The deformation at well-defined, narrow plate boundaries depends on the relative plate motion, but how the deformation takes place within a distributed plate boundary zone remains a conundrum. This was confirmed by the seismological analyses of the 2012 great Wharton Basin earthquakes [moment magnitude (Mw) 8.6], which suggested the rupture of several faults at high angles to one another. Using high-resolution bathymetry and seismic reflection data, we report the discovery of new N294°E-striking shear zones, oblique to the plate fabric. These shear zones are expressed by sets of normal faults striking at N335°E, defining the direction of the principal compressional stress in the region. Also, we have imaged left-lateral strike-slip faults along reactivated N7°E-oriented oceanic fracture zones. The shear zones and the reactivated fracture zones form a conjugate system of faults, which accommodate present-day intraplate deformation in the Wharton Basin.
- Great earthquakes
- Wharton Basin
- Intra-plate deformation
- Active Tectonics
- Indian Ocean
Plate boundaries are the key elements of plate tectonics. There are three types of plate boundaries: convergent, divergent, and transform or strike-slip boundaries. The deformation along these plate boundaries is controlled by the motion between the plates, evidenced by the occurrence of different types of earthquakes. For example, normal faulting earthquakes are generally produced at divergent plate boundaries, whereas dominantly thrust earthquakes occur at convergent boundaries. Therefore, the nature of deformation along plate boundaries is well understood, and the direction of maximum stress is more or less well defined. However, the deformation on plate boundaries sometimes induces stresses within a plate hundreds of kilometers away from the major plate boundaries, creating large-scale distributed deformation zones. One of the largest diffuse deformation zones lies in the equatorial Indian Ocean, extending from the Chagos-Laccadive Ridge in the west to the Investigator Ridge in the east for more than 3000 km (1–7). Near the Chagos-Laccadive Ridge, the deformation is expressed as a north-south extension, whereas in the Central Indian Ocean Basin (4), west of the Ninety East Ridge, the deformation takes place along east-west–trending thrust faults related to the north-south compression, resulting from the continental collision of India with Eurasia (8, 9). To the east of the Ninety East Ridge in the Wharton Basin, the deformation is suggested to take place along the north-south–trending reactivated fracture zones with left-lateral strike-slip motion (10, 11). This is due to both variable obliquity and rate of the subduction of the Indo-Australian composite plate beneath the Sunda plate in the Andaman-Sumatra region, creating a very large scale diffuse deformation zone in the Wharton Basin with poorly defined direction of maximum stress (6, 7, 12, 13). The Indo-Australian composite plate comprises the Indian, Capricorn, Australian, and Macquarie component of plates and multiple diffuse plate boundaries (2–5, 7).
The presence of active deformation in the Wharton Basin was confirmed by the occurrence of a series of strike-slip earthquakes in 2012 starting on 10 January 2012 with a moment magnitude (Mw) 7.2 earthquake, followed by the 11 April 2012 twin events of Mw 8.6 and Mw 8.2 (14–20) (Fig. 1), the largest intraplate earthquakes ever observed on Earth. More recently, an Mw 7.8 strike-slip earthquake occurred on 2 March 2016 (21) further south (~5°S). Although the foreshock of January 10 and the great aftershock of April 11 seem to have ruptured reactivated fracture zones (F6/F7) (22, 23), the seismological and geodetic inversions for the largest event of Mw 8.6 require a complex faulting pattern with several faults, up to six, oblique to each other (14–18), requiring the existence of a new set of faults (Fig. 1 and fig. S1). However, the existing seismic and bathymetric data from the region have not shown the presence of any such faults (22–26).
The lithosphere in this part of the Indian Ocean was created at the Wharton Spreading Centre. Seafloor spreading in the Indian Ocean initiated around 133 Ma, as India separated from Australia, with a spreading rate reaching 40 mm/year at 80 My and increasing to 150 mm/year at 67 My as India started to move rapidly northward (27, 28). As India started indenting Eurasia around 50 Ma, it slowed down to 50 mm/year at 45 My, and spreading ceased at 40 My when India and Australia became a single plate. The spreading ridge fabric (abyssal hill normal faults) is generally east-west, whereas the fracture zones are close to north-south. Present-day left-lateral motion along these reactivated fracture zones in the Wharton Basin is consistent with the increasing obliquity and decreasing convergence rate of the Indo-Australian plate from Java (orthogonal at 60 mm/year) in the east to Sumatra (54 mm/year) in the west, and nearly arc-parallel near the Andaman Islands (43 mm/year) (29, 30), leading to the reactivation of fracture zones capable of hosting great earthquakes.
