2011

U.S. Department of the Interior

U.S. Geological Survey

Aeromagnetic and Aeromagnetic-based Geologic Maps of 

the Coastal Belt, Franciscan Complex, Northern California

By V.E. Langenheim, R.C. Jachens, C.M. Wentworth, and R.J. McLaughlin

Pamphlet to accompany

Scientific Investigations Map 3188
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i

Contents

Introduction.....................................................................................................................................................1

Aeromagnetic Surveys..................................................................................................................................2

Merging ..................................................................................................................................................2

Aeromagnetic Map Processing and Anomaly Enhancement ................................................................2

Reduction to Pole..................................................................................................................................2

Anomaly Enhancement........................................................................................................................2

Locating Edges of Magnetic Bodies..................................................................................................3

Anomaly Sources and Magnetic Susceptibility Measurements ...........................................................3

Magnetic Anomalies .....................................................................................................................................4

Approximate Dips Inferred from Residual Aeromagnetic Anomalies...................................................4

Discussion.......................................................................................................................................................5

Conclusions.....................................................................................................................................................6

Acknowledgments.........................................................................................................................................6

References Cited............................................................................................................................................6

Figures

1. Index map of coastal California showing geology simplified from Jennings and others 

(1977)..........................................................................................................................................9

2. Geologic map of the study area ..................................................................................................10

3. Magnetic susceptibility measurements of various rock types in the Coastal belt.............11

4. Aeromagnetic, ground magnetic, and magnetic susceptibility data for the Fort Ross 

transect...................................................................................................................................12

5. Predicted magnetic anomalies (reduced to pole) for vertical and dipping magnetic 

layers.......................................................................................................................................13

6. Aeromagnetic map of Laguna Point area, showing ground-magnetic traverse, groundmagnetic data, magnetization boundaries, and locations of magnetic susceptibility 

measurements .......................................................................................................................14

7. Magnetic susceptibility measurements from the Laguna Point area...................................15

8. Model across the Laguna Point magnetic anomaly.................................................................16

9. Interpreted magnetic anomalies of the Coastal belt superposed on geology from 

figure 2.....................................................................................................................................17

Tables

1. Aeromagnetic survey specifications..........................................................................................18

2. Interpreted strikes and dips from magnetic anomalies. .........................................................19

Sheets

1. Map A. Mosaic aeromagnetic map of the northern Coast Ranges, California superposed 

on shaded-relief topography. Map B. Merged shaded-relief aeromagnetic map of 

the northern Coast Ranges, California. Mendocino, Redding, and Yorkville surveys 

upward continued 150 m to match the average height above ground for the Ukiah 

survey.

2. Residual, reduced-to-pole aeromagnetic map of the northern Coast Ranges, California.

3. Residual, reduced-to-pole aeromagnetic map of the northern Coast Ranges, California, 

with interpreted dips from asymmetry of residual magnetic anomalies.
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1

Introduction

The Coastal belt of the Franciscan Complex represents 

a Late Cretaceous to Miocene accretionary prism and overlying slope deposits. Its equivalents may extend from the offshore outer borderland of southern California (Vedder, 1987; 

McCulloch, 1987) to north of the Mendocino Triple Junction 

under the Eel River Basin (Bachman and others, 1984; fig. 1) 

and in the offshore of Cascadia (Aalto and others, 1995; 

McLaughlin and others, 2000; Stewart and Brandon, 2004). The 

Coastal belt is exposed on land in northern California, yet its 

structure and stratigraphy are incompletely known because of 

discontinuous exposure, structural disruption, and lithologically 

non-distinctive clastic rocks. The intent of this report is to make 

available, in map form, aeromagnetic data covering the Coastal 

belt that provide a new dataset to aid in mapping, understanding, and interpreting the incompletely understood geology and 

structure in northern California. 

Coastal belt rocks crop out from Bodega Bay in the south to 

Cape Mendocino in the north, and as much as 50 km east of the 

coast (fig. 2). The eastern margin of the belt is marked in northern 

California by the Coastal Belt Thrust, which places largely older, 

higher grade rocks of the Central belt of the Franciscan Complex 

(Bailey and others, 1964; Blake and others, 1988) above the 

Coastal belt. The Coastal belt also underlies the offshore region 

north of Point Arena east of the offshore San Andreas Fault 

(fig. 1). Some areas of the Coastal belt are mapped in varying 

degrees of detail (Kleist, 1974; O’Day, 1974; Kramer, 1976; 

Blake and others, 2002; McLaughlin and others, 2000; Beutner 

and others, 1980; Orchard, 1979; Jayko and others, 1989), but 

the region between the Sonoma County line (dashed black line in 

fig. 2) and latitude 39o30’ is mapped only at a scale of 1:250,000 

or smaller (Jennings and Strand, 1960; Wagner and Bortugno, 

1982; Jennings and others, 1977, at a scale of 1:750,000). The 

region is sparsely populated, a circumstance that has provided 

little incentive for modern detailed mapping. 

Following the convention of McLaughlin and others 

(2000), the Coastal belt can be divided into several terranes 

(fig. 2). The Coastal terrane is the largest and most lithologically diverse terrane of the belt. The terrane consists largely of 

pervasively sheared graywacke, shale, and minor conglomerate, with rare but characteristic exotic blocks of volcanic rock, 

pelagic limestone, chert and extremely rare blocks of blueschist. 

The Yager terrane is broadly coeval with the Coastal terrane 

but is less deformed and more lithologically homogeneous. The 

absence of exotic blocks and of Cretaceous fossils in the Yager 

terrane differentiates it from the Coastal terrane. The sandstones 

were deposited in a slope setting structurally above and likely 

interfingered with the accreted strata of the Coastal terrane 

(Bachman and others, 1984; McLaughlin and others, 1994). 

The Miocene King Range and False Cape terranes represent the 

youngest recognized accretion-related terranes of the Franciscan 

Complex (McLaughlin and others, 1982; Aalto and others, 

1995; McLaughlin and others, 2000); their age and presence of 

diatoms differentiate them from the Coastal and Yager terranes. 

