Tectonic Phenomenon in the Maltese Islands

The development of the Alps and the submarine elevation on which the Maltese Islands stand are the result of this process. The same mechanism was also responsable for the different rock strata found on the Islands. The Mediterranean area is one of the oldest synclinal depressions of the earth's crust. During the Oligocene and Miocene periods of the Cenozoic, the area was markedly effected by the Alpine cycle of earth movements. The third episode of the Alpine movements caused a fluctuation in sea-depth of the Islands' area hence resulting in different forms of rock strata. This same episode also resulted in the formation, filling and folding of the molasse foredeep. During the late Miocene (Samartian series), the area was again affected by Alpine movements. The fourth episode caused a progressive uplift of the body and forelands of the Alps, expelling the sea completely from the area. This uplift was marked by periods of stability, and even at times of sinking back. This episode of the Alpine cycle also saw the development of the elevation connecting the Maltese archipelago to Europe and Africa. Further techtonic activity in the region gave rise to a series of regional fault structures. The tectonic framework of offshore Malta belongs to that of the Pelagian domain and is the result of complex interactions between the African and Eurasian plates since the Triassic.

The principal tectonic phases are:

Triassic-Early Jurassic	Breakup of Pangea
Middle Jurassic - Early Cretaceous	Divergence and formation of Tethys
Cretaceous - Eocene	Plate Convergence and Collision
Eocene - Recent	Consumption of African and European Continental margins
Late Tertiary	Dextral movement of Europe with respect to Africa gave rise to renewed rifting

The area was a stable promontory of the African continental margin throughout this
tectonic regime. Opening of the Ionian Sea along the Sicily-Malta escarpment began in
Late Triassic-Early Jurassic time as a result of the sinistral movement of the European
plate. This gave rise to the NNW-SSE trends in the Ragusa basin, Melita basin and other
pull-apart basins of the area. Tectonism in the western Pelagian was also controlled by
movements of Late Triassic/Jurassic evaporites. Subsequent dextral movement of Europe
in latest Tertiary time gave rise to renewed extensive rifting and formation of pull-apart
basins across the area. This produced a second major tectonic trend in the area: the
NW-SE trending Plio-Quarternary Sicily Channel rift complex which extends from
Pantelleria to the Medina Bank.
 

Era	Period	Approximate duration (years)	Cycles of Earth movement	Years ago in millions
TERTIARY or CENOZOIC	Pliocene Miocene Oligocene Eocene	65,000,000		10 25 40 65
	Cretaceous	55,000,000		120

Earthquakes: The stresses caused by the movements of the oceanic crust builds up energy which is released through seismic activity. The Maltese Islands are completely composed of sedimentary rocks, and no evidence of igneous rock has been found. The Islands are however surrounded by a number of active or dormant volcanoes. To the north are Mt. Etna (Sicily); the volcanic islands of Stromboli and the Lipari Islands; Mt. Epomeo (Ischia, Bay of Naples); and Mt. Vesuvius, Mt. Albani and the Phlegraean Camps (Italian Mainland). To the northwest of the Maltese Islands lie the submarine Graham volcano and the young volcanic island of Panteleria. To the southwest are the volcanic islands of Linosa and Lampione; while much further away to the east is the Santorin volcano.
 

Some recorded earthquates which have in the past affected the Maltese Islands were accompanied by volcanic eruptions. In January 1692, an earthquake felt in Sicily and Malta was accompanied by the eruption of Mt. Etna. A previously quiescent Mt. Etna let off huge volumes of smoke during the widespread earthquake of October 1856 On September 1911, a locally felt earthquake appeared to have an epicenter in the region of Graham volcano or Pantelleria. Another earthquake with possible volcanic eruption occurred in January 1923. This earthquake was accompanied by a rumbling noise coming from a northerly direction. Flashes of "lightning" were noticed on the sea. The cause was attributed to a disturbance between Malta and the Ionian Islands. Volcanic quakes are normally due to the sudden release of steam or other volcanic gases under pressure and accompany volcanic eruptions. Their origin may lie at considerable depths under the sea and are then termed crypto-volcanic. The above recorded quakes could possibly have been volcanic in origin. However the volcanoes in question are situated a fair distance from the Maltese Islands, and it is very unlikely that their effects would be felt locally. It is more probable that the earthquakes were tectonic in origin, and the volcanic eruptions were the result of the widespread earthquakes in the region.

