Showing papers on "Mohorovičić discontinuity published in 1996"

The lunar crust: Global structure and signature of major basins

Abstract: New lunar gravity and topography data from the Clementine Mission provide a global Bouguer anomaly map corrected for the gravitational attraction of mare fill in mascon basins. Most of the gravity signal remaining after corrections for the attraction of topography and mare fill can be attributed to variations in depth to the lunar Moho and therefore crustal thickness. The large range of global crustal thickness (approx. 20-120 km) is indicative of major spatial variations in melting of the lunar exterior and/or significant impact-related redistribution. The 6l-km average crustal thickness, constrained by a depth-to-Moho measured during the Apollo 12 and 14 missions, is preferentially distributed toward the farside, accounting for much of the offset in center-of-figure from the center-of-mass. While the average farside thickness is 12 km greater than the nearside, the distribution is nonuniform, with dramatic thinning beneath the farside, South Pole-Aitken basin. With the global crustal thickness map as a constraint, regional inversions of gravity and topography resolve the crustal structure of major mascon basins to half wavelengths of 150 km. In order to yield crustal thickness maps with the maximum horizontal resolution permitted by the data, the downward continuation of the Bouguer gravity is stabilized by a three- dimensional, minimum-slope and curvature algorithm. Both mare and non-mare basins are characterized by a central upwarped moho that is surrounded by rings of thickened crust lying mainly within the basin rims. The inferred relief at this density interface suggests a deep structural component to the surficial features of multiring lunar impact basins. For large (greater than 300 km diameter) basins, moho relief appears uncorrelated with diameter, but is negatively correlated with basin age. In several cases, it appears that the multiring structures were out of isostatic equilibrium prior to mare emplacement, suggesting that the lithosphere was strong enough to maintain their state of stress to the present.

Variable crustal thickness in the Oman ophiolite: Implication for oceanic crust

Abstract: From 32 detailed cross sections through the gabbro unit of the Oman ophiolite, it is concluded that the thickness of this unit is on average 3.6 km. The lower layered gabbros represent two thirds, and the upper homogeneous foliated gabbros represent one third the gabbro unit. Assuming that the overlying basaltic lid (sheeted dikes and extrusives) is 1.5–2 km thick, the average crustal section in the Oman ophiolite is 0.5–1 km thinner than the standard 6-km-thick oceanic crust usually considered to be produced at fast spreading ridges, a point which is discussed. Variations in gabbro thickness between 5.4 km and 1.5 km are recorded. There is a general correlation throughout the ophiolite belt, particularly in the southeastern massifs, such that the thinnest gabbro units (2.2–2.5 km thick) overlay the thickest (300–700 m) transition zones which separate them from the mantle harzburgites and the thickest gabbro units (3.6–3.9 km thick) overlay the thinnest (5–100 m) transition zones. The combination of thinner gabbro units and thicker transition zones is observed above mantle diapirs or in domains which, following our structural models, were accreted above diapirs and have drifted in the spreading direction. If it is assumed that the extrusive basalt and the sheeted dike complex units have a constant thickness, such large variations indicate similar variations in the Moho level below the ridge of origin; in particular, the Moho above mantle diapirs should be some 1–1.5 km shallower than away from diapirs. As the Oman ophiolite is considered to derive from a fast spreading paleoridge, this doming should be detected in actual fast spreading ridges, as suggested by Barth and Mutter [this issue] and Wang et al. [this issue].

