Abstract
Large-magnitude intraplate earthquakes within the ocean basins are not well understood. The Mw 8.6 and Mw 8.2 strike-slip intraplate earthquakes on 11 April 2012, while clearly occurring in the equatorial Indian Ocean diffuse plate boundary zone, are a case in point, with disagreement on the nature of the focal mechanisms and the faults that ruptured. We use bathymetric and seismic reflection data from the rupture area of the earthquakes in the northern Wharton Basin to demonstrate pervasive brittle deformation between the Ninetyeast Ridge and the Sunda subduction zone. In addition to evidence of recent strike-slip deformation along approximately north-south–trending fossil fracture zones, we identify a new type of deformation structure in the Indian Ocean: conjugate Riedel shears limited to the sediment section and oriented oblique to the north-south fracture zones. The Riedel shears developed in the Miocene, at a similar time to the onset of diffuse deformation in the central Indian Ocean. However, left-lateral strike-slip reactivation of existing fracture zones started earlier, in the Paleocene to early Eocene, and compartmentalizes the Wharton Basin. Modeled rupture during the 11 April 2012 intraplate earthquakes is consistent with the location of two reactivated, closely spaced, approximately north-south–trending fracture zones. However, we find no evidence for WNW-ESE–trending faults in the shallow crust, which is at variance with most of the earthquake fault models.
Highlights
The breaking and fracturing of the Indo-Australian plate is a spectacular example of an active diffuse plate boundary within the ocean basins
In addition to evidence of recent strike-slip deformation along approximately north-south–trending fossil fracture zones, we identify a new type of deformation structure in the Indian Ocean: conjugate Riedel shears limited to the sediment section and oriented oblique to the north-south fracture zones
The Riedel shears developed in the Miocene, at a similar time to the onset of diffuse deformation in the central Indian Ocean
Summary
The breaking and fracturing of the Indo-Australian plate is a spectacular example of an active diffuse plate boundary within the ocean basins. 36.5 Ma and 83 Ma (Fig. 1A; Deplus et al, 1998; Carton et al, 2014; Jacob et al, 2014) These fracture zones recently attracted attention due to the 11 April 2012 Mw 8.6 and Mw 8.2 strike-slip intraplate earthquakes, which seem to have been promoted by stress transfer following the A.D. 2004 and 2005 Sunda megathrust events (Delescluse et al, 2012). These data provide an approximate chronology for the stratigraphy that we believe is sufficient to resolve relative fault activity but their absolute slip history. F: Model of strain for the Wharton Basin with spatial distribution pattern of Riedel shears and north-south fracture zones
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