Graywackes, which are by far the dominant rock type in 

the Coastal belt, are difficult to distinguish from one another in 

hand specimen or at outcrops. Underwood and Bachman (1986) 

were able to document five sandstone petrofacies within the 

Coastal and Yager terranes by using petrographic analyses. In 

addition, outcrops are discontinuous, much of the area is covered with vegetation, in places the regolith and soil exceed 10 

m in thickness (Kramer, 1976) and land access can be difficult 

in many places. Most of the fresh rock outcrops are limited to 

sea cliffs, stream channels, roads, and quarries. Of this subset, 

Kramer’s (1976) detailed work in the Fort Bragg-Willits area 

showed that road cuts and river channels often exhibit evidence 

of mass movement. Finally, the geology and structure of the 

Coastal belt are complicated due to its origin and assembly as 

a subduction zone accretionary complex, subsequently overprinted by deformation associated with the northward migration 

of the Mendocino triple junction. All of these factors, coupled 

with a dominantly graywacke and argillite composition, limit 

the ability to infer continuity between outcrops of the Coastal 

belt. Thus, interpretations based on widely scattered observations can be difficult.

Modern aeromagnetic surveys over northern California 

have been collected during the past few decades to aid in the 

understanding of the geology of the region and help evaluate 

the risks of natural hazards. These surveys were conducted from 

low-flying aircraft and measured local irregularities (anomalies) in the geomagnetic field that are caused by the presence of 

magnetic minerals, mostly magnetite, in the underlying rocks. 

Magnetic anomalies often can be related to rock type, and so the 

surveys provide a means to map remotely some aspects of the 

geology.

Some of the aeromagnetic surveys (for example, Santa 

Rosa/Napa Valley, Yorkville, sheet 1, map a, table 1) over 

the southern part of the Coastal belt have revealed puzzling magnetic anomalies that potentially imply a surprising 

degree of regularity, coherence, and spatial continuity of rock 

bodies within this terrane (Phelps and others, 2008a,b). Lowamplitude, curvilinear anomalies suggest near-surface or outcropping magnetic rock units 1–2 km wide and 10–40 km long. 

These magnetic anomalies are spatially pervasive, with anomalies generally 3–4 km apart. Subsequent surveys farther north 

(for example, Ukiah, table 1) show a continuation of this basic 

pattern over the Coastal belt, although the anomalies become 

somewhat less pervasive to the north and more widely separated—up to 5 or 6 km apart (sheet 1, map a). Some of the map 

anomaly patterns are suggestive of magnetic layers in plunging 

folds, as discussed subsequently.

With the completion of the Ukiah survey, it is appropriate 

to release these data in the form of a comprehensive aeromagnetic map (sheet 1, map b). In this report, we also include two 

derivative maps of low-amplitude anomalies, numerically 

enhanced to present the data more clearly for use in support of 

geologic mapping. As detailed in the following discussion, the 

first derivative map shows anomalies that arise from exposed 

or shallowly buried magnetic sources (sheet 2). The second 

derivative map includes the results of an automatic routine for 

defining near-surface ”edges” of magnetic bodies and a basic 

interpretation of these anomalies in terms of magnetic layers, 

with associated interpreted strikes and dips (sheet 3). 
2

Aeromagnetic Surveys

Six surveys cover the outcrop area of the Coastal belt 

(table 1). These surveys were collected during the past three 

decades as integral parts of U.S. Geological Survey investigations of the following: (1) mineral resources in proposed 

wilderness areas; (2) active faulting; (3) the plate tectonic 

development of northern California; and (4) regional geologic 

mapping.

Data from all surveys were corrected for diurnal and other 

short-period magnetic fluctuations resulting from atmospheric 

and ionospheric sources, and corrected for the Earth’s main 

magnetic field (the International Geomagnetic Reference Field 

[IGRF]; Langel, 1992) updated to the time period during which 

the data were collected. The resulting data were interpolated to 

a rectangular grid with nodes spaced 200 m apart according to 

an algorithm based on minimum curvature (Briggs, 1974). The 

resulting aeromagnetic survey maps are presented on sheet 1.

Merging

Some minor manipulation and processing of the aeromagnetic data shown in sheet 1, map a were required to merge the 

individual surveys into a relatively seamless map of the Coastal 

belt (sheet 1, map b). Even though the survey data were all collected at nominally the same height above the ground surface, 

differences in topographic relief between areas and differences 

in the ruggedness of topography between areas caused safety 

concerns that resulted in some surveys being flown, on average, 

slightly higher or lower than adjacent surveys. These differences 

were compensated by numerically adjusting the flight height of 

some surveys to a slightly different observation surface, a procedure known as upward continuation (Grant and West, 1965). 

The Cape Mendocino, Santa Rosa/Napa Valley, Yorkville, and 

Redding surveys were upward-continued 150 m to be consistent with the average terrain clearance of the Ukiah survey. In 

addition, although the data were theoretically reduced to the 

same magnetic datum by corrections for diurnal variations and 

the IGRF, in practical terms there is slight arbitrariness in the 

resulting magnetic datum. Therefore, the magnetic datum levels 

of adjacent surveys were compared in areas of overlap, and the 

individual surveys were adjusted by adding a constant to all 

magnetic values within individual surveys. Finally, the individual adjusted surveys were merged into a single data set by 

smooth interpolation across a 1-km-wide buffer zone between 

surveys, resulting in the unified map shown on sheet 1, map b.

Aeromagnetic Map Processing and 

Anomaly Enhancement

The long, curvilinear magnetic anomalies that characterize 

the Coastal belt are readily evident on sheet 1, maps a and b, but 

some simple numerical filtering and manipulation can enhance 

these anomalies and present them in a way that should be easier 

to use for investigators who are not experts in potential-field 

geophysics. We have applied three straightforward procedures 

to the data in sheet 1, map b, to produce the map shown on 

sheet 2.

Reduction-to-Pole Aeromagnetic Map

Magnetic anomalies produced by rocks with magnetic 

minerals as a result of induction by Earth’s main magnetic field 

tend to be asymmetric even when the bodies themselves have 

a symmetric geometry. Remanent magnetization probably does 

not contribute significantly to the magnetic anomalies, given 

the rock types within the Franciscan Complex. The asymmetry 

is caused by the non-vertical main magnetic field and thus is 

a function of magnetic latitude. In the Coastal belt region, for 

example, magnetic anomalies over symmetric, vertically dipping tabular bodies display magnetic lows on their north sides 

with the deepest magnetic lows just north of the north edges 

of the bodies and magnetic highs on their south sides with the 

strongest magnetic highs just north of the south edges of the 

bodies.