In May 2015, high-resolution bathymetry and seismic reflection data were acquired on board the Schmidt Ocean Institute (SOI) research vessel (R/V) Falkor in the epicentral region of the 2012 Mw 8.2 earthquake and in the southern edge of the inferred rupture zone of the Mw 8.6 earthquake (see Materials and Methods). The uninterpreted bathymetry image is shown in Fig. 2A, and the interpreted images are shown in Fig. 2 (B and C).
The two most important features on this map are as follows: (i) approximately north-south–oriented reactivated fracture zones (F6 and F7) accommodating left-lateral strike-slip movement and (ii) west-northwest–striking shear zones accommodating right-lateral motion. These fracture zones were previously identified using magnetic anomalies (28) and seismic reflection studies (22, 23). Bathymetry and seismic images show recent deformations associated with these structures. The easternmost fault of the reactivated fracture zone set, F6a, has a fresh and clear bathymetric expression on the seafloor, seems to be the most active, and strikes at N9°E (Figs. 2B and 3A) with some west-side down-dip slip component. This left-lateral fault shows en echelon fault segments and releasing step-overs, in particular a well-developed, rhomb-shaped, pull-apart basin that is 4 km long, 2.5 km wide, and 95 m deep. F6b is parallel to F6a at ~10 km further west. It consists of right-stepping, east-dipping en echelon faults that are 1 to 5 km long. The westward dip of F6a and the eastward dip of F6b, combined with the dominance of F6a, suggest that F6b might be the conjugate of the master fault F6a. The westernmost north-south feature, fault F7b located at 25 km west of F6b, also consists of en echelon faults striking at N6°E (Fig. 2B) with some eastward dip component. The epicenter of 11 April 2012 Mw 8.2 earthquake lies on F7b. These three reactivated fracture zones (F6a, F6b, and F7b) define a ~35-km-wide zone with an average strike of N7°E (Fig. 3A), where the left-lateral motion is localized.
On both east and west sides of the region of reactivated fracture zones, we observe many N294 ±4°–trending right-lateral shear zones (Figs. 2B and 3B) spaced at 5 to 10 km. These shear zones are, on average, ~2.5 km wide and appear as a series of elongated depressions bounded by sets of en echelon normal faults. The vertical offset on the seafloor along these normal faults is on the scale of 4 to 10 m, and their strike is N335 ±3° (Fig. 3C). Several north-south–oriented channels are also present west of the fracture zones. They flow southward, across the shear zones, and their bed is deflected by the presence of depressions within the shear zones. It is interesting to note that the shear zones do not cross the F6-F7 fracture zone system: They are imaged on both sides, but not in the intervening regions. We interpret the N294°-striking right-lateral shear zones and the left-lateral N7°E-striking reactivated fracture zones as conjugate faults along which the deformation is partitioned in the Wharton Basin. The left-lateral strike-slip faults are expressed by releasing step-overs or right-stepping en echelon faults, whereas the right-lateral shear zones are expressed as a set of en echelon normal faults defining the direction of extension in the region. Consequently, the strike of the small N335° normal faults represents the direction of the maximum horizontal compressional stress in the study area (Fig. 3C).
The age of the oceanic lithosphere across these four fracture zones is 52.6 to 54.8 My in the east and 63.1 to 64.2 My in the west (Fig. 2B), and the crustal thickness is 3.5 to 4.5 km in the east and 6 km in the west (22, 23), but the nature of the shear zones is similar on both sides. The strike of spreading-related normal faulting in the Wharton Basin is east-west, but the shear zones are oblique to these preexisting structures, suggesting that the shear zones are new and not reactivated spreading-related normal faults.
The sediment thickness over the normal oceanic crust, away from the fracture zones, is ~2.2 km (Figs. 4 and 5). A 200- to 300-m-thick, poorly reflective layer is imaged just above the basement, bounded at its top by what might be the pelagic/hemipelagic sediment transition, similar to observations reported further north (22, 25). Turbiditic sediments (~2 km) that have likely been deposited after the collision of India with Eurasia about 40 Ma lie above this lowermost unit. These sediments belong to the Nicobar Fan, similar to the Bengal Fan sediments but east of the Ninety East Ridge, carried by the Ganges and Brahmaputra river system, channeled between the Ninety East Ridge and Sunda Trench, and spread over thousands of kilometers (31). If we assume a constant sedimentation rate, then the sedimentation rate would be 50 m/My, rather high for such a distal abyssal plain, but similar to that observed further north (25).
