C. Prodehl

A crustal structure study of Jordan derived from seismic refraction data

Abstract The interpretation of a deep seismic refraction study in Jordan, performed in May 1984, shows that much of the country is underlain by continental crust, 32–35 km thick, and normal mantle with a velocity of 8.0–8.2 km/s. In the Aqaba region, southwest and central Jordan, east of Wadi Araba, the crustal thickness is of the order of 32–35 km, while in the Amman region it is not less than 35 km. In southeast Jordan the crust thickens to at least 37 km in what is probably the transition to the Arabian Shield type of crust. The boundaries between the upper and lower crust at about 20 km depth and the lower crust and uppermost mantle are probably transition zones. The upper crystalline crust has velocities of 5.8–6.5 km/s while the lower crust has velocities greater than or around 6.65 km/s. While the crystalline basement is exposed in southwest Jordan and is at a depth of 2–2.5 km north of Amman, it is at a depth of not less than 5 km in central Jordan. A comparison of the crustal type and structure of Jordan and the adjacent Dead Sea rift with that of the Black Forest and the Rhine valley yields a striking resemblance. The situation of the Jordan-Dead Sea rift is explained in terms of the continental crust of Arabia rifting in preference to the thin (?oceanic) crust of the Mediterranean Sea.

Crustal structure of the Fennoscandian Shield: A traveltime interpretation of the long-range FENNOLORA seismic refraction profile

Abstract The 1979 Fennoscandian Long-Range Project (FENNOLORA) was aimed at the determination of the detailed structure in the earths mantle down to a depth of about 400 km. Observation distances reached almost 2000 km within Scandinavia between shotpoints off the North Cape and the southern coast of Sweden. To achieve an unbiased regarding the upper mantle structure, a careful crustal survey was carried out along the entire profile at the same time. Beneath the Fennoscandian Shield, i.e. the central section of the profile, the crust is characterized by quite a smooth increase in P-wave velocity down to the Moho which lies at a depth of about 50 km in the southern half of the shield and at about 45 km further north. The mean crustal velocity is 6.6–6.7 km/s. At the base of the crust the velocity increases gradually from about 7 km/s to 8.0–8.4 km/s in a 5–10 km thick crust-mantle transition zone. Both in the south and in the north, the relatively homogeneous crust of the Baltic Shield borders on areas with a more differentiated velocity structure. First-order discontinuities at shallower depth characterize the crust-mantle boundary near the southeastern tip of Sweden (38 km) and under the Caledonides in the north (43 km).

Seismic Refraction Study of Crustal Structure in the Western United States

A network of 64 seismic-refraction profiles recorded by the U.S. Geological Survey in California and Nevada and adjacent areas of Idaho, Wyoming, Utah, and Arizona from 1961 to 1963 was re-interpreted. From record sections compiled for all profiles, a basic travel-time diagram can be derived. In addition to the first arrivals on profiles in the Snake River Plain, the northern Basin and Range province, and the middle Rocky Mountains, two dominant phases can be correlated in secondary arrivals, whereas the profiles in other areas show only one dominant phase in later arrivals. Based on velocity-depth functions calculated for each profile after the method of Giese, the crustal structure of the western United States is presented on contour maps and on a fence diagram that is composed of 15 crustal cross sections. Crustal thickness reaches maxima under the Sierra Nevada (42 km), the Transverse Ranges of southern California (37 km), and in southwestern Nevada (36 km), whereas the crust is relatively thin under the Coast Ranges of California (24–26 km), under the Mojave Desert (28 km), and under parts of the central Basin and Range province in Nevada and Utah (29–30 km). The base of the crust dips generally from the Basin and Range province toward greater depths in the Colorado Plateau (43 km), the middle Rocky Mountains (45 km), and the Snake River Plain (44 km). The upper-mantle velocity is less than 8.0 kmps under the Great Basin of the Basin and Range province, the Sierra Nevada, and the Colorado Plateau, but it is equal to or greater than 8.0 kmps under the Coast Ranges of California, the Mojave Desert, and the middle Rocky Mountains. Velocity inversions within the upper crust are indicated under the southern Cascade Mountains and the middle Rocky Mountains, but not under the Sierra Nevada. The average velocity of the upper crust beneath the Basin and Range province is 6.1 to 6.2 kmps to a depth of 15 to 20 km. Only beneath the middle Rocky Mountains, the Snake River Plain, and the northern part of the Basin and Range province can a boundary zone between upper and lower crust be determined confidently.

