Current research Interests
Metamorphic Time Scales and Metamorphic Facies Series
Deep processes within the Earth’s crust are not directly observable. Thus, geological evidence for crustal pressure (P) and temperature (T) conditions comes from snapshots provided by rocks that formed at great depth. The most common of archives of crustal conditions is metamorphic rock.
Associations of specific, diagnostic metamorphic mineral assemblages/rock types define the ‘metamorphic facies series’. Certain metamorphic facies series are found in association with specific tectonic settings; rocks with low T/P ratios (Franciscan-type) are associated with the tectonic process of subduction, intermediate T/P (Barrovian-type) rocks with continental collision and mountain building, and high T/P (Buchan-type) rocks with continental extension and/or large-scale magmatism. Plate tectonics give rise to crustal P and T conditions that are unique according to tectonic setting, meaning that metamorphic facies series provide an excellent tool for investigating tectonic histories.
That T/P conditions recorded by metamorphic facies series are relatively persistent within their respective tectonic setting has become a common assumption in metamorphic geology; the snapshots provided by metamorphic rocks are representative of typical, long-lived crustal conditions. However, recent studies in ‘geospeedometry’ have suggested that metamorphism can be extremely rapid, involving time scales as brief as 10–1000 kyr.
These findings suggest that metamorphic facies series need not represent quasi-steady state conditions resulting from crustal-scale interactions among heat advection, conduction and/or production. In some cases, metamorphic rocks may instead record transient periods of atypical crustal conditions. New findings on metamorphic time scales has prompted a reexamination of the significance of metamorphic facies series; are they representative of long-lived crustal conditions, or do they mark fleeting events affecting the tectonic settings to which they are allied? Techniques in geospeedometry and high-precision petrochronology are key to addressing this question.
Associations of specific, diagnostic metamorphic mineral assemblages/rock types define the ‘metamorphic facies series’. Certain metamorphic facies series are found in association with specific tectonic settings; rocks with low T/P ratios (Franciscan-type) are associated with the tectonic process of subduction, intermediate T/P (Barrovian-type) rocks with continental collision and mountain building, and high T/P (Buchan-type) rocks with continental extension and/or large-scale magmatism. Plate tectonics give rise to crustal P and T conditions that are unique according to tectonic setting, meaning that metamorphic facies series provide an excellent tool for investigating tectonic histories.
That T/P conditions recorded by metamorphic facies series are relatively persistent within their respective tectonic setting has become a common assumption in metamorphic geology; the snapshots provided by metamorphic rocks are representative of typical, long-lived crustal conditions. However, recent studies in ‘geospeedometry’ have suggested that metamorphism can be extremely rapid, involving time scales as brief as 10–1000 kyr.
These findings suggest that metamorphic facies series need not represent quasi-steady state conditions resulting from crustal-scale interactions among heat advection, conduction and/or production. In some cases, metamorphic rocks may instead record transient periods of atypical crustal conditions. New findings on metamorphic time scales has prompted a reexamination of the significance of metamorphic facies series; are they representative of long-lived crustal conditions, or do they mark fleeting events affecting the tectonic settings to which they are allied? Techniques in geospeedometry and high-precision petrochronology are key to addressing this question.
Colour-enhanced WDS raw x-ray count maps for Mn in garnet from Ring Mtn, California (blueschist–eclogite, T≈550°C; top left), Puerto Cabello, Venezuela (blueschist–eclogite, T≈550°C; top right, bottom left) and the Barrovian series, Scotland (pelite, T≈590°C; bottom right).
