Mendocino Triple Junction Region Research Articles

Strain Partitioning Among Forearc Faults in Southern Cascadia Inferred From GNSS

AbstractCrustal faults in the southern Cascadia forearc help to accommodate strains related to plate interactions near the Mendocino triple junction (MTJ). Geologic work has identified the Little Salmon fault, Mad River fault zone (MRFZ), and Grogan fault as Quaternary‐active structures; however, the slip rates on these major fault systems are poorly constrained. This work constructs elastic block models of the MTJ region and leverages GNSS velocity data to better understand how strain is partitioned among these three faults. We evaluate models with various forearc fault structures and apply a bootstrap analysis to provide independent slip rate distributions for each fault. Model results indicate that the Little Salmon fault zone is an important structure that accommodates both thrust and right‐lateral shear motion in the forearc. Appreciable right‐lateral slip rates on the MRFZ indicate that this system plays an important role in facilitating translation of the forearc and may serve as an extension of the northern San Andreas fault system. In contrast, the models place very little strike‐slip motion on the Grogan fault and prefer this structure to mainly host reverse slip. We observe the highest slip rates on the Bear River fault zone, indicating significant strain to the north of the MTJ.

Boulders as a lithologic control on river and landscape response to tectonic forcing at the Mendocino triple junction

AbstractConstraining Earth’s sediment mass balance over geologic time requires a quantitative understanding of how landscapes respond to transient tectonic perturbations. However, the mechanisms by which bedrock lithology governs landscape response remain poorly understood. Rock type influences the size of sediment delivered to river channels, which controls how efficiently rivers respond to tectonic forcing. The Mendocino triple junction region of northern California, USA, is one landscape in which large boulders, delivered by hillslope failures to channels, may alter the pace of landscape response to a pulse of rock uplift. Boulders frequently delivered by earthflows in one lithology, the Franciscan mélange, have been hypothesized to steepen channels and slow river response to rock uplift, helping to preserve high-elevation, low-relief topography. Channels in other units (the Coastal Belt and the Franciscan schist) may experience little or no erosion inhibition due to boulder delivery. Here we investigate spatial patterns in channel steepness, an indicator of erosion resistance, and how it varies between mélange and non-mélange channels. We then ask whether lithologically controlled boulder delivery to rivers is a possible cause of steepness variations. We find that mélange channels are steeper than Coastal Belt channels but not steeper than schist channels. Though channels in all units steepen with increasing proximity to mapped hillslope failures, absolute steepness values near failures are much higher (∼2×) in the mélange and schist than in Coastal Belt units. This could reflect reduced rock erodibility or increased erosion rates in the mélange and schist, or disproportionate steepening due to enhanced boulder delivery by hillslope failures in those units. To investigate the possible influence of lithology-dependent boulder delivery, we map boulders at failure toes in the three units. We find that boulder size, frequency, and concentration are greatest in mélange channels and that Coastal Belt channels have the lowest concentrations. Using our field data to parameterize a mathematical model for channel slope response to boulder delivery, we find that the modeled influence of boulders in the mélange could be strong enough to account for some observed differences in channel steepness between lithologies. At the landscape scale, we lack the data to fully disentangle boulder-induced steepening from that due to spatially varying erosion rates and in situ rock erodibility. However, our boulder mapping and modeling results suggest that lithology-dependent boulder delivery to channels could retard landscape adjustment to tectonic forcing in the mélange and potentially also in the schist. Boulder delivery may modulate landscape response to tectonics and help preserve high-elevation, low-relief topography at the Mendocino triple junction and elsewhere.

