General Characteristics Of Wide Rifts

One of the most commonly cited examples of a wide intracontinental rift is the Basin and Range Province of western North America (Fig. 7.1). In this region, large extensional strains have accumulated across a zone ranging in width from 500 to 800 km (Fig. 7.8). In the central part of the province, some 250-300 km of horizontal extension measured at the surface has occurred since ~16 Ma (Snow & Wernicke, 2000). In eastern Nevada and western Utah alone the amount of total horizontal surface extension is approximately 120150 km (Wernicke, 1992). These values, and the width of the zone over which the deformation occurs, greatly

Figure 7.8 Shaded relief map of the western United States showing topography and earthquakes with M > 4.8 in the northern and central sectors of the Basin and Range (image provided by A. Pancha and A. Barron and modified from Pancha et al., 2006, with permission from the Seismological Society of America). Circle radius is proportional to magnitude. The area outlined with a bold polygon encloses all major earthquakes that are associated with deformation of the Basin and Range.

Figure 7.8 Shaded relief map of the western United States showing topography and earthquakes with M > 4.8 in the northern and central sectors of the Basin and Range (image provided by A. Pancha and A. Barron and modified from Pancha et al., 2006, with permission from the Seismological Society of America). Circle radius is proportional to magnitude. The area outlined with a bold polygon encloses all major earthquakes that are associated with deformation of the Basin and Range.

exceed those observed in narrow continental rifts (Section 7.2).

The Basin and Range example thus shows that continental lithosphere may be highly extended without rupturing to form a new ocean basin. This pattern is characteristic of rifts that form in relatively thin, hot, and weak continental lithosphere. Here, the key features that distinguish wide rifts from their narrow rift counterparts are illustrated using the Basin and Range and the Aegean Sea provinces as examples:

1 Broadly distributed deformation. The Basin and Range Province is bounded on the west by the greater San Andreas Fault system and Sierra Nevada-Great Valley microplate and on the east by the Colorado Plateau (Figs 7.8, 7.9). Both the Sierra and the Plateau record comparatively low heat flow values (40-60 mW m-2) and virtually no Cenozoic extensional deformation (Sass et al., 1994; Bennett et al., 2003). In between these two rigid blocks Cenozoic deformation has resulted in a broad zone of linear, north-trending mountain ranges of approximately uniform size and spacing across thousands of square kilometers. The mountain ranges are about 15-20 km wide, spaced approximately 30 km apart, and are elevated ~1.5 km above the adjacent sedimentary basins. Most are delimited on one side by a major range-bounding normal fault. Some strike-slip faulting also is present. In the northern part of the province (latitude 40°N) roughly 20-25 basin-range pairs occur across 750 km.

The present day deformation field of the Basin and Range is revealed by patterns of seismicity (Figs 7.8, 7.10) and horizontal velocity estimates (Fig. 7.11) derived from continuous GPS data (Section 5.8) (Bennett et al., 2003). The data show two prominent bands of high strain rate along the eastern side of the Sierra Nevada and the western side of the Colorado Plateau. These are the eastern California/central Nevada seismic belt and the Intermountain seismic belt, respectively (Fig. 7.9). Focal mechanisms (Fig. 7.10) indicate that the former accommodates both right lateral and normal displacements and the latter accommodates mostly normal motion.

Figure 7.9 Map showing the various tectonic provinces of the Basin and Range determined from geodetic and geologic data (modified from Bennett et al., 2003, by permission of the American Geophysical Union. Copyright © 2003 American Geophysical Union). Rates given are relative to the North American plate.

Figure 7.9 Map showing the various tectonic provinces of the Basin and Range determined from geodetic and geologic data (modified from Bennett et al., 2003, by permission of the American Geophysical Union. Copyright © 2003 American Geophysical Union). Rates given are relative to the North American plate.

Figure 7.10 Shaded relief map of the northern Basin and Range showing major faults and earthquake focal mechanisms (image provided by R. Bennett and modified from Bennett et al., 2003, and Shen-Tu et al., 1998, by permission of the American Geophysical Union. Copyright © 2003 and 1998 American Geophysical Union). SAF, San Andreas Fault. Northward translation of the Sierra Nevada-Great Valley microplate is accommodated by strike-slip motion on the Owens Valley (OVF), Panamint Valley-Hunter Mountain (PVHM), and Death Valley (DVF) fault zones. Black boxes show approximate area of Fig. 7.13 (lower box) and Fig. 7.14 (upper box).

