Many hypotheses have been proposed to try to explain the changes observed in the geometry of subducting plates at subduction zones. Half of the displacement expected from the breakaway of Baja California from mainland Mexico has not yet been identified in the field.
Plate Tectonic Constraints on Flat Subduction
Abstract
Introduction
There is a possible flat plate segment in Ecuador that correlates with the subduction of the Carnegie Ridge (Gutscher et al., 1999a). Oceanic lithosphere on the Caribbean oceanic plateau may cause a flat plate in northwestern Columbia (Gutscher et al., 2000a).
Current state of subduction in central Mexico
In the Nankai Trough, for example, the Palau-Kyushu Ridge enters a trench at the southern boundary of the shoal. Breakup of the Cocos Plate allows smaller fragments to roll back more quickly, resulting in a change in dip along the trench (Billen, 2008).
History of subduction in Mexico
Associated with the extension is an ignimbrite flare that signals slab rollback or slab detachment (Ferrari et al., 2007). The southern Mexican margin has undergone major reshaping in Tertiary time (Moran-Zenteno et al., 1996).
Proposed causes of zones of shallow subduction
The origin and location of the Chortis block (present-day Nicaragua) through time is highly debated. 17 driven flow of the asthenosphere through the descending plate creates a zone of negative pressure in the mantle wedge (Tovish et al., 1978).
Discussion
Conclusions
Acknowledgements
The inset map shows the extent of the data used (cross) and the dashed line approximating the trough. Panel B shows the bathymetry of the area around the unnamed seamounts and the location of the representative bathymetric profile.
Tracking conjugate features
This makes it less likely to be the direct cause of the flat plate in Peru. Emperor Seamounts (4), Magellan Seamounts (3), Roo Rise (15), and Louisville Ridge (13) all subduct without any apparent change in the geometry of the associated subducting plate. The shallow segment of the Nankai subduction zone is centered over the Shikoku Basin (2), not over the Palau-Kyushu or Izu-Bonin subduction ridges (1).
Based on our analysis of the flat subduction in central Mexico (Skinner and Clayton, 2011), we favor a model of mantle hydration to induce shallow and flat plates (Billen and Gurnis, 2001; Manea and Gurnis, 2007). Relative motion of the Nazca (Farallon) and South American plates since the Late Cretaceous. The gray box represents the spatial and temporal extent of the flat plate from Ramos and Folguera (2009).
We expect impactors to pass through this target zone if the buoyancy hypothesis is the cause of the flat plate. The map shows the location of the flat plates along the South American margin (Ramos and Folguera, 2009).
Supplementary Material
This map shows synthetic fracture zones produced by the rotation model used in our reconstructions (Müller et al., 2008) in red and fracture zones as mapped by Matthews et al. Although it is unlikely that any model will be able to reproduce all the complexities of fracture zones, we believe that this model does an excellent job of reproducing the major observable trends. Poles of rotation for the Nazca plate relative to the Pacific plate used to test magnetic isochrone reconstructions.
Nazca/Farallon - South America Pacific- South America Age (Ma) Latitude Longitude Angle Latitude Longitude Angle. Rotation poles for the Pacific and Nazca plates relative to the South American plate used in the reconstruction and detection of conjugate features. The starting points on the Pacific plate (Latitude1, Longitude1), seafloor age, crustal volume in a strip centered on the starting point, and our reconstructed conjugate point on the Nazca plate (Latitude2, Longitude2).
Supplementary References
The thin black lines are 20 km depth contours to the top of the subducting plate. The lack of a gravity anomaly associated with the Nazca Ridge implies that the drag is compensated by a dense root, which cancels out any positive buoyancy due to thickening of the oceanic crust. If the cause of flat subduction is the positive buoyancy of bathymetric anomalies, then we expect a correlation between the volume of subducted liquid material and the resulting modification of the subduction zone geometry.
We then divide this extra volume by the feature's areal extent to get the average thickening associated with it. The average thickening is multiplied by the length of contact between the trench and anomaly to give us the subduction number. Intuition tells us that a larger subduction number will indicate a greater influence on the subduction geometry.
We have plotted the subduction rate for eighteen bathymetric anomalies visible today. The lack of clustering of the shallow subduction zones tells us that buoyancy from a bathymetric high is not a sufficient condition to predict a shallow slab.
Paleomagnetic Constraints on Rifting
Paleomagnetic studies of the Tuff of San Felipe on Isla Angel de La Guarda, Baja Califor-. Because of its widespread areal extent, the Tuff of San Felipe provides an important datum for reconstructing the rifting process that separated Baja California from the North American plate after the eruption of the tuff. These experiments have both characterized the magnetic mineralogy of the tuff at this new location and constrained the depositional flow directions.
