Approximately two-thirds of Earth’s outermost shell is composed of oceanic plates that form at spreading ridges and recycle back to Earth’s interior in subduction zones. A series of physical and chemical changes occur in the subducting lithospheric slab as the temperature and pressure increase with depth. In particular, olivine, the most abundant mineral in the upper mantle, progressively transforms to its high-pressure polymorphs near the mantle transition zone, which is bounded by the 410 km and 660 km discontinuities. However, whether olivine still exists in the core of slabs once they penetrate the 660 km discontinuity remains debated. Based on SKS and SKKS shear-wave differential splitting times, we report new evidence that reveals the presence of metastable olivine in the uppermost lower mantle within the ancient Farallon plate beneath the eastern United States. We estimate that the low-density olivine layer in the subducted Farallon slab may compensate the high density of the rest of the slab associated with the low temperature, leading to neutral buoyancy and preventing further sinking of the slab into the deeper part of the lower mantle.
more »
« less
Seismic Imaging Beneath Cascadia Shows Shallow Mantle Flow Patterns Guide Lower Mantle Upwellings
The mantle transition zone (MTZ) plays an important role in modulating material transport between the upper mantle and the lower mantle. Constraining this transport is essential for understanding geochemical reservoirs, hydration cycles, and the evolution of the Earth. Slabs and hotspots are assumed to be the dominant locations of transport. However, the degree of material transport in other areas is debated. We apply
- NSF-PAR ID:
- 10465686
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 128
- Issue:
- 9
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract Using recently collected high‐resolution seismic data along a dense linear transect across Ohio, West Virginia, and Virginia (called Mid‐Atlantic Geophysical Integrative Collaboration (MAGIC) profile), we analyze P‐to‐S receiver functions to investigate the undulations of the mantle transition zone (MTZ) discontinuities (410‐ and 660‐km) beneath the central Appalachian region. Our results incorporating the effects of local crustal and mantle structures suggest shallowing of both the 410‐ and the 660‐km discontinuities from the northwest (inland) to the southeast (coast) along MAGIC profile. Hydro‐thermal upwelling beneath the eastern U.S. coastal plain due to a hydrated MTZ and hot upwelling return flow associated with the descending lower mantle Farallon slab is consistent with our observations of MTZ structure considering 3D velocity heterogeneity. The inferred hydrous hot upwelling rising into the upper mantle may trigger dehydration melting atop the 410‐km discontinuity, which may help to explain the presence of a low velocity upper mantle anomaly beneath the region today.
-
more » « less
SUMMARY The Earth's mantle transition zone (MTZ) plays a key role in the thermal and compositional interactions between the upper and lower mantle. Seismic anisotropy provides useful information about mantle deformation and dynamics across the MTZ. However, seismic anisotropy in the MTZ is difficult to constrain from surface wave or shear wave splitting measurements. Here, we investigate the sensitivity to anisotropy of a body wave method, SS precursors, through 3-D synthetic modelling and apply it to real data. Our study shows that the SS precursors can distinguish the anisotropy originating from three depths: shallow upper mantle (80–220 km), deep upper mantle above 410 km, and MTZ (410–660 km). Synthetic resolution tests indicate that SS precursors can resolve $\ge $3 per cent azimuthal anisotropy where data have an average signal-to-noise ratio (SNR = 7) and sufficient azimuthal coverage. To investigate regional sensitivity, we apply the stacking and inversion methods to two densely sampled areas: the Japan subduction zone and a central Pacific region around the Hawaiian hotspot. We find evidence for significant VS anisotropy (15.3 ± 9.2 per cent) with a trench-perpendicular fast direction (93° ± 5°) in the MTZ near the Japan subduction zone. We attribute the azimuthal anisotropy to the grain-scale shape-preferred orientation of basaltic materials induced by the shear deformation within the subducting slab beneath NE China. In the central Pacific study region, there is a non-detection of MTZ anisotropy, although modelling suggests the data coverage should allow us to resolve at least 3 per cent anisotropy. Therefore, the Hawaiian mantle plume has not produced detectable azimuthal anisotropy in the MTZ.
-
more » « less
Abstract By taking advantage of the recent availability of a broadband seismic data set from Networks NR and BX covering the entire country of Botswana, we conduct a systematic receiver function investigation of the topography of the 410 and 660 km discontinuities beneath the incipient Okavango rift zone (ORZ) in northern Botswana and its adjacent Archean‐Proterozoic tectonic provinces in southern Africa. Similar to a previous mantle transition zone (MTZ) discontinuity study using data from a 1‐D profile traversing the ORZ, a normal MTZ thickness is observed in most parts of the study area. This is inconsistent with the existence of widespread positive thermal anomalies in the MTZ and further implies that active thermal upwelling from the lower mantle plays an insignificant role in the initiation of continental rifting. The results also suggest that cold temperature presumably associated with thick cratonic keels has indiscernible influence on the thermal structure of the MTZ. The expanded data set reveals several isolated areas of slight (~10 km or smaller) MTZ thinning. The largest of such areas has a NE‐SW elongated shape and is mostly caused by relative deepening of the 410 km discontinuity rather than shallowing of the 660 km discontinuity. These characteristics are different from those expected for a typical mantle plume. We speculate that the thinner‐than‐normal MTZ may be induced by minor thermal upwelling associated with late Mesozoic‐early Cenozoic lithospheric delamination, a recently proposed mechanism that might be responsible for the high elevation of southern Africa.
-
more » « less
Abstract The structure of the mantle transition zone (MTZ) in subduction zones is essential for understanding subduction dynamics in the deep mantle and its surface responses. We constructed the P (
V p ) and SH velocity (V s ) structure images of the MTZ beneath Northeast Asia based on two‐dimensional (2‐D) triplicated waveform modeling. In the upper MTZ, a normalV p but 2.5% lowV s layer compared with IASP91 are required by the triplication data. In the lower MTZ, our results show a relatively higher‐velocity layer (+2%V p and −0.5%V s compared to IASP91) with a thickness of ~140 km and length of ~1,200 km atop the 660‐km discontinuity. Taking this anomaly as the stagnant slab and considering the plate convergence rate of 7–10 cm/year in the western Pacific region during the late Cenozoic, we deduced that the stagnant slab has a subduction age of less than 30 Ma. This suggests that the observed stagnancy of the slab in the MTZ beneath Northeast Asia may have occurred no earlier than the Early Oligocene. From the constraints derived individually onV p andV s structures, highV p /V s ratios are obtained for the entire MTZ beneath Northeast Asia, which may imply a water‐rich and/or carbonated environment. Within the overall higher‐velocity stagnant slab, a low‐velocity anomaly was further detected, with a width of ~150 km,V p andV s reductions of 1% and 3% relative to IASP91. Such a gap may have provided a passage for hot deep mantle materials to penetrate through the thick slab and feed the Changbaishan volcano.