Figure 7.10 Shaded relief map of the northern Basin and Range showing major faults and earthquake focal mechanisms (image provided by R. Bennett and modified from Bennett et al., 2003, and Shen-Tu et al., 1998, by permission of the American Geophysical Union. Copyright © 2003 and 1998 American Geophysical Union). SAF, San Andreas Fault. Northward translation of the Sierra Nevada-Great Valley microplate is accommodated by strike-slip motion on the Owens Valley (OVF), Panamint Valley-Hunter Mountain (PVHM), and Death Valley (DVF) fault zones. Black boxes show approximate area of Fig. 7.13 (lower box) and Fig. 7.14 (upper box).

Figure 7.11 GPS velocities of sites in the Sierra Nevada-Great Valley microplate, northern Basin and Range, and Colorado Plateau with respect to North America (image provided by R. Bennett and modified from Bennett et al., 2003, by permission of the American Geophysical Union. Copyright © 2003 American Geophysical Union). Error ellipses represent the 95% confidence level. Velocity estimates were derived from continuous GPS data from GPS networks in and around the northern Basin and Range. SAF, San Andreas Fault.

Figure 7.11 GPS velocities of sites in the Sierra Nevada-Great Valley microplate, northern Basin and Range, and Colorado Plateau with respect to North America (image provided by R. Bennett and modified from Bennett et al., 2003, by permission of the American Geophysical Union. Copyright © 2003 American Geophysical Union). Error ellipses represent the 95% confidence level. Velocity estimates were derived from continuous GPS data from GPS networks in and around the northern Basin and Range. SAF, San Andreas Fault.

In the intervening area, deformation is diffusely distributed and, in some places, absent from the current velocity field. Three sub-provinces, designated the eastern, central and western Great basins, show distinctive patterns of strain (Fig. 7.9). Relative motion between the central Great Basin and Colorado Plateau occurs at a rate of 2.8 mm a-1 and is partly accommodated by diffuse east-west extension across the eastern Great Basin. Relative motion between the Sierra Nevada-Great Valley and the central Great Basin occurs at a rate of 9.3 mm a-1 toward N37°W and is accommodated by diffuse deformation across the western Great Basin (Section 8.5.2). The central Great Basin records little current internal deformation. Similar patterns of distributed deformation punctuated by zones of high strain rate occur in the extensional provinces of central Greece and the Aegean Sea (Goldsworthy et al., 2002).

Two other zones of deformation in the Basin and Range have been defined on the basis of Middle Miocene-Recent geologic patterns. The Walker Lane (Fig. 7.9) displays mountain ranges of variable orientation and complex displacements involving normal faulting and both left lateral and right lateral strike-slip faulting. This belt overlaps with the Eastern California Shear Zone (Fig. 8.1 and Section 8.5.2) to the south. Hammond & Thatcher (2004) reasoned that the concentration of right lateral motion and extension within the western Basin and Range results from weak lithosphere in the Walker Lane. Linear gradients in gravitational potential energy and viscosity also may concentrate the deformation (Section 7.6.3). Together these data suggest that the broad region of the Basin and Range currently accommodates some 25% of the total strain budget between the Pacific and North American plates (Bennett et al., 1999). The data also indicate that, at least currently, deformation in the Basin and Range involves a heterogeneous combination of normal and strike-slip displacements.

The depth distribution of microearthquakes also shows that the Basin and Range Province is characterized by a seismogenic layer that is thin relative to other regions of the continent. Approximately 98% of events occur at depths less than 15 km for all of

Utah (1962-1999) and 17 km for Nevada (1990-1999) (Pancha et al., 2006). This thickness of the seismogenic layer is similar to that displayed by most other rifts, including those in East Africa, except that in the Basin and Range it characterizes thousands of square kilometers of crust. The pattern implies that high geothermal gradients and crustal thinning have locally weakened a very large area. Because deformation is distributed over such a broad region, most of the major faults in the Basin and Range have recurrence times of several thousand years (Dixon et al., 2003). In the northern part of the province, several hundred faults show evidence of slip since 130 ka, yet contemporary seismicity and large historical earthquakes are clustered on only a few of them. This observation raises the possibility that a significant portion of strain is accommodated by aseismic displacements. Niemi et al. (2004) investigated this possibility by combining geologic data from major faults with geodetic data in the eastern Great Basin. The results suggest that both data types define a ~350 km wide belt of east-west extension over the past 130 ka. Reconciling deformation patterns measured over different timescales is a major area of research in this and most other zones of active continental tectonics.