A numerical procedure called “reduction to the pole” 

(Baranov and Naudy, 1964) transforms observed anomalies 

into those that would have been measured at the magnetic pole 

where the main field is vertical. This transformation has the 

effect of centering the magnetic highs over the centers of the 

magnetic bodies, with flanking lows on all sides of the highs. 

For the Coastal belt region, the inclination and declination 

(within one degree) of the Earth’s magnetic field is 62° and 15°, 

respectively.

Anomaly Enhancement

Aeromagnetic maps in general, and the maps a and b on 

sheet 1 in particular, constitute a set of superposed anomalies of 

many different spatial wavelengths. The amplitude of magnetic 

anomalies attenuates with increased distance to the magnetic 

source and short-wavelength anomalies attenuate more severely 

with increased distance compared to longer wavelength anomalies. In general, the shortest wavelength anomalies are caused 

by magnetic bodies at or close to the surface, whereas longer 

wavelength anomalies typically are caused by deeper magnetic 

source bodies. Thus, the shortest wavelength anomalies on 

sheet 1, map b are most likely produced by units that crop out or 

are only shallowly buried—those units that most likely would 

be found in outcrop or would reflect structures found in outcrop, 

and would be depicted on a traditional geologic map. These 

shortest wavelength anomalies are the anomalies of most direct 

interest to geologic mapping.

The more severe attenuation with distance of shortwavelength magnetic anomalies (relative to longer wavelength 

anomalies) provides the basis for a simple filter to enhance the 

anomalies from near-surface sources. If Mpole is the magnetic 

anomaly field shown in sheet 1, map b, reduced to the pole, 
3

then a magnetic anomaly field with short-wavelength anomalies enhanced with respect to longer-wavelength anomalies 

[Mpole]residual is given by

[Mpole]residual = Mpole– [Mpole] (up 200 m)

where [Mpole] up 200 m is the magnetic anomaly field of 

sheet 1, map b, reduced to the pole and upward continued to a 

surface 200 m above the original measurement surface, which 

produced the most continuous, short-wavelength anomalies. 

The procedure is essentially a discrete vertical derivative. This 

residual field is the magnetic anomaly field shown on sheets 2 

and 3.

Locating Edges of Magnetic Bodies

Cordell and Grauch (1985) devised a scheme for locating 

the near-surface sharp edges of magnetic bodies based on the 

anomalies magnetic bodies produce and a mathematical transformation called the pseudogravity transform (Baranov, 1957). 

Blakely and Simpson (1986) automated this scheme for application to digital representations of magnetic anomaly fields, and 

the result is a mapped distribution of closely spaced points that 

define the edges of magnetic bodies. 

The small, white-filled circles shown on sheet 3 represent 

the inferred surface contacts of outcropping magnetic bodies 

or the sharp, near-surface edges of shallowly buried magnetic 

bodies that produce the short-wavelength magnetic anomalies shown on sheet 1, map b; and sheet 2. These points were 

automatically generated by applying the method of Blakely 

and Simpson (1986) to [Mpole]residual. This method assumes that 

the magnetic bodies have vertical sides. For non-vertical sides, 

calculated points will shift in the direction of dip. For sides with 

moderate to steep dip (45 ̊ to vertical), however, the horizontal 

displacement of a gradient maximum from the top edge of an 

offset horizontal layer is always less than, or equal to, the depth 

to the top of the source (Grauch and Cordell, 1987). Because of 

the altitude that the data were collected, the resulting aeromagnetic anomaly may be wider than the magnetic source, even if 

the magnetic source is exposed at the surface. 

The various filters used to produce the magnetic anomaly 

field shown on sheets 2 and 3 (reduction to the pole, upward 

continuation, anomaly enhancement, and pseudogravity transform) are all linear filters so that the results are independent of 

the order in which the filters are applied (Blakely, 1995).

Anomaly Sources and Magnetic 

Susceptibility Measurements

Preliminary field investigations, comparisons between 

the magnetic anomaly map and available geologic maps, and 

magnetic susceptibility measurements suggest at least four different general sources for the long, narrow curvilinear magnetic 

anomalies that characterize the Coastal and Central belts of the 

Franciscan Complex.

1. Serpentinite and related ultramafic rocks, shown in 

many places to be dismembered parts of the Coast 

Range ophiolite. These rocks are complexly imbricated 

in the Central belt, notably along the Coastal Belt Thrust 

(McLaughlin and others, 2000; McLaughlin and others, 

1988). At the surface Coast Range ophiolite, remnants 

appear as thin, dipping, discontinuous, tabular bodies 

entrained in the country rock during the accretion 

process. These remnants were likely later modified by 

strike-slip faulting. They typically define suture zones, 

such as the Coastal Belt Thrust (McLaughlin and others, 

1988; 2000). Serpentinites contain abundant magnetite 

and typically are highly magnetic.

2. Franciscan Complex metavolcanic rocks. These have 

been shown to represent accreted fragments of oceanic 

basalt from subducted oceanic basement, including 

seamounts or oceanic plateaus (McLaughlin and others, 

1994; Sliter and others, 1986). Franciscan Complex 

metavolcanic rocks can be magnetic unless they 

have undergone extensive weathering, alteration, and 

metamorphism. 

3. Lenses of Coastal belt conglomerates whose cobbles 

retain enough magnetite that the bulk magnetic properties of the units are higher than those of the surrounding 

finer grained clastic rocks. Where dated, these conglomerates are the same age as and clearly are interbedded in 

the dominant graywackes. They are interpreted to represent submarine canyon or channel fills (McLaughlin and 

others, 1994; Underwood and Bachman, 1986). 

4. Lithic, volcanic-rich graywackes. These rocks most 

likely are a subset of the lithic Coastal belt petrofacies 

identified by Underwood and Bachman (1986) from 

samples near and north of Fort Bragg. Magnetically, 

these rocks appear to be thin, dipping, tabular bodies, 

commonly folded and fault-bounded.

