Sag-ponding and its Significance in determining Paleo-seismic events along the active strike- slip fault

During the development of one active fault, we really want to know how it behaves and what it will do next. This mostly depends on the record and preservation of the information showing the action of the fault. Sparse young sediments or sediments with coarse grain along most of big strike-slip faults make it hard record and preserve the vestige of the paleo-seismic events. This extremely restricts the development of the Paleo-seismic research. Sag-ponding as well as the deposits in ponds, which are formed by the movement of the fault, can help settling the difficulty. Periodic sag-ponding is a feature to which should be paid more attention along the strike-slip fault, it can develop a pond to capture plenty fine sediments which well record the action of the faults. Sag-ponding can easily be found on the main active strike-slip faults in northern and eastern Tibet. By disclosing the sag-ponding depositions with 3-D excavations, sediment distribution and characters of relevant sag-ponds, and the relation between the sag-ponding and faulting were discussed. 1. Mechanism of the formation of the sag-pond When the valleys and ridges intersecting with the fault are displaced, the fault scarps will block the flow of the streams cut by the fault, or make the gullies develop ancon-like bend. This would form a space for water-storage, and thus a sag-pond comes into being. If the fault behaves like this many times, multi-sag-ponding will occur. 2. Rhythmic sag-ponding deposition features and stratigraphic sequence (1) Vertical characteristics. Observed from the stratigraphic profiles disclosed by the excavation, stratigraphic sequence shows good rhythms. There are several rhythms in each pond, and one rhythm is composed of the lower coarse layers and the upper fine layers. That is, the grains are coarser below and finer upward. (2) Transverse variation. In the direction parallel to the fault, the deposition center of each sag-pond appears regular movement, or migration, associated with the slipping of the fault. 3. Faulting pattern of the sag-ponding Observations indicate that the deposition rhythm is concerned with the periodic behavior of the faulting. During the long-term activity of the fault, when the strain accumulates to some extent, the fault will move suddenly and violently, then come into a relative quiet period. This leads to the grains courser downside and finer upside in one rhythm in the sag-pond. If the fault acts like this several times, it will form a sequence composed of several deposition rhythms. 4. Slip amounts estimated by transverse variations of the depositional rhythm. One rhythm in a sag-pond represents a sag-ponding process. As a result of the strike-slipping, the pre-formed deposition center migrates with the movement of the fault in a direction parallel to the strike of the fault trace. By measuring the location variation of each of the deposition centers, we can determine the amounts of horizontal displacements of sag-ponding sediments in each pond. 5. Paleo-seismic events reflected by vertical variations of depositional rhythms Every rhythm of the deposition is the product of one slip event of the fault. These rhythmic structures actually reflect the abruptness and periodicity of the movement of the fault, which are really paleoearthquake events and their reoccur intervals. It can be inferred that each sag-ponding rhythm corresponds to a paleoearthquake event, thus the sag-ponding deposition sequence can be discussed in contrast with the paleoearthquake event sequence.