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Geospeedometry
Diffusion is a thermally activated process. Thus, the degree to which it progresses can provide information on the nature and duration of thermal processes. The term ‘geospeedometry’ refers to approaches that use diffusion length scales to investigate thermal time scales. Co-workers from the Australian National University (ANU) and I contributed to development of techniques in geospeedometry, including the use of length scales of Ar diffusion in white mica and Mn diffusion in garnet to investigate thermal durations for the Barrovian metamorphism, Scotland. Results from the work suggest that the Barrovian metamorphism occurred over a period of 1–10 Myr, approximately c. 470 Myr ago. Furthermore, geospeedometry has revealed that Mn diffusion in garnet occurred over multiple, distinct length scales, suggesting the (1–10 Myr) regional metamorphism was attended by distinct thermal excursions on time scales of 10–100 kyr. Such discrepancies in thermal time scales for the Barrovian metamorphism are accounted for by models in which regional heating involved incremental and episodic heat accumulation (leading to development of a regional-scale thermal anomaly). My recent work has expanded to consider variation in metamorphic tempo (with distance from heat source) across other Barrovian-type, progressive metamorphic sequences. This research has maintained focus on geospeedometry, particularly ‘comparative geospeedometry', involving comparison of diffusion length scales for an element–mineral system in different rocks, for different element–mineral pairs within a single rock, and/or for elements with different diffusivities from a single mineral. There is still great scope for development and application of geospeedometry techniques to investigate metamorphic time scales and drivers. However, uncertainty in the parameters used for diffusion modelling raises some doubts regarding the veracity of metamorphic duration estimates made from geospeedometry. Estimates obtained from geospeedometry must be validated by techniques in ‘petrochronology’. Such techniques may include TIMS dating of distinct sectors of zoned garnets and/or single-shot laser ablation split stream ICP–MS geochronology. |
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Single-shot laser ablation split stream (SS–LASS) ICP–MS petrochronology
SS–LASS petrochronology is a technique that involves simultaneous measurement of U/Th–Pb isotope ratios and trace elements (e.g. Eu, Y) in accessory minerals (e.g. zircon, monazite). Isotope ratios are used to obtain radiogenic age information and trace element values to provide information on the metamorphic context for accessory mineral growth. SS–LASS differs from conventional (continuous-pulse) LASS techniques because ICP–MS analysis is performed on analyte obtained from a single laser ablation pulse. The SS–LASS technique allows dating of metamorphic overgrowths from the surface of intact grains. SS–LASS petrochronology offers spatial resolution of < 1 µm per analysis, enabling the technique to interrogate metamorphic zircon overgrowths that are very thin, or which preserve a composite age structure. The technique can achieve accuracies of < 1.5%, meaning that it is capable of providing < 1 Myr resolution for Cenozoic rocks. Geospeedometry has calculated anomalously short time scales estimates of 10–1000 kyr for the duration of metamorphism. With high spatial and temporal resolution, SS–LASS offers tremendous promise for investigating the veracity of these claims. To demonstrate the advantages of SS–LASS, co-workers from UCSB and I applied the technique to metamorphic zircon overgrowths in five rocks from the Cordillera de la Costa, Venezuela. In these rocks, SS–LASS was able to decipher discrete, short-duration (< 1 Myr) zircon growth events at c. 33.0, c. 28.3, c. 23.0 and c. 18.2 Ma. Comparison with existing geo-/thermochronology suggests the SS–LASS dates represent (hydro)thermal events that mark distinct episodes of tectonism affecting the northern margin of South America. We are now in the process of developing projects to interrogate the development of the thermal anomalies recorded in Barrovian-type, progressive metamorphic sequences using the SS–LASS technique. The metamorphism must be as young as possible (Paleogene to Neogene) in order to resolve the fine details of the metamorphic tempo. |
Typical Tera–Wasserburg plot for SS–LASS petrochronology of a grain from a metasedimentary rock from Puerto Cabello, Venezuela (top left). U/Pb zircon (metamorphic) age distribution for all SS–LASS dates obtained from three metasedimentary rocks from Puerto Cabello (top right), and corresponding Y content (a proxy for garnet growth/breakdown; bottom).
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Continental tectonics
Extension during orogenesis
The ‘thermal relaxation’ model for regional metamorphism (e.g. England & Thompson, 1984) has provided strong support for the metamorphic facies series concept. According to the thermal relaxation model, intermediate T/P (Barrovian-type) regional metamorphism associated with mountain building (collisional orogenesis) results from a return to thermal equilibrium following thickening-related perturbation of the lithospheric thermal structure. Heat conduction occurs on the scale of the crust and thermal equilibration requires > 50 Myr (Thompson & England, 1984).
