Heat flow in southernmost California and the origin of the Salton Trough

Abstract

Heat ftow in the Imperial Valley and adjacent crystalline rocks is very high ( -140 mW m -1 ). Gravity and seismic studies suggest the crust is about 23.5 km thick with the lower half composed of gabbro and the upper fourth composed of low-density sediments. Conduction through such a crust resting directly on asthenosphere would give the observed heat ftow if there were no extension or sedimentation. However, both processes must have been active, as the Imperial Valley is part of the Salton Trough, a pull-apart sedimentary basin that evolved over the past 4 or 5 m.y. To investigate the interrelations of these factors, we consider a one-dimensional model of basin formation in which the lower crustal gabbro and upper crustal sediments accumulated simultaneously as the crust extended and sedimentation kept pace with isostatic subsidence. For parameters appropriate for the Salton Trough, increasing the extension rate has little effect o n surface heat ftow because it increases effects of heating by intrusion and cooling by sedimentation in a compensating manner; it does, however, result in progressively increasing lower crustal temperatures. Analytical results suggest that the average extensional strain rate during formation of the trough was -2(}-50%/ m.y. ( -10 14 s-1 ); slower rates are inadequate to account for the present composition of the crust, and faster rates would probably cause massive crustal melting. To achieve the differential velocities of the Pacific plate at one end of the trough and North American plate at the other with this strain rate, extension must have, on the average, been distributed (or shifted about) over a spreading region -150 lun wide. This is about 10 times wider than the present zone of active seismicity, suggesting that the seismic pattern is ephemeral on the time scale for the trough's formation. Narrow spreading zones are typical where sustained spreading is compensated by basaltic intrusion to form the thin oceanic crust, but where such spreading occurs in thicker continental crust, broader zones of distributed extension (with smaller strain rates) may be required for heat balance. The Salton Trough model suggests that distributed extension can be associated with substantial magmatic additions to the crust; their effect on crustal buoyancy has important implications for the relation between crustal extension and subsidence. I . I TRODUCTIOSouthernmost California is a region of high mountains, deep valleys, active faults, a nd locally intense geothermal activity. Within it, the central tectonic feature is the Salton Trough, a large complex pull-apart basin that lies ast ride the Pacific· North American plate boundary over the 300 km distance between the southern end of the San Andreas fault near the Salton Sea and the northern end of the Gulf of California in Mexico (Figures I and 2). Although the principal tectonic motion is shearing between the plates (represented by horizontal velocity differences of several centimeters per year), rapid uplift of rugged mountains nearby [Sha...

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“…[39] The observed high heat flow values within the Salton Trough (see section 4.1.1) and Basin and Range province to the north suggest that lower crust composition within this region is likely to be gabbroic [Lachenbruch et al, 1985], and dry, to achieve the relative LC to UM strength contrast that our models imply [Afonso and Ranalli, 2004]. This model is at odds with the jelly sandwich model for mature continental lithosphere, which is characterized by a strong uppermost 19 to 1 Â 10 21 Pa s (see legend) over a given lower crust viscosity (horizontal labels beneath their respective model group).…”

“…We did not test models in which the Salton Trough viscosity is greater than the adjacent crust, although it is unlikely that the decrease in m could be exactly compensated by increases in h. A higher viscosity would be inconsistent with the high heat flow values over the Salton Trough [Lachenbruch et al, 1985;Magistrale, 2002], that tend to suggest higher temperatures and lower viscosity at depth. However, it may be that sufficient material heterogeneity can increase (or maintain) viscosity despite increased temperatures [Thatcher and Pollitz, 2008].…”

“…[35] Lateral contrasts in rheology beneath the Salton Trough are likely to be transitional from continental to oceanic spreading on the basis of its tectonic setting, heat flow, and seismic velocities [Lachenbruch et al, 1985;Magistrale, 2002;Lin et al, 2007], and were considered for models of geodetic profiles across the SSAF [Fay and Humphreys, 2005;Fialko, 2006]. Layered crust and upper mantle rheology is important for understanding Earth material properties and geodetic data provide sensitivity to this to varying degrees on the basis of the timescales and processes observed [Bürgmann and Dresen, 2008;Thatcher and Pollitz, 2008], with earthquake cycle models providing some constraints on lower crust and upper mantle rheologies.…”

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“…Inland, Lachenbruch et al (1985) reported scattered locations of heat flows >100 mW/m 2 in San Bernardino County.…”

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“…Their values are 1.5 -2 times higher than the average continental heat flows in the rest of the world. In general, the surface heat flows are contributed by basement, radiogenic elements, exhumation, burial, fault/fluid, magma, and tectonic components (Blackwell 1978;Bodell and Chapman 1982;Lachenbruch et al 1985;Armstrong and Chapman 1998). Understanding these components provides information for the regionally thermal regimes needed that reveal the thermal structure.…”