B

Snake Range FOOT Detachment *

Snake Range FOOT Detachment *

CAST

EGAN

SMEL

reflector "F"

Egan Range Fault Spring Valley Fault

Egan Range Fault Spring Valley Fault

5 iG i5

5 iG i5

4 8 - 12

CAST

EGAN

SMEL

reflector "F"

Snake Range Fault House Range Fault

Snake Range Fault House Range Fault

Clear Lake Sevier Desert Reflector "F" Scarps Detachment

Figure 7.14 (a) Shaded relief map of a part of the eastern Basin and Range showing range-bounding faults and locations of seismic reflection profiles (black dashed lines) and GPS sites (white triangles) (image provided by N. Niemi and modified from Niemi et al., 2004, with permission from Blackwell Publishing). High-angle faults show ball and bar symbol in the hanging wall, low-angle faults show hachured pattern. Faults mentioned in the text include the Egan Range Fault (ERF), the Spring Valley Fault (SVF), the Sevier Desert Detachment (SDD), the Wasatch Fault Zone (WFS, WLS, WNS, WPS), and the Snake Range Detachment (SRD). Cross-section (b) constructed using seismic reflection data from (c) Hauser et al., 1987 and (d) Allmendinger et al., 1983 (with permission from the Geological Society of America). (e,f) Allmendinger et al., 1986 (redrawn from Allmendinger et al., 1986, by permission of the American Geophysical Union. Copyright © 1986 American Geophysical Union). SR, Snake Range Metamorphic Core Complex.

Clear Lake Sevier Desert Reflector "F" Scarps Detachment

Figure 7.14 (a) Shaded relief map of a part of the eastern Basin and Range showing range-bounding faults and locations of seismic reflection profiles (black dashed lines) and GPS sites (white triangles) (image provided by N. Niemi and modified from Niemi et al., 2004, with permission from Blackwell Publishing). High-angle faults show ball and bar symbol in the hanging wall, low-angle faults show hachured pattern. Faults mentioned in the text include the Egan Range Fault (ERF), the Spring Valley Fault (SVF), the Sevier Desert Detachment (SDD), the Wasatch Fault Zone (WFS, WLS, WNS, WPS), and the Snake Range Detachment (SRD). Cross-section (b) constructed using seismic reflection data from (c) Hauser et al., 1987 and (d) Allmendinger et al., 1983 (with permission from the Geological Society of America). (e,f) Allmendinger et al., 1986 (redrawn from Allmendinger et al., 1986, by permission of the American Geophysical Union. Copyright © 1986 American Geophysical Union). SR, Snake Range Metamorphic Core Complex.

basalts. These eruptions represent one major subcategory of a broad group of rocks known as Large Igneous Provinces (LIPs).

Large Igneous Provinces are massive crustal emplacements of mostly mafic extrusive and intrusive rock that originated from processes different from normal sea floor spreading. LIPs may cover areas of up to several million km2 and occur in a wide range of settings. Within oceanic plates, LIPs form oceanic plateaux such as Kerguelen and Ontong Java (Fig. 7.15). This latter example occupies an area two-thirds that of Australia. The Siberian and Columbia River basalts are examples that have erupted in the interior of continental plates. In East Africa, the Ethiopian and Kenyan flood basalts are associated with active continental rifting and the Deccan Traps in India and the Karoo basalts in southern Africa were emplaced near rifted continental margins. This diversity indicates that not all LIPs are associated with zones of extension. Within rifts their eruption can occur synchronously with rifting or million of years prior to or after the onset of extension (Menzies et al., 2002).

Estimation of the total volumes of lava in LIPs is complicated by erosion, dismemberment by sea floor spreading, and other tectonic processes that postdate their eruption. The Hawaiian Islands are well studied in this respect. Seismic data have shown that beneath the crust is a zone of rocks with a particularly high seismic velocity, which is probably derived from the same mantle source as the surface volcanic rocks. For Hawaii a basic relationship exists between velocity structure and the total volume of igneous rock (Coffin & Eldholm, 1994). This relationship has been applied to other LIPs to determine their volumes. For example, the Columbia River basalts are composed of 1.3 million km3, whereas the Ontong Java Plateau is composed of at least 27 million km3 of volcanic rock and possibly twice this amount (Section 5.5). These values are much higher than the continental flood basalts of East Africa. In Kenya the total volume of flood basalts has been

Coffin And Eldholm 1994

Figure 7.15 Map showing global distribution of Large Igneous Provinces (modified from Coffin & Edholm, 1994, by permission of the American Geophysical Union. Copyright © 1994 American Geophysical Union). Rifted volcanic margins are from Menzies et al. (2002). Labeled LIPs: Ethiopian flood basalts (ETHI); Deccan traps (DECC); Siberian basalts (SIBE); Kerguelen plateau (KERG); Columbia River basalts (COLR); Ontong Java (ONTO); North Atlantic igneous province (NAIP).

