The Extended Continental Crust West of Islas Marías (Mexico)

The crustal structure around the Islas Marías Archipelago has been debated for a long time. An important unresolved question is where the Rivera-North American plate subduction ends and the Tamayo fracture zone begins, from SE to NW. Results from the TsuJal project have shed light on the northwesternmost part of the Jalisco block structure. It is now clear that Sierra de Cleofas and the Islas Marías Escarpment comprise the northwestern continuation of the Middle America trench. However, other questions remain. In this paper, we present the structure of the shallow and deep crust and the upper mantle of the Islas Marías western region through the integration of multichannel seismic reflection, wide-angle seismic bathymetric and seismicity data, including records of an amphibious seismic network, OBS, and portable seismic stations, purposely deployed for this project, providing an onshore-offshore transect of 310 km length. Our findings disclose new evidence of the complex structure of the Rivera plate that dips 8°–9° underneath the NW Jalisco block as revealed by two seismic profiles parallel to the Islas Marías Escarpment. Moreover, we find five sedimentary basins and active normal faults at the edges of tectonic structures of the E-W oriented West Ranges and the N-S trending Sierra de Cleofas. Furthermore, the Sierra de Cleofas is the beginning of the active subduction of the Rivera plate beneath North America. The oceanic crust thickens and submerges towards the south while is coupled with the continental crust, from 6 km at the northern ends of the seismic profiles to 15 km in the contact region and 24 km at the coast and southern ends of them. The continental Moho was not fully characterized because of the geometry of the seismic transects, but a low-velocity layer associated with Rivera Plate subduction was observed beneath the Jalisco Block. Our results constrain the complexity of the area and reveal new structural features from the oceanic to continental crust and will be pivotal to assess geohazards in this area.

Crustal Structure Across the Northern Region of the Islas Marías Archipelago

The tectonic interaction between the Rivera and North American plates north of the Bahía de Banderas is poorly understood. The nature of the crust and where the subduction ends in the western part of the Islas Marias Archipelago are still controversial. Based on new geophysical data provided by the TsuJal project, we present the shallow and deep crustal structure of the Rivera–North American plate contact zone along two seismic transects, TS09b and RTSIM01b, and the bathymetry obtained across the northern region of María Madre Island. Detailed bathymetric analysis allowed mapping of a series of lineaments along the study region, with two main preferred tendencies (020–050° and 290–320°) associated with the evolution of the Pacific-Rivera rise and the transform faults of the Gulf of California, respectively. The shallow structure is characterized by five sedimentary basins without deformation, whose horizons are subparallel, suggesting that the sediment deposition occurred after the extension process ended. The deep structure corresponds to a transition between oceanic crust (Rivera Plate), with an average thickness of ∼10 km to the Islas Marías Escarpment, and a thinned continental crust, whose thickness increases toward the continent until it reaches 28 km, with a dip angle of 7–10°. The absence of an accretionary prism suggests that the subduction process of the Rivera Plate beneath the North American Plate to the north of Islas Marías has ceased. In this study, we determined that the morphological expression of the northern limit of the Rivera Plate corresponds to the Islas Marías Escarpment.

Interaction between interplate fault topography and tsunamigenic structures at the subduction zone offshore West Mexico

