Short-duration regional metamorphism is a recently observed and poorly understood phenomenon in metamorphic geology. In this review, it is defined as metamorphism on time scales that limit length scales (of the associated thermal anomaly) to significantly less than the thickness of the orogenic crust (<10 myr) or subducted oceanic lithosphere (<5 myr). Without appealing to exceptional heat sources, thermal models have been unable to account for peak metamorphic temperature during collisional orogenesis and subduction. This observation, combined with restricted time scales for regional metamorphism, suggests that metamorphic facies series can record atypical and transient thermal conditions (related to punctuated and localized heat advection and/or production), rather than normal, ambient conditions for the tectonic setting to which they are allied. High-precision geochronology can resolve short-duration metamorphic estimates of 1 – 10 myr. However, diffusion geospeedometry typically yields extremely short metamorphic durations (<1 myr); tools in metamorphic geology may have matured to the point that the discipline is beginning to recognize episodicity and criticality in deep processes. New, very high-precision petrochronology techniques offer great potential to probe the veracity of extremely short metamorphic durations being obtained from diffusion geospeedometry. Benchmarking of these new very high-precision petrochronology techniques must become a priority for metamorphic geology.
Early interpretations of the geological record leant heavily on catastrophism. As geology matured as a science, uniformitarianism came to supersede catastrophism. Uniformitarianism offers a more sophisticated, logical framework for interpretation of the rock record within the context of current and observable processes. However, the grand success of the uniformitarian philosophy has also resulted in a propensity toward interpretation of deep (and thus unobservable) geological phenomena in terms of slowly evolving or steady-state processes. A notable example of this preference for gradualism in deep processes has been ideas for the origins of regional metamorphism and metamorphic facies series, and implications for the nature of tectonism.
Barrow (1893, 1912) was the first to map regional metamorphism on the basis of diagnostic mineral assemblages. His Barrovian metamorphic sequence, exposed in NE Scotland, comprises a series of ‘isograds’ marking the first appearance of chlorite, biotite, garnet, staurolite, kyanite and sillimanite in pelitic lithologies, in the direction of increasing metamorphic grade. Regional metamorphism recorded by Barrovian-type isograds defines intermediate pressure/temperature (P/T) conditions (575 – 1250°C GPa−1) that have occurred in association with continental collisional since the Archaean (Jamieson et al. 1998).
Barrow (1893) originally proposed that his Barrovian metamorphism resulted from magmatic advection of heat from depth. Following development of the theory of plate tectonics, numerous models were proposed for the origins of specific (e.g. Alpine, Barrovian, Himalayan, Zagros) examples of regional metamorphism in convergent settings (e.g. Oxburgh & Turcotte 1974; Bickle et al. 1975; Bird et al. 1975; Graham & England 1976; Richardson & Powell 1976; England & Richardson 1977; Toksöz & Bird 1977; England 1978). From these models, ‘thermal relaxation’ (England & Thompson 1984) emerged as an elegant and universal paradigm to explain the association between Barrovian-type metamorphism and collisional orogenesis. The thermal relaxation model for orogenic metamorphism posits that intermediate P/T conditions recorded by Barrovian-type regional metamorphism result from a return to crustal-scale thermal equilibrium following collision-related crustal thickening (with mantle heat flux at the base of the lithosphere, and modest rates of erosional exhumation and internal radiogenic heating).
In essence, the thermal relaxation model takes orogenic metamorphism to reflect relatively steady-state tectonothermal processes; regional metamorphism results, simply, from the long-term interplay of burial, erosion and internal (distributed) radiogenic heating. However, more recent work has demonstrated that Barrovian-type regional metamorphism requires exceptional crustal heating scenarios, involving localized regions of elevated radiogenic (e.g. Jamieson et al. 1998; Engi et al. 2001) or mechanical (e.g. England et al. 1992; Treloar 1997; Stüwe 1998; Burg & Gerya 2005) heat production, or fluid (e.g. Camacho et al. 2005; Dragovic et al. 2012, 2015), magmatic (e.g. Baxter et al. 2002; Brouwer et al. 2004; Reverdatto & Polyansky 2004; Viete et al. 2013) or tectonic (e.g. Smye et al. 2011; Ashley et al. 2015) heat advection, to account for peak metamorphic T. Globally, exceptional heating scenarios are also necessary to explain P–T conditions of subduction-zone metamorphism at forearc depths corresponding to P < 2 – 2.5 GPa (Penniston-Dorland et al. 2015).
Peak T attained in the roots of collisional orogens suggests that thermal relaxation is not a primary driver of Barrovian-type regional metamorphism. Time scales of regional metamorphism also support this position. Durations required for thermal relaxation in collisional orogens (involving heat conduction on the scale of overthickened crust) are of the order of 50 myr (e.g. Thompson & England 1984). As tools to interrogate durations of tectonism and associated regional metamorphism have improved, it has been repeatedly shown that time scales of thermal events recorded by regional metamorphism are too short to allow the crustal-scale heat conduction required by thermal relaxation (e.g. Oliver et al. 2000; Baxter et al. 2002; Dachs & Proyer 2002; Faryard & Chakraborty 2005; Ague & Baxter 2007; Smye et al. 2011; Viete et al. 2011a,b, 2013; Spear et al. 2012; Spear 2014). Moreover, examples of temporal non-uniformity in rates of metamorphism have been related to extremely short-duration thermal instabilities (tectonometamorphic episodicity) within collisional orogens and subduction zones (e.g. Wijbrans & McDougall 1986, 1988; Keay et al. 2001; Kohn 2004; Camacho et al. 2005; Ague & Baxter 2007; Pollington & Baxter 2010; Viete et al. 2011b, 2015; Dragovic et al. 2012, 2015).
