The seismically active and topographically impressive eastern and western margins of the northern Basin and Range have been extensively studied but the northwestern margin of the province remains little-studied and incompletely understood.  This transition zone is characterized by Basin and Range extension in northwestern Nevada that gives way northwards and westwards to high volcanic plateaus in northwestern-most Nevada, northeastern California, and southern Oregon (Fig. 1). 

Figure 1: Map summarizing topography, and heat flow, and relevant geophysical data in the northern Basin and Range Province. Dashed black and white lines indicate COCORP seismic reflection profiles (Allmendinger et al. (1987)), solid black lines indicate PASSCAL experiment (Catchings and Mooney (1991)), Wyoming Ruby profile (Stoerzel and Smithson (1998)), USGS profiles in northern CA (Zucca et al. (1986)), and Mendocino profiles from Beaudoin et al. (1996). Dashed black line shows data from Hill and Pakiser (1976). Our 2004 experiment shown by white line (seismographs) and circles (shotpoints).  Numbers along seismic lines indicate crustal thickness in kilometers. Heat flow data from Blackwell et al. (1991).

Geologic and thermochronologic studies document a later (</=12 Ma), lower magnitude (</=20%) extensional history in northwestern Nevada (e.g., Colgan et al., 2004, Colgan et al., 2006, Lerch et al., in prep.) than in the central portion of the northern Basin and Range (</=30 Ma and 50-100%, e.g., Gans et al., 1989, Best and Christiansen, 1991, Smith et al., 1991, Christiansen et al., 1992).  Despite minor upper-crustal extension across the northwestern margin of the Basin and Range, previous nearby geophysical surveys suggested that crustal thicknesses in this region (~30-32 km) are as thin or thinner than the more highly extended region to the south (~30-36 km, Figure 1, e.g., Klemperer et al., 1986, Catchings and Mooney, 1991).  West of this, Moho depth increases to ~38 km in northern California (Figure 1, e.g., Fuis et al., 1987).  Because no data exists across northwestern Nevada, the structure of the crust and how it changes in thickness is unknown.

Two likely explanations for this phenomenon (thin crust in a region of low upper-crustal extension) are either that the crust in the northwestern corner of the Basin and Range was simply not as thick as that of central Nevada to begin with, or that strain has been heterogeneous over the crustal column (allowing for significant lower-crustal thinning with only minor upper-crustal extension).  Resolution of this question has important implications for the nature of extensional crustal deformation.  In the central portion of the northern Basin and Range, differential strain between the upper- and lower-crust is thought to maintain homogeneous crustal thicknesses despite local variations in upper-crustal extension (e.g., Gans, 1988).  On what length scales can lower-crustal deformation decouple from upper-crustal deformation?  If the northwestern margin of the Basin and Range possessed pre-extensional thicknesses comparable to central Nevada (~45-50 km, e.g., Smith et al., 1991), then large-scale lower-crustal transport would have been necessary to produce the modern Moho depths.

To address this apparent discrepancy between upper-crustal extension and crustal thickness, we collected ~300 km of seismic refraction/reflection/teleseismic data and ~260 km of potential fields data across this transition zone (Figure 2). 

Figure 2: Experiment layout and geology superimposed on a shaded relief map. Tan regions: Tertiary volcanic and sedimentary units. Blue regions: Paleozoic and Mesozoic igneous and metamorphic units. Grey regions: Quaternary deposits. Red circles: shotpoints (SPs 1 & 5 = 1800 kg; SPs 2, 3, 4, & 6 = 1100 kg) and mine blasts (Twin Creeks (TC) & Florida Canyon (FC) = 68 tonnes and 16 tonnes, respectively). Black circles: geophone locations. Yellow circles: passive deployment. Grey ellipses: Approximate PmP bounce points.  Black dotted line: 2 - D gravity profile. HR: Surprise Valley high-resolution reflection profile. Blue dashed lines correspond to basement thought to have > 50 % Cretaceous granitic material (Barton et al. (1988)). Geology simplified from Jennings et al. (1977), Stewart and Carlson (1978), and Walker and MacLeod (1991).

Our data (Figure 3) document ~20% crustal thinning associated with Basin and Range extension and are consistent with the low-magnitude extension (<25%) recorded at the surface, suggesting that extensional strain has been homogeneous over the entire crustal column. 

Figure 3: Velocity model. Velocities displayed in km/s, contoured every 0.25 km/s (heavy contours every 0.5 km/s). Geographical features labeled (MP = Modoc Plateau, SV = Surprise Valley, SP = Sheldon Plateau, BRR = Black Rock Range, JM = Jackson Mountains, DHM = Double H Mountains). Shotpoints (SP) 1 - 5, and Twin Creeks (TC) mine blast labeled. V.E. 3:1.

In addition, we observe no evidence for the distributed lower-crustal reflectivity commonly attributed to large-magnitude ductile strain (e.g., Catchings, 1992).  Using our data to reconstruct the evolution of the northwestern Basin and Range, we step backwards through time, estimating the impact of various tectonic and magmatic events from late Cretaceous to present (Figure 4).  Our simple reconstruction offers a significantly different image of crustal structure at ~90 Ma, when the crustal thickness of the western end of our survey area was less than the eastern end, opposite to its present-day configuration, and the thickest crust was only equal to the global continental average (~41 km, Christensen and Mooney, 1995).

Figure 4: Schematic crustal evolution from late Cretaceous - present. (a) Present - day crustal structure with west - dipping Moho, lower - and mid - crustal magmatic addition, and modern topography (MP = Modoc Plateau, SV = Surprise Valley, SP = Sheldon Plateau, BRR = Black Rock Range, SRR = Santa Rosa Range). (b) Crustal structure before Basin and Range extension, Moho topography is reduced. (c) Removing Cenozoic magmatic addition thins crust. (d) Restoring erosion (east) and removing sedimentation (west) produces an east - dipping Moho. Reconstruction uncertainty (RMS) is represented by the gray Moho outline. V.E.: 2:1, topography is shown schematically.

Additional geological studies are currently in progress across NW Nevada that will complement and add to the geophysical experiment:

1. The Warner Range is a large, isolated, tilted crustal block within the largely unextended Modoc volcanic plateau which is traversed by the seismic profile (Figs 1 and 2).   Geologic mapping and regional structural studies are being carried out to determine when the bounding Surprise Valley fault zone began its activity, how much slip it has accommodated, and how it dies out along strike.  This project has involved graduate and undergraduate students.  For more information, visit the Warner Range - Surprise Valley Project page.

2. PhD candidate Derek Lerch is carrying out detailed studies in the Black Rock Range to provide surface control on the geometry of faults and fault-bounded basins that were seismically imaged. This study has characterized the Tertiary volcanic succession across this region, documenting a previously unknown episode of Eocene-Oligocene volcanism (Lerch et al., 2004).

3. In the Pyramid Lake-Shawave Range region, PhD candidate Carrie Whitehill is studying the stratigraphy of Tertiary sedimentary and volcanic sequences, the age and amount of Basin-Range extension and its relation to the Quaternary record of deformation (Whitehill et al., 2004).

4. PhD candidate Julie Fosdick has begun work in central Nevada to better determine the nature and location of boundaries of strain domains exhibiting differing amounts of extension at different times in Basin and Range history.