<p>The megathrust topography is key in conditioning the structural integrity of the overriding plate, and thus, the generation of tsunamigenic structures. Our objective is to investigate the Rivera subduction zone, offshore the Mexican Pacific coast, known for hosting large megathrust tsunamigenic earthquakes (Mw > 7.5), and where little is known regarding the distribution of tsunamigenic structures along the margin. Our working hypothesis is that there is an interaction between the megathrust relief at the surface of the subducted slab (Rivera Plate) and the existence of tsunamigenic structures in the above unsubducted plate (North America). To investigate this interaction, we used seismic methods to characterize the variations of the physical properties of the overriding plate, generally related to tectonic (faults) structures that are sources of tsunamis, with the reliefs of the deeper subducted plate obtained with the same method. Here, we use spatially coincident 2D multichannel seismic (MCS, 5.85 km long-streamer) and active marine wide-angle seismic (WAS) data acquired during the TSUJAL survey in 2014 offshore west of Mexico to measure structural variations of the overriding plate and the megathrust interface. We have jointly inverted refracted and reflected travel-times (TT) from both MCS and WAS data to constrain the P-wave velocity (Vp) structure of the overriding plate and the geometry of the megathrust. Before the inversion and to increase the amount of refracted TT we have applied the downward continuation technique to MCS field data allowing to better image the refracted waves in the records. MCS data has a higher spatial sampling than OBS data, which translates into a higher density sampling of the refracted waves and hence the tomographic resolution. Therefore, the resulting tomographic model displays small-scale velocity structure variations of the overriding plate and the megathrust relief that would not be resolved with TT from OBS data only. We used further refracted and reflected TT from OBS data to constrain the Vp structure of the subducting oceanic plate and the geometry of the oceanic Moho. The inverted megathrust interface obtained with the tomography shows clear topographic features in its shallow portion (<~10 km from the trench). Such topographic variations are smaller than the average size of seamounts of the Rivera plate, but they are similar to the seafloor fabric generated by a relict East Pacific Rise segment identified west of the trench in the bathymetry map of the region. Time-migrated images were also obtained after processing the MCS data to constrain the tectonic framework of the shallow subduction zone regardless of the tomographic models. The seismic sections reveal the lack of an extensive accretionary prism, implying that subduction-erosion dominates the structure of the margin in this region. Integrating all the data results, we find that megathrust highs correlate with low-velocity anomalies, suggesting the presence of fluids, and correlate with the presence of extensional faults in the overriding plate as well. This correlation demonstrates the control that megathrust topography exerts on the formation of tsunamigenic structures along the Rivera plate boundary.</p>

Comparison of 3-D Raytracing and Finite Frequency Tomography

We performed 3-D seismic tomography using teleseismic arrival time at Southwest Mexico. The Mexican subduction zone results from successive fragmentation events that affected the ancient Farallon plate as various segments of the East Pacific rise approached the paleo-trench off western North America. The complexity in this region is related to two subducting oceanic plates, the Rivera and Cocos plates, that have different ages, compositions, convergence velocities and subduction dip angles. In this study, we compared the 3-D raytracing tomography model with finite frequency tomography model. Final models show the differences in amplitude and pattern between the raytracing and finite frequency. 3D raytracing models produced sharper images of fast velocity structures in the mantle. The deeper slabs are more coherent and show less broadening with depth than using 1D finite frequency kernels. However, although the finite frequency and 3-D ray tracing models show some differences in amplitude and pattern, the overall agreement of the models supports the interpretation of Yang et al. (2009) that slab rollback is occurring in South Western Mexico. One possible different interpretation between the raytracing and finite frequency theory results concerns the deep structure of the Rivera slab. The finite frequency models show that the Rivera slab is clearly observable at a depth of about 300km but fades away at greater depths. However, the 3-D ray tracing model shows a clear fast velocity band down to a depth of 400 km and thus our model does not support a slab tear of the Rivera plate above 400 km depth

Analysis of the Seismicity in the Jalisco Block from June to December 2015

ABSTRACT We present the first study of seismicity in the region of the Jalisco Block using data recorded by the Jalisco Seismic Accelerometric Telemetric Network between June and December 2015. During this period, 683 local earthquakes with magnitudes between 1.0<ML≤4.0 were identified and relocated with Hypo71PC. From this catalog, we identify a heterogeneous hypocentral distribution with six continental crustal seismogenic areas. We also observed seismicity associated with the subduction process that extends 180 km from the Mesoamerican trench, which suggests an estimated dip angle of the slab between 22° and 31°. A subtle dip also suggests oblique subduction toward the Colima rift zone and bending of the Rivera plate. These observations are in agreement with previous partial regional studies using local seismic networks. Two seismic swarms were observed in this period, one in the Bahia de Banderas seismogenic zone, and a second in the Guadalajara Metropolitan zone. We note two areas on the northern coast of Jalisco with meager rates of seismicity.