Previously unrecognized regional structure of the Coastal Belt of the Franciscan Complex, northern California, revealed by magnetic data

Abstract

Magnetic anomalies provide surprising structural detail within the previously undivided Coastal Belt, the westernmost, youngest, and least-metamorphosed part of the Franciscan Complex of northern California. Although the Coastal Belt consists almost entirely of arkosic graywacke and shale of mainly Eocene age, new detailed aeromagnetic data show that it is pervasively marked by long, narrow, and regularly spaced anomalies. These anomalies arise from relatively simple tabular bodies composed principally of magnetic basalt or graywacke confi ned mainly to the top couple of kilometers, even though metamorphic grade indicates that these rocks have been more deeply buried, at depths of 5-8 km. If true, this implies surprisingly uniform uplift of these rocks. The basalt (and associated Cretaceous limestone) occurs largely in the northern part of the Coastal Belt; the graywacke is recognized only in the southern Coastal Belt and is magnetic because it contains andesitic grains. The magnetic grains were not derived from the basalt, and thus require a separate source. The anomalies defi ne simple patterns that can be related to folding and faulting within the Coastal Belt. This apparent simplicity belies complex structure mapped at outcrop scale, which can be explained if the relatively simple tabular bodies are internally deformed, fault-bounded slabs. One mechanism that can explain the widespread lateral extent of the thin layers of basalt is peeling up of the uppermost part of the oceanic crust into the accretionary prism, controlled by porosity and permeability contrasts caused by alteration in the upper part of the subducting slab. It is not clear, however, how this mechanism might generate fault-bounded layers containing magnetic graywacke. We propose that structural domains defi ned by anomaly trend, wavelength, and source refl ect imbrication and folding during the accretion process and local plate interactions as the Mendocino triple junction migrated north, a hypothesis that should be tested by more detailed structural studies. KRt fc Cob Yg F i g . 5 B Fort Bragg s u r v e y e d g e WFt Fort Ross Point Arena Punta Gorda Point Delgada Figure 2. Filtered magnetic map of the Coastal Belt. See Langenheim et al. (2011) for details of fi ltering that places anomalies over magnetic sources and enhances anomalies for which sources are exposed or near surface. Magenta lines-margins of the belt, with the San Andreas fault on the west and the Coastal Belt thrust and other faults on the east. Dashed dark green lines-depositional contacts. Red linesboundaries between the terranes of the Coastal Belt: Coastal Belt, undivided (Cob), False Cape terrane (fc), King Range terrane (KRt), and Yager terrane (Yg). The Wheatfi eld Fork terrane (WFt) is too narrow to show at the scale of the fi gure, but its extent along the eastern boundary of the Coastal Belt is circled in dark blue. Thin dark blue dotted lines separate structural domains discussed in text and shown in fi gure 6. Blue line-profi le location of model shown in Figure 5B.

Slip rates and spatially variable creep on faults of the northern San Andreas system inferred through Bayesian inversion of Global Positioning System data

“…Castillo and Ellsworth [] noted the presence of earthquakes with strike‐slip mechanisms between the MF and SAF north of Point Arena as a possible indication of shear strain being accommodated on additional fault strands, and Langenheim et al . [] present aeromagnetic evidence for a complex fabric of fault and fold structures between the MF and BSF. Thomas et al .…”

Detrital zircon U–Pb reconnaissance of the Franciscan subduction complex in northwestern California

“…The Coastal Belt has been subdivided into the Yager, Coastal, King Range, and False Cape terranes (Underwood and Bachman 1986;McLaughlin et al 1994McLaughlin et al , 2000Langenheim et al 2013). Peak metamorphic conditions reported by Ernst and McLaughlin (2012) were 50-135°C and 1-2 kbar ( Figure 3).…”

“…As noted previously, such high Lv sandstones are rare in the terrane and are overrepresented in our data set because we specifically targeted them for analysis. Kleist (1974), Underwood and Bachman (1986), and Langenheim et al (2013) described distinctive high-Lv sandstones within the Coastal terrane, but this petrofacies did not form discrete, mappable bodies. Therefore, our estimate for the longest plausible time span for the bulk of the clastic strata in the Coastal terrane is from 55 Ma (YZP; pre-Challis) to post-32 Ma (YZP), our estimate for the shortest plausible time span is from 52 to 32 Ma, and our judgement of the actual age range is from 52 Ma to post-32 Ma.…”

Controls on the distribution and resilience ofQuercus garryana: ecophysiological evidence of oak's water‐limitation tolerance

“…), which underlies ≈50% of the Eel River watershed (Langenheim et al. ). The matrix of the mélange is argillaceous and encompasses coherent blocks of greywacke, chert, and minor high‐grade metamorphics and ultramafics (Blake and Jones , Cloos ).…”