5.1. Implications for the Timing and Longevity of Detachment Faulting at TAG
 Based on a near-bottom magnetic study,Tivey et al. proposed that the TAG ODF is an incipient feature that has been active since ∼0.35 Ma, in which case exhumed footwall rocks should only be present within a ∼4-km-wide region just east of the TAG active hydrothermal mound. In contrast,Smith et al.  and Schouten et al.  propose that detachment faulting has been occurring on the east flank of the TAG segment for at least the last 2 My based on the morphological characteristics of a series of lineated ridges on the eastern ridge flank interpreted to be rafted hanging wall blocks. Our new 3D velocity model of the TAG segment is more consistent with the hypothesis of Smith et al.  because we find that the asymmetric velocity structure characterized by high P-wave velocity in the eastern flank extends at least 15 km east of the present spreading axis (Figure 5). (We consider that the spreading axis is defined by the neovolcanic zones, which are located at X = −4 km for Y = 0 km, and that good model resolution at Y = 0 km is limited to the region where X < 10 km, Figure 6). If we assume symmetric spreading and a total spreading rate of 22 km/Myr [Tivey et al., 2003], our results suggest that detachment faulting has been active at TAG at least for the last ∼1.35 Myr. However, detachment faulting may be associated with asymmetric accretion [e.g., Okino et al., 2004; MacLeod et al., 2009], accommodating perhaps up to 80% of the plate separation [Baines et al., 2008], such that our estimate may represent an upper bound. Furthermore, we note that our data and models cannot discern whether extension has occurred on a single long-lived detachment fault as opposed to a series of consecutive, shorter-lived, detachment faults.
5.2. New Constraints on the Geometry of the TAG Detachment Fault
 The reflections we observed in some of the STAG record sections (Figures 9 and 10) are generated by impedance contrasts in the shallow crust, but the reflections themselves are not diagnostic of the source zone lithologies. The two most geologically probable reflectors are a low-angle fault zone (as per our synthetic models) and the transition from extrusive to intrusive basalt lithologies (i.e., seismic layer 2A/2B interface). Reflections from the seismic layer 2A/2B interface have been observed at other MAR segments, and the timing of these arrivals (e.g., 0.2–0.4 s below the seafloor at segment OH-1 near 35°N [Hussenoeder et al., 2002] and ∼0.4–0.6 s bsf at the Lucky Strike segment [Seher et al., 2010a]), are consistent with the reflector travel-times observed in the STAG data set (Figures 9 and 10). Therefore we cannot rule out the possibility that the reflections we observe correspond to the seismic 2A/2B boundary. However, given that the TAG segment is known to host an active oceanic detachment fault, and that the observed arrivals are qualitatively similar to those predicted by a model with a low-angle fault in the shallow crust (Figure 9b), we prefer the low-angle fault zone model for our observations.
 If our interpretation is correct, then the observation of shallow reflected arrivals at OBS D36 (Figure 10) suggests that the shallow portion of the fault may extend farther east than previously supposed. This supports the idea that the linear ridges forming the east flank of the TAG segment are tilted drafted blocks that cap the TAG OCC [Smith et al., 2008], a style of detachment faulting that is proposed to occur preferentially near segment centers [Reston and Ranero, 2011], as it does here. We can use the timing of zero-offset (i.e., when the air gun source is directly above an OBS on the seafloor) reflected arrivals to constrain the sub-seafloor depth of the source reflector beneath OBS D36. This instrument shows zero-offset reflections ∼90 ms behind the direct water wave (Figure 10), corresponding to a source depth of ∼150 m bsf (assuming shallow crustal velocities of ∼2400 m/s). This estimate is likely a maximum value because shallow crustal velocities tend to be overestimated in wide-angle velocity models. We do not consider the data quality for the reflections observed on OBSs D21, D35, and D62 to be sufficient to allow for arrival time picking, but it is nevertheless clear that the data trend toward later arrival times moving west across the profile, which also supports the idea of a westward dipping fault interface.
