Paul H. G. M. Dirks

Differential exhumation in response to episodic thrusting along the eastern margin of the Tibetan Plateau

Abstract Thermochronological data from the Songpan-Ganze˛Fold Belt and Longmen Mountains Thrust-Nappe Belt, on the eastern margin of the Tibetan Plateau in central China, reveal several phases of differential cooling across major listric thrust faults since Early Cretaceous times. Differential cooling, indicated by distinct breaks in age data across discrete compressional structures, was superimposed upon a regional cooling pattern following the Late Triassic Indosinian Orogeny. 40Ar/39Ar data from muscovite from the central and southern Longmen Mountains Thrust-Nappe Belt suggest a phase of differential cooling across the Wenchuan-Maouwen Shear Zone during the Early Cretaceous. The zircon fission track data also indicate differential cooling across a zone of brittle re-activation on the eastern margin of the Wenchuan-Maouwen Shear Zone during the mid-Tertiary, between ∼38 and 10 Ma. Apatite fission track data from the central and southern Longmen Mountains Thrust-Nappe Belt reveal differential cooling across the Yingxiu-Beichuan and Erwangmiao faults during the Miocene. Forward modelling of apatite fission track data from the northern Longmen Mountains Thrust-Nappe Belt suggests relatively slow regional cooling through the Mesozoic and early Tertiary, followed by accelerated cooling during the Miocene, beginning at ca. 20 Ma, to present day. Regional cooling is attributed to erosion during exhumation of the evolving Longmen Mountains Thrust-Nappe Belt (LMTNB) following the Indosinian Orogeny. Differential cooling across the Wenchuan-Maouwen Shear Zone and the Yingxiu-Beichuan and Erwangmiao faults is attributed to exhumation of the hanging walls of active listric thrust faults. Thermochronological data from the Longmen Mountains Thrust-Nappe Belt reveal a greater amount of differential exhumation across thrust faults from north to south. This observation is in accord with the prevalence of Proterozoic and Sinian basement in the hanging walls of thrust faults in the central and southern Longmen Mountains. The two most recent phases of reactivation occurred following the initial collision of India with Eurasia, suggesting that lateral extrusion of crustal material in response to this collision was focused along discrete structures in the LMTNB.

Extensional structures during deposition of the 3460 Ma Warrawoona Group in the eastern Pilbara Craton, Western Australia

Abstract Structures in the granite-greenstone terrain of the eastern Pilbara were mapped in both the Coongan Belt greenstones and Shaw Batholith granitoids. Important structures that are coeval with an early magmatic event, in both the granitoids and the greenstones, were identified. In the Coongan greenstones a brittle extensional fault array is syn-kinematic with felsic volcanics of the Duffer Formation, dated at 3452 ± 16 Ma (Pidgeon, 1978a). In the underlying Shaw Batholith a major ductile shear zone was mapped at the contact with the Coongan Belt. Although the foliation in the shear zone forms part of a domal structure, the stretching lineations constantly plunge to the ENE. The sense of shear as recorded in the shear zone is consistently east up with subsidiary sinistral strike slip movement on the EW-trending part and dextral movement on the NS-trending part. A wedge-shaped granitoid intruded into the shear zone during deformation. This granitoid is part of the North Shaw Suite which has been dated at 3467 ± 6 Ma (McNaughton et al., 1988). The structures in the greenstones and in the batholith have therefore formed during the same event at different crustal levels. The structural/kinematic, magmatic, metamorphic and geochronological data are consistent with a model in which these structures represent the downdip part of a dome with a core complex type of geometry. In turn, this implies that the domal geometry of the Shaw Batholith was initiated, at mid crustal levels, at a very early stage (namely during deposition of the Warrawoona Group) in the evolution of the Pilbara craton. As extension occurred during intrusion and extrusion of large volumes of magma, the original distribution and thickness of granitoid bodies and greenstones were probably largely determined by the extensional geometry. The initial domal shape of the Shaw Batholith has been enhanced during later deformation phases, which caused steepening of NS-trending structures.

