Himalayan sub-Moho earthquakes suggest crustal faults trigger eclogitized-drip tectonics

doi:10.21203/rs.3.rs-5051130/v1

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Himalayan sub-Moho earthquakes suggest crustal faults trigger eclogitized-drip tectonics

https://doi.org/10.21203/rs.3.rs-5051130/v1

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Along the 1400-km Himalayan arc, 100 + sub-Moho earthquakes have been detected from their Sn/Lg amplitude ratios or S-P delay times, concentrated densely beneath a ~ 300-km segment in south Tibet where they reach ~ 110-km depth. Explanations for these earthquakes include Moho-penetrating faults and the dripping of eclogitized lower crust. We estimate the geological strain-rates, temperatures, and timescales for these two processes from seismological, thermal, geological, and geodetic datasets. We constrain the eclogite viscosity to \(\:\lesssim\:\)1–5×10²¹ Pa⋅s through numerical modeling of viscous Rayleigh-Taylor dripping within the available geological timescale (~ 20 Ma). Thermal analysis shows it is unlikely that brittle failure in olivine-dominated lithospheric mantle occurs below the 70-km Moho, hence a deeply penetrating fault cannot by itself explain the 70–110-km seismicity. Stronger eclogitized lower crust at upper-mantle depths would enable brittle failure, but an eclogite drip by itself cannot explain the dominating dextral-slip focal mechanisms. We propose that eclogitization of mafic granulites in the Indian lower crust occurs along lower-crustal shear zones associated with active faults and fluid intrusion, creating the density anomaly that drives Rayleigh-Taylor instability. As the eclogite drip grows, high strain within the drip creates brittle faulting to upper-mantle depths, albeit in what are crustal lithologies.

Earth and environmental sciences/Solid Earth sciences/Tectonics

Earth and environmental sciences/Solid Earth sciences/Geodynamics

Earth and environmental sciences/Solid Earth sciences/Seismology

The Moho discontinuity, defined by a large increase in seismic wavespeed and accompanied by increased density, is commonly assumed to represent the petrological crust-mantle boundary. Near-Moho seismicity is rare in most continental lithosphere, but is well-known across the Tibetan plateau, where competing claims as to whether these earthquakes are in the crust or in the mantle stimulated development of first the “jelly-sandwich” model ^1,2 and then the competing “crème-brulée” model ³. The former suggests an aseismic lower-crust above a brittle upper-mantle, while the latter argues all earthquakes occur within the crust ⁴. However, large uncertainties inherent in time-to-depth conversions have been overcome by modern methods including S-minus-P delay time comparison ^5,6, fitting mantle-converted phases ^7,8, and Sn/Lg amplitude ratio analysis ^9,10, so that the existence of Tibetan sub-Moho earthquakes now seems incontrovertible. Furthermore, these sub-Moho earthquakes are so concentrated in south and northwest Tibet, in each area spanning < 300 km along strike, that they cannot be fully explained by a single, orogen-parallel model like slab bending ¹¹ or underthrusting cold cratonic Indian mantle ¹². Proposed hypotheses to accommodate the localized distribution of these sub-Moho earthquakes include faults penetrating the whole crust and dripping of eclogitized lower crust ^11,13–15.

We focus on south Tibet where the penetrating-fault hypothesis draws support from the shape of the deep seismicity cluster, elongated SE-NW along strike of the Dhubri-Chungtang fault (DCF) from the Shillong plateauthe Himalaya, and the match of many of the deep and shallow focal mechanisms to the DCF’s dextral strike-slip ^15,16 (Fig. 1). Some deep extensional focal mechanisms may also suggest the Moho-penetrating extension of southern Pumqu-Xainza graben (PXG) ¹⁴. The penetrating-fault hypothesis requires common upper-mantle compositions to remain brittle at Tibetan mantle temperatures ( ≳ 600℃ ¹⁷).

