Mantle Lid Research Articles

Tomography of the western United States from regional arrival times

First arrival times from regional distances (200–1200 km) in the western United States extracted from the International Seismological Centre (ISC) data set were inverted in a tomographic study to map the laterally varying Pn velocity structure of the uppermost mantle (i.e., the mantle lid) and to estimate the crustal static delay at each seismograph station. Synthetic data were used to evaluate resolution. Results correlate well with major tectonic features. We find low uppermost mantle velocities (V < 7.9 km/s) centered in the Basin and Range Province surrounded by uppermost mantle of higher, more normal Pn velocities (V > 7.9 km/s). These low apparent Pn velocities are primarily due to the effects of isostasy, hotspot volcanism, and crustal extension. We interpret the low apparent Pn velocities beneath the Sierra Nevada (7.6–7.7 km/s) to be the result of time delays introduced by ray paths tunneling through the deep crustal root. The Yellowstone hotspot, migrating along the Snake River Plain, has left low velocity (<7.6–7.8 km/s), hot upper mantle in its wake. Lower Pn velocities (∼7.8 km/s) beneath the extensional Basin and Range are also presumably due to hotter uppermost mantle. Based on station static delays for the Basin and Range, we infer that Moho depths there do not vary significantly from 30 km. Normal continental Pn velocities (∼8.0 km/s) as well as thicker crust (40–50 km) underlying the Rocky Mountains, Wyoming Basin Province, Colorado Plateau, and the northern Columbia Plateau indicate the presence of relatively cool uppermost mantle beneath these regions.

堆積盆地の地史情報に基づく沈降モデルと構造形成 日本海東部と中国北部湾を例にして

Concerning the origin and structural developments of sedimentary basins, detailed case studies were made in the eastern part of the Sea of Japan and the Beibu Gulf of China where enough well data and seismic data were accumulated. Through these studies, a quantitative analysis for the subsidence of sedimentary basin was investigated. Three patterns were recognized for the subsidence of these basins such as 1) the isostatic initial subsidence, 2) the thermal subsidence, and 3) the flexure. Each of them is deeply associated with the evolution of sedimentary basins.This analysis seems to be an effective approach for the comprehensive evaluation of the sedimentary basins, therefore a detailed procedure for the analysis will be introduced.From this study, the developments of the basins in the Sea of Japan and the Beibu Gulf were summarized as follows:1. Phase of the isostatic initial subsidence.a. The eastern part of the Sea of Japan subsided with rates of 16 times larger in comparison with that of the Beibu Gulf. In the eastern part of the Sea of Japan, subsidence with such a large scale in short time could be resulted from prominent volcanic activities of the final stage of the Green Tuff movement.b. The both basins subsided to about 3, 000m in depth. Main structural elements such as grabens were formed in this phase. It is believed that the important source rocks deposited in the end of this phase.c. Some important reservoirs are developed in the Green-tuff Formation in Japan. In the Beibu Gulf, similar reservoirs are not developed because of weak volcanic activities there, and sandstones are important reservoirs of this phase.2. Phase of the thermal subsidence.a. Crustal thinning (24-32km) caused the conversion of relatively thickened asthenosphere to slightly more dense mantle lid and the basins subsided slowly in accordance with the movement.b. The ultimate amount of thermal subsidence (Do) of the eastern part of the Sea of Japan is larger than that of the Beibu Gulf. Thus the sediments in the Sea of Japan are greater than the Beibu Gulf.c. Remarkable structural movements in large scale during this phase did not take place in both areas. Therefore, only stratigraphic traps and anticlines which resulted from differential compaction are expected in this phase.3. Phase of the flexure.a. Subsidence in this phase was more vigorous because of plate deflections accelerated by thick sedimentary load in comparison with that of the preceding phase.b. Flexural rigidity in the eastern part of the Sea of Japan is smaller than that of the Beibu Gulf, therefore, deflections of the crust of the Sea of Japan are larger.c. Tilting of the basin led to reactivation of faults, decollement movements and gravity gliding, and structural traps were formed in this phase.

