the Continuous Alteration of Earth Materials From One Rock Type to Another is Known as the

Metamorphism

Metamorphism is a process of mineral assemblage and texture variation that results from the physical-chemical changes of solid rocks, caused by factors such as crust movement, magma activity, or thermal fluid change in the earth.

From: Unconventional Petroleum Geology , 2013

Oil and Gas in Metamorphic Reservoirs

Caineng Zou , in Unconventional Petroleum Geology, 2013

Section 1 Metamorphic Rock Type and Tectonic Setting

Metamorphism is a process of mineral assemblage and texture variation that results from the physical-chemical changes of solid rocks, caused by factors such as crust movement, magma activity, or thermal fluid change in the earth. The metamorphism comprises recrystallization, metamorphic crystallization, deformation, fragmentation, and alternation. The product of the existing rock (igneous and sedimentary rocks) suffering metamorphism is called metamorphic rock ( Winkler, 1975) whose chemical composition is related not only to original rock, but also to metamorphism. During the process of metamorphism, whether the deformation of original rock or re-composition of material is mostly finished under solid state is not known; thus the metamorphic rock can keep some aspects of the original rock or layer. The metamorphic rocks resulting from metamorphism are often characterized by unique mineral composition, particular texture, and structure to differentiate them from igneous and sedimentary rocks.

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Volume 2

Somnath Dasgupta , Santanu Kumar Bhowmik , in Encyclopedia of Geology (Second Edition), 2021

Burial metamorphism

Burial metamorphism ( Coombs, 1961) is a type of very low-grade metamorphism, which results from partial to complete recrystallization of deeply buried sedimentary and volcanic rocks at depths and temperatures in excess of 10   km and 200–300   °C respectively and along low to medium-P/T ratios (Spear, 1993; Smulikowski et al., 2007; Bucher and Grapes, 2011). Although of regional extent, the geological setting of burial metamorphism characteristically lacks orogenesis, synchronous magmatism and deviatoric stress. The resultant rocks largely preserve original rock fabrics and display incomplete mineral transformations. The metamorphic minerals are commonly restricted to veins, vesicles, interstices and alteration zones. At deeper levels of burial metamorphism, there is circulation of hot hydrous fluids at an elevated geothermal gradient and the metamorphism overlaps with hydrothermal metamorphism.

Although, it is complicated to accurately constrain where diagenesis ceases and burial metamorphism initiates, the increasing intensity of metamorphism progressively stabilizes zeolites, prehnite and/or pumpellyite and sub-greenschist facies mineral assemblages in rocks of appropriate composition. At the northern end of the Bay of Bengal, Curray (1991) predicted a greenschist facies metamorphic condition (P  ~   0.6   GPa, T  ~   395–480   °C) at the base of the 22   km thick Bengal fan sediments.

However, categorization of burial metamorphism in any scheme of classification is problematic. The type area of Coombs (1961) in South Island, New Zealand was later interpreted to be an accretionary complex related to convergent plate margin, and not anorogenic (Robinson and Merriman, 1999). This type is included in convergent plate margin metamorphism in Table 3 as A.2.1 under the heading, burial metamorphism (accretionary). On the other hand, there is ample evidence elsewhere of anorogenic burial of sediments and low grade metamorphism, following the original attributes of burial metamorphism (Michigan basin, present day passive margin sediments in the Bay of Bengal and Gulf of Mexico). In Table 3, burial metamorphism is kept under the plate interior setting in line with the original description, albeit not from the type area. This implies that burial metamorphism in other tectonic settings needs to be distinguished to avoid confusion.

Burial metamorphism under extensional tectonic settings, such as those in the Welsh basin, southwest England basin, and English Lake district with characteristic anticlockwise P-T-t paths (Merriman and Frey, 1999) was given the term diastathermal metamorphism (Robinson, 1987). This term is retained in Table 3 under the category, continental extension (Section "Metamorphism in continental extension zone").

