21 research outputs found
Seismic anisotropy indicates organised melt beneath the Mid-Atlantic Ridge aids seafloor spreading
Skip Nav Destination
RESEARCH ARTICLE| AUGUST 04, 2023
Seismic anisotropy indicates organized melt beneath the Mid-Atlantic Ridge aids seafloor spreading
J.M. Kendall; D. Schlaphorst; C.A. Rychert; N. Harmon; M. Agius; S. Tharimena
Author and Article Information
Geology (2023) 51 (10): 968â972.
https://doi.org/10.1130/G51550.1
Article history
Standard View
Open thePDFfor in another window
Cite
Share Icon Share
Permissions
Abstract
Lithospheric plates diverge at mid-ocean ridges and asthenospheric mantle material rises in response. The rising material decompresses, which can result in partial melting, potentially impacting the driving forces of the system. Yet the geometry and spatial distribution of the melt as it migrates to the ridge axis are debated. Organized melt fabrics can cause strong seismic anisotropy, which can be diagnostic of melt, although this is typically not found at ridges. We present anisotropic constraints from an array of 39 ocean-bottom seismometers deployed on 0â80 Ma lithosphere from March 2016 to March 2017 near the equatorial Mid-Atlantic Ridge (MAR). Local and SKS measurements show anisotropic fast directions away from the ridge axis, which are consistent with strain and associated fabric caused by plate motions with short delay times, ÎŽt (<1.1 s). Near the ridge axis, we find several ridge-parallel fast splitting directions, Ï, with SKS ÎŽt that are much longer (1.7â3.8 s). This is best explained by ridge-parallel sub-vertical orientations of sheet-like melt pockets. This observation is much different than anisotropic patterns observed at other ridges, which typically reflect fabric related to plate motions. One possibility is that thicker sub-ridge lithosphere with steep sub-ridge topography beneath slower spreading centers focuses melt into vertical, ridge-parallel melt bands, which effectively weakens the plate. Associated buoyancy forces elevate the sub-ridge plate, providing greater potential energy and enhancing the driving forces of the plates
Seismic Constraints on the Thickness and Structure of the Martian Crust from InSight
NASAÂżs InSight mission [1] has for
the first time placed a very broad-band seismometer on
the surface of Mars. The Seismic Experiment for
Interior Structure (SEIS) [2] has been collecting
continuous data since early February 2019. The main
focus of InSight is to enhance our understanding of the
internal structure and dynamics of Mars, which includes
the goal to better constrain the crustal thickness of the
planet [3]. Knowing the present-day crustal thickness of
Mars has important implications for its thermal
evolution [4] as well as for the partitioning of silicates
and heat-producing elements between the different
layers of Mars. Current estimates for the crustal
thickness of Mars are based on modeling the
relationship between topography and gravity [5,6], but
these studies rely on different assumptions, e.g. on the
density of the crust and upper mantle, or the bulk silicate
composition of the planet and the crust. The resulting
values for the average crustal thickness differ by more
than 100%, from 30 km to more than 100 km [7].
