Where are the Missing Faults in Far-Traveled Terranes? Lessons from Baja California and the San Andreas Fault

Umhoefer, P J (1)

(1) Geology, Northern Arizona University, Flagstaff, AZ 86011-4099 United States

Abstract:
A central dilemma in understanding far-traveled terranes is that paleomagnetic studies indicate translations of 1000's of kilometers, which are fully compatible with modern plate tectonics, but the faults along which the terranes moved these great distances are not found. It has been suggested that missing faults means that the paleomagnetic data are being interpreted wrongly. But relations long known from geology and from recent studies of modern analogues suggest that preserving faults that record 1000-km-scale translations is unlikely and paleomagnetism may be our only reliable indicator of how far terranes move (a point made long ago by Myrl Beck). Important reasons why large-offset faults are likely to be "missing" in terranes are that (1) faults in continents commonly are partially or wholly destroyed, (2) 1000-km-scale offsets are likely to occur on many 100-km-scale faults, many of which are "left behind" by fault jumps and not found in the translated terrane, (3) major faults in oblique-rift oceans have a low potential for preservation, and (4) faults in the margins of oblique rifts are dominantly normal faults and not strike-slip faults.

Each of these reasons has a pertinent example in British Columbia, the San Andreas fault system, or Gulf of California oblique rift. (i) Only $\sim$100 km of over 1000 km of offset on Cretaceous - early Tertiary strike-slip faults in northern British Columbia can be accounted for in southern British Columbia within the Rocky Mountain thrust belt. The largely missing strike-slip faults probably continued south into the hinterland of the thrust belt and then were obliterated by Eocene exten-sional faulting. (ii) The greater San Andreas plate boundary has accumulated $\sim$1200 km of transform motion that is distributed on numerous faults from the offshore borderland to eastern California. There is likely a narrow "terrane" off of California that has moved 1200 km relative to North America, but no single fault has nearly that much offset. (iii and iv) Baja California is a continental block that is likely to be preserved in any future accretion. In contrast, the dense oceanic crust of the Gulf of California with transform faults has little preservation potential. There is little strike-slip faulting in central Baja California even though the plate boundary has a rift angle ($\alpha$) of only $20\deg$. Recent field data and modeling show that at $\alpha$ = $0\deg$ to $20\deg$ strike-slip faults will dominate the secondary faults, at $\alpha$ = $\sim$$20\deg$ to $\sim$$35\deg$ there are both strike-slip and normal faults, and at $\alpha$ $>$$\sim$$35\deg$ there are few or no strike-slip faults.

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