COULD LAURENTIA'S DOUBLE-WIDE, WESTERN MIOGEOCLINE BE A LEFT-LATERAL REPEAT OF ITSELF?

An integral part of Laurentia’s western Cordillera is a pair of parallel, west-facing, Neoproterozoic - Cambrian carbonate platforms separated by a shale basin, the inner platform always younger. Although inconsistent with how rifted margins are thought to form, the prevailing view is that prolonged rifting culminating with two distinct rift-drift transitions created the pair. Since the outer platform’s Paleozoic fauna is so different from the inner, some have reasonably suggested that the outer platform, and the thick siliciclastic wedge that underlies it, is exotic.

Devonian sinistral deformation documented along Laurentia's northern margin combined with growing evidence that the rift-drift transition in both carbonate platforms gets younger to the south, begs a question. Could a 6,000 km-long, Paleozoic, sinistral strike-slip fault in the shale basin account for the faunal contrasts, and double-wide nature of the miogeocline? The south end of the ~530 Ma outer platform near Death Valley, California would hypothetically be the northern continuation of the inner platform of the Mackenzie Mountains,Yukon. The margin, before sinistral faulting, would have extended at least 10,000 km between the oldest (~560 Ma) Cassiar Mountains section of the outer platform, and the youngest (~500 Ma) section of the inner platform of Arizona.

Laurentia's Pacific margin is, and has been the site of diachronous rifting, collision, and strike-slip faulting for some time. Large amounts of Mesozoic and younger dextral displacements have been proposed, among other things, to account for the faunal juxtapositions of the outer and inner platforms. Sinistral displacement during the Paleozoic, if large enough, might accommodate the same thing, and offer a more feasible explanation for why the south end of the outer platform's Ordovician Eureka Quarztite has such a distinctive northern Canadian shield (Wopmay orogen) detrital zircon provenance.

Considering that the Cordillera has enjoyed multiple phases of deformation since the Mesozoic, the trace of any Paleozoic, margin-parallel, strike-slip fault would be, predictably, highly disjunct. Faults of this ilk are known, but their lack of continuity make them hard to interpret relative to each other, and to the patchwork of upper Paleozoic orogenic belts that populate the same areas.