In contrast, the lower-crustal drip hypothesis assumes crème-brulée rheology, and follows potentially analogous observations in Vrancea ¹⁸ and the Hindu Kush ¹⁹. South-Tibetan lower-crustal eclogitization, widely inferred from the presence of a receiver-function doublet ^5,14,20,21, has been suggested to have a critical connection with sub-Moho earthquakes ^13,22, so provides the basic condition for this hypothesis. Mafic lower crust when transformed to eclogite becomes denser than ultramafic upper-mantle so may initiate dripping ²³ while also remaining brittle to higher temperatures than either parent mafic lower crust or ultramafic upper mantle ²⁴. Local thinning of the receiver-function lower-crustal doublet ¹⁴, radial SKS splitting directions ²⁵ surrounding the proposed dripping center (Fig. 1a), and some localized receiver-function deep converters ⁶ are possible seismological evidence for this hypothesis. Surface rebound, a consequence of completed delamination in the absence of compensating lower-crustal flow ^26,27, is not observed in south Tibet, but the continuity of the deep seismicity from near-Moho to ≳ 100 km suggests that any eclogite drip has not yet detached from the lower crust.

Discriminating between faulting and dripping hypotheses requires analysis of multiple data constraints and re-evaluation of Tibetan rheology. We first estimate strain rates and temperatures within the sub-Moho seismic zone, then build lithospheric rheology profiles to identify lithologies that can remain brittle (seismogenic) to ⪆100 km. Our geodynamic simulation of Rayleigh-Taylor instability ²⁸ constrains the viscosity range that permits dripping to ⪆100 km in the < 10 Myr timescale constrained by Himalayan geological history. Eclogitized lower crust can satisfy temperature and viscosity criteria to ≳ 100 km, but likely requires water input to meet the temporal constraint, perhaps facilitated by crustal-scale faulting transecting the Moho and the eclogite layer.

The sub-Moho seismicity is a window into present-day temperatures and viscosities, either for this specific location within the Indian plate that is moving northward beneath the Himalaya at the plate convergence rate, and/or for a stress and strain condition approximately fixed with respect to the topographic crest of the Himalaya that affects different rocks at different times.

The earthquakes span ~ 50–100 km across the DCF (N53ºW, Fig. 1c), ~ 200 km along strike (Fig. 1d), and from ~ 50–100 km vertically (Fig. 1c&d). They lie ~ 100–200 km north of the Himalayan thrust front, where temperatures range from ~ 400–800°C ¹⁷ (Fig. 2a). Lithologies of the deep earthquake zone likely include pyroxene-plagioclase rocks (diabase/gabbro or mafic granulite), garnet-pyroxene rocks (mafic eclogite, transformed from mafic lower crust by high-pressure phase change), and olivine-dominated rocks (ultramafic upper mantle).

The sub-Moho epicenters cluster beneath the intersection of the dextral DCF and extensional PXG (Fig. 1). The geodetic slip rate of DCF is \(\:\dot{s}\) = 0.4–0.8 mm/yr ³⁶; the geochronological slip rate across PXG is \(\:\dot{s}\) ~1 mm/yr ³⁷. Under the faulting hypothesis, shearing spans the sub-Moho seismicity orthogonal to the fault for a distance \(\:{L}_{h}\) ~100 km, so the characteristic seismogenic strain-rate is \(\:\dot{ϵ}\) = \(\:\dot{s}/{L}_{h}\) ≈ 0.3×10⁻¹⁵s⁻¹. Strain rates will be faster if shearing is accomplished in a high-strain zone narrower than \(\:{L}_{h}\), so we take \(\:\dot{ϵ}\) ~ 1×10⁻¹⁵ s⁻¹ as an order-of-magnitude estimation.

However, in the sub-Moho seismogenic region, receiver-function images ^5,14 support a \(\:H\) \(\:\lesssim\:\) 20 km thick eclogite layer above a ~ 70 km Moho. A Rayleigh-Taylor instability or eclogite drip may have been triggered by the last major slab breakoff/rollback event in the southern Lhasa terrane at ~ 20 Ma that is indicated by the consequent ~ 18–8 Ma adakitic magmatism ³⁸ characteristic of melting of delaminated garnet-bearing lower-crust ²⁷. All recent seismic tomographies ^39–41 recognize high-velocity anomalies within the asthenosphere beneath the underthrust Indian mantle lithosphere, and interpret them as evidence of this ~ 20 Ma slab break-off.

