Late Neogene and Quaternary Landscape Evolution of the Northern California Coast Ranges: Evidence for Mendocino Triple Junction Tectonics
Total Page：16
File Type：pdf, Size：1020Kb
Late Neogene and Quaternary landscape evolution of the northern California Coast Ranges: Evidence for Mendocino triple junction tectonics Jane Lock† Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Harvey Kelsey‡ Department of Geology, Humboldt State University, Arcata, California 95521, USA Kevin Furlong§ Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Adam Woolace# Department of Geology, Humboldt State University, Arcata, California 95521, USA ABSTRACT the double-humped pattern of uplift and our understanding of the lithospheric forces subsidence migrates, and the Coast Ranges that have built the orogen, and recognizing the A landscape records the surface response emerge. Smaller drainages develop and tectonic signal recorded by the landscape. We to tectonics at time scales intermedi- evolve by stream capture and fl ow reversal, describe how the surface responds to tectonics ate between short time-scale information and the two main divides migrate in concert in northern California, and we use the tectonic derived from seismic imaging and global with the triple junction. In contrast to the signal contained in the landscape to test and positioning systems and the long-term geo- systematic development of the small streams, develop our understanding of the geodynamics. logic record. We link late Neogene and the largest trunk streams can maintain grade A variety of mechanisms has been proposed Quaternary deposits and landforms in the through regions of high uplift, and coastal to explain the timing of uplift of the northern northern California Coast Ranges to the tec- river mouths remain stationary despite the California Coast Ranges. Dumitru (1989), using tonics of the Mendocino triple junction. In uplift moving north. Before ca. 2 Ma, the results of fi ssion-track analyses, argued that the northern California Coast Ranges, the majority of the Coast Range drainage fl owed primary uplift of the Coast Ranges occurred Mendocino crustal conveyor geodynamic to a southern coastal outlet near the present in association with Cretaceous subduction. model describes crustal thickening, thin- mouth of the Russian River. At 2 Ma, facili- Abundant late Neogene marine sediments that ning, and dynamic topography that produce tated by headwater stream capture at key outcrop throughout the area indicate that while a “double-humped” pattern of uplift that locations, the drainage direction reversed, Cretaceous uplift may refl ect a period of sub- migrates northward with the Mendocino tri- and the majority of Coast Range rivers now stantial exhumation, it cannot be responsible ple junction. The tectonics are manifest in the drain into the north-fl owing Eel River. The for the development of the present area of high drainage system and elevation pattern of the major drainage reorganization at 2 Ma high- elevation. Alternatively, uplift of the Coast Coast Ranges. At long wavelengths, the ele- lights the potential for complexity in geomor- Ranges could be driven by transpression along vation pattern closely matches the predicted phic response to tectonics. the developing San Andreas fault system, in a double-peaked shape of Mendocino crustal fashion similar to the Southern Alps of New conveyor topography, and the high points of Keywords: northern California Coast Ranges, Zealand (Walcott, 1998). However, at latitudes uplift control the location of drainage divides. landform evolution, Mendocino triple junction, north of the San Francisco Bay, plate motion on Presently, the divide between the Russian drainage evolution, geodynamics, tectonic geo- the San Andreas fault system is almost entirely and Eel Rivers and the divide between the morphology. parallel to the trend of the fault (DeMets et al., Eel and Van Duzen Rivers approximately 1990) (Fig. 1A). correspond to the peaks of uplift predicted INTRODUCTION Rather than transpression-driven or subduc- by the Mendocino crustal conveyor model. tion-related uplift, the underlying cause for the As the triple junction migrates northward, An orogen and its landscape develop in direct formation of the Coast Ranges is more likely response to underlying tectonic driving forces. processes associated with the passage of the †Present address: Department of Earth and As a result, the geomorphology and geology of Mendocino triple junction (Zandt and Furlong, Space Sciences, University of Washington, Seattle, a region record a tectonic history that contains 1982; Furlong et al., 1989; Merritts and Bull, Washington 98195, USA; e-mail: janelock@ess. information about deep-seated geodynamic 1989). The Mendocino triple junction lies at the washington.edu. ‡E-mail: [email protected]. processes. In this paper, we explore the links junction between the Pacifi c, North American, §E-mail: [email protected]. between the surface and tectonics in the north- and Juan de Fuca (or Gorda) plates (Fig. 1A). #E-mail: [email protected]. ern California Coast Ranges (Fig. 1A) using The triple junction is migrating to the northwest GSA Bulletin; September/October 2006; v. 118; no. 9/10; p. 1232–1246; doi: 10.1130/B25885.1; 9 fi gures; 1 table. 