(A–C) Topography of the Pitman (A), Heezen (B), and Tharp (C) transform faults (TFs). Topography and earthquakes at Pitman, Heezen, and Tharp transform faults Since these three transform systems are located close to the Euler pole for relative motion between the Pacific and the Antarctic plates, their morphology is especially sensitive to small changes in relative plate motion. The Heezen, Tharp, and Pitman TFs, located at ∼54°S, 56°S, and 65°S along the PAR, have full slip rates of 78.9, 79.0, and 54.0 mm/year, 9, 10, 11 and mega to moderate transform offset lengths of 390, 460, and 80 km ( Figure 1), respectively. 8 The significant changes in the direction of plate motion, diverse spreading rates, and diverse transform offset lengths along the PAR make it an ideal laboratory to study transform dynamics. 7 The spreading direction along the PAR has been gradually changing in a clockwise rotation since ∼12 Ma, with an abrupt change occurring at ∼5.9 Ma. Striking examples of broad transform zones are seen along the Pacific-Antarctic Ridge (PAR), which separates the Pacific and Antarctic plates ( Figure 1), which formed ∼68 mya. 1, 2, 3, 4, 5, 6 These ubiquitous changes in relative plate motion and their induced deformation along and around transform faults (TFs) should be considered to be fundamental processes along ridge-transform systems. In reality, spreading centers frequently experience minor changes in plate motion, changes that can lead to extension or compression along transform plate boundaries. In a simplified steady-state view, a single, narrow strike-slip fault would be sufficient to accommodate the differential motion between the two adjacent mid-ocean ridge segments. The new conceptual model provides a unifying theory to explain the observed broad deformation at global transform systems. We illustrate that such progressive complex deformation is an inherent feature of oceanic TFs. With further extension, these normal faults can develop into off-transform rift zones (ORZs), some of which can further develop into transform plate boundaries. Off-transform normal faults (ONFs) form when across-transform deviatoric extensional stresses exceed the yield strength of the adjacent oceanic lithosphere. We argue that, under across-transform extension due to a change in plate motion, plate deformation occurs first along high-angle transtensional faults (TTFs) within the transform valleys. Here, we propose a new conceptual model for the progressive deformation within broad zones at oceanic TFs through detailed morphological, seismic, and stress analyses. However, broad zones of complex deformation are ubiquitous at TFs. Oceanic transform faults (TFs) are commonly viewed as single, narrow strike-slip seismic faults that offset two mid-ocean ridge segments.
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