Fluvial dissection, isostatic uplift, and geomorphological evolution of volcanic islands (Gran Canaria, Canary Islands, Spain)
Introduction
Erosion in volcanic islands basically produces both a loosening of subaerial material, which promotes subsequent isostatic uplift, and a correlative reactivation of the fluvial incision, together with the rejuvenation of the landscape (Watts, 2000). In addition, hot-spot islands are subject to differential, and progressive, vertical movements, due to the lithospheric flexure induced by new adjacent volcanic islands (Watts, 2000). This is a prominent process in most of the oceanic volcanic chains such as in the Marquesas (Cares et al., 1995), Tuamotu (Ito et al., 1995) and Hawaii (Grigg and Jones, 1997, Keating and Helsey, 2002) in the Pacific, and the Reunion islands (de Voogd et al., 1999) in the Indian Ocean. The Canary Islands, located on the Atlantic oceanic lithosphere of the African plate (Fig. 1a), is a special case of a slow moving (2 mm/yr) hot-spot volcanic archipelago (Carracedo et al., 2002). Seismic reflection lines from the Canary Island region (Watts et al., 1997, Yee et al., 1999, Watts, 2000, Collier and Watts, 2001) indicate that lithospheric flexure is an active process in this archipelago. In particular, flexural processes are relevant for the contiguous islands of Tenerife and Gran Canaria (Watts, 2000).
Stratigraphic data from the island of Gran Canaria reveal a continuous West to East differential uplift of the present elevation of raised Pliocene pillow-lavas horizons (c.a. 4 Ma), from 46 to 143 m above the present sea level (Pérez-Torrado et al., 2002). This fact reveals a broad post-Pliocene westwards tilting of Gran Canaria, consistent with the lithospheric flexure exerted by the younger island of Tenerife, located around 60–65 km to the West (Fig. 1a). In fact, the growth of Tenerife (12 Ma old) induced the generation of a flexural moat of about 2–3 km depth, filled with sediments coming from the fluvial and gravitational erosion of the islands (Watts et al., 1997). This depression in the lithosphere extends over 250 km from the centre of Tenerife (Watts et al., 1997) and, at present, the sea floor is depressed a maximum of about 500 m within the moat (Collier and Watts, 2001). The older (14.5 Ma old) island of Gran Canaria is presently located within the flexural moat generated by Tenerife, and has been subject to differential uplift throughout its development. Therefore, assuming the same climatic history and sea-level changes for the entire Canary archipelago, variations in fluvial dissection throughout a particular island can be linked to the occurrence of differential uplift in space and time.
Geomorphological studies of the deep set of ‘barrancos’ dissecting the various volcanic edifices of the Canary Islands (Paris, 2002, Menéndez et al., 2004) clearly reveal the occurrence of contrasting fluvial incision within individual islands. In detail, Menéndez et al. (2004) describe linear longitudinal profiles as relevant features of the ‘barrancos’ dissecting the Northern slope of the Island of Gran Canaria. Similar linear profiles have also been described in the volcanic archipelago of Hawaii (Seidl et al., 1994). This kind of linear profile can be considered to be typical of oceanic volcanic islands, where the adjustment of incision to the inherited slopes of the volcanoes is a striking factor, backfed by sustained uplift, thus reflecting a fine balance between incision and uplift (Seidl et al., 1994, Menéndez et al., 2004).
Over the last few decades morphometric studies of drainage networks have been demonstrated to be a powerful tool in the evaluation of uplift (e.g. Gilchrist et al., 1994, Whipple et al., 1999). Most of the works focus on the analysis of large-scale mountain chains such as the Himalayas, the Alps (Montgomery, 1994, Gilchrist et al., 1994, Burbank et al., 1996, Hovius, 2000) and the Rocky Mountains (Wohl and Merrit, 2001, Brocklehurst and Whipple, 2002). Some other works have explored the uplift-dissection relationships of large-plate marginal islands, such as New Zealand or Taiwan (Bull and Knuepfer, 1987, Hovius, 2000), but few works have analyzed the subject in oceanic volcanic islands (Seidl et al., 1994, Paris, 2002, Menéndez et al., 2004) focusing on the volcanic archipelagos of Hawaii and the Canary islands.
