Mineral Geochemistry of Basaltic Rocks from IODP Expeditions 334 and 344: Implications for Magmatic Processes of Cocos Ridge Segment Being Subducted Beneath the Middle America Trench

The Cocos Ridge, which is subducted beneath the Central American Volcanic Arc, has a complex tectonic evolution history due to plume-ridge interaction between the Galápagos plume and the Cocos—Nazca spreading center. This study presents major and trace element analyses of plagioclase and clinopyroxenes hosted by Cocos Ridge basaltic rocks that were drilled in three holes (U1381A, U1381C and U1414A) of Sites U1381 and U1414 on the Cocos Ridge close to the Middle America Trench during the Integrated Ocean Drilling Program (IODP) Expeditions 334 and 344. The results show that (1) plagioclases are mainly bytownite and labradorite with subordinate andesine, which are enriched in light rare earth elements (LREE) and some large-ion lithophile elements (LILE) and exhibit marked positive Eu anomalies; and (2) that clinopyroxenes are augites, which are depleted in highly incompatible elements such as LREE and LILE, have nearly flat heavy rare earth elements patterns (HREE) and lack Eu anomalies in chondrite-normalized rare earth element (REE) diagrams. During the ascent to the surface, the primary magmas experienced fractional crystallization of plagioclase, clinopyroxene, Ti-Fe oxides and possibly olivine (complete replacement of olivine by secondary minerals). The crystallization temperatures of plagioclase phenocrysts and microlites are 1050 to 1269 °C, and 866 to 1038 °C, respectively, and the pressures of plagioclase phenocrysts are 0.3–0.7 GPa. The crystallization temperatures of clinopyroxene phenocrysts/micro-phenocrysts is 1174–1268 °C, similar to those of plagioclase phenocrysts, suggesting some of clinopyroxene and plagioclase phenocrysts cotectic crystallized during early stage of magmatic evolution. In addition, the equilibrium pressures of clinopyroxene phenocrysts/micro phenocrysts are 0.02–0.97 GPa, implying that the clinopyroxene started to crystallize within the mantle, and magma evolution has undergone an early crystallization stage with clinopyroxene and no plagioclase.

Nowcasting and multifractal features of the seismicity in the subduction zone of Tehuantepec Isthmus, southern México

<p>After the M8.2 earthquake occurred on September 07, 2017 at Isthmus of Tehuantepec, notable spatial and temporal changes where<br>registered, the temporal rate of occurrence increased and the spatial seismicity distribution showed a clear clusterization along<br>the region of collision of the Tehuantepec Transform/Ridge with the Middle America Trench off Chiapas. Also, the b-value in the<br>Gutenberg-Richer law showed changes in time. On the basis of that behavior we studied the sequence of magnitudes of the<br>earthquakes occurred within the Isthmus of Tehuantepec at southern Mexico from 2010 to 2020, by using the nowcasting method<br>and the multifractal detrended fluctuation analysis. Our findings suggest the b-value could depend on time and after the main-shock<br>M8.2, the underlying dynamics in the Tehuantepec ridge has been changed, which is clearly described by our analyses based on<br>nowcasting method and in the multifractality estimated changes.</p>

Subduction May Recycle Less Water Than Thought

A new analysis of seismic data from the Middle America Trench suggests that previous calculations have vastly overestimated the total amount of water transported to the mantle worldwide.

A Simplified Method to Invert Slow Slip Events: Examples for the 2002, 2006 and 2014 events in Guerrero, Mexico

An efficient method to obtain a simple model of the slip distribution on a fault plane was developed from estimates of static offsets from continuous GPS measurements. Transfer functions were computed for the displacement due to a unit force on a fault plane (with a predetermined geometry, subdivided in a den-se grid of subfaults) for all combinations of stations and subfaults, considering a layered media. The forward problem is built as a linear combination of elliptical slip-patches over the fault plane and the transfer functions between each patch and each station. To test the method, the 2002, 2006 and 2014 slow slip events (SSE) were modeled in Guerrero, Mexico, assuming that slip occurred along the subduction interface between the Cocos and North America plates. This subduction fault was modeled as two adjacent planes, both striking 289° for 600 km approximately parallel to the Middle America trench and with a width of 120 km (240 km total width). The shallower plane starts at the trench and dips 14° and the deeper plane is sub-horizontal (2° dip). The merged fault plane is segmented in 120x48, 5x5 km2 subfaults. A pre-calculated set of transfer functions is kept for future slow slip events, so that these only need to be computed for new GPS stations. A strong agreement between observations and synthetic data was found for all analyzed events, and the results are consistent with those reported by previous studies.