Thrust Faults and Thin-Skinned Tectonics: Evidence of a Widespread Decollement 
Under the Cordillera Mountain Ranges of Luzon, Philippines

Emmanuel G. Ramos & Arriane C. Tabanao

Tekton GeoMetrix, Inc.


Large horizontal faults under tall ridges that are linked to shallow thrusts in low foothills suggest the widespread presence of a decollement under Luzon’s Cordillera Mountain Ranges. These proofs for active thin-skinned tectonics will now require rethinking of not only for the academic knowledge of orogenic stresses and tectonic deformation in the Cordilleras, but also for the practical information on how earthquake hazards are affected by such conditions, and how ore bodies and geothermal resources can be massively displaced by horizontal faults.

In several areas in the Cordilleras, thrust faults are observed around low hills and valleys but then fading out as the fault traces approach the tall ridges. These shallow thrust faults are readily observed in Kapangan-Kibungan area, and in several areas in and around Baguio City. In conjunction, horizontal faults are observed under some of the tall ridges, seen as exposures of horizontal faults along some river channels and in some mine shafts and drill holes. The genetic relationship between the shallow thrust structures and the horizontal faults suggests the presence of a decollement along which the gliding movement of the massive mountain ranges are facilitated.

The thrust faults generally strike N to NE wrapping around the north or northwest edges of the elevated terrain that forms the overriding block. The dip is commonly to the east or southeast, appearing listric and becoming shallower at depth where it merges with a horizontal fault. The shallow thrust faults that can be linked to a horizontal fault suggest that the phenomenon cannot be isolated—since mountains can only move either by displacing the adjacent ridges, or by all the ridges moving together in unison. Since there is no identifiable set of convergent zones in the Cordilleras, the only reasonable conclusion is that the horizontal fault is widespread and that the mountains are moving in unison over an extensive decollement.

The existence of the thrust faults and the decollement brings to fore several items that need to be seriously considered:

  1. How does orogeny and crustal movement in Luzon’s Cordillera Rangess proceed under such thin-skinned tectonics? What is happening under the horizontal fault?
  2. Did the decollement influence the massive damage of the earthquake in Baguio during the 1990 earthquake?
  3. Does the thin-skinned tectonics extend beyond to the east to the Sierra Madres? To the south to Central Luzon?
  4. How do you explore for the extension of ore bodies and geothermal systems that are truncated and massively displaced by a decollement?

Probing for the answers will first require a paradigm shift: That a decollement can exist in the backyards of Luzon’s island arcs.

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