ABSTRACT This field trip examines contrasting lines of evidence bearing on the timing and structural style of Cenozoic (and perhaps late Mesozoic) extensional deformation in northeastern Nevada. Studies of metamorphic core complexes in this region report extension beginning in the early Cenozoic or even Late Cretaceous, peaking in the Eocene and Oligocene, and being largely over before the onset of “modern” Basin and Range extension in the middle Miocene. In contrast, studies based on low-temperature thermochronology and geologic mapping of Eocene and Miocene volcanic and sedimentary deposits report only minor, localized extension in the Eocene, no extension at all in the Oligocene and early Miocene, and major, regional extension in the middle Miocene. A wealth of thermochronologic and thermobarometric data indicate that the Ruby Mountains–East Humboldt Range metamorphic core complex (RMEH) underwent ~170 °C of cooling and 4 kbar of decompression between ca. 85 and ca. 50 Ma, and another 450 °C cooling and 4–5 kbar decompression between ca. 50 and ca. 21 Ma. These data require ~30 km of exhumation in at least two episodes, accommodated at least in part by Eocene to early Miocene displacement on the major west-dipping mylonitic zone and detachment fault bounding the RMEH on the west (the mylonitic zone may also have been active during an earlier phase of crustal extension). Meanwhile, Eocene paleovalleys containing 45–40 Ma ash-flow tuffs drained eastward from northern Nevada to the Uinta Basin in Utah, and continuity of these paleovalleys and infilling tuffs across the region indicate little, if any deformation by faults during their deposition. Pre–45 Ma deformation is less constrained, but the absence of Cenozoic sedimentary deposits and mappable normal faults older than 45 Ma is also consistent with only minor (if any) brittle deformation. The presence of ≤1 km of late Eocene sedimentary—especially lacustrine—deposits and a low-angle angular unconformity between ca. 40 and 38 Ma rocks attest to an episode of normal faulting at ca. 40 Ma. Arguably the greatest conundrum is how much extension occurred between ca. 35 and 17 Ma. Major exhumation of the RMEH is interpreted to have taken place in the late Oligocene and early Miocene, but rocks of any kind deposited during this interval are scarce in northeastern Nevada and absent in the vicinity of the RMEH itself. In most places, no angular unconformity is present between late Eocene and middle Miocene rocks, indicating little or no tilting between the late Eocene and middle Miocene. Opinions among authors of this report differ, however, as to whether this indicates no extension during the same time interval. The one locality where Oligocene deposits have been documented is Copper Basin, where Oligocene (32.5–29.5 Ma) conglomerates are ~500 m thick. The contact between Oligocene and Eocene rocks in Copper Basin is conformable, and the rocks are uniformly tilted ~25° NW, opposite to a normal fault system dipping ~35° SE. Middle Miocene rhyolite (ca. 16 Ma) rests nonconformably on the metamorphosed lower plate of this fault system and appears to rest on the tilted upper-plate rocks with angular unconformity, but the contact is not physically exposed. Different authors of this report interpret geologic relations in Copper Basin to indicate either (1) significant episodes of extension in the Eocene, Oligocene, and middle Miocene or (2) minor extension in the Eocene, uncertainty about the Oligocene, and major extension in the middle Miocene. An episode of major middle Miocene extension beginning at ca. 16–17 Ma is indicated by thick (up to 5 km) accumulations of sedimentary deposits in half-graben basins over most of northern Nevada, tilting and fanning of dips in the synextensional sedimentary deposits, and apatite fission-track and (U-Th)/He data from the southern Ruby Mountains and other ranges that indicate rapid middle Miocene cooling through near-surface temperatures (~120–40 °C). Opinions among authors of this report differ as to whether this period of extension was merely the last step in a long history of extensional faulting dating back at least to the Eocene, or whether it accounts for most of the Cenozoic deformation in northeastern Nevada. Since 10– 12 Ma, extension appears to have slowed greatly and been accommodated by highangle, relatively wide-spaced normal faults that give topographic form to the modern ranges. Despite the low present-day rate of extension, normal faults are active and have generated damaging earthquakes as recently as 2008.

