Topography of upper mantle discontinuities beneath the North Atlantic using precursors to PP and SS waves

We are mapping the topography of upper mantle seismic discontinuities beneath the North Atlantic and surrounding regions by using precursor arrivals to PP and SS seismic waves that reflect off the seismic discontinuities. Numerous source-receiver combinations have been used in order to collect a large dataset of reflection points beneath our investigation area. We analysed over 1700 seismograms from MW>5.8 events using array seismic methods to enhance the signal to noise ratio. The measured time lag between PP (SS) arrivals and their corresponding precursors on robust stacks are used to measure the depth of the transition zone boundaries. The reflectors’ depths show a correlation between the location of known hotspots and a significantly depressed 410 km discontinuity indicating a temperature increase of 50-300 K compared to the surrounding mantle (Figure 1.a). For the 660 km discontinuity three distinct behaviours are visible: i) normal depths beneath Greenland and at a distance of a few hundred kilometres away from known hotspots, ii) shallower 660 km discontinuity compared with the global average value near hotspots closer to the Mid-Atlantic Ridge and iii) very few observations of a 660 km discontinuity at the hotspot locations (Figure 1.b). We interpret our observations as a large upwelling beneath the southern parts of our study region, possibly due to the South Atlantic convection cell. The thermal anomaly may be ponding beneath the endothermic 660 km phase transformation and likely does not extend through the top of the transition zone as a whole, except for those branches which appear as the thinner upwellings of Azores, Canaries and Cape Verde hotspots at the surface (Figure 1.c).

Topography Map

Figure 1: Topography map of the 410 km discontinuity (a) and the measured depths for the 660 km discontinuity at the location of each reflection point (b) beneath the Northern Atlantic. c) Cartoon illustrating our interpretation: a large low velocity region reaches up to the lower part of the transition zone beneath the south-western part of the studying area, ponding beneath the 660 km discontinuity. From there three small upwellings extend through the transition zone, pierce the upper boundary of the transition zone and deflect the 410 km discontinuity downward, eventually reach the surface of the Earth at the three hotspot areas.

Relevant Papers

Saki, M., Thomas, C., Nippress, S., and Lessing, S. (2015). Topography of upper mantle seismic discontinuities beneath the North Atlantic: The Azores, Canary and Cape Verde plumes. Earth Plan. Sci. Lett., 409: 193-202.

Lessing, S., Thomas, C., Saki, M., Schmerr, N., and Vanacore, E. (2015). On the difficulties of detecting PP precursors. Geophys. J. Int., 201:1666-1681.

Wave velocities 1d

Figure 1 : P wave velocity (red), S wave velocity (blue) and density in the upper mantle and mantle transition zone in the 1-D Earth model ak135(Kennett et al., 1995). (Lessing, 2014, PhD thesis)

The mantle is the largest region in the Earth and extends from the crust to the core mantle boundary (CMB). The Earth's mantle is divided into a upper and lower mantle by the mantle transition zone (MTZ), a region that stretches from a depth of 410 km to 660 km. The structure of MTZ has been investigated with several seismic waves. Seismic reflectors at ~410 km and at ~660 km appear as globally distributed seismic discontinuities and are therefore called the 410 km and 660 km seismic discontinuities. The 410 km and 660 km seismic discontinuities are part of commonly used 1-D Earth reference models (Figure 1).

The 410 km and 660 km discontinuities are generally attributed to phase transformations in the olivine mineral system. Phase transformations are rearrangements of the crystal lattice (solid-solid phase transitions) or mineral reactions accounting for thermodynamically stable states at given pressure and temperature conditions (Mineral physicists and mineralogists prefer pressure and temperature conditions over depth and temperature conditions).

Volume fractions

Figure 2 : Volume fractions of mineral species assuming pyrolitic bulk mantle composition. Abbreviations: Ol = olivine, Wad = wadsleyite, Rw = ringwoodite, Mw = magnesiowuestite, Pv = perovskite, Cpx = Clinopyroxene, Opx = Orthopyroxene, Gt = Garnet, Ca-Pv = Calcium perovskite. (Lessing, 2014, PhD thesis)

Apart from a large fraction of olivine bearing minerals, the mantle's bulk composition comprises of pyroxene and garnet minerals (Figure 2). Minerals of the pyroxene garnet systems undergo several phase transformation in a similar depth range as the 410 km and 660 km discontinuities. These phase transformations may generate additional seismic discontinuities depending on local thermal and compositional variations within the MTZ. Seismic waves that encounter those discontinuities and can be detected at seismic stations, may give additional constraints on the thermal and compositional constraints of the mantle.

One way to investigate seismic discontinuities in the upper mantle and mantle transition is looking at underside reflections of PP and SS waves off those seismic discontinuities. The underside reflections are reflected at the underside of a seismic discontinuity at a depth d, halfway between the source and the receiver (Figure 3). The signals are denoted as PdP or SdS. The underside reflections have a shorter ray path through the mantle than the PP or SS waves. Thus they arrive before the PP and SS waves at the receiver and are therefore also called precursors to PP or SS (Figure 4).

Ray paths

Figure 3: Ray paths of Pdiff, PP and its precursors off the 410 km and 660 km seismic discontinuities, P410P and P660P.

Figure 4: Slowness vespagram of the event 1994/07/13, 11:45 UTC (source location: 7.53°S, 123.24°E, 158 km), recorded at stations of the German Regional Seismic Network (GRSN).

By studying the properties of seismic discontinuities in the mantle transition zone with PP and SS waves, a variety of fields can be brought together, including seismology, mineral physics and geodynamics. Combining observations and insights from various disciplines allows to further constrain the composition, thermal structure and dynamics of the mantle.