2 Heterogeneous crustal thinning in previously thickened crust. Wide rifts form in regions where extension occurs in thick, weak continental crust. In the Basin and Range and the Aegean Sea the thick crust results from a history of convergence and crustal shortening that predates rifting. Virtually the entire western margin of North America was subjected to a series of compressional orogenies during Mesozoic times (Allmendinger, 1992). These events thickened sedimentary sequences that once formed part of a Paleozoic passive margin. The ancient margin is marked now by an elongate belt of shallow marine sediments of Paleozoic and Proterozoic age that thicken to the west across the eastern Great Basin and are deformed by thrust faults and folds of the

Mesozoic Sevier thrust belt (Fig. 7.12). This deformation created a thick pile of weak sedimentary rocks that has contributed to a delocalization of strain (Section 7.6.1) during Cenozoic extension (Sonder & Jones, 1999). Some estimates place parts of the province at a pre-rift crustal thickness of 50 km, similar to that of the unextended Colorado Plateau (Parsons et al., 1996). Others have placed it at more than 50 km (Coney & Harms, 1984). This pre-extensional history is one of the most important factors that has contributed to a heterogeneous style of extensional deformation in the Basin and Range. The uniformity in size and spacing of normal faults in the Basin and Range, and the apparent uniform thickness of the seismogenic layer, at first suggests that strain and crustal thinning, on average, might also be uniformly distributed across the province. However, this assertion is in conflict with the results of geologic and geophysical surveys. Gilbert & Sheehan (2004) found Moho depths ranging from 30 to 40 km beneath the eastern Basin and Range (Plate 7.1a), with the thinnest crust occurring in northern Nevada and Utah (Plate 7.1b) and thicknesses of 40 km in southern Nevada (Plate 7.1c) (Plate 7.1a-c between pp. 244 and 245). Louie et al. (2004) also found significant variations in Moho depths with the thinnest areas showing depths of only 19-23 km beneath the Walker Lane and northwest Nevada. This southward thickening of the crust coincides with variations in the pre-Cenozoic architecture of the lithosphere, including differences in age and pre-extensional thickness. Similar nonuniform variations in crustal thickness occur beneath the Aegean Sea (Zhu et al., 2006). These results illustrate that crustal thinning in wide rifts is nonuniform and, like narrow rifts, is strongly influenced by the pre-existing structure of the lithosphere. The nonuniformity of crustal thinning in the Basin and Range is expressed in patterns of faulting within the upper crust. The Death Valley region of eastern California contains some of the youngest examples of large-magnitude extension in the world adjacent to

Intermountain Seismic Belt

□ Extent of the Paleozoic p I Mesozoic passive margin IZ_I Batholiths m Sevier Thrust Belt M Physiographic IXAXi !____! Provinces

Figure 7.12 Map of the western United States showing extent of Paleozoic passive margin sequences and the Sevier thrust belt (after Niemi et al., 2004, with permission from Blackwell Publishing).

areas that record virtually no upper crustal strain. East-west extension beginning about ~16 Ma has resulted in ~250 km of extension between the Sierras and the Colorado Plateau (Wernicke & Snow, 1998). The intervening region responded to this divergence by developing a patchwork of relatively unextended crustal blocks separated by regions strongly deformed by extension, strike-slip faulting, and contraction (Fig. 7.13a). The heterogeneous distribution of extension is illustrated in Fig. 7.13b, which shows estimates of the thickness of the pre-Miocene upper crust that remains after extension assuming an original thickness of 15 km. In some areas, such as the Funeral and Black mountains, the upper crust has been dissected and pulled apart to such a degree that pieces of the middle crust are exposed (Snow & Wernicke, 2000).

One of the most enigmatic characteristics of the Basin and Range Province involves local relationships between large-scale extension in the upper crust and the distribution of strain in the lower part of the crust. Some studies have shown that despite highly variable patterns of upper crustal strain, local crustal thickness appears to be surprisingly uniform (Gans, 1987; Hauser et al., 1987; Jones & Phinney, 1998). This result implies that large strains have been compensated at depth by lateral flow in a weak lower crust, which acted to smooth out any Moho topography (Section 7.6.3). Park & Wernicke (2003) used magnetotelluric data to show that this lateral flow and flattening out of the Moho in the

I I Alluvial cover (<2.5 Mai ~l Rocks >2.5 Ma \//\ Relatively minor Neogene tectonlsm