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Resources in the Near-Surface Earth

B.M. Moskowitz , ... V. Chandler , in Treatise on Geophysics (Second Edition), 2015

11.05.5.3 Metamorphic Rocks

Metamorphism can have a profound effect on magnetic properties of sedimentary and igneous rocks, and a proper magnetic interpretation should take into account metamorphic grade and its possible variation across a study area. The effects of metamorphism on magnetic properties have been discussed at length ( Clark, 1997; Grant, 1985a; McIntrye, 1980), and the reader is referred to these references for any information beyond the treatment that is presented here.

Mafic igneous rocks such as basalts usually have sufficient accessory (titano)magnetite so that they are moderately to strongly magnetic when fresh ( Figures 16 and 17 ). These magnetite grains tend to remain during zeolite to prehnite–pumpellyite-grade metamorphism, in the absence of hydrothermal fluids. Upon greenschist facies metamorphism, the rock becomes nonmagnetic as the magnetite grains are converted into chlorite, epidote, and hematite (Purucker and Whaler, 2007). In turn, these minerals give way to biotite and amphibole in the amphibolite facies of regional metamorphism. In mafic plutonic rocks such as gabbro, magnetite may survive where it occurs as fine grains within silicate hosts or as an abundant cumulus phase, so that the magnetism of mafic plutonic rocks may be less sensitive to low- and medium-grade metamorphism than that in their extrusive and hypabyssal equivalents (Clark, 1997). Magnetite reappears during granulite-grade metamorphism, and at the highest metamorphic grade (eclogite), the iron returns to silicates such as clinopyroxene and garnet (Purucker and Whaler, 2007). Uplift and decompression of granulites can produce fine-grained magnetite by breakdown of garnet and clinopyroxene, and this magnetite can carry a strong, stable remanence. In general, however, metamorphic magnetite occurs in coarse, multidomain grains that are associated with low Q _n values ( Figure 17 ). In contrast, hematite–ilmenite intergrowths in granulite-facies rocks from the Adirondacks (New York, USA), Labrador (Canada), Norway, and Sweden have been shown to have high magnetizations with stable remanence and Q _n  =   10–100 (Kletetschka and Stout, 1998; Kletetschka et al., 2002; McEnroe and Brown, 2000; McEnroe et al., 2001b). The source of this stable remanence and associated remanent-dominated anomalies is lamellar magnetism in the nanoscale intergrowths of hematite–ilmenite (McEnroe et al., 2009).

A wide variety of factors affect the magnetization of metamorphosed sedimentary rocks. The iron content of sediments, which is generally higher for pellites than for psammites, and the redox conditions during deposition and diagenesis will largely determine the capacity of sedimentary rocks to develop secondary magnetite during metamorphism (McIntrye, 1980). Significant chemical input by exhalative metal-bearing solutions, including iron, also enhances likelihood for magnetite formation during subsequent metamorphism. Magnetite formation also depends on the oxidation state during metamorphism, which should be intermediate between that which favors ilmenite and that which favors hematite. Magnetite can be produced from high-grade metamorphism of hematite-bearing sediments (Clark, 1997). Finally, metamorphism of organic-rich sediments usually produces graphitic rocks that lack magnetite but may include magnetic pyrrhotite. In the end, magnetic anomaly patterns observed over metasedimentary rocks reflect both sedimentary-facies variations as well as metamorphic conditions (McIntrye, 1980).

