GEOLOGICAL SOCIETY OF THE PHILIPPINES

 
 

Earthquakes, faults and transverse ridges—The Collision of Benham Rise into Luzon

 

Emmanuel Ramos and Raymundo Savella

 

Abstract


 

The Benham Rise has docked into N Luzon, causing:

  • the collapse of what used to be the East Luzon Trench into a deformed shallow trough;

  • seismicity that lacks the depth and structure of a normal Benioff Zone;

  • a series of earthquakes which suggests that thrust faulting in central Sierra Madre (or along the E Luzon Trough) leads to strike-slip faulting in Central Luzon;

  • the creation/conversion of curved, imbricate strike-slip faulting along N- and NW-trending faults into a plate margin; and

  • deformation of the northern edge of Luzon into transverse ridges that terminate the N-S grain of the Cordilleras, Cagayan Basin and northern Sierra Madre.

Aside from supporting the concept that Benham Rise has sutured with N Luzon, these features define the block that is moving NNW with the Philippine Sea Plate due to this collision.

 

A large section of N Luzon is therefore moving with the Philippine Sea Plate. This makes the western boundary of the Philippine Sea Plate not on the E Luzon Trough but further west than previously known. This boundary is now being defined as the broad range of deformation marked by the set of N- and NW-trending strike-slip faults where the NNW motion of the plate is accommodated. This plate boundary includes the Digdig Fault which caused the 1990 Luzon Earthquake.

 

On the north, the plate boundary of the Philippine Sea Plate is terminated by another wide zone of deformation—this time marked by imbricate transverse E-W trending ridges near the northern coast of Luzon. This section of Luzon is deforming by crustal shortening and thickening, providing the buffer that reduces the effect of the NNW vector of the Philippine Sea Plate in the Taiwan convergence zone.

 

Although moving as part of the Philippine Sea Plate, N Luzon is also internally deforming through faulting on the western edge and in its interiors, collisional orogeny on the northern edge, continuing crustal shortening on its contact with the Benham Rise, and an en echelon set of faults within the block. These tectonic processes define the nature of seismicity, uplift rates, convergence and deformation and possibly the distribution of mineral and hydrocarbon resources within this region of the Philippine archipelago.

 

 
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