| dc.description |
A large, damaging earthquake in 1944 on a blind thrust fault caused 60 cm of surface rupture on the subsidiary La Laja fault and additional unmeasured growth of an associated backlimb fold. Both the fold and fault are components of the La Laja Fault System (LLFS) located 25 km northeast of San Juan, Argentina. Deformed, syntectonic growth strata were investigated in detail to determine whether such units preserve evidence of individual paleoearthquakes. The objective of this study is to provide a clearer understanding of the signal of fold growth on short timescales, and show the extent to which growth strata can provide value in deducing the paleoseismic record of thrust tectonic regimes.
Two large (50 m x 6 m x 7 m) trenches were excavated within and outside of active drainages to 1) analyze stratigraphic geometries to determine the mechanism of fold growth, and 2) compare the deformational history contained within the growth strata with fold growth in an historic earthquake and with earthquake chronologies from the La Laja fault. Contact relationships of exposed trench strata indicate that a mixed-mode of fold growth involving progressive limb rotation and kink-band migration characterize the evolution of the fold. Mixed-mode fold growth results from hangingwall displacement along a broad, curved shallow ramp, which is driven by earthquakes on a larger, deep blind thrust fault. The growth strata record three distinct fold events. These fold events are expressed through three unconformity-bounded stratal packages of units with similar tilt. The most recent fold event recorded by the youngest strata is associated with twice as much shortening as uplift (100 cm [60%] shortening to 70 cm [30%] uplift). Uplift exceeds shortening in the two older folding events. Restoration of folded growth strata yields cumulative shortening of 0.5 m - 2 m and cumulative uplift of 8.2 m - 9.7 m. Overall, uplift dominates the tectonic activity since > 64 kyr. An earthquake deformed the youngest unit in the trench exposures, Unit A (5.88 kyr) in 1944. Relief on the base of Unit A is 130 cm, greater than twice the 60 cm of uplift recorded in the 1944 event. Therefore, the deformed Unit A likely records multiple paleoearthquakes. Capture of individual paleoearthquakes by growth strata in this semi-arid environment requires either a relatively higher rate of sediment deposition or larger degrees of folding in order to be stratigraphically differentiable. This study finds that an understanding of both the fold mechanism and the geomorphic setting of sediment accumulation are required to unambiguously identify individual paleoearthquakes in growth strata through paleoseismic trenching. |
|