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Volume 2

John D. Winter , in Encyclopedia of Geology (Second Edition), 2021

Abstract

Metamorphism involves the solid state processes by which rocks adjust chemically (mineralogically) and/or texturally to conditions that differ from those under which they initially formed. It takes place between the domains of weathering/diagenesis and that of large-scale melting and is thus the dominant geological process taking place throughout most of the earth's crust and mantle. The principal agents of metamorphism are temperature, pressure, infiltrating fluids and deformation. Metamorphic processes occur well beneath the earth's surface, so they cannot be observed directly. It is generally accepted that some metamorphic rocks can be created by simple burial to sufficient depth, but that most metamorphic rocks now exposed at the surface were affected by some additional process such as mountain-building (orogeny), alteration by an adjacent igneous body, intense deformation, etc. In the field, differential uplift and erosion may produce a surface exposure that cuts across an initial gradient in temperature and pressure with depth that accompanied initial metamorphism. A series of metamorphic rocks may thus be revealed that were exposed to differing grades of metamorphism. One might then walk across rocks ranging from unmetamorphosed into progressively higher metamorphic grades. Characteristic metamorphic index minerals may be used to distinguish zones of progressively higher grade across such a transect. The pre-metamorphic parent rock is called the protolith. Metamorphic rocks are classified on the basis of mineralogy and texture. The protolith, temperature and pressure of metamorphic equilibration, and effects of deformation are the principal controls governing the resulting rock and its name. A proper study of metamorphic rocks allows interpretation of the physical conditions developed at a geographic area during one or more metamorphic events. Understanding the relations between the metamorphic rocks and the protoliths of each further allows a researcher to infer such things as sedimentary environments that existed prior to metamorphism. When combined with age dating and structural analysis the geologic history of various areas can be interpreted.

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Volume 2

Yong-Fei Zheng , in Encyclopedia of Geology (Second Edition), 2021

Abstract

Metamorphism in subduction zones transforms crustal rocks into given mineral assemblages under given pressure (P)-temperature (T) conditions. This gives rise to metamorphic rocks with different peak thermobaric ratios, which can be translated into metamorphic thermal gradients. As a consequence, regional metamorphism along convergent plate boundaries is classified into three types. The first is low T/P Alpine type at low thermal gradients, resulting in blueschist to eclogites facies series at high to ultrahigh pressure. The second is moderate T/P Barrovian type at moderate thermal gradients, leading to amphibolite to granulite facies series at medium to high pressures. The third is high T/P Buchan type at high thermal gradients, giving rise to amphibolite to granulite facies series at medium through high to ultrahigh temperatures. As such, three metamorphic facies series can be produced within different P/T fields, providing a genetic link to dynamic regimes at convergent plate boundaries. Whereas compressional heating is responsible for prograde Alpine and Barrovian type metamorphism at lower thermal gradients during subduction, extensional heating is responsible for Buchan type metamorphism at elevated thermal gradients during rifting. This yields contrasting thermal gradients for bimodal metamorphism in ongoing and fossil subduction zones. Consequently, their products show different relationships in space to different types of orogen. Whereas paired metamorphic belts are separated from each other along accretionary orogens, polymetamorphic products are superimposed on each other along collisional orogens. Therefore, the change of thermal gradients with time is a key to the understanding of regional metamorphism at convergent plate boundaries.

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Principles of Geology

Nicholas P. Cheremisinoff Ph.D. , in Groundwater Remediation and Treatment Technologies, 1997

Metamorphic Rocks

Metamorphism is a process that changes preexisting rocks into new forms because of increases in temperature, pressure, and chemically active fluids. Metamorphism may affect igneous, sedimentary, or other metamorphic rocks. The changes brought about include the formation of new minerals, increase in grain size, and modification of rock structure or texture, all of which depend on the original rock's composition and the intensity of the metamorphism.

Some of the most obvious changes are in texture, which serves as a means of classifying metamorphic rocks into two broad groups, the foliated and non-foliated rocks. Foliated metamorphic rocks typify regions that have undergone severe deformation, such as mountain ranges. Shale, which consists mainly of silt and clay, is transformed into slate by the change of clay to mica. Mica, being a platy mineral, grows with its long axis perpendicular to the principal direction of stress, forming a preferred orientation. This orientation, such as the development of cleavage in slate, may differ greatly from the original bedding.

With increasing degrees of metamorphism, the grains of mica grow to a larger size so that the rock has a distinct foliation, which is characteristic of the metamorphic rock, schist. At even higher grades of metamorphism, the mica may be transformed to a much coarser-grained feldspar, producing the strongly banded texture of gneiss.