New independent constraints from InSight will be
based on seismically determining the crustal thickness
at the landing site. This single firm measurement of
crustal thickness at one point on the planet will allow to
constrain both the average crustal thickness of Mars as
well as thickness variations across the planet when
combined with constraints from gravity and topography
[8]. Here we describe the determination of the crustal
structure and thickness at the InSight landing site based
on seismic receiver functions for three marsquakes
compared with autocorrelations of InSight data [9].We acknowledge NASA, CNES, partner agencies and institutions (UKSA, SSO,DLR, JPL, IPGP-CNRS, ETHZ, IC, MPS-MPG) and the operators of JPL, SISMOC, MSDS, IRIS-DMC and PDS for providing SEED SEIS data. InSight data is archived in the PDS, and a full list of archives in the Geosciences, Atmospheres, and Imaging nodes is at https://pds-geosciences.wustl.edu/missions/insight/. This work was partially carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. ©2021, California Institute of Technology. Government sponsorship acknowledge
The interior of Mars as seen by InSight (Invited)
InSight is the first planetary mission dedicated to exploring the whole interior of a planet using geophysical methods, specifically seismology and geodesy. To this end, we observed seismic waves of distant marsquakes and inverted for interior models using differential travel times of phases reflected at the surface (PP, SS...) or the core mantle-boundary (ScS), as well as those converted at crustal interfaces. Compared to previous orbital observations1-3, the seismic data added decisive new insights with consequences for the formation of Mars: The global average crustal thickness of 24-75 km is at the low end of pre-mission estimates5. Together with the the thick lithosphere of 450-600 km5, this requires an enrichment of heat-producing elements in the crust by a factor of 13-20, compared to the primitive mantle. The iron-rich liquid core is 1790-1870 km in radius6, which rules out the existence of an insulating bridgmanite-dominated lower mantle on Mars. The large, and therefore low-density core needs a high amount of light elements. Given the geochemical boundary conditions, Sulfur alone cannot explain the estimated density of ~6 g/cm3 and volatile elements, such as oxygen, carbon or hydrogen are needed in significant amounts. This observation is difficult to reconcile with classical models of late formation from the same material as Earth. We also give an overview of open questions after three years of InSight operation on the surface of Mars, such as the potential existence of an inner core or compositional layers above the CM
Constraints on the shallow elastic and anelastic structure of Mars from InSight seismic data
Marsâs seismic activity and noise have been monitored since January 2019 by the seismometer of the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander. At night, Mars is extremely quiet; seismic noise is about 500 times lower than Earthâs microseismic noise at periods between 4 s and 30 s. The recorded seismic noise increases during the day due to ground deformations induced by convective atmospheric vortices and ground-transferred wind-generated
lander noise. Here we constrain properties of the crust beneath InSight, using signals from atmospheric vortices and from the
hammering of InSightâs Heat Flow and Physical Properties (HP3) instrument, as well as the three largest Marsquakes detected
as of September 2019. From receiver function analysis, we infer that the uppermost 8â11 km of the crust is highly altered and/
or fractured. We measure the crustal diffusivity and intrinsic attenuation using multiscattering analysis and find that seismic
attenuation is about three times larger than on the Moon, which suggests that the crust contains small amounts of volatiles
Imaging lithospheric discontinuities beneath the Northern East African Rift using S -to-P receiver functions
Imaging the lithosphere is key to understand mechanisms of extension as rifting progresses. Continental rifting results in a combination of mechanical stretching and thinning of the lithosphere, decompression upwelling, heating, sometimes partial melting of the asthenosphere, and potentially partial melting of the mantle lithosphere. The northern East African Rift system is an ideal locale to study these processes as it exposes the transition from tectonically active continental rifting to incipient seafloor spreading. Here we use SâtoâP receiver functions to image the lithospheric structure beneath the northernmost East African Rift system where it forms a triple junction between the Main Ethiopian rift, the Red Sea rift, and the Gulf of Aden rift. We image the Moho at 31 ± 6 km beneath the Ethiopian plateau. The crust is 28 ± 3 km thick beneath the Main Ethiopian rift and thins to 23 ± 2 km in northern Afar. We identify a negative phase, a velocity decrease with depth, at 67 ± 3 km depth beneath the Ethiopian plateau, likely associated with the lithosphereâasthenosphere boundary (LAB), and a lack of a LAB phase beneath the rift. Using observations and waveform modeling, we show that the LAB phase beneath the plateau is likely defined by a small amount of partial melt. The lack of a LAB phase beneath the rift suggests melt percolation through the base of the lithosphere beneath the northernmost East African Rift system
Evolution of the Oceanic Lithosphere in the Equatorial Atlantic From Rayleigh Wave Tomography, Evidence for SmallâScale Convection From the PIâLAB Experiment
The oceanic lithosphere is a primary component of the plate tectonic system, yet its evolution and its asthenospheric interaction have rarely been quantified by in situ imaging at slow spreading systems. We use Rayleigh wave tomography from noise and teleseismic surface waves to image the shear wave velocity structure of the oceanic lithosphereâasthenosphere system from 0 to 80 My at the equatorial MidâAtlantic Ridge using data from the Passive Imaging of the LithosphereâAsthenosphere Boundary (PIâLAB) experiment. We observe fast lithosphere (VSV > 4.4 km/s) that thickens from 20â30 km near the ridge axis to ~70 km at seafloor >60 My. We observe several punctuated slow velocity anomalies (VSV 400 km from the ridge. We observe a high velocity lithospheric downwelling drip beneath 30 My seafloor that extends to 80â130 km depth. The asthenospheric slow velocities likely require partial melt. Although melt is present off axis, the lack of offâaxis volcanism suggests the lithosphere acts as a permeability boundary for deeper melts. The punctuated and offâaxis character of the asthenospheric anomalies and lithospheric drip suggests smallâscale convection is active at a range of seafloor ages. Smallâscale convection and/or more complex mantle flow may be aided by the presence of large offset fracture zones and/or the presence of melt and its associated lowâviscosities and enhanced buoyancies
First seismic constraints on the Martian crust - receiver functions for InSight
EGU2020: Sharing Geoscience Online, 4-8 May, 2020NASA's InSight mission arrived on Mars in November 2018 and deployed the first very broad-band seismometer, SEIS, on the planet's surface. SEIS has been collecting data continuously since early February 2019, by now recording more than 400 events of different types. InSight aims at enhancing our understanding of the internal structure and dynamics of Mars, including better constraints on its crustal thickness. Various models based on topography and gravity observed from the orbit currently vary in average crustal thickness from 30 km to more than 100 km, with important implications for MarsÂż thermal evolution, and the partitioning of silicates and heat-producing elements between different layers of Mars.
