Lawrence W. Braile

The Yellowstone hotspot

Abstract Direct evidence for a plume-plate interaction as the mechanism responsible for the Yellowstone-Snake River Plain (YSRP), 16-Ma volcanic system is the observation of a linear age-progression of silicic volcanic centers along the Snake River Plain 800 km to the Yellowstone caldera — the track of the Yellowstone hotspot. Caldera-forming rhyolitic volcanism, active crustal deformation, extremely high heat flow (about 30 times the continental average), and intensive earthquake activity at Yellowstone National Park mark the surface manifestations of the hotspot. Anomalously low P-wave velocities in the upper crust of the Yellowstone caldera are interpreted as solidified but still hot granitic rocks, partial melts, hydrothermal fluids and sediments. Unprecedented deformation of the Yellowstone caldera of up to 1 m of uplift from 1923 to 1984, followed by subsidence of as much as ∼ 12 cm from 1985 to 1991, clearly reflects a giant caldera at unrest. The regional signature of the Yellowstone hotspot is highlighted by an anomalous, 600-m-high, topographic bulge centered on the caldera and that extends across a ∼ 600-km-wide region. We suggest that this feature reflects long-wavelength tumescence of the hotspot. Yellowstone is also the center of a + 10 m to + 12 m geoid anomaly, the largest in North America, and extends about 500 km laterally from the caldera, similar in width to the geoid anomalies of many oceanic hotspots and swells. The 16-Ma trace of the Yellowstone hotspot, the seismically quiescent Snake River Plain, is surrounded by “bow-wave” or parabolic-shaped regions of earthquakes and high topography. The systematic topographic decay along the Snake River Plain, totaling 1,300 m, fits a model of lithospheric cooling and subsidence which is consistent with passage of the North American plate across a mantle heat source. The 16-0 Ma rate of 4.5 cm/yr silicic volcanic, age progression of the YSRP includes a component of southwest motion of the North American plate, modeled at ∼ 2.5 cm/yr, and a component of concomitant crustal extension estimated to be 1 to 2 cm/yr. The YSRP also exhibits anomalous crustal structure which we believe is inherited from magmatic and thermal processes associated which the Yellowstone hotspot. This includes a thin, 2–5-km-thick surface layer composed of basalts and rhyolites and an unusually high-velocity (6.5 km/s), mid-crustal mafic layer that we suggest reflects extinct “Yellowstone” magma systems that have replaced much of the normal granitic upper crust. Direct evidence for a mantle connection for the YSRP system is from anomalously low, P-wave velocities that extend from the crust to depths of ∼ 200 km. These properties and the kinematics of the YSRP are consistent with an analytic model for plume-plate interaction that produces a “bow-wave” or parabolic pattern of upper-mantle flow southwesterly from the hotspot, similar to the systematic patterns of regional topography and seismicity. Our unified model for the origin of the YSRP is consistent with the geologic evidence where basaltic magmas ascend from a mantle plume to interact with a silicic-rich continental crust producing partial melts of rhyolitic composition and the characteristic caldera-forming volcanism of Yellowstone. Cooling and contraction of the lithosphere follows the passage of the plate over the hotspot with continuing episodic eruptions of mantle-derived basalts along the SRP.

Tectonic development of the New Madrid rift complex, Mississippi embayment, North America

Abstract Geological and geophysical studies of the New Madrid Seismic Zone have revealed a buried late Precambrian rift beneath the upper Mississippi Embayment area. The rift has influenced the tectonics and geologic history of the area since late Precambrian time and is presently associated with the contemporary earthquake activity of the New Madrid Seismic Zone. The rift formed during late Precambrian to earliest Cambrian time as a result of continental breakup and has been reactivated by compressional or tensional stresses related to plate tectonic interactions. The configuration of the buried rift is interpreted from gravity, magnetic, seismic refraction, seismic reflection and stratigraphic studies. The increased mass of the crust in the rift zone, which is reflected by regional positive gravity anomalies over the upper Mississippi Embayment area, has resulted in periodic subsidence and control of sedimentation and river drainage in this cratonic region since formation of the rift complex. The correlation of the buried rift with contemporary earthquake activity suggests that the earthquakes result from slippage along zones of weakness associated with the ancient rift structures. The slippage is due to reactivation of the structure by the contemporary, nearly E-W regional compressive stress which is the result of plate motions.