Initial field investigations in the area south of Ukiah 

(Phelps and others, 2008a,b) found that the sources of many 

long, narrow, curvilinear magnetic anomalies over the Coastal 

belt are associated at the surface with graywackes rich in 

andesitic detritus. Magnetic susceptibility measurements of the 

graywacke (fig. 3; magenta and green stars on sheet 2) indicate 

many graywackes are weakly magnetic, with values less than 

1 x 10-3 SI units. Significantly, however, about a quarter of graywackes measured have susceptibilities large enough to produce 

magnetic anomalies measured at the height of the aeromagnetic 

surveys, with values greater than 2 x 10-3 SI units and reaching 

values as high as 28 x 10-3 SI units. 

Field investigations also indicate that basaltic metavolcanic rocks are sources of many of the anomalies, particularly 

in the Fort Bragg-Willits area, where mapping (Kramer, 1976) 

indicates thin layers of metavolcanic rocks. Farther north, 

sparse outcrops of metavolcanic rocks are associated with 

magnetic anomalies in the Usal-Dugans and Julius’ opening 

area (McLaughlin and others, 1994; O’Day, 1974; anomaly 5c, 

table 2), Parkhurst Ridge (PR on fig. 2), and Whitethorn areas 
4

(anomaly 5b, table 2; McLaughlin and others, 2000; Jayko and 

others, 1989). Magnetic susceptibility measurements indicate 

that the metavolcanic rocks in the Coastal belt can be very magnetic (fig. 3). Lower values measured on the metavolcanic rocks 

are due to extensive weathering or alteration, or intermixing of 

limestone and argillite with the volcanic rocks. 

Mapped Tertiary conglomerate appears to coincide 

with several weakly positive magnetic anomalies north of 

Garberville; however, these conglomerates are a relatively 

minor constituent in the Coastal belt, and the conglomerates 

vary widely in their clast contents. A few measurements of 

conglomerate indicate generally low magnetic susceptibilities 

(fig. 3), but one Yager terrane conglomerate and one Yager terrane sandstone have magnetic susceptibilities that could account 

for the weak anomalies north of Garberville (sheet 2). 

To illustrate the effectiveness of these magnetic data to 

map features on the ground, we used the aeromagnetic map to 

locate a previously undiscovered outcrop of basalt along the San 

Andreas Fault near Fort Ross. The aeromagnetic map shows 

a long, linear anomaly along and nearly parallel to the San 

Andreas Fault (fig. 4). We then performed a ground magnetic 

survey and acquired susceptibility measurements across the 

feature. The ground magnetic survey reveals an approximately 

300-m-wide anomaly with an amplitude of 500 nT (see fig. 4). 

The ground anomaly is much narrower and higher amplitude 

than that measured from the airplane (~ 1 km wide, 20 nT 

amplitude), an expected result due to the aeromagnetic survey 

height. Although outcrop quality was poor, we did locate a 

small outcrop of basalt that was highly magnetic (K=38–50 

x10-3 SI units) within the positive ground magnetic anomaly 

that had not been previously mapped. The absence of a prominent ground magnetic anomaly coincident with the magnetic 

graywacke along the Fort Ross road suggests that the magnetic 

graywackes may not be voluminous enough to produce the 

aeromagnetic anomaly in that area.

Magnetic Anomalies

The magnetic maps reveal regions that share anomaly 

characteristics. Although slightly magnetic terrain can produce 

measureable aeromagnetic anomalies, the anomaly patterns 

do not appear to coincide with topographic features (sheet 1, 

map a). The magnetic patterns can be related to terranes in the 

Coastal belt. The King Range terrane is marked by a smooth 

magnetic field with generally lower values; Griscom (1980) 

attributed this pattern to a substantial thickness of nonmagnetic 

sandstones. In general, the Yager terrane is characterized by 

a smooth magnetic field, except for the region northwest of 

Garberville that locally includes magnetic conglomerate and 

associated sandstone, where residual anomalies exceed 2 nT 

in amplitude, and along the eastern margin of the terrane. The 

relatively smooth magnetic field is likely due to the absence 

of metavolcanic rocks. Along the eastern margin of the Yager 

terrane, prominent magnetic anomalies mark the location of 

the Coastal Belt Thrust where it is well mapped outside of the 

Ukiah 1° by 2° quadrangle. These anomalies are caused by 

interleaving of Coast Ranges ophiolite within and along the 

Coastal Belt Thrust. This correlation suggests that it may be 

possible to more accurately map the location of the thrust fault 

within the Ukiah 1° by 2° quadrangle.

The Coastal terrane is marked by numerous, west-northwest- to north-northwest-trending, linear to arcuate positive 

anomalies that locally extend to the west offshore. These 

anomalies do not extend across the San Andreas Fault Zone, 

which is characterized by a set of north-trending anomalies. The 

eastern margin of the terrane is marked by a subtle transition 

to the generally smoother magnetic field of the Yager terrane, 

generally north of Laytonville. 

Changes in regional magnetic patterns within the Coastal 

terrane are located at about lat 39°37.5' N. and at about lat 

39° N. The anomalies over the southern part of the terrane, 

south of about lat 39° N., appear to coincide with closely spaced 

structural repetition of Coastal and Central belt rocks and by the 

complex folding and compression of the Coastal Belt Thrust by 

younger west-northwest-trending reverse faults. The anomaly 

pattern is marked by closely spaced (3–4 km) anomalies and 

reflects three known anomaly sources: graywacke, metabasalt, 

and serpentinite. Serpentinite is associated with rocks derived 

from the Coast Range ophiolite entrained within the Coastal 

Belt Thrust Zone and shear zones within the Central belt. The 

region between latitudes 39°37.5' N. and 39° N. is characterized by a prevalent N 40–60° W anomaly trend, with a wider 

spacing between anomalies than those to the south. Both basalt 

and graywacke are known magnetic sources in this part of the 

Coastal terrane. North of lat 39°37.5' N. to the latitude of the 

town of Petrolia, magnetic anomalies are characterized by more 

northerly trends of N. 10–30° W. Many of these anomalies coincide with small, discontinuous and sheared exposures of basalt. 

Near the northern extent of exposed Coastal belt rocks, anomaly 

trends swing to the west and extend offshore along the south 

side of the Eel River Basin; see also figures 2 and 9.

Approximate Dips Inferred from 

Residual Aeromagnetic Anomalies

The magnetic anomalies shown in sheet 2 provide an 

opportunity to estimate the dip of magnetic graywacke or 

metavolcanic layers within the Coastal belt. On sheet 2, a vertical tabular body should produce a symmetric magnetic anomaly 

high centered over the body, with adjacent lows on both side of 

the high that are the same shape and amplitude. If the tabular 

body is not vertical but instead dips, then the low on the side 

toward the dip direction will be diminished relative to the low 

on the opposite side (fig. 5). The discrepancy becomes more 

pronounced as the dip shallows. Thus, anomaly asymmetry on a 

“reduced to pole” magnetic anomaly map can yield information 

about both dip direction and an approximate dip angle. 

Interpretations of dip direction and dip angle from maps 

such as sheet 3 must be made with caution because other factors 
5

can influence magnetic anomaly asymmetry. Primary among 

possible complications are interference between overlapping 

anomalies and non-tabular source geometry, but other factors 

such as non-uniform distribution of magnetic minerals, possible 

remanent magnetization not oriented along the present Earth’s 

main magnetic field direction, and others not yet discovered 

could lead to erroneous interpretations. This method should be 

applicable to the Franciscan Complex because remanent magnetization in the Franciscan Complex rocks is weak. Despite these 

potential issues, many of the anomalies show surprising lateral 

continuity and consistency in dip direction along strike, suggesting that some of these complicating factors may not be major 

factors.

We have qualitatively examined the long curvilinear 

magnetic anomalies over the Franciscan Complex Coastal belt 

and have interpreted source body dip direction, approximate 

dip angle, and measurement confidence for nearly 50 individual 

anomalies (sheet 3; table 2). These data are presented on sheet 3 

by symbols described in the map legend. This exercise was conducted as a blind test without reference to the generally sparse 

dip information on available geologic maps. The dips were estimated by determining the amplitude asymmetry from the grid 

and were not the result of forward modeling of each anomaly.

Subsequent to our interpretation, we performed a groundmagnetic survey in the Laguna Point area (fig. 2), where one of 

the curvilinear magnetic anomalies crosses the coastline (anomaly 25b, sheet 3). This area was a good place to test the airborne 

results because graywacke and metavolcanic rocks along the 

coast are well exposed and are easily accessible. Groundmagnetic measurements indicate a magnetic high that is about 

200 m wide, much narrower than the anomaly measured at the 

height of the airborne survey (~ 1000 m; fig. 6) as expected. 

Magnetic susceptibility measurements indicate that the metavolcanic rocks are the source of the anomaly. The graywackes are 

weakly magnetic; the contacts between the blocks of metavolcanic rocks and the graywackes are highly sheared, locally incorporating thin streaks of argillite and pink, pelagic limestone 

and leading to intermediate magnetic values for both rock types 

(fig. 7). We projected the width of the outcrop of exposed basalt 

from the coast to the profile to help constrain the forward model 

of the ground-magnetic data. With this constraint, the model 

(fig. 8) indicates a steep south dip for the magnetic volcanic 

rocks, consistent with structural attitudes measured on the graywacke (David Andersen and Jeana Lopez, written commun., 

2010; fig. 6) and consistent with our interpreted dip from the 

airborne data (25b on sheet 3 and table 2). The measured strikes 

of the graywacke are also broadly consistent with the strike of 

the aeromagnetic anomaly. This test indicates that the method 

of using the asymmetry of the reduced-to-pole aeromagnetic 

anomaly can be successfully applied and that the magnetization 

boundaries derived from aeromagnetic anomalies may be not 

only considerably wider than the magnetic rocks exposed at the 

surface, but also shifted.

Discussion 

The magnetic anomalies point to coherent structures within 

the Coastal belt, despite its characterization of the dominant 

structure within the Coastal belt as “broken formation”. For the 

Fort Bragg area, three deformational episodes have been discussed. Although several workers (for example, Kramer, 1976; 

O’Day, 1974; Kleist, 1974) have described a first generation of 

deformation as consisting of submarine landsliding and softsediment deformation, these features have chaotic orientations, 

and thus do not appear responsible for the coherent magnetic 

anomalies of the Coastal belt.

The second phase of deformation described by Kramer 

(1976) and others produced the most noticeable and severe 

deformation, resulting in northwest-trending folds and faults. 

Many of the magnetic bodies trend northwest and dip to the 

northeast (sheet 3; fig. 9), consistent with this second phase 

of deformation, which Kramer (1976) attributed to structural 

interleaving of tectonic blocks and slabs of ocean floor within 

an accretionary complex during subduction of the Farallon Plate 

beneath the North American Plate. Kramer’s third generation of 

deformation produced broad, east-west trending folds that also 

are not evident in the magnetic data. 

Other studies of the Yager and Coastal terranes north of 

the Fort Bragg area point to an early generation of large-scale 

recumbent folding overprinted by later upright isoclinal folds in 

the Coastal belt (Beutner and Hanson, 1975; Beutner and others, 

1980; McLaughlin and others, 1994). Superposed on these folds 

is pervasive brittle deformation (boudinage, shearing, and crush 

zones) that breaks up and transposes fold limbs. The magnetic 

signature of the Coastal belt, however, suggests that the brittle 

deformation that is so apparent in surface outcrops apparently 

was not extensive enough to obliterate the record of folding in 

the Coastal belt accretionary complex. 

The eastern margin of the Coastal belt is marked by 

anomalies that trend more northerly than those in the western 

margin of the belt and whose source bodies are interpreted to 

dip predominantly to the northeast. These anomalies likely are 

caused by serpentinites within the Coastal Belt Thrust, which 

is poorly mapped in the southern half of the Ukiah 1° x 2° 

quadrangle, but which coincides with magnetic anomalies 

north of lat 39°30' N. (Jayko and others, 1989; McLaughlin and 

others, 2000) and has been mapped using aeromagnetic anomalies north of the study area (Clarke, 1992). The northeast dips 

of these sources are consistent with the northeast dip for the 

Coastal Belt Thrust. 

The magnetic anomalies in the Coastal belt are caused by 

multiple sources. Along its southern margin, anomalies associated with the Coast Ranges ophiolite can be traced into the 

Coastal belt and are largely due to the northwestward translation of Coast Range ophiolite in the Coastal Belt Thrust along 

steeply dipping reverse faults that disrupt the Coastal Belt 

Thrust. Previously, serpentinite slices imbricated with rocks 

of the Franciscan Complex were considered to be Franciscan 

Complex basement; however, recent work argues that all serpentinites within the Franciscan Complex represent basement of 

the Coast Ranges ophiolite that has been tectonically interleaved 
6

with the Franciscan Complex during accretion and subsequent 

faulting (M.C. Blake, Jr., written commun., 2011). Metavolcanic 

or basalt blocks clearly are responsible for magnetic anomalies 

throughout the Coastal terrane. At least two to three distinct 

“layers” of graywacke produce magnetic anomalies suggestive 

of plunging folds. For example, a horseshoe-shaped anomaly 

(26a on sheet 3, table 2) may represent a plunging antiform, 

with anomaly 27 representing another layer that is part of this 

structure. 