Differences in channel and hillslope geometry record a migrating uplift wave at the Mendocino triple junction, California, USA

AbstractTectonic plate motion, and the resulting change in land surface elevation, has been shown to have a fundamental impact on landscape morphology. Changes to uplift rates can drive a response in fluvial channels, which then drives changes to hillslopes. Because hillslopes respond on different time scales than fluvial channels, investigating the geometry of channels and hillslopes in concert provides novel opportunities to examine how uplift rates may have changed through time. Here we perform coupled topographic analysis of channel and hillslope geometry across a series of catchments at the Mendocino triple junction (MTJ) in northern California, USA. These catchments are characterized by an order-of-magnitude difference in uplift rate from north to south. We find that dimensionless hillslope relief closely matches the uplift signal across the area and is positively correlated with channel steepness. Furthermore, the range of uncertainty in hillslope relief is lower than that of channel steepness, suggesting that it may be a more reliable recorder of uplift in the MTJ region. We find that hilltop curvature lags behind relief in its response to uplift, which in turn lags behind channel response. These combined metrics show the northward migration of the MTJ and the corresponding uplift field from topographic data alone.

Erosion of coastal drainages in the Mendocino Triple Junction region (MTJ), northern California

The Mendocino Triple Junction (MTJ) region in northern California is an archetype for studying landscape response to varying rock uplift rates as they increase toward the triple junction. Underpinning these studies lies the assumption that this landscape has reached a dynamic equilibrium in which rock uplift and erosion rates are equal; however, no study has shown that such an equilibrium actually exists around the MTJ region. Here, we report 10Be- and 26Al-derived erosion rates calculated from isotope concentrations in detrital, fluvial sediment from coastal drainage basins; we then compare these rates with uplift rates inferred from marine terraces that were formed and preserved by uplift during the last ∼305 ka. Erosion rates in the more slowly uplifting southern part of the region range from 0.21–0.32 mm/yr and are consistent with rock uplift rates since 305 ka. However, in the northern transition zone, where uplift rates apparently started to increase about ∼100 ka due to northward migration of the MTJ, erosion rates are higher, 0.43 to 0.69 mm/yr. These rates are similar to uplift rates from 96–305 ka, but substantially less than recent uplift rates of ∼3.5–4 mm/yr inferred for the past ∼72 ka. In the central part of the King Range, finite erosion rates cannot be determined due to (i) the presence of excess 10Be that does not appear to have originated from in-situ cosmic-ray production during erosion and transport of the sediment, and (ii) 26Al concentrations below detection limits. The difference between catchment-wide erosion and recent uplift rates in coastal catchments may be related to a lag in response of hillslopes and tributaries to changes in rates of tectonically-driven relative base-level fall or the uncertainties in rock uplift rates inferred from marine terraces. This study suggests that measurements of both erosion and uplift rates are crucial in assessing the equilibrium state of landscapes and in understanding the topographic features made by surface and tectonic processes.

Stress rotation across the Cascadia megathrust requires a weak subduction plate boundary at seismogenic depths

The Mendocino Triple Junction region is the most seismically active part of the Cascadia Subduction Zone. The northward moving Pacific plate collides with the subducting Gorda plate causing intense internal deformation within it. Here we show that the stress field rotates rapidly with depth across the thrust interface from a strike-slip regime within the subducting plate, reflecting the Pacific plate collision, to a thrust regime in the overriding plate. We utilize a dense focal mechanism dataset, including observations from the Cascadia Initiative ocean bottom seismograph experiment, to constrain the stress orientations. To quantify the implications of this rotation for the strength of the plate boundary, we designed an inversion that solves for the absolute stress tensors in a three-layer model subject to assumptions about the strength of the subducting mantle. Our results indicate that the shear stress on the plate boundary fault is likely no more than about ∼50 MPa at ∼20 km depth. Regardless of the assumed mantle strength, we infer a relatively weak megathrust fault with an effective friction coefficient of ∼0 to 0.2 at seismogenic depths. Such a low value for the effective friction coefficient requires a combination of high fluid pressures and/or fault-zone minerals with low inherent friction in the region where a great earthquake is expected in Cascadia.