Horizontal accretion and stabilization of the Archean Zimbabwe Craton

Structural-metamorphic data and mineral ages from the northern parts of the Zimbabwe craton indicate that Archean crustal formation and stabilization evolved in two stages. In the Shamva-Bindura greenstone belt, early layer-parallel shear zones began to form at 2670 Ma and accommodated imbricate stacking of oceanic and volcanic arc material between large nappe structures with felsic gneiss cores. The resultant crustal pile of anomalously hot felsic and mafic crustal slices reached isostatic and mechanical equilibrium at a thickness of 35 km. Further shortening of this pile caused strain partitioning into vertical strike-slip zones. The subsequent establishment of an equilibrium geotherm resulted in large-scale crustal melting and diapirism between 2620 and 2600 Ma. The rise of the melt and diapirs caused a second transient metamorphic imprint and much of the strain pattern regarded as typical for the Zimbabwe craton. This diapiric stage led to cooling and stabilization of the craton.

Geothermobarometric evidence for a metamorphic core complex in Sinai, Egypt

Blasband et al. [Geologie en Mijnbouw 76 (1997) 247, J. Geol. Soc. Lond. 157 (2000) 615] postulated a metamorphic core-complex model for the Wadi Kid area, south Sinai, Egypt. This core complex was formed in an extensional setting after gravitational collapse of the East African Orogen in the Late Proterozoic. Arc-accretion was responsible for the closure of the Mozambique Ocean. Blasband et al. [Geologie en Mijnbouw 76 (1997) 247, J. Geol. Soc. Lond. 157 (2000) 615] based their theory mainly on structural data. In order to justify this model geothermobarometric evidence is crucial. Therefore, the metamorphic rocks of the Wadi Kid area were the subject of a detailed metamorphic study. The M1 metamorphic phase is characterized by greenschist-facies conditions and is related to arc-accretion. The M 2 metamorphic phase is the main subject of this paper. The petrologic characteristics of pelitic and mafic rocks that were metamorphosed during M2 show that the lower crustal rocks in the Wadi Kid area were subjected to upper greenschist to upper amphibolite conditions of the low-pressure/high-temperature type at the end of the Late Proterozoic. Garnet-biotite and amphibole-plagioclase geothermometry reveal temperatures of 488–684 ◦ C. Plagioclase-biotite-muscovite-garnet geothermometry and amphibole-plagioclase geobarometry indicate pressures of 3.42–4.28 kbar. The M2 metamorphic phase is associated with a D2 deformation phase. This phase is a relict of gravitational collapse during the final stages of the collision of East and West Gondwanaland.

Pb- and Nd-isotope systematics of stromatolitic limestones from the 2.7 Ga Ngezi Group of the Belingwe Greenstone Belt: constraints on timing of deposition and provenance

Abstract Pb–Pb isochrons have been obtained for stromatolitic limestones from the late Archaean Belingwe Greenstone Belt of Zimbabwe, providing direct age constraints on the deposition of these shallow water marine sediments. Samples from the Manjeri Formation and stratigraphically higher Cheshire Formation yield age estimates of 2706±49 Ma (MSWD=11.5) and 2601±49 Ma (MSWD=0.93), respectively. These data are in agreement with published U–Pb zircon and Pb–Pb whole rock ages of associated volcanics, and we, therefore, interpret our Pb–Pb ages as representing the timing of early diagenesis, thus providing a minimum age for carbonate precipitation. A 2543±70 Ma age (MSWD=5.1) for one sample from the Cheshire Formation is considered to reflect either late-stage diagenesis, a craton wide thermal/chemical disturbance or tectonic activity along the crustal-scale Mtshingwe fault. Calculated model μ1-values for Manjeri and Cheshire limestones are 8.40±0.02 and 9.02±0.01, similar to values for approximately 3.5 and 2.9 Ga Archaean basement units adjacent to the Belingwe Belt. Negative eNd (Tdeposition) and fSm/Nd suggest derivation of the REE from old, LREE enriched continental crust. Two-stage Nd model ages for the carbonates indicate that precursor rocks were extracted from the mantle at around 3.5 Ga (Cheshire Formation) and 3.3 Ga (Manjeri Formation), in good agreement with mantle-extraction ages for local basement units (model TDM: 3.5–3.3 Ga).