Azimuthal anisotropy of velocity in the mantle lid beneath the Basin and Range province

THE Basin and Range province in western North America is one of the largest and best-studied regions of Cenozoic continental extension1,2, yet there is still considerable debate surrounding the geological history of the province, and the cause of the observed widespread rifting. Although recent geological and crustal geophysical observations2 constrain crustal-scale processes within the Basin and Range, more information is needed on the structure and rheology of the mantle part of the lithosphere (the mantle lid), to distinguish between competing geodynamic models1,2. Here we present seismic data showing an azimuthal variation of ∼3.2% in the P-wave velocity in the shallowest mantle beneath the Basin and Range. The direction of maximum seismic velocity coincides with the NW–SE direction of Neogene and present-day tectonic extension, suggesting that penetrative deformation associated with this extension has induced a preferred orientation of the minerals in the mantle lid. Our results thus imply that deformation associated with present-day surface rifting extends at least 30 km into the underlying mantle.

Upper mantle structure of southern Africa from Pnl waves

We modeled Pnl waveforms from two moderate‐sized normal‐fault earthquakes in Zambia to determine an upper mantle P wave velocity model for southern Africa. The event parameters are May 15, 1968, mb = 5.7, depth = 28 km; and December 12, 1968, mb = 5.9, depth = 6 km. Teleseismic body wave inversions by previous researchers constrain the focal parameters for these events. We compute synthetic seismograms for six upper mantle velocity models using a wave number integration algorithm until an acceptable fit to the data is obtained. These models have different mantle P wave velocity gradients and low‐velocity zone (LVZ) depths. Quality of fit is measured primarily by the Pn/PL amplitude ratio. Source‐station geometry allows for the independent sampling of the upper mantle beneath the Kaapvaal‐Zimbabwe craton and mobile belt structural provinces. Synthetics from a constant velocity mantle model do not contain prominent precursor arrivals seen in the data; these are interpreted as P waves turning in the upper mantle. These synthetics also produce a too low Pn/PL amplitude ratio. Synthetics for a model with a mantle P wave velocity gradient of 0.0033 s−1 and a LVZ no shallower than 170 km fit the cratonic path data adequately. A slightly lower gradient, with a LVZ no shallower than 150 km, is indicated for the mobile belt regions. All the data preclude a mantle lid velocity gradient as high as 0.0066 s−1. Different velocity gradients between the two provinces implies different thermal gradients, which supports the hypothesis that a deep, cool lithospheric root exists beneath the Kaapvaal‐Zimbabwe craton. Cratonic thermal gradients derived from our velocity gradients agree closely with those derived from thermal modeling by Ballard and Pollack [1987].

Crustal thickness and upper mantle velocities in the Tibetan Plateau Region from the inversion of regional Pnl waveforms: Evidence for a thick upper mantle lid beneath southern Tibet

The crustal thickness and upper mantle velocity variation in Tibet and surrounding regions was mapped through the inversion of 130 individual long‐period Pnl waveforms. The 130 paths crisscross the Tibetan Plateau, Hindu Kush, Karakoram, southwest China, Burma, and northern India regions. We regionalized these areas on the basis of topographic and geologic expression and determined the average upper mantle velocity and crustal thickness value for each block or region through a linear least squares inversion of the 130 parameter values. Crustal thicknesses in the Tibetan plateau, Karakoram, and Hindu Kush regions range from 63 to 72 km and Pn velocities are 0.10–0.20 km/s faster beneath these regions than those beneath northern India. We observed impulsive first arrivals (evidence for positive upper mantle gradients) on seismograms recorded at distances between 1000 and 1400 km. The paths of these waves crossed portions of Tibet, the Hindu Kush, and east India regions. Simple modeling of the long‐period Pn wave‐form and amplitude requires fairly high upper mantle gradients of 0.18–0.25 km/s per 100 km beneath the Pamir‐Hindu Kush region and requires an upper mantle lid in excess of 100 km thickness. The southern half of Tibet also requires a lid of about 100 km thickness. A thinner upper mantle lid (50 km) provided a poor match to the Tibetan observations, even with a very high gradient of 0.2 km/s per 100 km. The upper mantle structure of east India appears to be similar to the southern half of Tibet (100‐km‐thick positive gradient zone). Both the increase in Pn velocity from India into Tibet and the evidence for a thick upper mantle lid beneath Tibet are consistent with the model in which Indian lithosphere has underthrust southern Tibet. Due to poor path coverage, the structure beneath northern Tibet cannot be resolved, and thus underthrusting beyond central Tibet cannot be constrained. The rapid increase in upper mantle velocity from northern India into Tibet can be explained by the pressure increase alone, produced by the double‐thick crust, and indicates that the Indian lithosphere, which underlies Tibet, is out of thermal equilibrium.