5.3. Footwall Structure and Hydrothermal Convection
 Early studies hypothesized that heat extracted from cooling hot but solidified mid-crustal plutonic intrusions is the main driver of high-temperature hydrothermal convention at the TAG segment [Kong et al., 1992; Rona et al., 1993; Tivey et al., 2003] (although energy and chemical balances seem to require heat from crystallization of a melt reservoir [Humphris and Cann, 2000]). These ideas were modified in light of the results from the STAG experiment, wherein microseismicity was found to extend deep into the crust/uppermost mantle (>7 km) [deMartin et al., 2007], and no evidence for a mid-crustal magma chamber (i.e., low-velocity anomaly) beneath the valley floor neovolcanic zones was found in the seismic refraction data [Canales et al., 2007]. These results led to the hypothesis that the heat driving hydrothermal convection is sourced out of a magma body at the root zone of the detachment fault, in the lower crust/upper mantle.
 Our 3D tomographic velocity model contains a relatively low velocity anomaly (6.2–6.5 km/s) embedded within the high-velocity body forming the footwall of the detachment fault (Figures 5, 6, and 7). This feature extends from 1.5 km bsf to at least 4 km bsf (Figures 6 and 7). The low-velocity feature was recognized byCanales et al.  in their 2D model of Profile 1, but the new 3D model provides better constraints on its location and lateral extent. The velocity anomaly is centered beneath the active TAG hydrothermal mound (Figure 6), and is surrounded by high-velocity zones whereP-waves travel at 7.0–7.2 km/s (Figures 5c, 6, and 7). At greater depths, the low velocity anomaly becomes wider, about 8 km in diameter, eventually occupying most of the footwall at 4 km bsf (Figure 6), but its lowest value remains centered beneath the TAG hydrothermal mound ([X, Y] = [0, 0] km). Relatively low velocities are also present outside of the detachment footwall (e.g., along the valley floor, X < −4 km, and Y < 10 km, Figure 6), indicating that some caution is required when interpreting the low-velocity feature in terms of processes associated with the detachment footwall. However, a negative velocity gradient (i.e., velocity inversion,Figure 7) at a depth of ∼1.75 km bsf overlies most of the low-velocity region, and it is spatially restricted to central and southern parts of the footwall (Figure 8). The region of velocity inversion is located beneath the TAG active hydrothermal mound, extending roughly SW-NE across a slightly elongated ∼5 km × 8 km section of the footwall. Our resolution tests indicate that features of this size within this part of the 3D model are resolvable with our data (Figures S3g and S3h). The striking spatial correlation of this feature with the detachment footwall and the location of the active hydrothermal mound lead us to conclude that the velocity inversion and low-velocity anomaly underneath it are features associated with hydrothermal processes at TAG detachment fault system.
 We consider three possible scenarios for the origin of this region of anomalously low velocities: (1) a hot, perhaps partially molten, gabbro pluton intruding the detachment fault footwall; (2) serpentinized mafic/ultramafic rocks, with hydration of the footwall being enhanced by hydrothermal fluid flow; or (3) a highly fissured zone produced by extensional stresses during footwall exhumation.
5.3.1. Thermal/Melt Origin
 Canales et al.  argued against a thermal source for the velocity anomaly based on thermal models of OCC formation [Tucholke et al., 2008; Williams et al., 2006], and the assumption that the observed seismic activity beneath the TAG active hydrothermal mound, which extends below ∼5 km bsf, precludes the occurrence above this depth of temperatures higher than those associated with the brittle-plastic transition. These arguments remain valid, but they do not account for the complicated thermal structure of a detachment footwall that is likely to arise from the interplay of complex hydrothermal flow paths along the fault and within the footwall [McCaig and Harris, 2012; McCaig et al., 2010], nor do they account for potential time-space variations in the emplacement of magma within the detachment fault system [Olive et al., 2010]. Therefore we cannot rule out the possibility that the velocity anomaly (Figure 8) has a thermal origin possibly representing a hot, partially molten pluton. If this scenario is correct, then it may not be necessary to invoke deep heat extraction and along-fault fluid circulation for the TAG system [deMartin et al., 2007]. A heat source located within the footwall could drive hydrothermal circulation at TAG and promote high-temperature alteration of footwall lithologies, as has been documented at Atlantis Massif OCC [Nozaka and Fryer, 2011].
