CRYPTIC STRIKE-SLIP FAULTS AND DISMEMBERED OPHIOLITES - POTENTIAL TRACERS OF THE ALASKA RANGE SUTURE ZONE

Suture zones between continental and oceanic crust are often identified by ophiolites and/or high P/T metamorphic rocks, separating geologic sections with markedly different histories. Based on these criteria alone, no suture zone would be found in the central Alaska Range between the Wrangellia composite terrane, juvenile mafic crust, and the Yukon-Tanana terrane, a composite terrane of continental character. We propose that the cryptic nature of this suture reflects the long evolution of this suture zone, with both strike-slip and convergent deformation occurring over the last ~150 m.y.

Surface geology only constrains the suture in the Alaska Range, between 147° and 150° W, to a zone ~ 130 km wide, with the predominant rock units being Upper Jurassic-Albian clastic rocks overlain unconformably by Upper Cretaceous-early Cenozoic clastic and volcanic rocks. The modern Denali strike-slip fault transects these basins but does not appear to mark the location of the old suture, based on the absence of a significant geologic or magnetic anomaly contrast on either side of the Denali. Numerous thrust faults, both north- and south-vergent, deform the clastic rocks but these faults record a relatively young convergent event, latest Cretaceous to early Cenozoic. Previous workers identified a dismembered ophiolite in the Chulitna block, in the middle of the suture zone, but it would appear to be too old (Triassic) to record initial collision along the Wrangellia composite terrane margin. The Chulitna serpentinite belt, however, is subparallel to and just west of a major topographic low which also contains serpentine, the Broad Pass mélange. This in turn is subparallel to a major magnetic anomaly boundary, between the southern Alaska magnetic trough and deep magnetic high. Surface geology does not record a major structure above this magnetic boundary, suggesting subsequent convergence has buried an original high-angle structure, possibly a cryptic strike-slip fault. All of these fabrics strike NE to NNE, at a significant angle to the modern Denali fault system, and are cut by the latter at the apex of the bend in the Denali. We propose that these belts of serpentinite and the sub-parallel magnetic boundary are the best clues for locating the original suture between continental and oceanic crust in southern Alaska.