Geological development of and nature of fractures in the sleeted Dyke complex of the Spilia-Politiko area, Troodos ophiolite, Cyprus: implications for permeablity in oceanic crust

Van Everdingen, David Allard (1993) Geological development of and nature of fractures in the sleeted Dyke complex of the Spilia-Politiko area, Troodos ophiolite, Cyprus: implications for permeablity in oceanic crust. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The presence of hydrothermal vents on the seafloor at present-day spreading centres, and the lower than expected conductive heat flow values at ridge crests requires the convective circulation of fluids in oceanic crust. The fractured nature of oceanic crustal rocks further suggests that fractures rather than the rock matrix control the fluid circulation and hydrothermal alteration. Study of the nature and role of these fractures in the sub-surface below the modem seafloor is at present done only through one-dimensional boreholes and seismic refraction studies. These fracture networks are better described through analogues such as the ancient obducted oceanic crust of the Troodos ophiolite, Cyprus, which provide a three-dimensional, well-exposed cross-section through the oceanic crust. -- The Spilia-Politiko area, Cyprus, was chosen to investigate the characteristics of fractures in the sheeted dyke complex of the Troodos ophiolite. Sheeted dykes in the study area were intruded along a fast spreading, ridge in the Tethys Sea approximately 90 Ma. A jump of the spreading axis from the Solea graben to the Spilia-Politiko area, a second ridge jump further to the east, and movement along the South Troodos Transform Fault have contributed to the formation of five, partly fault-bounded, dyke domains which are defined by their strike orientations. The chemistry of the dykes and the distribution of hydrothermal alteration zones are indicative of multiple, small, transient magma chambers. A later phase of amagmatic extension, post-dating hydrothermal alteration, resulted in the formation of the Mitsero graben structure through a roughly 12% extension of the crust. -- Fracture characteristic data for roughly 3200 fractures were systematically collected at 41 locations in the different dyke domains and throughout a vertical section of the Troodos crust, using a scanline mapping method. The mapped characteristics included fracture type, orientation, aperture, length, termination mode, and mineral filling. Fractures within the sheeted dyke complex of the Spilia-Politiko area are sub-divided into two distinct sets: (1) fractures parallel to dyke margins; and (2) those related to columnar jointing. The dyke-parallel fractures tend to exhibit longer trace lengths and larger apertures than those related to columnar jointing. In addition, the dyke-parallel fractures tend to contain epidote whereas the columnar joints were filled by calcite which formed at a later time. Both fracture types formed as the result of contraction during cooling of the dykes, such that fracture orientations are related to the paleo-orientation of the dykes. Other fracture characteristics, such as trace length, aperture, and mineral filling, are generally consistent across the different dyke domains suggesting that the tectonic and hydrothermal processes during the period immediately following dyke emplacement were homogeneous. Fracture aperture and trace length decrease with paleo-depth through the sheeted dyke complex, whereas the occurrence of hydrothermal minerals increases with depth. -- Paleo-permeability of the sheeted dyke complex, numerically modelled on the basis of fracture radius, aperture, orientation, and density data from the study area, ranges from 10⁻⁸ to 10⁻¹²m². An observed decrease in the calculated paleo-permeability with depth corroborates studies of modern ocean crustal permeability as reported from Deep Sea Drilling Project borehole packer tests. Areas of intense hydrothermal alteration are more fractured than unaltered dykes, therefore their calculated permeabilities are higher, but only by 6%. The intensely altered zones are 50-100 metres wide and contain fluid inclusion and mineralogical evidence for the passage of hot, circa 350ﾟC, seawater-salinity fluids. They are considered to be the up-flow portions of hydrothermal circulation cells which resulted in the formation of epidosite bodies. Preferential fracturing along the dyke margins favours vertical permeability, which facilitates vertical hydrothermal convection driven by heat from the small magma chambers. In the study area, the distribution of epidosite bodies suggests that the hydrothermal cells have a mean cross-strike spacing of 5.3 kilometres. Restriction of hydrothermal alteration to well defined zones higher in the sheeted dyke section and an increase in the occurrence of hydrothermal mineral precipitation with depth implies focussing of hydrothermal up-flow higher in the section. Because the amount of fracturing in the diabase does not vary significantly from that of well-defined epidosite zones it is concluded that the location of hydrothermal circulation cells is controlled by the position of the heat source. -- Study of the variation in dyke orientations provides a useful basis for establishing a tectonic framework for sheeted dyke complexes. The study of fractures, alteration and permeability in ophiolites within such a tectonic framework provides valuable information about the fracture characteristics and permeability structure of the upper oceanic crust. The permeabilities calculated for the study area give an indication of the magnitude of permeability in oceanic crust at a spreading axis during hydrothermal circulation, whereas sub-sea boreholes (e.g. DSDP hole 504B), with present technology, can only provide information on permeability at off-axis locations and may not be as useful for estimating permeabilities in active hydrothermal systems.

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