Lloyd T. White

Pre-existing basement structure and its influence on continental rifting and fracture zone development along Australia’s southern rifted margin

Palaeogeographical reconstructions of the Australian and Antarctic margins based on matching basement structures are commonly difficult to reconcile with those derived from ocean-floor magnetic anomalies and plate vectors. Following identification of a previously unmapped crustal-scale structure in the southern part of the early Palaeozoic Delamerian Orogen (Coorong Shear Zone), a more tightly constrained plate reconstruction for these margins is proposed. This reconstruction places the Coorong Shear Zone opposite the Mertz Shear Zone in Antarctica and lends itself to a revised interpretation of continental rifting along Australia’s southern margin in which rift basin architecture, margin segmentation and the formation of ocean-floor fracture zones are all linked to pre-existing basement structure and the reactivation of a few deep-rooted crustal structures inherited from the Delamerian Orogeny in particular. Reactivation of the Coorong Shear Zone and other basement structures (Avoca–Sorell Fault Zone) during the earlier stages of rifting was accompanied by the partitioning of extensional strain and formation of late Jurassic–Early Cretaceous normal faults and half-graben in the Bight and Otway basins with opposing NE–SW and NW–SE structural trends. Previously, the Mertz Shear Zone has been correlated with the Proterozoic Kalinjala Mylonite Zone in the Gawler Craton but this positions Australia 300–400 km too far east relative to Antarctica prior to breakup and fails to secure an equally satisfactory match in both basement geology and the superimposed extension-related structures.

Is the switch from I- to S-type magmatism in the Himalayan Orogen indicative of the collision of India and Eurasia?

The switch from I- to S-type magmatism in the Trans-Himalayan Batholith is one of the criteria used to determine when India and Eurasia collided. We present new 238U/206Pb SHRIMP data from the Karakorum Batholith, NW India. Our results suggest that an I-type hornblende–biotite granodiorite was emplaced at 31.4 ± 0.4 Ma and crosscut by S-type granite dykes at 18.0 ± 0.4 Ma. We interpret these data to indicate that volatile-fluxed melting of a mantle wedge above an active subduction zone and/or decompression melting of sub-arc asthenosphere was possible until at least 31 Ma. This interpretation uses the same rationale as previous workers have adopted to propose that the India–Eurasia collision occurred between 57 Ma and 47 Ma. A review of published data provides further evidence of <50 Ma magmatism and Cu–Au–Mo porphyry deposit mineralisation north of the Himalayan range. These rocks would be interpreted to have formed in an arc setting in other parts of the world. However, most workers consider that <50 Ma magmatism and porphyry deposit formation in the Himalaya is unrelated to subduction of oceanic lithosphere and are instead considered to form owing to ‘post-collisional’ processes. We argue that these interpretations have been based on an established model where the India–Eurasia collision is deemed to occur at ca 50 Ma, instead of using the data to question and refine existing tectonic models.

Ripping and tearing the rolling-back New Hebrides slab

The three-dimensional (3D) geometry of subducting lithospheric slabs has four-dimensional (4D) implications, i.e. in three dimensions plus time. To understand the implications of present 3D geometry, we need to consider what has gone before, what is happening now, and what will happen in the future. We illustrate this point by examining the 4D evolution of the New Hebrides slab, concluding that the Australian lithosphere tore as it began to subduct, and is still ripping in the present day. Southward motion of a north-dipping flap has been enabled as the result of westward propagation of an active rip, accompanied by southward foundering of newly created transform segments. Foundering was progressive but the rate appears to have been episodic. Additional transforms formed in the upper-plate in consequence. Subduction transform foundering is reflected by steps in the height of the subducted slab, and this needs to be taken into account so as not to significantly overestimate the area of subducted material. The principles illustrated are of general interest because foundering of subduction transforms may be a common occurrence in curved subduction zones worldwide.