Figure 7.15 Map showing global distribution of Large Igneous Provinces (modified from Coffin & Edholm, 1994, by permission of the American Geophysical Union. Copyright © 1994 American Geophysical Union). Rifted volcanic margins are from Menzies et al. (2002). Labeled LIPs: Ethiopian flood basalts (ETHI); Deccan traps (DECC); Siberian basalts (SIBE); Kerguelen plateau (KERG); Columbia River basalts (COLR); Ontong Java (ONTO); North Atlantic igneous province (NAIP).

Figure 7.16 Map showing the location of Cenozoic flood basalts of the Ethiopian Plateau and East African Plateau (Kenya Dome) (after Macdonald et al., 2001, by permission of Oxford University Press).

estimated at approximately 924,000 km3 (Latin et al., 1993). In Ethiopia (Fig. 7.16), layers of basaltic and felsic rock reach thickness of >2 km with a total volume estimated at 350,000 km3 (Mohr & Zanettin, 1988). The eruption of such large volumes of mafic magma has severe environmental consequences, such as the formation of greenhouse gases, the generation of acid rain, and changes in sea level (Coffin & Eldholm, 1994; Ernst et al., 2005). The eruptions also make significant contributions to crustal growth.

Some LIPs appear to form very quickly. For many continental flood volcanics, 70-80% of the basaltic rock erupted in less than 3 million years (Menzies et al., 2002). Geochronologic studies have shown that the main flood event in Greenland (Tegner et al., 1998), the Deccan Traps (Hofmann et al., 2000), and the bulk of the Ethiopian Traps (Hofmann et al., 1997) all erupted in less than one million years. Nevertheless, this latter example (Section 7.2) also shows that pulses of volca-nism between 45 and 22 Ma contributed to the formation of the flood basalts in the Afar region. Submarine plateaux probably formed at similar rates, although less information is available from these types of LIPs. The North Atlantic Province and Ontong Java Plateau formed in less than 3 Ma and the Kerguelen Plateau in 4.5 Ma. Most of the volcanic activity occurred in short, violent episodes separated by long periods of relative quiescence. Estimates of the average rates of formation, which include the periods of quiescence, are 1218 km3 a-1 for the Ontong Java Plateau and 2-8 km3 a-1 for the Deccan Traps. Ontong Java's rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid-ocean ridge system (Coffin & Eldholm, 1994).

The outpouring of large volumes of mafic magma in such short periods of time requires a mantle source. This characteristic has encouraged interpretations involving deep mantle plumes (Sections 5.5, 12.10), although the existence and importance of these features are debated widely (Anderson & Natland, 2005). Mantle plumes may form large oceanic plateaux and some continental flood basalts also may be attributed to them. Beneath the Ethiopian Plateau and the Kenya Dome (in the East African Plateau), extensive volcanism and topographic uplift appear to be the consequences of anomalously hot asthenosphere (Venkataraman et al., 2004). The isotopic characteristics of the volcanic rock and the large volume of mafic lava erupted over a short period of time (Hofmann et al., 1997; Ebinger & Sleep, 1998) suggest that a plume or plumes below the uplifts tap deep undegassed mantle sources (Marty et al., 1996; Furman et al., 2004). As the deep plumes ascend they undergo decompression melting with the amount of melt depending on the ambient pressure (Section 7.4.2). Consequently, less melting is expected under thick continental lithosphere than under thick oceanic lithosphere. Nevertheless, the sources of magma that generated many LIPs are not well understood and it is likely that no single model explains them all. Ernst et al. (2005) review the many aspects of LIP research and models of their formation, including links to ore deposits (Section 13.2.2).