If eclogite drips to the deepest seismogenic depth (~ 110 km), it crosses the depth range of the seismogenic zone, \(\:{L}_{z}\) ~60 km. The timing of any putative eclogite drip is controlled by the convergence rate between India and the drip across the Main Himalayan Thrust (MHT) (~ 15 mm/yr ⁴²) and the low-angle geometry of the MHT that implies Indian lower crust does not reach depths (pressures) appropriate for eclogitization until passing beneath the Lesser Himalayan thrust ramp ~50 km north of the thrust front where the MHT reaches ⪆20-km depth ⁴³. The sub-Moho earthquakes occur 100–200 km NNE of the Himalayan thrust front, in rocks that cannot have begun to eclogitize until ~5–10 Ma. If the available timescale for an eclogite drip is \(\:T\) = 5–10 Myr, the strain rate \(\:\dot{ϵ}\) ~ \(\:{L}_{z}/\left(HT\right)\) ~10⁻¹⁴ s⁻¹ in the putative drip.

In summary, we seek a geologic scenario in which brittle earthquakes occur at ~ 400–800°C at 50–100 km depth at averaged strain rates of 10^− 15–10^− 14 s^− 1 within ~ 5–10 Myr of entry into the subduction zone, in a region where lithosphere removal may have occurred at 20 Ma.

We next investigate the range of viscosities that permit a drip of the size of the sub-Moho seismogenic zone to form in the available 5–10 Myr, using pure viscous modeling of Rayleigh-Taylor instabilities ²⁸ (Fig. 2). We run simulations with different lithospheric-mantle viscosity \(\:{\eta\:}_{m}\), eclogite viscosity \(\:{\eta\:}_{e}\), and eclogite density \(\:{\rho\:}_{e}\). We require eclogite to form a drip length \(\:{d}_{e}\) > 40 km from a static environment at 20 Ma.

The eclogite drip length \(\:{d}_{e}\) is a strong function of viscosity. For eclogite density \(\:{\rho\:}_{e}\) = 3.40 × 10³ kg/m³, corresponding to ~ 40–60% lower-crustal eclogitization ⁴⁴, in general \(\:{d}_{e}\) ≥ 40 km for \(\:{\eta\:}_{e}\) \(\:\lesssim\:\) 5 × 10²¹ Pa⋅s (Fig. 2c). Modeling with our default parameters (supplementary material S1), we show that drips initiate simutaneously in the upper-mantle lithosphere and eclogite layers, but the upper mantle drips much faster because it has lower viscosity and is settling into the asthenosphere that has even lower viscosity (Fig. 2b). After 10 Myr the eclogite drip has barely initiated, but lithospheric mantle has already reached the bottom of our model (400 km), inducing inward lithospheric and asthenospheric mantle flow at the tail of the drip. From 10–20 Myr mantle flow continues as the trailing stem of the drip gradually closes, while the eclogite drip grows faster, reaching > 100 km depth by the end of the simulation (supplementary material S2). Therefore, although our modeling timescale is 20 Myr, the effective timescale for the eclogite drip is \(\:{T}_{e}\) ≲10 Myr. Our 5-Myr simulations is also conducted (Fig. 2d) follow a linear scaling, and require \(\:{\eta\:}_{e}\) ≲ 1 × 10²¹ Pa⋅s.

The lithospheric mantle viscosity \(\:{\eta\:}_{m}\) has little influence on the eclogite drip rate if \(\:{\eta\:}_{m}\lesssim\:\) 0.2\(\:{\eta\:}_{e}\). The mantle-eclogite density difference is also crucial: setting \(\:{\rho\:}_{e}\) = 3.30 g/cm³, only 50 kg/m³ above the lithospheric-mantle density, corresponding to just 10–30% eclogitization ⁴⁴ barely changes the maximum \(\:{\eta\:}_{e}\), from ~ 5.10²¹ Pa⋅s (Fig. 2c) to ~ 4.10²¹ Pa⋅s (Fig. 2e). This small decrease is far less than predicted by the linear relation \(\:\eta\:\) ~ \(\:{\Delta\:}\rho\:g\lambda\:T\) for Rayleigh-Taylor instabilities ⁴⁵. Our supplementary simulations (S3) simulating eclogite dripping in a static lithospheric mantle show that eclogite drips much faster in the presence of lithospheric delamination, due to the mantle flow field created by earlier lithospheric delamination. If viscous drag is the major driving force for an eclogite drip, a large density difference (or equivalently, a high degree of eclogitization) is not required, so that an eclogite drip initiates at the very beginning of eclogitization. Viscous drag from large-scale mantle flow has recently been inferred to trigger lithospheric dripping at the base of the North American craton ⁴⁶, and beneath southern Tibet ⁴⁷.

