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Duplex kinematics reduces both frontal advance and seismic moment deficit in the Himalaya
Geology ( IF 5.8 ) Pub Date : 2022-10-01 , DOI: 10.1130/g50229.1 Wan-Lin Hu 1 , Victoria L. Stevens 2, 3
Geology ( IF 5.8 ) Pub Date : 2022-10-01 , DOI: 10.1130/g50229.1 Wan-Lin Hu 1 , Victoria L. Stevens 2, 3
Affiliation
Duplexing plays important roles in the evolution of fold-and-thrust belts and accretionary wedges, and causes internal shortening of the system, which then impacts both rates of frontal advance and seismic-moment deficit. Nevertheless, the significance of this internal shortening has not yet been highlighted in previous studies in the Himalaya or elsewhere. We invoke geometric solutions to constrain the ratio of transferred slip (R; i.e., the ratio of updip slip to downdip slip) for the midcrustal ramp—the most active ramp within the midcrustal duplex—in the Himalayan wedge. We find that R is ~0.9, and then used this ratio to calculate the accumulating seismic moment. The reduction in seismic-moment accumulation over the past 1000 yr along the entire Himalayan arc (~2200 km) is equivalent to at least one ~Mw 8.72 earthquake, and potentially reduces the seismic moment deficit by ~23%–54%, which may reconcile the long-term unbalanced seismic moment in the Himalaya.The megathrust underlying the Himalayan orogen stretches more than 2000 km along strike, accommodating convergence of the Eurasian and Indian plates. This megathrust can generate great (Mw >8) earthquakes, which threaten millions of inhabitants living above the shallow décollement, the Main Himalayan thrust (MHT; Fig. 1A). The MHT is a gently northward-dipping décollement, with a flat–ramp–flat geometry, and forms the main seismogenic structure in the Himalayan wedge. It reaches the surface along the Main Frontal thrust (MFT), the most active strand of the MHT in the Holocene (Lavé and Avouac, 2000). The upper décollement, stretching roughly ~100 km northward from the surface trace of the MFT, is connected by a midcrustal ramp to the lower décollement beneath the High Himalayan Range and Tibet. This ramp corresponds to the thermally controlled brittle-ductile (kinematically locked-creeping) transition zone highlighted by a band of microseismicity, showing an area of high stress buildup; the midcrustal ramp also corresponds to local anomalies in erosion and thermochronometric ages at the surface, where it has been suggested to be associated with a midcrustal duplex below (Fig. 1B) (e.g., Bilham et al., 1997; Bollinger et al., 2006; Ader et al., 2012; Grandin et al., 2012; Avouac, 2015; Bilham, 2019; Ghoshal et al., 2020; Johnston et al., 2020). The Mw 7.8 2015 CE Gorkha (Nepal) earthquake highlighted the seismic hazard in the Himalaya and revealed a more accurate picture of the geometry of the MHT (e.g., Elliott et al., 2016; Wang et al., 2017), enabling us to perform detailed calculations in this study.Decadal geodetic shortening rates across the Himalayan wedge are commonly used to estimate the seismic-moment accumulation rate. A comparison with instrumental and historical catalogs of earthquakes and paleoseismological observations suggests that the seismic moment released over the past 500 yr represents <~20% of the seismic moment accumulated over the same period (Bilham and Ambraseys, 2005; Ader et al., 2012). The number of Mw >8.0 earthquakes required for balance between seismic moment accumulation and release greatly exceeds their known occurrence rate, resulting in a surprisingly large moment deficit (Stevens and Avouac, 2016). This moment deficit has been explained by an underestimation of seismic-moment release due to incomplete estimates of released seismic moment (Stevens and Avouac, 2016) or by an overestimation of seismic-moment accumulation. This overestimation could be due to the assumption that all interseismic deformation is elastic, i.e., released in earthquakes, with no permanent internal shortening. However, internal shortening may be caused by off-MHT inelastic deformation (e.g., Bilham et al., 1997; Meade, 2010), out-of-sequence thrusting (e.g., Whipple et al., 2016; Dey et al., 2019), or midcrustal duplexing (e.g., Cannon et al., 2018; Johnston et al., 2020). Here, we studied the effects of duplex-related internal shortening on the seismic-moment budget of the Himalaya.The midcrustal duplex (i.e., Lesser Himalayan duplex) presents along the entire Himalaya and has been substantiated by abundant evidence (e.g., Mitra and Boyer, 2020; Long and Robinson, 2021) (see the Supplemental Material1); while it has been suggested that internal shortening in the form of a midcrustal duplex could reduce the seismic-moment accumulation and therefore decrease the moment-release deficit (e.g., Cannon et al., 2018; Johnston et al., 2020), its relative contribution has not been estimated. If the internal shortening occurs, Himalayan convergence rates should exceed the south-ward advance rate of the Himalaya over the Indian plate observed at the MFT. However, constraining this midcrustal duplex–related internal shortening geodetically and geologically is challenging because the internal shortening occurs near the interseismically locked portion of the MHT (Fig. 2A) and because the field-estimated geomorphic slips along the MFT at the surface have low spatial resolution and large uncertainties (Fig. 1C). Our study takes advantage of geometric solutions to theoretically estimate the slip change across the midcrustal duplex, which