7.4.2 Petrogenesis of rift rocks

The geochemistry of mafic volcanic rocks extruded at continental rifts provides information on the sources and mechanisms of magma generation during rifting. Rift basalts typically are enriched in the alkalis (Na2O, K2O, CaO), large ion lithophile elements (LILE) such as K, Ba, Rb, Sr, Pb2+ and the light rare earths, and volatiles, in particular CO2 and the halogens. Tholeiitic

Figure 7.17 (a) Total alkali-silica diagram showing the geochemical characteristics of lavas from Ethiopia (after Kieffer et al., 2004, by permission of Oxford University Press). Dashed line separates alkaline from tholeiitic basalts. Rare earth element (b) and spider diagram (c) showing a typical alkaline oceanic island basalt (OIB) and a typical tholeiitic mid-ocean ridge basalt (MORB) (from Winter, John D, An Introduction to Igneous and Metamorphic Petrology, 1st edition © 2001, p. 195. Reprinted by permission of Pearson Education, Inc., Upper Saddle River, NJ).

Figure 7.17 (a) Total alkali-silica diagram showing the geochemical characteristics of lavas from Ethiopia (after Kieffer et al., 2004, by permission of Oxford University Press). Dashed line separates alkaline from tholeiitic basalts. Rare earth element (b) and spider diagram (c) showing a typical alkaline oceanic island basalt (OIB) and a typical tholeiitic mid-ocean ridge basalt (MORB) (from Winter, John D, An Introduction to Igneous and Metamorphic Petrology, 1st edition © 2001, p. 195. Reprinted by permission of Pearson Education, Inc., Upper Saddle River, NJ).

flood basalts also are common and may be associated with silicic lavas, including rhyolite. Observations in East Africa indicate that a continuum of mafic rocks generally occurs, including alkaline, ultra-alkaline, tholeiitic, felsic, and transitional compositions (Fig. 7.17a). This diversity reflects both the compositional heterogeneity of mantle source regions and processes that affect the genesis and evolution of mafic magma.

There are three ways in which the mantle may melt to produce basaltic liquids beneath rifts. First, melting may be accomplished by heating the mantle above the normal geotherm (Fig. 7.18a). Perturbations in the geotherm could be related to the vertical transfer of heat by deep mantle plumes. It is probable, for example, that the volcanism and topographic uplift associated with the Ethiopian and East African plateaux reflect anomalously hot mantle. Investigations of Pn wave attenuation beneath the Eastern branch of the East African Rift suggest sublithospheric temperatures that are significantly higher than those in the ambient mantle (Venkataraman et al., 2004). A second mechanism for melting the mantle is to lower the ambient pressure (Fig. 7.18b). The ascent of hot mantle during lithospheric stretching (Section 7.6.2) or the rise of a mantle plume causes a reduction in pressure that leads to decompression melting at a variety of depths, with the degree of melting depending on the rate of ascent, the geotherm, the composition of the mantle, and the availability of fluids. A third mechanism of melting involves the addition of volatiles, which has the effect of lowering the solidus temperature. All three of these mechanisms probably contribute to generation of basaltic melts beneath continental rifts.

Once formed, the composition of mafic magmas may be affected by partial melting. This process results in the separation of a liquid from a solid residue, which can produce a variety of melt compositions from a single mantle source. Primary mafic melts also tend to fractionate, whereby crystals are physically removed from melts over a wide range of crustal pressures, resulting in suites of compositionally distinctive rocks. Current models generally favor fractional crystallization of basaltic melts in shallow magma chambers as the dominant process that generates rhyolite.

2000

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Figure 7.18 (a) Melting by raising temperature. (b) Melting by decreasing pressure (from Winter, John D, An Introduction to Igneous and Metamorphic Petrology, 1st edition © 2001, p. 195. Reprinted by permission of Pearson Education, Inc., Upper Saddle River, NJ). In (b) melting occurs when the adiabat enters the shaded melting zone. Percentages of melting are shown.

Compositional variability also reflects the assimilation of crustal components and magma mixing. The bimodal basalt-rhyolite eruptions are thought to reflect combinations of mantle and silica-rich crustal melts.

A comparison of trace element concentrations and isotopic characteristics indicates that basalts generated in continental rifts are broadly similar to those of oceanic islands (Section 5.5). Both rock types preserve evidence of a mantle source enriched in incompatible trace elements, including the LILE, and show relatively high radiogenic strontium (87Sr/86Sr) and low neodym-ium (143Nd/144Nd) ratios. These patterns are quite different to those displayed by mid-ocean ridge basalts, which are depleted in incompatible trace elements (Fig. 7.17b,c) and display low strontium and high neodym-ium ratios. Trace elements are considered incompatible if they are concentrated into melts relative to solid phases. Since it is not possible to explain these differences in terms of the conditions of magma genesis and evolution, the mantle from which these magmas are derived must be heterogeneous. In general, the asthe-nosphere is recognized as depleted in incompatible elements, but opinions diverge over whether the enriched sources originate above or below the asthenosphere. Undepleted mantle plumes offer one plausible source of enriched mantle material. Enrichment also may result from the trapping of primitive undepleted asthe-nosphere at the base of the lithosphere or the diffusion of LILE-rich volatiles from the asthenosphere or deeper mantle into the lithosphere.