Crustal evolution of the Rhinegraben area. 1. Exploring the lower crust in the Rhinegraben rift by unified geophysical experiments

Abstract Unified geophysical investigations of the lithosphere in the Rhinegraben rift system have revealed new details of the lower crust and its role in the rifting process. The new findings allow an assessment of the compatibility of four different geophysical notions and properties of the lower crust: 1. (1) the lower crust as the layer beneath the Conrad discontinuity with a P-wave velocity of about 6.5 km/s or greater (refraction seismics); 2. (2) the laminated band of reflections, as seen in the near-vertical reflection seismic experiments in many parts of the continents; 3. (3) the ductile part of the crust below the brittle-ductile transition, devoid of earthquakes in seismically active regions; 4. (4) the electrical conductivity of the lower crust indicative of dry or wet conditions, or still unknown conduction phenomena. In the Rhinegraben area the lamination of the lower crust serves as an outstanding marker of deep tectonic activity during the rifting process, in which the crust of the Rhinegraben rift system has been subjected to three different natural dynamic processes: (1) Uplift by 2 to 3 km with subsequent erosion of the Rhinegraben shoulders (Black Forest and Vosges Mountains) caused decompression possibly leading to the formation of a low-velocity/high-electrical-conductivity zone right on top of the laminated lower crust beneath the elevated shoulders of the Black Forest. (2) The brittle crystalline wedge of the graben proper subsided nearly undeformed into the lower crust, which became about 5 to 7 km thinner below the graben than below the shoulders. (3) The deepest hypocentres in the Black Forest (Dinkelberg area), if projected onto the neighbouring reflection profile, would be located 7 to 8 km within the laminated lower crust beneath the southern Black Forest, indicating a discrepancy between the top of the lower crust as defined by the brittle-ductile transition as seen by the deepest earthquakes and by the top of the laminated reflection band. The Rhinegraben rift system reveals the properties and behaviour of the lower crust under a wide variety of tectonic situations.

LISPB–III. Upper crustal structure of northern Britain

Interpretation of upper crustal data obtained during the LISPB seismic experiment reveals the velocity structure of the pre-Caledonian basement in northern Britain. Lewisian-like basement with a relatively high seismic velocity (> 6.4 km/s) extends from the Caledonian foreland into the Midland Valley and probably terminates at the Southern Uplands fault. To the south, beneath northern England, the basement has a lower velocity (< 6.3 km/s). We suggest that a horizontal discontinuity may exist in the pre-Caledonian basement between the Southern Uplands fault and the Stublick Line though we cannot yet determine the exact nature of this discontinuity.

VRANCEA99—the crustal structure beneath the southeastern Carpathians and the Moesian Platform from a seismic refraction profile in Romania