The emerging picture of short-duration and episodic metamorphism may have fundamental implications for exactly what metamorphism records (i.e. the ‘everyday’ versus the exceptional) and for what is ‘typical’ in terms of tectonic evolution. In this paper, the nature of regional metamorphism will be discussed and the concept of metamorphic facies series reviewed. First, however, to establish the geological significance of short-duration regional metamorphism, the following three questions must be addressed. (1) What qualifies as short-duration regional metamorphism? (2) Why is it geologically important? (3) Where and when has it occurred?
What qualifies as short-duration regional metamorphism?
Short-duration orogenic regional metamorphism, for the purpose of this review, is considered to involve a thermal excursion of duration 107 yr (10 myr) or less. (Metamorphism here is considered to involve thermal drivers and thus a T excursion. This need not always be the case; metamorphism may also be driven by P fluctuations (e.g. García-Casco et al. 2002; Forster & Lister 2005; Beltrando et al. 2007; Kabir & Takasu 2010; Rubatto et al. 2011; Lister & Forster 2016) and/or fluid fluxing (e.g. Bjørnerud et al. 2002; John et al. 2004; Bjørnerud & Austrheim 2006). Alternative metamorphic drivers and implications for large-scale, extremely short-duration metamorphism are briefly discussed later in this review.) Short-duration regional metamorphism in subduction zones is defined by a maximum thermal duration of 5 myr. The choice of 10 and 5 myr as upper limits for short-duration orogenic and subduction-zone metamorphism, respectively, is not arbitrary and the reasons relate to the geological importance of short-duration metamorphism.
Why is short-duration regional metamorphism geologically important?
Orogenic regional metamorphism involving durations less than 10 myr records a sub-crustal-scale thermal anomaly. Similarly, subduction-zone regional metamorphism with duration less than 5 myr records a sub-lithospheric-scale thermal anomaly. Thus, fundamentally, the associated thermal anomaly is transient and not reflective of the long-term tectonothermal environment.
There is currently little consensus on how the duration of a regional metamorphic event should be described. It is commonplace in diffusion geospeedometry to define a thermal event by a square-wave or triangular temperature–time (T–t) path. Although these approaches are relatively arbitrary, they are computationally simple and are valid for order-of-magnitude duration estimates. An alternative approach, used by Viete et al. (2011a), is to model conductive growth–decay of a thermal anomaly. The major advantage of this approach is that a T–t path (of fixed form) can be defined by a characteristic length scale of heat conduction, which can then be compared with field-determined dimensions of the regional metamorphic sequence (see Viete et al. 2011a).
Carslaw & Jaeger (1959) provided analytical solutions for the conduction of heat in solids, for various initial and boundary conditions. For most conductive heating scenarios, however, solutions are computationally awkward and require numerical modelling approaches. An alternative approach to modelling T–t evolution involves approximation of an infinite series solution to a conductive heating scenario of interest (e.g. Viete et al. 2011a). This approach can yield a simple equation that can be used to calculate a T–t curve for a given thermal diffusivity value and system size. Figure 1a describes temperature evolution at distance x = L, within an infinite half-space (i.e. 1D heating) composed of a material with thermal diffusivity a, following a heat pulse applied to the boundary at x = t = 0. A formula to describe the T–t curve of Figure 1a has been given by Viete et al. (2011a).
Because the conductive heating–cooling curve (Fig. 1a) displays exponential decay, time scales must be defined by some ‘near-peak’ duration above a T between ambient (Tamb) and maximum (Tmax). As demonstrated in Figure 1b, the definition used for the ‘near-peak’ thermal duration (t) influences the associated conduction length scale (L). Figure 1c and d gives the relationship between t and L for the conductive heating–cooling curve of Figure 1a, a thermal diffusivity of a = 10−6 m2 s−1 and ‘near-peak’ thermal durations defined by time spent with T within the uppermost 30, 50, 70 and 90% of the total temperature excursion (Fig. 1b). For these conditions, a heating–cooling curve with defined duration t = 10 myr is produced by L = 15 – 30 km (see Fig. 1d).
The continental crust in active orogenic settings is typically >60 km in thickness (e.g. Pasyanos et al. 2014), meaning that a metamorphic event with ‘near-peak’ duration <10 myr will record a sub-crustal-scale thermal anomaly. Subducting oceanic lithosphere is typically >40 km in thickness (e.g. Turcotte & Schubert 2002; Pasyanos et al. 2014), meaning that lithospheric-scale thermal equilibrium in subduction settings must involve length scales >20 km and thus metamorphic time scales >5 myr (see Fig. 1d).
Consideration of T-dependent thermal diffusivity (e.g. Whittington et al. 2009), or internal radiogenic or mechanical heating, will lead to longer thermal time scales for a given conduction length scale. Similarly, protracted heating at x = 0, as may be expected for incremental (non-instantaneous) heat advection by thrusting, or episodic magmatism or fluid activity, will produce longer time scales for a given conduction length scale. On the other hand, advective cooling (e.g. erosional exhumation) may shorten thermal time scales for a given conduction length scale. Notwithstanding assumptions inherent in the approach outlined above and in Figure 1, definition of short-duration regional metamorphism as <10 myr for orogenic settings and <5 myr for subduction settings limits length scales of the associated thermal anomaly to significantly less than the thickness of the continental crust (in orogenic settings) or oceanic lithosphere (in subduction settings). Therefore, short-duration regional metamorphism is associated with crustal and/or lithospheric thermal disequilibrium marked by a localized thermal anomaly, meaning that it has fundamentally different geological significance to longer-duration metamorphism; additional heat sources (e.g. punctuated and localized heat advection and/or production) are required, and metamorphism is transient and anomalous rather than representing normal, ambient conditions for the tectonic setting to which it is allied.