5.3.2. Lithological Origin
 Geological and geophysical studies have documented the lithological heterogeneity of detachment fault footwalls, with gabbros and serpentinized peridotites being the dominant lithologies [e.g., Blackman et al., 2011; Canales et al., 2008; Dick et al., 2008; Henig et al., 2012; Xu et al., 2009]. The footwall anomaly in our model has P-wave velocities in the range of 6.2–6.5 km/s, which are consistent with 50–60% serpentinized peridotite, while the high-velocity sections of the footwall (7.0–7.2 km/s) are consistent with either gabbro or 25–35% serpentinized peridotite [Miller and Christensen, 1997]. It is thus plausible that the velocity heterogeneity that we observe within the TAG footwall has a lithological origin, with the lower velocities corresponding to altered ultramafics and the higher velocities corresponding to gabbroic and/or less-altered ultramafic rocks. The velocity inversion (Figure 7) could thus represent a shallow gabbroic body overlying serpentinized peridotite. A similar velocity structure and stratigraphic relationship has been proposed for the Kane OCC [Canales, 2010], and is consistent with proposed models of OCC formation by intrusion of a gabbro body into a peridotite host [Ildefonse et al., 2007].
 The arguments against the presence of altered ultramafics lithologies within the TAG footwall come primarily from the lack of observations of ultramafic exposures in the area [Zonenshain et al., 1989], and from the chemistry of hydrothermal plume fluids [e.g., Charlou et al., 1991]. The ratio of Total Dissolved Manganese (TDM) to methane (CH4) concentration in fluids sampled from the water column above the TAG segment indicate that present-day hydrothermal fluids principally interact with basalt as opposed to peridotite [Campbell et al., 1988; Charlou and Donval, 1993]. However, native Ni0 particles recovered from sediments surrounding the TAG hydrothermal field have been interpreted as resulting from the tectonic disturbance of a serpentinized fault zone [Dekov, 2006], and low boron levels in TAG fluids have been attributed to serpentinization in the downgoing convection limb [Palmer, 1996]. In addition, hydrothermal discharge at TAG has been episodic, with numerous periods of sporadic venting from the various mounds located on the detachment footwall over the past ∼140,000 yrs [Lalou et al., 1995, 1993], and we do not know how the fluid chemistry varied over this period. Thus it seems possible that previous episodes of hydrothermal discharge may have involved serpentinization of ultramafic rocks within the detachment fault zone, even though present-day fluids appears to react primarily with basalt.
5.3.3. Fracturing Origin
 The footwall of a detachment fault experiences bending stresses as it is exhumed and rotated to lower-angle dips in the shallow crust [Buck, 1988], and extension of the upper surface of the footwall has the potential to reduce seismic velocities via fracturing. This process would also increase permeability in the upper section of the footwall, which is likely to be exploited by circulating hydrothermal fluids. Fracturing of the footwall is thus a plausible hypothesis for the low velocity zone, but fracturing should be most intense in the region where the footwall is rotating, which is 2–3 km west of the active mound [deMartin et al., 2007], and thus not co-located with the region of reduced velocities in our model. Also, this hypothesis would predict that bending-related fractures extend all the way up to the seafloor, which seems inconsistent with the velocity inversion (Figure 7).
 In summary, we cannot rule out any of the three potential mechanisms (i.e., thermal, serpentinization, fracturing) on the basis of the extant data, and a combination of any/all of them is also possible given the causal relationships between heat, permeability, and hydrothermal circulation. Regardless of the process responsible for generating the footwall velocity anomaly in our 3D model, it seems likely that the conceptual model of deMartin et al.  and its generalization and broader implications put forth in McCaig et al.  will need to be revised to include additional complexity in the detachment footwall. Additional research will be required to determine the mechanism(s) that have created the seismic velocity structure of the detachment footwall, and this is an important topic for future work at TAG.
5.4. Comparison With Other MAR Segments
 We compare our 3D velocity model for the TAG segment with results from other similar and contrasting MAR segments where seismic studies of comparable scale and resolution have been conducted: at 22°19′N where an OCC forms the western MAR flank [Dannowski et al., 2010], and the magmatically robust segments OH-1 at 35°N [Magde et al., 2000; Dunn et al., 2005] and Lucky Strike at 37°20′N [Seher et al., 2010b].
