Where are the missing faults in translated terranes?

Abstract

A central dilemma in understanding terranes that have translated long distances is that modern plate tectonics and paleomagnetic studies indicate translations of thousands of kilometers, while the faults along which the terranes moved these great distances are not found. Relations long known from geology and from recent studies of modern transform-plate boundaries suggest that preserving strike-slip faults that record 1000-km-scale translations is unlikely, and therefore, in many cases, paleomagnetism may be our only reliable indicator of how far terranes translated. Four reasons are explored here for why large-offset strike-slip faults are likely to be ‘missing’ in translated terranes. (1) Strike-slip faults in continents commonly are partially or wholly destroyed, or reactivated. Only ∼400–500 km of over 1000 km of offset on Cretaceous–early Tertiary strike-slip faults in northern British Columbia are accounted for in southern British Columbia. The missing strike-slip faults probably continued south into the hinterland of the Rocky Mountain thrust belt as oblique-thrust faults, much like the modern Alpine–Marlborough fault system of New Zealand. The major strike-slip faults in southeastern British Columbia were obliterated by Eocene extensional faulting, uplift, and erosion. (2) Offsets of 1000 km scale are likely to occur on many anastomosing faults with offsets of 100 km scale, many of which are ‘left behind’ and not found in the translated terrane. The greater San Andreas plate boundary has accumulated ∼1200 km of transform motion that is distributed on numerous faults from the offshore borderland to eastern California. There is a narrow ‘terrane’ in the offshore borderland of California that has moved ∼1200 km relative to North America, but no single fault has nearly that much offset. (3) Major faults in narrow oceans formed by oblique-divergent plate boundaries have a low potential for preservation. For example, the deep, dense oceanic crust of the Gulf of California, where the major transform faults of the oblique rift lie, has a low potential for preservation. In contrast, Baja California is currently a continental terrane that has translated ∼300 km relative to North America along the plate boundary in the Gulf of California. Because of the relative buoyancy of continental crust, Baja California is likely to be preserved in any future accretion event. (4) Faults in the margins of oblique-divergent plate boundaries are dominantly normal faults and not strike-slip faults. There is little strike-slip faulting in central Baja California, even though the plate boundary has a rift angle ( α) of only 20°. Recent field data and modeling show that at α=0–20° strike-slip faults will dominate the secondary faults. Surprisingly, at α=∼20 to ∼35°, there are both strike-slip and normal faults, and at α>∼35–45°, there are few or no strike-slip faults, even though the relative motion of these types of oblique rifts is dominated by transcurrent motion over extensional motion.