I I Alluvial cover (<2.5 Mai ~l Rocks >2.5 Ma \//\ Relatively minor Neogene tectonlsm

> 15 km 15-10 km 10-5 km I 1 5-2.5 km I 1 < 2.5 km

Figure 7.13 Maps showing (a) major Cenozoic faults (heavy black lines) in the central Basin and Range Province and (b) the distribution of upper crustal thinning estimated by reconstructing Cenozoic extension using pre-extensional markers (images provided by B. Wernicke and modified from Snow & Wernicke, 2000. Copyright 2000 by American Journal of Science. Reproduced with permission of American Journal of Science in the format Textbook via Copyright Clearance Center). Symbols in (a) indicate strike-slip faults (arrows), high-angle normal faults (ball and bar symbols), low-angle normal faults (tick marks), and thrust faults (teeth). Large-magnitude detachment faults in metamorphic core complexes include the Eldorado-Black Mountains (eb), the Mormon Peak (mp), Tule Springs (ts), and the Kingston Range (kr) detachments. Contours in (b) represent the remaining thickness of a 15-km-thick pre-extensional Cenozoic upper crust, such that the lightly shaded areas represent the areas of greatest thinning. Black dots are points used in the reconstruction.

> 15 km 15-10 km 10-5 km I 1 5-2.5 km I 1 < 2.5 km

Figure 7.13 Maps showing (a) major Cenozoic faults (heavy black lines) in the central Basin and Range Province and (b) the distribution of upper crustal thinning estimated by reconstructing Cenozoic extension using pre-extensional markers (images provided by B. Wernicke and modified from Snow & Wernicke, 2000. Copyright 2000 by American Journal of Science. Reproduced with permission of American Journal of Science in the format Textbook via Copyright Clearance Center). Symbols in (a) indicate strike-slip faults (arrows), high-angle normal faults (ball and bar symbols), low-angle normal faults (tick marks), and thrust faults (teeth). Large-magnitude detachment faults in metamorphic core complexes include the Eldorado-Black Mountains (eb), the Mormon Peak (mp), Tule Springs (ts), and the Kingston Range (kr) detachments. Contours in (b) represent the remaining thickness of a 15-km-thick pre-extensional Cenozoic upper crust, such that the lightly shaded areas represent the areas of greatest thinning. Black dots are points used in the reconstruction.

Basin and Range probably occurred during the Miocene. By contrast, other regions, such as the Aegean Sea and the D'Entrecasteaux islands (Section 7.8.2), do not show this relationship, implying a more viscous lower crust that resists flow beneath highly extended areas.

3 Thin mantle lithosphere and anomalously high heat flow. Like most wide rifts, the Basin and Range is characterized by high surface heat flow, negative long-wavelength Bouguer gravity anomalies, and low crustal Pn and Sn velocities (Catchings & Mooney, 1991; Jones et al., 1992; Zandt et al., 1995; Chulick & Mooney, 2002). Regional topography in the Basin and Range also is unusually high with an average of 1.2 km above mean sea level. Low seismic velocities are discernible down to 300-400 km depth. Seismic tomographic models indicate that adiabatic mantle temperatures of 1300°C occur as shallow as 50 km under most of the province. For comparison, temperatures at 50-100 km in the cratonic mantle beneath the stable eastern part of North America are on average 500°C cooler than under the Basin and Range. All of these characteristics indicate a shallow asthenosphere and very thin, warm upper mantle (Goes & van der Lee, 2002). Temperatures at 110 km depth inferred from seismic velocity models suggest the presence of small melt and fluid pockets in the shallow mantle beneath the Basin and Range (Goes & van der Lee, 2002). Warm, low-density subsolidus mantle also may contribute to the high average elevation and large-scale variations in topography of the region. Other factors contributing to the high elevations probably include isostatic effects caused by previously thickened continental crust and magmatic intrusions. However, a lack of correlation between crustal thickness variations and surface topography indicates that simple Airy isostasy is not at play and the high elevations across the southwestern United States must involve a mantle component (Gilbert & Sheehan, 2004). Volcanic activity is abundant, including eruptions that occurred both before and during extension. This activity is compatible with evidence of high heat flow, elevated geotherms, and shallow asthenosphere. Pre-rift volcanism is mostly calc-alkaline in composition. Magmatism that accompanied extension is mostly basaltic. Basalts from Nevada have an isotopic signature suggesting that they were derived from sublithospheric mantle. This pattern matches evidence of mantle upwelling beneath the rift (Savage & Sheehan, 2000).

