R. J. Holcombe

Apatite fission-track thermochronology of the Sierras Pampeanas, central western Argentina: Implications for the mechanism of plateau uplift in the Andes

Over much of its length, the Andean orogen is characterized by a generally east-vergent geometry and a progressive eastward (cratonward) migration of individual arc-orogenic elements. A departure from this model occurs in the Sierras Pampeanas region of central western Argentina where a terrane of active basement uplifts is currently developing to the east of the main Cordillera. Apatite fission-track data from uplifted Precambrian and Phanerozoic basement rocks of the Sierras Pampeanas constrain the tectonic development of this terrane and indicate that deformation associated with exhumation may have propagated broadly westward since the late Miocene. Two pre-Andean cooling events—during the Carboniferous–Permian (ca. 300–280 Ma) and the early Jurassic–middle Jurassic (ca. 200–174 Ma)—have been identified. The onset of Andean deformation is represented by a cooling event during the late Paleocene–middle Eocene. This cooling was followed by a period of middle Miocene–late Miocene reheating, during foreland basin-style sedimentation. Exhumation, and possible westward migration of the exhumation “front” in the Sierras Pampeanas, commenced during the late Miocene-Pliocene to the east of the dominantly east-vergent Precordillera fold-and-thrust belt. The apparent convergence of deformation in these two terranes and the progressive closure of Miocene-Pliocene intermontane basins in the Sierras Pampeanas may reflect the early stages of Andean plateau uplift. The relative timing of plateau development along strike suggests that lateral thickening of the orogen is progressing southward at least from the latitude of central Bolivia (ca. 20°S). Furthermore, the time-space coincidence between basement uplift and flattening of the subducted slab beneath the Sierras Pampeanas suggests that a relationship exists between westward displacement of the terrane and the dynamics of plate interaction.

Tectono‐metamorphic evolution of the Mary Kathleen fold belt, northwest Queensland: A reflection of mantle plume processes?

The Proterozoic Mary Kathleen Fold Belt is one of the most intensely deformed fold belts within the Mt Isa Inlier (northwest Queensland), and contains key evidence for both extension and shortening, as well as a protracted thermal history culminating in a high temperature, low pressure amphibolite facies metamorphism. An early phase of extension (1780–1730 Ma) may have initially been responsible for basin development, and was subsequently associated with intense ductile shearing and multiple magma injection in a lower plate, and minor folding, extensional fracturing and emplacement of discrete intrusions in a more brittle upper plate. The subsequent regional deformation (D2) and metamorphism (1600–1500 Ma) involved east‐west directed compression, locally producing a variety of fold interference patterns by overprinting of F1 folds. This deformation resulted in tight folding of the early zone of high D1 strains into a large anticlinorium (the Wonga Belt). East of this zone, the effects of D2 dominate. The ...

Ductile fabrics in the zone of active oblique convergence near the Alpine Fault, New Zealand: identifying the neotectonic overprint ☆

The mid-crustal Alpine Schist in central Southern Alps, New Zealand has been exhumed during the past similar to3 m.y. on the hanging wall of the oblique-slip Alpine Fault. These rocks underwent ductile deformation during their passage through the similar to 150-km-wide Pacific-Australia plate boundary zone. Likely to be Cretaceous in age, peak metamorphism predates the largely Pliocene and younger oblique convergence that continues to uplift the Southern Alps today. Late Cenozoic ductile deformation constructively reinforced a pre-existing fabric that was well oriented to accommodate a dextral-transpressive overprint. Quartz microstructures below a recently exhumed brittle-ductile transition zone reflect a late Cenozoic increment of ductile strain that was distributed across deeper levels of the Pacific Plate. Deformation was transpressive, including a dextral-normal shear component that bends and rotates a delaminated panel of Pacific Plate crust onto the oblique footwall ramp of the Alpine Fault. Progressive ductile shear in mylonites at the base of the Pacific Plate overprints earlier fabrics in a dextral-reverse sense, a deformation that accompanies translation of the schists up the Alpine Fault. Ductile shear along that structure affects not only the 12-km-thick section of Alpine mylonites, but is distributed across several kilometres of overlying nonmylonitic rocks

