Deep seismic crustal studies:Case study from Calabria(Italy)
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- Tez No: 28737
- Danışmanlar: DOÇ. DR. MUSTAFA ERGÜN
- Tez Türü: Doktora
- Konular: Jeofizik Mühendisliği, Geophysics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1993
- Dil: İngilizce
- Üniversite: Dokuz Eylül Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Jeofizik Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 215
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Özet (Çeviri)
SUMMARY The data of complete refraction-reflection seismic profiles were acquired with the goal to investigate the crustal structure of the Serre mountains, Calabria (Southern Italy). The Calabrian arc located where continent-continent collision between the African and European plates already occured at the end of the Eocene. The tectonics are ruled by the convergence system under the forces of subduction and collision. The importance of this area is that it represents a nearly complete crustal section. The outcropping lithological sequence is a tilted block that comprises interlayered acidic, mafic and pelitic rocks metamorphosed to granulite and amphibolite fades. It has been uplifted into middle crust after Hercynian metamorphism. Outcropping and tilting occured afterwards during the Appennic orogeny. In the Serre area, the lower crustal rocks outcropped for an area of about 400 km2 and expose a 7-8 km thick section. The Continental Mohorovicic discontinuity (MOHO) has been recognized worldwide from refracted (Pn) and wide- angle reflected (PmP) phases from earthquake and explosive sources. Studies utilizing Pn and PmP arrivals have generally interpreted the Moho to be a simple discontinuity in velocity between lower crust and upper mantle rocks which is laterally continuous and which is exists everywhere at the base of the continental crust. Deep seismic reflection profiling data have shown that the near-vertical reflection signuture of the Moho is more complex. Those data have been interpreted to indicate that the Moho consists of a transition zone of several kms in thickness and that is laterally variable. The characteristics of the seismicty in the southern Italy permit that the Benioff zone has a concave shape and and arc-like structure with almost vertical dipping as far as to the depth of 250 km then with a 50 ° dip from 250 to 340 km depth and quasi horizontal dipping for greater depths. Bouguer gravity anomalies suggest a thin crust which may underlain by a differentiated mantle. The area is characterised by magnetic anomalies having being connected to the basement intrasedimentary sequences which are controlled structurally. The seismic characteristics of the lower crust have been investigated with a Deep Seismic Sounding (DSS) experiment. A main profile (N-S) of about 30 km long hasbeen acquired. In this profile 31 dynamite shots (10 to 95 kg of charge) have been recorded on three different layouts by five recording units (receivers; one vertical and two horizontal components). Also four shorter (about 1 km) profiles, orthogonal to the main line, have been recorded. TWT records up to 30 s were acquired. Seismic reflection data have been processed with a nearly standard processing sequence. The main reflecting structures of the complete crust can be seen on the NMO-corrected section. The line drawings of interpreted zero-offset unmigrated time sections were performed on the vertical component. For the upper 2 km of the section, the first breaks of refraction data were used as inputs for tomographic inversion calculations. The velocity data for these upper layers were determined through tomographic inversion processes. These and as well as velocity data for the deeper layers were used combinedly for the raytracing modelling purposes. There are two ways of creating depth model; one of them is time model that is created in 2D with the model builder MIMIC, following the line drawing; then it could be converted to depth partially automatically, partly interactively. The depth model was used as input for the raytracing, in order to obtain the synthetic sections. Afterwards the section was extended in a cylindrical way to obtain 3D models. On this quasi-3D model, a Raytracing was again performed, putting the original locations of the acquisition, in terms of coordinates x,y of geophones and shots (it could also be possible to include the heights). The second routine was also performed for a better 3D model with support of RAYMAP which is a more realistic method of converting travel times into depths. Then the 3D model was subjected to the Raytracing, and it was modified and studied in detail in order to obtain the best fit. Finally 3D model which contains the real positions of shots and receivers, was created. The Raytracing programs QUIK(SIERRA) allow the simulation of a zero-offset sections as well as shot gathers. All these have been obtained and displayed for both 2D and 3D cases. The Shot gathers raytracing was performed on these models. Field sections and models have shown very good correlations for seven or eight layers such as Hercynian lower crustal reflections, LVZ reflections, strongly reflective Calabrian crust, diffrentiated Moho etc. Different layers were identified from the zero-offset section. This LVZ zone is the contact zone in between the Hercynian lower crust and strongly reflective the Calabrian crust underlying the Alpine metamorphic units. LVZ layer is about 5-10 km. The crust-differentiated moho boundary, which dips to the South, can be seen in between 19-24 km depth.The results of raytracing, kinematic raytracing and the previous marine seismic records were used for the gravity modelling purposes. Previous marine seismic records were also re-interpreted before two gravity profiles were modelled. These previous marine seismic profiles known as with high resolution, but only the layers of water depth, sediments and basement were calculated and the continiuties of these horizons were followed along the profiles. Anomalies which are originated from subsurface layers or bodies, can be simulated as the geometric models for the gravity modelling purposes. In Tyrrhenian Sea, high Bouguer gravity anomalies show the thin crust around 18-19 km depth. The morphology of the crust/mantle boundary in the southern Italian area including the adjacent sea regions appears to be influenced less by the mountain chain of the Appennines than by the transition from the region of the Tyrrhenian Sea to that the Adriatic Plate. The Ionian Basin, which is located in tectonicaily most complex area of the Mediterranean Basin, has been played a crucial role in the interaction between the African and European margins. High Bouguer gravity anomalies are in fact consistent with the thin crustal thickness in the southern Ionian abyssal plain. There is a definite evidence of reduced crustal thickness in the middle of the Ionian basin. Also, there is a result in Moho depth of less than 25 km, both in the southernmost part of the Tyrrhenian Basin and beneath Calabria. Sediments were identified by the densities as 2.20, 2.35, 2.40 and 2.62 g/cm3. Upper crust can be implified by the mean density of 2.74 g/cm3. Lower crust's density can be around 2.82 g/cm3. A differentiated moho or a transitien zone can be explained by a density of 3.00 g/cm3.
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