STRIKE-SLIP TRANSLATIONS IN EVOLVING OROGENS: THE NORTH AMERICAN CORDILLERA EXAMPLE AND IMPLICATIONS FOR PALEOTECTONIC AND PALEOGEOGRAPHIC ANALYSES

Plate convergence, the driving force of most continental orogenesis, generally occurs at an oblique angle to continental margins. Displacement partitioning resulting from convergence obliquity may lead to a component of margin-parallel strike-slip faulting. Over time, hundreds of kilometers of strike-slip offset may occur and result in substantial margin-parallel translation of outboard crustal fragments (terranes). This process is especially likely where continental margins have a long-lived history of offshore convergence, such as the North American Cordillera. If strike-slip terrane displacements are not taken into account when constructing tectonic and paleogeographic models for the orogen, then substantial errors in interpretation can occur. For example, contractional tectonism in an outboard oceanic arc terrane could be used to argue for an episode of arc-continent collision in that area, but that interpretation would be incorrect if the arc terrane was deformed elsewhere along the margin and only brought to its present location by younger strike-slip faulting. The North American Cordillera is an exceptional example of a continental margin that has experienced abundant, margin-parallel strike-slip displacements, mostly of dextral-sense offset, over a protracted time interval, from at least the late Mesozoic through the Cenozoic. Relations and reconstructions from this system provide insight into processes and consequences with broad relevance for orogenic belts in other parts of the globe.

In this talk, we present palinspastic restorations of the Canadian and U.S. Cordillera that account for major strike-slip offsets of Cretaceous and Cenozoic age (building on Wyld and Wright, 2001; Wyld, Umhoefer, and Wright, 2006; Wright and Wyld, 2007). The resulting reconstructions place many of the Paleozoic and Mesozoic terranes of the Cordillera hundreds of kilometers south of their current locations, with minimum offsets of 700-1200 km since the Early Cretaceous. These displacements are sufficient to translate Cretaceous and older provinces of the Cordillera into positions that are far outside the paleogeographic and paleotectonic realm in which they are currently found. When the offsets are taken into account, linkages between terranes (sedimentary basins, orogenic belts, and magmatic provinces) must be critically re-evaluated, and new interpretations have the potential to solve long outstanding problems and generate exciting new avenues of research. In general, offset analysis in the Cordillera has impacted interpretation of time-space variation in the evolution of basins and orogenic belts, driving forces of sedimentary basin development, sediment provenance, terrane linkages, and tectonic models.

In summary, oblique convergence can lead to margin-parallel, strike-slip terrane transport over distances large enough to render in situ paleotectonic interpretations potentially meaningless. It is thus crucial to account for strike-slip offsets as much as possible when interpreting an orogenic belt. Additional results of our study include: (1) Strike-slip boundaries can be reactivated by younger tectonism or serve as a locus for plutonism, and may thereby become quite obscure. Their presence, however, should be expected in long-lived orogenic belts regardless of whether direct structural evidence is preserved. Many terrane boundaries should thus be considered suspect, particularly when the boundary juxtaposes terranes that can't be readily linked by accordion-style tectonics. Wherever this type of relation exists, it may be fruitful to search along the orogen for other matching terranes or features, and consider what these matches might indicate about cryptic strike-slip displacements. (2) Timing and sense of regional strike-slip displacement can provide important constraints on relative plate motions during development of an orogen, and are a useful compliment to other sources of data (e.g., paleomagnetic) on plate motions. (3) Strike-slip faulting along an active margin can result in a complex distribution of terranes, particularly when multiple fault strands are active and/or when the sense of motion switches over time. In addition, long-lived episodes of strike-slip faulting can result in a deficit of terranes in one part of an orogen and a pile-up of terranes elsewhere.