Where and when has short-duration regional metamorphism occurred?
As a reference to the interested reader, and to support the arguments made in this review, two tables are provided. Table 1 gives a list of examples of regional metamorphism shown by high-precision geochronology to have developed over <10 myr for orogenic metamorphism and <5 myr for subduction-zone metamorphism. Estimates of age and P–T conditions for the metamorphism are also provided. Table 2 is equivalent to Table 1, but considers only duration estimates made on the basis of diffusion geospeedometry. Figure 2 shows the location and age of examples of short-duration metamorphism from Tables 1 and 2. Figure 3a and b illustrates metamorphic durations and P–T estimates for the studies from Tables 1 and 2, respectively. It should be noted that Tables 1 and 2, and Figures 2 and 3, although providing a relatively complete account of all published examples of short-duration regional metamorphism, are not exhaustive.
Although there are several examples of short-duration ultrahigh-temperature (UHT) metamorphism associated with large-scale magmatism (e.g. Platt et al. 1998; Schmitz & Bowring 2003; Kemp et al. 2007; Pownall et al. 2014), these are not included in Tables 1 and 2, and Figures 2 and 3. UHT metamorphism occurs at T > 900°C (Harley 1998) and, in the absence of obvious advective heat sources, requires highly radioactive source rocks, slow erosion rates and tens of million years to overcome the latent heat of partial melting (e.g. Clark et al. 2011, 2015; Horton et al. 2016). Moreover, even the most robust geochronometers (e.g. zircon, monazite, rutile) experience dissolution–precipitation and post-crystalline resetting at UHT conditions (e.g. Kelsey 2008; Kelsey & Hand 2015; Harley 2016), adding great complexity to efforts to constrain time scales for UHT metamorphism. Durations and drivers for UHT metamorphism are beyond the scope of this review, and the interested reader is referred to the detailed reviews of Kelsey & Hand (2015) and Harley (2016).
Tables 1 and 2 list dozens of examples of regional metamorphism with duration so brief as to record transient, anomalous thermal conditions. These include many classic examples of regional metamorphism (e.g. the Alps, Greek Cyclades, Himalaya, New England, Scotland). The metamorphic facies series concept considers regional metamorphism to record relatively long-lived tectonothermal environments. Thus, the prevalence of short-duration regional metamorphism has significant implications for the metamorphic facies series concept, which links metamorphic geology and plate tectonics.
The metamorphic facies series concept
Metamorphic rocks provide the most reliable record of thermal conditions in the crust. Thermodynamic datasets amassed from extensive experimental work (e.g. Berman 1988; Holland & Powell 1990, 1998, 2011) are routinely used to calculate metamorphic P and T from the metastable minerals and mineral assemblages that form metamorphic rocks. Despite firm understanding of P–T conditions for metamorphism, important questions remain regarding why metamorphism occurs when it does and exactly what it records.
Current techniques for estimating metamorphic P–T conditions follow from the early concept of metamorphic facies (Eskola 1915, 1920). Barrow (1893, 1912) demonstrated that an increasing degree of metamorphism is reflected in a succession of mineral assemblages in pelites, defining his Barrovian metamorphic sequence. Eskola (1915, 1920) extended Barrow's findings, proposing that fields in P–T space (metamorphic facies) can be delineated by commonly observed metamorphic mineral associations. Figure 3 illustrates the P–T range and distribution of the most commonly accepted metamorphic facies (e.g. Miyashiro 1973; Turner 1981; Spear 1993).
With the advent of the theory of plate tectonics, the metamorphic facies concept was extended to that of metamorphic facies series (Miyashiro 1961, 1973). Miyashiro (1961, 1973) recognized that specific tectonic settings are associated with metamorphic facies defining distinct P/T arrays. With respect to an assumed stable crustal geotherm (e.g. Rudnick et al. 1998), facies series associated with regional metamorphism can be divided into low P/T, intermediate P/T, intermediate–high P/T and high P/T (Miyashiro 1961, 1973). Rates of tectonic processes outstrip those of the conduction of heat in rock, meaning that specific quasi-steady-state thermal environments would logically emerge from the processes of large-scale advection (e.g. crustal thickening, thinning, subduction) that characterize plate tectonics. Quantitative modelling of tectonic advection and crustal heat flow supports this idea (e.g. McKenzie 1967, 1969; England & Thompson 1984).
In this review paper, low, intermediate, intermediate–high and high P/T are referred to with respect to the classic examples of metamorphism of low P/high T (Buchan of Read 1923, 1952), medium P/medium T (Barrovian of Barrow 1893, 1912), high P/medium T (Sanbagawan of Miyashiro 1961, 1973) and high P/low T (Franciscan of Bailey 1962; Ernst 1963; Bailey et al. 1964). In Figure 3, Buchan-type (low P/T) metamorphism has T–P ratios >1250°C GPa−1, meaning that andalusite (rather than kyanite) occurs as the low-T aluminosilicate phase and the metamorphic facies progression is typified by zeolite–greenschist–amphibolite–granulite. Franciscan-type (high P/T) metamorphism is considered to involve T–P ratios <425°C GPa−1 and zeolite–prehnite pumpellyite–blueschist–eclogite facies assemblages (Fig. 3). Barrovian-type (intermediate P/T) metamorphism has T–P ratios of 575 – 1250°C GPa−1, restricting aluminosilicate phases to kyanite and sillimanite, and metamorphic assemblages to zeolite–(prehnite pumpellyite–)greenschist–epidote amphibolite–amphibolite–granulite facies (Fig. 3). Finally, this review paper considers Sanbagawan-type (intermediate-high P/T) metamorphism to comprise a narrow band of T–P ratios (425 – 575°C GPa−1) that produces greenschist- or blueschist-facies assemblages at T = 250 – 450°C and eclogite-facies assemblages at T > 675°C (Fig. 3). The type Sanbagawan metamorphic series preserves a progression with increasing T from blueschist to greenschist and/or epidote amphibolite facies then eclogite facies (see Miyashiro 1994, rfig. 8.3, p. 203). Unlike for the other metamorphic facies series, intermediate-high P/T metamorphism will not necessarily preserve the exact metamorphic facies progression observed for its type locality.
Eskola (1939) and various researchers since (e.g. Miyashiro 1973, 1994; Turner 1981; Yardley 1989) recognize an extremely low P/T contact metamorphic facies series comprising a facies progression of albite epidote hornfels–hornblende hornfels–pyroxene hornfels–sanadinite. Low P/T, Buchan-type metamorphism is related to magmatism and associated advection of heat into the upper crust (e.g. Wickham & Oxburgh 1985; Barton & Hanson 1989; De Yoreo et al. 1991; Sandiford & Powell 1991; Collins 2002; Viete et al. 2010). The contact metamorphic facies series may thus be viewed as a very localized and extreme member of the Buchan metamorphic facies series.
Barrovian-, Sanbagawan- and Franciscan-type metamorphism is often taken to reflect relatively steady-state thermal processes within the tectonic environments with which they are associated. As discussed above with respect to the importance of short-duration regional metamorphism, brief heating (i.e. <5 – 10 myr) limits thermal length scales, meaning that associated metamorphic facies series do not represent a stable tectonothermal environment. Instead, metamorphic facies series produced by short-duration regional metamorphism represent some P–T conditions arising in response to punctuated and localized processes of heat advection and/or production. Jamieson et al. (1998, p. 23), with reference to global examples of orogenic regional metamorphism, asked: ‘Barrovian regional metamorphism: where's the heat?’ Penniston-Dorland et al. (2015, p. 243), with reference to global examples of subduction-zone metamorphism, exclaimed ‘rocks are hotter than models’. Metamorphic rocks record excessive T for the steady-state tectonothermal scenarios with which they are associated, and short-duration regional metamorphism, fundamentally, records thermal transience.
The picture emerging from P–T conditions and durations of regional metamorphism suggests that metamorphic facies series do not necessarily record steady-state thermal processes for the tectonic environment to which they are allied. If what is recorded by regional metamorphism is typically ‘exceptional’ rather than ‘normal’, then this leads to questions concerning the significance of metamorphism and metamorphic rocks, and potential bias in the exhumed rock record. Are metamorphic rocks really representative of the long-term/quasi-steady-state thermal picture of the crust? Is exhumation/preservation of transient thermal conditions (metamorphism) preferred and, if so, why? Do metamorphic facies series have the geological significance they are thought to have?
Metamorphic facies series are used as a primary tool for understanding the thermal state of the crust and how the processes of plate tectonics move heat; the concept of metamorphic facies series links metamorphic geology and plate tectonics. Thus, answers to the questions above have potential to reframe the entire discipline of metamorphic geology and its role in the understanding of our planet.
Figure 2 shows that metamorphic duration estimates from diffusion geospeedometry are significantly shorter than those obtained from high-precision geochronology. Additionally, Tables 1 and 2 and Figure 3 contain no Precambrian examples of short-duration metamorphism. These observations highlight limitations in existing approaches to estimating metamorphic duration. Below, the phenomenon of extremely short-duration (i.e. <1 myr) metamorphism is introduced, and its implications for the nature of metamorphism and episodicity in deep processes are discussed. Within the context of extremely short-duration metamorphism, opportunities offered by new techniques for quantifying short-duration metamorphism are also reviewed.
Extremely short-duration metamorphism
Most examples of short-duration metamorphism in Figure 2 are very young (late Mesozoic–Tertiary), and there are no Archaean or Proterozoic examples. Secular changes in thermal state and/or crustal chemistry may have limited intermediate–high and high P/T metamorphism during the Precambrian (e.g. Stern 2005; Palin & White 2016). In addition, with reworking of the continental crust, one may expect higher preservation potential for younger low and intermediate P/T regional metamorphic sequences. However, perhaps the most convincing explanation for the lack of known examples of Precambrian short-duration metamorphism is that uncertainties in geochronology limit temporal resolution in old rocks. For example, 2% uncertainty precludes recognition of short-duration metamorphism (<10 myr) in rocks older than c. 500 Ma. Although short-duration metamorphism may have been commonplace in the Precambrian, current tools may not allow its recognition.
The key to extending knowledge of short-duration metamorphism into the Precambrian may be diffusion geospeedometry. However, diffusion geospeedometry approaches also have their pitfalls. A comparison of Figure 3a and b is revealing; duration estimates from diffusion geospeedometry are significantly and systematically shorter than those obtained from high-precision geochronology. This observation conflates two potential influences: limitations in temporal resolution may cause approaches in geochronology to overestimate metamorphic time scales (particularly for Palaeozoic rocks), whereas uncertainties in the Arrhenius parameters that govern diffusion or invalid assumptions inherent in diffusion models (such as the form of the initial composition profile, redox conditions during metamorphism or premise of mechanical equilibrium) may cause approaches in diffusion geospeedometry to underestimate metamorphic time scales.
Metamorphism involving time scales <1 myr, obtained exclusively from diffusion geospeedometry (Fig. 3), is here referred to as extremely short-duration metamorphism. It is important to establish whether the short time scales of regional metamorphism obtained from high-precision geochronology (typically 1 – 10 myr; Fig. 3a) or extremely short time scales obtained from diffusion geospeedometry (typically 10 – 1000 kyr; Fig. 3b) are typical of short-duration regional metamorphism. This is because extremely short-duration metamorphism may have great implications not only for the significance of metamorphic facies series and the tectonic picture provided by metamorphic rocks, but also for metamorphic drivers. The key to exploring this issue will be more detailed experimental work to explore the full range of variables that influence diffusion rates, in addition to the development of new very high-precision, petrologically controlled techniques in geochronology that are capable of estimating metamorphic duration and verifying estimates obtained from diffusion geospeedometry.
Diffusion rates: experiments and models versus nature
Metamorphic duration estimates obtained from diffusion geospeedometry are shorter than those obtained from high-precision geochronology (Fig. 3). Thus, if down-temperature extrapolation of Arrhenius parameters is contributing to this mismatch, then the associated errors must arise from systematic experimental overestimation of natural diffusion rates (e.g. Baxter 2003; Villa 2006, 2016); either mechanisms for diffusion are fundamentally different from what we envision and reproduce in experiments, or experiments consistently misrepresent the chemical and physical conditions found in natural scenarios (e.g. see discussions of the influence of fO2 on diffusion rates by Carlson (2006), Chu & Ague (2015) and Ferry et al. (2015)). Additionally, new concepts in metamorphic petrology offer an alternative explanation relating to maintenance of sharp compositional gradients as a result of significant, long-lived, grain-scale pressure variations and their influence on chemical activity and diffusion (e.g. Tajčmanová et al. 2014; Wheeler 2014).
Diffusion is central to geospeedometry, but is also a fundamental consideration for geochronology. In geochronology, diffusion is often treated by the ‘closure temperature’ concept of Dodson (1973), which calculates the T, for a mineral grain size and shape, cooled at a prescribed (uniform) rate, at which diffusive loss of the daughter isotope is fully compensated by radiogenic ingrowth; closure temperature gives the T during cooling to which a radiogenic age corresponds. The closure temperature model is an idealization that can misrepresent many real-world complexities, including multimodal grain size, non-linear or highly protracted cooling, and the effects of recrystallization during cooling (e.g. Lister & Baldwin 1996). Moreover, diffusion in geochronology (and geospeedometry) has been shown to be governed not only by T, but also by P (e.g. Harrison et al. 2009; Forster & Lister 2010, 2014; Warren et al. 2012a), deformation (e.g. Hames & Cheney 1997; Mulch et al. 2002; Kramer et al. 2003; Cosca et al. 2011) and the availability and nature of intergranular fluids (e.g. Rubie 1986, 1990; Kühn et al. 2000; Glodny et al. 2008; Smye et al. 2013).
To resolve the mismatch between metamorphic duration estimates produced from geochronology versus diffusion geospeedometry, future work must include more diffusion experiments that consider a broader set of conditions and the influence of additional variables (e.g. metamorphic P, presence of fluids, chemical activity, etc.). Ideally, future work would also utilize new, very high-precision approaches for the estimation of metamorphic time scales. These new approaches must allow assessment of the veracity of claims of extremely short-duration metamorphism that have been made on the basis of diffusion geospeedometry (e.g. Camacho et al. 2005; Ague & Baxter 2007; Viete et al. 2011b; Spear et al. 2012; Spear 2014).
Development of new very high-precision, petrologically controlled approaches to explore extremely short-duration metamorphism
The duration of thermal metamorphism is broadly governed by time scales for dissipation of the thermal anomaly that it records (see the introduction) and the thermal diffusivity of rock is far better characterized than rates of chemical diffusion. Thus, the method outlined in Figure 1 can provide a first-order check that the thermal length scale associated with a chemical diffusion feature matches the length scales of metamorphism observed in the field (e.g. Viete et al. 2011a). Such an approach may be used in cases where metamorphism is unambiguously driven by heating and the regional metamorphic sequence is relatively intact. However, it is of limited use where metamorphism occurs in isolated, dismembered blocks (e.g. high P/T blocks in a subduction mélange). For such cases, very high-precision, petrologically controlled geochronology techniques are necessary to verify the estimates of extremely short-duration metamorphism being made on the basis of diffusion geospeedometry.
