpretation but we favour a Late Proterozoic quasi-continental lower crustal layer, that has deformed independently of the upper crust by either homogeneous flattening or crustal duplexing during Silurian-Carboniferous convergence, and provided a source for Silurian-Devonian granites.

Detrital zircon ages in Neoproterozoic to Ordovician siliciclastic rocks, northeastern Australia: implications for the tectonic history of the East Gondwana continental margin

U–Pb detrital zircon ages in variably metamorphosed, dominantly fine-grained clastic successions are used in northeastern Australia to identify two major successions along the East Gondwana margin. The older succession is of probable Late Neoproterozoic age and is considered part of a passive margin associated with rifting at c. 600 Ma. Most detrital zircons have ages in the range 1000–1300 Ma and were probably derived from an extension of a Late Mesoproterozoic (1050–1200 Ma) orogenic belt from the central Australian Musgrave Complex located 1500 km to the west. No evidence has been found for 600–800 Ma rifting of a Rodinian supercontinent and therefore it is suggested that breakup must have occurred well outboard of the present Early Palaeozoic East Gondwana margin. The younger succession is of Early Palaeozoic age and contains the distinctive 500–600 Ma detrital zircon signature that is widespread in East Gondwana in addition to some samples with ages in the range 460–510 Ma consistent with local igneous sources. The younger succession is related to the active margin of Gondwana that developed on the former passive margin in a back-arc setting, and the source of 510–600 Ma zircons is considered to be a composite of rift-related and back-arc volcanic sources.

Ordovician-Silurian accretion tectonics of the Lachlan Fold Belt, southeastern Australia

The Lachlan Fold Belt of southeastern Australia contains a remnant ocean basin with Cambrian igneous basement of intra‐arc and backarc boninitic to tholeiitic volcanics and island‐arc calc‐alkaline volcanics overlain by ?Upper Cambrian to dominantly Ordovician siliciclastic turbidite successions derived from Gondwana and deposited in a huge submarine turbidite fan(s). Much of the remnant ocean basin is preserved in accretionary subduction complexes that are characterised by abundant coherent successions with relatively sparse mélanges. The lack of chaotic rocks reflects the accretion of thick (2–5 km) siliciclastic turbidite successions and low rates of underthrusting. Three previously recognised accretionary subduction complexes are located in western Victoria (Stawell and Bendigo Zones), eastern Victoria (Tabberabbera Zone) and along the eastern coastline of southeast Australia (Narooma subduction complex). In addition, a short‐lived west‐dipping subduction zone is proposed to account for the areally restricted Howqua River Zone along the eastern margin of the Melbourne Zone in eastern Victoria. The Howqua River Zone contains gently dipping mélanges and subduction‐related blueschist fragments. The subduction complex in the Tabberabbera Zone is considered to extend throughout the remnant ocean basin succession of the Wagga‐Omeo Zone of the central Lachlan Fold Belt. Apart from the Howqua River Zone, these subduction complexes are an end‐member in the spectrum of accretionary complexes that contrast with more chaotic assemblages as preserved in southwest Japan.

Proterozoic–Cambrian detrital zircon and monazite ages from the Anakie Inlier, central Queensland: Grenville and Pacific‐Gondwana signatures*

The Anakie Metamorphic Group is a complexly deformed, dominantly metasedimentary succession in central Queensland. Metamorphic cooling is constrained to ca 500 Ma by previously published K–Ar ages. Detrital‐zircon SHRIMP U–Pb ages from three samples of greenschist facies quartz‐rich psammites (Bathampton Metamorphics), west of Clermont, are predominantly in the age range 1300–1000 Ma (65–75%). They show that a Grenville‐aged orogenic belt must have existed in northeastern Australia, which is consistent with the discovery of a potential Grenville source farther north. The youngest detrital zircons in these samples are ca 580 Ma, indicating that deposition may have been as old as latest Neoproterozoic. Two samples have been analysed from amphibolite facies pelitic schist from the western part of the inlier (Wynyard Metamorphics). One sample contains detrital monazite with two age components of ca 580–570 Ma and ca 540 Ma. The other sample only has detrital zircons with the youngest component between 510 Ma and 700 Ma (Pacific‐Gondwana component), which is consistent with a Middle Cambrian age for these rocks. These zircons were probably derived from igneous activity associated with rifting events along the Gondwanan passive margin. These constraints confirm correlation of the Anakie Metamorphic Group with latest Neoproterozoic ‐ Cambrian units in the Adelaide Fold Belt of South Australia and the Wonominta Block of western New South Wales.