Crust–mantle decoupling and the growth of the Archaean Zimbabwe craton

Abstract Based on the Zimbabwe craton, it is suggested that, during the Archaean, full decoupling between a strong upper crust and a strong upper mantle across a weak detachment zone at the Moho allowed the independent development of crustal and mantle geometries in response to lithospheric shortening. This is an effective way to explain the field observations made in the Zimbabwe craton, which suggest a late-Archaean interplay between lateral accretionary processes through low angle thrust stacking and underplating and deep seated lineament zones with a possible mantle origin. The lineament zones play an important role in the localisation of mineral deposits such as base metals, gold, and possibly diamonds. Thickening of the mantle lithosphere occurred independently from the crust, through early Archaean melt segregation and/or lithospheric underplating.

Structural relations and PbPb zircon ages for the Makuti gneisses: evidence for a crustal-scale Pan-African shear zone in the Zambezi Belt, northwest Zimbabwe

Abstract The Makuti Group of northwest Zimbabwe is composed of mafic and intermediate biotite-rich gneisses interlayered with quartzofeldspathic gneisses of granitic composition, and minor sedimentary units. The gneisses have experienced a multi-staged metamorphic history, including an early high temperature-high pressure event and subsequent reworking at upper- to mid-amphibolite-facies conditions. They are positioned along the strongly deformed, southern margin of the east-west trending Zambezi Belt, and have been correlated with supracrustal gneiss units along the northern margin of the Zimbabwe Craton. The Makuti Group is characterised by an intensely developed gneissic layering and complex disharmonic folds that resulted from non-coaxial deformation involving repeated stages of transposition. The basal contact of the g roup coincides with a decrease in strain intensity, but not with a directional change of characteristic structural elements (e.g. lineations, fold axes), nor with a clear change in rock types. Pink quartzofeldspathic gneisses of granitic composition are typical for the Makuti Group, but locally intrude basement gneiss as well. The quartzofeldspathic gneisses occur as porphyritic and non-porphyritic varieties that are, invariably, intensely sheared. The age and nature of the basal contact of the Makuti Group and its relationship to the quartzofeldspathic gneisses has been investigated. Samples for single zircon PbPb dating were collected from a felsic biotite gneiss just below (2704 ± 0.3 Ma) and above (2510 ± 0.4 Ma) the lower contact of the Makuti Group at an ‘unconformity’ 2 km northwest of Vuti. Further samples were collected from pink quartzofeldspathic units at the base (737 ± 0.9 Ma), central part (764 ± 0.9 Ma; 797 ± 0.9 Ma) and top (794 ± 0.5 Ma; 854 ± 0.8 Ma) of the Makuti Group. Two samples of Kariba orthogneiss (1920 ± 0.4 and 1963 ± 0.4 Ma) underlying the Makuti Group in the northwest were also collected. In all samples, long-prismatic, colourless to brown, igneous zircon grains were selected. Dates were obtained using a stepwise single-grain evaporation technique. Although this technique only allows minimum age estimates, the dates are highly reproducible, indicating that they approximate emplacement ages. The ages conform with the field observations that the basement has been reworked in the Makuti Group and that the quartzofeldspathic units may have been emplaced as granites. It is proposed that the Makuti Group represents a crustal scale shear zone that partly reworked basement gneisses and acted as a conduit for granite emplacement. Shearing took place in an extensional setting around 800 Ma ago, and may have resulted in the exhumation of lower crustal rocks.

Silicic Layer-Parallel Shear Zones in a Zimbabwean Greenstone Sequence: Horizontal Accretion Preceding Doming

