Deep Seismic Experiments Research Articles

Crustal velocity structure across the Tornquist and Iapetus Suture Zones — a comparison based on MONA LISA and VARNET data

The Tornquist and Iapetus Suture Zones result from amalgamation of three plates (Laurentia, Baltica and Eastern Avalonia) during the Early Paleozoic Caledonian orogeny (∼440Ma). We present a comparison of the velocity structure of the contrasting Proterozoic and Paleozoic lithosphere across the margins of Eastern Avalonia based on two deep seismic experiments, MONA LISA and VARNET in the SE North Sea and SW Ireland, respectively. Both velocity models show three different crustal types: (a) a high-velocity, three-layered shield type Proterozoic crust (in Baltica and Laurentia) to the north; (b) a transitional crust in the central part across the suture zones; and (c) Eastern Avalonian crust to the south. However, the sub-Moho velocities are ∼7.8km/s under the ∼34-km-thick Baltica crust and ∼8.2km/s under the 26-km-thick Eastern Avalonian crust on the MONA LISA-1 profile, in contrast to ∼7.8km/s under the ∼31-km-thick Eastern Avalonian crust and ∼8.1km/s under the ∼33km thick Laurentian crust on the VARNET profile. These differences in the sub-Moho velocity structure are interpreted to be related to a change in subduction polarity between the Tornquist Sea and the Iapetus Ocean or in the direction of shearing in the mantle during collision tectonics.

Seismic evidence for deep Palaeozoic sedimentary units in the Ringkøbing-Fyn High offshore Denmark

Interpretation of refracted and reflected arrivals in seismic data from the MONA LISA combined wide-angle and normal incidence deep seismic experiment in the south-eastern North Sea has revealed deep sediments in the basement high between the Central Graben and the Horn Graben. Evidence for such deep sediments in the Ringkøbing-Fyn system of basement highs is most clearly found in refracted arrivals in wide-angle seismic data. P-waves refracted in the deep sedimentary units show velocities between ~4.5 km/s and ~5.1 km/s which are significantly below the observed velocities of about 6.0 km/s for crystalline basement. Outline of the shape and fine structure of sedimentary units are constrained by coincident normal incidence seismic reflection data. A seismic velocity model and a reflectivity section for a 150 km section of the east-west trending MONA LISA line 3 are presented. The velocity model is consistent with both the wide-angle data and the normal incidence section. Pre-Zechstein sedimentary units locally up to 4 km in thickness are observed, and locally the top of the crystalline basement is found at depths of about 7 km to 10 km. The boundary between the deepest sediments and the crystalline basement is characterized by several large block faults. On the flanks of the Horn Graben and the Central Graben a pronounced thinning of the pre-Zechstein sedimentary strata is observed indicating uplift and erosion of rift shoulders. A correlation between free-air gravity anomalies, local basement highs and deep basins is observed, and the gravity data indicate that Pre-Zechstein sedimentary structures of similar thicknesses are present in the offshore part of the Ringkøbing-Fyn High both north and south of the analyzed seismic line.

Seismic reflection investigations in the Willochra Basin, South Australia

Cainozoic sediments of the Willochra Basin overlie the Delamerian Fold Thrust Belt, a north-south arcuate tectonogene formed during a major Cambro-Ordovician Orogeny. Seismic experiments were undertaken in an attempt to provide information about the large scale tectonic features of the Fold Belt below the Willochra Basin, to help resolve some of the uncertainty regarding the mechanism of formation and the internal structure of the orogen. Two small scale seismic reflection experiments successfully identified reflecting horizons in the younger Cainozoic basin sediments overlying the Neoproterozoic rocks. The acoustic impedance contrasts of these generally unconsolidated sediments resulted in significant transmission loss and absorption of the seismic signals, restricting the depth of penetration into the Neoproterozoic strata; however, useful reflections were recorded down to two-way travel times of 1/2 second. A small 1 km wide basin structure resting unconformably in the Neoproterozoic rocks was successfully mapped. The Cainozoic and Neoproterozoic reflectors were identified by correlation with geological units in a nearby stratigraphic borehole and local outcrops. The information obtained confirms that it should be possible to conduct a deep seismic profile experiment across the Flinders Ranges, perhaps quelling the decades of debate about the internal structure of the Delamerian Fold Belt.

Wide-angle vibroseis test across the Rhine graben

Summary. As a joint operation of ECORS and DEKORP, a deep wide-angle seismic experiment using vibrators was carried out in the autumn of 1984. The object was to get information on the deep crust under the Rhine graben without crossing through sedimentary layers. Offsets were in the range of 50 to 89 km. In the first phase, two vibration points were executed in the Vosges mountains. A signal was received in the Black Forest from solely the farthest VP. In the second phase, fourteen VPs were executed in the Black Forest. No stacking or correlation was performed in the field in France. The quality of the results is good only if an equalization is applied before vertical stacking and correlation in the computing centre.

Jemez Mountains volcanic field, New Mexico: Time term interpretation of the CARDEX seismic experiment and comparison with Bouguer gravity

The Jemez Mountains volcanic field (JVF) is centered astride the western flank of the Rio Grande rift in north central New Mexico. Near the center of the volcanic field are the Valles and Toledo calderas, the sites of two cataclysmic eruptions of silicic tephra and subsequent caldera collapses. These calderas are among the largest and most recent (1.1 and 1.4 Ma) of the world's giant volcanic calderas. The Valles caldera, especially, has associated highly developed hydrothermal systems and may still contain residual magma within a cooling, compositionally zoned pluton at shallow crustal depths. The main goal of our study was to estimate the complex shallow crustal structure beneath the Jemez volcanic field using data from a seismic refraction experiment that encompassed the central part of the field. A time term technique was used to process first arrival time data from the network of temporary seismic stations of the Caldera and Rift Deep Seismic Experiment (CARDEX) refraction experiment. The data set, consisting of 290 time‐distance pairs, was processed to yield time terms at 84 locations (78 recording stations and 6 explosion sites). The P wave velocity of the Precambrian basement refractor was estimated to be 5.86 ± 0.01 km/s. Station time terms are highly correlated to Bouguer gravity anomalies and thus tend to corroborate models for which the crystalline basement outcrops along the western margins of the rift and western flank of the JVF and dips generally eastward to depths of at least 600 m below sea level beneath sediment‐filled axial basins of the rift.

Deep seismic reflection profile across the northern Appalachians

A marine seismic reflection line across the northeast extremity of the Canadian Appalachians in Newfoundland indicates a collisional suture(?) in the lower crust beneath the central ophiolitic Dunnage terrane. The thrust and fold belt (miogeocline) above the Grenville basement, and the Dunnage and Gander tectonostratigraphic terranes all appear to be allochthonous with respect to lower crustal basement. The Gander-Avalon terrane boundary to the east is a near-vertical feature that penetrates the crust. The data also suggest that the ancient passive margin of North America extends eastward under the Dunnage terrane for about 70 km. The Newfoundland deep seismic experiment indicates major tectonic differences compared to results for the southern Appalachians.

SHOOTING FOR DEEP REFRACTION EXPERIMENTS

Ammonium Nitrate—Fuel Oil explosive loaded into an abandoned mine shaft can be a convenient energy source for a deep refraction seismic experiment. Two such sites have been shot successfully in Australia.