Μελέτη της τεκτονικής δομής της λεκάνης Μεσσαράς, Κεντρικής Κρήτης με τη βοήθεια τεχνικών τηλεπισκόπησης και συστημάτων γεωγραφικών πληροφοριών

Abstract

The wider region of central Crete, is a particular neotectonic structure that is constituted by two separate post-alpine basins, that of Messara's to the south and Heraklion to the north, each one with different orientation of development (E-W and N-S respectively). After a quick study of this area by observing older geological maps, satellite images, and also the place of Crete along the Hellenic Arc, is that the Messara basin has a quite geometrical provision that might be related with surfacial extensional strain field, which trends perpendicular to the subduction zone. It could be a symmetrical structure trending E-W, with active marginal faults parallel to the basin, separating the uplifted basement rocks from the post-alpine formations, which cover unconformably the Alpine basement in the intermediary area of the basin. However, a second glance creates divergences from the ideal model of the symmetrical basin, as continuous marginal structures do not appear to exist in neither the northern nor the southern fringe. On the contrary, the northern margin of the basin seems to be a large fault zone which is interrupted abruptly and instead of a similar tectonic structure, the basin of Heraklion is developed trending N-S, perpendicular to that of Messara's. The margins of the latter are not evident, even if the appearance of Alpine rocks is quite frequent in the intermediary area. Additionally, in the south-western utmost of the Messara basin, post-alpine sediments are found outside the ideal area of this neotectonic basin and this creates scepticism as for the exact location of the southern margin. In the same region, it appears that the dominant structures trend NE-SW and have strong influence on the post-alpine tectonic regime which is different than the one that created the bigger structures of E-W. The same directions seem to coexist in all the extent of the basin which complicates the thoughts regarding to the estimation of time of the activity of the various fault zones. Finally, based on the topographical data, one could observe that there should be a single hydrologic basin in the region of Messara, trending E-W, beginning from the western slopes of Lasithi Mts and leading to the eastern coast of the Messara bay. The existence of this particular morphology is the one that differentiates the southern region of central Crete from the remainder southern coasts of island, where the rivers flow generally to the south. The contribution of Asteroussia Mts in this is very important as their linear growth of E-W, isolates the basin from the open sea of Liviko. After these points, important issues are opening and are called to be explained via the systematic study of the existing data, and also newly collected field data which are acquired and processed with the aid of relatively new technological methods. These methods concern in the statistical analysis of morphotectonic parameters, the tectonic analysis of field data with special techniques and the creation of databases in a Geographic Information System (GIS) environment. This geodatabase includes all kinds of data, aiming to their essential control and classification in the various time periods. The latter is very important as major changes of the strain field, along the Messara basin, seem to have happened during the post-alpine period. A brief summary of the publications found in the international literature is made in the 2nd Chapter. Most of these publications are referring to the description of the Alpine units and secondly to the post-alpine formations of Neogene age. Also, the prevailing case scenarios of tectonic development of the wider region of central Crete are described based on papers published on this subject. The basic methodology of designing and using a GIS is analyzed in the 3rd Chapter. This also concerns in the proposed structure and contents of a geodatabase that will be used as an interactive digital geological map. The way of using new technological tools is described here as well as how these can contribute in the more detailed, objective, and rapid importing of data collected by the fieldwork. In this chapter is also presented the way of using a GIS for the palaeo-geographic representation of central Crete and especially the Heraklion and Messara basins, based on the palaeo-environmental conditions during the time of deposition of the Neogene sediments. The background images of a GIS are usually satellite images of various resolutions so in the 4th Chapter the main methodologies that were followed for the analysis and interpretation of remote sensing data are presented. The techniques that are used and described are distinguished in basic and advanced, depending on the degree of difficulty. Additionally the final products that were used for geological and tectonic observations are also presented. In the 5th Chapter, the detailed and systematic methodology of studying the morphotectonic parameters is described as well as the precise calculation of several morphometric indices, with the use of the GIS as a tool for tectonic geomorphology. The structure and the design of the used databases are described, while the methodology that was followed is analyzed. Finally a discussion is made about the results, concerning the hydrologic basins that are included in the neotectonic basins of Messara and Heraklion. The analysis of the fieldwork data is described in the next chapters. In every chapter the data for each one of the key areas, which were mapped in high detail, are analyzed by using also high resolution remote sensing images and creating spatial databases in GIS environment. The Matala – Kaloi Limenes area is located at the south-western utmost of the basin and although its location is outside the "theoretical" margins of the postalpine graben, large areas are covered by Neogene sediments. Several fault zones have been studied and analyzed based on field measurements. The dynamic analysis led to the calculation of the local strain field which was required for every fault zone to operate and finally to the gradual changes of the regional strain field in order to be classified by the age of activation. The results were also combined with seismological data as evidence of the present strain field. The amount of deformation was quantified by using two different methodologies and the results showed more or less the same quantity of extension along the given trends. Finally a model of evolution, for the area, was extracted. The Ano Viannos area is located at the easternmost area of the Messara basin where large marginal fault zones are crossing each other, separating the Alpine basement rocks from the Neogene sediments. The tectonic analysis which was made by using the collected field data revealed the strain field that was dominant in several periods and the sequence of activation for each fault zone. The third area is located near Agia Varvara village on the watershed divide between Messara and Heraklion basins. The area is crossed by a very large and active fault zone that is representing the northern margin of Messara graben although it disappears towards east possibly because it is covered by post-alpine sediments. The main reason seems to be the activity of a number of faults with perpendicular strike (N-S) and trans-tensional sense of movement. Various tectonic analysis methods were applied on synsedimentary structures and the strain field during several time periods was calculated. The combination of the results at the above mentioned areas led to a model regarding to the evolution of Messara basin since Late Miocene. The present dominant strain field seems to be extensional after the phases of trans-pression and trans-tension respectively. During the later phase two left lateral strike slip fault zones were active by segmenting the northern margin of Messara basin into three main segments. These segments compose a low angle normal fault dipping to the south, creating a half-graben, whilst on the top of its hanging-wall the Neogene sediments are deposited. The strike slip movement is migrating from the east to the west starting from Tortonian and ending during Messinian, as a result of the transpression that started at the eastern part of the Hellenic trench during the Late Miocene.