Abstract Detailed structural metamorphic data from the Trojan nickel mine area in the Shamva-Bindura Greenstone Belt (SBGB) show that two groups of ductile structures associated with separate amphibolite facies metamorphic assemblages can be distinguished. D 1 /M 1 structures are related to a network of anastomosing shear zones that accommodated W-directed imbricate stacking of the stratigraphy, while metamorphic conditions in the mine area reached about 500°C at pressures of 3-4 kbar. D 1 shears are locally strongly silicified and preserve fine-grained mylonitic textures. They have been wrongly identified in the past as banded iron stones or stratigraphic chert horizons. Truncations of primary layering associated with such fine-grained mylonitic quartzites are tectonic and not stratigraphic in origin, and the units can not be used as tectonic marker horizons, but instead represent glide planes across which tectonic imbrication of the stratigraphy has occurred. An important silicified shear zone of this nature occurs along the boundary of the Iron Mask and Arcturus Formations. D 2 /M 2 structures are related to doming of the Chinamora Batholith, and a contact metamorphic overprint and recrystallisation of M 1 assemblages. M 2 temperatures in the mine area reached 565°C. Such high contact metamorphic temperatures at 3 km from the contact of the batholith can only be explained if the entire Chinamore Batholith was emplaced as a relatively hot (>750°C) intrusive body in an already anomalously hot greenstone sequence. Metamorphic fluids during M 1 and M 2 where CO 2 -rich and carbonate alteration was pervasive. D 1 /M 1 and D 2 /M 2 events may have been separated by as much as 70 Ma. D 1 structures in the Trojan area can be related to a large mantled-gneiss, nappe structure represented by the Madziwa Batholith and a mantle of mafic greenstones to the N of the SBGB. The footwall thrust of this nappe occurs along the N boundary of the Shamvaian sediments in the centre of the SBGB. The imbricate stacking of the stratigraphy in the Trojan area can be interpreted as secondary structures in the footwall of the nappe. Horizontal accretion and stacking of crustal fragments during D 1 was followed by thermal perturbations that resulted in the diapiric rise of domes like the Chinamora Batholith during D 2 . After doming the terrain appears to have cooled and stabilized, with further deformation partitioned into narrow strike-slip shear zones.

Horizontal tectonic deformation geometries in a late Archaean sedimentary sequence, Belingwe greenstone belt, Zimbabwe

In the Belingwe greenstone belt of Zimbabwe, structural evidence from the circa 2.65 Ga old sedimentary Cheshire Formation which overlies and is imbricated with a mafic volcanic unit is consistent with thrusting of the greenstone sequence. The Cheshire Formation consists of a karstified carbonate ramp sequence overlain by siliciclastic turbidite deposits that formed in a southeast deepening basin. The earliest deformational structures formed during a syndepositional to postdepositional, thin-skinned thrusting event (D1) that affected poorly consolidated sediments and is recorded in bedding-parallel ductile shear zones, boudins, folds, and block-in-matrix structures. D1 shear zones separate the volcanic sequence and the overlying sediments and occur between the carbonate and siliciclastic units. Syntectonic sulphide mineralization and silicification of mainly fine-grained sediments along the main thrust faults gave rise to the formation of rocks similar in appearance to banded iron formations. Stratigraphic units were locally duplicated along the D1 thrust faults, including a tectonic slice of mafic volcanics that was emplaced onto carbonates along a chaotic unit similar to a tectonic melange. The elongation of inclusions in the melange zone and lineations in ironstones together with kinematic indicators suggest that stratigraphic duplication resulted from northwestward directed tectonic transport. Deposition of the Cheshire Formation took place in an asymmetric, foreland-type basin contemporaneously with thrusting. Soon after deposition the formation was incorporated into the thrust stack. Subsequent deformation events include tight upright folding, gentle cross-folding, and dextral strike-slip faulting of the greenstone succession. Evidence for early thrusting suggests that horizontal tectonic processes played an important role in the evolution of the Belingwe greenstone belt.

U-Pb zircon ages from a craton-margin archaean orogenic belt in northern Zimbabwe

Abstract In northern Zimbabwe, the Archaean Pfunzi Orogen comprises an east-west-trending belt of migmatitic gneisses separating the Zimbabwe Craton from the Pan-African Zambezi Belt farther north. Previously available Rb-Sr dates for the Pfunzi Orogenic Belt have been interpreted to record formation of granulites at ca 3.0 Ga and subsequent amphibolite-facies metamorphism at ca 2.6 Ga. Here the first conventional IDTIMS U-Pb zircon dates for the Pfunzi Belt are reported, from orthogneisses in the eastern part of the belt in northeastern Zimbabwe. Two intrusive masses of granitic and tonalitic gneiss, together with a granitic leucogneiss inferred to have formed by leucosome segregation during migmatisation, yield crystallisation ages of ca 2.62 Ga. These results are interpreted to date a major tectonothermal event along the northern margin of the Zimbabwe Craton, involving regional amphibolite-facies metamorphism and migmatisation, as well as emplacement of compositionally diverse granitoid plutons and retrogression of older granulites. This event is roughly coeval with orogenesis in the Limpopo Belt along the southern craton margin and with emplacement of the craton-wide ca 2.6 Ga Chilimanzi Granite Suite.