It would therefore appear that earthquakes felt in the Maltese Islands owe their origin to Global Tectonics. Earthquakes in the immediate region of the Maltese arcipelago are rare, and rarer still are earthquakes of sufficient magnitude to cause extensive damage. This is very surprising since the Mediterranean region is an active area due to the young age of the surrounding mountain ranges in one of the earth's oldest synclinal depressions. Though earthquakes may be a rare occurence in the Maltese archipelago, the disturbances arising from them may occur more frequently than originally thought. One such disturbance is a change in sea-level. In the destructive earthquake of 1692, the sea at Xlendi (Gozo) is supposed to have receeded a whole mile and then rushed back again causing further damage. A sudden recession of water on the shoreline, followed by a giant wave indicates a "tsunani", resulting from an earthquake with a magnitude of at least 6.5 on the Richter's scale. A tsunani may occur without the obvious accompanment of a tremor, and may not be identified with a tectonic origin. Such an occurrance took place on the night of July 9, 1973. Fishermen and residents in the Salina Bay area (Malta) reported a marine disturbance, which old fishermen called "il-milghuba", and which was alleged to have occurred a few years previously. At about 3 am, the sea level went down by couple of feet. A short while later, the sea rose a couple of feet above the normal level, before settling down to its original level. Some boats, especially those anchored in shallow water, were seen resting on the seabed. When the sea rose again, a "rumbling noise" was heard by several people in the area, and the resulting wave covered up normally dry land up to 400 feet inshore. Mt. Etna in Sicily was reported to be very active a few days before.

Regional Tectonics: Tremors in the Maltese Islands are not solely due to tectonic factors. Localised tremors of variable magnitude may be caused by local geological events, such as cave-ins of subterranean hollows - such as the cave-in at Bahrija (Malta) which killed a boy and several sheep in 1923; and landslips - such as that which occurred at Ghajn Tuffieha (Malta) in 1980.

The effects of tectonic phenomenon are very often seen in the rock strata in the form of flexures and fractures. The whole island of Malta appears to be in a process of sinking, this being more pronounced on the eastern side. This has resulted in a southeastwards tilting of the Island, raising the cliffs of the western coast and giving the deep harbours on the eastern side. This tilting may be assumed from the constant dip of the strata towards the east and northeast of the Island. The marked sinking on the eastern side can be noted from the existence of "cartruts" running along the rocky bottom of the sea for some distance from the shore at the inlet of St. George's Bay at Birzebbugia (Malta). It is further confirmed by the report that during the laying of the Grand Harbour breakwater foundations, divers detached stalagmites from the bottom of the sea. Stalagmites are formed in caves by the redeposition of dissolved limestone in water. This suggests that the area was at one time above sea-level. If Malta continues to sink, the Island will eventually become smaller and more elongated.

Regional Tilts also occur locally. Hence for example, the stratification of the larger island of the St. Paul's Islands group shows a tilt with a dip of about 12^o. Regional tilts may also be responsible for the presence of raised beaches round the Maltese coast. The erosive properties of waves result in platform formation of the rock strata. At some sites, the formed platform appears to have been lifted above sea-level - an effect which is probably accounted for by regional tilts. On a regional dip may be found wrinkles and folds which may have been present before or formed at the same time as a consequence of the tilt. A terrace may be described as a local lessening of the inclination in an otherwise uniformly tilted series, while a local steepening of an inclination is termed a monocline. The various combinations of folds may result in wave-like upwraps (anticlines) or downwraps (synclines) as independant or complimentary structures. Geological investigations have shown that gentle synclines and anticlines do exist in Malta, the Mizieb syncline being the largest completely closed basin structure known in the Maltese Islands. A striking example of a combined anticline - syncline formation in stratified Upper Coralline Limestone can be seen on the road from Rabat to Mtarfa. This formation is only an example of pseudo-folding, and is not purely tectonic in origin. It very probably owes its origin to the dissolving properties of local limestone.
 

