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Ocean Drilling Program Initial Reports Volume Schmincke, H.-U., Weaver, P.P.E., Firth, J.V., et al., 1995 Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 157 2. BACKGROUND, OBJECTIVES, AND PRINCIPAL RESULTS OF DRILLING THE CLASTIC APRON OF GRAN CANARIA (VICAP)1 Shipboard Scientific Party2 INTRODUCTION rise to debris flows and turbidity currents (Masson and Watts, in press). Ocean islands have played a prominent role in the history of the The VICAP drilling project was designed to monitor the subma- earth sciences, despite their small number, surface area, and volume, rine and subaerial temporal, compositional, volcanic, and chemical compared to other major geological units of the eartiYs crust. The growth and mass wasting evolution of a well studied ocean island first fundamental petrological ideas were formulated by Robert Bun- system by drilling through its peripheral clastic apron and through the sen and Charles Darwin in their studies on Iceland and the Azores. edge of the submarine shield phase deposits. We chose Gran Canada The decrease in age of oceanic islands, as deduced from the degree of because it has a very long history with magmas of greatly contrasting erosion from west to east in the Hawaiian island chain (Dana, 1849), compositions and because the temporal, volcanic, and compositional and, among other Atlantic island groups from east to west in the Ca- evolution of the island has been studied in detail. nary islands (von Fritsch, 1867), were the cornerstones for Wilson's Our aim was to understand the way ocean islands evolve and de- 1963 model of hot spots and plate motions. Subsequently Morgan cay from the beginning of the submarine shield stage through the (1972) expanded this idea in his plume model of ascending mantle highly evolved and erosional phases. The evolution of the volcanic material, which was believed to feed the oceanic islands, with plumes apron around Gran Canada and the detailed correlation with the land serving as a reference frame for determining directions and rates of record is designed as a case history study, a model that will allow us plate motions. Ocean islands remain the premier geological site to to assess the past volcanic, petrologic, and plate tectonic environ- test fundamental models for magma genesis and plate dynamics. ments of sedimentary basins adjacent to productive volcanic source Charles Lyell (1865) held that the large type caldera on La Palma areas as well as marine volcaniclastic successions drilled in the ODP in the western Canary Islands, postulated by Leopold von Buch program. (1825) to have formed by "updoming," was instead carved by erosion generating huge amounts of sediments. Alexander von Humboldt BACKGROUND considered the celebrated Orotava Valley on northern Tenerife below the Canadas "caldera" at 2000 meters above sea level (masl) to have Volcanic aprons resulted from a giant landslide. The role of ocean islands as a significant sediment source was not A volcanic apron peripheral to an oceanic island consists of three appreciated, however, until Menard (1956) introduced the concept of main facies above the central or core facies and is made up of extru- archipelagic aprons surrounding ocean islands, deduced entirely sive and intrusive igneous rocks (Schmincke, 1994; Fig. 1): from bathymetry. Menard argued that the volume of clastic material shed from the islands into the basins would in many cases even sur- pass the volume of the islands themselves. Such aprons contain sig- nificant amounts of material representing the evolution of the Sea level volcanic complex, including material no longer present on the island (Figs. 1 and 2). Most importantly, this includes material from the un- exposed and inaccessible submarine stages. Two recent discoveries highlight the significance of ocean islands as prototype earth systems in which the growth of the islands as a re- sult of mantle processes and their degradation by mass wasting are in- timately interwoven. Watts and ten Brink (1985) recognized that layers of volcaniclastic sediments, that could be traced by multichan- nel seismic methods up to >200 km from the Hawaiian island of Oahu, dip toward the island in its immediate surroundings rather than away from the island. They used this inclination to estimate the de- gree of loading of the lithosphere by the islands and thus the elastic thickness of the lithosphere. The enormous scale of mass wasting is evident from GLORIA side-scan records that reveal that the Hawai- ian islands have spawned giant slumps up to 10,000 km3 in volume with blocks to 20 km wide. These, in turn, have generated major de- bris flows and distal turbidites that can be traced for several 100 km from the source volcanoes (Moore et al., 1989; Lipman et al., 1988). Figure 1. Schematic drawing of ocean island and its clastic apron. The volca- Recent work has shown that the Canary Islands erode in similar ways nic apron consists of the "seismically chaotic" flanks of the island, made up to the Hawaiian Islands, namely through voluminous slumps giving dominantly of hard volcanic rocks including pillow lavas, pillow breccias, debris-flow deposits, and slump blocks, overlying the core facies of extrusive and intrusive igneous rocks, the basin facies, characterized by widespread 'Schmincke, H.