GPS-derived strain in northwestern California: Termination of the San Andreas fault system and convergence of the Sierra Nevada–Great Valley block contribute to southern Cascadia forearc contraction

GPS-derived velocities (1993–2002) in northwestern California show that processes other than subduction are in part accountable for observed upper-plate contraction north of the Mendocino triple junction (MTJ) region. After removing the component of elastic strain accumulation due to the Cascadia subduction zone from the station velocities, two additional processes account for accumulated strain in northern California. The first is the westward convergence of the Sierra Nevada–Great Valley (SNGV) block toward the coast and the second is the north–northwest impingement of the San Andreas fault system from the south on the northern California coastal region in the vicinity of Humboldt Bay. Sierra Nevada–Great Valley block motion is northwest toward the coast, convergent with the more northerly, north–northwest San Andreas transform fault-parallel motion. In addition to the westward-converging Sierra Nevada–Great Valley block, San Andreas transform-parallel shortening also occurs in the Humboldt Bay region. Approximately 22 mm/yr of distributed Pacific–SNGV motion is observed inland of Cape Mendocino across the northern projections of the Maacama and Bartlett Springs fault zones but station velocities decrease rapidly north of Cape Mendocino. The resultant 6–10 mm/yr of San Andreas fault-parallel shortening occurs above the southern edge of the subducted Gorda plate and at the latitude of Humboldt Bay. Part of the San Andreas fault-parallel shortening may be due to the viscous coupling of the southern edge of the Gorda plate to overlying North American plate. We conclude that significant portions of the upper-plate contraction observed north of the MTJ region are not solely a result of subduction of the Gorda plate but also a consequence of impingement of the western edge of the Sierra Nevada–Great Valley block and growth of the northernmost segments of the San Andreas fault system.

Channel response to tectonic forcing: field analysis of stream morphology and hydrology in the Mendocino triple junction region, northern California

An empirical calibration of the shear stress model for bedrock incision is presented, using field and hydrologic data from a series of small, coastal drainage basins near the Mendocino triple junction in northern California. Previous work comparing basins from the high uplift zone (HUZ, uplift rates around 4 mm/year) to ones in the low uplift zone (LUZ, ∼0.5 mm/year) indicates that the HUZ channels are about twice as steep for a given drainage area. This observation suggests that incision processes are more effective in the HUZ. It motivates a detailed field study of channel morphology in the differing tectonic settings to test whether various factors that are hypothesized to influence incision rates (discharge, channel width, lithology, sediment load) change in response to uplift or otherwise differ between the HUZ and LUZ. Analysis of regional stream gaging data for mean annual discharge and individual floods yields a linear relationship between discharge and drainage area. Increased orographic precipitation in the HUZ accounts for about a twofold increase in discharge in this area, corresponding to an assumed increase in the erosional efficiency of the streams. Field measurements of channel width indicate a power-law relationship between width and drainage area with an exponent of ∼0.4 and no significant change in width between the uplift rate zones, although interpretation is hampered by a difference in land use between the zones. The HUZ channel width dataset reveals a scaling break interpreted to be the transition between colluvial- and fluvial-dominated incision processes. Assessments of lithologic resistance using a Schmidt hammer and joint surveys show that the rocks of the study area should be fairly similar in their susceptibility to erosion. The HUZ channels generally have more exposed bedrock than those in the LUZ, which is consistent with protection by sediment cover inhibiting incision in the LUZ. However, this difference is likely the result of a recent pulse of sediment due to land use in the LUZ. Therefore, the role of sediment flux in setting incision rates cannot be constrained with any certainty. To summarize, of the four response mechanisms analyzed, the only factor that demonstrably varies between uplift rate zones is discharge, although this change is likely insufficient to explain the relationship between channel slope and uplift rate. The calibrated model allows us to make a prediction of channel concavity that is consistent with a previous estimate from slope–drainage area data. We show that the inclusion of nonzero values of critical shear stress in the model has important implications for the theoretical relationship between steady-state slope and uplift rate and might provide an explanation for the observations. This analysis underscores the importance of further work to constrain quantitatively threshold shear stress for bedrock incision.

