The Role of the Orano Dike Swarm in Generating High-Yield Bedrock Springs on Elba Island (Italy)
Daniel Pellegrini with David Westerman (mentor)
Brittle fractures patterns and intrusive dikes of the Orano dike swarm were mapped on Elba Island during the summer of 2009. More than 1,700 fracture measurements revealed intersecting patterns at all locations, with no apparent control on the development of high-yield bedrock springs within the M. Capanne granite. Of the nine high-elevation, high-yield springs studied, eight had a cross-cutting Orano dike below the spring that inhibited groundwater flow and produced the significant discharge directly from brittle fractures in the bedrock. The ninth spring system had cross-cutting roof pendant of country rock that served as a similar barrier to flow.
Linear features and major fracture zones are often examined as potential water supplies (Drury et al., 2001), and new methods of exploration are continuously being evaluated (Saxena et al., 2005). The people of Elba, who depend extensively on the tourist industry for economic support, need more water; they currently supplement local supplies by piping water from the mainland at a substantial cost.
This study focuses on groundwater, particularly it’s movement through the ground and back to the surface with sufficient yield to make its capture realistic. Pores and fractures in earth materials are saturated below a certain depth in the ground; this level is known as the water table. When erosion intersects this level, ground water becomes surface water and forms a stream, the head of which is a spring. Sometimes springs have minimal yield and the stream “grows” with continuous minor addition along its length, often resulting in significant contamination from animal activity. Other times, the yield at the source point is high and can be captured as a freshwater spring.
Numerous high-yield, bedrock springs have been exploited for hundreds of years at the higher elevations of M. Capanne on Elba Island, Italy (Fig. 1), an arid mountainous island in the Tyrrhenian Sea. Many of these springs were previously connected with aqueducts to transport water to the main harbor of Portoferraio at the peak of the iron smelting activity on Elba Island in the late 1800’s (S. Rocchi, pers. comm. 2009).
Recent detailed mapping of the Orano dike swarm on M. Capanne (Dini et al., 2009) suggests a correlation between the positions of dikes crossing stream valleys and the locations of ancient springs. Above a critical thickness, similar dikes have been shown to behave as barriers to the flow of groundwater, producing areas of groundwater build up (Bromley et al. 1996). This work examines the questions of 1) whether these dikes actually correlate with the locations of high-yield springs, and 2) whether they serve as aquifers to transport groundwater to the surface by providing a permeable pathway, or serve as aquitards inhibiting groundwater flow.
Successive intrusive pulses of granite into the upper crust 7 Ma formed the M. Capanne pluton, which is now exposed to form a mountain rising 1,000 m out of the sea. This pluton is part of the Elba intrusive complex (A. Dini et al., 2002; Westerman et al., 2004), which culminated with intrusion of the Orano dike swarm. Emplacement of the Orano dike swarm occurred as a result of fracturing of the M. Capanne pluton and its surrounding country rock due to stresses in a NE-SW trending transform fault zone (Dini et al, 2009). Regional uplift caused by intrusion of the Elba intrusive complex in western Elba led to the overlying country sliding off to the east to form central Elba (Fig. 2; Westerman et al., 2004).
Almost all two hundred dikes in the Orano dike swarm trend approximately N80E (roughly ENE) as nearly vertical sheets of fine-grained, intrusive rock cutting through the coarse-grained granite pluton, although two subsets trending N20W and N10E are locally common in discrete zones (Fig. 3).
During formation of these dikes, magma flowed through a network of systematic fractures to form rapidly-cooling sheets ranging in thickness up to fifty meters. These dikes vary widely in structure; some have more well-defined, fine-grained, porphyritic borders, whereas others have more homogenous makeup (Fig. 4). Figure 5 presents the map distribution of Orano dikes projected onto the surface of a digital elevation model.
Drainage Basin Trends: Preliminary stream orientation research was done to ascertain the dominant direction of stream valley systems and determine whether or not stream orientations were controlled by fracture patterns or by erosion in the direction of topographic gradient (i.e. directly away from the highest ground). Drainage basins were named based on the town closest to mouth of the stream where it reaches the sea. The basins proceed counterclockwise around the center of pluton. Their names and directions of orientation are as follows:
St. Andrea zone- Dominant NNW, Secondary NW
Orano zone- Dominant NW, Secondary WNW
Guardia zone- Dominant WNW, Secondary NNE and WSW
Chiesse zone- Dominant SW, Secondary W
Pomonte zone – Dominant SW, Secondary W
In general the basins originate from the highest location or the summit of the M. Capanne pluton and and radiate down gradient towards the sea.