The thermal relaxation model was proposed on the understanding that collisional orogens experience perpetual shortening (and thickening) over time scales of 50 Myr or more. It is now known that orogenesis can be as brief as 10–20 Myr and that orogenic evolution can be punctuated, with enhanced thermo-tectonic activity accounting for only a fraction (e.g. 1–10 Myr) of the total duration of orogenesis.
Orogenic regional metamorphism can be short enough in duration to require length scales of thermal conduction significantly less than the thickness of the crust. This means, in crustal-scale terms, orogenic regional metamorphism records conditions of thermal disequilibrium; regional metamorphism must be driven by thermal processes operating on sub-crustal length scales (e.g. advection of heat by focused magmatism/fluid activity, localised mechanical heating, etc.).
Extension of thickened lithosphere provides a simple means of producing thermal anomalies with sub-crustal length scales. Magmatic heat advection, following mantle melting in response to decompression of the asthenosphere (during lithospheric extension), may augment heat budgets already elevated by lithospheric stretching (and attenuation of isotherms). Extension is often accommodated by large-scale, mid-crustal shear zones. Such shear zones can provide a focus for pulsed magma emplacement and/or hot fluid fluxing, concentrating advective heat sources. They will also produce heat by mechanical work. Large-scale, mid-crustal shear zones can thus account for incremental development of a thermal aureole (and metamorphic sequence) simply through a punctuated deformation/permeability history.
In Scotland, the classic Barrovian and Buchan metamorphisms developed during a 473–465 Ma phase of the broader (488–461 Ma) Grampian Orogeny. Structural mapping, microstructural analysis, and igneous and metamorphic petrology, which my co-workers and I performed whilst at the ANU, demonstrated that large-scale, mid-crustal extensional shear zones were the focus for emplacement of Grampian-age, decompression-related magmas. The classic Barrovian and Buchan metamorphisms developed as the results of lithospheric-scale extension and regional-scale contact metamorphism during the Grampian Orogeny.
The ‘thermal relaxation’ model for regional metamorphism (e.g. England & Thompson, 1984) has provided strong support for the metamorphic facies series concept. According to the thermal relaxation model, intermediate T/P (Barrovian-type) regional metamorphism associated with mountain building (collisional orogenesis) results from a return to thermal equilibrium following thickening-related perturbation of the lithospheric thermal structure. Heat conduction occurs on the scale of the crust and thermal equilibration requires > 50 Myr (Thompson & England, 1984).
The thermal relaxation model was proposed on the understanding that collisional orogens experience perpetual shortening (and thickening) over time scales of 50 Myr or more. It is now known that orogenesis can be as brief as 10–20 Myr and that orogenic evolution can be punctuated, with enhanced thermo-tectonic activity accounting for only a fraction (e.g. 1–10 Myr) of the total duration of orogenesis.
Orogenic regional metamorphism can be short enough in duration to require length scales of thermal conduction significantly less than the thickness of the crust. This means, in crustal-scale terms, orogenic regional metamorphism records conditions of thermal disequilibrium; regional metamorphism must be driven by thermal processes operating on sub-crustal length scales (e.g. advection of heat by focused magmatism/fluid activity, localised mechanical heating, etc.).
Extension of thickened lithosphere provides a simple means of producing thermal anomalies with sub-crustal length scales. Magmatic heat advection, following mantle melting in response to decompression of the asthenosphere (during lithospheric extension), may augment heat budgets already elevated by lithospheric stretching (and attenuation of isotherms). Extension is often accommodated by large-scale, mid-crustal shear zones. Such shear zones can provide a focus for pulsed magma emplacement and/or hot fluid fluxing, concentrating advective heat sources. They will also produce heat by mechanical work. Large-scale, mid-crustal shear zones can thus account for incremental development of a thermal aureole (and metamorphic sequence) simply through a punctuated deformation/permeability history.