Geometry of a Middle Proterozoic extensional décollement in northeastern Australia

Abstract An 80 km-long belt of gneissic and mylonitic rocks is exposed in the core of a major anticlinorium in the Middle Proterozoic Mt. Isa Inlier of northeastern Australia. These rocks, in the Mary Kathleen Fold Belt, represent an early, originally horizontal, mid-crustal shear zone which is interpreted as an extensional decollement between a lower ductile sheet and an upper, more brittle plate. The decollement was subhorizontal over a distance of at least 80 km and its depth was perhaps as shallow as 7 km. Shearing was accompanied by the syn-tectonic intrusion of a bimodal complex of granitic and basic rocks over a period of 70–90 m.y. Moderate, to extreme, ductile shearing was pervasive throughout the 1–1.5 km thickness of the lower plate that is now exposed. Massive dilation accompanied shearing and approximately 70% of the volume of-the shear zone is occupied by igneous intrusive rocks (which have been variably deformed to gneissic granite, mylonite, and amphibolite). Strain softening driven by this high thermal input is responsible for the pervasive ductile response at such shallow depths. Peak metamorphic conditions at the decollement were approximately 625°C to 675°C at 2 kbar. Intense ductile shearing in the upper plate is restricted to the lower few hundred metres. Above this zone, brittle processes dominate, although evidence is presented that the sheet as a whole has undergone a component of ductile shearing, probably heterogeneously distributed. Large, sheet-like granite and dolerite bodies were introduced into dilation sites associated with synthetic faults in this plate but, unlike their synchronous counterparts in the lower plate (about 1 km below), these are internally undeformed. The Middle Proterozoic history of the area involves a cycle of extension and shortening with the shortening axis perpendicular to the earlier extension axis. We speculate that the driving mechanism for both extension and shortening is mantle convection, with switches in stress related to mantle upwelling.

A sensitive vorticity gauge using rotated porphyroblasts, and its application to rocks adjacent to the Alpine Fault, New Zealand

Variable aspect ratio porphyroblasts deformed in non-coaxial flow. and internally containing rotated relicts of an external foliation, can be used to characterise plane strain flow regimes. The distribution obtained by plotting the orientation of the long axis of such grains, classified by aspect ratio, against the orientation of the internal foliation is potentially a sensitive gauge of both the bulk shear strain (as previously suggested) and kinematic vorticity number. We illustrate the method using rotated biotite porphyroblasts in the Alpine Schist: a sequence of mid-crustal rocks that have been ramped to the surface along the Alpine Fault. a major transpressional plate boundary. Results indicate that, at distances greater than or equal to similar to1 km from the fault, the rocks have undergone a combination of irrotational fattening and dextral-oblique, normal-sense shear, with a bulk shear strain of similar to0.6 and kinematic vorticity number of similar to0.2. The vorticity analysis is compatible with estimates of strongly oblate bulk strain of similar to 75% maximum shortening. Dextral-reverse transpressional flow characterises higher strain S-tectonite mylonite within similar to1 km of the Alpine Fault. These relationships provide insight into the kinematics of flow and distribution of strain in the hangingwall of the Alpine Fault and place constraints on numerical mechanical models for the exhumation of these mid-crustal rocks

U–Pb zircon geochronology of Late Devonian to Early Carboniferous extension‐related silicic volcanism in the northern New England Fold Belt*

Laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) analysis of zircons confirm a Late Devonian to Early Carboniferous age (ca 360–350 Ma) for silicic volcanic rocks of the Campwyn Volcanics and Yarrol terrane of the northern New England Fold Belt (Queensland). These rocks are coeval with silicic volcanism recorded elsewhere in the fold belt at this time (Connors Arch, Drummond Basin). The new U–Pb zircon ages, in combination with those from previous studies, show that silicic magmatism was both widespread across the northern New England Fold Belt (>250 000 km2 and ≥500 km inboard of plate margin) and protracted, occurring over a period of ∼15 million years. Zircon inheritance is commonplace in the Late Devonian — Early Carboniferous volcanics, reflecting anatectic melting and considerable reworking of continental crust. Inherited zircon components range from ca 370 to ca 2050 Ma, with Middle Devonian (385–370 Ma) zircons being common to almost all dated units. Precambrian zircon components record either Precambrian crystalline crust or sedimentary accumulations that were present above or within the zone of magma formation. This contrasts with a lack of significant zircon inheritance in younger Permo‐Carboniferous igneous rocks intruded through, and emplaced on top of, the Devonian‐Carboniferous successions. The inheritance data and location of these volcanic rocks at the eastern margins of the northern New England Fold Belt, coupled with Sr–Nd, Pb isotopic data and depleted mantle model ages for Late Palaeozoic and Mesozoic magmatism, imply that Precambrian mafic and felsic crustal materials (potentially as old as 2050 Ma), or at the very least Lower Palaeozoic rocks derived from the reworking of Precambrian rocks, comprise basement to the eastern parts of the fold belt. This crustal basement architecture may be a relict from the Late Proterozoic breakup of the Rodinian supercontinent.

Exhumation of late Paleozoic blueschists in Queensland, Australia, by extensional faulting

Blueschists in southeastern Queensland, Australia, record a Carboniferous history of subduction and metamorphism at >6 kbar. A later thermal overprint associated with intrusion of S type granitoids ca. 306 Ma and regional greenschist facies metamorphism accompanied formation of ductile fabrics in the lower plate of an oceanic-protolith metamorphic core complex. Late Carboniferous extensional deformation in this part of the New England orogen may have accompanied outstepping of the subduction trench, or rollback and steepening of the paleo-Pacific plate during development of a wide zone of asthenospheric upwelling and continental back-arc extension. -Authors

Petrological, geochemical and geochronological evidence for a Neoproterozoic ocean basin recorded in the Marlborough terrane of the northern New England Fold Belt*

Petrological, geochemical and radiogenic isotopic data on ophiolitic‐type rocks from the Marlborough terrane, the largest (∼700 km2) ultramafic‐mafic rock association in eastern Australia, argue strongly for a sea‐floor spreading centre origin. Chromium spinel from partially serpentinised mantle harzburgite record average Cr/(Cr + Al) = 0.4 with associated mafic rocks displaying depleted MORB‐like trace‐element characteristics. A Sm/Nd isochron defined by whole‐rock mafic samples yields a crystallisation age of 562 ± 22 Ma (2σ). These rocks are thus amongst the oldest rocks so far identified in the New England Fold Belt and suggest the presence of a late Neoproterozoic ocean basin to the east of the Tasman Line. The next oldest ultramafic rock association dated from the New England Fold Belt is ca530 Ma and is interpreted as backarc in origin. These data suggest that the New England Fold Belt may have developed on oceanic crust, following an oceanward migration of the subduction zone at ca540 Ma as recorded by deformation and metamorphism in the Anakie Inlier. Fragments of late Neoproterozoic oceanic lithosphere were accreted during progressive cratonisation of the east Australian margin.

Yarrol terrane of the northern New England Fold Belt: Forearc or backarc?

