Abstract Seismic mapping indicates that Lasuen Knoll in the Inner Borderland, offshore southern California, is a pop-up structure associated with a restraining stepover of the Palos Verdes Fault. Dextral shear is apparently transferred southeast through a complex zone of faults that includes the Carlsbad Ridge and Coronado Bank faults. This model is supported by comparison with other pop-up structures in the Inner Borderland and analogs to published sandpack laboratory simulations. The Palos Verdes Fault along Lasuen Knoll has existed at least since the late Miocene Epoch (5 to 8 Ma) and is contemporaneous with the segment of the Palos Verdes Fault on San Pedro Shelf and adjacent to the Palos Verdes Peninsula. Isochore maps of stratigraphic intervals indicate that extension occurred locally along the Palos Verdes Fault adjacent to the present Lasuen Knoll pop-up during the Mohnian Stage and became more widespread during the Delmontian Stage (approximately 7.5 to 5 Ma). Repettian strata onlapping Lasuen Knoll indicate that the knoll began to form as a pop-up structure by the early Repettian (entire stage approximately 5 to 2.3 Ma) and has been active possibly until the Holocene. Transtensional zones occur along the Palos Verdes–Carlsbad Ridge–Coronado Bank Fault Zone north and south of Lasuen Knoll. The San Gabriel Transtensional Zone, north of Lasuen Knoll, separates the knoll from the Palos Verdes Anticlinorium, the other major uplift structure along the Palos Verdes Fault. The Lasuen Knoll basement high, the overall structural high that includes Lasuen Knoll, is similar in dimension and shape to the Palos Verdes Anticlinorium, which has been interpreted to be underlain by a low-angle ramp fault. Recent models of low-angle ramp faults as major causes of the uplift of Palos Verdes, however, cannot be readily applied to Lasuen Knoll.

Abstract Hot springs along Sulphur Creek in Colusa County, California, have been recognized for about 130 years. Several researchers have proposed that the hot spring fluid there is derived from mixing of “connate” or “evolved connate” water which is derived from ancient seawater deposited in the Mesozoic sedimentary rocks. This water, which is similar in composition to Complexion Spring, mixes with meteoric water to form Wilbur Springs and other hot spring waters along Sulphur Creek. A δD - δ 18 O plot shows that Complexion Spring really does not plot along this trend; it must be isotopically modified to plot along the trend. Tuscan Springs, which is located 140 km NNE of Wilbur Springs, just NE of Red Bluff, has chemical and isotopic characteristics which are similar to the Sulphur Creek hot springs. Tuscan Springs vent from the Chico Formation of the Great Valley sequence and indicate that Tuscan Springs and Wilbur Springs are both derived from waters originating in the Great Valley sequence. Also δ 11 B correlates well with Cl, δD and δ 18 O, which originate in the Great Valley sequence, suggesting a similar source for the higher d 11 B values. Chemical geothermometry of the Sulphur Creek hot springs indicates a reservoir temperature of ~ 180 °C. This temperature agrees with measured homogenization temperatures from fluid inclusion which range from 150 to 180 °C. The calculated cation geothermometer temperatures are affected by the presence of dissolved Mg, even though the concentrations appear low.