4.1. Prism Structure and Influence of the Incoming Oceanic Plate Section
 From these analyses of the Makran margin, the outer prism appears to be relatively structurally homogeneous across and along strike at a regional scale (with the exception of local changes over the Little Murray Ridge). The structures seen across the margin are simple, with a consistent imbricate structure, which may be partly a function of the normal convergence [Kukowski et al., 2001]. There is no evidence for large scale segmentation of the upper plate, changes in fault vergence, or major out-of-sequence thrusts and, with the exception of the region overlying the Little Murray Ridge, fault spacing and taper are very consistent along the margin. The extensive frontal thrust (length ≥250 km) further illustrates the continuity of the prism structures. Comparisons between accretionary margins by McAdoo et al.  highlight the unusual simplicity and along-strike continuity of the Makran structure when compared to other margins such as Cascadia and Nankai. There is however a gradual change in prism characteristics toward the eastern extent of the subduction zone: with increasing proximity to the Murray Ridge and the Eurasian/Arabian/ Indian triple junction, the deformation front begins to rotate to the north and strain becomes more focused in the outermost prism. The subduction of the Little Murray Ridge also causes local changes in fault spacing and reduces the incoming sediment thickness to less than 1 km.
 To investigate how the thick accreted sediment section of the Makran affects two of its structural parameters (taper and fault spacing), the results from this study have been compared with those from other margins with significant accretionary prisms: Lesser Antilles, North and South Hikurangi, Cascadia and Nankai (Table 2). What is evident from this comparison is that, despite its high sediment input, the taper and fault spacing of the Makran are comparable to these other margins, suggesting that the high incoming sediment thickness of the Makran is not generating unusual structural behavior in the offshore prism and is not the main control on structural development. The Makran, with an accreted sediment thickness of ∼6 km, has similar fault spacing to N Cascadia (Washington) where the accreted section is only 2–3 km. However, there does appear to be a potential lower cut-off of <1 km accreted sediment, below which the fault spacing drops to low values (3–4 km) such as in Nankai (Muroto) and possibly North Hikurangi (Table 2). An alternative interpretation for fault spacing in the Makran is proposed by Ding et al. , who suggest that the primary fault spacing is 12 km, and that faults which form within this across-strike span are secondary, out-of-sequence thrust faults. We find no categorical evidence from our data set for such regular out-of-sequence thrusting, or any clear distinction between ‘primary’ and ‘secondary’ thrusts, however uncertainties in the exact sequence of faulting should be noted as a possible source of error in the fault spacing measurements as a function of their spacing at initiation.
Table 2. Comparison of Structural Parameters Calculated for the Makran From This Study, With Published Data From Selected Other Marginsa
| ||Max Accreted Sediment Thickness at Trench (km)||Taper Values (°)||Average Fault Spacing (km)|
|Surface Slope (α)||Basement Dip (β)||Taper (α + β)|
|Makran (this study)||6||2 (avg.)||1.5–2.5 (avg.)||3.5–4.5||6|
|Southern-Central Lesser Antilles (13.5°N) [Bangs et al., 1990]||5||2||1||3||8|
|Hikurangi (South-Central) [Barnes et al., 2010]||2–4||1||3.0||4||5–6|
|North Cascadia (Washington) [Adam et al., 2004]||2||2||1.8||3.8||6|
|Hikurangi (North) [Barker et al., 2009]||0–1||3||8||11||∼3 (frontal 3 thrusts)|
|Nankai (Muroto) [Gulick et al., 2004]||0.7||1.5||1.6||3.1||4 [Ikari and Saffer, 2011]|
 The average prism taper of the Makran, without the effects of the Little Murray Ridge is 4.5°. This classes it as a low taper margin among global subduction zones, comparable to Southern-Central Lesser Antilles, Southern Hikurangi and Cascadia-Washington [Lallemand et al., 1994]. According to Coulomb tapered wedge theory, the low taper of the Makran is likely to indicate either a weak basal surface, or high internal strength [Davis et al., 1983]. However, analogue models have suggested that accretionary prisms which experience very high sediment input may have mechanical differences to classic Coulomb wedges and may be prevented from reaching the critical taper their basal friction would predict, forming sub-critical wedges [Storti and McClay, 1995]. Therefore the Makran may represent an example of a sub-critical wedge which is actively deforming in response to high levels of frontal accretion. This mechanism has also been suggested for the Southern Hikurangi margin, which experiences a similarly low taper (4°) and high rates of frontal accretion [Barnes et al., 2010].
