下载掌阅APP，畅读海量书库
立即打开
Deep-sea Sediment Source Areas: Implications of Variable Rates of Movement between California and the Pacific Plate
J. R. Hein
Editor’s Note
The San Andreas Fault is the most conspicuous source of earthquakes in the US state of California. The fault is the boundary between the tectonic plate to the west which carries the floor of the Pacific Ocean and to the east which carries the continental United States. Here James Hein of the University of California at Santa Cruz presents an ingenious method of working out how the plates had moved relative to each other over the past 25 million years, using data gathered on expeditions of a deep-sea drilling ship supported by the US National Science Foundation. The relative displacement of the two plates is estimated at 5.5-7.0 centimetres per year. 中文
RECENT work has indicated acceleration of motion on the San Andreas Fault 1 , but previous studies have not delineated variable rates of motion with time between the Pacific and American plates. Offsets on the San Andreas and associated fault systems yield a mean rate of motion for a given period of time and deal with motion within the area of a wide plate boundary; therefore, these do not show total relative motion or variations in velocity between the two plates 2 . Magnetic anomalies can be used to measure the total relative motion and variations in velocity between the two plates. But magnetic anomalies at the mouth of the Gulf of California represent only the past few million years of movement which has been going on for at least 25 m.y. (ref. 3). Here I describe a method by which the amount and rates of motion between California and the Pacific plate with time can be determined. 中文
The Delgada submarine fan lies off northern California, south of the Mendocino Fracture Zone, and is at least Oligocene in age 4 . The fan lies in a unique position on the eastern edge of the Pacific plate directly adjacent to the varied lithology of source areas on the American plate to the east. If the fan has passed source areas of distinctive lithologies on the American plate, then its stratigraphy may indicate the amount and rate of movement between the two plates. This area is particularly well located for such a study because there is no land mass (source area) between the fan and the margin of the Pacific plate and because the Mendocino Fracture Zone inhibits the introduction of sediment into the fan area from the north 4 . Therefore changes in sediment composition may indicate the position of the fan relative to these distinctive source area rocks on the American plate. 中文
Semi-quantitative X-ray diffraction studies of the less than 2 µm fraction 5 from sites 32, 33 and 34, Leg V, of the DSDP, reveal significant changes in the percentages of the various clay minerals with time. The changes in percentages of kaolinite + chlorite/ illite/montmorillonite occur at approximately 4 m.y., 5 m.y., 9 m.y. and 15 m.y. (Table 1). 中文
Table 1. Computer Analysis of Maximum Variations in Percentages of Clay Minerals from Leg V, Sites 32, 33 and 34, DSDP
Changes in the detrital coarse fraction mineralogy, amphibole and pyroxene assemblages and changes in degrees of stress and strain indicated by stretching and undulatory extinction show significant changes during these same time periods (manuscript in preparation). 中文
The Delgada fan is assumed to be one basin of deposition and to have been built by turbidity current and hemipelagic deposition from continental source areas. A simple model of evolution of the fan can be constructed so that the changes in mineralogy and style of deformation of mineral grains are consistent with the changing source areas (Fig. 1). Important source areas in this model, schematically represented in Fig. 1, include: Northern Coast Ranges from Holocene to 4 m.y.; north-central Sierra Nevada from 4 m.y. to 5 m.y.; eastern Santa Cruz Mountains-northern Diablo Range from 5 m.y. to 9 m.y.; and southern Diablo Range-southern Sierra Nevada from 9 m.y. to 15 m.y. ago. 中文
The greatest interpretative problem is: how far the fan travelled into a proposed source area before the influence of that source area dominated the fan sediment. The transition from one lithology to another in fan sediment is variable, spanning as little as 200,000 yr at the 4 m.y. change to as much as 1.5 m.y. at the 9 m.y. lithology change. Arbitrarily using the center point of these time intervals and the palaeogeographic lithologic boundaries allows a preferred interpretation of rates of motion with time between California and the Pacific plate (Fig. 2). In all cases distances to varying lithologies shown in Fig. 1 are measured from the head of Delgada Canyon. 中文
Fig. 1. Palaeogeography of central California 6-8 . Cross-lined represents northern Coast Ranges, lined the Diablo Range and eastern Santa Cruz Mountains, the dotted line is the present coast line. The Sierra Nevada Mountains would be outside the north margin of parts A and B . A , The fan at 4 m.y.; B , at 5.5 m.y.; C , at 9 m.y. (—) and 15 m.y. ago (- - -).
The rate of motion of 5.5 to 7.0 cm yr –1 for the past 4 m.y. yielded by this method agrees with the rate calculated from marine magnetic anomalies 3 and adds validity to the choice of space-time boundaries. Reasonable variations in the placement of the palaeogeographic boundaries and possible errors in time boundaries would give ranges of rates of motion shown in Fig. 2. Even if the preferred interpretation is not correct, the data strongly suggest an increase in the rate of motion between California and the Pacific plate from about 16 m.y. to the present. 中文
Fig. 2. Increasing rate of motion between California and the Pacific Plate. The ordinate is distance along the San Andreas Fault from the head of Delgada Canyon; the abscissa is absolute age of the fan sediment. The curve represents my preferred interpretation. Shaded areas are possible range of values: 5.5 to 7.0 cm yr –1 from Holocene to 4 m.y., 3.4 to 5.5 cm yr –1 from 4 to 9 m.y. and 2.8 to 4.0 cm yr –1 from 9 to 15 m.y. ago.
I thank T. Atwater, D. Bukry, R. Coe, R. Garrison, G. Griggs, J. Sumner, and S. Wright (Leg V, DSDP) for help. 中文
( 241 , 40-41; 1973)
James R. Hein
Earth Sciences Board, University of California, Santa Cruz, California 95060
Received September 21, 1972.
References: