Thermal structure of the Costa Rica – Nicaragua subduction zone

Estudiamos la distribución frecuencia-magnitud a lo largo de la trinchera mesoamericana (MAT), usando 2345 eventos del periodo 1964-1994. Utilizamos el catalogo regional MIDAS con magnitud de completitud de 4.2. Para mapear el valor b como función de la profundidad (enfoque unidimensional), aplicamos el procedimiento de ventanas deslizantes en la vertical. Cada ventana contiene un número constante de eventos. Para obtener más detalles en la distribución del valor b, proyectamos los hipocentros del catálogo en tres regiones (aproximadamente Guatemala-El Salvador, Nicaragua, Costa Rica), hacia planos perpendiculares a la trinchera. Luego, calculamos el valor b en volúmenes cilíndricos deslizantes (enfoque bidimensional) que contienen un número constante de eventos y con centros en los nodos de un enrejillado de 5 km x 5 km. El valor b varía significativamente a lo largo de la MAT. Identificamos valores altos de b en la parte superior de la litosfera subducida, a profundidades de 80-110 km por debajo de Guatemala y El Salvador, y a profundidades de 130-170 km por debajo de Nicaragua. Localizamos valores anómalos (altos) de b en la parte inferior de la litosfera, a profundidades de 50-90 km y 50-160 km por debajo de Guatemala-El Salvador y Nicaragua, respectivamente. Las anomalías observadas en la parte superior de la litosfera pueden estar relacionadas con deshidratación e incremento sucesivo de la presión de poro en la litosfera descendiente. Estos, a su vez, producirían el volcanismo que ocurre sobre las anomalías en la parte superior de la litosfera. Las anomalías en la parte inferior de la zona de Wadati-Benioff podrían estar asociadas con el alto gradiente térmico entre la litosfera y el manto.

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Exhumation of subducted mafic rocks in a dynamically evolving thermal structure: constraints from phase equilibria modelling

10.5194/egusphere-egu21-13868 ◽

2021 ◽

Author(s):

Rilla C. McKeegan ◽

Victor E. Guevara ◽

Adam F. Holt ◽

Cailey B. Condit

Keyword(s):

Phase Equilibria ◽

Subduction Zone ◽

Subduction Zones ◽

Mantle Wedge ◽

Dynamic Models ◽

Thermal Structure ◽

Temporal Dependence ◽

Subduction Channel ◽

Mafic Rocks ◽

Point Of No Return

The dominant mechanisms that control the exhumation of subducted rocks and how these mechanisms evolve through time in a subduction zone remain unclear. Dynamic models of subduction zones suggest that their thermal structures evolve from subduction initiation to maturity. The series of metamorphic reactions that occur within the slab, resultant density, and buoyancy with respect to the mantle wedge will co-evolve with the thermal structure. We combine dynamic models of subduction zone thermal structure with phase equilibria modeling to place constraints on the dominant controls on the depth limits of exhumation. This is done across the temporal evolution of a subduction zone for various endmember lithologic associations observed in exhumed high-pressure terranes: sedimentary and serpentinite m&#233;langes, and oceanic tectonic slices.Initial modeling suggests that both serpentinite and sedimentary m&#233;langes remain positively buoyant with respect to the mantle wedge throughout all stages of subduction (up to 65 Myr), and for the spectrum of naturally constrained ratios of mafic blocks to serpentinite/sedimentary matrix. In these settings, exhumation depth limits and the &#8220;point of no return&#8221; (c. 2.3 GPa) are not directly limited by buoyancy, but potentially rheological changes in the slab at the blueschist-eclogite transition stemming from: the switch from amphibole-dominated to pyroxene-dominated rheology and/or dehydration embrittlement. These mechanisms may increase the possibility of brittle failure and hence promote detachment of the slab top into the subduction channel. For the range of temperatures recorded by exhumed serpentinite m&#233;langes, the locus of dehydration for altered MORB at the slab top coincides with the point of no return (2.3 GPa) between 35 and 40 Myr, suggesting a strong temporal dependence on deep exhumation in the subduction channel.&#160;Tectonic slices composed of 50% mafic rocks and 50% serpentinized slab mantle show a temporal dependence on the depth limits of positive buoyancy. For the range of temperatures recorded by exhumed tectonic slices, the upper pressure limit of positive buoyancy is ~2 GPa, and is only crossed between ~30 and 40 Myr after subduction initiation. Some exhumed tectonic slices record much higher pressures (2.5 GPa); thus, other mechanisms or lithologic combinations may also play a significant role in determining the exhumation limits of tectonic slices.&#160;Future work includes constraining how the loci of dehydration vary through time for different degrees of oceanic crust alteration, how exhumation limits and mechanisms may change with different subducting plate ages, and calculating how initial exhumation velocities may vary through time. Further comparison with the rock record will constrain the parameters that control the timing and limits of exhumation in subduction zones.

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