The only study to have directly compared time scales of metamorphism obtained by diffusion geospeedometry with those estimated using high-precision geochronology has been that of Philippot et al. (2001). They showed that Lu–Hf garnet and 40Ar/39Ar white mica dates were within error, for metamorphism of both the c. 259 Ma Yukon–Tanana Terrane and c. 310 Ma Blyb Metamorphic Complex, Great Caucasus. For both cases, this limits the time scales for thermal metamorphism, during exhumation from intermediate–high P/T conditions, to less than the uncertainties on single Lu–Hf garnet and 40Ar/39Ar phengite dates; that is, a few million years. Diffusion geospeedometry performed on the same rocks yielded extremely short-duration metamorphic durations of a few hundred thousand years (Perchuk & Philippot 1997; Perchuk et al. 1999). Thus, Philippot et al. (2001) were able to demonstrate short-duration metamorphism for the Yukon–Tanana Terrane and Blyb Metamorphic Complex using high-precision geochronology, but not confirm the extremely short durations estimated from diffusion geospeedometry (Perchuk & Philippot 1997; Perchuk et al. 1999). They concluded that ‘time-scale resolution required for unravelling rates of high-pressure metamorphism [estimated from diffusion geospeedometry] remains out of reach of current [geo]chronological methods’ (Philippot et al. 2001, p. 24).
Cooper & Kent (2014) compared time scales of volcanic processes obtained from both diffusion geospeedometry and U-series geochronology, for 16 volcanic provinces. Geochronology was found to provide duration estimates 1 – 2 orders of magnitude longer than diffusion geospeedometry. Issues relating to errors in the Arrhenius parameters used for diffusion geospeedometry are less important for magmatic systems that, owing to high T, do not require significant down-temperature extrapolation from the conditions of experimental determination (e.g. Costa et al. 2008). However, diffusion is a T-dependent process and thus diffusion geospeedometry provides information on the time-integrated thermal response, but not the exact T–t history, which may include significant T fluctuations (particularly for a volcanic chamber experiencing periodic eruption and/or recharge). Cooper & Kent (2014) reasoned that the discrepancy in time scales between high-precision geochronology and diffusion geospeedometry may relate to episodicity in thermal processes. New approaches in geochronology capable of verifying extremely short-duration metamorphic time scales obtained from diffusion geospeedometry must use techniques with very high temporal resolution on young rocks (to ensure access to time scales <1 myr), but must simultaneously offer the spatial resolution necessary to decipher episodicity in thermal (and metamorphic) processes.
Results obtained from geochronology can be influenced by elemental and/or isotopic disequilibrium relating to inheritance of ‘isotopically old’ or daughter-element-rich chemistry as a result of (relict) mineral breakdown (e.g. Thöni 2002; Romer & Xiao 2005; Pollington & Baxter 2011), persistence of sub-microscopic relict mineral domains (e.g. Beltrando et al. 2013) or stagnant fluid conditions (e.g. Cosca et al. 2000; Warren et al. 2012b; Smye et al. 2013) during metamorphism. Dates obtained from geochronology may also be affected by elemental and/or isotopic fractionation during rapid, diffusion-limited mineral growth (e.g. Skora et al. 2006; Watson & Müller 2009). The various influences of P, deformation, fluids and reaction histories on results obtained from geochronology also illustrate a need for detailed petrological understanding and characterization to stand at the forefront in geochronology endeavours (e.g. Vance et al. 2003; Villa & Williams 2013). ‘Petrochronology’ is a term that has been coined to describe geochronology performed with detailed petrological context in terms of P–T and reaction history.
There are three existing techniques in petrochronology with the spatial and temporal resolution to resolve multiple metamorphic episodes at 100 – 1000 kyr. These are: (1) Sm–Nd or Lu–Hf thermal ionization mass spectrometry (TIMS) performed on single growth sectors of metamorphic garnet (e.g. Baxter et al. 2002; Ducea et al. 2003; Lancaster et al. 2008; Pollington & Baxter 2010; Dragovic et al. 2012, 2015), (2) the ‘asymptotes and limits’ approach to identifying the effects of brief heating and/or limited recrystallization on 40Ar/39Ar step-heating spectra (e.g. Forster & Lister 2004, 2005; Lister & Forster 2016), and (3) petrologically controlled U–Pb depth profiling approaches, including single-shot laser ablation split stream (SS-LASS) inductively coupled plasma mass spectrometry (ICP-MS) petrochronology, capable of deciphering distinct, fine-scale episodes of accessory mineral growth (e.g. Smye & Stockli 2014; Viete et al. 2015; Stearns et al. 2016). U–Pb TIMS approaches that use partial acid digestion or focused ion beam (FIB) milling to isolate extremely small volumes of zircon or monazite (for analysis) may also offer necessary spatial and temporal resolution, but are still in the embryonic stages of their development (R. Ickert, pers. comm.). Figure 4 provides a summary of the three established, high-resolution and very high-precision petrochronology techniques listed above, with images and plots for each demonstrating how the metamorphic minerals record growth episodes at the microscopic scale, and the results that can be obtained.
The high-resolution and very high-precision petrochronology techniques of Figure 4 have never been applied in combination (and in parallel with detailed diffusion geospeedometry) on a common set of young rocks. Such work would allow the various techniques to be benchmarked and calibrated. It thus represents a crucial next step for knowledge development on the origins and significance of short-duration metamorphism. Further development of high-resolution and very high-precision petrochronology techniques (including the above-mentioned work to compare results obtained from the three established techniques) will assist not only in assessing the veracity of diffusion geospeedometry results and the integrity and relevance of extremely short-duration metamorphic time scale estimates, but also in providing data from nature that can be used to verify diffusion experiments.
Drivers for short- and extremely short-duration metamorphism
If new, high-resolution techniques in very high-precision petrochronology yield results in agreement with the extremely short time scales for metamorphism being obtained from diffusion geospeedometry, then regional metamorphism may occur on time scales that belie thermal length scales. There are two potential explanations for such observations. First, metamorphism involving thermal drivers may, locally, record small-scale (extremely short-duration) thermal perturbations that overprint the regional (short-duration) metamorphic event (e.g. Baxter et al. 2002; Ague & Baxter 2007; Viete et al. 2011b). Second, extremely short-duration metamorphism may be driven not by heating but by alternative metamorphic drivers whose influence dissipates more quickly than heat can by conduction.