4.2. Fault Activity Analysis
 Fault activity analysis indicates that much of the outer Makran accretionary prism is currently deforming, with strain distributed over a ∼50 km wide region of the prism, and up to 9 imbricate thrusts. Our analysis suggests that a given thrust in this environment will be continuously active until approximately 30 km from the deformation front (3rd–4th thrust), when it will begin to experience periods of inactivity, and then will likely become fully inactive ∼50–60 km from the deformation front. It is not possible here to establish the activity of the onshore portion of the prism, though it has been suggested that active folding and faulting also occurs in this region [Farhoudi and Karig, 1977; Platt et al., 1988]. Our analysis divides the outer prism into three zones across strike, a seaward continuously active zone, a central intermittently active zone and a landward predominantly inactive zone (Figure 14). Detailed studies of fault activity in active accretionary prisms are relatively rare; however published analysis of the Nankai Muroto 3D volume indicates that the majority of prism thrusts retain some level of activity seaward of the out-of-sequence thrust (∼35 km from the deformation front), with increased recent relief on the frontal 5–6 thrusts [Gulick et al., 2004]. As a further example, in the onshore imbricate fault system of Taiwan, GPS data indicate that maximum deformation is concentrated in a frontal zone ∼15 km wide, but that moderate deformation continues across the prism [Chang et al., 2003].
 The thick input section of the Makran, the majority of which is accreted, and the moderate convergence rate (∼4 cm/yr) imply that frontal accretion rates will be high. This may require enhanced deformation across a wider region of the prism relative to other margins. Diffuse fault activity may also be a result of low basal friction: analogue models [Koyi et al., 2000] suggest that fold-thrust belts which form over a low basal friction décollement are more prone to sustained activity over a broad region than those which form over a high basal friction surface. It is also important to consider the scale of the Makran subduction zone. At ∼400 km wide, it is the widest accretionary prism on earth, and therefore may be expected to have a wider active prism toe than narrower prisms if the active prism occupies a certain fraction of its total width. The division of the accretionary prism into three discrete zones of activity is similar to what is produced by analogue models [e.g., Hardy et al., 1998; Lohrmann et al., 2003]. The intermittent activity of the central zone thrusts may be in response to changes in accretion or sedimentation rate, for example an increase in the rate of frontal accretion may trigger increased deformation as the prism deforms to reach/maintain its critical taper [Lohrmann et al., 2003]. The reduction in fault activity in the east of the study area may be due to increased sediment blanketing (as observed from our MCS data), which has been suggested by analogue models to inhibit fault reactivation through increasing the normal stress on the fault [Storti and McClay, 1995; Hardy et al., 1998; Bigi et al., 2010]. The proximity of the Murray Ridge triple junction and onshore strike-slip systems may also act to reduce fault activity in the east.
4.3. Response of the Décollement to Subducting Basement Topography
 The position of the décollement affects many aspects of subduction zone dynamics [Moore, 1989; von Huene and Scholl, 1991; Saffer and Bekins, 2006]. The proportion of sediment subducted is important for global sediment budgets while the stratigraphic position of the décollement will affect its shear strength, and hence its development and seismogenic behavior. For the majority of the E. Makran Subduction Zone, the décollement is located within the sediment section, with along-strike variations in the thickness of underthrust sediment as shown in Figure 10. In the central section where the Little Murray Ridge intersects with the deformation front, the décollement forms at the basement-sediment interface, accreting the entire (albeit thinner) sediment section. As a result, 100% of the incoming sediment is accreted in this region. The same phenomenon has been observed on the Central Hikurangi margin where the décollement position changes from within the sediment to the basement-sediment interface where a seamount is being subducted [Barnes et al., 2010; Bell et al., 2010].
4.4. Implications for Décollement Properties and Prism Mechanics
 The Makran can be characterized as a fluid and sediment rich margin, supported by its low prism taper (4.5°), high sediment input (up to 7 km), and evidence for fluids. The identification of a BSR, fluid seeps, bubble plumes and regions of high backscatter in side scan sonar data (likely seafloor carbonate crusts at cold seep sites) across the prism suggest that although dewatering must be significant due to compaction in the input section and accretion in the outermost prism, significant fluid and fluid flux persist throughout the prism [Spiess et al., 2007; Ding et al., 2010]. These fluids also form mud volcanoes both offshore and onshore, suggesting a degree of overpressure [Schlüter et al., 2002]. Velocity-porosity conversions along the Cam-30 Profile [Fruehn et al., 1997] showed no significant evidence for overpressure in the incoming section, but increased porosity and slight inferred overpressure at depth landward of the deformation front. Similar conclusions were reached by Fowler et al.  who noted from sonobuoy data that a high degree of tectonic consolidation occurs at the prism toe, where the incoming section is initially deformed. The normally compacted nature of the incoming sediment section [Fruehn et al., 1997] may be due to the presence of permeable layers in the sediment which facilitate vertical fluid loss and compaction, enhanced by the presence of widespread normal faults. It is important to note that overpressure measurements from velocity models are for the bulk sediment volume, and will therefore not resolve localized overpressure variations at a discrete horizon (e.g., the décollement).