Middle Palaeozoic thrusting in the eastern Lachlan Fold Belt, southeastern Australia

Abstract Early to Middle Palaeozoic clastic and volcanic strata in the Goulburn-Bungonia region, 150 km southwest of Sydney, are divided by the N-trending Yarralaw Fault into two domains. In the western domain major E-W shortening formed folds, axial-planar cleavage and W-dipping contraction faults in all pre-Upper Devonian units. This deformation formed an anticlinorium cored by a W-dipping thrust system with an overturned E-younging limb. In the eastern domain upright flattened chevron folds occur in Ordovician strata with stratal repetition along steep contraction faults spaced at intervals of 1 km or less. Zones of tectonic melange occur in the hangingwalls of these faults and have a well-developed scaly fabric, with a steeply plunging striation, overprinting a bedding-parallel cleavage in mudstone. In interbedded Silurian shale and limestone a classical stair-step trajectory is recognized for the Frome Hill Fault which duplicates the succession and has undergone back-rotation to its present steep dip. Overall, thrusting progressed from east to west with deformation in the eastern domain pre-dating intrusion of the Early Devonian Marulan Batholith, whereas farther west deformation continued into the Middle Devonian. The folds and thrusts in the Goulburn-Bungonia region are part of a major fold-thrust zone that extends throughout the eastern Lachlan Fold Belt, and which was formed in the Middle Silurian to Middle Devonian during a period of plate convergence between Gondwana and the palaeo-Pacific Ocean. The fold-thrust zone is inferred to have an arc-frontal arc setting above a W-dipping subduction zone, and the deformation relates to underthrusting of an allochthonous terrane with major shortening in the hangingwall block.

Late Ordovician stratigraphy, zircon provenance and tectonics, Lachlan Fold Belt, southeastern Australia*

Ordovician quartz turbidites of the Lachlan Fold Belt in southeastern Australia accumulated in a marginal sea and overlapped an adjoining island arc (Molong volcanic province) developed adjacent to eastern Gondwana. The turbidite succession in the Shoalhaven River Gorge, in the southern highlands of New South Wales, has abundant outcrop and graptolite sites. The succession consists of, from the base up, a unit of mainly thick‐bedded turbidites (undifferentiated Adaminaby Group), a unit with conspicuous bedded chert (Numeralla Chert), a unit with common thin‐bedded turbidites (Bumballa Formation (new name)) and a unit of black shale (Warbisco Shale). Coarse to very coarse sandstone in the Bumballa Formation is rich in quartz and similar to sandstone in the undifferentiated Adaminaby Group. Detrital zircons from sandstone in the Bumballa Formation, and from sandstone at a similar stratigraphic level from the upper Adaminaby Group of the Genoa River area in eastern Victoria, include grains as young as 453–473 Ma, slightly older than the stratigraphic ages.The dominant detrital ages are in the interval 500–700 Ma (Pacific Gondwana component) with a lessor concentration of Grenville ages (1000–1300 Ma). This pattern resembles other Ordovician sandstones from the Lachlan Fold Belt and also occurs in Triassic sandstones and Quaternary sands from eastern Australia. The Upper Ordovician succession is predominantly fine grained, which reflects reduced clastic inputs from the source in the Middle Cambrian to earliest Ordovician Ross‐Delamerian Fold Belts that developed along the eastern active margin of Gondwana. Development of subduction zones in the Late Ordovician marginal sea are considered to be mainly responsible for the diversion of sediment and the resulting reduction in the supply of terrigenous sand to the island arc and eastern part of the marginal sea.