Non-foliated rocks include the hornfels and another group formed from rocks that consist mainly of a single mineral. The hornfels occur around an intrusive body and were changed by "baking" during intrusion. The second group includes marble and quartzite, as well as several other forms. Marble is metamorphosed limestone and quartzite is metamorphosed quartz sandstone.

There are many different types of metamorphic rocks, but from a hydrogeologic viewpoint they normally neither store nor transmit much water and are of only minor importance as aquifers. Their primary permeability is notably small, if it exists at all, and fluids are forced to migrate through secondary openings, such as faults, joints, or other types of fractures.

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Geological Maps and Some Basic Terminology

Graham Borradaile , in Understanding Geology Through Maps, 2014

Metamorphic rocks

Metamorphic rocks are sedimentary or igneous rocks that have been changed in mineralogy and texture, usually at great depth due to the effects of any combination of heat, pressure, strain, and aggressive fluid interaction. They are not "melted" rock; the original minerals are partly or completely transformed to new minerals by solid-state diffusion, although this may be aided by diffusion through a fluid (not a melt).

Metamorphism:

1.

Increases the density of rocks.

2.

Reduces the water content of minerals.

3.

Reduces the number of minerals.

4.

Introduces minerals with more varied elemental composition.

Metamorphic rocks mostly (exception is contact metamorphism) show an alignment of minerals (due to strain) and the ornaments on maps sometimes show this. Most metamorphic rocks are folded. However, folding is not restricted to metamorphic rocks; many sedimentary rocks fold without any metamorphism, simply by rearranging the positions and orientations of mineral grains, aided by fluid action. Map ornaments often indicate metamorphic rocks with folded layers (bent bedding planes).

Typical metamorphic lithologies with their protoliths [original rock] are as follows:

Slate	[Mudstone]
Pelite	[Mudstone]
Quartzite	[Quartz sandstone]
Greywacke	[Arkose, feldspathic sandstone]
Psammite	[Any sandstone]
Schist	[Slate, mudstone]
Marble	[Limestone]
Greenstone	[Basalt]
Amphibolite	[Basalt or basic tuff]
Paragneiss	[Schist, slate, mudstone]
Orthogneiss	[Some igneous rock]
Gneiss	[Coarse grained of unknown protolith]

Occasionally, the prefix "meta" is added to a protolithology name to indicate that it is now in a metamorphic state, e.g., metagreywacke.

Metamorphism is the most difficult aspect of petrology that a student will meet. A few generalizations will help at this stage. Progressive or prograde regional metamorphism produces

1.

denser minerals,

2.

fewer minerals (at a maximum six due to the application of Gibb's phase rule),

3.

less hydrous minerals (H₂O is always driven out),

4.

minerals with more elements (greater solid solution),

5.

alignments of minerals (textures or fabrics) related to finite strain or at very high temperatures due to syncrystallization stress,

6.

groups of minerals (assemblages) associated with ranges of pressure and temperature (metamorphic facies). As examples, subduction zones are characterized by blueschist facies, orogenic metamorphism on continental margins or at collision zones shows greenschist and amphibolite facies metamorphism, and

7.

reversal of metamorphism (regression) to "less metamorphic" assemblages is very rare and incomplete; thus metamorphism invariably records the ultimate change.

Plutonic Rocks

There are many examples, particularly in the Canadian Shield and other Archean terranes (>2500 Ma), where it is difficult, even for an experienced geologist, to decide whether a rock is igneous or metamorphic. Invariably, these rocks are coarse grained, if fossils or sedimentary structures were present, they could not have survived the changes; only rarely does a ghost stratigraphy survive to prove a sedimentary origin. Almost always, any convincing primary sedimentary or igneous character is lost. Although they are commonly banded, the layers may commonly be shown to result from tectonic deformation and metamorphic differentiation. The unique characteristic that distinguishes plutonic rock from sedimentary and igneous ones is the presence of deformational-metamorphic textures, especially preferred lattice orientations of crystals. Even if they are observable in the field, without the benefit of microscope work, it may be difficult to distinguish preferred lattice orientation due to metamorphism from that caused by magmatic flow.