Variability in oceanic crustal thickness and structure: Multichannel seismic reflection results from the northern East Pacific Rise

Abstract: Multichannel seismic reflection data acquired between 8°50′ and 9°50′N and between 12°30′ and 13°30′N along the East Pacific Rise provide a three-dimensional view of the young oceanic crust. Seafloor-to-Moho reflection travel times vary by up to 0.9 s within our study areas; the total range of crustal travel times in the 9°N area is 1.55 to 2.45 s; the total range in the 13°N area is 1.60 to 2.05 s. The variation is systematic, indicating thinner crust locally associated with overlapping spreading centers (OSCs) and, in the 9°N area, segment-scale variation along crustal isochrons. Crustal travel time is found to be a valid proxy for oceanic crustal thickness. Outside of the axial low-velocity volume, thickness can be calculated from time to ∼500 m. Even in the axial region thickness can be calculated to <1 km, if low-velocity zone position is known. Crustal thicknesses calculated from travel times vary by 2.6 km in the 9°N area, and by 1.5 km in the 13°N area. The majority of this variation is attributed to seismic layer 3 (the lower crust). Segment-scale variation of ∼1.8 km (∼5.5 to 7.3 km thickness) is observed in the 9°N area, with thinnest crust formed between ∼9°40′ and 9°50′N and thickest formed between ∼9°15 and 9°20′N. Results imply a three-dimensional pattern of magma supply to the 9°N segment. The OSC at 9°03′N is associated with major disruptions of the segment-scale pattern, in the form of local thin areas within the discordant zone; the smaller OSC at 12°54′N is not associated with dramatic changes in thickness of the surrounding crust. In the absence of OSCs, the process of crustal formation displays more temporal uniformity along flow lines than spatial uniformity along isochrons within a segment. Thicker crust does not always correlate with shallower ridge bathymetry, broader axial cross section, or more negative mantle Bouguer or subcrustal gravity anomaly. Variable thickness of the crust-mantle transition region as well as crustal flow in the axial region may be responsible for this unexpected result. We hypothesize that the geophysical signature of diapiric mantle upwelling beneath a fast spreading ridge is relatively thin crust associated with a thick Moho transition zone and a subcrustal gravity low. Such a diapiric upwelling center appears to be now located beneath the East Pacific Rise near 9°40′ to 9°50′N.

Crustal Root Beneath the Urals: Wide-Angle Seismic Evidence

Abstract: Wide-angle reflection and refraction data acquired as part of the URSEIS 995 geophysical experiment across the southern Uralide orogen provide evidence for a 12- to 15-kilometer-thick crustal root, yielding a total crustal thickness of 55 to 58 kilometers. Strong reflections from the Mohorovicic discontinuity (Moho) at relatively small precritical distances suggest that the crust-mantle transition beneath the crustal root is a sharp feature. The derived P- and S-wave velocity models constrain key physical properties of the crust, including the depth of the mafic rocks of the Magnitogorsk volcanic arc and the existence of a lower crustal zone of possible basic rock enrichment beneath the East Uralian zone.

Crustal structure of the Colorado Plateau, Arizona: Application of new long‐offset seismic data analysis techniques

Abstract: The Colorado Plateau is a large crustal block in the southwestern United States that has been raised intact nearly 2 km above sea level since Cretaceous marine sediments were deposited on its surface. Controversy exists concerning the thickness of the plateau crust and the source of its buoyancy. Interpretations of seismic data collected on the plateau vary as to whether the crust is closer to 40 or 50 km thick. A thick crust could support the observed topography of the Colorado Plateau isostatically, while a thinner crust would indicate the presence of an underlying low-density mantle. This paper reports results on long-offset seismic data collected during the 1989 segment of the U.S. Geological Survey Pacific to Arizona Crustal Experiment that extended from the Transition Zone into the Colorado Plateau in northwest Arizona. We apply two new methods to analyze long-offset data that employ finite difference travel time calculations: (1) a first-arrival time inverter to find upper crustal velocity structure and (2) a forward-modeling technique that allows the direct use of the inverted upper crustal solution in modeling secondary reflected arrivals. We find that the crustal thickness increases from 30 km beneath the metamorphic core complexes in the southern Basin and Range province to about 42 km beneath the northern Transition Zone and southern Colorado Plateau margin. We observe some crustal thinning (to ∼37 km thick) and slightly higher lower crustal velocities farther inboard; beneath the Kaibab uplift on the north rim of the Grand Canyon the crust thickens to a maximum of 48 km. We observe a nonuniform crustal thickness beneath the Colorado Plateau that varies by ∼15% and corresponds approximately to variations in topography with the thickest crust underlying the highest elevations. Crustal compositions (as inferred from seismic velocities) appear to be the same beneath the Colorado Plateau as those in the Basin and Range province to the southwest, implying that the plateau crust represents an unextended version of the Basin and Range. Some of the variability in crustal structure appears to correspond to preserved lithospheric discontinuities that date back to the Proterozoic Era.