The VRANCEA99 seismic refraction experiment is part of an international and multidisciplinary project to study the intermediate depth earthquakes of the Eastern Carpathians in Romania. As part of the seismic experiment, a 300-km-long refraction profile was recorded between the cities of Bacau and Bucharest, traversing the Vrancea epicentral region in NNE–SSW direction. The results deduced using forward and inverse ray trace modelling indicate a multi-layered crust. The sedimentary succession comprises two to four seismic layers of variable thickness and with velocities ranging from 2.0 to 5.8 km/s. The seismic basement coincides with a velocity step up to 5.9 km/s. Velocities in the upper crystalline crust are 5.9–6.2 km/s. An intra-crustal discontinuity at 18–31 km divides the crust into an upper and a lower layer. Velocities within the lower crust are 6.7–7.0 km/s. Strong wide-angle PmP reflections indicate the existence of a first-order Moho at a depth of 30 km near the southern end of the line and 41 km near the centre. Constraints on upper mantle seismic velocities (7.9 km/s) are provided by Pn arrival times from two shot points only. Within the upper mantle a low velocity zone is interpreted. Travel times of a PLP reflection define the bottom of this low velocity layer at a depth of 55 km. The velocity beneath this interface must be at least 8.5 km/s. Geologic interpretation of the seismic data suggests that the Neogene tectonic convergence of the Eastern Carpathians resulted in thin-skinned shortening of the sedimentary cover and in thick-skinned shortening in the crystalline crust. On the autochthonous cover of the Moesian platform several blocks can be recognised which are characterised by different lithological compositions. This could indicate a pre-structuring of the platform at Mesozoic and/or Palaeozoic times with a probable active involvement of the Intramoesian and the Capidava–Ovidiu faults. Especially the Intramoesian fault is clearly recognisable on the refraction line. No clear indications of the important Trotus fault in the north of the profile could be found. In the central part of the seismic line a thinned lower crust and the low velocity zone in the uppermost mantle point to the possibility of crustal delamination and partial melting in the upper mantle.

SEISMIC-REFRACTION STUDIES OF THE AFRO-ARABIAN RIFT SYSTEM : A BRIEF REVIEW

Abstract The crustal and uppermost-mantle structure of major units of the Afro-Arabian rift system has been consecutively investigated by seismic-refraction surveys in the Jordan-Dead Sea rift, the Red Sea, the Afar depression and the East African rift of Kenya. With the exception of the Jordan-Dead Sea transform, the entire Afro-Arabian rift system is underlain by anomalous mantle with Pn-velocities less than 8 km/s, while under the rift flanks the velocity is clearly equal to or above 8.0 km/s. Various styles of rifting have been found. Oceanic crust floors the axial trough of the southern Red Sea rift, thinned continental crust underlies the margins of the Red Sea as the Afar depression and the northern Kenya rift. On the other hand, 30–35-km-thick continental crust is found both under the Jordan-Dead Sea rift, where strike-slip rifting is active and thinning towards the Mediterranean occurs, and under the central Kenya rift, where updoming is apparently the controlling feature. While the transition from thinned continental to 5–6-km-thick oceanic crust in the centre of the Red Sea appears to be more gradual, the transition from rift-related structure to undisturbed continental crust of 40 ± 5 km thickness is mostly rather abrupt. The seismic data indicate various stages of rifting evidenced by different styles of crustal strucre and they imply the presence of heated uppermost-mantle under most parts of the rift system, possibly related to plume activity. Local volcanism may disrupt and/or underplate the crust in places, altering in particular the structure of the lower crust. Progressive thickening of the rifted crust away from the oceanized centres in the southern Red Sea and Gulf of Aden towards north and south may be viewed as an evolutionary sequence which, however, may be difficult to explain when viewing the Afro-Arabian rift system as an active rift controlled by plume activity.