However, characteristic remanence directions isolated by principal component analysis reveal rotations of the vertical axis of the magnetic remanence vector within the tuff. Studies have focused on the seismically defined structure of the northern Gulf of California and the location of rifts through time (Aragon-Arreola and Martin-Barajas, 2007), movement at the southern end of the remaining unpublished movement suggests that some 300 km occurred outside the Gulf 6 million years ago moves.
This proved invaluable in reconstructing Isla Tiburon to its counterpart on the Baja California Peninsula (Oskin et al., 2001), and constraining the opening of the northern Gulf. Work on locations in the Sierra Libre of Sonora (Vidal-Solano et al., 2005; Vidal Solano et al., 2008) aims to detect some of the slip deficit.
Field expeditions and sample collection
In 2010, a field team conducted similar reconnaissance samples of mesas on the Baja California Peninsula in the area around Cataviña to follow up on a 2008 expedition that identified the Tuff of San Felipe in that region. Again using remote sensing to guide our sampling, we proposed 50 target sites where there were indications of a tuff similar to the Tuff of San Felipe. An individual inclined block, about 200 meters in diameter, covered by the Tuff of San Felipe, produced six paleomagnetic sample sites distributed around its perimeter.
Field identification of the Tuff of San Felipe is complicated by the lateral variations in appearance and the incorporation of the underlying substrate. Correlations of the Tuff of San Felipe are based on geochronology, petrology, and most often the unique paleomagnetic signature (Stock et al., 1999). The Tuff of San Felipe records a low dip and southwesterly declination due to a geomagnetic excursion (Bennett, 2013; Lewis and Stock, 1998b).
88 magnetic signature (Slope 212.4°, Slope -3.0°), at the type locality established by Bennett (2013), is an essential way to identify the San Felipe Tuff for further study. Pumice is characterized by 3%–15% phenocrysts, with alkali feldspar being the most abundant (Stock et al., 1999).
Geologic setting
The Tuff of San Felipe sits deposits on top of the volcanic conglomerate, with a maximum thickness on the island of ~12 meters. Tilt corrections were determined using the basal contact of the tuff or of the foliation. Where the base of the tuff was exposed, the height of the core was measured directly.
Before and after each insertion of the sample into the coils, the empty coils are measured. Components of the characteristic remanent magnetization (ChRM) were measured by stepwise erasing the natural remanent magnetization (NRM) of a sample by low-temperature cycling, alternating field (AF), or thermal demagnetization. The intersection of the IRM collection and IRM demagnetization curves gives us information about the interacting fields in the sample (Cisowski, 1981).
We used this technique to estimate emplacement temperatures in selected samples of the San Felipe Tuff. The AMS of the samples was measured in a field of 200 A/m with the rotation mode of the AGICO MFK1-FA Kappabridge.
Data Analysis
Almost all samples lie in the flattened sector of the Flinn diagram (Flinn, 1962) (Fig. 6). All thermal sensitivity curves (Figure 19) show that the dominant magnetic phase is magnetite. Large variations in the paramagnetic content of the matrix may be a contributing factor to the changes measured in the AMS ellipsoid (Richter and van der Pluijm, 1994).
This behavior is a clear sign of the presence of a superparamagnetic fraction of grains in the sample (Carter-Stiglitz et al., 2006). The most likely remaining explanation for the scatter in AMS axes is turbulent flow of ignimbrite during deposition. Therefore, the rotation of the upper part of the unit relative to the lower part is a true signal.
There is no apparent difference between the cores in the rotated section and the outcrops in the rest of the island. A test of the correlation between the declination of the ChRM and the direction of the AMS line casts doubt on the idea that we have measured a CRM (Figure 35).
Conclusion
E., 1990, Preisach diagrams and anhysteresis: do they measure interactions?: Physics of the Earth and Planetary Interiors, v. M., 2004, Sandstone detrital modes support Magdalena Fan displacement from the mouth of the Gulf of California: Geology , v .A key relationship to note is the nature of the contact between the Tuff of San Felipe and lower units.
Figure B shows the upper welded tuff zone in contact with dacitic lavas. Panel B is a Jelinek plot of shape parameter (T) versus Jelinek degree of anisotropy (Pj). This diagram shows a clear decrease in sensitivity with increasing frequency of the applied field.
A stereonet of equal area with the lower half shows the K1 (red) and K3 (blue) axes of the AARM ellipsoid. We can see that the vertical axis rotation tectonic correction does not remove all the scatter in the AMS directions. This plot shows the absence of ChRM rotation along a known (blue) and inferred (red) fault that cuts through the pumice.
This photograph shows a welded upper layer of the tuff in contact with the dacite lavas.
