In this chapter we are concerned with underwater volcaniclastic eruptions and products, as well as pyroclastic flows and debris generated on land and transported by mass flowage into the sea. Submarine fallout deposits derived from land-based eruptions are treated in Chapter 7. Clastic materials redistributed as mass flows or turbidity currents originating from the rapid build-up of tephra and epiclastic volcanic debris along coastal parts of volcanoes are also discussed in various places throughout the text (e.g. Chap. 13).

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Submarine volcaniclastic rocks

Tectonic framework of the New Hebrides island arc

The New Hebrides islands overlie an eastward-dipping Benioff zone and are bordered by a submarine trench. They consist of volcanic and volcaniclastic rocks, intrusions, mudstones and limestones. Eastern and Western Belts of islands containing Neogene rocks are bisected by a Central Chain of active and recently extinct volcanoes. In the Western Belt intense early Miocene basaltic and andesitic volcanism resulted in a thick succession consisting largely of quartz-free marine volcaniclastic rocks; intrusion of hornblende andesites and diorites preceded middle Miocene block faulting and uplift. Late Miocene turbidites derived from this succession and rare tufts were faulted and eroded prior to a Pliocene transgression. In the Eastern Belt undated basalt and andesite lavas were faulted and intruded by gabbros and norites before the middle Miocene. Emplacement of serpentinites containing amphibolite rafts pre-dated accumulation of pillow basalts and breccias of late Miocene to early Pliocene age. Most islands of the Central Chain consist of late Pliocene to Recent subaerially-erupted basaltic lavas; some islands in the south contain similar basalts and andesites overlying thick Pliocene successions of pillow lavas and volcaniclastic rocks. Massive reef limestones deposited in all three Belts during the late Pliocene and Quaternary now form tilted plateaux and terraces. The Western Belt was the site of an early Miocene volcanic arc; a parallel submarine trench may have existed either near the Eastern Belt, or to the west of the Western Belt. In many other island arcs volcanic episodes were contemporaneous with those in the Western Belt and Central Chain of the New Hebrides.

Geological evolution of the New Hebrides island arc

The Upper Jurassic–Lower Cretaceous strato-tectonic belts of the southern Andes and South Georgia, 2000 km apart, can be correlated and explained as the products of an island-arc–back-arc system. From the Pacific Ocean to the Atlantic, these belts, which exhibit structural and metamorphic differences, are: (1) a pyroclastic belt developed on an ensialic volcanic arc; (2) a back-arc flysch sequence underlain in the southern Andes by a basic complex with oceanic affinities; this was intruded into continental crust as a result of sea-floor spreading which created a marginal basin; (3) a slate sequence deposited on a continental shelf. The pyroclastic and marginal basin belts and the adjacent part of the continental shelf were folded and uplifted during the early Upper Cretaceous, whereas the foreland part of the continental shelf assemblage underwent deformation during the early Tertiary.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0016756800047592

An Upper Mesozoic island-arc–back-arc system in the southern Andes and South Georgia

Modern active island arcs and their surroundings consist of two main physiographic zones-a volcanic ridge or arc and a submarine trench. Rocks in island arcs fall into three groups, the origin of which can be interpreted in terms of plate tectonics and of geological processes in the two zones and surrounding ocean floor. The groups are (1) volcanic arc deposits, consisting of undeformed subaerially erupted volcanic rocks, block-faulted volcanic and volcaniclastic rocks, diorites, granites, and low-pressure/high-temperature meta-morphic rocks; (2) submarine trench deposits, consisting of trench-fill sediments, represented by thick successions of quartz-deficient turbidites and pelagic sediments, and high-pressure/low-temperature meta-morphic rocks and melange deposits indicating a Benioff zone, (3) ocean-floor oceanic crustal rocks consisting of tholeiitic lavas, spilites, gabbros, ultrabasic rocks, and pelagic sediments, and including fine-grained turbidites rich in tholeiitic detritus. Most island arcs o...

Evolution of Island Arcs

SUMMARY

On Malekula Island pre-Miocene pelagic red mudstones are in tectonic contact with a thick marine succession of Early Miocene age, which is intruded by basaltic and andesitic dykes and sills with a calc-alkaline composition and by plutons. This succession is in faulted contact with Middle Miocene sedimentary rocks.



The Lower Miocene succession consists of quartz-free volcanoclastic rocks, detrital limestones and pelagic sediments, and rare lava flows. Fifteen sedimentary facies have been recognized, all of which accumulated below the zone of strong wave or current action. Vertical changes in facies are common; facies relationships and associations and internal sedimentary structures indicate that many facies were transported by mass gravity processes. Texture and composition of clasts in the volcanoclastic rocks show that some facies were derived from subaerially erupted volcanic rocks, and others from submarine volcanoes. Clastic limestones consist largely of algal and coral fragments and benthonic foraminifers.



The Lower Miocene palaeogeography resembled that in and around volcanic chains in present island arcs. Subaerial and submarine volcanoes fed by minor intrusions supplied epiclastic, pyroclastic and autoclastic debris which accumulated in deep water around the volcanic chain. Carbonate detritus was derived from reefs bordering the volcanic islands.



The Middle Miocene succession consists of volcanic and reef-derived turbid-ites, pelagic sediments and tuffs. The epiclastic rocks in this succession were derived from the uplifted rocks of Early Miocene age and from bordering reefs.



The presence of some ancient volcanic arcs, now located either within existing ares or within geosynclinal successions lying in or on the borders of continents, may be indicated only by quartz-poor turbidites containing abundant calc-alkaline volcanic detritus.

A. H. G. Mitchell

Papers

Facies of an early miocene volcanic arc, malekula island, new hebrides