528 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



MENDOCINO FRACTURE ZONE 



M il l I ' 



170 M 



PIONEER RIDGE- 



Correlotable 

 magnetic 

 intensity 

 zone—* 



'fracturFzone "*"97~M 



Fig. 32.16. Horizontal displacements along fracture zones indicated by the offset magnetic 

 intensity field. Horizontal displacement along San Andreas fault also shown. After Menard (pri- 

 vate map). Murray fracture zone offset by Mason (1958) and Pioneer Ridge offset by Vacquier, 

 letter to Nature, 1959. Distances are in miles. 



block of oceanic crust south of the Murray fracture zone has moved 97 

 statute miles westward. Likewise, the intensity pattern is offset along the 

 Pioneer Ridge 170 statute miles (see Fig. 32.16) with the north block 

 having moved west (Menard and Vacquier, 1958). The north block of 

 the Mendocino fracture zone has moved the astonishing distance west- 

 ward of 1250 kilometers, according to Vacquier et al. (1961). These con- 

 siderable horizontal displacements are immediately thought of in 

 connection with postulated strike-slip movement of the San Andreas fault, 



and the relation of the several postulated movements is shown in Fig. 

 32.12. 



The magnetic expressions of the volcanoes are puzzling. Most all yield 

 positive magnetic impressions in the intensity contours, but in no way are 

 they as striking as the relief contours of the volcanic cones would suggest. 

 Compare Figs. 32.14 and 32.15. They deflect the intensity contours of 

 the dominant linear features or are superposed on them but are not 

 sufficiently strong to make much of an impression. The magnetic effect 

 is also variable according to Menard and Vacquier, who propose the 

 variability to be due to the fact that some cones are built of fragmental 

 material of lower intensity and some of massive flows of higher inten- 

 sity. 



The topography of the ocean floor has an irregular north-south fabric 

 but it is of low relief and in striking contrast to the relief of the volcanic 

 cones; yet its intensity contours are sharp and strong. 



Regarding the cause of the anomalies it is evident that the distribution 

 of rocks with different magnetic intensities must match the intensity 

 pattern, with allowance made for depth and several magnetic factors. 



In analyzing the profiles across the linear magnetic positive features 

 the seismic refraction data of the area were first considered (Mason, 

 1958). The velocities and interpreted rock layers are shown in Fig. 32.17. 

 The magnetic values are concluded to be compatible with those of basic 

 igneous rock, which is here characterized by a high susceptibility and 

 also a high intensity of remanent magnetization. As a consequence the 

 "volcanics" layer is taken as the most likely seat of the anomalies, and 

 calculations made to determine the depth, thickness, and lateral extent 

 of basic igneous rock masses to produce the observed profiles. If the 

 tabular mass is flat-bottomed, it would appear as in R, Fig. 32.17; if flat- 

 topped, as in C, to produce the anomaly shown in A. 



Rut how can we manage on a sound geologic basis the elongate tablets 

 of basalt, diabase, or gabbro of the required shape and magnitude prop- 

 erly spaced and in parallel arrangement? The structure must be com- 

 patible with the subdued relief of the ocean floor. It should be pointed 

 out that the topography of the northeast part of the area of Figs. 32.14 

 and 32.15 is particularly smooth and appears to be a graded alluvial 