In summary, for an eclogite drip to reach and create seismicity at 110 km depth since 5–10 Ma, \(\:{\eta\:}_{e}\) ≤ 1–5.10²¹ Pa⋅s, and likely \(\:{\eta\:}_{m}\) ≤ 10²¹ Pa⋅s. These maximum viscosities are higher than most previous estimates from Tibet and India, of lower-crustal viscosity 10^18–21 Pa⋅s ^48–51 and lithospheric-mantle viscosity 10^18–21 Pa⋅s ^52,53. Delamination modeling studies also use similar or smaller viscosities: eclogite 10^19–21 Pa⋅s ⁵⁴, and dry/wet olivine 10^16–21 Pa⋅s ^55,56.

Earthquakes occur in materials that are brittle at the imposed strain rates, pressures, and temperatures, because their ductile failure modes cannot dissipate the build-up of stress. The differential stress required for brittle failure increases with confining pressure (depth), whereas the stress needed to reach a specific ductile flow rate decreases with temperature (depth). The intersection of the curves representing these two strength laws (Fig. 3) is the lithospheric brittle-ductile-transition depth, hence maximum depth of seismicity, which we estimate using the strain rates for both tectonic hypotheses, laboratory-measured constitutive equations for different rock types (supplementary material S4), and temperature models ¹⁷ (supplementary material S5).

The penetrating-fault hypothesis requires the dominant upper-mantle mineral, olivine, to remain brittle beneath a ~ 70-km Moho, and even down to 110-km depth to satisfy the sub-Moho seismicity observations. For strain rate ~ 10^− 15 s^− 1, in the modeled temperature range the olivine brittle realm is almost entirely above the Moho, whether olivine is wet, dry or undergoing Peierls creep, for all faulting mechanisms for a 30º frictional angle (Fig. 3b, with the deepest transition point at 65 km). Even at 10^− 14 s^− 1 strain rate (Fig. 3c) the deepest brittle-ductile transition for strike-slip faulting is barely below the Moho (71 km). Only if the frictional angle is as low as ~ 2º ⁵⁴ due to decreased friction at high pressure and temperature ⁵⁷ or viscous strain softening at high strain rates ⁵⁸, does the brittle-ductile transition occur well below the Moho, but still only at ~ 80 km, far shallower than the deepest located seismicity. Hence an olivine-rich mantle below the ~ 70 km Tibetan Moho is likely to be aseismic, as in the “crème-brûlée” model ³.

The eclogite-drip hypothesis, in contrast, requires eclogite to remain brittle at typical upper-mantle depths. For strain rate ~ 10^− 14 s^− 1, the brittle-ductile transition of eclogite is 5–10 km below the 70-km Moho for the three kinds of faulting (Fig. 3c). Note that stress types in Fig. 3 are described horizontally, and therefore, horizontal compression is equivalent to vertical extension which represents the dripping condition. An eclogitic region lying on the modelled geotherm therefore may also not predict seismicity to the observed ~ 110 km depth. However, as discussed above, if there is an eclogite drip beneath the Himalaya, it has dripped ~ 40 km downward in only ~ 5–10 Myr, much faster than its thermal-diffusion timescale (~ 50 Ma, supplementary material S6). The core of the drip is effectively isothermal for ~ 50 Myr, allowing it to remain brittle to 110-km depth (adiabatically shifted dashed curve, Fig. 3c). In addition, constitutive equations for clinopyroxenite, likely the controlling component of eclogite, show even deeper brittle-ductile transition depths, also permitting seismicity below 100 km. Although the creep curves have large uncertainty due to compositional variability (supplementary material S5&7), clearly an eclogite drip is more likely to remain seismogenic to > 100 km than olivine-dominated upper mantle.

