Properties of the Subduction System in Mexico

Citation

Kim, YoungHee (2011) Properties of the Subduction System in Mexico. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/BB7N-QP03. https://resolver.caltech.edu/CaltechTHESIS:04272011-150709139

Abstract

This thesis presents seismic imaging results of the structure of the Mexican subduction zone using receiver function (RF) based on teleseismic P-to-S converted waves, in order to gain insight into the physical and chemical factors associated with internal geodynamic processes. More specifically, this thesis investigates (1) the nature of tectonic processes involved in the buildup and subsequent modification of continental and oceanic lithosphere, and (2) the determination of mineralogy/petrology and fluidphase reactions in the subducting Cocos oceanic crust.

Utilizing the data acquired from two dense broadband seismic lines in Mexico, the geometries and seismic properties of the interface of the subducting Cocos plate beneath Mexico are determined from the RFs. The RF image for central Mexico shows that the subducting oceanic crust dips shallowly north at 15 degrees for a distance of 80 km from Acapulco at the Pacific coast, and then horizontally underplates the continental crust for approximately 200 km to the Trans-Mexican Volcanic Belt (TMVB). Modeling of the RF conversion amplitudes and timings of the underplated features reveals a thin very-low velocity zone between the plate and the continental crust that appears to absorb nearly all of the strain between the upper plate and the slab. The migrated image of the RFs shows that the slab dips steeply into the mantle at an angle of about 75 degrees beneath the TMVB.

The RF results for southern Mexico in the Isthmus of Tehuantepec show an image of the Cocos slab down to about 100 km depth. The same cross-section image also reveals a slab-like south-dipping structure interpreted to be subducted from the Gulf of Mexico. This anomalous slab with the opposite dip direction of the Cocos slab appears to cut off the Cocos slab at 150 km depth. There is no tectonic explanation for the south-dipping slab under the current paradigm of Caribbean plate reconstructions. We present in this thesis the case for a new reconstruction of the Gulf of Mexico and propose that the slab may be due to the collision of the Yucatan Block into Mexico in the Miocene, and may also be responsible for the Cocos plate truncation imaged from previous tomography studies. This hypothesis explains the Chiapas Fold and Thrust Belt to the south of the Yucatan Block and may explain the unusual volcanic arc configuration in southern Mexico.

We formulate and apply a new inversion technique based on the plane wave conversion to obtain the seismic parameters (S wave velocity, Vs, and density) of the oceanic crust. We use such parameters to infer mineralogical properties of subducting oceanic crust. From this effort, we provide tighter constraints on physical properties of the subducting Cocos oceanic crust, and explain the difference in the slab geometries betweeen central and southern Mexico from the mineral physics point of view. Anomalously low Vs (2.4−3.4 km/s) in the upper part of the flat oceanic crust in central Mexico points to elevated Poisson’s and Vp/Vs ratios of the oceanic crust. This directly relates to the presence of water and hydrous minerals or high pore pressure; the mechanically weak hydrous layer may explain current subduction geometry at very shallow depth of about 45 km without strong coupling between the plates.

Using Vp/Vs as a function of Vs in a range of likely pressure and temperature for candidate hydrous phases, we identify the major hydrous mineral phases present in the upper (3−5 km thickness) and lower parts (3−5 km thickness) of the subducted oceanic crust of central and southern Mexico. In central Mexico, the upper oceanic crust in the flat slab region is enriched with hydrous minerals such as talc over the normal oceanic crustal compositions such as MORB-like gabbro. Petrologically, the generation of talc during subduction of the oceanic crust is nearly impossible. One possible mechanism to produce such a low-velocity anomaly at the upper oceanic crust is that lower crustal rocks are hydrated with 15-20 percent of free water to reduce the seismic velocities significantly. We thus propose that the thin low-velocity, talc-rich layer in the upper oceanic crust is then generated from the mantle wedge side during the slab flattening process coupled with trench rollback. The talc-rich rocks at the slab interface can be formed in the mantle by the addition of silica transported by rising fluids via the dehydration reaction from the subducting oceanic crust and by mechanical mixing of mantle and siliceous rocks. The evolution of the thin low-strength zone, which decouples the horizontal slab from the continental crust, originating from the mantle wedge side rather than the trench side, has important implications for the dynamics of the subduction system, including the flattening process of the slab, as well as the geochemistry of the mantle wedge and arc in central Mexico. After passing through the flat segment, the major compositions of the steeply subducting oceanic crust underneath the TMVB are zoisite and lawsonite from 60 to 100 km in depth. The eclogitization occurs at the depth of about 100 km. The dominant mineral phase in the upper oceanic crust of southern Mexico from 45 to 120 km depth is amphibole on top of unaltered gabbroic oceanic crust.

Thesis Files