The Maacama fault system and fracture zone lies at the heart of Mendocino County, defining the landscape of the central valleys and framing the geography, ecosystems, industry and culture of inland Mendocino.
A sweeping and formidable tectonic corridor, this sister of the San Andreas Fault system delineates the valleys and towns of Laytonville, Willits, Ukiah, Hopland, and others along its path. Historic toponyms such as Long Valley and Little Lake Valley allude to the geography of the fractious Maacama.
A tectonic fracture zone is just that, not a single linear fault line but a highly splintered system of fault strands. Recognizable tectonic landforms of the Maacama include the primary wider northwest trending strike-slip valleys (transtensional basins) of the 101 corridor at the heart of the fault system, and a multitude of smaller tectonic strand valleys branching out at angles from the central basins.
Two of the more recognizable strand valleys of the Maacama fault system and fracture zone include the terminal tectonic basin of Redwood Valley, and the narrow linear fault valley east of Largo which runs roughly from the Ukiah Rancheria in the north to the UC Hopland Research Center in the south. And at the shattered northern reaches of the Maacama fracture zone several tectonic strand basins splinter out through the hills of western Long Valley from the Laytonville area toward the town of Branscomb.
Fault creep roils parking lots and sidewalks but is undetectable in real time
Another distinct feature, or phenomenon, of seismic geomorphology found on the landscape of the Maacama fracture zone is known as fault creep. Telltale evidence of fault creep can be traced throughout old-town Willits along the northwest trending main-stem of the Maacama as it cuts through and rumples parking lots, sidewalks, backyards and historic neighborhoods. In fact, much of the displacement along the Maacama fault system is the result of a-seismic fault creep, defined as steady tectonic slippage undetectable in real time without instrumentation, as opposed to the abrupt seismic behavior of earthquakes.
The Maacama Fracture Zone is a major component of the greater San Andreas Fault system in Northern California, and formed between six to ten million years ago as a segment splintered off toward the northeast of the Rodgers Creek Fault. This is believed to have been caused by the shifting motion of the migrating Mendocino Triple Junction at the intersection of the Pacific, Gorda, and North American tectonic plates, which lies approximately 100 miles to the northwest.
Recent studies of the complexities of Northern California’s multi-component tectonic mechanics, both above and beneath the earth’s crust, reveals the Maacama Fracture Zone as the primary boundary at depth between the Pacific and North American tectonic plates in Northern California, and not the San Andreas fault as previously assumed.
The Maacama Fracture Zone also delineates a western boundary of the migrating slab window of the Mendocino Triple Junction, a region of very thin lithosphere and upwelled magmatic activity close to the earth’s surface, which is responsible for the signature volcanic landforms and geothermal activity of Mendocino, Lake, and Napa counties.
The rugged foothills of two distinct and rapidly uplifting geologic belts of the northern Coast Ranges frame the Maacama’s sweeping rift valleys and transtensional basins. To the west of the Maacama, the foothills of the Coastal Belt of the northern Coast Ranges display a steeply rolling silhouette of rumply outcrops of uplifted and deformed sedimentary rock, peridotite, serpentinite, and other geologic units of earth’s mantle and ocean crust origin.
These western slopes of the Coastal Belt are draped in mixed oak-madrone-gray pine woodlands, with fingers of mixed evergreen forest cascading down from the higher elevations, and interspersed by an understory of perennial bunch grasslands as well as mixed chaparral.
The massive blocky foothills and ridgelines of the Central Belt rise to the east of the valleys of the Maacama fracture zone, and are composed of volcanic materials as well as older Franciscan geologic units. The lower foothills of the Central Belt propagate sweeping oak savannahs, and the higher elevations of these eastern foothills are uniformly cloaked by a massive swath of old growth chaparral.
Chaparral is California’s primary and emblematic ecosystem of dense woody shrublands widely dispersed throughout the Coast Ranges, featuring a rich assemblage of winter and early spring-blooming endemic plant species, such as buck brush (Ceonothus cuneatus) and a multitude of manzanita species (Arctostaphylos). Northern California’s chaparral forms a vast ecosystem mosaic harboring rare pollinator and animal communities, and in Mendocino County provides vital wildlife corridors connecting the wilderness areas of the Bureau of Land Management and National Forestlands of the northern Coast Ranges.
Rowena Forest is a physical geographer and a science writer at Cal Geographic.com.