With respect to metamorphism driven by heating on multiple time and length scales, a model incorporating self affinity in thermal behaviour is outlined below. This model appeals to focused, episodic heating to account for both extremely short-duration and short-duration metamorphism within the one tectonothermal framework. A brief background on alternative metamorphic drivers is also provided below.
Self affinity in metamorphic time scales?
The Barrovian sillimanite isograd occurs in close association with mafic igneous intrusions in NE Scotland (e.g. Fettes 1970; Pankhurst 1970; Ashworth 1975). Chinner (1966), Harte & Johnson (1969) and Harte & Hudson (1979) showed that the highest-T isograds (e.g. sillimanite, migmatite) appear to have been ‘superimposed’ on the regional Barrovian isograd pattern. Diffusion geospeedometry applied to the high-grade Barrovian rocks has yielded extremely short estimates for the duration of thermal metamorphism (Ague & Baxter 2007; Viete et al. 2011b).
Chinner (1966) considered the Barrovian sillimanite overprint to have a fundamentally different thermal origin from the broader regional Barrovian metamorphism. Harte & Hudson (1979) proposed an alternative model in which the sillimanite overprint developed within the same thermal context as the rest of the Barrovian metamorphic sequence; the sillimanite overprint represents the Barrovian thermal climax. Viete et al. (2013) reconciled the Harte & Hudson (1979) view on the sillimanite overprint with the spatial association between sillimanite-grade rocks and the Grampian gabbros, suggesting that the entire Barrovian thermal anomaly, from the chlorite zone to the sillimanite zone, resulted from regional contact metamorphism driven by punctuated and focused magmatic heat advection.
The punctuated heat advection model of Viete et al. (2013) followed Viete et al. (2011b), who showed that Mn diffusion is recorded at multiple, discrete length scales within single garnets from the Barrovian sillimanite zone. Viete et al. (2011b) suggested that multimodal distribution of length scales of chemical diffusion in high-grade garnets mirrors the multimodal distribution of length scales of the Barrovian isograds, and records thermal activity on multiple time scales in the high-grade core of the Barrovian metamorphic series. Viete et al. (2011b, 2013) proposed that the sillimanite overprint was, in effect, a ‘last gasp’ pulse of an episodic advective heating regime that produced the entire Barrovian metamorphic series.
Figure 5 illustrates a series of T–t paths that may develop in response to the episodic heating regime proposed for the Barrovian metamorphism by Viete et al. (2011b, 2013). The conceptual curves of Figure 5 demonstrate decreasing self affinity with distance from the heat source. Rocks immediately adjacent to the zone of punctuated heating (i.e. the sillimanite zone) would record extremely short-duration (small-length-scale) thermal excursions and thus a punctuated T–t history (Fig. 5). On the other hand, the signature of each individual, extremely short-duration thermal excursions would dissipate with distance from the central heating zone, meaning that rocks at greater distance (e.g. the biotite zone) record only a single, regional thermal excursion (Fig. 5).
The prograde phase of the (short-duration) regional metamorphism develops in response to extremely short-duration heating episodes that are closely spaced in time. Regional cooling begins when extremely short-duration heating episodes are no longer of great enough frequency to maintain the regional thermal anomaly. As the regional thermal anomaly develops and both thermal and chemical diffusion length scales grow, evidence of single, extremely short length-scale thermal or chemical perturbations in the Barrovian high-grade core would be consumed. However, records of extremely short-duration thermal excursions that occur during regional cooling (Fig. 5) would survive.
Diffusion geospeedometry may be applied to extremely short length-scale diffusion features related to single and retrograde, extremely short-duration thermal pulses. Such diffusion features are expected for the self-affine, punctuated heating regime illustrated in Figure 5. However, if the extremely short length-scale features are mistakenly thought to represent the regional metamorphism, then abnormally short metamorphic durations would be calculated from diffusion geospeedometry. Thus, a possible explanation for the contradictory (extremely short) durations being obtained for regional metamorphism is that diffusion geospeedometry has focused on records of small, very localized thermal perturbations rather than the regional thermal anomaly itself.
The above-described thermal regime requires a regional heat source that develops incrementally, as the result of extremely short-duration thermal pulses. Focused and episodic hot fluid pulses and/or mechanical heating represent alternative heat sources to episodic (sheeted) magmatism in the self-affine, punctuated heating model. Large-scale shear zones, whose episodic deformation history could result in punctuated heating relating to elevated permeability (hot fluid fluxing, magma emplacement) and/or strain rate (mechanical work), provide a logical environment for a self-affine thermal regime (Viete et al. 2011b, 2013). Non-uniform thermal tempo cannot, however, be explained by radiogenic heat production (which is a relatively constant-rate process), and can be explained by tectonic advection only if it involves very small-scale tectonic slices and is therefore associated with very thinly sliced lithostratigraphy. Thus, formal recognition of self-affine metamorphic tempo, involving metamorphic heating at multiple time and length scales, would help to rule out certain thermal drivers. An alternative explanation for extremely short-duration regional metamorphism is that metamorphism is triggered by drivers other than heating.