Fractures are the result of some applied stress, and are never formed accidentally or without due cause. They are classified according to the direction of the stress. Joints are fractures in the earth's crust along which movement has been normal to the fracture surface. This movement is analogous to a pulling or tearing movement. It may be the result of desiccation; temperature, mineral or physical changes; or due to an active dynamic stress. Fractures along which some movement has occured parallel to the plane of fracture are termed faults. This involves a shifting or sliding displacement, wherein the masses of rock on opposite sides of the break ride by one another in a shearing action. This action may smoothen and polish the rock surfaces resulting in slickensides. A very good example of slickenside in Malta can be seen along the coast from the cliffs under Mnaidra to Lapsi along the Maghlaq fault. Fracturing may also involve extensive crushing of the fault sides, the fissure being later filled by what is termed fault breccia. A major fault structure is usually developed not by one break alone, but instead comprises several individual fractures. Each is a component part of the whole. Very often, faults are not isolated phenomenon but follow each other at irregular intervals in the same direction forming parallel sets or step-faults, one adding to the dislocation of the other. A portion of land may be displaced in relation to the faultlines. Thus an upward displacement to form a plateau is termed a Horst, while a downward displacement is termed a Grapen or Rift Valley.

Faults are classified according to the direction and angle of movement. Thus a normal fault is one wherein the handing wall has apparently moved downward with respect to the footwall. A movement in the upward direction results in a reverse fault. The fault angle may also be used to sub-classify faults. Faults whose plates are inclinded at 45^o or less from the horizontal are termed low-angled faults, while faults inclined at more than 45^o to the horizonatal are termed high-angled faults.

The Maltese Islands are cut up into small portions by numerous faults of varying importance. These were first enumerated by C. Rizzo (1932). In general, it may be stated that there is a principal fault from which the more important faults of the Maltese Islands proceed almost parallel to one another.

The principal fault <Maghlaq fault> is thought to start from the Hurd Bank to the east of Malta off Benghisa Point, proceeds in a northwest direction along the southwest coast of Malta, cutting off a slice of the coast in the region of Lapsi (Malta). This fault accounts for the fact that while Filfla is composed of Upper Coralline Limestone, the coast of Malta in the same region is made up of Lower Coralline and Globigerina Limestones. After crossing the continuation of the Great Fault off Ras Ir-Raheb (Malta), the Maghlaq Fault is believed to continue in a northwest direction to a point off Dimitri Point (west of Gozo). The same tectonic force which resulted in the Maghlaq Fault also resulted in other faults running almost parallel to this fault. The most important of these secondary faults is the Grand Fault of Gozo running from Mgarr ix-Xini to Dwejra.

diagramatic cross-section of the Great Fault (after Spratt, 1854)

Extending from the Maghlaq fault at right angles to it, are the more important of the faults of the Islands. The most conspicuous of these faults is the Great Fault (Victoria Lines). starting at Madliena Bay (east of Malta), this fault crosses the Island in an east-west direction to meet the Maghlaq Fault in the west of Malta. In Gozo too, running from the Grand Fault of Gozo, is another major fault running in an east-west direction parallel to the Victoria Lines. Between these two parallel faults are a sequence of step faults, which have cut the area up into a system of "semi-Horsts" and Graben. Thus the inner side of the land between the faults has formed a plateau, while towards the outer side a Graben has resulted, the deepest of which is the Straits of Comino. Tectonic features are also present towards the southeast of Malta, however they are here less pronounced. The southeastern area of Malta is noteworthy because of the sea-inlets running in a northeast-southeast direction resulting from a system of parallel faults.

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