On the basis of trace element concentrations and isotopic characteristics, Macdonald et al. (2001) inferred that mafic magmas in the Eastern branch of the East African Rift system were derived from at least two mantle sources, one of sublithospheric origin similar to that which produces ocean island basalts and one within the subcontinental lithosphere. Contributions from the subcontinental mantle are indicated by xenoliths of lithospheric mantle preserved in lavas, distinctive rare earth element patterns, and by the mineralogy of basaltic rock. In southern Kenya, the presence of amphibole in some mafic lavas implies a magma source in the subcontinental lithosphere rather than the astheno-sphere (le Roex et al., 2001; Späth et al., 2001). This conclusion is illustrated in Fig. 7.19 where the experimentally determined stability field of amphibole is shown together with a probable continental geotherm and adiabats corresponding to normal asthenospheric mantle and a 200°C hotter mantle plume. It is only in the comparatively cool lithospheric mantle that typical hydrous amphibole can exist. The additional requirement of garnet in the source, which is indicated by distinctive rare earth element patterns, constrains the depth of melting to 75-90 km. These and other studies show that the generation of lithospheric melts is common in rifts, especially during their early stages of development. They also indicate that the identification of melts derived from the subcontinental lithosphere provides a potentially useful tool for assessing changes in lithospheric thickness during rifting.

In addition to compositional variations related to source regions, many authors have inferred systematic relationships between basalt composition and the depth and amount of melting in the mantle beneath rifts

Temperature (°C) 1000 1200 1400

CL C5

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Figure 7.19 Pressure-temperature diagram showing the stability field of amphibole (after le Roex et al., 2001, Fig. 10. Copyright © 2001, with kind permission of Springer Science and Business Media). Amphibole is stable in the subcontinental mantle but not under conditions characteristic of the asthenospheric mantle or a mantle plume. Gt, garnet; Sp, spinel.

1600

-120

Figure 7.19 Pressure-temperature diagram showing the stability field of amphibole (after le Roex et al., 2001, Fig. 10. Copyright © 2001, with kind permission of Springer Science and Business Media). Amphibole is stable in the subcontinental mantle but not under conditions characteristic of the asthenospheric mantle or a mantle plume. Gt, garnet; Sp, spinel.

(Macdonald et al., 2001; Späth et al., 2001). Tholeiitic basalts originate from relatively large amounts ofmelting at shallow mantle depths of 50 km or less. Transitional basalts are produced by less melting at intermediate depths and highly alkaline magmas originate at even greater depths (100-200 km) by relatively small amounts of melting. These relationships, and the general evolution of mafic magmas toward mid-oceanic ridge compositions as rifting progresses to sea floor spreading, imply a decrease in the depth of melting and a coincident increase in the amount of melting with time. In support of this generalization, tomographic images from East Africa show the presence of small melt fractions in relatively thick mantle lithosphere below juvenile rift segments, such as those in northern Tanzania and Kenya (Green et al., 1991; Birt et al., 1997). Larger melt fractions occur at shallower depths beneath more mature rift segments, such as those in northern Ethiopia and the Afar Depression (Bastow et al., 2005). However, as discussed below, compositional trends in basaltic lavas erupted at continental rifts may not follow a simple progression, especially prior to lithospheric rupture.

Although there may be broad trends of decreasing alkalinity with time, defining systematic compositional trends in basalts is often difficult to achieve at the local and regional scales. For example, attempts to document a systematic decrease in the degree of lithospheric contamination as rifting progresses have proven elusive. Such a decrease might be expected if, as the lithosphere thins and eventually ruptures, melts from the sublitho-spheric mantle begin to penetrate the surface without significant interaction with lithosphere-derived melts. However, studies in Kenya and Ethiopia show no systematic temporal or spatial patterns in the degree of lithospheric contamination in rift basalts (Macdonald et al., 2001). This indicates that rift models involving the progressive evolution of alkaline magmas toward more tholeiitic magmas during the transition to sea floor spreading are too simplistic. Instead, the data suggest that the full compositional range of mafic melts can coexist in continental rifts and that magma genesis may involve multiple sources at any stage of the rifting process. Tholeiites, for example, commonly are present during all stages of rifting and can precede the generation of alkaline and transitional basalts.

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