contributes to internal shortening of the Himalayan wedge.A duplex developed in a contractional setting grows as the lower décollement breaks forward into the footwall, creating a new horse and forming fault bends. The footwall materials are underplated and accreted onto the hanging wall. As the new (foreland) fault slips, this horse is rotated and folded by the fault bends connecting the ramp to the décollements (e.g., Boyer and Elliott, 1982). Meanwhile, the hinterland faults and the overlying thrust sheets are refolded by the fold formed below, and the refolded fault segments may be reactivated due to lengthening (Suppe, 1983; Shaw et al., 1999, 2005; Connors et al., 2021).During duplex development, slip changes across two orders of structures. First, slip decreases across the anticlinal upper bend of the ramp on the foreland fault—the first-order structure—due to fold formation; this allows us to quantify changes in slip across the upper fault bend by applying geometric solutions to fault-bend folding (Suppe, 1983) (Fig. 2B). According to the fault-bend fold model, for a parallel-kink fault-bend fold, the slip ratio (R) for any given fault bend (defined as the ratio of the slip beyond the bend to the slip before the bend) can be determined based on three geometric parameters: the hanging-wall cutoff angle before the fault bend (θ), the axial angle (γ), and the change in fault dip (Φ) as shown by (after Suppe, 1983, his equation 16)The geometric relations show that if a fault obliquely cuts across bedding (θ ≠ 0°), then the slip across the fault bend is not preserved: the slip may increase (R > 1) or decrease (R < 1) as a synclinal or an anticlinal fault-bend fold, respectively, is formed (Suppe, 1983). For details of the geometric derivations, see the Supplemental Material and Suppe (1983).Second, reactivations of hinterland faults—second-order structures—could further change the slip transferred across bends formed during imbrication (Connors et al., 2021); moreover, additional reactivated slip may be required to accommodate the change of lengths of refolded fault segments to maintain parallel behavior (Suppe, 1983; Shaw et al., 1999, 2005; Connors et al., 2021). For example, ~10% of input slip is needed for reactivating the refolded fault segment at the back limb in the case shown in Figure 2C; i.e., an additional ~10% of input slip is consumed compared to the case where only the first-order structure is considered. For details of the underlying analysis, see Connors et al. (2021).In the context of the Himalaya, the midcrustal duplex is presumably a forward-breaking sequence (Grandin et al., 2012), and the first-order structure—the most active ramp within the midcrustal duplex—is the midcrustal ramp on the MHT, revealed by the 2015 Gorkha earthquake. However, geometry of the second-order structures—the hinterland faults within the midcrustal duplex—is varied along the arc and has multiple viable solutions across the same section (e.g., Ghoshal et al., 2020). Thus, we primarily constrain slip decrease across the first-order structure by calculating the slip ratio for the upper bend of the midcrustal ramp, and assume a similar fault geometry extends along the entire Himalaya.The midcrustal ramp in the central Himalaya dips at ~26° and the upper décollement at ~8° (Elliott et al., 2016; Wang et al., 2017), so the change in fault dip (Φ) is ~18°. Because the midcrustal ramp steps up from one décollement to another, (1) the ratio of transferred slip (i.e., the ratio of the updip slip to the downdip slip) for the midcrustal ramp can be defined as the slip ratio crossing the upper bend of the mid-crustal ramp; and (2) the change in the fault dip and the cutoff angle are identical and the axial angle can be calculated by (after Suppe, 1983, his equation 12)Using Equations 1 and 2 and assuming that φ and θ have a value of 18° ± 3°, we find that R for the midcrustal ramp is ~0.91, which means that ~91% of slip could be transferred from the lower to upper décollement. In other words, ~9% of the slip could translate into internal shortening in the wedge. Because the slip along the subsurface structure is inferred from the geodetic shortening at the surface, correspondingly, R can also be applied to the geodetic shortening rate (Fig. 2A) and thus to the following applications of calculations of seismic-moment accumulation. We studied the R in two end-member models of the geodetic shortening rate, with an upper bound from Stevens and Avouac (2015) and a lower bound from Lindsey et al. (2018). Both those studies used geodetic measurements to calculate the far-field shortening rates across the MHT. Across the central Himalaya, Lindsey et al. (2018) found shortening rates of ~15 mm/yr compared to ~20 mm/yr in Stevens and Avouac (2015) (Fig. 1C); this difference reduces moment accumulation by ~30%. The discrepancy in shortening rates in the two studies may have several causes, such as the use of differing data sets, modeling techniques, and assumptions. For example, Lindsey et al. (2018) took a two-dimensional approach and assumed a binary coupling of either 0 or 1, while Stevens and Avouac (2015) used a three-dimensional model and allowed for a continuous range of coupling values from 0 to 1.We calculated how the use of R and different shortening rates influences the seismic-moment accumulation and deficit when compared to those of Stevens and Avouac (2015) (Fig. 3). If we use their shortening rates and coupling coefficients, applying R shows that