Where does India end and Eurasia begin

The Indus Suture Zone is defined as the plate boundary between India and Eurasia. Here we document geochronological data that suggest that Indian rocks outcrop to the north of this suture zone. The inherited age spectrum of zircons from mylonitic gneiss collected in the southern part of the Karakorum Batholith is similar to those obtained from the Himalayan Terrane, the Pamir and is apparently Gondwanan in its affinity. These data are taken to indicate that the Karakorum Terrane was once a component of Gondwana, or at least derived from the erosion of Gondwanan material. Several continental ribbons (including the Karakorum Terrane) were rifted from the northern margin of Gondwana and accreted to Eurasia prior to India-Eurasia collision. Many therefore consider the Karakorum Terrane is the southern margin of Eurasia. However, we do not know if rifting led to the creation of a new microplate(s) or simply attenuated crust between Gondwana and these continental ribbons. Thus there is a problem using inherited and detrital age data to distinguish what is “Indian” and what is “Eurasian” crust. These findings have implications for other detrital/inherited zircon studies where these data are used to draw inferences about the tectonic history of various terranes around the world.

Can the size of K-feldspar phenocrysts be used to map the occurrence of mylonitic shear zones? Results from the Wyangala Granite, Eastern Lachlan Fold Belt, New South Wales

We report the results of mapping the high-strain (shear) zones in the Wyangala Granite. The high-strain zones are part of the Wyangala Fault System and primarily occur along the eastern margin of the Wyangala Granite, Eastern Lachlan Fold Belt in New South Wales. The aim of this study was to determine if the size of K-feldspar phenocrysts could be used to map variations in strain intensity in the Wyangala Granite and map the location of these high-strain shear zones. The high-strain zones were classified into components of relative strain intensity on the basis of K-feldspar phenocryst grainsize, the presence of S and C fabrics (and the angle between the foliations) and the width of mylonite and ultramylonite outcrops. Our results suggest that there is no correlation between the size of K-feldspar phenocrysts and recognised indicators of high strain, such as the acute angle between S and C fabrics, and thickness of bands of mylonite/ultramylonite. This may indicate that the size of K-feldspar in the Wyangala Granite was not homogeneous after crystallisation and prior to deformation. We therefore suggest that the size of K-feldspar phenocrysts is not a good guide to the strain intensity. However, the high-strain shear zones in the Wyangala Granite can be mapped using the acute angle between S and C fabrics and the intensity and thickness of mylonites/ultramylonites. This work suggests that the high-strain zones are not continuous along strike.

Phylogeography and population genomics of a lotic water beetle across a complex tropical landscape

The habitat template concept applied to a freshwater system indicates that lotic species, or those which occupy permanent habitats along stream courses, are less dispersive than lentic species, or those that occur in more ephemeral aquatic habitats. Thus, populations of lotic species will be more structured than those of lentic species. Stream courses include both flowing water and small, stagnant microhabitats that can provide refuge when streams are low. Many species occur in these microhabitats but remain poorly studied. Here, we present population genetic data for one such species, the tropical diving beetle Exocelina manokwariensis (Dytiscidae), sampled from six localities along a ~300 km transect across the Birds Head Peninsula of New Guinea. Molecular data from both mitochondrial (CO1 sequences) and nuclear (ddRAD loci) regions document fine‐scale population structure across populations that are ~45 km apart. Our results are concordant with previous phylogenetic and macroecological studies that applied the habitat template concept to aquatic systems. This study also illustrates that these diverse but mostly overlooked microhabitats are promising study systems in freshwater ecology and evolutionary biology. With the advent of next‐generation sequencing, fine‐scale population genomic studies are feasible for small nonmodel organisms to help illuminate the effect of habitat stability on species’ natural history, population structure and geographic distribution.

Comment on “Dextral transpression and late-Eocene magmatism in the trans-Himalayan Ladakh Batholith (North India): implications for tectono-magmatic evolution of the Indo-Eurasian collisional arc”

We would like to congratulate Sen and Collins (2012) for producing a detailed study of the structural history of the Ladakh Batholith according to new LA-ICPMS U/Pb analyses of zircon and AMS measurements. This work provides some significant findings to the evolution of this part of the Himalaya. However, we wanted to draw the author’s attention to the omission of a discussion of other geochronological results from the region (White et al. 2012). This is because the results are particularly important for understanding the India–Asia collision. We also raise concerns about some of the conclusions that were drawn from AMS measurement in regard to postcrystallization deformation of the Ladakh Batholith granitoids. Switches between Iand S-type granites during the Himalayan orogeny