The nature of petrologic and structural properties and processes that characterize the middle and lower continental crust is a long-standing problem in the earth sciences. During the past several decades significant progress has been made on this fundamental problem by synthesizing deep-crustal seismic-reflection imaging, laboratory-based seismic-velocity determinations, xenolith studies, and detailed geologic studies of exposed crustal cross sections. Geological, geochemical, and geophysical studies of crustal sections provide a crustal-scale context for a variety of important problems in the earth sciences. Crustal sections are widely used to evaluate crustal composition and petrogenesis, including lateral and vertical variations in rock types. Evidence from deep levels of crustal sections suggests seismic shear-wave anisotropy and seismic lamination result from widespread subhorizontal contacts, shear zones, and transposition fabrics, and in some sections from metamorphosed m- to km-thick, intraplated and/or underplated mafic magmatic sheets and plutons. Crustal sections also facilitate the evaluation of crustal rheology in natural settings from regional to outcrop scale. Magmatism, metamorphism, partial melting, and relatively small lithological differences control rheology, localize strain, and lead to markedly heterogeneous deformation over a wide range of crustal levels. Finally, crustal sections provide unique views of the architecture and deformation patterns of fault zones in the deep crust. As a guide to the growth and evolution of continental crust in the past 0.5 Ga, we summarize the salient features of some examples of crustal cross sections from Phanerozoic orogens. These crustal sections represent different tectonic settings, although the variation in magmatic arcs from intra-oceanic to continental-margin settings is a major theme in our synthesis. Another theme is the importance of attenuated crustal sections in reconstructing the hinterland of orogens that have experienced large-magnitude crustal extension after an earlier history of crustal contraction. The Phanerozoic crustal cross sections summarized in this chapter developed during a polyphase deformational and magmatic history that spanned 10–100s of Ma and resulted in overprinting of different events. Consequently, we conclude that there is no “typical” Phanerozoic continental crustal section, and the overall crustal composition varies markedly between sections. The thickness of lower crust that existed below an exposed crustal section is difficult to quantify. Only a few sections are in contact (typically faulted) with mantle rocks, and although xenoliths can provide important information about the unexposed parts of the deep crust and upper mantle, they are absent for most sections. The exhumation of relatively intact crustal cross sections and lower-crustal rocks probably requires an unusual sequence of tectonic events, and almost all of the sections evaluated in this chapter were exhumed by multiple mechanisms. Major exhumation is most commonly attributed to normal faults and extensional shear zones.

Petrologic, geochemical, and metamorphic data on gneissic xenoliths derived from the middle and lower crust in the Neogene Bering Sea basalt province, coupled with U-Pb geochronology of their zircons using sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG), yield a detailed comparison between the P-T-t and magmatic history of the lower crust and magmatic, metamorphic, and deformational history of the upper crust. Our results provide unique insights into the nature of lithospheric processes that accompany the extension of continental crust. The gneissic, mostly mafic xenoliths (constituting less than two percent of the total xenolith population) from lavas in the Enmelen, RU, St. Lawrence, Nunivak, and Seward Peninsula fields most likely originated through magmatic fractionation processes with continued residence at granulite-facies conditions. Zircon single-grain ages (n = 125) are interpreted