The magnetic data allow for differentiating the generally monotonous graywacke within the Coastal terrane into 

two groups that reflect potentially different source regions. 

Preliminary work (Phelps and others, 2008a,b) shows that the 

magnetic properties of graywackes, primarily from the southern 

half of the Coastal terrane, are due to andesitic debris. Further 

work is needed to determine if these observations apply to the 

entire Coastal belt and to pinpoint the source(s) of the andesitic 

detritus. The aeromagnetic data also may help delineate lithofacies within the Yager terrane that were described by Underwood 

and Bachman (1986). Their petrographic data suggest that the 

northern part of the Yager terrane contains a higher percentage of feldspar and volcanic lithic clasts than does the southern 

part of the terrane. To first order, the magnetic data support 

such a distinction in that the southern part of the Yager terrane 

is characterized by a smoother magnetic field than that of the 

northern Yager terrane. The boundary between the Yager and 

Coastal terranes as mapped by Underwood and Bachman (1986) 

is not in the same place as depicted by Jayko and others (1989) 

and suggests that the location of the terrane boundary should be 

reexamined in the area of mismatch and particularly to the south 

of the Covelo quadrangle mapped by Jayko and others (1989). 

A smoother magnetic field may indicate the presence of weakly 

magnetic Yager terrane; the transition to the linear belt of the 

anomalies could mark the Yager-Coastal terrane boundary.

Conclusions

Newly merged aeromagnetic data over the Coastal belt 

of the Franciscan Complex reveal long, linear anomalies that 

indicate remarkably coherent structure within a terrane where 

mapping at the surface indicates complex deformation and that 

has been described as “broken formation” and, even locally 

as “mélange”. The anomalies in the Coastal belt are primarily sourced by volcanic-rich graywackes and exotic blocks of 

basalt. Some anomalies along the contact of the Coastal belt 

with the Central belt are likely caused by local interleaving of 

components of the Coast Ranges ophiolite. These data can be 

used to map additional exotic blocks within the Coastal belt and 

to distinguish lithologically indistinct graywackes within the 

Coastal terrane. Using anomaly asymmetry allows projection 

of these “layers” into the subsurface. This analysis indicates 

predominant northeast dips consistent with tectonic interleaving 

of blocks within a subduction zone.

Acknowledgments

We thank Geoff Phelps and Jeana Lopez for providing 

additional magnetic susceptibility measurements from the 

Coastal belt. We also are grateful to Mike Underwood who 

provided samples from his thesis for physical property measurements. Jeana Lopez and Dave Andersen provided valuable 

structural attitudes from the Laguna Point area. We appreciate 

comments from Richard Blakely and Ray Wells that improved 

the manuscript and maps.

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8

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resource potential of the continental margin of Western 

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Division of Mines and Geology, Regional Geologic Map 

2A, scale 1:250,000.
9

Figure 1. Index map of coastal California, showing geology simplified from Jennings and others (1977). 

Heavy black lines are plate boundaries. Black and gray lines are terrane boundaries generalized from 

McCulloch (1987), Vedder (1987), Clarke (1992), and Jennings and others (1977). Red lines are faults 

generalized from Jennings and Bryant (2010). BSF, Bartlett Springs Fault; MF, Maacama Fault; MTJ, 

Mendocino Triple Junction.

124o  122o  120o  118o 

34o

38o 

40o

? 

36o 

38o 

40o

36o 

34o

0 100 KM

EXPLANATION 

SIERRA 

NEVADA 

KLAMATH 

MTNS 

SAN ANDREAS FAULT

Tertiary sedimentary rocks 

GREAT VALLEY 

Cretaceous sedimentary rocks 

Central belt and other Franciscan 

Eel Complex terranes

River MTJ basin 

Coastal Belt terrane (Franciscan Complex) 

Plutonic and metamorphic rocks 

Ultramafic rocks 

Onshore 

Offshore 

Coastal belt or equivalent (Patton terrane)

Other Franciscan Complex equivalents 

Salinia terrane 

Nicolas terrane (Great Valley equivalent?) 

Cascadia trench 

Coastal Belt thrust 

paleosubduction zone 

Area of

map

NEVADA 

CALIFORNIA 

MF 

BSF 

Bodega 

Head 

Cape 

Mendocino 

Point 

Arena 

PACIFIC

OCEAN

Coastal belt

0 100 MI
10

38°30'

38°45'

39° 

39°15'

39°30'

39°45'

40° 

40°15'

40°30'

124°15' 124°  123°45' 123°30' 123°15' 123° 

Central Belt or other Franciscan Complex

Serpentinite/ultramafic

Metavolcanic rocks

Gabbro

Coastal terrane

Jurassic-Cretaceous (Great Valley)

Mesozoic/Paleozoic

Cenozoic volcanic rocks

Tertiary sedimentary 

rocks

Yager terrane

Quaternary deposits

 or covered 

Granitic rocks

0 10 KM

SAN ANDREAS FAULT

MAACAMA FAULT

BARTLETT SPRINGS FAULT

Point

Arena

Bodega Bay

Ukiah

Willits

Jenner

Mendocino

Fort 

Bragg

Clear

Lake

Eel River Basin

KLAMATH

MOUNTAINS

Point

Delgada

Cape 

Mendocino

Boonville

Laytonville

Garberville

Elk

Petrolia

Julius volcanics

of O’Day (1974)

volcanics mapped

by Kramer (1976)

COASTAL BELT

THRUST

Lake

Pillsbury

MF-2336

MF-2402

Laguna Point

SAN ANDREAS FAULT

King Range terrane

False Capeterrane

Coastal belt of Franciscan Complex

Fort Ross

PR

PACIFIC

OCEAN

38°30'

38°45'

39° 

39°15'

39°30'

39°45'

40° 

40°15'

