Mexico Subduction Experiments (MASE and VEOX)

The Surveys

We conducted two seismic surveys in central and southern Mexico in order to examine the subduction of the Cocos Plate under the North American Plate. The two surveys are:
MASE - Meso-America Subduction Experiment. This array consisted of 100 bradband sensors deployed along a 600 km line that runs from Acapulco on the Pacific to near Tampico on the Gulf of Mexico, passing through Mexico City. The stations al had Guralp 3T broadband sensors (120 sec to 50 Hz) and were deployed from 2005/01 to 2007/06. This line samples the flat-slab portion of the subduction system. More details of the MASE survey are available here VEOX - Veracruz-Oaxaca line. This array consisted of 45 sensors across the Isthmus of Techuantepec (narrowest part of Mexico). This 300 km line also had Gurlap 3T broadband sensors and was deployed from 2007/08 to 2009/03. This survey was a partnership between the TO and IG/UNAM. It samples a normal-dip portion of the subduction. More details of the VEOX survey are available here
Collaborators This survey was a colaboration among: the Tectonics Observatory (TO) at Caltech, the Center for Embedded Network Sensors (CENS) at UCLA, the Instiute of Geophsics at University National Autonomous of Mexico (IG/UNAM), and the Geoscience Center at UNAM

Some Results

Cross-sections beneath the MASE line. The left panel shows a composite of the lithosphere and mantle models. The slab clearly terminates at about 500 km, which means it has been truncated and the deeper portion has sunk out of view. The right panel shows the detail of the crust and upper mantle based on receiver functions. The continuous red interface is the Moho which does not show topography commensurate with the 2 km elevation of the Trans-Mexican Volcanic Belt (TMVB). To the south (left) of the TMVB, the Moho appears to be in direct contact with the Cocos slab. The panels on the lower right show the presence of a thin but extremely low-velocity layer. Results in these figure are taken from Perez-Campos et al (2008) and Husker and Davis (2009).

Supporting the TMVB The upper plot shows the shear wave velocities in the lower crust and upper mantle obtain by the surface wave study of Iglesias et al (2009). In particular, the velocities are very sklow beneath the TMVB. The middle plot shows the resistvities obtain by the MT profile reported by Jodicke et al (2006), which also have high resistivity associated the TMVB. The lower plot shows the apparent Pratt compensation that support the 2-km high TMVB assuming various conversion (R factors) between velocity variation and density variations. This can explain how the TVMB is supported despite having almost no crustal root.

Model for Subduction History Along the Mase Line Prior to 30 Ma, the major portion of the forarc disappears, possibly by being subducted as suggested by Keppie (2009). Thwn start at 29 Ma, he volcunism in the arc along the coast extinquishes, as the flab flattens by an unknown mechanism. The flatten extends to the north edge of the present day TVMB where volcanism resumes. Then starting at 19 Ma, the slab start to roll back as evidenced by the pregeressively younger volcanism to the south of the TMVB. At a point near Mexico City and at 7 Ma, the slab is cut off, exposing it to the athenosphere. Ferrari suggests that this tear start near the Rivera Plate at the time of its break away from the Cocos Plate. This tear propagated W->E across the TVMB reaching M.C. at 7 Ma. This accounts for the observation of Adakites near M.C. The slab continues to rolback until the hinge reaches its present position just south of the southern edge of the TVMB where the current arc (i.e. Popo Volcano) is. The present configuration has a low velocity lower crust beneath the TVMB, possibly created by generated by the injection of water from the underplated slab. The observed NVT in the crust may be a symptom of the process. The hydration would reduce the density and could be the source of buoyancy to hold up the TMVB. The underplated slab is separated from the continental crust by a thin layer with very low shear-wave velocities, and presumeably very low shear strength. This layer allows the subduction of the slab (at 6 cm/yr) to be completely decoupled from the upper plate. There are very few earthquakes in the crust, but the are a number of normal events in the oceanic lithosphere indicaing that it is in tension. These events appear to be similar to the standard "outer rise" earthquakes.

3D Tomographic Results A 3D model of P-wave velocity, S-wave velocity, Qp and Qs is constructed for central Mexico using local earthquakes. The station coverage is dense along the MASE and VEOX lines, in inbetween it is filled in with station from the SSN network (Seismic Service National) operated by UNAM. The maximum depth the local events is 90 km, so the model is restricted to depth above that level. The Cocos slab is clearly evident in the P image, with its dip increasing southward. Somewhat surprising, an even stronger image (but a bit less resolved) is evident in the shear attenuation (Qs). The is a strong high attenuation zone associated with the Veracuz the is disussed below.

