关键词: 南口-孙河断裂带, 钻孔探测, 隐伏活动断层, 层序地层学

Abstract: In water area,shallow groundwater area and plains with thick Quaternary sediments,drilling is necessary for active fault survey.Due to complex facies change of Quaternary terrestrial formation,there is no mature scheme for the key problems such as borehole arrangement,decision and correlation of isochron surfaces and deciphering of episodic activity of fault.A test of drillhole exploration was implemented across the northern segment of Nankou-Sunhe Fault buried under Beijing plain.Based on the data of shallow seismic investigation,we drilled a row of boreholes.A combined borehole section was built by sequence stratigraphy,lithology and facies analysis,magnetic susceptibility and absolute chronology,which can define the location,geometry and accumulated displacements in several time spans.The result shows that the fault has an episodic movement since 60ka BP.The active stages of the fault are 60ka to 47ka BP,36ka to 28ka BP,and 16ka to present,respectively.Other intervals are relatively stable.The average vertical slip rate is 0.35mm/a from 60 ka to 37ka BP,0mm/a from 37ka to 32ka BP,0.78mm/a from 32ka to 12ka BP,and 0.35mm/a since 12ka BP.Stratigraphic cyclicity is the main issue of sequence stratigraphy.The cyclicity is of multilevel character.Controlled by climate fluctuations,tectonic movements and matter source factors,each sequence has a specialty itself as effective index to the correlation of borehole strata.Correlation of borehole strata from big to small sequence in turn can effectively reduce the blindness and the uncertainty.Hiatus of sediment strata of low base level such as low-water-level system territory and transgression territory on uplifted side indicates existence of fault scarp and active stage of fault slip.Along with the rise and later fall of base level,together with the attenuation of fault activity,high-water-level system territory and regression system territory may deposit on the uplifted side.Contrarily,homo sequence on both sides indicates weak activity of fault.Magnetic susceptibility reflects well the size difference of terrestrial formation and its vertical fluctuation corresponds well with sequence cyclicity.United application of sequence stratigraphy,lithology and facies analysis,magnetic susceptibility and age dating can achieve high-precision correlation of strata across boreholes.Since the late Quaternary shows distinct climate fluctuation of millenary scale,the substitute index of climate has a potential in high-precision correlation of strata.The borehole correlation of thick stratigraphic succession with a distance of 20～40m is easy in alluvial plains away from the affected zone of fault action.Whereas within rupture zone of fault,the correlation of strata thinner than 1m with a distance no more than 5m is usually difficult if there are no evident horizon markers,although such correlation is necessary for deciphering single surface rupture event.Some fault-scarp-derived colluvial wedges with a width less than the distance of neighbour holes may be missed.Therefore once the fault zone is limited within a bound less than 20m,hole distance less than 2～3m is necessary for probing single event.For the limit of borehole distance and the precision of stratigraphic subdivision,the correlation of borehole strata by sequence stratigraphy cannot reach sub-meter precision,and therefore can only decipher active and quiet stages of fault,but not single surface rupture event.An active stage may include several single surface rupture events.

Key words: Nankou-Sunhe Fault, drilling exploration, buried active fault, sequence stratigraphy