Interactions between onshore bedrock‐channel incision and nearshore wave‐base erosion forced by eustasy and tectonics

AbstractWe explore the response of bedrock streams to eustatic and tectonically induced fluctuations in base level. A numerical model coupling onshore fluvial erosion with offshore wave‐base erosion is developed. The results of a series of simulations for simple transgressions with constant rate of sea‐level change (SLR) show that response depends on the relative rates of rock uplift (U) and wave‐base erosion (ɛw). Simple regression runs highlight the importance of nearshore bathymetry. Shoreline position during sea‐level fall is set by the relative rate of base‐level fall (U‐SLR) and ɛw, and is constant horizontally when these two quantities are equal. The results of models forced by a realistic Late Quaternary sea‐level curve are presented. These runs show that a stable shoreline position cannot be obtained if offshore uplift rates exceed ɛw. Only in the presence of a relatively stable shoreline position, fluvial profiles can begin to approximate a steady‐state condition, with U balanced by fluvial erosion rate (ɛf). In the presence of a rapid offshore decrease in rock‐uplift rate (U), short (∼5 km) fluvial channels respond to significant changes in rock‐uplift rate in just a few eustatic cycles. The results of the model are compared to real stream‐profile data from the Mendocino triple junction region of northern California. The late Holocene sea‐level stillstand response exhibited by the simulated channels is similar to the low‐gradient mouths seen in the California streams.

Landscape response to tectonic forcing: Digital elevation model analysis of stream profiles in the Mendocino triple junction region, northern California

Landscape response to tectonic forcing: Digital elevation model analysis of stream profiles in the Mendocino triple junction region, northern California

Landscape response to tectonic forcing: Digital elevation model analysis of stream profiles in the Mendocino triple junction region, northern California

The topographic evolution of orogens is fundamentally dictated by rates and patterns of bedrock-channel incision. Quantitative field assessments of process-based laws are needed to accurately describe landscape uplift and denudation in response to tectonics and climate. We evaluate and calibrate the shear stress (or similar unit stream-power) bedrock-incision model by studying stream profiles in a tectonically active mountain range. Previous work on emergent marine terraces in the Mendocino triple junction region of northern California provides spatial and temporal control on rock-uplift rates. Digital elevation models and field data are used to quantify differences in landscape morphology associated with along-strike northwest to southeast changes in tectonic and climatic conditions. Analysis of longitudinal profiles supports the hypothesis that the study-area channels are in equilibrium with current uplift and climatic conditions, consistent with theoretical calculations of system response time based on the shear-stress model. Within uncertainty, the profile concavity (𝛉) of the trunk streams is constant throughout the study area (𝛉 ≈ 0.43), as predicted by the model. Channel steepness correlates with uplift rate. These data help constrain the two key unknown model parameters, the coefficient of erosion ( K ) and the exponent associated with channel gradient ( n ). This analysis shows that K cannot be treated as a constant throughout the study area, despite generally homogeneous substrate properties. For a reasonable range of slope-exponent values ( n ), best-fit values of K are positively correlated with uplift rate. This correlation has important implications for landscape-evolution models and likely reflects dynamic adjustment of K to tectonic changes, due to variations in orographic precipitation, and perhaps channel width, sediment load, and frequency of debris flows. The apparent variation in K makes a unique value of n impossible to constrain with present data.

Fluids in the lower crust following Mendocino triple junction migration: Active basaltic intrusion?

Geodynamic and plate tectonic models for the Mendocino triple junction, a fault-fault-trench triple junction in northwestern California, predict a slab-free zone south of the triple junction in which asthenospheric mantle upwells to the base of the crust. A variety of geological and geophysical data support this model, although fine-scale (<20 km) details of the lithospheric structure have been unknown previously. Seismic investigations in the onshore transform regime south of the Mendocino triple junction region reveal very strong short-offset reflections from the lower crust and at the crust-mantle boundary beneath the entire width of the Coast Range, particularly near Lake Pillsbury, California. Seismic analysis suggests that these reflections are from discrete zones of fluid. The reflector geometry implies that the source of the fluid is within the upper mantle. In this tectonic context it is likely that the fluids are largely partial melt, segregated from asthenospheric mantle upwelling into the slab-free zone. The tectonic setting and the location of Lake Pillsbury relative to the estimated position of the southern edge of the Gorda slab and the Clear Lake volcanic field suggest that volcanism may initiate in this region within the next 400 k.y.

