| Popis: |
Determination of paleocurrent directions is a robust approach to establish the mechanism of sediment transport and of deposition. However, in tectonically controlled (or affected) basins where rotations are suspected, it is of critical importance to correct previously calculated paleocurrent directions to establish the original (pre-tectonic) transport and deposition patterns. In order to illustrate this, we determined the orientations of the anisotropy of magnetic susceptibility (AMS) fabrics from 14 sites distributed along the sedimentary deposits interbedded within the Late Miocene Main Volcanic Unit (MVU) of the Taitao ophiolite, Southern Chile. Data were used to estimate characteristics (directions and relative velocities) of the sediment transporting paleocurrents and orientations of the depositional slope. Sedimentary deposits consist of a rhythmic sequence of conglomerate (base), sandstone and mudstone (top) beds that present a systematic decrease in grain size up-sequence as a whole, and within individual beds. Sedimentary deposits of the MVU, as well as overlaying effusive volcanic products, are disrupted by small and discrete dextral/normal brittle faults. The distribution of fault planes and their kinematic characteristics suggest the development of a dextral negative flower-like fault zone in the area. Previously reported NW-ward paleocurrent directions obtained from the orientations of sedimentary structures roughly agree with our calculations based on the orientations of AMS fabrics. Calculated paleocurrent directions are sub-parallel to the NW-SE trend of Estero Cono channel, the main lineament in the area. The spatial distribution of AMS axes between and within sites suggest that coarse grain deposits were transported by relatively high velocity flows whereas fine grained deposits were by low to nearly null velocity flows. Previously reported paleomagnetic analyses indicate that counterclockwise rotations affected the whole Taitao ophiolite, including the MVU. By applying the necessary corrections we obtained paleocurrent directions yielding mostly NNW-ward directions. Restoration of the orientations of beddings indicates that the depositional surface dipped steeply at its easternmost edge and shallowly at the westernmost end. We propose here that sediments were transported by turbidite flows generated by slope instabilities in the innermost part of the Estero Cono. These turbidites followed the Estero Cono lineament while they filled the channel and transported sedimentary material (eroded from the upper portions of the ophiolite and nearby units) towards the Pacific Ocean. Sediments were deposited on a NW- to W-facing slope and sorted in decreasing grain size along the channel. The instability of the slope was most probably triggered by the activity of a transtensional fault zone developed along the mid-axis of the Estero Cono channel. |