Spherical earth gravity and magnetic anomaly analysis by equivalent point source inversion

Abstract To facilitate geologic interpretation of satellite elevation potential field data, analysis techniques are developed and verified in the spherical domain that are commensurate with conventional flat earth methods of potential field interpretation. A powerful approach to the spherical earth problem relates potential field anomalies to a distribution of equivalent point sources by least squares matrix inversion. Linear transformations of the equivalent source field lead to corresponding geoidal anomalies, pseudo-anomalies, vector anomaly components, spatial derivatives, continuations, and differential magnetic pole reductions. A number of examples using 1°-averaged surface free-air gravity anomalies and POGO satellite magnetometer data for the United States, Mexico and Central America illustrate the capabilities of the method.

CRUSTAL STRUCTURE, GEOPHYSICAL MODELS AND CONTEMPORARY TECTONISM OF THE COLORADO PLATEAU

Abstract Surface wave dispersion and seismic refraction data show that the crust of the interior of Colorado Plateau is approximately 45 km thick. This thickness is significantly greater than that found in the Basin and Range Province (~30 km) which bounds the plateau on the west and south. Results from recent seismic studies indicate that the Rio Grande rift, which bounds the plateau on the east, also has a thinner crust (30—35 km) than the plateau. The northern boundary of the plateau is apparently not associated with a major change in crustal thickness. In general, belts of active seismicity and late Cenozoic faulting and volcanism are associated with those boundaries of the Colorado Plateau which involve substantial crustal thinning. At both the northwestern and southwestern boundaries of the plateau, seismic data indicate that thinner Basin and Range crust extends as much as 100 km into the plateau. Thus, it appears that zones of extension (rifting?) bounding the plateau appear to be growing at the plateaus expense. Surface wave and seismic refraction data indicate that the crustal structure of the interior of the Colorado Plateau is typical of stable continental areas. However, Pn (upper mantle) velocities appear to be lower (7.8 km s−1) than would be expected in a stable region. Silica geothermometry indicates that the average heat flow for the plateau is 55 mWm−2 (1.3 HFU). Thermal and gravity models of the plateau indicate the thickness of the lithosphere to be approximately 60 km, a thickness which is intermediate between those of the Basin and Range and Great Plains. This thickness for the lithosphere is consistent with both seismic and electrical conductivity data and may explain the elevation difference between the Colorado Plateau and the Great Plains.

Crustal structure beneath the Kenya Rift from axial profile data

Abstract Modelling of the KRISP 90 axial line data shows that major crustal thinning occurs along the axis of the Kenya Rift from Moho depths of 35 km in the south beneath the Kenya Dome in the vicinity of Lake Naivasha to 20 km in the north beneath Lake Turkana. Low Pn velocities of 7.5–7.7 km/s are found beneath the whole of the axial line. The results indicate that crustal extension increases to the north and that the low Pn velocities are probably caused by magma (partial melt) rising from below and being trapped in the uppermost kilometres of the mantle. Along the axial line, the rift infill consisting of volcanics and a minor amount of sediments varies in thickness from zero where Precambrian crystalline basement highs occur to 5–6 km beneath the lakes Turkana and Naivasha. Analysis of the Pg phase shows that the upper crystalline crust has velocities of 6.1–6.3 km/s. Bearing in mind the Cainozoic volcanism associated with the rift, these velocities most probably represent Precambrian basement intruded by small amounts of igneous material. The boundary between the upper and lower crusts occurs at about 10 km depth beneath the northern part of the rift and 15 km depth beneath the southern part of the rift. The upper part of the lower crust has velocities of 6.4–6.5 km/s. The basal crustal layer which varies in thickness from a maximum of 2 km in the north to around 9 km in the south has a velocity of about 6.8 km/s.