4 Small- and large-magnitude normal faulting. Large extensional strains and thinning of the crust in wide rifts is partly accommodated by slip on normal faults. Two contrasting patterns are evident. First, the deformation can involve distributed normal faulting where a large number of more or less regularly spaced normal faults each accommodate a relatively small amount (<10 km) of the total extension. Second, the strain may be highly localized onto a relatively small number of normal faults that accommodate large displacements of several tens of kilometers. Both patterns are common and may occur during different stages of rift evolution.

Many of the range-bounding normal faults in the Basin and Range record relatively small offsets. These structures appear similar to those that characterize narrow rift segments. Asymmetric half graben and footwall uplifts are separated by a dominant normal fault that accommodates the majority of the strain. The morphology of these features is governed by the elastic properties of the lithosphere (Section 7.6.4) and the effects of syn-rift sedimentation and erosion. The asymmetry of the half graben and the dips of the range-bounding faults also commonly change in adjacent basin-range pairs. Many of the tectonically active faults maintain steep dips (>45°) that may penetrate through the upper crust. However, unlike the border faults of East Africa, some of the range-bounding faults of the Basin and Range exhibit geometries that involve low-angle extensional detachment faults. A few of these low-angle normal faults accommodate very large displacements and penetrate tens of kilometers into the middle and, possibly, the lower crust.

Extensional detachment faults are low-angle (<30°), commonly domed fault surfaces of large areal extent that accommodate displacements of 10-50 km (Axen, 2004). The footwalls of these faults may expose a thick (0.1-3 km) ductile shear zone that initially formed in the middle or lower crust and later evolved into a frictional (brittle) slip surface as it was unroofed during the extension (Wernicke, 1981). In the Basin and Range, these features characterize regions that have been thinned to such an extent (100-400% extension) that the upper crust has been completely pulled apart and metamorphic rocks that once resided in the middle and lower crust have been exhumed. These domed regions of deeply denuded crust and detachment faulting are the hallmarks of the Cordilleran extensional metamorphic core complexes (Crittenden et al., 1980; Coney & Harms, 1984). Core complexes are relatively common in the Basin and Range (Figs 7.13, 7.14), although they are not unique to this province. Their ages are diverse with most forming during Late Oligocene-Middle Miocene time (Dickinson, 2002). Similar features occur in many other settings, including the southern Aegean Sea, in rifts that form above subduction zones, such as the D'Entrecasteaux Islands (Section 7.8.2), near oceanic spreading centers (Section 6.7), and in zones of extension within collisional orogens (Section 10.4.4).

Most authors view core complexes as characteristic of regions where weak crustal rheologies facilitate lateral flow in the deep crust and, in some cases, the mantle, causing upper crustal extension to localize into narrow zones (Sections 7.6.2, 7.6.5). Nevertheless, the mechanics of slip on low-angle normal faults is not well understood. Much of the uncertainty is centered on whether specific examples initially formed at low angles or were rotated from a steep orientation during deformation (Axen, 2004). The consensus is that both types probably occur (Section 7.8.2). Some low-angle, large-offset normal faults may evolve from high-angle faults by flexural rotation (Section 7.6.4). As the hanging wall is removed by slip on the fault, the footwall is mechanically unloaded and results in isostatic uplift and doming (Buck, 1988; Wernicke & Axen, 1988). The doming can rotate the normal fault to gentler dips and lead to the formation of new high-angle faults.

The variety of Cenozoic fault patterns that typify the Basin and Range is illustrated in Fig. 7.14, which shows a segment of the eastern Great Basin in Utah and eastern Nevada (Niemi et al., 2004). The 350 km long Wasatch Fault Zone is composed of multiple segments with the largest displaying dips ranging from 35° to 70° to the west. Its subsurface geometry is not well constrained but it probably penetrates at least through the upper crust. The Sevier Desert Detachment Fault dips 12° to the west and can be traced continuously on seismic reflection profiles to a depth of at least 1215 km (Fig. 7.14b). The range-bounding Spring Valley and Egan Range faults penetrate to at least 20 km depth and possibly through the entire 30 km thickness of the crust at angles of ~30°. The Snake Range Detachment also dips ~30° through most of the upper crust. Large-magnitude extension along the Snake Range (Miller et al., 1999) and Sevier Desert (Stockli et al., 2001) detachment faults began in Early Miocene time and Late Oligocene or Early Miocene time, respectively. In most areas, high-angle normal faults are superimposed on these older structures.

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