Mike Sandiford

Deep crustal metamorphism during continental extension: modern and ancient examples

Granulite facies metamorphism in the lower levels of continental crust which is undergoing extension is indicated by unusually high heat flow in modern-day extensional regimes. For certain geometries of extension, particularly those involving crustal-penetrative detachment zones, this metamorphism may occur on a regional scale. The predicted pressure-temperature-time (P-T-t) paths for such metamorphism involve heating into the granulite facies at constant or decreasing pressure during extension, followed by cooling at constant or increasing pressure after extension stops, and thus they differ considerably from P-T-t paths of metamorphic terrains formed by continental convergence. Many granulite terrains from both the Precambrian and Phanerozoic record preserve P-T-t paths which involve substantial, essentially isobaric, cooling. In such terrains granulite facies metamorphism is typically associated with recumbent structures characterised by subhorizontal stretching lineations which are attributed to intense non-coaxial deformation. Such deformation may be expected for the deep crustal expression of detachment zones. These terrains may provide ancient examples of deep crustal metamorphism during extensional tectonics. Extensional tectonics is attracting widespread interest in the geological community, and geologists, in particular structural geologists, have begun to investigate the features which may be considered diagnostic of ancient extensional tectonic regimes [1]. Recently, extension associated with continental rifting has been proposed as a mechanism for the generation of high-T/low-P metamorphic terrains [2-4], and there is now a need to define the criteria by which metamorphism associated with extensional tectonics may be distinguished from the metamorphism of the more familiar terrains of continental convergence zones. In this contribution we discuss the evidence for deep crustal metamorphism during continental extension, the pressure-temperature-time (P-T-t) paths associated with such metamorphism, and, finally, a number of examples of deep crustal metamorphism which may have been associated with extensional tectonics, The observation that heat flow through con

Asymmetric extension associated with uplift and subsidence in the Transantarctic Mountains and Ross Embayment

Abstract Apatite fission track data combined with regional geological observations indicate that the uplift of the Transantarctic Mountains has been coeval with thinning and subsidence of the crust beneath the Ross Embayment. In the Dry Valleys region of south Victoria Land, the mountains have been uplifted about 5 km since the early Cenozoic at an average rate of about 100 m/Ma. During uplift, the crust remained at constant thickness or was slightly thickened by magmatic underplating. In contrast, the crust beneath the Ross Embayment has been extended and consequently thinned beginning in the Late Cretaceous but mainly during Cenozoic times. We suggest here that the uplift of the Transantarctic Mountains and the subsidence of the Ross Embayment are a result of passive rifting governed by a fundamental structural asymmetry defined by a shallow crustal penetrative detachment zone that dips westward beneath the Transantarctic Mountain Front. The localization and asymmetry of this detachment and its unusually deep level expression are attributed to a profound crustal anisotropy inherited from an early Palaeozoic collision along the present site of the mountain range.

Topography, boundary forces, and the Indo-Australian intraplate stress field

The relative contribution of topographic (e.g., ridge push, continental margins, and elevated continental crust) and plate boundary (e.g., subduction and collisional) forces to the intraplate stress field in the Indo-Australian plate (IAP) is evaluated through a finite element analysis. Two important aspects of the IAP intraplate stress field are highlighted in the present study: (1) if substantial focusing of the ridge push torque occurs along the collisional boundaries (i.e., Himalaya, New Guinea, and New Zealand), many of the first-order features of the observed stress field can be explained without appealing to either subduction or basal drag forces; and (2) it is possible to fit the observed SHmax, (maximum horizontal stress orientation) and stress regime information with a set of boundary conditions that results in low tectonic stress magnitudes (e.g., tens of megapascals, averaged over the thickness of the lithosphere) throughout the plate. This study therefore presents a plausible alternative to previous studies of the IAP intraplate stress field, which predicted very large tectonic stress magnitudes (hundreds of megapascals) in some parts of the plate. In addition, topographic forces due to continental margins and elevated continental material were found to play an important role in the predicted stress fields of continental India and Australia, and the inclusion of these forces in the modeling produced a significant improvement in the fit of the predicted intraplate stresses to the available observed stress information in these continental regions. A central focus of this study is the relative importance of the boundary conditions used to represent forces acting along the northern plate margin. We note that a wide range of boundary conditions can be configured to match the large portion of the observed intraplate stress field, and this nonuniqueness continues to make modeling the IAP stress field problematic. While our study is an important step forward in understanding the sources of the IAP intraplate stress field, a more complete understanding awaits a better understanding of the relative magnitude of the boundary forces acting along the northern plate margin.