Mapping high Pn velocity beneath the Colorado Plateau constrains uplift models

The massive International Seismological Centre data set of the past 20 years and the two‐station method are used to determine Pn velocities in the mantle lid beneath the Colorado plateau. In this method the event is located at distances where Pn is the first arrival (2°–16°) and the path is in or very near the azimuth of the two‐station pair and crosses the plateau. This technique to a large extent minimizes the hypocenter mislocation effect and possible errors due to variations in the crustal structure near the source, since only the difference in travel times at the two stations is used. However, this technique has a few underlying assumptions and possible sources of errors (such as the quality of the Pn data base and station delays caused by varying crustal structure) that require an extremely careful application of the method. A detailed study of the source of errors and a methodology of selection of the data are presented. Application of this method to the Colorado plateau using all possible two‐station pairs from 53 stations located within or along the margin of the plateau yields an average high Pn velocity of 8.12±0.09 km/s. This value is considerably larger than the average value of 7.83 km/s based on available but very limited seismic refraction profiles but is remarkably similar to the average value of 8.1 km/s for the relatively stable midcontinent region. Our new Pn velocity for the Colorado plateau eliminates the paradox in the literature that emphasizes the rather close similarity between average Pn velocities beneath the Colorado plateau and the Basin and Range Province while their tectonic and magmatic Cenozoic history is dramatically different. Previous models for the structure and evolution of the plateau have used the low Pn velocity as an important constraint on density and thermal state of the lithosphere. Hence such models should be reexamined on the basis of this new uppermost mantle Pn velocity determination. There are two main models that have been proposed to explain the 2‐km uplift of the Colorado plateau. One is based on a combination of thermal thinning of the lithosphere and crustal thickening, and the other involves a combination of the delamination of the subducted, subhorizontal Farallon oceanic plate from the overriding North American plate and crustal thickening. We show that the delamination model is more readily consistent not only with our velocity determination and the elevation of the plateau but also with varied geological observations reported in the literature that concern the Cenozoic evolution of western North America.

Deep crustal structure and flexure of the Arabian Plate Beneath the Zagros collisional mountain belt as inferred from gravity observations

The Zagros mountain belt region, a Neogene continental collisional zone between Arabia and Iran that is characterized by a suture, a folded belt, and a foreland basin, is unusual among active mountain belts in that its 6–12 km thick sedimentary cover is shortening predominantly by folding while the basement is apparently thickening along numerous high‐angle reverse faults. We have used over 9000 gravity measurements in the region in conjunction with limited seismological observations and geological cross sections of the uppermost 10 km of crust to infer the deeper crustal structure beneath the region. Crustal models constrained by the Bouguer anomalies and other seismological and geological data are characterized by a Moho that dips about 1° to the northeast beneath the folded belt and increases in dip to about 5° near the Main Zagros Thrust (MZT); the Moho depth increases from 40 km beneath the leading edge of the foreland basin (the Mesopotamian foredeep and Persian Gulf) to as much as 65 km beneath the MZT. Alternative models that incorporate a subducted oceanic crust that is attached to the underthrusting Arabian crust deviate little in their Moho configurations from the above simpler models. Negative isostatic residual anomalies are interpreted to indicate local overcompensation beneath the foreland basin and near the folded belt and beneath the MZT. We show that underthrust sedimentary rocks of the converging margins along the suture zone may account for the negative isostatic residual anomalies near the MZT. However, to explain the rest of the isostatic anomalies requires other forces to be acting on this collisional plate boundary. For example, elastic flexure models required a combination of topography and subsurface loads or downward force (the origin of which is not clear) to approximate the Bouguer or isostatic residual anomalies associated with the foreland basin and the folded belt. The topography load of the Zagros mountain belt is insufficient to cause the required deflection of the underthrusting Arabian plate. A combination of isostatic, elastic flexure, and horizontal compression forces acting on the edge of the Arabian craton and the transitional lithosphere of central Iran appears to best model the crust of the Zagros region. We propose a model in which the lower crust of the converging Arabian plate located beneath the Zagros is being shortened and thickened plastically between the seismogenic, rigid upper crust and a rigid uppermost mantle lid. The mantle lid, probably decoupled from the lowermost crust, un der thrusts the Iranian crustal blocks. Thus confined from below, the horizontally compressed, plastic lower crust hydraulically depresses the Moho and raises the faulted and folded, brittle upper crust in isostatic equilibrium.

Upper mantle velocity of Fiji region from surface wave dispersion.