the seismic moment accumulated during the past 1000 yr in the Himalaya (~2200 km) could be reduced by a Mw ~8.72 earthquake (~1.39 × 1022 N·m); i.e., by ~23% of the moment deficit compared to Stevens and Avouac (2016) (Fig. 3B). If we use the shortening rates from Lindsey et al. (2018), the total accumulated seismic moment is reduced by a Mw ~8.94 earthquake (2.93 × 1022 N·m); i.e., by ~36% of the moment deficit (Fig. 3C) compared to Stevens and Avouac (2016). If we then apply R to Lindsey et al.'s (2018) shortening rates, the reduction increases to a Mw ~9.06 earthquake (~4.37 × 1022 N·m); i.e., by ~54% of the moment deficit (Fig. 3D). Additionally, R is sensitive to fault geometry (Fig. S1; see the Supplemental Material); if we apply R5 (the cut-off angle is, on average, 5° steeper than the previously assumed ~18°) to Lindsey et al.'s (2018) shortening rate, the consequent reduction in seismic moment accumulation could be as much as a Mw ~9.11 earthquake (~5.21 × 1022 N·m); i.e., by ~66% of the moment deficit (Fig. 3E).Due to the perceived similarity between far-field geodetic measurements and geomorphic measurements made at the MFT, previous studies have argued for little internal shortening in the Himalayan wedge (e.g., Lavé and Avouac, 2000), so the seismic-moment accumulation for the MHT could be derived from geodetic shortening rates across the Himalayan wedge. In our study, we highlight the role of the first-order structure within the midcrustal duplex in internal shortening of the Himalayan wedge. We estimate the corresponding reduced seismic-moment accumulation along the MHT over the past 1000 yr to be at least one Mw ~8.72 earth-quake (i.e., moment deficit reduced by ~20%–50%) (Fig. 3). Our result implies that even a small amount of internal shortening can impact the overall accumulated seismic moment significantly. Furthermore, the slip rates derived from geomorphic studies along the MFT have large uncertainties because of poorly constrained near-surface fault geometry (e.g., Drukpa et al., 2018), fluctuating base levels and stream power in response to climate changes during the Holocene (Dey et al., 2016), or other non-tectonic processes (Schanz et al., 2018). Therefore, the field-estimated geomorphic slip rates do not conflict with an ~9% reduction in the geodetic shortening rate (Fig. 1C).Note that we consider only the simplified first-order structure in the midcrustal duplex; more realistic structures, such as multiple bends within a fault, and reactivations of the second-order structures may further reduce transferred slip (e.g., Connors et al., 2021). Other factors, such as pervasive deformation as a result of small-scale faults and folds (Groshong et al., 2012), the frontal imbricates in the Himalayan thrust belt (e.g., Long and Robinson, 2021), and multiple active ramps within the duplex (e.g., Harvey et al., 2015; Laporte et al., 2021), could further reduce the frontal advance or the overall seismic-moment accumulation along the MHT. Geodetically, small-scale fault patches with lower coupling may not be identified due to the limited spatial resolution of measurements (e.g., Mukul et al., 2018); the choice of geodetic model also impacts the estimates of moment accumulation significantly (Fig. 3C). Additionally, the after-shocks of the 2015 Gorkha earthquake and microseismicity reveal previously unidentified faults, indicating that the more complicated structures and sporadic out-of-sequence thrusting cannot be disregarded (e.g., Whipple et al., 2016; Wang et al., 2017; Dey et al., 2019; Laporte et al., 2021).In this study, we estimated the duplexing-related internal shortening of the Himalayan wedge. We constrained the slip reduction across the first-order structure within the midcrustal duplex in the Himalaya by applying geometric solutions to fault-bend folds (Suppe, 1983). The reductions in transferred slip and the associated seismic-moment accumulation can impact the Himalayan frontal advance and moment deficit; the moment deficit could be reduced by ~20%–50%, potentially explaining the unbalanced seismic moment reported in previous studies. However, the calculations are sensitive to fault geometry and limited by necessary simplifications of current geological cross sections. This implies that a more realistic geological architecture can improve the overall understanding of seismic potential. Because duplexes and fault-bend folds are integral to the growth of fold-and-thrust belts, accretionary wedges, and subduction zones (e.g., Boyer and Elliott, 1982; Madella and Ehlers, 2021), the impacts of internal shortening caused by these structures on the issues mentioned above should be considered.We thank the editor and the reviewers (C. Connors, R. Bilham, and an anonymous reviewer) for their useful comments; we also thank P. Adamek, J. Suppe, S. Wei, S. Haines, J. Hubbard, and R. Almeida for helpful discussions. W.-L. Hu is supported by a Singapore International Graduate Award (SINGA) awarded by A*STAR, Singapore. V.L. Stevens was supported by the Claude Leon Postdoctoral Fellowship (South Africa). This research was also supported by a Singapore National Research Foundation (NRF) Investigatorship grant (award number NRFI05-2019-0009) and the Earth Observatory of Singapore via its funding from the NRF Singapore and the Singapore Ministry of Education under the Research Centres of Excellence initiative. This work comprises EOS contribution number 453.
更新日期：2022-09-18
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