EARTHQUAKE CLUSTERING INFERRED FROM PLIOCENE GILBERT-TYPE FAN DELTAS IN THE LORETO BASIN, BAJA-CALIFORNIA-SUR, MEXICO

A stacked sequence of Pliocene Gilbert-type fan deltas in the Loreto b asin was shed from the footwall of the dextral-normal Loreto fault and deposited at the margin of a marine basin during rapid fault controll ed subsidence. Fan-delta parasequences coarsen upward from marine silt stone and sandstone at the base, through sandy bottomsets and gravelly foresets, to gravelly nonmarine topsets. Each topset unit is capped b y a thin shell bed that records marine flooding of the delta plain. Se veral mechanisms may have produced repetitive vertical stacking of Gil bert deltas: (1) autocyclic delta-lobe switching; (2) eustatic sea-lev el fluctuations; (3) climatically controlled fluctuations in sediment input; and (4) episodic subsidence produced by temporal clustering of earthquakes. We favor hypothesis 4 far several reasons, but hypotheses 2 and 3 cannot be rejected at this time. Earthquake clustering can re adily produce episodic subsidence at spatial and temporal scales consi stent with stratigraphic trends observed in the Loreto basin. This mod el is supported by comparison with paleoseismological studies that doc ument clustering on active faults over a wide range of time scales. Ea rthquake clustering is a nem concept in basin analysis that may be hel pful for understanding repetitive stratigraphy in tectonically active basins.