The Geology and Gravity Anomalies of the Troodos Massif, Cyprus

I. G. Gass, D. Masson-Smith

Abstract

Over Cyprus there is one of the largest recorded gravity anomalies which reaches a maximum of over +250 mgal. This paper records the main geological features of the island, investigates the source of the gravity anomaly and correlates both lines of evidence in support of an hypothesis on the evolution and structure of the area. The topography of Cyprus, which lies in the north-eastern Mediterranean, is dominated by two east-west mountain ranges separated by the low-lying central plain of Mesaoria. The northern, Kyrenia range is part of the southernmost arc of the Tauro-Dinaric Alps, whilst the southern Troodos range is an igneous massif composed of basic and ultrabasic rocks of plutonic and extrusive character. The Troodos rocks fall logically into three main units: (a) the Sheeted Intrusive Complex; (b) the Troodos Plutonic Complex; and (c) the Troodos Pillow Lava Series. The Sheeted Intrusive Complex forms the major part of the Troodos massif and is a north-south basic dyke swarm cutting basic lavas. The dykes range in thickness from 1 to 15 ft. and form over 90% of the complex. Abundant evidence is available to substantiate the intrusive nature of this dyke complex. Its unique concentration and regularity is attributed to repeated intrusion coupled with intense erosion. The north-south orientation of the intrusives is thought to be due to the east-west tensional stress that was dominant throughout the evolution of the massif. The central part of the massif is occupied by the Troodos Plutonic Complex, a layered ultrabasic complex of batholithic dimensions in which the rock types range from central dunites and peridotites outwards through melagabbros and olivine-gabbros to gabbros and granophyres. Field, mineralogical and geophysical data indicate that the parent material was of peridotitic composition. Although gabbros are, by far, the most abundant rocks exposed, it is considered that these represent but a minor percentage of a vast mass of underlying, high-density, ultrabasic material. Differentiation of the ultrabasic parent material is thought to have resulted in the gradual upward and outward change from central dunites and peridotite through melagabbros and olivine-gabbros to overlying gabbros and granophyres. Forming an incomplete ring around the Sheeted Intrusive Complex is the Troodos Pillow Lava Series, a very thick sequence of pillow lavas and their related intrusives. Although divided into two units on the presence of a partial unconformity, and petrographic differences, the general basaltic nature of the series persists throughout. The series shows an increase in basicity with decreasing age, the main rock type in the lower unit being basalt, whilst olivine-basalts predominate in the upper division. There is evidence that this series has resulted from the partial fusion of a rock of peridotitic composition and that the relationship between age and basicity is due to the progressively more complete fusion of the parent material. Serpentines of post-Lower Triassic age and considered to be the initial phase in the igneous activity of the Alpine orogeny are also present in Cyprus, where they appear to have been emplaced as a serpentine 'magma'. Cyprus is covered by a strong positive gravity anomaly mainly between 100 and 250 mgal. The axis of the anomaly lies over the Troodos massif, runs parallel to the Kyrenia range and extends from Pomos in the west, eastwards to Famagusta; superimposed upon the main anomaly are smaller local anomalies. The gravity field falls off all round Cyprus to less than 100 mgal; no other gravity anomalies of this size have, so far, been found in the eastern Mediterranean. The high-density rocks, which appear to have produced this large anomaly, have the form of a rectangular, near-surface, subhorizontal slice, which measures 120 miles east-west by 70 miles north-south and whose centre is displaced about 20 miles to the north-west of the centre of Cyprus. This high-density mass must be at least 7 miles thick under Mount Olympus, whilst at Pomos a thickness of over 20 miles is estimated; its maximum elevation is at Mount Olympus where the dunites and peridotites of the Troodos Plutonic Complex crop out. A correlation between the ultrabasic rocks of the Troodos Plutonic Complex and the high-density material causing the main anomaly is well substantiated. The geological and geophysical evidence suggests that the Troodos massif evolved in pre-Triassic times as an oceanic volcanic pile situated between the then more widely spaced continental masses of Africa and Eurasia. During the Alpine orogeny these continental masses converged, the southern mass underthrusting the Troodos volcanic pile, and parts of the Eurasian hinterland. The underthrusting took place at such a level that not only the volcanic pile but also part of the upper mantle was uplifted above sea level as an undeformed slice. Intense erosion has denuded the volcanic pile almost to its roots. It is thought that the stratiform Troodos Plutonic Complex might represent upper mantle material, partly fused and differentiated to provide the basic volcanic rocks of the Troodos massif.

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