Newly identified strike-slip plate boundary in the northeastern Arabian Sea

The first regional swath-bathymetry survey of the Makran accretionary wedge revealed a sinistral strike-slip fault, named herein the Some fault, obliquely crossing the wedge and continuing into the abyssal plane. This fault separates the western part of the Makran subduction zone where plate boundary events are absent from the eastern part that does show plate boundary seismicity; most events are concentrated along the Some fault. Little Murray Ridge (a basement high) and related magnetic anomalies are offset along the Some fault. Together, these observations identify the newly discovered Some strike-slip fault as a plate boundary that has been active similar to 2 m.y. This finding suggests that what has been considered the northeasternmost part of the Arabian plate is actually a separate microplate, named herein the Ormara plate, the formation of which resulted from tearing of the Arabian plate along the Some fault. With this concept, the different dips of the downgoing plate below the western and eastern parts of the Makran margin and the related different distances between the trench and Quaternary are volcanic centers can be unequivocally explained.