The Role of Rifting in the Tectonic Development of the Midcontinent, U.S.A.

Abstract Recent studies have proposed the existence of several major ancient rift zones in the midcontinent region of North America. Although the dating of some of these rifts (and even the rift interpretations) are subject to question, an analysis of these “paleo-rifts” reveals three major episodes of rifting: Keweenawan (~ 1.1 b.y. B.P.), Eocambrian (~ 600 m.y. B.P.), and early Mesozoic (~ 200 m.y. B.P.). The extent of these events documents that rifting has played a major role in the tectonic development of the midcontinent region. This role goes well beyond the initial rifting event because these features display a strong correlation with Paleozoic basins and a strong propensity for reactivation. For example, the Eocambrian Reelfoot rift was reactivated in the Mesozoic to form the Mississippi embayment and is the site of modern seismicity which suggests reactivation in a contemporary stress field of ENE compression. Even though the importance of rifting can be established, recognition of rifts and delineation of their complexities remain a major problem which requires more study.

Seismic structure of the uppermost mantle beneath the Kenya rift

A major goal of the Kenya Rift International Seismic Project WRISP) 1990 experiment was the determination of deep li~ospheric structure. In the re~action/wide-angle reflection part of the KRISP effort, the experiment was designed to obtain arrivals to distances in excess of 400 km. Phases from interfaces within the mantle vyere recorded from many shotpoints, and by design, the best data were obtained along the axial profile. Reflected arrikals from two thin (< 10 km), high-velocity layers were observed along this profile and a refracted arrival was observed from the upper high-velocity layer. These mantle phases were observed on record sections from four axial profile shotpoints so overlapping and reversed coverage was obtained. Both high-velocity layers are deepest beneath Lake Turkana and become more shallow southward as the apex of the Kenya dome is approached. The first layer has a velocity of 8.05-8.15 km/s, is at a depth of about 45 km beneath Lake Turkana, and is observed at depths of about 40 km to the south before it disappears near the base of the crust. The deeper layer has velocities ranging from 7.7 to 7.8 km/s in the south to about 8.3 km/s in the north, has a similar dip as the upper one, and is found at depths of 60-65 km. Mantle arrivals outside the rift valley appear to correlate with this layer. The large amoungs of extrusive volcanics associated with the rift suggest comp~itional anomalies as an explanation for the observed velocity structure. However, the effects of the large heat anomaly associated with the rift indicate that composition alone cannot explain the high-velocity layers observed. These layers require some anisotropy probably due to the preferred orientation of olivine crystals. The seismic model is consistent with hot mantle material rising beneaith the Kenya dome in the southern Kenya rift and north-dipping shearing along the rift axis near the base of the lithosphere beneath the northern Kenya rift. This implies lithosphere thickening towards the north and is consistent with a thermal thinning of the lithosphere from below in the south changing to thinning of the lith~phere due to stretching in the north.