The drip hypothesis clearly best fits the brittle-failure criterion. An eclogite drip can achieve 40-km deepening in 5–10 Myr enabled by viscous drag from mantle flow caused by the mid-Tertiary mantle-lithospheric delamination. This drag is an additional pre-requisite for rapid drip formation to occur, but recent data assimilation studies show the detached Indian slab creates large drag forces with maximum shear stress inside the lithospheric root ⁴⁷. Our dripping model also explains why, despite the eclogite doublet being observed ^21,59 for 1400-km along-strike, sub-Moho Himalayan earthquakes are restricted to the part of the orogen where seismic tomography has found deep high-velocity anomalies^39,60 likely representing the lithospheric delamination at ~ 20 Myr ^38,61. Although the locus of Miocene mantle delamination is ~ 150-km north of our proposed eclogite drip ³⁹, additional modeling (supplementary material S8) suggests this is immaterial.

However, our drip hypothesis has apparent inconsistencies. We locate the drip ~ 100–200-km north of the Himalayan front, but gravity profiles have previously been used to suggest that Indian lower-crustal density only reaches upper-mantle density, enabling drip initiation, ~ 200-km north of the front ^20,62, consistent with the slow kinetics of anhydrous eclogitization ⁶³. Most the deep earthquakes beneath south Tibet have strike-slip focal mechanisms ^15,16,31, in contrast to the vertical extension nominally created by dripping and observed beneath Vrancea ^18,19 and the Hindu Kush ^18,19.

An isolated eclogite drip is therefore an inadequate explanation of our observations. The mismatch between our proposed early eclogitization and the later eclogitization inferred from gravity anomalies ²⁰ seems well within the resolution limit of density models: in supplementary material S9 we show the trade-off between deepening the eclogite layer by 5 km (the potential uncertainty in Moho depth) and densifying the eclogite layer by ~ 200 kg/m³. In order to expedite eclogitization within only a few million years, grain-boundary water is certainly required, though perhaps only a few ppm ⁶³ or a few hundred ppm ²³. This water may be present in the Indian lower-crustal granulites ⁵⁵ or may be introduced by seismic pumping through an active fault reaching the eclogitizing layer ^23,63. The seismogenic middle crust below the MHT implies the DCF remains active beneath the High Himalaya, so may pump surface fluid down to the lower crust as well as create dextral shear within the proposed eclogite drip.

We therefore propose that the sub-Moho seismogenesis results from crustal-scale faulting enabling lower-crustal eclogitization that triggers dripping. In this hypothesis, pre-existing Moho-penetrating faults (DCF and possibly PXG) guide fluid into the lower crust, triggering fast eclogitizaton ²³ (Fig. 4a). When the eclogitized lower crust becomes denser than the upper mantle, the mantle flow field created by lithospheric delamination ~ 20 Myr ago triggers an eclogite drip to \(\:\gtrsim\:\)110 km depth, bringing brittle materials down to today’s sub-Moho seismogenic zone. The deeply-penetrating faults allow the sub-Moho eclogite to break via weaker strike-slip or extensional faulting mechanisms, enabling brittle faulting to greater depth (Fig. 3b) and explaining the observed strike-slip and normal-fault mechanisms. The receiver-function Moho offsets further north along the PXG may mark the earlier lower-crustal dripping triggered by the graben ¹⁴ (Fig. 4b).

Using constraints from geologic history and active seismotectonics, together with strain-rate, temperature, and viscosity estimates, our viscous-drip modeling and brittle-ductile-transition analysis suggests the sub-Moho Himalayan earthquakes occur within an eclogite drip that required crustal-scale faulting for its initiation. Although each data constraint has significant uncertainty, collectively they are sufficient to begin to test different models for this unusual, spatially isolated, sub-Moho seismic zone. We infer these continental mantle earthquakes depend on the rare spatial coincidence of subduction of an active strike-slip fault cutting mafic lower crust in a region of geologically recent slab breakoff or delamination.