Location and source parameters of the 19 June 1994 (MW = 5.0) offshore Petrolia, California, earthquake

Abstract The MW = 5.0, 19 June 1994 offshore Petrolia, California, earthquake was well recorded by nine ocean-bottom hydrophones (OBH) and seismometers (OBS), providing an opportunity to precisely locate an earthquake in the tectonically active Mendocino triple junction region. Adding the offshore data improves the azimuthal station coverage and essentially removes the epicenter's sensitivity to the choice of inversion parameters and velocity models. The hypocentral parameters, assuming an oceanic upper-mantle velocity of 7.9 km/sec, are 10:39:33.2 UTC for origin time, 40.376° N latitude, 124.441° W longitude, and a depth of 18.8 km. The moment-tensor solution obtained by modeling of low-frequency regional waveforms indicates predominantly strike-slip faulting with a north-south-trending P axis, as is typical for Gorda plate earthquakes, and confirms the depth estimate from the P-wave travel-time data.

The Mendocino triple junction: Active faults, episodic coastal emergence, and rapid uplift

A complex zone of rapid Holocene surface uplift and deformation occurs at the Mendocino triple junction, the juncture of three plate‐bounding faults: the Cascadia subduction zone, San Andreas fault, and Mendocino fault. Within this mountainous structural knot, up to 1.4 m of coastal emergence occurred during the 1992 Cape Mendocino MS 7.1 earthquake. Surveying and radiometric dating of ancient marine strandlines (&lt;8000 years) along the trace of the southernmost Cascadia subduction zone indicate that the Holocene pattern of net surface uplift is very similar to the 1992 coseismic uplift pattern. Results of this investigation also indicate that episodic emergence occurred at least four times between about 600 and 7000 years ago and that some past events might have resulted in larger amounts of uplift (∼2.5 m) than the 1992 earthquake, perhaps during great earthquakes (M&gt;7.5) along the Cascadia subduction zone megathrust. However, another plausible interpretation of the data is that multiple earthquakes resulting in smaller amounts of net surface uplift per event (similar to the 1992 earthquake) occurred closely spaced in time, giving the appearance in the geologic record of less frequent and larger events. Regardless of the number and timing of paleoearthquakes, the number of platforms is a minimum of the number of events that resulted in sudden, rapid uplift, because platform preservation is also a function of rising Holocene sea level. At present, rates and patterns of net surface uplift are better constrained than the timing and magnitude of paleoearthquakes. Periods of rapid Holocene emergence also are identified as far south of the area of 1992 uplift and the Mendocino fault as ∼30 km, along the unlocated San Andreas fault. These also might be associated with coseismic uplift. Based on new mapping of active faults in the region, it is proposed here that this uplift is the result of multiple discontinuous thrust and strike‐slip fault segments which distribute plate boundary deformation along a restraining bend at the termination of the San Andreas fault. This interpretation is consistent with the fact that a single, prominent, strike‐slip fault has not been identified in the Mendocino triple junction region, while several small reverse faults crossing Holocene deposits and geomorphic surfaces have been mapped and dated.

Seismotectonics of the April 25, 1992, Petrolia earthquake and the Mendocino triple junction region