Alternative metamorphic drivers
Metamorphism need not record thermal activity. Numerous studies have observed episodic metamorphism as a result of near-isothermal burial or exhumation (e.g. García-Casco et al. 2002; Forster & Lister 2005; Beltrando et al. 2007; Kabir & Takasu 2010; Rubatto et al. 2011; Li et al. 2016; Lister & Forster 2016). Additionally, work on high P/T metamorphism has suggested that sudden metamorphism of otherwise metastable rocks can be triggered by episodic fluid fluxing (e.g. Kühn et al. 2000; Bjørnerud et al. 2002; John et al. 2004; Bjørnerud & Austrheim 2006; Glodny et al. 2008). Diffusion geospeedometry has typically been performed under the assumption that metamorphism is driven solely by heating, with the influence of P and fluid activity ignored. It may be that the extremely short-duration values being obtained from diffusion geospeedometry relate to a false assumption that metamorphism is principally driven by heating.
Episodicity, criticality and catastrophism
The science of geology is underpinned by the philosophy of uniformitarianism, which holds that the rock record should be interpreted in the context of current and observable processes, and which has entrenched observation as the primary tool for interpreting the Earth. Many natural phenomena involve processes observable at the Earth's surface and on human time scales. For example, it is accepted that earthquakes, volcanism and mass slides punctuate the longer-term processes of mountain building, secular cooling and topography creation. Although not directly observable, processes that occur at depth, such as regional metamorphism, may also involve inherently non-uniform and episodic activity.
Tables 1 and 2 and Figure 2 contain no record of Precambrian short-duration metamorphism. As discussed above, it is likely that limitations in geochronology have not allowed recognition of Archaean or Proterozoic short-duration metamorphism. Indeed, it could be argued that the evolution of geochronological precision has been integral in shaping ideas on the nature of metamorphism. When techniques in geochronology were first applied to metamorphism (in the 1960s–1980s), dates were calculated with precision >10 Ma. Metamorphism was thus thought to occur over time scales comparable with those generally accepted for orogenesis (i.e. 10 – 100 myr), and metamorphic facies series were considered to record relatively steady-state tectonothermal processes. Is it instead possible that orogenesis may persist only for the time scales of short-duration regional metamorphism (i.e. 1 – 10 myr)? Or that orogenesis is typified by bursts of short-duration tectonothermal activity that punctuate longer-term periods of relative tectonic quiescence (e.g. Lister et al. 2001)?
Techniques in geochronology are now resolving fine-scale metamorphic growth events on time scales of 1 – 10 myr, and the community appears to have accepted that regional metamorphism may not record long-lived tectonothermal environments. But extremely short-duration metamorphism involves episodicity at even shorter time scales (i.e. <1 myr). As tools in petrochronology and diffusion geospeedometry continue to develop, will we observe metamorphic episodicity at time scales of millennia, or less? For example, is it possible that rocks respond metamorphically to events as sudden and catastrophic as earthquakes? And, if so, why should metastability also occur, suggesting that rocks will respond to transient P–T pulses (on time scales 1 myr) yet fail to respond to ambient P–T conditions persisting over 10 – 100 myr?
In nature, reactions may require significant overstepping of equilibrium reaction boundaries, and therefore reaction rates may significantly lag rates of change in P and/or T (e.g. Pattison & Tinkham 2009; Gaidies et al. 2011; Pattison et al. 2011; Spear et al. 2014). Rocks at the surface preserve assemblages formed at great P and T, suggesting that metamorphic transformation may be the exception rather than the rule. In a recent conversation with Paddy O'Brien, he quipped ‘not only is metastability common, it is central to plate tectonics’. If metamorphism was slow and steady, rather than typified by metastability and episodicity, the density contrasts that drive plate tectonics may not occur. One may ponder whether plate tectonics (as we know it) would even exist if metamorphic facies series were to represent long-lived, quasi-steady-state tectonothermal environments.
Although the processes of plate tectonics undoubtedly lead to heat advection, the normal, ambient tectonothermal environment is not recorded in many regional metamorphic sequences. Instead, short-duration regional metamorphism records transient, atypical thermal episodes marking localized heat advection and/or production. The growing list of worldwide examples of short-duration regional metamorphism may necessitate a reassessment of the significance of metamorphic facies series.
Current techniques to quantify time scales of metamorphism yield conflicting results. Although metamorphic durations are commonly short, diffusion geospeedometry typically yields time scales 1 – 2 orders of magnitude shorter than high-precision geochronology. In some cases, the extremely short-duration estimates obtained from diffusion geospeedometry belie the thermal length scales of regional metamorphism. This raises questions regarding sources and tempo for metamorphic heating and/or the nature of metamorphic drivers. Short- and extremely short-duration metamorphism also raises questions relating to episodicity and criticality in deep and thus unobservable processes.
Techniques in very high-precision petrochronology that are capable of resolving short- and even extremely short-duration metamorphic activity exist, but are yet to be applied in combination to a single set of young rocks. Such work represents a crucial next step for metamorphic geology, as it will allow comparison and appraisal of these new tools in very high-precision petrochronology, whilst simultaneously probing the veracity of diffusion geospeedometry's findings of extremely short-duration metamorphism.
Acknowledgements and Funding
Research funding from Durham University and Marie Curie actions of the European Commission, in the form of a Durham International Junior Research Fellowship to D.R.V., is gratefully acknowledged. Discussions with B. Hacker helped identify contradictions inherent in extremely short-duration metamorphism, and the need for development of new techniques in petrochronology. Feedback from L. White on an early draft helped improve the paper. The paper benefited greatly from thoughtful reviews by A. Smye and Subject Editor C. Clark. M. Allen both supported and encouraged the work. This paper was written in memory of Marco Beltrando.