Seismic reflection images document the activity along faults F6 and F7 (Fig. 4), whose different strands can be traced from the seafloor down to the basement. Basement reliefs of 0.6 to 1.7 km, generally containing a basement high (ridge) and low (depression), are imaged in relation to these faults. Seismic reflection data confirm that F6a is the most active fault of the reactivated fracture zone set. In the south, fault F6a is vertical and its trace is located 2 to 3 km east of the basement high. It crosscuts the sediments like a knife (fig. S2B), possibly breaking through the oceanic crust that forms new faults (fig. S2B). Further north, sediment deformation at depth records the development of a pull-apart basin already identified from the bathymetry (fig. S2C). North of the main pull-apart basin, the fault lies close to the basement high (fig. S2D).
Fault F6b is best expressed in the northern part of the study area, consisting of eastward dipping fault segments. It is associated with a basement high flanked by a bathymetric low (fig. S3), and the fault extends at depth through this bathymetric low. Fault F7b is present along the whole north-south extent of the survey area and is associated with a prominent (1.5-km) basement ridge (fig. S4). Another basement ridge with a 1.2-km relief is imaged in seismic data 10 km east of F7b, associated with a fault strand that we call F7a (Fig. 4). F7a is less active, with little surface expression, and F7a, at depth, cuts across the basement high instead of the slope (Fig. 4).
The 294° azimuth shear zones are expressed by flower structures with a set of faults dipping inward with a dip of 65° to 70° (Fig. 5), defining the location of strike-slip faults at depth, with the innermost pair of faults joining at a depth of ~5.6 km (Fig. 5). Some of the faults can be traced down to the basement, which they seem to offset. The number of faults seems to increase below ~5.6 km due to the complex interaction between the normal faulting, the shear zone, and the faults at depth (Fig. 5B). These shear zones are different from the Riedel shears observed by Geersen et al. (25) further north in the outer rise area that might have been influenced by the plate bending, but some of the Riedel shears striking 330° to 360° might be genetically related to the 335°-striking normal faults that we observe in our study. Similarly, faults west of fracture zone F5 (fig. S5) have strikes between 330° and 340°, indicating that they might also have a similar origin.
Although the basement highs and lows along the traces of F6 and F7 might represent the original fracture zone morphology, the sediment deposition patterns indicate a westward flexural rotation of the basement ridges and subsidence in the adjacent lows (fig. S4). Regions of active faulting along the reactivated fracture zones are typically 3 to 4 km wide and contain several en echelon fault segments, which make it difficult to characterize the age and evolution of these faults from limited two-dimensional seismic reflection data (Fig. 4). F6b seems to contain only two major fault branches (F6b1 and F6b2), allowing us to get some idea about the evolution of fault activity over time (Fig. 6). The cumulative vertical offset across the two branches of F6b measured along profile WB03 increases with depth up to ~5.6 km, corresponding to an age of 17.5 My, and then remains constant below this depth, suggesting that the reactivation along these fracture zones started ~17.5 My, similar to that in the Central Indian Ocean (32), coinciding with the uplift of the Himalayas (33). For F6b1, we find two distinct slopes, corresponding to the age variations from 0 to 7 My and 7 to 17.5 My, which could be due to a change in either the sedimentation or the deformation rate. This transition at ~7 My corresponds to the uplift of Tibet (32, 33), although there is still some debate about the timing of the uplift of Tibet (34). A similar pattern is observed along some of the other profiles (fig. S6).
Vertical offsets on the seafloor along the normal faults associated with the shear zones are small (4 to 10 m), but one can clearly observe growth with depth reaching up to 20 to 30 m (fig. S7), with a maximum at a depth of 5.6 km (17.5 My) (fig. S7), which coincides with the sudden increase in the number of faults below this depth (Fig. 5B), suggesting that these shear zones also initiated at the same time.
The conjugate nature of deformation along the reactivated fracture zones (F6/F7) and the N294°-striking shear zones might be able to reconcile the discrepancy between the rupture models of the 2012 Mw 8.6 earthquake and the seafloor observations. The total width of the reactivated fracture zone regions in our study area is 35 km, and the one north of the Mw 8.6 epicenter is ~42 km (23). Also in that area, we observe four faults along reactivated fracture zones with strike varying from 0° to N6°E, suggesting that fracture zones F6 and F7 continue all the way to the subduction front. Furthermore, there is also some evidence of N335°-striking normal faulting in the north on the outer rise at 60 km from the subduction front (fig. S5), suggesting that the deformation along this azimuth is common in the Wharton Basin. The Mw 8.6 earthquake likely ruptured several distinct fault segments, among which at least one was along the north-northeast–south-southwest segment in the vicinity of F6/F7 (Fig. 7). We suggest that the Mw 8.6 earthquake ruptured F6a to F7b in an en echelon form, south stepping and breaking segments of ~75- to 100-km length each (Fig. 7). Because the inversion of seismological and geodetic data record can only use long wavelength signals hundreds and thousands of kilometers away from the rupture location, these data cannot resolve the slip along each segment separately but instead provide an average solution (Fig. 7 and fig. S1). Similarly, the closely spaced N294°-striking shear zone might have ruptured in an en echelon form, westward stepping and breaking in smaller segments. The en echelon rupture along these faults can explain the discrepancies between the seafloor observation and the different rupture models (Fig. 7).