Η λεκάνη της Μεσσαράς αποτελεί μια ιδιαίτερη νεοτεκτονική δομή με πολύ γεωμετρικήδιάταξη, με διεύθυνση εγκάρσια ως προς το Ελληνικό τόξο. Θα μπορούσε να πρόκειταιγια μία συμμετρική λεκάνη (Α-Δ), με ενεργά περιθωριακά ρήγματα ίδιας διεύθυνσης,που να διαχωρίζουν αλπικούς από μεταλπικούς σχηματισμούς, οι οποίοι να καλύπτουνασύμφωνα το αλπικό υπόβαθρο στον ενδιάμεσο χώρο της λεκάνης. Μια δεύτερη ματιάδημιουργεί αποκλίσεις από το ιδανικό μοντέλο της συμμετρικής λεκάνης, καθώς δεφαίνεται να υπάρχουν συνεχή περιθωριακά ρήγματα είτε στο βόρειο είτε στο νότιοπεριθώριο. Η μεθοδολογία στηρίζεται στη συστηματική μελέτη υπαρχόντων στοιχείων,αλλά και νέων στοιχείων υπαίθρου που συλλέχθηκαν και επεξεργάστηκαν με τηβοήθεια νέων τεχνολογικών μεθόδων. Αφορά στη στατιστική επεξεργασίαμορφοτεκτονικών παραμέτρων, την τεκτονική ανάλυση δεδομένων υπαίθρου, με ειδικέςτεχνικές και τη δημιουργία βάσεων δεδομένων σε Συστήματα ΓεωγραφικώνΠληροφοριών (ΣΓΠ). Η συσχέτιση τους με δομές διαφορετικής κλίμακας, γίνεταιαντιληπτή μέσα από ψηφιακή επεξεργασία δεδομένων τηλεπισκόπησης και προϊόντωνπου προκύπτουν από την εφαρμογή των νέων τεχνικών. Η εφαρμογή της παραπάνωμεθοδολογίας οδήγησε στην διατύπωση ενός μοντέλου εξέλιξης της λεκάνης Μεσσαράςαπό το Αν. Μειόκαινο μέχρι σήμερα. Το σημερινό εντατικό πεδίο είναι καθαράεφελκυστικό (Β-Ν) αφού έχουν προηγηθεί οι φάσεις της διατμητικής συμπίεσης και τουδιατμητικού εφελκυσμού. Η τελευταία φάση σχετίζεται με ζώνες αριστερόστροφηςοριζόντιας ολίσθησης, οι οποίες έχουν τμηματοποιήσει τη βόρεια περιθωριακή ρηξιγενήζώνη της Μεσσαράς, σε τρία τμήματα. Αυτά συνθέτουν την επιφανειακή έκφραση ενόςκανονικού ρήγματος μικρής κλίσης προς τα νότια, δημιουργώντας μια ασύμμετρητάφρο. Στο κατερχόμενο ρηξιτέμαχος αυτής, αποτέθηκαν τα Νεογενή ιζήματα και στησυνέχεια περιστράφηκαν λόγω της συνεχιζόμενης δράσης του βόρειου περιθωρίου τηςλεκάνης.