A comparative study of the Rio Grande and Kenya rifts

Abstract Since they are two of the prominent continental rifts which are active today, the Kenya and Rio Grande rifts have been the subject of many recent studies. There are many gaps in our knowledge, but the data available make a comparative analysis worthwhile. Although they are part of much larger extensional regimes, these rifts are of similar dimensions and extent. In addition to these obvious similarities, geophysical data suggest the crustal and upper mantle structures of these features are also similar. They both are associated with relatively localized crustal thinning and much broader zones of lithospheric thinning. Seismic and gravity data indicate that the basement structure beneath both rift valleys is very complex, and recent studies have stressed the role of low-angle normal faulting in their structural development. However, geologic data and earthquake focal mechanisms indicate that high-angle normal faults and strike-slip faulting are also important. However, as one looks at the tectonic evolution and volcanic histories of the rifts, many differences are apparent. Volcanism has been virtually continuous in the Kenya rift area for over 20 Ma and the total volume of extrusives is over 200,000 km 3 . Unlike the Kenya rift, the Rio Grande rift formed in a region which was already both tectonically and magmatically active. Thus, there is uncertainty about what constitutes rift volcanism in the Rio Grande rift. However, the volume of extrusives generally accepted to be associated with this rift is about 5–10% that of the Kenya rift. The compositions of the volcanics are also very different in these rifts, and the Kenya rift displays migration of volcanic activity, and compositional variations in time which have not been recognized in the Rio Grande rift. The volcanic activity in the Rio Grande rift post-dated most of the faulting, whereas faulting and volcanism display complex interactions in Kenya. The nature and timing of uplift are important but difficult questions in both rifts.

Paleozoic continent-ocean transition in the Ouachita Mountains imaged from PASSCAL wide-angle seismic reflection-refraction data

A wide-angle reflection-reflection experiment, sponsored by the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL), was conducted in the Ouachita Mountains area of southwestern Arkansas and northwestern Louisiana. This experimental employed 400 state-of-the-art seismic recorders and overlapped the southern one-third of the COCORP (Consortium for Continental Reflection Profiling) deep seismic reflection profile in the area. A wide variety of data processing and interpretation techniques was employed to derive an Earth model from these data. The model depicts a preserved early Paleozoic continental margin buried beneath allochthonous Paleozoic strata and younger sedimentary rocks. The southern part of the model indicates the presence of oceanic or highly extended continental crust overlain by about 15 km of mostly Paleozoic sedimentary rock. These results are consistent with little if any shortening of crystalline continental crust during the Ouachita orogeny.

The East African rift system in the light of KRISP 90

Abstract On the basis of a test experiment in 1985 (KRISP 85) an integrated seismic-refraction/teleseismic survey (KRISP 90) was undertaken to study the deep structure beneath the Kenya rift down to depths of 100–150 km. This paper summarizes the highlights of KRISP 90 as reported in this volume and discusses their broad implications as well as the structure of the Kenya rift in the general framework of other continental rifts. Major scientific goals of this phase of KRISP were to reveal the detailed crustal and upper mantle structure under the Kenya rift, to study the relationship between mantle updoming and the development of sedimentary basins and other shallow structures within the rift, to understand the role of the Kenya rift within the Afro-Arabian rift system and within a global perspective and to elucidate fundamental questions such as the mode and mechanism of continental rifting. The KRISP results clearly demonstrate that the Kenya rift is associated with sharply defined lithospheric thinning and very low upper mantle velocities down to depths of over 150 km. In the south-central portion of the rift, the lithospheric mantle has been thinned much more than the crust. To the north, high-velocity layers detected in the upper mantle appear to require the presence of anistropy in the form of the alignment of olivine crystals. Major axial variations in structure were also discovered, which correlate very well with variations in the amount of extension, the physiographic width of the rift valley, the regional topography and the regional gravity anomalies. Similar relationships are particularly well documented in the Rio Grande rift. To the extent that truly comparable data sets are available, the Kenya rift shares many features with other rift zones. For example, crustal structure under the Kenya, Rio Grande and Baikal rifts and the Rhine Graben is generally symmetrically centered on the rift valleys. However, the Kenya rift is distinctive, but not unique, in terms of the amount of volcanism. This volcanic activity would suggest large-scale modification of the crust by magmatism. Although there is evidence of underplating in the form of a relatively high-velocity lower crustal layer, there are no major seismic velocity anomalies in the middle and upper crust which would suggest pervasive magmatism. This apparent lack of major modification is an enigma which requires further study.