THE ORIGIN OF MODERN CHIAPANECAN VOLCANIC ARC IN SOUTHERN MEXICO INFERRED FROM THERMAL MODELS

http://www.geo.mtu.edu/~raman/papers2/Manea_Chiapas_2005.pdf

see http://plate-tectonic.narod.ru/volcanoam13k1photoalbum.html


MODEL CONSTRAINTS

To gather all these assumptions together, we developed a thermal model with a fixed mantle wedge tip which simulates the decoupling of the subducting slab due to mantle serpentinization, and a hot source (1200° C) beneath the MCVA which represents the NW-SW asthenospheric intake. The results show a well developed serpentinized mantle wedge tip. It can be seen that the mantle wedge beneath Chiapas lies just in front of a cold area which corresponds to the flat slab. In order to investigate the existence of a cold and serpentinized mantle wedge beneath Chiapas, we need to look for some external constraints for our model. The serpentinization process has two major effects on the physical properties of peridotite: the density decreases from ~ 3000 kg/m3 to 2500 kg/m3 (Christensen, 1966; Saad, 1969), and the remnant magnetization increases by at least an order of magnitude (Saad, 1969). A recent study of Blakely et al. (2004) demonstrates that magnetic mantle is a common feature in subduction zones like Cascadia, Japan and southern Alaska. The serpentinized mantle can be enlightened by the presence of a long-wavelength magnetic anomaly above subduction zones. The availability of a new magnetic map for North America (NAMAG, 2002) allows us to prove the existence of a large amount of serpentine in Chiapas. Aeromagnetic data display a distinctive long wavelength positive anomaly (~ 500 nT) offshore Chiapas. Unfortunately, the maximum extent inland of the serpentinized wedge is not possible to be inferred because the magnetic data lack onshore Chiapas. Instead, the contact between Moho and the subducting Cocos slab is obtained using the onset of the strong positive magnetic anomaly. Since the magnetic anomaly has not been migrated to the pole, the source is not located just beneath. 2D magnetic profile migrated to pole anomaly. The onset of the serpentinized mantle wedge is located at ~ 125 km from the trench, and runs parallel to the MAT. This provides a Moho depth beneath the Chiapas coast of ~ 35 km which is consistent with the results of Bravo et al. (2004) who give a Moho average depth beneath the Gulf of Tehuantepec of 28.5 +/- 3.5 km. Also, the maximum extent of the rupture areas of past subduction earthquakes are in good agreement with the onset of the positive magnetic anomaly, and are therefore being controlled by the ductile serpentinized mantle wedge tip rather than by temperature. On the other hand, the updip seismogenic limit is controlled by temperature, the 100° C isotherm being in good agreement with the onset of rupture areas

DISCUSSION AND CONCLUSIONS

In this study we inferred the steady-state thermal structure beneath Chiapas using the numerical scheme proposed by Manea et al. (2005-C). The thermal models beneath the northernmost edge of CAVA successfully explain the calc-alkaline character of the magmatic products. Also the thermal model for the flat slab just northwest of TR predicts the origin of the calc-alkaline and alkaline magmas of the TVF . The only model which is not consistent with the position of the active volcanic arc is the model located just southeast of the TR beneath MCVA . Also the position and the alkaline character of the El Chichón volcano cannot be explained by this model. To address these issues, we proposed an alternative model in which a mantle wedge flow perturbation is produced by a strong intake related with the space-time history of the subducting slab beneath Central Mexico (Ferrari et al., 1999). The flattening of the slab 17 Ma ago, would have produced a strong mantle wedge flow from NW to SE, forming the MCVA. The migration of the intake further southeast can be seen in the age distribution of magmatic rocks through the MCVA (Damon and Montesinos, 1978. The cessation of the ancient Miocene Sierra Madre volcanic arc suggests that the mantle wedge beneath should be cold enough to prevent mantle wedge melting. We argue that the proximity of the flat slab is responsable for this cooling down. To constrain such model we use the availability of aeromagnetic data offshore Chiapas. A good correlation between the strong positive magnetic anomaly and the serpentinized mantle wedge suggests that the mantle wedge beneath the extinct Miocene Sierra Madre arc is sufficiently cold (T < 600° C) to prevent melting.

Although these models offer a reasonable explanation for the origin of the MCVA, there is still an important unknown left: El Chichon volcano. De Ignacio et al. (2003) proposed an astenospheric intake through major faults in the subducting Cocos slab. This intake would have melted the oceanic slab and produced adakitic magmas. Unfortunately, this scenario is very difficult to prove, because we have to assume that the Cocos slab beneath Chiapas is segmented and decoupled in different blocks along slab dip faults. If this is the case, then the two parts of the subducting slab, northwest and southeast of TR, are completely decoupled. But an interesting thing can be readily observed in: although in front of Chiapas the oceanic plate is older ( 29 +/- 3 Ma from Manea et al., 2005-B), and therefore denser, just southeast TR, its dip is smaller than the slab dip further south where the slab is younger ( 23 Ma). This paradox can be easily explained by the existence of the flat subduction just northwest of TR, which actually does not allows the steeper slab to sink freely into the asthenosphere, therefore proving the nonexistence of such faults in the Cocos slab. Since the model of De Ignacio et al. (2003) looks quite unrealistic, we have to look for an alternative model in order to explain the singularity of El Chichon volcano within MCVA. There is a striking correlation between the TR and the position of El Chichon volcano, also pointed out by. It is known that the long-offset fracture zones are subject to a double serpentinization process due to bend-faulting at the outer rise (East pacific Rise for the TR) and at the trench axis. These faults might be conduits for seawater to react with the slab lithosphere and serpentinize it. If so, then such long fracture zones might produce tears beneath volcanic arc because they are water rich, therefore favoring more melting just above the slab surface. Indeed, a strong positive magnetic anomaly is observed along the TR ,suggesting that the slab-lithosphere might be highly serpentinized. The slab geotherm through the slab-lithosphere for the cold thermal model shows that 10 %wt might be released into the mantle at greater depths between 100 and 150 km, close beneath the El Chichón volcano. Since El Chichon volcano is very young ( 0.2 Ma), we argue that this might reflect the arrival of the serpentinized TR beneath the MCVA at least 0.2 Ma ago, resulting from an instantaneous magma rise through the mantle. Assuming a constant convergence rate of 7 cm/yr, the TR would have commenced subduction at least 6 Ma ago.


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