The April 25, 1992, Petrolia earthquake (M s 7.1) occurred at the southern tip of the Cascadia subduction zone. This is the largest thrust earthquake ever recorded instrumentally in the Cascadia subduction zone. The earthquake was followed by two large strike‐slip aftershocks (both Ms 6.6). Moment release of each of the earthquakes is as follows: 4.0 × 1019 Nm in the first 10 s for the mainshock, 0.7 × 1019 Nm in the first 8 s for the first aftershock, and 0.9 × 1019 Nm in the first 2 s for the second aftershock. These indicate that the mainshock and each of the aftershocks may have different tectonic backgrounds. The best depth estimates of the mainshock and the two aftershocks are 14 km, 18 km, and 24 km, respectively. The slip direction of the mainshock is between N75°E and N80°E. This slip direction is not consistent with either the relative motion of the North American and Juan de Fuca plates (N60°E) or between the North American plate and the Gorda deformation zone (N40°E). It has been suggested that the North American‐Pacific plate motion is accommodated by right‐lateral slip on both the San Andreas and Maacama‐Rodgers Creek‐Hayward fault systems; the intervening block is the Humboldt plate. If we modify the relative motion of the southernmost Gorda deformation zone to conform with the seismicity trends and allow the Humboldt‐Pacific plate motion to be about half the total North American‐Pacific motion, then the Gorda deformation zone‐Humboldt relative motion matches the direction of the Petrolia slip vector. Also, the mixture of focal mechanisms in the two distinct aftershock clusters can be explained by motion between the Gorda deformation zone and Pacific plate and the Humboldt and North American plates. The Gorda deformation zone is subducting beneath the Humboldt plate in the Cape Mendocino area, and the Petrolia earthquake ruptured the entire subduction segment between the Gorda deformation zone and Humboldt plate.

Pulling the rug out from under California: Seismic images of the Mendocino Triple Junction Region

The active tectonic regime of northwestern California changes abruptly from transform motion to subduction at the Mendocino Triple Junction. Northward migration of the triple junction has been a major factor in the tectonic history of the continental margin of California since the Oligocene and continues at present. Understanding the effects of triple junction migration on the structure of the crust and upper mantle in this region is therefore necessary for reconstructing the geologic evolution of the continental margin of California and accurately assessing seismic hazards associated with the San Andreas fault system and the Cascadia subduction zone.In 1993 and 1994 a network of large‐aperture seismic profiles was collected to image the crustal and upper‐mantle structure beneath northern California and the adjacent continental margin. The data include approximately 650 km of onshore seismic refraction/reflection data, 2000 km of off‐shore multichannel seismic (MCS) reflection data, and simultaneous onshore and offshore recording of the MCS airgun source to yield large‐aperture data. Scientists from more than 12 institutions were involved in data acquisition.

Three‐dimensional crustal structure of the Mendocino Triple Junction region from local earthquake travel times

The large‐scale, three‐dimensional geometry of the Mendocino Triple Junction at Cape Mendocino, California, was investigated by inverting nearly 19,000 P wave arrival times from over 1400 local earthquakes to estimate the three‐dimensional velocity structure and hypocentral parameters. A velocity grid 175 km (N‐S) by 125 km (E‐W) centered near Garberville, California, was constructed with 25 km horizontal and 5 fan vertical node spacing. The model was well resolved near Cape Mendocino, where the earthquakes and stations are concentrated. At about 40.6° N latitude a high‐velocity gradient between 6.5 and 7.5 km/s dips gently to the south and east from about 15 km depth near the coast. Relocated hypocenters concentrate below this high gradient which we interpret as the oceanic crust of the subducted Gorda Plate. Therefore the depth to the top of the Gorda Plate near Cape Mendocino is interpreted to be ∼15 km. The Gorda Plate appears intact and dipping ∼8° eastward due to subduction and flexing downward 6°–12° to the south. Both hypocenters and velocity structure suggest that the southern edge of the plate intersects the coastline at 40.3° N latitude and maintains a linear trend 15° south of east to at least 123° W longitude. The top of a large low‐velocity region at 20–30 km depth extends about 50 km N‐S and 75 km E‐W (roughly between Garberville and Covelo) and is located above and south of the southern edge of the Gorda Plate. We interpret this low velocity area to be locally thickened crust (8–10 km) due to either local compressional forces associated with north‐south compression caused by the northward impingement of the rigid Pacific Plate or by underthrusting of the base of the accretionary subduction complex at the southern terminous of the Cascadia Subduction Zone. South of Cape Mendocino and southwest of the Garberville fault, high velocities indicative of oceanic crust are detected at 15 km depth. We interpret this evidence as tectonic underplating of the Pacific oceanic crust beneath the coastal Franciscan Complex, similar to the situation seismically imaged in central coastal California (Page and Brocher, 1994). This geometry indicates that the westernmost portion of the slab window has been filled by tectonic underplating and crustal thickening, while deep‐seated mantle upwelling is confined to an area centered about 75 km north of Clear Lake (Benz et al, 1992; Zandt et al., 1992).

