The Capo Castello Shear Zone (Eastern Elba Island): Deformation at the Contact between Oceanic and Continent Tectonic Units
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Low-grade mylonitic shear zones are commonly characterized by strain partitioning, with alternating low strain protomylonite and high strain mylonite and ultramylonite, where the shearing is most significant. In this paper the capo Castello shear zone is analyzed. It has developed along the contact between continental quartzo-feldspathic, in the footwall, and oceanic ophiolitic units, in the hangingwall. The shear zone shows, mostly within the serpentinites, a heterogeneous strain localization, characterized by an alternation of mylonites and ultramylonites, without a continuous strain gradient moving from the protolith (i.e., the undeformed host rock) to the main tectonic contact between the two units. The significance of this mylonitic shear zone is examined in terms of the dominant deformation mechanisms, and its regional tectonic frame. The combination of the ultramafic protolith metamorphic processes and infiltration of derived fluids caused strain softening by syntectonic metamorphic reactions and dissolution–precipitation processes, leading to the final formation of low strength mineral phases. It is concluded that the strain localization, is mainly controlled by the rock-fluid interactions within the ophiolitic level of the Capo Castello shear zone. Regarding the regional setting, this shear zone can be considered as an analogue of the initial stage of the post-collisional extensional fault, of which mature stage is visible along the Zuccale fault zone, a regional structure affecting eastern Elba Island.Keywords:
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Greenschist
Abstract Late Gothian (c. 1.58 Ga) and Sveconorwegian (1.1–0.9 Ga) structures outline a 35 km long, NNE‐oriented, open gneiss synform in the Varberg‐Horred region of SW Sweden. This is a region of the Southwest Scandinavian Domain, within which a major shear zone and tectonic boundary, the Mylonite Zone, forms a branching shear zone system which converges in the eastern part of the synform. A subdivision between the Gothian and Sveconorwegian events is made by using the intervening anorogenic intrusions as structural markers. This, and the non‐recognition of a previously assumed orogenic event, results in a geodynamic model which is similar for the crustal segments on both sides of the largely N‐S trending Mylonite Zone, except for the higher grade Sveconorwegian metamorphism to the east. The evolution is characterised by one or more major Gothian gneiss‐forming events, followed by intermittent anorogenic magmatism and a later Sveconorwegian development that, outside discrete shear zones, gave rise to moderate fabric‐forming deformation and only localised formation of migmatitic leucosomes. The final Gothian orogenic episode at c. 1.58 Ga and three distinct anorogenic events between 1.51 and 1.20 Ga are correlated across the Mylonite Zone, thus supporting models where the Mylonite Zone constitutes an intracratonic Sveconorwegian shear zone. The Sveconorwegian development is interpreted to include eastward thrusting on the Mylonite Zone, followed by dominantly static metamorphism prior to 0.98 Ga, due to the thickened crust. Subsequent uplift and rapid cooling preserved granulite‐facies assemblages in the southern Eastern Segment. Late Sveconorwegian extensional movements occurred until c. 0.92 Ga along the largely west‐dipping Mylonite Zone system. Åhäll, K.‐L, 1995: Crustal units and role of the Mylonite Zone system in the Varberg‐Horred region, SW Sweden. GFF, Vol. 117 (Pt. 4, December), pp. 185–198. Stockholm. ISSN 1103–5897.