5.4.1. The OCC at the MAR 22°19′N
 The MAR segment between 22°05–25′N is characterized by a dome-shaped massif located ∼10 km off-axis forming the western valley wall, interpreted as an OCC. Here a 2D wide-angle seismic profile across the OCC and MAR valley shows a pronounced structural asymmetry in the upper 3–4 km of the lithosphere, with the west flank and OCC having higher seismic velocities than the eastern flank [Dannowski et al., 2010]. The structural asymmetry is very similar to that found across the TAG segment, indicating that at both locations lithospheric structure is strongly controlled by uplift of deep lithological units and OCC formation. Perhaps the main difference is that the OCC at 22°19′N is more developed and probably more mature than the TAG OCC, although whether or not detachment faulting at 22°19′N remains active is unclear [Dannowski et al., 2010]. Dannowski et al.  reported observations of PmP reflections from the Moho beneath the OCC, which they interpreted as indicating continuous magmatic accretion during OCC formation. In our TAG data set we do not observed PmP arrivals from beneath the footwall, perhaps because the impedance contrast at the Moho beneath the TAG OCC may be less developed than at 22°19′N because of the TAG OCC is younger and not fully formed yet.
 Crustal thickness differences between the 22°19′N OCC and its conjugate flank were interpreted as resulting from slip along the fault and OCC rotation [Dannowski et al., 2010] and not necessarily from asymmetric crustal accretion as has been proposed for other OCCs [e.g., Baines et al., 2008]. Lack of PmP observations in our TAG data set and insufficient data coverage on the conjugate western flank prevent us from quantifying the amount of crustal asymmetry at TAG and its origin.
5.4.2. Magmatically Robust Segments
 The Lucky Strike segment, located just south of the Azores hot spot, has a well-developed axial valley and rift faults characteristic of the MAR, but it hosts an unusually large central volcano, indicating a robust magma supply [e.g.,Langmuir et al., 1997]. Here, an upper crustal low velocity anomaly extends along most of the segment, and at the segment center a lower crustal velocity anomaly is present beneath the central volcano [Seher et al., 2010b]. Segment OH-1 lacks a well-developed central volcano as Lucky Strike does, but this segment is also considered to be magmatically robust based on its morphological and geophysical characteristics [Detrick et al., 1995; Hooft et al., 2000]. Upper crustal structure at OH-1 is characterized by an elongated (in the along-axis direction) axial low velocity zone above a center-of-segment, mid-crustal low-velocity zone [Magde et al., 2000]. The axial low velocity zone extends into the uppermost mantle, and it is accompanied by more than 2 km of crustal thickening [Dunn et al., 2005]. The lower and mid-crustal anomalies at both Lucky Strike and OH-1 have been interpreted as resulting from elevated temperatures and possibly some partial melt [Dunn et al., 2005; Seher et al., 2010b], which are thought to arise from focused magmatic accretion at the center of the segment [e.g., Magde and Sparks, 1997]. Anomalies in the upper crust likely represent increased porosity in the axial valley caused by extrusive volcanics and ridge-parallel fractures and fissures [Dunn et al., 2005; Seher et al., 2010b].
 In contrast to these two magmatically robust MAR segments, our model of the crustal structure at TAG does not contain upper and mid-crustal features that could be attributed to focused magmatic accretion and/or upper crustal diking zone. The lowest crustal velocity we find at 4 km bsf is 6.5 km/s (near X = −8 km, Y = −5 km,Figure 5c). This value is comparable to the lowest velocity reported at similar depth in OH-1 (6.4 km/s [Magde et al., 2000; Dunn et al., 2005]), but it is located beneath the western valley walls, suggesting that it does not correspond to an accretionary center. The upper crust at TAG also lacks the elongated low velocity anomalies of Lucky Strike and OH-1. With the exception of the southernmost neovolcanic zone (X ≈ 0 km, Y ≈ [−13, −21] km), none of the other mapped neovolcanic zones overly a zone of marked low velocity anomalies (Figure 5c). These observations indicate that mid- and upper crustal magmatic accretion at TAG is not focused at the segment center as other magmatically robust MAR segments, but instead is probably more diffuse and distributed along the segment. The absence of a high-amplitude mantle Bouguer gravity anomaly at the TAG segment [Fujimoto et al., 1996] also suggests that accretion is not focused at the segment center at lower crustal or mantle levels. Thus our results suggest that supply of magma to the crust at the TAG segment is more distributed along-axis compared to magmatically robust segments like Lucky Strike and OH-1. It is unknown at this time whether a distributed magma supply is a general feature of MOR segments characterized by detachment faulting, or if this is specific to the TAG segment. Additional studies of the crustal architecture of segments with active detachments will be needed to assess this issue.