Tectonic geomorphology of a large normal fault: Akşehir fault, SW Turkey

Abstract

In order to better understand the activity of the Akşehir normal fault in SW Turkey and the associated seismic hazard, we investigated the tectonic geomorphology of a 60-km stretch of the 100-km-long Akşehir fault block. The fault can be separated into seven geomorphic segments (1 to 7 from NW to SE) along the mountain front. Segment length varies from about 9 to 14km, and relief of the horst block varies from about 0.6km in the SE to 1.0km in the NW. Analysis of the tectonic geomorphology of 32 drainage basins and mountain front facets using a combination of geomorphic indices reveals a general pattern of high slip rates in the northern and central segments and low slip rates in the southern, probably older, segments. We show that mountain front sinuosity varies from about 1.1 to 1.4 on segments S1-S6 to 2.4 on segment S7, suggesting that the six northern segments are more active than the southernmost segment. Similarly, χ analysis and slope-area analysis of streams reveal a pattern of steepest channels draining the central and northern segments of the horst. The ratio of valley floor width to valley height varies from 0.2 to 0.6, which are typical values for tectonically active mountain fronts; and alluvial fans along segments S1, S2, and S4 are back-tilted. Finally, we show that (1) shapes of the ~100–900m high mountain front facets are mostly triangular (~80%) and partly trapezoidal (~20%); (2) facet slopes range from 6 to 22°; (3) facets at the NW and SE segment ends are larger than the intervening facets; and (4) steepest facets occur along the central segments. Uplift rates estimated from the slope of mountain front facets range from about 0.06m/ky on the southernmost fault segment (S7) to 0.23m/ky on the more central S5 and 0.16m/ky on the northern segment (S1). The estimated pattern of uplift is consistent with the pattern of geomorphic indices. The vertical relief of the facets suggests that uplift of the mountain front initiated in the late Miocene-early Pliocene and continues to the present, with the earliest surface-rupturing faults on the southernmost fault segment (S7).Large normal faults with a similar slip rate of 0.2–0.3m/ky typically have strong earthquakes every few thousand years. Therefore, the many moderate to strong earthquakes on the Akşehir fault in the past few hundred years may be misleading. A paleoseismic evaluation could answer questions concerning the area's earthquake hazard. The tectonic geomorphology suggests that the Akşehir fault is active, and larger earthquakes than those of the historic period are a potential threat.