as both magmatic and metamorphic and are entirely Cretaceous to Paleocene in age (ca. 138–60 Ma). Their age distributions correspond to the main ages of magmatism in two belts of supracrustal volcanic and plutonic rocks in the Bering Sea region. Oscillatory-zoned igneous zircons, Late Cretaceous to Paleocene metamorphic zircons and overgrowths, and lack of any older inheritance in zircons from the xenoliths provide strong evidence for juvenile addition of material to the crust at this time. Surface exposures of Precambrian and Paleozoic rocks locally reached upper amphibolite-facies (sillimanite grade) to granulite-facies conditions within a series of extension-related metamorphic culminations or gneiss domes, which developed within the Cretaceous magmatic belt. Metamorphic gradients and inferred geotherms (~30–50 °C/km) from both the gneiss domes and xenoliths are too high to be explained by crustal thickening alone. Magmatic heat input from the mantle is necessary to explain both the petrology of the magmas and elevated metamorphic temperatures. Deep-crustal seismic-reflection and refraction data reveal a 30–35-km-thick crust, a sharp Moho and reflective lower and middle crust. Velocities do not support a largely mafic (underplated) lower crust, but together with xenolith data suggest that Late Cretaceous to early Paleocene mafic intrusions are likely increasingly important with depth in the crust and that the elevated temperatures during granulite-facies metamorphism led to large-scale flow of crustal rocks to produce gneiss domes and the observed subhorizontal reflectivity of the crust. This unique combined data set for the Bering Shelf region provides compelling evidence for the complete reconstitution/re-equilibration of continental crust from the bottom up during mantle-driven magmatic events associated with crustal extension. Thus, despite Precambrian and Paleozoic rocks at the surface and Alaska’s accretionary tectonic history, it is likely that a significant portion of the Bering Sea region lower crust is much younger and related to post-accretionary tectonic and magmatic events.

The Kodiak Border Ranges ultramafic complex, Afognak batholith, and Shuyak Formation on Kodiak and Afognak Islands together form the lower, middle, and upper portions, respectively, of a Jurassic–Triassic island-arc crustal section. The Kodiak section exhibits structural and geochemical trends similar, but not identical to, the Tonsina-Nelchina segment of the Talkeetna arc, located >500 km to the northeast. Exposed at the base of the Kodiak section is cumulate clinopyroxenite with associated dunite, wehrlite, and layered gabbro. In the inferred middle to upper crust, tonalite and quartz diorite of the Afognak batholith intrude Shuyak Formation basaltic flows, basaltic pillow lavas, and volcaniclastic sedimentary rocks. Despite the fault-bounded nature of the lower crustal and mantle rocks, continuous chemical trends in elements such as MgO, Ni, Cr, Nb, Sr, Y, and rare-earth elements exist across all three units. Modeling of these data suggest that Kodiak arc evolution occurred in two main stages: (1) a gabbroic initial melt underwent fractional crystallization that produced a pyroxenitic root and a gabbroic lower crust, and (2) melt in equilibrium with the gabbroic lower crust underwent assimilation-fractional crystallization to produce mid-crustal plutonic and upper-crustal volcanic rocks. Kodiak Island exposes the oldest and thinnest portion of the Talkeetna arc, with ages from the Afognak batholith ranging from ca. 215–185 Ma. In the eastern and western Talkeetna arc, magmatism migrated northward after ca. 180 Ma in response to inferred forearc erosion. Forearc erosion coupled with differential subduction-channel movement juxtaposed blueschist-facies rocks with middle and lower crustal arc rocks. These processes occurred earlier and to a greater degree in the western Talkeetna arc, causing the arc to split in half, separating the Kodiak and Alaskan Peninsula parts of the Talkeetna arc.