Single station group velocities over the period range 15-100 s are computed for nine fundamental mode Rayleigh wave propagation paths that cross tectonic provinces of the Fiji region. These computations are based on recordings of the earthquakes around the Fiji Islands by the WWSSN (World Wide Standard Seismic Network) stations AFI (Afiamalu), HNR (Honiara), and WEL (Wellington). The inversion of the derived dispersion curves for shear velocity-depth structure shows that uppermost mantle velocity beneath South Fiji basin, Lau basin, and North Fiji basin is slightly lower than that derived for the Pacific ocean region of similar age by Yu and MITCHELL (1979). The uppermost mantle lid is almost absent beneath these marginal basins, and the shear velocity is between 4.0-4.3 km/s in the depth range from Moho down to 220 km.

Q structure of the basin and range from surface waves

Regionalized Rayleigh and Love wave attenuation coefficients have been measured across the Basin and Range province of western United States in the frequency range 0.02–0.2 Hz. The measurements were made by using the methods of Tsai and Aki (1969) and Yacoub and Mitchell (1977) adapted to work on any number of events simultaneously. Rayleigh wave Q values at low frequency approach values near 40, which are significantly lower than previous measurements in the western United States. Love wave Q values, on the other hand, are quite high at low frequencies. We suggest that interference between fundamental and higher modes may explain the Love wave observations. The Rayleigh and Love wave attenuation measurements have been inverted simultaneously for shear‐wave attenuation as a function of depth. A frequency‐independent Q model is consistent with both Rayleigh wave and short‐period Love wave attenuation data. The shear‐wave Q (Qβ) model is characterized by low Qβ in the lower crust (Qβ∼100) and Qβ decreasing in the upper mantle with lowest values (Qβ∼30) beneath 60 km depth. Forward modeling shows that a high‐Q lower crust or upper mantle lid is inconsistent with the data. Our interpretation of these results is that the lithosphere is poorly developed beneath the Basin and Range and that the partially molten asthenosphere may reach very shallow depths, possibly to the base of the crust. This interpretation of the Q model in conjunction with a number of geological and geophysical evidences suggests that attenuation mechanisms involving partial melt predominate in the lower crust and upper mantle of the Basin and Range.

Shear wave structure of the Southern Volcanic Plateau of Oregon and Idaho and the Northern Great Basin of Nevada from surface wave dispersion

Fundamental mode Rayleigh wave phase velocities (to 100‐s period) have been measured for two paths across the northern Great Basin of Nevada and Utah and southern Volcanic Plateau of Oregon and Idaho. The average dispersion for these regions is similar to that previously observed for the central Great Basin of Nevada and Utah. Average structural models were derived for these paths from the observed dispersion data and existing refraction data. The model for the southern Volcanic Plateau and western Snake River consists of a three‐layer, 40‐km‐thick crust and a 10±10 km thick high‐velocity mantle lid above a broad low‐velocity zone for shear waves. Higher phase velocities are observed at long periods (>40 s) for the path through the northern Great Basin than for parallel paths in the central Great Basin or Volcanic Plateau, indicating higher average mantle shear wave velocities. The model for the northern Great Basin consists of a three‐layer 30‐km‐thick crust and a 30±10 km thick high‐velocity mantle lid above a low‐velocity zone for shear waves. The average shear wave velocity within the low‐velocity zone beneath the northern Great Basin is 4.3 km/s compared with 4.1 km/s within the low‐velocity zone beneath the central Great Basin. These higher mantle shear wave velocities postulated for the northern Great Basin are at odds with the observed heat flow but in agreement with teleseismic P wave results. Both the higher velocities and the high heat flow may be the result of upper mantle differentiation during a recent volcanic event.

Petrologic variations in quaternary volcanic rocks, British Columbia, and the nature of the underlying upper mantle

Volcanic activity has produced Late Tertiary and Quaternary cinder cones and flows between the Snake River Plain, U.S.A. and the Yukon Territory, Canada. The rock types include basanites, alkali olivine basalts, high-iron basalts, hawaiites, ankaramites, nephelinites, and olivine tholeiites. The alkali olivine basalts, basanites and hawaiites sampled are chemically similar to rocks from the mid-Atlantic islands. Associated with the volcanic rocks are xenoliths of ultramafic rocks, gabbros, granites and granulites. Seismic data indicate that the Moho throughout the region dips eastward at a very shallow angle. The low velocity zone has been located beneath southern British Columbia and displays a topographic high trending northwest-southeast. The nephelinite was erupted from near the crest of this high with less undersaturated lavas erupted from along its flanks. The suite of ultramafic xenoliths spans a greater variety of rock types than can be generated by maximum amounts of partial melting of a uniform source material to produce the lavas in the region. Calculated residual olivine compositions in equilibrium with the magmas at low velocity zone depths and liquidii temperatures are more iron-rich than the typical lherzolite xenolith olivine. This suggests that the residua from the partial melting episodes which produced the volcanic rocks are different from the upper mantle lid above the low velocity zone as represented by the ultramafic xenoliths.