1232 For permission to copy, contact [email protected] © 2006 Geological Society of America Landscape evolution of the northern California Coast Ranges at ~5 cm/yr (Sella et al., 2002). Zandt and Fur- A Mad River A Gor Eureka long (1982) proposed that the high elevations Plate Tr i nity R da ive of the Coast Ranges could be a response to the Van Duzen R. r Eel infl ux of asthenosphere and resulting high tem- MTJ River peratures in the “slab window” that forms in Nor M the wake of a triple junction. Since these initial King Rangeattole R N. Fk. Eel River thern C studies, seismic images have shown that crustal iver 40º N thickness beneath the Coast Ranges varies spa- S. Fk. tially and reaches thicknesses of up to 40 km 2 Ma alifornia Coas E (Beaudoin et al., 1996, 1998; Villasenor et al., el R 1998; Verndonck and Zandt, 1994). Furlong iver M. Fk. and Govers (1999), in an attempt to explain Pacific Eel River Plate Nor the variable crustal structure, developed the th Mendocino crustal conveyor (MCC) model 4 Ma t Range Pla (Fig. 1B). The MCC is built on a numerical Ame geodynamic model in which uplift is driven te 0 40 rican by a combination of crustal thickening and s Russian River Clear dynamic topography, with a minor component km 6 Ma Lake of thermally driven uplift. San Andreas Fault B The purpose of this paper is to depict the landscape evolution of the northern California Coast Ranges by integrating the MCC geody- San namic model with geomorphic and geologic Francisco data from the Coast Ranges. We review geologic Santa Rosa Los Angeles and geomorphic data that provide evidence of 123ºW paleocoastlines, drainages, and topography in the late Neogene Coast Ranges, then outline the Southern Edge Area of active Pacific-Sierra Nevada salient points of the MCC geodynamic model. of Gorda Slab strike-slip seismicity relative plate motion We test whether the MCC model predictions (Jennings, 1994) calculated from are refl ected in the topography and geomorphic Line of MCC REVEL (Sella et al, 4 Ma Location of triple evolution of the Coast Ranges. First, the MCC model 2002) ~ 5 cm/yr junction through model predicts that there is a double-peaked Drainage divide time (after Atwater uplift in the Coast Ranges that follow in the and Stock, 1998) wake of the triple junction. Second, streams should respond to the MCC predicted topogra- MTJ SE NW phy by lengthening longitudinally (in a N-NW– S-SE direction) in the wake of the triple junc- B Crust Thins North American Crust tion. Third, E-W–trending stream divides in the Crust Thickens Coast Ranges should migrate N-NW in the wake of the triple junction. Finally, the Coast Ranges Viscous Coupling ate should sequentially emerge with passage of the a Pl ord triple junction such that remnant marine sedi- G ments that outcrop in the now-emergent Coast Upwelling Mantle Ranges young in age to the north. If the topog- Asthenosphere raphy and geomorphic evolution is inconsis- Looking from the northeast. Gorda plate is subducting out of the tent with the MCC model and more consistent page. with Coast-Range-axis-normal convergence, then there will be no northward-younging age progression to drainage development or Coast Figure 1. (A) Location map of the northern Coast Ranges of California showing the Men- Ranges emergence and no evidence for N-NW docino triple junction (MTJ), which marks the intersection of the Pacifi c, North America, stream lengthening or the migration of E-W– and Gorda (or Juan de Fuca) plates. The triple junction has been migrating to the northwest trending stream divides. at a steady rate for the last 8 m.y. Line A–B delineates the location of the two-dimensional The discussion integrates the geologic and Mendocino crustal conveyor (MCC) model (Furlong and Govers, 1999), which predicts a geomorphic attributes of the Coast Ranges with pattern of uplift of the northern California Coast Ranges. (B) Schematic cross section show- our understanding of the tectonics to depict the ing the main geodynamic processes in the MCC model along line A–B in part A. Deforma- landscape evolution of the northern California tion occurs as hot asthenosphere fi lls the gap left by the migrating Gorda plate, causing Coast Range. By making the link between the viscous coupling between the Gorda slab and the base of the North American crust. As the surface and tectonics, we achieve two goals. Gorda plate moves to the north, the North American plate thickens, then thins, driving One, we can use the paleogeomorphology isostatic uplift. Flow in the mantle causes dynamic topography that adds to the uplift and inferred from the preserved geologic deposits subsidence. Geological Society of America Bulletin, September/October 2006 1233 Lock et al.