Fortuna

Covelo

0 10 MI

cgl

cgl

King Range 

terrane

Coastal terrane

Yager

terrane

False

Cape

terrane

Figure 2. Geologic map of the study area. Dotted black lines show margins of more detailed, digital geologic maps (McLaughlin and others, 

2000, [MF 2336]; Blake and others, 2002, [MF 2402]) that were simplified for this figure. Dotted orange line shows western contact of the 

Yager complex from Underwood and Bachman (1986), which differs significantly from the extent of the Yager terrane from Jayko and others 

(1989). Abbreviations: PR, Parkhurst Ridge; cgl, conglomerate.
11

90

80

70

60

50

40

30

20

10

0

> -1 -0.8 -0.6 -0.4 -0.2 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0

Metavolcanic rocks N=67

Yager terrane sandstones N=16

Coastal terrane graywacke N=526

Yager terrane conglomerate N=2

Coastal terrane conglomerate N=2

Log10 (Magnetic Susceptibility in 10-3 SI units)

Number of Magnetic Susceptibility Measurements

< 1.6

non-magnetic to magnetic

weakly magnetic

-1.0

Figure 3. Magnetic susceptibility measurements of various rock types in the Coastal belt. N, number of measurements.
12

500 nT 

-135

-134

-133

-132

-131

-130

-129

-128

-127

-126

-125

-124

-123

-122

-121

-120

-119

-118

-117

-116

Magnetic Field, 

in nanoteslas

012 KM

Fort Ross 

0 1 MI

Ground magnetic survey 

track

Ground magnetic anomaly

Coastline

Magnetization boundary 

from aeromagnetic data

Basalt (k= 38-50 10-3 SI)

Magnetic sandstone 

(k > 2 10-3 SI)

Non-magnetic sandstone

(k < 10-3 SI) 

123°15' 123°14' 123°13'

38°30'

38°31'

38°32'

magnetic graywacke 

along Fort Ross Rd 

38°30'

38°31'

38°32'

Figure 4. Aeromagnetic (color background), ground magnetic (brown line), and magnetic susceptibility data (red, green, and blue 

circles) for the Fort Ross transect. White dotted lines, flight lines.
13

-50 -40 -30 -20 -10 0 10 20 30 

0

-100

100

200

300

400

0

5

DISTANCE, IN KILOMETERS

DEPTH, IN 

KILOMETERS

MAGNETIC FIELD, IN nT

SW 

magnetic intensity

0.001 emu/cc

For vertical layer, lows are symmetric around highFor a dipping layer, the amplitude of the low southwest ofthe high increases relative to the amplitude of the high and relative to the low on the northeast side of the high.

reduced to pole anomaly

Figure 5. Predicted magnetic anomalies (reduced to pole) for vertical and dipping magnetic layers. For a vertical layer (shown in red), the lows are high. For layers that dip to the northeast (shown in green and blue), the amplitude of the low southwest of the high increases relative to the amplituamplitude of the low on the northeast side of the high.
14

123°48.5' 123°48' 123°47.5' 123°47'

39°29'

39°29.5'

-160

-158

-156

-154

-152

-150

-148

-146

-144

-142

-140

-138

-136

-134

-132

-130

-128

-126

-124

-122

Magnetic Field,

in nanoteslas

A

A'

39°29'

39°29.5'

62

51

76

65

66

64

LAGUNA

POINT

coastline

flightline

flightline

magnetic 

51

Sandstone, argillite, 

or limestone

Basalt

Average attitude

of bedding

0 1 KM

ground 

traverse

Magnetization 

boundary determined

from aeromagnetic

data

64

Average attitude

of fractures, shearing

0 1 MI

Figure 6. Aeromagnetic map of Laguna Point area, showing ground-magnetic traverse (heavy dotted magenta line), ground-magnetic 

data (thin magenta line), magnetization boundaries (white circles), and locations of magnetic susceptibility measurements. Flight lines 

marked by green dotted lines.
15

Sedimentary Rocks at Laguna Point

Volcanic Rocks at Laguna Point

Log10 (Magnetic Susceptibility in 10-3 SI units)

0

20

5

30

Number of Measurements 10

intermixed,

sheared 

15

25

-1.0 -0.8 -0.6 -0.4 -0.2 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0 <1.6

Figure 7. Magnetic susceptibility measurements from the Laguna Point 

area.
16

Magnetics Field,

in nanoteslas

49400

49600

49800

50000

1500 1000 500 0

Distance, in meters

V.E.=1 

Depth, in meters

500

250

0

Magnetics Field,

in nanoteslas

Observed

SOUTH NORTH

Ground magnetic profile

calculated at 349 m

above ground

calculated at 290 m

above ground

aeromagnetic gridded value

Calculated 

Basalt

0.025 (SI)

-200

-150

-100

flightline

flightline

noisy because

of nearby railroad 

trestle

terrace deposits

(not magnetic)

Figure 8. Model across the Laguna Point magnetic anomaly.
17

38 30¬

38 45¬

39 

39 15¬

39 30¬

39 45¬

40 

40 15¬

40 30¬

38 30¬

38 45¬

39 

39 15¬

39 30¬

39 45¬

40 

40 15¬

40 30¬

38°30'

38°45'

39° 

39°15'

39°30'

39°45'

40° 

40°15'

40°30'

124°15' 124°  123°45' 123°30' 123°15' 123° 

Central belt or other Franciscan 

Complex

Serpentinite/ultramafic

Metavolcanic rocks

Gabbro

Coastal terrane

Jurassic-Cretaceous (Great Valley 

Sequence)

Mesozoic/Paleozoic

Cenozoic volcanic rocks

Tertiary sedimentary 

rocks

Yager terrane

Quaternary deposits

 or covered

Granitic rocks

0 10 KM

SAN ANDREAS FAULT

MAACAMA FAULT

BARTLETT SPRINGS FAULT

KLAMATH

MOUNTAINS

King Range terrane

False Cape terrane

Coastal belt of Franciscan Complex

38°30'

38°45'

39° 

39°15'

39°30'

39°45'

40° 

40°15'

40°30'

t

r

t

r

cgl

cgl

best---------------least confidence in dip determined from 

asymmetry of reduced-to--pole anomalies (see figure 4).