The Coast orogen of western coastal British Columbia and southeastern Alaska is one of the largest batholithic belts in the world. This paper addresses the structure and composition of the crust in the central part of this orogen, as well as the history of its development since the mid-Cretaceous. The core of the orogen consists of two belts of metamorphic and plutonic rocks: the western metamorphic and thick-skinned thrust belt comprising 105–90-Ma plutons and their metamorphic country rocks, and the Coast Plutonic Complex on the east, with large volumes of mainly Paleogene magmatic rocks and their high-temperature gneissic host rocks. These two belts are separated by the Coast shear zone, which forms the western boundary of a Paleogene magmatic arc. This shear zone is subvertical, up to 5 km wide, and has been seismically imaged to extend to and offset the Moho. Lithologic units west of the Coast shear zone record contractional deformation and crustal thickening by thrusting and magma emplacement in the mid-Cretaceous. To the east, the Coast Plutonic Complex records regional contraction that evolves to regional extension and coeval uplift and exhumation after ca. 65 Ma. Igneous activity in the Complex formed a Paleogene batholith and gave rise to high crustal temperatures, abundant migmatite and, as a result, considerable strain localization during deformation. In both belts, during each stage of the orogeny, crustal-scale deformation enabled and assisted magma transport and emplacement. In turn, the presence of magma, as well as its thermal effects in the crust, facilitated the deformation. After 50 Ma, the style of crustal evolution changed to one dominated by periods of extension oriented approximately perpendicular to the orogen. The extension resulted in tilting of large and small crustal blocks as well as intra-plate type magmatic activity across the orogen. Seismic-reflection and refraction studies show that the crust of this orogen is unusually thin, probably due to the periods of orogen-perpendicular stretching. Magmatic activity west of the Coast shear zone in the Late Oligocene and Miocene was related to one period of orogen-parallel transtension along the margin. Small-scale, mafic, mantle-derived volcanic activity continues in the region today. The change from convergence to translation and extension is related to a major plate reorganization in the Pacific that led to a change from subduction of an oceanic plate to northwestward translation of the Pacific plate along the northwest coast of North America. Although it has been proposed that this orogen is the site of major (up to 4000 km) pre-Eocene northward terrane translation, there is little evidence for such large-scale displacement or for the kind of discontinuity in the geological record that such displacement would entail.

The crystalline core of the North Cascades preserves a Cretaceous crustal section that facilitates evaluation of pluton construction, emplacement, geometry, composition, and deformation at widely variable crustal levels (~5–40-km paleodepth) in a thick (≥55 km) continental magmatic arc. The oldest and largest pulse of plutonism was focused between 96 and 89 Ma when fluxes were a minimum of 3.9 × 10 ‒6 km 3 /yr/km of arc length, but the coincidence with regional crustal thickening and underthrusting of a cool outboard terrane resulted in relatively low mid- to deep-crustal temperatures for an arc. A second, smaller peak of magmatism at 78–71 Ma (minimum of 8.2 × 10 ‒7 km 3 /yr/km of arc length) occurred during regional transpression. Tonalite dominates at all levels of the section. Intrusions range from large plutons to thin (<50 m) dispersed sheets encased in metamorphic rocks that record less focused magmatism. The percentage of igneous rocks increases systematically from shallow to middle to deep levels, from ~37% to 55% to 65% of the total rock volume. Unfocused magmas comprise much higher percentages (~19%) of the total plutonic rock at deep- and mid-crustal depths, but only ~1% at shallower levels, whereas the largest intrusions were emplaced into shallow crust. Plutons have a range of shapes, including: asymmetric wedges to funnels; subhorizontal tabular sheets; steep-sided, blade-shaped bodies with high aspect ratios in map view; and steep-sided, vertically extensive (≥8 km) bodies shaped like thick disks and/or hockey pucks. Sheeted intrusions and gently dipping tabular bodies are more common with depth. Some of these plutons fit the model that most intrusions are subhorizontal and tabular, but many do not, reflecting the complex changes in rock type and rheology in arc crust undergoing regional shortening. The steep-sheeted plutons partly represent magma transfer zones that fed the large shallow plutons, which were sites of intermittent magma accumulation for up to 5.5 m.y. Downward movement of host rocks by multiple processes occurred at all crustal levels during pluton emplacement. Ductile flow and accompanying rigid rotation were the dominant processes; stoping played an important secondary role, and magma wedging and regional deformation also aided emplacement. Overall, there are some striking changes with increasing depth, but many features and processes in the arc are similar throughout the crustal section, probably reflecting the relatively small differences in peak temperatures between the middle and deep crust. Such patterns may be representative of thick continental magmatic arcs constructed during regional shortening.