In Scotland, the classic Barrovian and Buchan metamorphisms developed during a 473–465 Ma phase of the broader (488–461 Ma) Grampian Orogeny. Structural mapping, microstructural analysis, and igneous and metamorphic petrology, which my co-workers and I performed whilst at the ANU, demonstrated that large-scale, mid-crustal extensional shear zones were the focus for emplacement of Grampian-age, decompression-related magmas. The classic Barrovian and Buchan metamorphisms developed as the results of lithospheric-scale extension and regional-scale contact metamorphism during the Grampian Orogeny.
Model for the Barrovian and Buchan metamorphisms of Scotland during Grampian lithospheric-scale extension, at 473–465 Ma.
Tectonic model for Eocene–Oligocene development of the Caribbean–South America boundary zone, involving collision of the Caribbean and Proto-Caribbean subduction trenches, and development of the Puerto Cabello eclogites and Villa de Cura blueschists. Modified after Pindell et al. (2006, Fig. 17, p. 331).
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Subduction zones
More to come... |
Size effects in rock mechanics
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Ever considered how the body shape of an elephant or rhinoceros compares to that of a mouse or marmoset? Galileo Galilei did. He reasoned that larger animals require more robust body shapes—“the weakness of the giants”—because the proportional strength of a solid decreases with increasing size. Over a century earlier, Leonardo da Vinci had performed experiments on the strength of iron wires, producing similar evidence for a size effect in the strength of materials.
In simplest terms, the origin of the size effect may be considered statistical; a solid is only as strong as its weakest element and a larger solid is more likely to contain a weaker element. However, materials such as rock, ice, concrete and timber (which are not perfectly brittle) accommodate significant cracking (energy dissipation) prior to failure, meaning that models for the size effect in bodies of finite volume must incorporate energetic considerations. Alternative explanations for the size effect in rock and concrete strength have also been proposed on the basis of the fractality of rock flaws. My most recent research in rock mechanics, performed with co-workers at Monash University (Melbourne) and Northeastern University (Shenyang), involved experiments investigating the size effect on the compressive strength of rock. Energetic–statistical explanations for the size effect in rock mechanics can generally account for the results of experiments. However, the results of our testing suggest that the influence of crack fractality is non-trivial for small rock volumes. This has significant implications for approaches to upscaling results from laboratory testing to values applicable for engineering design of rock slopes, tunnels, etc. |
Examples of fractals applied to the size effect in rock mechanics. (a) and (b) show self similar geometries, and (c) shows a set of self affine curves. (d) demonstrates the fractal cracking process, within the fracture process zone, during quasi-brittle rock failure. Modified after Bazant & Yavari (2005, Fig. 7, p. 15).
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References
Pindell, J., Kennan, L., Stanek, K.P., Maresch, W.V. & Draper, G., 2006. Foundations of Gulf of Mexico and Caribbean evolution: eight controversies resolved. Geologica Acta 4, 303–341.
Bazant, Z.P. & Yavari, A., 2005. Is the cause of size effect on structural strength fractal or energetic–statistical? Engineering Fracture Mechanics 72, 1–31.
England, P.C. & Thompson, A.B., 1984. Pressure–temperature–time paths of regional metamorphism I. Heat transfer during the evolution of regions of thickened crust. Journal of Petrology 25, 894–928.
Thompson, A.B. & England, P.C., 1984. Pressure–temperature–time paths of regional metamorphism II. Their inference and interpretation using mineral assemblages in metamorphic rocks. Journal of Petrology 25, 929–955.
Bazant, Z.P. & Yavari, A., 2005. Is the cause of size effect on structural strength fractal or energetic–statistical? Engineering Fracture Mechanics 72, 1–31.
England, P.C. & Thompson, A.B., 1984. Pressure–temperature–time paths of regional metamorphism I. Heat transfer during the evolution of regions of thickened crust. Journal of Petrology 25, 894–928.
Thompson, A.B. & England, P.C., 1984. Pressure–temperature–time paths of regional metamorphism II. Their inference and interpretation using mineral assemblages in metamorphic rocks. Journal of Petrology 25, 929–955.