We present P-to-S and S-to-P receiver functions, which are available for 4 and 3 marsquakes, respectively, up to now. Out of all of the marsquakes recorded to date, these are the only ones with clear enough P- or S-arrivals not dominated by scattering to make them suitable for the analysis. All of the quakes are located at comparatively small epicentral distances, between 25° and 40°. We observe three consistent phases within the first 10 seconds of the P-to-S receiver functions. The S-to-P receiver functions also show a consistent first phase. Later arrivals are harder to pinpoint, which could be due to the comparatively shallow incidence of the S-waves at the considered distances, which prevents the generation of converted waves. Identification of later multiple phases in the P-to-S receiver functions likewise remains inconclusive. To obtain better constraints on velocity, we also calculated apparent velocity curves from the P-to-S receiver functions, but these provide meaningful results for only one event so far, implying a large uncertainty. Due to difficulties in clearly identifying multiples, the receiver functions can currently be explained by either two crustal layers and a thin (25-30 km) crust or three crustal layers and a thicker (40-45 km) crust at the landing site. This model range already improves the present constraints by providing a new maximum value of less than 70 km for the average crustal thickness. Information from noise autocorrelations as a complementary method, identification of P-reverberations and S-precursors in the event recordings, and more extensive modeling, ultimately including 3D-effects, are considered to further our understanding of the waveforms and tighten the constraints on the crust
First Receiver Functions on Mars - Constraints on the Martian Crust from InSight
2020 SSA Annual Meeting, 27-30 April, Albuquerque, New MexicoNASA¿s InSight mission has for the first time deployed a very broad-band seismometer, SEIS, on the surface of Mars, which has been collecting data continuously since early February 2019. The main focus of InSight is to enhance our understanding of the internal structure and dynamics of Mars, including better constraints on its crustal thickness. Various models based on topography and gravity observed from the orbit currently differ by more than 100% for the average crustal thickness. Here, we present P-to-S and S-to-P receiver functions, which are available for 4 and 3 marsquakes, respectively, up to now. All of these quakes are located at comparatively small epicentral distances, between 25° and 40°. We observe three consistent phases within the first 10 seconds of the P-to-S receiver functions. The S-to-P receiver functions also show a consistent first phase. Later arrivals are harder to pinpoint, which could be due to the comparatively shallow incidence of the S-waves at the considered distances, which prevents the generation of converted waves. To obtain better constraints on velocity, we also calculated apparent velocity curves from the P-to-S receiver functions, but these provide meaningful results for only one event so far, implying a large uncertainty. Due to difficulties in clearly identifying multiples, the receiver functions can currently be explained by either two crustal layers and a thin (25-30 km) crust or three crustal layers and a thicker (40-45 km) crust at the landing site. This model range already improves the present constraints by providing a new maximum value of less than 70 km instead of more than 100 km for the average crustal thickness. Information from noise autocorrelations as a complementary method, identification of P-reverberations and S-precursors in the event recordings and more extensive modeling, ultimately including 3D-effects, are investigated to tighten the constraints