Neogene doming in the north-central Klamath Mountains, California, tilted the Rattlesnake Creek terrane, chiefly an ophiolitic mélange, exposing an oblique cross section through disrupted and metamorphosed oceanic crust and mantle. The deepest section of the tilted terrane, in the Kangaroo Mountain area near Seiad Valley, contains tectonic slices of ultramafic, mafic, and sedimentary rocks that were penetratively deformed and metamorphosed under upper-amphibolite- to granulite-facies conditions. This section, called the Seiad complex, is the ophiolitic basement of an accreted Mesozoic island arc, and its polygenetic history reflects the magmatic and tectonic processes that occur during island-arc construction and evolution. The presence of metarodingite and metaserpentinite, and the concordance of structural elements and metamorphic grade among all units of the Seiad complex, indicate that initial tectonic disruption of the ophiolitic suite occurred in the upper crust and subsequent penetrative deformation and metamorphism occurred under high-temperature conditions in the deep crust. Crustal granulite-facies metamorphism is indicated by two-pyroxene metagabbroic bodies and two-pyroxene metasedimentary paragneiss. Geothermobarometric data from garnet amphibolite and granulite-facies metagabbro within the ophiolitic suite yielded pressure and temperature conditions of ~5–7 kb and ~650–750 °C. Geochemical data from samples of granulite, amphibolite, and leucotrondhjemite suggest a supra-subduction origin, although there is significant variation among the amphibolite samples, indicating multiple magma types. Crosscutting, radiometrically dated plutons and the regional geologic context suggest that high-grade metamorphism and deformation of these disrupted ophiolitic rocks occurred in the Middle Jurassic (ca. 172–167 Ma). This time interval broadly corresponds with contraction along several regional thrust faults in the Klamath Mountains province and juxtaposition of the Rattlesnake Creek terrane with terranes to the east. A U-Pb zircon age of 152.7 ± 1.8 Ma on a sample of a crosscutting leucotrondhjemitic dike swarm and published 40 Ar/ 39 Ar hornblende age spectra of ca. 150 ± 2 Ma from amphibolite indicate that magmatism and an accompanying thermal flux continued to affect this region into the Late Jurassic. Compared with the deep-crustal sections of the well-studied Kohistan and Tal-keetna arc complexes, the widespread mélange character of the Rattlesnake Creek terrane (including the Seiad complex) is unique. However, ophiolitic rocks, including mantle ultramafic rocks, are common components in the basal parts of these classic arc crustal sections. Hornblende gabbro/diorite and clinopyroxenite in the Seiad complex may be small-scale melt conduits that fed middle- and upper-crustal components of the arc, analogous to the relationship seen in Kohistan between deep-crustal ultramafic-mafic bodies and mid-crustal magma chambers.

The eastern Transverse Ranges provide essentially continuous exposure for >100 km across the strike of the Mesozoic Cordilleran orogen. Thermobarometric calculations based on hornblende and plagioclase compositions in Mesozoic plutonic rocks show that the first-order distribution of rock units resulted from differential Laramide exhumation. Mesozoic supracrustal rocks are preserved in the relatively little exhumed eastern part of the eastern Transverse Ranges and south-central Mojave Desert, and progressively greater rock uplift and exhumation toward the west exposed rocks originating at mid-crustal depths. The eastern Transverse Ranges thus constitute a tilted, nearly continuously exposed crustal section of the Mesozoic magmatic arc and framework rocks from subvolcanic levels to paleodepths as great as ~22 km. The base of this tilted arc section is a moderately east-dipping sheeted magmatic complex >10 km in width by 70 km in length, constructed structurally beneath, yet synchronous with Late Jurassic and Cretaceous upper-crustal plutons. Geochronology and regional structural relations thus suggest that arc magmas generated in the lower crust of this continental arc interacted in a complex mid-crustal zone of crystallization and mixing; products of this zone were parental magmas that formed relatively homogeneous upper crustal felsic plutons and fed lavas and voluminous ignimbrites.

