An Analysis of thermal data from the vicinity of Cajon Pass, California

Sass, J.H.; Lachenbruch, Arthur H.; Galanis, S.P.; Munroe, Robert J.; Moses, T.H.

doi:10.3133/ofr86468

Cited by 9 publications

(7 citation statements)

References 8 publications

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“…[13] To compute near-surface heat flow from model simulations, we extract temperatures at 150-300 m below land surface from the simulated steady state temperature fields. This depth range is comparable to that used to determine heat flow in most boreholes for the California data set [Lachenbruch and Sass, 1980;Sass et al, 1986Sass et al, , 1994Sass et al, , 1997Saffer et al, 2003;Fulton et al, 2004] (Table 1). In addition, we remove conductive topographic refraction effects from the simulated heat flow values in order to compare them with the data, which have been previously corrected for these effects [e.g., Fulton et al, 2004].…”

Section: Model Domain and Boundary Conditionssupporting

confidence: 68%

“…In this region, heat flow ranges from ∼63 to 94 mW m −2 [ Lachenbruch and Sass , 1980; Sass et al , 1997; Fulton et al , 2004] (Table 1 and Figure 2) and varies by up to 20 mW m −2 over distances as small as 5 km. In contrast, surface heat flow data elsewhere in the California data set displays substantially less variability [ Sass et al , 1986, 1994]; for example, in the western Mojave Desert, most heat flow measurements range from 62 to 69 mW m −2 [ Sass et al , 1986]. With the exception of a suggested gradual 20 mW m −2 decrease in surface heat flow along strike of the SAF from Coalinga southeastward to Cholame that may reflect a long‐wavelength regional change in basal heat flux, and which corresponds to a deepening of the base of the seismogenic zone [ Sass et al , 1997; Williams et al , 2004], the observed variations in surface heat flow do not exhibit clear trends with lithology or location.…”

Section: Introductionmentioning

confidence: 99%

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Heat advection by groundwater flow through a heterogeneous permeability crust: A potential cause of scatter in surface heat flow near Parkfield, California

Popek¹,

Saffer²

2011

J. Geophys. Res.

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[1] Surface heat flow in the California Coast Ranges near Parkfield, California, exhibits substantial scatter, with differences as large as 20 mW m −2 over lateral distances of 5-70 km. This scatter has been an important limitation in using the heat flow data set to constrain geodynamic processes, but to date it has not been explained. Here we use a numerical model of coupled fluid and heat transport to test the hypothesis that heat advection by groundwater flow can generate the scatter. Our study significantly extends previous investigations in that we consider realistic and heterogeneous permeability architecture and topographic driving forces. We find that the magnitude and spatial characteristics of the scatter, including the standard deviation, variation in heat flow as a function of separation distance, and patterns of heat flow and elevation can be generated if the Tertiary sediments in the upper 2-3 km of the crust have a permeability ≥3 × 10 −16 m 2 , allowing recharge of ∼0.5 cm yr −1 or higher. These permeabilities and recharge rates are consistent with existing constraints on both quantities, suggesting that groundwater flow offers a plausible explanation for the observed scatter in the heat flow data set. Last, although not the primary focus of this study, we demonstrate that for a range of reasonable permeability architectures, topographically driven groundwater flow would not mask a thermal anomaly associated with frictional heating on the San Andreas Fault.

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Section: Model Domain and Boundary Conditionssupporting

confidence: 68%

Section: Introductionmentioning

confidence: 99%

Heat advection by groundwater flow through a heterogeneous permeability crust: A potential cause of scatter in surface heat flow near Parkfield, California

Popek¹,

Saffer²

2011

J. Geophys. Res.

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“…Mean heat flow in this region is 63 mW m À2 , and is remarkably consistent over a large area [e.g., Lachenbruch and Sass, 1980]. Heat flow measurements used in this study were made at depths ranging from 107-1067 m (Table 1) [Sass et al, 1986]. In general (and where possible), heat flow stations are sited in areas of moderate elevation, relatively flat topography, and in crystalline rock to avoid hydrologic and topographic effects on heat flow.…”

Section: Geologic Setting and Backgroundmentioning

confidence: 75%

“…We consider this 15% range as a probable upper bound to the uncertainty in comparing model results and observations because (1) most heat flow corrections are less than $5% [e.g., Sass et al, 1997] (for example, the smooth topography northeast of the San Andreas Fault in the Mojave study area should result in a relatively small correction) and (2) the 1.5 km topographic sampling we use for our models is comparable to the distance over which much of the topographic heat flow correction is applied. However, because the magnitude of slope orientation and vegetation effects are poorly characterized and can approach several Sass et al [1986] and C. Williams (personal communication, 2001)) are given. Stations are ordered according to location, from SW to NE on the profiles shown in Figures 2b, 4c, 4d, 5b, and 6. percent in some cases [e.g., Blackwell et al, 1980], we use an uncertainty of ±15% for all data points.…”