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Low-grade mylonitic shear zones are commonly characterized by strain partitioning, with alternating low strain protomylonite and high strain mylonite and ultramylonite, where the shearing is most significant. In this paper the capo Castello shear zone is analyzed. It has developed along the contact between continental quartzo-feldspathic, in the footwall, and oceanic ophiolitic units, in the hangingwall. The shear zone shows, mostly within the serpentinites, a heterogeneous strain localization, characterized by an alternation of mylonites and ultramylonites, without a continuous strain gradient moving from the protolith (i.e., the undeformed host rock) to the main tectonic contact between the two units. The significance of this mylonitic shear zone is examined in terms of the dominant deformation mechanisms, and its regional tectonic frame. The combination of the ultramafic protolith metamorphic processes and infiltration of derived fluids caused strain softening by syntectonic metamorphic reactions and dissolution–precipitation processes, leading to the final formation of low strength mineral phases. It is concluded that the strain localization, is mainly controlled by the rock-fluid interactions within the ophiolitic level of the Capo Castello shear zone. Regarding the regional setting, this shear zone can be considered as an analogue of the initial stage of the post-collisional extensional fault, of which mature stage is visible along the Zuccale fault zone, a regional structure affecting eastern Elba Island.
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A high-strain ductile shear zone trending NEE with southeasterly dipping mylonitic foliation, has been recognized affecting a Mesoproterozoic (ca. 1105 Ma) peraluminous garnet-bearing two mica granitoid (El Tigre granitoid: 31o31'30''S- 68o15'12''W) which is part of the crystalline basement of the Sierra de Pie de Palo, Western Sierras Pampeanas. Kinematic analysis indicates a main strike-slip component and provides evidence that the relative movement within the shear zone have dextral sense. Relict igneous and peak amphibolite facies metamorphic mineral assemblages and textures are preserved in the granitoid protolith although, within the shear zone, deformation obliterates those generating typical mylonitic fabrics. Mylonitization operated under open-system conditions, provoking mobilitization (either enrichment or depletion) of almost all major and trace elements, including rare earth elements and Rb/Sr and Sm/Nd isotopes. Observed chemical variations are mostly controlled by syntectonic fluid-transport processes and decrease in the garnet, biotite and accessory minerals amounts during mylonitization, and the neoformation of white-mica in the fine-grained mylonite matrix. Moreover, the different iso- topic signatures between the protolith and the mylonites could be a consequence of mechanisms of deformation-driven pro- cesses assisted by fluid flow with different fluid-host rock interaction ratios. Textural, modal and chemical changes between the El Tigre granitoid protolith and its mylonites, allow reconstructing its tectono-metamorphic evolution and the metamor- phic conditions achieved. Paragenetic associations and deformation textures on mylonites suggest that El Tigre granitoid she- ar zone was developed under low-T greenschist facies conditions, probably at temperatures below 400 °C. This deformatio- nal event took place at ca. 473 ± 10 Ma during the Famatinian orogeny.
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The near NS ductile shear zone was first discovered in Proterozoic Bendong granodiorite pluton. It dips toward 240°～280° at the dip angle of 40°～63°. The lineations, which represent the shear direction, plunge to 216°～226° at the plunge angle of 39°～46°. The kinematics of Bendong ductile shear zone is characterized by sinistral-normal shearing, and shows a sliding from NE to SW. Typical granitic mylonites were developed in the ductile shear zone and clearly show a zoning from phyllonite through mylonite to initial mylonite from center to both walls. The discovery of Bendong ductile shear zone and mylonite indicates an important structural event superimposed on Proterozoic Bendong pluton.
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The eastern margin of the Chitradurga schist belt is marked by a NNW-SSE trending sub vertical crustal scale ductile shear zone. The kinematic indicators indicate a predominant sinistral sense of strike-lip movement along the shear zone. Syntectonically emplaced granitic rocks are converted to mylonites and ultramylonites as a result of crystalplastic deformation in the shear zone. In contrast, there are localized zones of brittle failure with attendant functional heat generation exemplified by the development of thin but conspicuous bands and veins of pseudotachylytes, which are emplaced either subparallel with or transgressing the C-planes of the mylonites. From our field and petrographic studies it is interpreted that these two coexisting rock types, namely the mylonite and pseudotachylyte, which are the results of contrasting deformational mechanisms, have generated near synchronously in a progressively developed ductile shear zone. The pseudotachylytes represent the brief interlude of sudden increase in strain rate in an overall ductile regime.
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