Inversion of three-dimensional wide-angle seismic data from the southwestern Canadian Cordillera

Abstract: Seismic refraction/wide-angle reflection data were recorded on a triangular array in southwestern British Columbia centered on the boundary between the Coast Belt to the southwest and the Intermontane Belt to the northeast. The experiment, part of the Lithoprobe Southern Cordillera transect, enabled determination of the three-dimensional (3-D) velocity structure of the crust and upper mantle. An algorithm for the inversion of wide-angle seismic data to determine 3-D velocity structure and depth to reflecting interfaces is developed. The algorithm is based on existing procedures for the inversion and forward modeling of first arrival travel times and forward modeling of reflection travel times, including (1) forward modeling using a 3-D finite difference algorithm; and (2) a simple velocity model parameterization for the inversion which eliminates the need to solve a large system of equations. The existing procedure is extended to allow (1) the inversion of reflection times to solve for depth to a reflecting interface and/or velocity structure; (2) the inversion of first arrival travel times to solve for depth to a refracting interface; and (3) layer stripping. Application of the algorithm to southern Cordillera data uses Pg to constrain upper crustal velocity structure, PmP to constrain lower crustal velocity structure and depth to Moho, and Pn to constrain upper mantle velocities and depth to Moho. The 3-D velocity model for the southwestern Canadian Cordillera is characterized by (1) significant lateral velocity variations at all depths that do not, in general, correlate with surface geological features or gravity data; (2) a relatively high velocity middle and lower crust in the southwestern part of the study area which correlates with a strong relative gravity high and outlines the eastern extent of lower Wrangellia, an accreted terrane forming the Insular Belt to the west; (3) a narrow zone of slower velocity in the lower crust and change in crustal thickness associated with the Fraser Fault system, lending additional support to the view that it is a crustal penetrating fault; (4) an average upper mantle velocity of 7.85 km/s; and (5) a depth to Moho of 33–36 km in the Intermontane Belt and 36–38 km throughout most of the Coast Belt, decreasing in the west to 33 km near the Insular-Coast contact. Horizontal velocity structure slices and an interpreted cross section based on these and other results show the complexity of crustal structure in the region.

Crustal thickness variations beneath the peninsular ranges, southern California

Abstract: We investigate the crustal thickness under the Peninsular Ranges using P-to-S converted phases of teleseismic body waves recorded on a temporary broadband seismometer array and isolated by the receiver function method. Ps minus P times at sites west of a compositional boundary that separates the Peninsular Ranges batholith into east and west zones indicate a relatively flat, deep Moho. Ps minus P times at sites east of the compositional boundary decrease eastward, Moho depth estimates (made from the Ps delays and crustal velocities from seismic tomography) indicate a relatively constant 36 to 41 km thick crust in the western zone. In the eastern zone the crust thins rapidly from 35 km thick at the compositional boundary to 25 km at the edge of the Salton trough, a lateral distance of 30 km. The lack of correlation between topography and Moho depths suggests compensation via lateral density variations in the lower crust or upper mantle. We propose that the compositional boundary decouples the eastern and western portions of the batholith, and that the eastern portion has thinned in response to regional Miocene extension, or Salton trough rifting, or both.