The Upper Devonian to Lower Carboniferous volcanosedimentary rocks of the Yarrol terrane of the northern New England Fold Belt have previously been ascribed to a forearc basin setting. New data presented here, however, suggest that the Yarrol terrane developed as a backarc basin during the Middle to early Late Devonian. Based on field studies, we recognise four regionally applicable stratigraphic units: (i) a basal, ?Middle to Upper Devonian submarine mafic volcanic suite (Monal volcanic facies association); (ii) the lower Frasnian Lochenbar beds that locally unconformably overlie the Monal volcanic facies association; (iii) the Three Moon Conglomerate (Upper Devonian ‐ Lower Carboniferous); and (iv) the Lower Carboniferous Rockhampton Group characterised by the presence of oolitic limestone. Stratigraphic and compositional differences suggest the Monal volcanic facies association post‐dates Middle Devonian silicic‐dominated magmatism that was coeval with gold‐copper mineralisation at Mt Morgan. The Lochenbar beds, Three Moon Conglomerate and Rockhampton Group represent a near‐continuous sedimentary record of volcanism that changed in composition and style from mafic effusive (Late Devonian) to silicic explosive volcanism (Early Carboniferous). Palaeocurrent data from the Three Moon Conglomerate and Rockhampton Group indicate dispersal of sediment to the west and northwest, and are inconsistent with derivation from a volcanic‐arc source situated to the west (Connors‐Auburn Arch). Geochemical data show that the Monal volcanic facies association ranges from tholeiitic subalkaline basalts to calc‐alkaline basaltic andesite. Trace and rare‐earth element abundances are distinctly MORB‐like (e.g. light rare earth element depletion), with only moderate enrichment of the large‐ion lithophile elements in some units, and negative Nb anomalies, suggesting a subduction‐related signature. Basalts of the Monal volcanic facies association are best described as transitional between calc‐alkali basalts and N‐MORB. The elevated high field strength element contents (e.g. Zr, Y, Ti) are higher than modern island‐arc basalts, but comparable to basalts that floor modern backarc basins. This geochemical study, coupled with stratigraphic relationships, suggest that the eruption of backarc basin basalts followed widespread Middle Devonian, extension‐related silicic magmatism (e.g. Retreat Batholith, Mt Morgan), and floored the Yarrol terrane. The Monal volcanic facies association thus shows similarities in its tectonic environment to the Lower Permian successions (e.g. Rookwood Volcanics) of the northern New England Fold Belt. These mafic volcanic sequences are interpreted to record two backarc basin‐forming periods (Middle ‐ Late Devonian and Late Carboniferous ‐ Early Permian) during the Late Palaeozoic history of the New England Orogen. Silicic‐dominated explosive volcanism, occurring extensively across the northern New England Fold Belt in the Early Carboniferous (Yarrol terrane, Campwyn Volcanics, Drummond and Burdekin Basins), reflects another period of crustal melting and extension, most likely related to the opening of the Drummond Basin.

40Ar/39Ar thermochronology of epidote blueschists from the North D'Aguilar block, Queensland, Australia: Timing and kinematics of subduction complex unroofing

Mechanisms proposed for exhumation of blueschists include whole-crust uplift and erosion, denudation by crustal-scale extensional faulting or ductile strain, and transport of blocks entrained in rising diapiric masses. Insight into the unroofing process, and the role of granitic plutonism, can be gleaned from study of North D9Aguilar block in the New England orogen of southeastern Queensland, Australia. Epidote blueschist facies rocks occur as structurally coherent schists and as blocks in serpentinite matrix melange. These subduction-underplated rocks formed at depths >18 km and occur in the lower plate of a metamorphic core complex. A slate from the upper plate has a whole-rock total-gas age of 315 Ma, interpreted as a minimum age for subduction along this part of the paleo-Pacific margin. Exhumation of lower plate schists was coeval with overprinting of early high-pressure (M 1 ) metamorphic fabrics by a greenschist facies fabric (M 2 ) and was accomplished in part by ductile stretching and normal faulting. Phengites from most lower plate schists yield 40 Ar/ 39 Ar plateau ages of ca. 299–296 Ma, recording the time elapsed since the schists cooled below ∼350 ± 50 °C following M 2 . Plateau-shaped age spectra and small structural depth-related age gradients indicate rapid cooling of schists in the footwall of a present-day low-angle normal fault. Samples from deeper levels of the lower plate remained at >350 °C for 35 m.y. longer than the other samples and contain extraneous Ar possibly absorbed from fluids originating at depth. At ca. 307 Ma a granodiorite pluton was intruded into the lower plate near the peak of M 2 . A synkinematically intruded, in part mylonitic pluton increased the regional M 2 fabric to amphibolite facies in its aureole. Heat from the granitic pluton, and perhaps others like it at depth, may have played a strain-softening role in formation of the core complex. Similar 40 Ar/ 39 Ar cooling ages for different blueschist and greenschist blocks in serpentinite matrix melange support the view that the Australian melange was uplifted by extensional tectonic processes unrelated to serpentinite diapirism.