关键词: 全新世, 郯庐断裂带, 逆断层陡坎, 古地震

Abstract:

The Chishan section of Tan-Lu fault zone is located in Sixian County, northern Anhui Province. Research on the characteristics of Quaternary fault activity of this section began in the 1990s, which includes microgeomorphology survey, trench excavation, dating sample collection and measurement, and so on. Through these studies, many valuable data and results were accumulated, which laid a good foundation for the current research. Based on the field geological survey and previous studies, two geological trenches were excavated, which are named trench XJ1 and XJ2 respectively. Among them, very rich remains of ancient earthquakes were found in trench XJ1 and analyzed as major contents in this paper, and few relics of ancient earthquake were found in trench XJ2, which are not involved in this paper.

In the trench XJ1, ten strata units were revealed, labeled as U₁ to U₁₀ from old to young, respectively. Layer U₁ is the Cretaceous sandstone with a thickness about 0.5~1.0m, lying on the bottom of the west wall of the trench. Layer U₂ is yellowish brown clay with a thickness of 1~2.5m, located at the bottom of the eastern side of trench profile. One OSL sample is collected in the middle of this layer with an age more than 150k {\mathrm{a}}^{}BP, which indicates the layer was deposited before the Mid Pleistocene. Layer U₃ is purple clay-sand, which is wide at the bottom around 6.5m and narrow at the top around 2.5m, and the top extends about 7m continuously from west to east. Layer U₄ is motley gravel with a thickness about 2.0~2.5m, which is below layer U₉ and above layer U₄ on the west side of the trench wall. Layer U₅ is gravel containing a lot of clay and a few of sandstone clumps, wide at the top about 3m and narrow at the bottom about 2m. Layer U₆ is light green gravel containing some sand and clay, thick in the west about 0.8m and thin in the east about 0.2m, extending around 7m discontinuously from west to east. Layer U₇ is grayish white gravel with sand and clay, thick in the west around 1.0m and thin in the east around 0.2m, extending about 5m continuously from west to east. Layer U₈ is yellow clay with a thickness of 0.5~2.0m, located below layer U₉ and above U₇. One peat sample was taken from the top of the layer and the age of this sample is 21.57~21.22k {\mathrm{a}}^{}BP measured by Beta Analytic Inc in the United States, which indicates this layer was deposited in Late Epipleistocene. Layer U₉ is black clay with a thickness of 0.5~1.5m, which is located above Layer U₄, U₅, U₇ and U₈ and is the latest disturbed layer in the trench. One peat sample was taken from the bottom of this layer and the age of this sample is 11.10~10.75k {\mathrm{a}}^{}BP measured by Beta Analytic Inc in the United States, which indicates this layer was deposited in the early Holocene. Layer U₁₀ is the cultivation layer with a thickness of 0.2~0.5m, located on the topmost of the trench wall.

Three faults were revealed in these layers, named as F₁ and F₂ and F₃ respectively from east to west. Three paleoseismic events were identified, which are labeled as E₁ and E₂ and E₃ respectively from old to new. The E₁ represents a thrust activity of fault F₁. After the deposition of layers U₅, U₃ and U₂ finished, the hanging wall U₅ of fault F₁ thrust upward above the footwall U₈, and the soft layer U₃ in between was squeezed and rubbed upward, forming lenticles in the layer, which indicates the movement direction of the hanging wall of F₁ is thrust upward. A compressional overfall scarp was formed by this event, then the layer U₆ was deposited on the east side of the scarp, whose age is not measured. But the dating of layer U₂ beneath the fault F₁ yields an age before Mid Pleistocene, which constrains the lower limit age of E₁ to be after Mid Pleistocene. The E₂ represents a thrust faulting of fault F₂. After the deposition of layer U₆, a new thrust faulting occurred on fault F₂, which cut through layer U₅ and formed a thrust fault scarp. Later, U₇ and U₈ were deposited on the east of the scarp. The layer U₇ is gravel, whose age is not measured, but the layer U₈ is dated as the Late Epipleistocene, which constrains the upper limit age of events E₁ and E₂ to be after Late Epipleistocene. The E₃ represents a strike-slip normal faulting of Fault F₃, which faulted the layer U₃. According to the age of the layer U₃, we can constrain the lower limit age of E₃ to be the Early Holocene, which indicates that the Chishan section of the Tan-Lu fault zone is still active after the Early Holocene.

To sum up, two geological trenches were excavated at the Chishan section of Tan-Lu fault zone, named as trench XJ1 and XJ2 respectively, and three main faults were revealed on the wall of trench XJ1, named as F₁, F₂ and F₃ from east to west, and three paleoseismic events were identified, which are labeled as E₁ and E₂ and E₃ respectively from old to new. The latest ancient seismic event faulted the Early Holocene layer, indicating the Chishan section of the Tan-Lu fault zone is still active after the Late Holocene, and the latest activity is of strike-slip normal faulting, which provides new evidence for the presence of Holocene activity of this fault section and new information for long-term seismic risk assessment in this area.

Key words: Holocene, Tan-Lu fault zone, reverse fault scarp, paleoearthquake