Seismic refraction investigation of the Black Forest

Abstract Within the framework of site surveys of the Deep Continental Drilling Program (KTB), deep seismic sounding experiments were carried out in the Black Forest, southwest Germany, in August 1984. These site surveys comprised seismic refraction as well as seismic reflection measurements. This paper will concentrate on the interpretation of the refraction data which were observed along a 240 km long line, approximately following the morphological axis of the Black Forest in a N-S direction and extending in the south into the Swiss Molasse Basin. From the available seismic refraction data, a detailed velocity-depth model of the crust and uppermost mantle was derived. A clear phase refracted in the basement with a velocity of 5.9 km/s is observed for all shots. This phase is followed by a reflection from the upper boundary of a low-velocity zone. This zone is very accentuated under the northern and central Black Forest and is characterized by a velocity decrease from 6.2 to 5.4 km/s between depths of about 8 and 14 km. Towards the south the velocity inversion gradually loses its intensity. Under the center of the southern Black Forest velocities of about 6 km/s prevail throughout the upper crust. The lower crust, which starts at the lower boundary of the low-velocity zone shows a very complicated response in the record sections. Due to long reverberations between travel-times of 1.5–3.5 s (reduced by 6.0 km/s) at an epicentral distance of approximately 30–70 km, a classical phase correlation is not possible. Based on a combined interpretation of near-angle and wide-angle reflection data, the lower crust is characterized by a stack of thin layers with alternating high and low velocities. The average velocity of the laminated zone is 6.7 km/s. The reflection from the Moho at a depth of 25–26 km is the most prominent phase at epicentral distances greater than 60 km.

Interpretation of a seismic-refraction survey across the Arabian Shield in western Saudi Arabia

Abstract In February 1978 seismic-refraction profiles were recorded by the U.S. Geological Survey along a 1000 km line across the Arabian Shield in western Saudi Arabia. This report presents a traveltime and relative amplitude study in the form of velocity-depth functions for each individual profile assuming horizontally flat layering. The corresponding cross section of the lithosphere showing lines of equal velocity reaches to a depth of 60–80 km. The crust thickens abruptly from 15 km beneath the Red Sea Rift to about 40 km beneath the Arabian Shield. The upper crust of the western Arabian Shield yields relatively high-velocity material at about 10 km depth underlain by velocity inversions, while the upper crust of the eastern Shield is relatively uniform. The lower crust with a velocity of about 7 km/s is underlain by a transitional crust-mantle boundary. For the lower lithosphere beneath 40 km depth the data indicate the existence of a laterally discontinuous lamellar structure where high-velocity zones are intermixed with zones of lower velocities. Beneath the crust-mantle boundary of the Red Sea rift most probably strong velocity inversions exist. Here, the data do not allow a detailed modelling, velocities as low as 6.0 km/s seem to be encountered between 25 and 44 km depth.

Fault systems of the 1971 San Fernando and 1994 Northridge earthquakes, southern California: Relocated aftershocks and seismic images from LARSE II

We have constructed a composite image of the fault systems of the M 6.7 San Fernando (1971) and Northridge (1994), California, earthquakes, using industry reflection and oil test well data in the upper few kilometers of the crust, relocated aftershocks in the seismogenic crust, and LARSE II (Los Angeles Region Seismic Experiment, Phase II) reflection data in the middle and lower crust. In this image, the San Fernando fault system appears to consist of a decollement that extends 50 km northward at a dip of ∼25° from near the surface at the Northridge Hills fault, in the northern San Fernando Valley, to the San Andreas fault in the middle to lower crust. It follows a prominent aseismic reflective zone below and northward of the main-shock hypocenter. Interpreted upward splays off this decollement include the Mission Hills and San Gabriel faults and the two main rupture planes of the San Fernando earthquake, which appear to divide the hanging wall into shingle- or wedge-like blocks. In contrast, the fault system for the Northridge earthquake appears simple, at least east of the LARSE II transect, consisting of a fault that extends 20 km southward at a dip of ∼33° from ∼7 km depth beneath the Santa Susana Mountains, where it abuts the interpreted San Fernando decollement, to ∼20 km depth beneath the Santa Monica Mountains. It follows a weak aseismic reflective zone below and southward of the main-shock hypocenter. The middle crustal reflective zone along the interpreted San Fernando decollement appears similar to a reflective zone imaged beneath the San Gabriel Mountains along the LARSE I transect, to the east, in that it appears to connect major reverse or thrust faults in the Los Angeles region to the San Andreas fault. However, it differs in having a moderate versus a gentle dip and in containing no mid-crustal bright reflections.