Tectonic feedback and the ordering of heat producing elements within the continental lithosphere

The distribution of the heat producing elements within the lithosphere provides an important control on continental thermal regimes and the mechanical strength of the lithosphere. Moreover, the strong temperature dependence of lithospheric rheology suggests the possibility of an important feedback between deformation and the distribution of heat producing elements. Simple models for lithospheric rheology are used to illustrate how such feedback might serve as an important control on both the characteristic abundance of, and spatial variation in, the heat production elements in the crust. These models also imply that the organisation of heat producing elements is essential for the long-term tectonic stabilisation of the continental crust. This is particularly relevant to the evolution of cratons in early Earth history, wherein lies the most dramatic evidence for the role played by tectonic processes in achieving a stable ordering of the heat producing elements. ( 2002 Elsevier Science B.V. All rights reserved.

Regional geochemistry and continental heat flow: implications for the origin of the South Australian heat flow anomaly

Abstract Existing measurements from South Australia define a broad (>250 km wide) zone of anomalously high surface heat flow (92±10 mW m −2 ). This zone is centred on the western margin of the Adelaide Fold Belt (Neoproterozoic to early Phanerozoic cover floored by Palaeoproterozoic to Mesoproterozoic basement), where it borders the eastern Gawler Craton and Stuart Shelf (Palaeoproterozoic–Mesoproterozoic). To the west, in the western Gawler Craton (Archaean to Palaeoproterozoic), heat flow averages ∼54 mW m −2 while to the east in the Willyama Inliers (Palaeoproterozoic) heat flow averages ∼75 mW m −2 . We use a regional geochemical dataset comprising >2500 analyses to show that the anomalous heat flow zone correlates with exceptional surface heat production values, mainly hosted in Palaeoproterozoic to Mesoproterozoic granites. The median heat production of Precambrian ‘basement’ rocks increases from −3 west of the anomalous zone to ∼6 μW m −3 within the anomalous zone. In the highest known part of the heat flow anomaly, Mesoproterozoic gneisses and granites of the Mount Painter Province in the northern Adelaide Fold Belt yield an area-integrated mean heat production of 9.9 μW m −3 . These data suggest that the anomalous heat flow reflects an unusual enrichment in U and Th in this part of the Proterozoic crust, with the total complement of these elements some 2–3 times greater than would be expected for Proterozoic crust on the basis of the global heat flow database. This extraordinary enrichment has played an important role in modulating the thermal regime of the crust in this region, and particularly its response to tectonic activity.

The origins of the intraplate stress field in continental Australia

The ridge push force acting on the Indo-Australian plate exerts a significant torque (8.5 × 1025N m) about a pole at 30.3°N, 34.5°E. The angular difference between this torque pole and the observed pole of rotation for the plate (19.2°N, 35.6°E) is less than 12° and suggests that the ridge push force plays an important role in the dynamics of the Indo-Australian plate. We have used an elastic finite-element analysis to study the predicted intraplate stress field in continental Australia for four models which employ different boundary conditions to balance the ridge push torque acting on the plate. The modeling indicates that a number of important features of the observed stress field within the Australian continent can be explained in terms of balancing the ridge push torque with resistance imposed along the Himalaya, Papua New Guinea, and New Zealand collisional boundaries segments. These features include NS-to NE-SW-oriented compression in the northern Australia and E-W-oriented compression in southern Australia. Our analysis also shows that subduction processes along the northern and eastern boundaries provide only second-order controls on the intraplate stress field in continental Australia.

Intraplate deformation in central Australia, the link between subsidence and fault reactivation

Abstract Central Australia has experienced two intraplate orogenic events involving significant north–south shortening: the late Neoproterozoic to Early Cambrian Petermann Orogeny and the Devonian to Carboniferous Alice Springs Orogeny. In each event pre-existing structures inherited from Mesoproterozoic terrain amalgamation were reactivated and basement rocks exhumed from beneath thick sedimentary successions accumulated in the Centralian Superbasin. The pattern of fault reactivation during these events shows a striking similarity to the pattern of subsidence in the overlying basin. Immediately prior to the Petermann Orogeny, the Centralian Superbasin was thickest in the vicinity of the Musgrave Block, the region in which deformation was subsequently localised. At the same time crustal-scale faults elsewhere in central Australia that were covered by a relatively thin sheet of sediment remained inactive despite being favourably oriented to accommodate the north–south shortening. Between the Petermann and Alice Springs Orogenies, subsidence patterns shifted, such that fault systems in the Arunta Block and also those in the southern Musgrave Block were buried by significant thicknesses of sediment, whereas the major structures that were exhumed during the Petermann Orogeny were not significantly buried. During the Alice Springs Orogeny reactivation once again occurred along the most deeply buried faults, even in the instances where those faults had remained inactive during the earlier Petermann Orogeny. Importantly the major Petermann-aged structures that were not buried during renewed subsidence remained inactive during the Alice Springs Orogeny. The record of reactivation implies that the presence of pre-existing crustal-scale faults alone was insufficient to localise deformation. Rather, fault reactivation appears to have required a priming process that modulated the strength of the lithosphere on a regional scale. The correspondence between the distribution of basement fault reactivation and subsidence patterns during both the Petermann and Alice Springs Orogenies implies a link between relatively thick sedimentation and long-term lithospheric weakening. We show that this link is compatible with the thermal effects of a thick sedimentary blanket. In the context of central Australia the mechanical impact of basin formation is likely to be enhanced by the presence of regionally elevated heat production in the Proterozoic basement.

