A small-scale illustration of the principle of transform faulting
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GEOSITES
A small‑scale illustration of the principle of transform faulting
In 1965, J. Tuzo Wilson introduced a ‘new class of faults’, that connected different segments of mid-ocean ridges, arcs and mountain belts in a new way. Faults with large horizontal movements like the San Andreas but also oceanic fracture zones were known before from the Pacific, the Atlantic and the Red Sea; however, ‘the fact that they appeared to end abruptly was puzzling’. The new faults, termed transform faults by Wilson (1965), did not displace pre-existing structures like the usual strike-slip or trans-current faults but were acting simultaneously with extension or contraction to create new kinematic boundaries. Wilson (1965, p. 343) proposed that ‘these features are not isolated, but connected into a continuous network of mobile belts about the Earth which divide the surface into several large rigid plates’. The new faults, as conceived by Wilson (1965), connect zones of extension (rifts or spreading zones) with each other or with zones of contraction (island arcs, active margins, collision zones) as well as zones of contraction with each other (Wilson 1965, Figs. 3–4). Typically, transform faults do not affect the interior of crustal blocks or plates, but are restricted to the plate boundaries. In particular, Wilson’s (1965) explanation of the Atlantic fracture zones as transform faults not dissecting the ridge, but transferring movement from one segment of the ridge to the next one (Fig. 1) explained convincingly the kinematics of the opening of the Atlantic Ocean. His concept was the first formulation of plate tectonics. Although Wilson’s concept was associated with lithospheric plates, transform faults occur at various scales and materials, including lava lakes (Duffield 1972) or floating ice (Vogt et al. 1977). Figure 2 shows a small-scale example of a transform fault occurring in the Macchia Vecchia Formation, a Lower to Middle Liassic tectono-sedimentary breccia related to rifting in the future southern margin of the
Alpine Tethys. During the latest Triassic to Early Jurassic, the continental crust of the western Southern Alps underwent several phases of rifting and a number of asymmetric basins and submarine highs evolved (Bernoulli 1964; Bernoulli et al. 1990; Bertotti et al. 1993; Berra et al. 2009; Fig. 3). On one of these highs, the Lugano High, the pre-rift sediments are cut by NW- or NNE-trending narrow grabens, ranging in width a few to 150 m across. These fault zones and crevices were filled by submarine cements, sediments and complex polyphase breccias, called Macchia Vecchia. The breccias show up to six phases of in-filled carbonate sediments (Wiedenmayer 1963), whereby the older phases occur as clasts of the host formations, the younger ones occur as matrix, forming a complex network of discordant sedimentary ‘neptunian’ dikes and sills intruded from above and cutting across older clasts and dykes (Fig. 2). There is a continuum of breccias in which fractured, cyclic units of the Dolomia principale can be followe
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