6.1 Viscous drip modeling

We model pure viscous 2D Rayleigh-Taylor dripping ²⁸ using Underworld2 ⁶⁴, finite-element geodynamics software which has been widely applied in lithospheric viscous simulations ^65,66 (supplementary material S10).

Our model is 400-km deep × 800-km wide, with four layers: crust, eclogite, ultramafic upper mantle, and asthenosphere (Fig. 2a). The thickness of each layer is set based on the receiver-function doublet, and Moho and LAB depths beneath south Tibet ^34,67. Densities are set at 2600 kg/m³ (crust), 3400 or 3300 kg/m³ (eclogite), 3250 kg/m³ (lithospheric mantle), and 3200 kg/m³ (asthenosphere). Viscosities are set at 50 and 0.01 × 10²² Pa⋅s for the crust and asthenospheric mantle respectively. We vary the eclogite and lithospheric-mantle viscosities from 0.2–1.0 × 10²² Pa⋅s and 0.02–0.4 × 10²² Pa⋅s. In order to trigger instabilities, we assign a Gaussian perturbation of 20 km to the base of the eclogite (Moho) and lithospheric-mantle (LAB) using horizontal standard deviations of 100 km (length scale of the deep earthquake cluster) and 200 km (length scale of the tomographic + 2% shear-velocity anomaly ³⁹). For detailed parameters see supplementary materials S1.

Modeling commences from a static state (a fixed-India reference frame) and runs for 20 Myr, the suggested timing of Indian lithospheric delamination, with a focus on processes requiring only 5 Myr, our more restrictive constraint from the plate-convergence rate. The depth reached by the bottom of the eclogite drip (i.e. 70 km (Moho depth) plus its elongation below Moho, \(\:{d}_{e}\)), is taken to represent the extent of the brittle seismogenic zone. We adjust the viscosity of eclogite \(\:{\eta\:}_{e}\) from 2–10 × 10²¹ Pa⋅s (0.5–2 × 10²¹ Pa⋅s for 5-Myr model runs) and the upper mantle \(\:{\eta\:}_{m}\) from 0.2–4 × 10²¹ Pa⋅s (0.1–2 × 10²¹ Pa⋅s for 5-Myr model runs), with the additional constraint \(\:{\eta\:}_{e}\ge\:{\eta\:}_{m}\) because eclogite is stronger than upper mantle (Fig. 2c,d,e).

6.2 Rheology profiles

We use existing south-Tibet lithospheric thermal models ¹⁷ to establish temperature profiles and uncertainties at the location of the sub-Moho seismicity (supplementary material S5). We take creep laws from representative rocks, using felsic granulite and diabase ^68,69 for the upper and lower crust, wet and dry olivine ^70,71 for the lithospheric mantle and asthenosphere, and eclogite/clinopyroxene ^24,72 for an eclogite drip, where present. For detailed rock ductile parameters see supplementary material S4. We also plotted envelopes for Peierls creep that is thought to be the dominant low-temperature failure mode for olivine ⁷⁰.

In Fig. 3 we plot brittle-failure curves using the minimum differential stress for brittle-slip laws ⁷³ for extension (across PXG), strike-slip (along prolongation of DCF), and compressional (equivalent to vertical extension caused by dripping) fault mechanisms (supplementary material S11). We plot the ductile curves for the preferred ‘Western Dharwar’ temperature model ¹⁷ for strain rates of 10^− 15 s^− 1 (faulting hypothesis) and 10^− 14 s^− 1 (dripping hypothesis). We show equivalent curves for a cooler model in supplementary material S7.

Acknowledgements

We acknowledge Underworld2 technique support from Jiaqi Fang and helpful discussions with William Shinevar, Jing-hui Tong, and Ji-Chin Chen. György Hetényi kindly shared his gravity-fitting code and original density models.

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Himalayan sub-Moho earthquakes suggest crustal faults trigger eclogitized-drip tectonics

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Abstract

Figures

1. Main

2. Physical conditions in sub-Moho seismogenic zone

3. Temporally constrained viscosity of a Rayleigh-Taylor instability

4. Brittle and ductile strengths of south Tibetan lithosphere

5. Discussion: eclogite drip or Moho-penetrating fault?

6. Methods

6.1 Viscous drip modeling

6.2 Rheology profiles

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