 The potential effects of a thick sediment section, such as in the Makran Subduction Zone, on pore pressures are debated. On the one hand, thicker sediment is likely to suggest a higher sedimentation rate, which could increase pore pressures if sedimentation rates prevent sufficient fluid expulsion, alongside reduced vertical permeability due to increased drainage length [Saffer and Bekins, 2006]. Conversely high sedimentation may indicate a higher proportion of coarser sediment, with likely increases in overall permeability (although not necessarily vertically) and hence reduced pore pressure. It is likely that reduced vertical permeability and insufficient fluid expulsion dominates, as margins with thick incoming sections (e.g., Southern Lesser Antilles, South Hikurangi) tend to be characterized by low taper angles indicative of high basal fluid pressures [Dahlen et al., 1984].
 Low taper, fluid-rich subduction zone forearcs are often inferred to have weak basal surfaces, commonly observed as high-amplitude, negative polarity décollement reflectors in seismic data suggesting reduced bulk density, increased porosity and likely increased pore fluid pressure [e.g., Moore et al., 1995; Zhao et al., 2000; Bangs et al., 2004]. Prism fault geometry analysis of Kukowski et al.  suggest that the décollement in the Makran is >3 times weaker than the overlying prism sediments, however they also note that the slight observed overpressure [Fruehn et al., 1997] may only account for ∼30% of the inferred décollement weakness, with the remainder a function of lithology or fracturing. The observed overpressure may also be distributed through the lower sediment section, rather than being concentrated at a particular interface (such as the décollement).
 The décollement in the E Makran appears to be unreflective in seismic reflection data across the study area, which is unexpected considering other prism characteristics which may suggest a weak décollement. Kopp et al.  point out a high amplitude horizon deep in the input section in the west of the study area on one seismic profile but this is a localized feature and there is no clear indication that this represents the pre-décollement horizon. High amplitude, negative polarity reflectors may either be indicative of a discrete décollement horizon with specific properties, or be due to an impedance contrast between the accreted and underthrust sediment, and ordinarily these two cannot be distinguished. On the Muroto Nankai transect fluid overpressure and associated reduction in effective stress in the underthrust section were observed from drilling and direct sampling up to 20 km landward of the trench, in a region of high décollement reflectivity in seismic data [Tobin and Saffer, 2009].
 The high sediment velocities at the base of the Makran sedimentary section [Fruehn et al., 1997; Kopp et al., 2000] may imply that there is an insufficient velocity contrast between the accreted and underthrust sediment to form a coherent reflector. The Makran has the thickest accreted sediment section of any accretionary margin, and it is therefore difficult to compare with other localities in this respect, however, seismic reflection data from the Southern Lesser Antilles accretionary prism (where the incoming section is ∼4 km thick) also show little velocity contrast between accreted and underthrust sediment, though the reflectivity of the décollement here is unclear [Bangs et al., 1990]. However, data from the Sumatran margin show similarly high (>4 km/s) velocities to the Makran in the lower sediment section, and do resolve a high amplitude, negative polarity basal surface [Dean et al., 2010].
 The Makran therefore represents an intriguing contradiction between evidence for overpressure, and a weak basal surface, with an unreflective décollement. These observations might be explained by a weak décollement which is either fluid rich, but not producing a reflector in the MCS data, or not fluid rich, but weak due to other sediment properties which also do not produce a significant impedance contrast. However, these explanations do not truly provide an adequate explanation of the seismic properties observed when compared with other margins and data sets. These results may instead be explained by a sub-critical taper which therefore may not reflect basal strength, and distributed overpressure through the thick accreted sediment section, rather than at a discrete horizon. In this scenario the décollement in the Makran may be stronger than expected.
4.5. Implications for Seismogenic Potential
 The high velocities (<4.4 km/s) observed in the deepest sediments of the Makran accretionary prism may indicate that the sediments at décollement level are sufficiently consolidated to support seismic rupture. These sediment velocities are comparable to those seen at depth in the southern December 2004 Sumatra rupture area [Dean et al., 2010] where shallow/seaward rupture beneath the prism has been suggested. As discussed above, the basal surface itself may also be stronger than expected, therefore increasing the potential for seismogenic rupture beneath the outer forearc, with implications for tsunamigenesis. Our MCS data show that the structures seen in this portion of the Makran are fairly homogenous along strike and show little evidence for large scale segmentation (although the basement topography of the subducting Little Murray Ridge may affect the outermost prism). Integrating all evidence, it therefore seems reasonable to suggest that the low level of seismicity currently seen in the Makran could be a reflection of a long recurrence interval of plate interface earthquakes, and short historical record, rather than simply due to aseismic subduction, and that there may be a significant seismogenic potential. The occurrence of the 1945 and 1947 earthquakes, and reports of previous historical events [Byrne et al., 1992] emphasizes this point. This discussion illustrates how an increased knowledge of the Makran though a systematic, spatially extensive investigation of prism structure, can help to elucidate information regarding seismogenic potential.