The coarse grain size of plutonic rocks does readily permit a mineralogical classification, however, even in the field. Regardless of whether the rocks had a sedimentary protolith (paragneiss) or an igneous protolith (orthogneiss), we may readily apply an "igneous" label to the mineral assemblage. For example, many such rocks in the Canadian Shield have the same minerals as granite, granodiorite, syenite, or tonalite but advanced studies using petrography, structure, and geochemistry can be prove they were originally sedimentary, subsequently modified by severe metamorphism. To distinguish a plutonic rock from a mineralogically similar igneous one, many geologists would substitute granitoid for of "granite". A purely descriptive term would avoid confusion and would not prejudice future discussion. (The history of geology and biology abounds in the precocious adoption of a genetic label.) Preferred, less biased plutonic rock terms include the following, with the original rock [protolith] indicated in brackets. These are reasonable generalizations:

Granitoid	[Any continent-derived sediment or igneous granitic rock]
Charnockite	[Idem]
Eclogite	[A basic igneous rock, commonly basalt or mafic tuff]
Granulite	[Any continent-derived sediment]
Anorthosite	[Feldspar-rich gneiss of arguable genesis]
Migmatite	[Any clastic sediment, subject to extreme metamorphic differentiation; partial melting could be involved in some cases]
Anatectite	[Quite rare, any clastic sediment or granitic rock, convincingly affected by melting]

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Convergent margins and orogenic belts

Dietrich. Roeder , in Regional Geology and Tectonics: Principles of Geologic Analysis, 2012

6.42 Metamorphism: Concepts and methods

Metamorphism comprises the solid-state reactions in rocks undergoing changes of pressure and temperature ( Winkler, 1976). Within known and experimentally verified ranges of temperatures and pressures, chemical reactions in solid state generate stable key minerals that may survive the cooling down to the earth's surface. Key minerals and their reaction paths have been used to define grades of metamorphism and their ranges of temperature and pressure. Metamorphic grades range between diagenesis or rock solidification and anatexis or melting. The depth range of a typical Cordilleran belt easily spans the metamorphic temperature range.

Almost all metamorphic rocks record the burial and cooling by unroofing down through the geothermal gradient. By volume, most of the rocks in a typical orogen are metamorphic. Erosion truncates the geothermal field of the orogen (Barrow, 1912; Frey et al., 1980; and many others), and one-half or more of its mappable area exposes metamorphic rocks.

Tectonic flow temporarily deforms the geothermal field. The range of rock densities is narrow, and pressure varies almost linearly with depth. Fields of key metamorphic minerals or boundaries of metamorphic isograds can be mapped in the field and plotted into exposed structures. When plotted in a Cartesian field of temperature and pressure, solid-state reactions can quantify the tectonics of orogenic belts, their compression, up-ramp transport, and uplift against erosion. Mineralogically established and radiometrically dated cooling paths of orogenic terranes (Verhoogen et al., 1970) are the most important tools to describe the tectonic process. Their use in a known field of strain will generate dated strain paths (Means, 1976).

Simpler and more widely used tools include the pressure-defined ranges of geothermal gradients (Miyashiro, 1973; Winkler, 1976). Low-pressure metamorphism is centred at a gradient of 50 °C/km, medium-pressure metamorphism at 25 °C/km, and high-pressure metamorphism at 10 °C/km. Extensional tectonics (Figs. 6.47, 6.48) is the favoured domain of low-pressure metamorphism.

Figure 6.48. Five sketch cross-sections illustrating extensional tectonics as it may affect Cordilleran regions. The extended block has no internal structure, no composition, and no scale. The five sketches show possible tectonic increments. A: Coulomb-Navier criterion of horizontal extension in equilibrium with basal traction. B: Brittle extension by about twice the thickness of the block. C: Isostatic upward adjustment of extended block. D: More extension within mobile hangingwall, no extension in footwall. Cover of low-angle rift valley with associated ignimbrites or clastic fill. E: Shift of extension into asthenospheric space and scale. Random dissection of formerly extended block into tilted fault blocks. Distinction of hangingwall and footwall is no longer possible.