Palaeozoic synorogenic sedimentation in central and northern Australia: A review of distribution and timing with implications for the evolution of intracontinental orogens

The Palaeozoic Alice Springs Orogeny was a major intraplate tectonic event in central and northern Australia. The sedimentological, structural and isotopic effects of the Alice Springs Orogeny have been well documented in the northern Amadeus Basin and adjacent exhumed Arunta Inlier, although the full regional extent of the event, as well as lateral variations in timing and intensity are less well known. Because of the lack of regional isotopic data, we take a sedimentological approach towards constraining these parameters, compiling the location and age constraints of inferred synorogenic sedimentation across a number of central and northern Australian basins. Such deposits are recorded from the Amadeus, Ngalia, Georgina, Wiso, eastern Officer and, possibly, Warburton Basins. Deposits are commonly located adjacent to areas of significant basement uplift related to north‐south shortening. In addition, similar aged orogenic deposits occur in association with strike‐slip tectonism in the Ord and southern Bonaparte Basins of northwest Australia. From a combination of sedimentological and isotopic evidence it appears that localised convergent deformation started in the Late Ordovician in the eastern Arunta Inlier and adjacent Amadeus Basin. Synorogenic style sedimentation becomes synchronously widespread in the late Early Devonian and in most areas the record terminates abruptly close to the end of the Devonian. A notable exception is the Ngalia Basin in which such sedimentation continued until the mid‐Carboniferous. In the Ord and Bonaparte Basins there is evidence of two discrete pulses of transcurrent activity in the Late Devonian and Carboniferous. The sedimentological story contrasts with the isotopic record from the southern Arunta Inlier, which has generally been interpreted in terms of continuous convergent orogenic activity spanning most of the Devonian and Carboniferous, with a suggestion that rates of deformation increased in the mid‐Carboniferous. Either Carboniferous sediments have been stripped off by subsequent erosion, or sedimentation outpaced accommodation space and detritus was transported elsewhere.

Horizontal structures in granulite terrains: A record of mountain building or mountain collapse?

In many high-temperature (>800 °C) granulite terrains, the development of characteristic horizontal structures occurred during the metamorphic culmination and was followed by isobaric cooling. The absolute magnitude of isobaric cooling (commonly >300 °C) implies cooling intervals of the order of the thermal time constant of the continental lithosphere (∼100 m.y.). Such prolonged isobaric cooling implies that no significant erosional denudation followed the development of the horizontal structures and thus precludes the prograde deformation being responsible for significant crustal thickening. Rather, the prograde deformation more probably records bulk crustal thinning during extensional collapse of a previously thickened crust possibly triggered by detachment of a thickened thermal boundary layer at the base of the lithosphere.

High radiogenic heat–producing granites and metamorphism—An example from the western Mount Isa inlier, Australia

The origins of metamorphism (∼600 °C and 3–4 kbar) in the western Mount Isa inlier, Australia, remain controversial for a number of reasons. (1) No synmetamorphic intrusive bodies can be recognized; (2) high temperatures appear to be sustained for periods >100 m.y.; and (3) metamorphism follows an extended phase of thermal subsidence. We show that the burial of granite batholiths enriched in radiogenic elements beneath the thick insulating sedimentary succession of the Mount Isa Group (deposited in response to repeated rift-sag cycles) was capable of generating steep upper crustal thermal gradients immediately prior to the Isan orogeny. These gradients are appropriate to peak metamorphic conditions, such that the ensuing Isan orogeny required no significant additional heat input. This result is significant in that it may provide a mechanism for understanding the origins of high-temperature metamorphism in other terranes where the involvement of transient heating is not obvious.