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confidence: 99%

Topographically driven groundwater flow and the San Andreas heat flow paradox revisited

2003

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[1] Evidence for a weak San Andreas Fault includes (1) borehole heat flow measurements that show no evidence for a frictionally generated heat flow anomaly and (2) the inferred orientation of s 1 nearly perpendicular to the fault trace. Interpretations of the stress orientation data remain controversial, at least in close proximity to the fault, leading some researchers to hypothesize that the San Andreas Fault is, in fact, strong and that its thermal signature may be removed or redistributed by topographically driven groundwater flow in areas of rugged topography, such as typify the San Andreas Fault system. To evaluate this scenario, we use a steady state, two-dimensional model of coupled heat and fluid flow within cross sections oriented perpendicular to the fault and to the primary regional topography. Our results show that existing heat flow data near Parkfield, California, do not readily discriminate between the expected thermal signature of a strong fault and that of a weak fault. In contrast, for a wide range of groundwater flow scenarios in the Mojave Desert, models that include frictional heat generation along a strong fault are inconsistent with existing heat flow data, suggesting that the San Andreas Fault at this location is indeed weak. In both areas, comparison of modeling results and heat flow data suggest that advective redistribution of heat is minimal. The robust results for the Mojave region demonstrate that topographically driven groundwater flow, at least in two dimensions, is inadequate to obscure the frictionally generated heat flow anomaly from a strong fault. However, our results do not preclude the possibility of transient advective heat transport associated with earthquakes.

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“…One core was obtained near the bottom of the well, and a complete suite of drill cuttings was available for measurements of thermal conductivity. Conductivity measurements on chips at 30-m intervals were combined with temperature gradients to produce an estimate of heat flow [Lachenbruch et al, 1986a, b;Sass et al, 1986]. The initial heat flow result of--•90 mW m -2 was surprising in that it was higher than data from shallow (100-200 m) wells in the region, which were tightly grouped about a mean of about 70 mW m -2 [cf.…”

Section: Introductionmentioning

confidence: 99%

Heat flow from a scientific research well at Cajon Pass, California

Sass¹,

Lachenbruch²,

Moses³

et al. 1992

J. Geophys. Res.

156

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The long‐standing “stress/heat flow paradox” was the primary scientific motivation for the Cajon Pass borehole. For nearly two decades, the absence of a fault‐centered heat flow anomaly from measurements to relatively shallow (∼200 m) depths had indicated low average shear stresses (≤20 MPa) on the San Andreas fault, while laboratory data on rock strength and in situ stress determinations to about a kilometer had indicated high stress (∼100 MPa). Initial results from an unsuccessful 1.7‐km‐deep oil well at the site gave a high heat flow (∼90 mW m−2) consistent with a strong San Andreas fault; however, the late Cenozoic geologic history of the Cajon Pass area suggested that the anomalous heat flow was the transient effect of rapid erosion. Theoretical studies predicted that the ∼30% surface anomaly would be substantially reduced at depths of 3–5 km. The research borehole reached a total depth of 3.5 km. Below a superficial covering of Tertiary sedimentary rocks, it penetrated gneissic rocks with composition ranging from gabbroic to granodioritic. Core recovery amounted to only about 3% of the total depth, necessitating the use of drill cuttings to characterize thermal conductivity. This, in turn, resulted in much higher uncertainties in average conductivity (±10–15%) than would have occurred with a continuously cored hole (±3–5%). From a time series of temperature logs, equilibrium temperature gradients were established over selected intervals of 250–500 m to within 95% confidence limits of 2%. These gradients were combined with harmonic mean thermal conductivities having larger uncertainties to give interval heat flows, which vary systematically from 100 ± 5 mW m−2 in the uppermost 400 m to 75 ± 3 mW m−2 in the lowermost 300 m. Thus, at the Cajon Pass site, heat flow is decreasing with depth at a mean rate of more than 7 mW m−2 per kilometer, consistent with a frictionless fault and with theoretical predictions based on local erosional history.

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An Analysis of thermal data from the vicinity of Cajon Pass, California

Cited by 9 publications

References 8 publications

Heat advection by groundwater flow through a heterogeneous permeability crust: A potential cause of scatter in surface heat flow near Parkfield, California

Heat advection by groundwater flow through a heterogeneous permeability crust: A potential cause of scatter in surface heat flow near Parkfield, California

Topographically driven groundwater flow and the San Andreas heat flow paradox revisited

Heat flow from a scientific research well at Cajon Pass, California

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