Brittle Fracture Analyses: Fracture orientations were collected from the perimeter of that part of the pluton containing Orano dikes. Higher elevation stream valley regions were studied to determine fracture pattern in areas of known springs. Results are presented with stereograms for all zones in Figure 6. Results of lower elevation fracture analyses are presented in sequence moving counterclockwise around the center of the pluton, starting in the northeastern zone.
Mt. Perone zone- Dominant N34E, 63E; secondary N80E, 62S
Nivera zone- Dominant N79E, 63S; secondary N5E, 80W
Marciana zone- Dominant N35W, 68W; secondary N13E, 82E
St. Andrea zone- Dominant – N84E, 89S; secondary N9E, 53E
Orano zone- Dominant – N73W, 76S; secondary N54W, 88 N
Chiesse zone- Dominant – N86E, 86N; secondary N22E, 56E
Pomonte zone- Dominant – N87E, 88N; secondary N18 E, 61E
Dominant sets of fractures locally parallel down gradient (radial) directions, however, the overall strongest direction of fracture parallels the dominant trend of the Orano dike swarm (N80E). This suggests that the fracture system is not controlled by the common development of radial and concentric cooling joints as the pluton contracted. Nevertheless, the abundance of joints crossing in orthogonal patterns provides easy access for groundwater to flow down gradient.
The spring zone names, and their dominant and secondary orientations are as follows:
Campo Sportiva- Dominant N49E, 74N; secondary N71W, 25N and N49W, 88N
Marciana- Dominant N90E, 89S; secondary N21W, 86W
Tavola- Dominant set N68W, 88S extending to N4W, 85W.
Napoleone- Dominant N89W, 87S; secondary N55E, 83S and N37W, 87W
Nivera- Dominant N40E, 80E; secondary N85W, 80S
Chiesse- Dominant N78E, 89S; secondary N4W, 88W
Troppollo- Dominant N89W, 89S; secondary N55W, 65S
Madonna Dell Monte- Dominant N79E, 84N; secondary N9W, 87E
Bolero- Dominant N51W, 79S; secondary N57E, 80S
Marconi- Dominant N30W, 76W; secondary N22E, 65E
These results, shown stereographically in Figure 6, indicate two basic relationships. First, the fractures at high elevations are similar to those at lower elevations with correlation to trends of Orano dikes. Second, there is an abundance of fractures intersecting at steep angles such that groundwater has good access to flow down gradient within the granite of M. Capanne.
Detailed Mapping at Spring Localities: Nine zones containing ancient springs formerly connected via an aqueduct were mapped in detail to determine the distribution of Orano dikes in the associated valley for each spring. Results of these studies are presented in block diagrams (Fig. 7). Note that in all but one case, one or more Orano dikes (shown in purple) is located downstream from the spring (shown as a blue box).
Fractures Allowing Groundwater Flow: In all studied locations there are distinct sets of dominant and secondary fractures that intersect at steep angles. This relationship would provide a fracture network for groundwater to move though the rock formation. While in some areas there is a positive relationship between the trend of the valleys (notably west-draining valleys) and the orientations of the dominant sets of fractures, but this relationship is not clear for NW-, N- and NE-draining valleys. The most dominant fracture trend is oriented N80E, parallel to the dominant orientation of Orano dikes. This suggests that the fractures formed during the regional shear that allowed emplacement of the dikes, rather than as the result of cooling and contraction of the granite.
The Influence of Dikes on Groundwater Flow: Exploitable springs in this region require that they be relatively high yield bedrock springs, where groundwater comes directly out of a brittle fracture in the granite. Water is captured in these instances by building an enclosed spring box (Fig. 8) where yield is high, because weeping springs or extended zones of discharge are quickly contaminated by animals.
Intersecting brittle fractures are abundant in the M. Capanne granite, and allow groundwater to easily flow in the down-gradient direction. These brittle fractures are dominantly parallel to the Orano dikes that were emplaced in shear-induced fractures shortly after intrusion of the M. Capanne pluton. The Orano dikes on Elba Island behave as aquitards, inhibiting the flow of groundwater due to their reduced permeability. Their orientation relative to the stream valleys they cross, and the high yield springs that are present above them, provide evidence confirming this relationship. The blockage forces the water table to rise behind it (Fig.9), and spring from the ground. The extra water flow in these locations provide increased ability to capture the springing water.
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