The thermal structure of the external, sedimentary, and non-metamorphic zones of the orogen is studied as a one-dimensional burial history at many points. Commercial hydrocarbons form from organic matter at a temperature range of 100–200 °C, and therefore, the method of one-dimensional burial history (Jeffreys, 1962; Van Hinte, 1978) is used in assessing the hydrocarbon potential of non-metamorphic basins (Tissot and Welte, 1978; Waples, 1980).

Thermal disturbances in orogenic belts are of geologically short duration, but significant because of the low thermal dispersivity of rock (Turcotte and Schubert, 1982). Intracrustal faults can dislocate isotherms at significantly faster rates than their thermal re-equilibration by conductance. This mechanism can create tectonic temperature inversions above subthrust settings for commercial hydrocarbon deposits.

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Tectonic settings

Kent C. Condie , in Earth as an Evolving Planetary System (Fourth Edition), 2022

Convergent margin metamorphism

Metamorphism associated with convergent margins provides a mineral record that may be inverted to yield apparent thermal gradients for different orogenic belts, which in turn may be used to infer details of tectonic setting ( Brown, 2009). Generally, peak metamorphic mineral assemblages are robust recorders of metamorphic pressure (P) and temperature (T), particularly at high P-T conditions, because prograde dehydration and melting produce anhydrous mineral assemblages that are difficult to retrogress or overprint without more fluid influx. Lower thermal gradients are usually associated with subduction and the early stages of collision, whereas higher thermal gradients are characteristic of back-arcs and orogenic hinterlands. This duality of thermal regimes gives rise to paired metamorphic belts. Paired metamorphic belts are pene-contemporaneous belts of contrasting types of metamorphism that record different apparent thermal gradients: one warmer and the other cooler, juxtaposed by plate tectonics processes. The duality of metamorphic belts is the characteristic imprint of asymmetric or one-sided subduction in the geological record.

Low-temperature, high-pressure metamorphism, known as blueschist metamorphism, is important in subduction zones, where high-pressure, relatively low-temperature mineral assemblages form. Glaucophane and lawsonite, both of which have a bluish color, are common minerals in this setting. In subduction zones, crustal fragments can be carried to great depths (> 50   km), yet remaining at rather low temperatures, usually <   400°C (Fig. 2.11). A major unsolved question is how these rocks return to the surface. One possibility is by continual underplating of the accretionary prism with low-density sediments, resulting in fast, buoyant uplift during which high-density pieces of the slab are dragged to the surface (Cloos, 1993). Another possibility is that blueschists are thrust upward during later collisional tectonics.

One of the most intriguing fields of research at present is seeing just how far crustal fragments are subducted before returning to the surface. Discoveries of coesite (a high pressure silica phase) and diamond inclusions in pyroxenes and garnet from eclogites from high-pressure metamorphic rocks in eastern China record astounding pressures of 4.3   GPa (about 150   km burial depth) at 740°C and are now known to have developed during ultrahigh-pressure (UHP) metamorphism (Zheng, 2008; Hacker and Gerya, 2013). Several other localities have reported coesite-bearing assemblages recording pressures of 2.5–3   GPa. Also, new high-pressure hydrous minerals have been identified in these assemblages, indicating that some water is recycled into the mantle and that not all water is lost by dehydration to the mantle wedge. Perhaps the most exciting aspect of these findings is that for the first time we have direct evidence that crustal rocks (both felsic and mafic) can be recycled into the mantle. There are at least five tectonic settings by which quartzofeldspathic rocks can reach depths of 150   km or more (Hacker and Gerya, 2013): (1) continental margin subduction, (2) partial subduction of a microcontinent, (3) sediment subduction, (4) subduction erosion, and (5) foundering of a crustal root. Whether the upper plate is oceanic or continental is irrelevant to whether UHP rocks form, and subduction erosion of continental rocks can even be caused by an oceanic lower plate. That most UHP rocks have been found in collisional orogens may have something to do with how these rocks are exhumed, but says nothing a priori about how UHP rocks form.