Higher‐mode surface waves and structure of the Great Basin of Nevada and western Utah

Observed seismograms and dispersion data for crust and mantle higher mode surface waves in the Great Basin are compared with theoretical seismograms and dispersion curves computed for the Great Basin model of Priestley and Brune [1978]. This structure was originally derived from fundamental mode surface wave and refraction data. Phases identified as Sa [T∼13 s] are observed to have a phase velocity of 4.50±0.03 km s−1. Crustal second Rayleigh mode (first shear mode) waves have predominant periods varying from about 5 s on some paths to about 8 s on others. The observed excitation and phase velocity of the Sa phase are in agreement with theoretical seismograms and computed phase velocities for a modified Great Basin model. The agreement provides added support, in an unexpected way, for the existence of a mantle lid of velocity about 4.5 km s−1 in the Great Basin. The crustal higher mode group velocity observations, i.e., the predominant periods of the second Rayleigh mode along various paths, reflect variations in crustal thickness within the Great Basin. Crustal thicknesses of approximately 25 km are indicated for some paths in northwestern Nevada and southeastern Oregon, whereas crustal thicknesses of greater than 35 km are indicated for east central Nevada. The crustal thickness of 35 km in the Great Basin model is probably most appropriate for the central part of the Great Basin.

Seismic velocity and Q‐structure of the upper mantle lid and low velocity zone for the Eastern Great Basin

A 100‐km‐long record section of NTS explosions recorded in the eastern Snake River Plains lpar;7°<Δ<8°) shows the cusp of critical refractions from the steepened P velocity gradient at the bottom of the upper mantle LVZ. Synthetic seismograms calculated with a modified reflectivity program have been used to derive a regional velocity model of the upper mantle beneath the eastern Great Basin. The model suggests that observed very weak Pn arrivals are due to a slight negative velocity gradient below the Moho and that no high velocity mantle lid exists in this region.

Crust‐mantle whispering gallery phases: A deterministic model of teleseismic Pn wave propagation

Teleseismic Pn waves are modeled as a sum of whispering galley waves, which propagate in a waveguide composed of a simple high velocity mantle lid underlain by a low velocity zone. This model is able to account for those Pn wave propagation properties that are not dominated by scattering, i.e., their apparent velocity and lack of long period energy. Pn apparent velocity data constrain the P wave velocity gradient in the upper 100 km of mantle to be low; dvp/dz<0.001 s−1. The whispering gallery rays are shown to have significant amplitude (when compared to the direct P wave) and to have spectra that rapidly fall off at long periods. Yet this model cannot account for certain details of the observed Pn amplitude spectra. Most important among these is the spectral ratio of Pn to P, which the model underestimates by a factor of 10. Nevertheless, the model presents a useful framework for understanding some characteristics of Pn wave propagation and provides an estimate of the distribution of energy with depth in the Pn waves.

Toward an understanding of Sn: Normal modes of love waves in an oceanic structure

abstract The transverse component of the seismic phase Sn is considered in terms of normal mode propagation of Love waves. Calculations of dispersion, attenuation and particle amplitude versus depth are presented for a realistic model of oceanic structure which includes the effects of sphericity and the upper mantle zone of low velocity and high attenuation. We find modes with group velocity maxima near the Sn velocity of about 4.7 km/sec, and with attenuation sufficiently low to explain the predominance of short-period (about 1 sec) motions of Sn at epicentral distances up to 35° to 40°. For these modes, the particle displacements are largely restricted to depths above the low-velocity zone of the upper mantle. These modes are found for a structure in which velocities in the uppermost mantle lid are constant as a function of depth; sphericity alone is sufficient to provide a “wave guide” for Sn . The most interesting result of the calculations is that at periods longer than about 1 1 2 sec, modes with group-velocity maxima near 4.7 km/sec have substantial particle displacements in the low-velocity, low-Q zone in the upper mantle and are thus subject to severe attenuation. The results therefore indicate a filtering effect, which could explain the well-observed predominance of short-period motions of Sn . The particle displacement profiles calculated for the oceanic case also help to explain the efficiency with which Sn is excited by intermediate-depth sources, and provide a basis for accounting for the conversion of Sn into Lg across a continental margin.