Vertical

Steep dip

Medium dip

Shallow dip

Arrow points in dip direction of

flap 

Magnetic body 

Clear

Lake

Point

Delgada

Laytonville

Fort Ross

Fortuna

Covelo

PACIFIC

OCEAN

0 10 MI

Figure 9. Interpreted magnetic anomalies of the Coastal belt superposed on geology from figure 2
18

Survey name Archive # Date flown Flight-line spacing Flight-line direction Flight height above average terrain Position control Senso

Cape Mendocino 4157, 4158 08/93–02/94 800 m N45E 150 m offshore, 

300 m onshore

local transponder Proton Prec

Clear Lake 4168 02/95–07/95 450 m EW 300 m GPS ? Cesium Vap

Redding 4134 01/85–02/85 800 m EW 300 m Cesium 

Santa Rosa/ Napa Valley

4188 12/2/96–1/12/97 530 m EW 300 m GPS/PNAV Cesium Vap

Ukiah 5/7–5/22/2009 800 m EW 300 m GPS Cesium Vap

Yorkville Jan–08 800 m EW 300 m GPS Cesium Vap

Table 1. Aeromagnetic survey specifications.[See area of survey on sheet 1, map A. GPS, Global Positioning System;; PNAV, Precision Navigation.; USGS, U.S. Geological Survey]
19

Map 

Number 

(on sheet 3)

Anomaly 

amplitude 

1=>20 nT 

2=10-20 nT 

3=<10 nT

Dip confidence 

1=best

2=intermediate 

3=least

Strike Dip direction Dip angle Anomaly amplitudes 

(south or west-side 

low/central high/north 

or east-side low)

Rock type (Bold

indicates rock type is 

associated with outcrop; 

italic indicates rock type 

is inferred); comments

1a 1 2 NNW n/a vertical -8/20/-8 metavolcanic

1b 2 2 NNW n/a vertical -2/17/-2 metavolcanic

2a 1 2 NS E shallow -28/100/-14 metavolcanic

2b 1 2 EW n/a vertical -10/26/-10 metavolcanic

3a 1 2 WNW n/a vertical -8/34/-8 metavolcanic

3b 3 3 NNW n/a vertical? -3/5/-2 metavolcanic

4 1 2 NNW WSW medium -10/45/-16 metavolcanic; Griscom 

(1980) model is WSW 

dip of 35–40°

5a 1 1 NNW ENE medium -9/14/-6

5b 1 1 NNW ENE steep -9/45/-9 metavolcanic

5c 1 1 NNW ENE steep -20/40/-9 metavolcanic

5d 1 2 NNW ENE medium -10/32/-8 --

6a 2 2 NNW n/a vertical -4/12/-4 --

6b 3 2-3 NS E steep -2/4.4/-1.7 --

7 3 3 NS E steep -4.95/4.6/-4.85 --

8 1 2 NW NE steep -9/28/-7 --

9 2 3 WNW n/a vertical -2/12/-2 --

10 1 2 WNW NNE steep -6/17/-4 --

11 1 1 NS E shallow -19/40/-10 metavolcanic

12 1 1 NS E shallow -6/32/0 --

13 1 1 NS E medium -9/23/-7 --

14 1 1 NW NE shallow -14/35/-6 metavolcanic

15 1 1 NW NE shallow -16/28/-6 metavolcanic

16 2 2 NW NE shallow -6/13/-2 --

17 1 2 NW NE shallow -9/20/-3 --

18 1 2 WNW ENE shallow -12/23/-2 metavolcanic

19 3 2 NW SW shallow -0.5/4/-1.5 metavolcanic

20a 1 1 NW SW steep -5/14/-7 --

20b 2 3 NW SW shallow -3/7/-5 --

21 3 3 NW NE steep -1.5/5.5/-1 metavolcanic

22 1 1 NW? NE medium -5/21/-3 metavolcanic

23 2 2 NS E steep -4.5/8.8/-4.2 --

24 1 1 NW NE medium -6/18/-3.5 --

25a 2 2 NW n/a vertical -5/6/-5 --

25b 2 2 NW SW steep -2/16/-3 metavolcanic

26 1 2 NW SW shallow -6/25/-11 graywacke

27 3 3 NW NE medium -7/2/-6 --

28 1 2 NW NE shallow -6/16/-2 --

29 2 2 NW NE medium -6/10/-4 --

30 1 3 NW SW medium -3/15/-5 --

41b 1 2 NNW n/a vertical -13/15/-14 graywacke

Table 2. Interpreted strikes and dips from magnetic anomalies

[n/a, not applicable; nT; nanotesla; --, no data]
20

Map 

Number 

(on sheet 3)

Anomaly 

amplitude 

1=>20 nT 

2=10-20 nT 

3=<10 nT

Dip confidence 

1=best

2=intermediate 

3=least

Strike Dip direction Dip angle Anomaly amplitudes 

(south or west-side 

low/central high/north 

or east-side low)

Rock type (Bold

indicates rock type is 

associated with outcrop; 

italic indicates rock type 

is inferred); comments

42 2 2 NW NE steep -6/8/-5 —

43 3 3 NW NE medium -5/2/-3 --

44 3 3 NW n/a vertical -3/6/-3 --

45 1 1 NW NE steep -6/29/-5 metavolcanic

46 2 3 WNW NE shallow -5/11/-2 metavolcanic?

47 3 3 NNW ENE medium -0.9/0.9/-0.7 --

48 3 3 NW NE shallow -1.6/1.5/-0.6 Coastal Belt Thrust

49 3 3 NW NE shallow -1.8/1.2/-1.0 Coastal Belt Thrust

50a 3 3 NNW ENE medium -2/3/-1 Coastal Belt Thrust

50b 3 3 NNW NE shallow -2/2/0 Coastal Belt Thrust

51 1 1 NNW ENE steep -11/40/-9 --

52 2 2 NNW n/a vertical -2/8/-2 metavolcanic

53 3 3 NS n/a vertical -1.4/2.3/-1.3 Coastal Belt Thrust

54 2 2 EW S shallow -1/9/-5 metavolcanic

55 2 3 NNW ENE steep -3/10/-2 --

56 3 3 NNW ENE shallow -1.2/1.8/-0.3 --

57 3 2 NW NE shallow -2.7/0.3/-1.2 --

58 1 3 NW NE shallow -5/19/3 --

Table 2. continued—Interpreted strikes and dips from magnetic anomalies