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Tectonic Style, Rock Successions, and Tectonic Provinces

Joseph A. DiPietro , in Landscape Evolution in the United States, 2013

Foreland-Hinterland Transition

Metamorphism within both the Appalachian and Cordilleran mountain belts, intrusions, and exposure of crystalline shield rock at the rear of the foreland fold-and-thrust belt allow us to define a foreland-hinterland boundary. In an idealized orogenic system the foreland-hinterland boundary separates sedimentary from crystalline rock ( Figure 21.1). The change in rock type creates a topographic step that coincides with a marked change in landscape. Such a step is seen in the Appalachians between the sedimentary Valley and Ridge and the higher, more resistant crystalline topography of the Blue Ridge. The topographic step will retreat over time if the contact is close to horizontal but will remain relatively stationary if the contact is close to vertical as outlined long ago in Figure 3.13. The most distinctive foreland-hinterland boundary is one marked by a major thrust fault. Again we can cite the southern Appalachians as an example where a series of thrust faults, collectively referred to as the Blue Ridge thrust, forms the foreland-hinterland boundary. The boundary, however, can also be marked by an intrusive contact or by a nonconformity. All three are present in the Appalachians.

In the Cordillera, in order to define a foreland-hinterland boundary we need to: (1) ignore the fact that crystalline rock is widespread across the reactivated western craton in front of the foreland fold-and-thrust belt, and (2) ignore the fact that sedimentary rocks are present across the thrust belt all the way to the boundary with accreted terranes and beyond. Crystalline rocks crop out within the hinterland thrust belt as isolated anticlinal culminations, granitic intrusions, and normal fault exhumed crystalline core complexes surrounded by miogeoclinal sedimentary rock. We can, therefore, define a foreland-hinterland boundary as the first appearance of crystalline rock west of the Sevier foreland fold-and-thrust belt. Given the presence of sedimentary rock in the hinterland, we can conclude that the Cordillera does not display a fully developed, mature foreland-hinterland transition.

There are marked differences in both the landscape and the style of deformation across the foreland-hinterland thrust belt. If you were to look at rocks along the side of the road within a foreland fold-and-thrust belt, they probably would not look terribly deformed. Perhaps all you would see are tilted layers of sedimentary rock. If you are lucky enough to see a fault, you would notice that the rocks are clearly broken across the fault surface. If you are really lucky and have a grand view of a mountainside, you might see giant folds that broadly deform thick sedimentary layers. What you likely would not see is any evidence of metamorphism, granitic intrusion, or abundant small-scale folds. The reason for this is that rocks in a foreland fold-and-thrust belt are deformed under brittle conditions where temperatures are too low to cause obvious metamorphism and where rocks tend to break rather than bend. The foreland fold-and-thrust belt landscape is one of asymmetrical, linear mountains that are steep on the cratonic side and tilted with a more gentle dip on the oceanward side. This type of landscape was described in Chapter 12 (Figure 12.6). The thrust faults themselves impart the asymmetry to the mountain belt because they transport rock layers mostly in one direction: toward the craton.

Mountains in the hinterland are higher, massive, and more rugged, primarily because of the presence of hard, poorly layered, crystalline rock. Here, if you were to look at rocks along the side of the road, you would likely see multiple small-scale folds, granitic intrusions, and evidence of metamorphism. These rocks reached temperatures hot enough to bend without breaking. They were sheared, folded, and intruded under warm, ductile, metamorphic conditions. The difference between brittle and ductile deformation is a little bit like the difference between a stick of cold wax and a stick of warm wax. Cold wax will snap and break. Warm wax will bend but not break. The change from cold, brittle-deformed sedimentary rock at the front of a mountain belt, to warm, ductile-deformed crystalline rock at the rear, can be expressed in terms of a foreland–hinterland transition.

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