Tomography Along the MASE line The slab descending to 45 km is eveident in the P-wave image although it is somewhat smeared upwards near the coasr (left edge of the image). The horizontal portion is not visible in the image because the raypaths are insensitive to horizontal strucures. The low velocity in the crust, particularly under the TMVB agree with the surface-wave derived S-wave velocities. The P-wave and P-wave attenuation are also high under the TMVB. Both attenuation fields show thin zones of higher attenuation coincidental with the two clusters on Non-Volcanic Tremor (NVT) denoted by the arrows. The NVT comes from the Payero et al *2008) study. The bottom image is the resistivity drived from the MT survey along the MASE line (Jodicke et al, 2006) and shows higher resistance in the crust beneath the TMVB.

Tomography Along the VEOX line The Cocos Slab is evident in the P, Qp and Qs images, and to some extent in the Vp/Vs image. There is also a well-defined fast and low attenuation structure dipping to the south. The is the same feature that shows up in the teleseimic tomographic image discussed below. Also evident is major slow, attenuating region located under the Veracruz Basin. Note that this is distinctly south of the Los Tuxtlas Volcanic Field. The slow feature is likely due to the deep sediments in the Veracruz Basin. The tomographic image is likely blurred downward.

Structure Along the VEOX Line. The structure along the VEOX line was a surprise. The Cosos slab can be found in the receiver functions (with some dificulty). It is also well defined by the seismicity. The surprise is the presense of a south dipping structure (much clearer the the Cocos slab) that extends to 250 km depth. It appears to cut off the Cocos slab at 150 km, and there appears to be a "nest" of seismicity at the cut-off point. Thew upper right plot is the tomography for the line obtained from teleseismic events. It also clearly shows the south dipping structure. The difference in slope is reconciled in Chen and Clayton (2012). The situation appears not unlike the scenario proposed by Rogers et al (2002) for a region to the south of the VEOX line, that is based on global tomograhy models.

Model For Structure Along the VEOX Line. Proposed tectonic reconstruction diagram for southern Mexico. (a) A schematic model showing the collision of the Yucatan in Mexico. The onset of subduction (25-20 Ma) is estimated assuming a subduction rate of 2-5 cm/yr and a slab length of 250 km. The date of truncation of the Cocos slab is taken from Rogers et al. (2002). The volcanism is extinguished at the Miocene arc due to the collision, and reappears after 3 Ma (Manea & Manea 2008). The last stage at 0 Ma shows a present-day model for southern Mexico, and the volcano symbol indicates the MCVA. It is also directly analogous to Fig. 9(b). (b) Map view of the Yucatan Block rotations. The black line arrow represents the counter-clockwise movement of the Yucatan Block based on the standard model by Pindell & Kennan (2009). The red solid-line arrow represents our proposed model. We note that arrow positions are only indicative, not based on true tectonic reconstruction.

Estimating Minerology of the Subducted Slabs The amplitudes of the Receiver Functions (RFs) depend on the transmission coefficients of the interfaces where the P-S conversion takes place. By examining the ampitudes of the RFs we can estimate the transmission coeffients, and with some assumptions about about the avergae velocities above and below the interface, the quantities Vs and Vp/Vs can be inevrted for. In practice we use a Bayersian inversion to take the assumed velocities into account. The inversion for Vs is much better constrained than Vp/Vs, because the transmission is only weakly dependent on Vp. We then compare the Vs and Vp/Vs is those measured for various minerals and rocks to try to estimate the minerals present in the subduction slab. The left panel deals with the horizontal portion of the slab along the MASE line, and the comparison indicates that talc is a good candidate for the ultra-slow velcoity layer, and that some combination of lizardite, antigorite, and gabbro match the layer beneath it. The right panel shows the results for the dipping portion along MASE and VEOX lines. The MASE line shows the upper layer is zoisite/lawsonite transitioning to eclogite, indicating the slab hase dehydrated. The VEOX line shows the upper layer is likely amphibole transitioning to gabbro at depth (indicating a lack of dehydration).

Possible Tear in the Slab along the Orosco Fracture Zone

Seismicity and Other Evidence for Tear

Determining the edge of the Ultra Slow Velocity Layer Based on Waveforms