Along‐axis variability in crustal accretion at the Mid‐Atlantic Ridge: Results from the OCEAN study

Abstract: The OCEAN experiment is an integrated geophysical study of a region of the Cape Verde abyssal plain that formed at 140 Ma. Deep seismic reflection and ocean bottom hydrophone (OBH) refraction data were acquired along lines parallel and perpendicular to the paleoridge axis trend identified from a detailed magnetic anomaly survey. The igneous basement is overlain by about 1.3 km of sediment which enables improved imaging of intracrustal structure beyond that possible near the Mid-Atlantic Ridge axis. We describe the results of a 150-km long profile oriented parallel to magnetic anomalies M15 and M16, along which deep seismic reflection data collected by the British Institutions Reflection Profiling Syndicate are complemented by refraction data constrained by four OBHs. The line spans an entire spreading segment between two fracture zones; the northern of which has an offset of 40 km and the other (central) has an offset of only 10 km. Away from the fracture zones, the mean igneous crustal thickness is 7.2 km; near both fracture zones, thinning of up to 4 km is observed, giving a mean igneous crustal thickness over the whole segment of approximately 6.5 km. Differences are seen between the two fracture zones in their seismic velocity structure, in the associated basement topography, and in the presence of a strong reflection extending into the mantle beneath the northern fracture zone. The boundary between oceanic layers 2 and 3 correlates with variably coherent normal incidence reflections and a change in the character of the reflectivity. A number of planar reflections up to 10 km in length are present within the middle and lower crust, dipping outward from beneath low-amplitude basement highs at ∼15°; these appear to be present only within layer 3. The Moho has several expressions in the reflection data, including isolated reflection events, a local increase in reflected amplitudes, and a downward decrease in coherent reflections. At the center of the segment there is a zone at the base of the crust within which both high- and low-velocity materials are present. This zone shows an enhanced level of discontinuous normal incidence reflectivity and may represent an initial fractionation event as melt was emplaced at the spreading ridge.

Velocity structure of the Andes of central Peru from locally recorded earthquakes

Abstract: Arrival-times of local events recorded during two short term seismic fied investigations in central Peru are used to determine the P and S velocity structures above the slab, in a segment of the Andes where the subduction is horizontal. The velocities in the upper-crust can be related to the sedimentary cover. The Moho depth is estimated to be about 50 km below the western part of the sub-Andean zone, and deepens gently to the west, reaching 60 km under the Western Cordillera. An uplift of high velocity material is observed under the coastal zone, associated with a cluster of earthquakes in and above the slab. The asymmetrical structure of the crust below the Eastern Cordillera reflects the convergence process in this part of the Andes: the shortening occurs along active reverse steeply west dipping faults involving the crust in its entirety on the eastern border of the cordillera, the compression then decreases toward the east.

A 3D gravimetric crustal model for the northeastern region of the Iberian Peninsula

Abstract: We combine different methodologies normally used separately in exploration geophysics to obtain the topographies of the Moho discontinuity and the boundary between the upper and lower crusts by separating their effects in the Bouguer anomaly map. The SFM method was used to define the average depth of the layers causing the anomalies. The resulting three-dimensional model which reproduces the observed gravity anomaly was estimated by inverting the corresponding domains of the power spectrum. A good correlation exists between the calculated depths and the upper and lower boundaries of the layered lower crust observed along the reflection seismic lines in the area.

Origine des rides 90E et Investigator : analyse de données gravimétriques et bathymétriques

Abstract: The 90E Ridge and the Investigator Ridge are two major North-South linear bathymetric features in the Eastern Indian Ocean. Their origin is still poorly known. Bathymetric and gravimetric data, acquired on a long profile perpendicular to these features, allow study of the mode of compensation of the ridges. The 90E Ridge appears to be locally compensated as shown by the large Moho deflection, and was probably emplaced on a young lithosphere, i.e. near a spreading centre. This result agrees with the ages determined for basalts dredged both on the 90E Ridge and on the adjacent crust. On the contrary the free-air anomaly over the Investigator Ridge is well explained with a model assuming a crust with constant thickness. This implies that the ridge is not a volcanic construction.