Finite fault models of the 2012 earthquakes suggest that these earthquakes ruptured down to the base of the lithosphere at a depth of 40 to 50 km (14, 16). Qin and Singh (26) have imaged faults down to a depth of 45 km in this area. Because the oceanic lithosphere in this area was formed at a fast spreading environment, the original fracture zone–related faults should have been shallow (~6 km) (35), overlying a thin lithosphere when they formed, indicating that the deep lithospheric faulting in the Wharton Basin is related to the recent intraplate deformation.
The direction of principal compression in the Wharton Basin is poorly constrained, varying from 315° to 350° (7, 12, 13). In our survey area, the strike of the reactivated fracture zones is ~N7 ±2°E, whereas the strike of the shear zones is N294 ±4°E. Hence, the angle between the two principal directions of motion is ~75 ±4.5°, slightly larger than the 60° expected for conjugate shear faults. The strike of the normal faults bounding the shear zones is N335 ±3°E, that is, ~34° from the reactivated fracture zones and 41° from the average orientation of the shear zones, nearly half (37.5 ±4°) of the angle between the two conjugate faults, suggesting that the 335° azimuth is the principal stress direction in the Wharton Basin with an average static friction of 0.26 along conjugate faults (36). However, it could be as small as 0.14 along the reactivated fracture zones and up to ~0.40 along the shear zones. The weakness of the reactivated fracture zones may be due to the accumulation of large co-seismic slip with strong dynamic weakening (37). Experimental investigation of frictional resistance at high slip rates indicates that thermally activated processes may largely reduce fault friction (38–40). This would be possible if the fracture zones repeatedly hosted great earthquakes such as the 2012 sequence. Furthermore, these fracture zones with a large age and crustal thickness contrast (F6/F7 and F8) can act as a boundary for the differential plate motion, enhancing the strain accumulation across them, as was suggested by the Coulomb stress change estimated after the 2004 great megathrust earthquake (41). These effects, combined with the 335° azimuth principal compressive stress, would strongly encourage rupture along the north-south fracture zones, acting as the main zone of deformation in the Wharton Basin and possibly leading to the creation of a new plate boundary in the Indian Ocean. The presence of strike-slip earthquakes along the fracture zone F6/F7 over a length of 1000 km (23) suggests that these fracture zones could be a nascent plate boundary.
MATERIALS AND METHODS
High-resolution bathymetry and seismic reflection data were acquired on board the SOI R/V Falkor. The R/V Falkor was equipped with a multibeam swath bathymetry system with a dominant frequency of 27 kHz (EM 321), leading to a very high resolution image of the seafloor. At a water depth of 4500 m in the study area, the vertical resolution is 3 to 4 m and the lateral resolution is 25 m, providing an unprecedented image of the seafloor. In 1 week, we covered an area of 750 km2 that includes about a 75-km segment of F6/F7 fracture zones and about 50 km of oceanic crust on either side of these fracture zones (Fig. 2A). Coincidently, high-resolution seismic reflection data were acquired at a line spacing of 6 km (Fig. 2A). A two-gun cluster, with a total volume of 8.19 liters, fired at a 25-m interval, was used as the source. The guns were deployed at a water depth of 3 m to generate energy up to 200 Hz. A streamer, containing 96 channels at a group interval of 12.5 m, deployed at a depth of 4.5 m, recorded the seismic energy. The seismic data were processed using conventional processing steps, including swell noise removal, binning, trace editing, velocity analysis, stacking, and post-stack time migration (42). The data were converted to depth using seismic velocity information that was obtained further north (43). After processing, the lateral trace spacing is 6.25 m, and the vertical resolution is 4 m for a dominant frequency of 100 Hz.
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/1/e1601689/DC1
fig. S1. Different models of the 2012 Mw 8.6 earthquake.
fig. S2. Seismic reflection images of F6a.
fig. S3. Seismic reflection images of F6b.
fig. S4. Seismic reflection images of F7b.
fig. S5. Bathymetry in the outer rise region.
fig. S6. Fault offset with age.
fig. S7. Blow-up seismic image of shear zone.
This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.
REFERENCES AND NOTES
- Copyright © 2017, The Authors