This work focuses on the morphometric characterization of the radial drainage network dissecting the Island of Gran Canaria, in order to extract the uplift–dissection relationships. Multivariable morphometric analyses on the whole drainage basins have been performed in order to assess the influence of lithology, chronology and climate on drainage development. The uplift values, as compared to the fluvial unloading of the island, have been calculated by individual basins. Different morphometric techniques, such as are mainly used in orogens, have been combined and implemented in a digital elevation model of the island to evaluate isostatic uplift. This new methodology combines geology and morphometry, together with a well-constrained geomorphological and geochronological control of the pre-incision surfaces.
Section snippets
Geological setting
The Canary Islands are on the oceanic Jurassic lithosphere of the Atlantic, adjacent to the passive continental margin of Africa (Carracedo et al., 2002). The Island of Gran Canaria is approximately at the centre of the volcanic hot-spot chain (Fig. 1a). This circular-shaped island (ca. 45 km in diameter) covers an area of 1532 km2 with a maximum elevation of 1949 m (Pico de Las Nieves) near its geometrical centre. A dense radial network of deep ‘barrancos’ dissects the island, forming a rugged
Methodology
Using the digital cartography of Gran Canaria, (scale 1:5000; GRAFCAN, 1996), 58 drainage basins with thalwegs of over 4 km in length were extracted (Fig. 1, Fig. 3). Basins less than this length were rejected from this study, since they offer information about very local (lithological, structural, microclimatic) factors and, thus, generate an anomalous noise in the morphometric analysis of the island overall. Each selected basin was analyzed in order to extract the individual drainage basin
Geomorphological sectorization of the island
The successive volcanic edifices, lava flows and platforms, generated over the various stages of development of Gran Canaria are distributed differentially over the island according to the location of their corresponding volcanic centers and type of activity. This differential distribution of the volcanism has printed a contrasting geomorphology on both sides of the NW–SE rift system which cuts across the entire island (Fig. 1b).
The SW half of the island displays deeply incised ‘barrancos’ (up
Morphometric assessment of climatic and lithological controls in drainage development
Morphometric relationships, comparing thalweg length, maximum basin elevation and drainage basin area vs maximum dissection (Fig. 5), display evident clustering of ‘barrancos’ according to the age (old or young) of the volcanic substratum in which they are carved, but a weak correlation for the total data set of ‘barrancos’ studied. Barrancos of the old sector display deeper incision than those carved in the young one. This seems to be a logical response to the longer erosive history recorded
Uplift record on the island of Gran Canaria
It is possible to compare incision and uplift rates with the present elevation of raised paleo-shoreline markers in the NE quadrant of the island. Pérez-Torrado et al. (2002) described and levelled the transition between sub-aerial and sub-marine (pillow-lavas) facies of basaltic lava flows between 4.3 and 3.97 Ma (Guillou et al., 2004) (Fig. 1b). This ancient sea-level marker runs from 57 to 80 m above the sea level, from West to East, with peak values of + 143 m in the NE corner of the island (
Discussion
The total removed material evaluated here (223.95 km3) only represents approximately 0.5–0.6% of the total bulk volume of the island, but around 26% of its subaerial volume (859.77 km3). However, even the removal of around a quarter of the subaerial mass of the island is not enough to explain the apparent uplift of over one hundred meters displayed by the raised pillow-lava horizons. Values of uplift triggered by fluvial unloading, as computed in this study, only explain 70–83% of the uplift
Conclusions
The Geomorphological and morphometric analysis of the set of deep ‘barrancos’ dissecting Gran Canaria allows for the following conclusions to be reached:
- (a)
The island can be divided into four different environmental sectors, as defined by the age of the volcanic materials (NE half young, and SW half old) and the contrasting climatic zonation produced by the orographic-rain shadow effect (SE half dry, and NW half wet) as a result of the Atlantic wind patterns.
- (b)
Relationships between thalweg length
Corollary
Oceanic islands may be used as natural laboratories to study drainage development and fluvial incision. In addition, oceanic islands offer accurate delineation of pre-incision surfaces over the ancient volcanic slopes which can, in turn, be precisely dated by radiometric methods. Therefore, this kind of island offers interesting and challenging landscapes where we may evaluate geomorphological rates over a variety of climatic conditions which are difficult to find within continental settings.
Acknowledgments
This work is part of PI2002/148 project of the Autonomous Government of Canary Islands. The authors are grateful to the constructive comments of Anne Mather (U.K.), Raphael Paris (France) and Adrian Harvey (U.K.), and also to Margaret Hart Robertson (English revision of the text).
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