Depth-dependent variations in the structure and composition of continental crust can be studied via integrated investigations of isobaric terranes. In this contribution, we summarize three isobaric terranes in Archean to Proterozoic crust. In western Canada, 35–45-km-deep lower crust is exposed over an area of more than 20,000 km 2 . The Upper Granite Gorge of Grand Canyon, Arizona, provides a transect of 20–25-km-deep middle crust. The Proterozoic basement of central Arizona represents an isobaric exposure of 10–15-km-deep middle crust. Isobaric terranes yield a conceptual image of continental crust that can be compared to seismic images, xenolith data, and drill core data to clarify rheology, coupling/decoupling of crustal levels, and the interplay between deformation, metamorphism, and plutonism. General observations include: (1) The crust is heterogeneous at all levels and cannot be accurately modeled as a simple progression from quartz-rich to feldspar-rich lithologies or from felsic to mafic bulk compositions. (2) The crust is segmented into foliation domains that alternate between steeply dipping and shallowly dipping. (3) Magmatism is expressed differently at different depths due to different background temperatures and a general upward distillation from mafic to felsic composition, and may be the most important control on crustal architecture and rheology. The strength of continental crust (and its potential for low-viscosity flow) is not simply a function of temperature, depth, and compositional layering, but is controlled by the size and distribution of rheological domains. The rheological character of a particular layer can vary in space and time at any crustal level.

A deeply eroded orogen in southwest New Zealand preserves a record of changing flow patterns in the middle and lower crust during a transition from contraction and crustal thickening to extension and crustal thinning. The New Zealand exposures show that deformation patterns at mid-lower crustal depths were strongly influenced by local variations in crustal structure, temperature, composition, magmatic activity, and rheology. Kinematic parameters, including the orientation of shear zone boundaries, the degree of non-coaxiality and kinematic partitioning, strain symmetry, and whether shear zones were thickening or thinning in different planes of observation, were extremely variable spatially and changed repeatedly over an 8–10 Ma period. However, despite this variability, several aspects of superposed deformations remained constant and can be assigned to distinctive tectonic settings. All shear zones that formed during the 119–111 Ma period in Northern Fiordland record flow involving bulk horizontal (layer-parallel) shortening, vertical (layer-perpendicular) thickening, and >50% pure shear regardless of shear zone orientation, degree of non-coaxiality, strain symmetry, and temperature conditions. In contrast, all shear zones that formed during the 114–90 Ma period in Central Fiordland record flow involving vertical thinning, subhorizontal stretching, and 40%–50% pure shear. These patterns are correlative with regional contraction and regional extension, respectively. The data suggest that at length scales of ~100 km and time scales of ca. 10 Ma, the effects of changing plate boundary dynamics on deformation patterns in the middle and lower crust can be distinguished from the effects of changing local boundary conditions, including steep temperature gradients and variable rheology.

The Mount Hay block is a ~12-km-thick, deep continental crustal section exposed in the Arunta inlier in central Australia. The ~4-km-wide, granulite-facies (770–776 ± 38 °C) Capricorn ridge shear zone cross-cuts the dominant granulite-facies fabric of the Mount Hay block. In its present geometry, the Capricorn ridge shear zone contains a steeply south-southeast-dipping foliation, steeply east-southeast-plunging lineation, and south-side-up shear-sense indicators. When post-granulite-facies tilting is removed, the shear zone restores to a shallowly to moderately (30–50°) dipping, normal shear zone in which the lineation is oblique to the inferred Proterozoic plate boundary, suggesting oblique divergence. The field observations and reconstruction indicate that strain can be localized in the high-temperature, deep-crustal roots of extensional fault systems. This geometry of a discrete, moderately dipping, deep-crustal shear zone is consistent with simple-shear conceptual models of crustal extension.