Geomorphic response of coastal streams to low, intermediate, and high rates of uplift, Medocino triple junction region, northern California

Research Article| November 01, 1989 Geomorphic response of coastal streams to low, intermediate, and high rates of uplift, Medocino triple junction region, northern California DOROTHY MERRITTS; DOROTHY MERRITTS 1Geology Department, Franklin and Marshall College, P.O. Box 3003, Lancaster, Pennsylvania 17604-3003 Search for other works by this author on: GSW Google Scholar KIRK R. VINCENT KIRK R. VINCENT 2Geosciences Department, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1989) 101 (11): 1373–1388. https://doi.org/10.1130/0016-7606(1989)101<1373:GROCST>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DOROTHY MERRITTS, KIRK R. VINCENT; Geomorphic response of coastal streams to low, intermediate, and high rates of uplift, Medocino triple junction region, northern California. GSA Bulletin 1989;; 101 (11): 1373–1388. doi: https://doi.org/10.1130/0016-7606(1989)101<1373:GROCST>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Analysis of three-dimensional morphological properties of 24 coastal drainage basins that have evolved in areas of low (<1 m/1,000 yr), intermediate (1-3 m/1,000 yr), and high (>3 m/1,000 yr) rates of uplift near the Mendocino triple junction (MTJ), coastal northern California, identified channel gradients as the best indicator of tectonism in the landscape. Lower-order tributaries best reflect tectonically controlled differences. The largest streams examined, of third order, are able to adjust to most base-level change and maintain their profile form, whereas lower-order streams farther upstream tend to accumulate the effects of net base-level fall and have steepest profiles in the areas of highest uplift rates. Variations in steepness of first-order channel gradients indicate that (1) high uplift rates in the wake of the MTJ have existed for at least 73,000 yr and (2) differential tilt of the region to the north and south of the current locus of highest uplift rate is occurring in association with regional uplift. Although first-order streams are excellent indicators of areas of both high uplift rates and regional differential tilting, they are less useful in distinguishing between areas of low and intermediate uplift rate. Analysis of the longitudinal profile of the main trunk stream of ten of the 24 drainage basins, using the stream-gradient index, was more useful to categorize broadly the uplift rates and to distinguish between streams in low- and intermediate-uplift-rate areas. Although the hypotheses tested herein do not identify the exact mechanism off uplift in the MTJ region, they do indicate that the nature of deformation is most likely regional tilt rather than crustal shortening with localized compressional folding and thrust faulting. They also indicate that geomorphic responses trail in the wake of the uplift-rate response caused by development of a slab window beneath the North American plate; (1) maximum uplift rates occur ∼9 km south of the northern boundary of the slab window, (2) maximum mean first-order channel gradient occurs 16 km south of the northern edge of the slab window, and (3) maximum drainage-basin relief occurs 20 km south of the northern edge of the slab window. Associated lag times between passage of the southern edge of the subducted Gorda plate slab and geomorphic responses are ∼160,000 yr for maximum uplift rates, ∼290,000 yr for maximum channel gradients, and ∼370,000 yr for maximum relief. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

JGR special issue: Geologic and Tectonic Evolution of the Mendocino Triple Junction Region

A special section of the Journal of Geophysical Research—Solid Earth and Planets will be dedicated to the publication of papers on the geologic and tectonic evolution of the Mendocino triple junction region, as an outgrowth of the symposium held at the 1986 AGU Fall Meeting.