-U., Weaver, P.P.E., Firth, J.V., et al., 1995. Proc. ODP, Init. Repts., 157: College Station, TX (Ocean Drilling Program). reflectors representing volcaniclastic mass-flow deposits (debris flows, tur- 2 Shipboard Scientific Party is given in the list preceding the Table of Contents. bidites), and ash fallout layers. From Schmincke (1994). 11 SHIPBOARD SCIENTIFIC PARTY Leg 47A Site 397 Shield stage E -12 Submarine stage Figure 2. Three schematic growth stages of a volcanic ocean island as reflected in three main types of subma- rine volcaniclastic rocks drilled at DSDP Site 397 (Leg - 18 47A) ~ 100 km south-southeast of Gran Canaria. From Schmincke (1982). 1. Flank facies comprising the seismically chaotic apron. on the slope and destabilized by earthquakes, sea level changes, or The higher velocity and "seismically chaotic" flank with veloci- other causes. The distal facies extends for hundreds of km. Very dis- ties around 3.4-4 km/s is characterized by discontinuous reflectors, tal fine-grained volcanic turbidites from the Canaries can be traced to rough topography, and has been called chaotic seismic facies or vol- the area of the Madeira Abyssal Plain -1000 km to the west (e.g., canic apron by some authors (e.g., Wissmann, 1979; Holik et al., Masson, 1994). 1991). The interior of the seamount stage of a volcanic island is thought Geophysical and Geological Setting to consist largely of intrusives, pillow lavas (densely packed and non- vesicular at depth, becoming increasingly vesicular and interlayered The Canarian archipelago built on the African continental slope with pillow breccias and pillow fragment breccias), debris flows, and and rise consists of seven major islands: Lanzarote and Fuerteventura primary and resedimented hyaloclastites upwards. Rocks from the to the east, Gran Canaria, Tenerife and Gomera in the center and La core facies and the clastic cover (flank facies) are well exposed in an Palma and Hierro in the west (Fig. 3). Three islands (Gran Canaria, uplifted section of the submarine portion of La Palma (Staudigel and Tenerife, La Palma) are >2000 m high and Tenerife, dominated by Schmincke, 1984). Pico de Teide, is the third largest oceanic volcano on earth. The ar- The dominantly clastic envelope around the core can be subdivid- chipelago extends between 115 and 600 km west of the African coast ed into pillow breccias and various types of hyaloclastites and flow and for 100-200 km north-south. Total surface area of the islands is deposits that were generated during the shallow submarine, emer- -7500 km2. The very small volcanic island remnants of the Selvagens gent, and subaerial stages and were redeposited down the flank of the Islands, Concepcion Bank to the north and seamounts to the south- seamount. The clastic hyaloclastite cover may extend appreciably be- west are regarded as part of the Canary Island volcanic province. yond the core facies. The outer limits of the seismically defined There are few groups of volcanic islands with such long histories of "basement" flanks are identifiable in the profiles down the slopes of eruptions (Fig. 3), spanning >15 m.y. on some individual islands— Gran Canaria, Tenerife, and Fuerteventura, although the lithologic and such an enormous variety of volcanic and plutonic rocks, ranging and seismic boundaries between the flank and the basin facies are not from highly silica—undersaturated melilite nephelinites to peralka- always clear (Funck et al., in press). line rhyolitic welded tuffs. The seismically defined flanks are overlain by the sedimentary The Canary Islands are oceanic intraplate islands in many ways part of the volcanic apron which consists of two main facies: resembling the evolutionary stages of the Hawaiian Islands 2. Slope facies. (Schmincke, 1976). Differences between the Hawaiian and Canarian The slope facies, above the seismically chaotic flank facies, are evolution include the following: characterized by slumps, discontinuous bedded and massive units, debris flows, erosional channels, etc. It corresponds approximately to 1. The volume of lavas from the shield stages in Gran Canaria is the submarine morphological slope of an island. This facies extends smaller than on Hawaii. roughly from the present shore line of Gran Canaria to -45 km. 2. The types of shield volcanoes differ among the Canaries and 3. Basin facies. therefore between the Canaries and the Hawaiian Islands. The slope facies grades laterally into the basin facies which is 3. The number of post-erosional or rejuvenated stages in the Ca- characterized by well developed, more continuous and flat reflectors. nary Islands is higher than in the Hawaiian Islands. It consists of diverse types of volcaniclastic deposits including ash 4.
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