Paleomagnetism of Late Jurassic to Early Cretaceous red beds from the Cardamom Mountains, southwestern Cambodia: Tectonic deformation of the Indochina Peninsula

- Jurassic to Cretaceous paleomagnetic direction is identified in southern Cambodia.
- It is characterized by easterly deflected direction: D=43.4°, I=31.9° (α95=3.6°).
- Southern Cambodia experienced southward displacement as a part of Indochina Block.
- It rotated clockwise ∼15° larger than the Khorat Basin.
- Lithosphere is not rigid beneath the southern tip of the Indochina Block.

Late Jurassic to Early Cretaceous red beds of the Phuquoc Formation were sampled at 33 sites from the Sihanoukville and Koah Kong areas of the Phuquoc-Kampot Som Basin, southwestern Cambodia. Two high-temperature remanent components with unblocking temperature ranging 650°-670 °C and 670-690 °C were identified. The magnetization direction for the former component (D=5.2°, I=18.5° with α95=3.1° in situ) reveals a negative fold test that indicates a post-folding secondary nature. However, the latter component, carried by specular hematite, is recognized as a primary remanent magnetization. A tilt-corrected mean direction of D=43.4°, I=31.9°(α95=3.6°) was calculated for the primary component at 11 sites, corresponding to a paleopole of 47.7°N, 178.9°E (A95=3.6°). When compared with the 130 Ma East Asian pole, a southward displacement of 6.0°±3.5° and a clockwise rotation of 33.1°±4.0° of the Phuquoc-Kampot Som Basin (as a part of the Indochina Block) with respect to East Asia were estimated. This estimate of the clockwise rotation is ∼15° larger than that of the Khorat Basin, which we attribute to dextral motion along the Wang Chao Fault since the mid-Oligocene. The comparison of the herein estimated clockwise rotation with the counter-clockwise rotation reported from the Da Lat area in Vietnam suggests the occurrence of a differential tectonic rotation in the southern tip of the Indochina Block. During the southward displacement of the Indochina Block, the non-rigid lithosphere under its southern tip moved heterogeneously, while the rigid lithosphere under the Khorat Basin moved homogeneously.