Heat flow data and thermochronologic derived paleotemperature gradient data are examined to calculate heat flow ~25 Ma and, at present, for a southern Basin and Range location north of Tucson, Arizona. An increase in the surface heat flow is estimated from ~25 Ma to the present; changing from ~47 to ~83 mW m-2. Steady-state conduction temperature vs. depth profiles provide estimates of lithosphere thicknesses both for the present and for ~25 Ma. Different heat transfer models for present heat flow predict present LAB depth that agrees with seismic studies. From these temperature profiles, lithosphere thinning from ~184 km to ~70 km is suggested during the Neogene. Mantle lithosphere thinning caused by thermal phenomena is likely a fundamental driving force for southern Basin and Range extension. Because the mantle lithosphere has likely thinned much more than the crust, it is shown that additional vertical advection, such as an asthenosphere plume, delaminating part of the mantle lithosphere, convection cells, and rising magmas along conduits, add to the vertical advection component of upper mantle lithosphere extension. Interestingly, values of heat flow 25 Ma, lithosphere thicknesses 25 Ma, and Neogene lithosphere thinning are somewhat similar for the Four Corners area of the Colorado Plateau and the southern Basin and Range, even though Neogene tectonic development was quite different, i.e., no Neogene extension in the Colorado Plateau vs. ~57% in the southern Basin and Range. Neogene lithosphere thinning phenomena are likely different in the two regions.Obtaining data-driven estimates of lithosphere thickness ~25 Ma and Neogene lithosphere thinning in different geological provinces of the western United States may provide valuable boundary conditions for geologic models of Neogene tectonics. Geothermal information providing subsurface temperature estimates relevant to a past geologic time can be considered with present heat flow data to estimate the related past lithosphere thickness as well as lithosphere thinning over the corresponding geologic interval. For example, mineralogy and heat flow studies were used to estimate heat flow ~25 Ma for the Four Corners Area of the Colorado Plateau [1, 2] and to suggest Neogene lithosphere thinning of ~100 km for the area. A heat flow-temperature-depth expression, the same as in this study, was used to match upper mantle depth-temperature estimates at ~25 Ma consistent with the mineralogy data relevant at ~70 and~140 km depth [1, 2]. From this analysis, a heat flow estimate of ~47 mW m-2 for 25 Ma was possible, and therefore, an estimate of lithosphere thickness could be made [1]. Heat flow analyses also provided estimates of present lithosphere thickness from the present heat flow data for the Colorado Plateau, which was consistent with seismic studies [1]. Lithosphere thinning as proposed for the Colorado Plateau results from a mantle delaminating process or upper mantle layer separation [1, 3]. Interestingly, Axen et al. [4] present a very different mechanism for Laramide tectonics in which flat-slab subduction plows off 20-50 km of lower continental lithosphere across large regions of the western United States.In the present study, I employ thermochronologic derived near-surface geothermal gradients relative to the beginning of extension at a location in the southern Basin and Range ~87 km north of Tucson, Arizona (the Grayback normal fault block in the Tortilla Mountains; [5, 6]; Figure 1). Extension is thought to have begun ~25 Ma or 25-20 Ma [5, 6]. This location is quite unique in that more than a dozen present-day heat flow measurements are also available in the area. Differing from the study in the Four Corners area, near-surface paleogeothermal gradients combined with appropriate thermal conductivity data (from [7, 8]) allow preextension heat flow to be estimated at the study location in the southern Basin and Range. Using the derived heat flow estimate for ~25 Ma, the cotemporal lithosphere thickness is calculated. From present-day heat flow data over the study area, the present lithosphere thickness is also calculated, which agrees with seismic studies of lithosphere depth. These calculations show that a significant increase in heat flow over the past ~25 Myr is likely coupled with a major decrease in lithosphere thickness.The present study area in the southern Basin and Range has experienced considerable Neogene extension and crustal thinning whereas the Four Corners area of the Colorado Plateau has not experienced Neogene extension [9, 10]. In this study, it will be shown that these two very different geologic regions have comparable pre-Neogene (25 Ma) heat flow and rather similar pre-Neogene (25 Ma) lithosphere thickness, as well as a large amount of Neogene lithosphere thinning. Lithosphere separation is proposed as the Neogene lithosphere thinning process in the Four Corners region and may have occurred in the southern Basin and Range as well. However, additional advective mechanisms such as a primary thermal plume, convection cells, and magma migration along conduits are likely present for southern Basin and Range development [11, 12]. Crustal and mantle lithosphere extensions are considered to show the necessity of upper mantle asthenosphere and lithosphere advection in the southern Basin and Range. The present study will develop numerical estimates for southern Basin and Range Neogene lithosphere thinning in Arizona.Near-surface geothermal gradients, determined by thermochronologic methods, relating to pre-Neogene (just before crustal extension began ~25 Ma) in the study area (Figure 1) are rather low (⁠17±5°C km−1⁠, [5]; 14±7°C km−1⁠, [6]). The two values average 15.5±2.1±1.5°C km−1 (⁠average±standard deviation±standard error of the mean; more study values may provide a more certain mean and reduce the mean uncertainty). Although the study location has undergone a high degree of extension, the location is not within a metamorphic core complex [6, 13]. The present crustal thickness at the study area is ~28.5 km, almost the same as the characteristic thin crust of 28.2±0.5 km for the southern Basin and Range in Arizona [10]. The location is amagmatic, as opposed to two other paleogeotemperature gradient study sites (with higher calculated temperature gradients) located in the lower Colorado River area in a region of major extension [5, 14]. It is suggested therefore that the study locale is a rather representative location of Neogene geologic development occurring in many areas of the southern Basin and Range of Arizona.Sixteen present-day heat flow measurements at sites within ~50 km of the study locale yield a mean value of 83.1±14.4±3.7 mW m−2 [7, 8, 13]. Including values within the 99.9% confidence range, the mean of thirteen heat flow values is 84.5±7.7±2.2 mW m−2 (Figure 1). This value happens to be very similar to the average of seven heat flow values taken over much of the southern Basin and Range in Arizona in plutonic rocks to depths greater than 500 m (⁠84.5±6.1±2.5 mW m−2⁠; [7, 8]; Figure 1; deeper tests in plutonic rocks typically allow longer conductive temperature gradient intervals to be observed). Neogene erosion of ~1 km is suggested for the study locale but is also questioned [15]. The present heat flow value for the study area is therefore reduced by about 0-3% (calculated after [16]), giving a heat flow of ~83 mW m-2. This value is quite similar to the average of a large number of values across the southern Basin and Range (⁠82 mW m−2⁠; [13]; 157 values). Therefore, the study location may also be rather representative of geothermal evolution over the past 25 Myr in many areas of the southern Basin and Range in Arizona.Although the mean thermal conductivity for the thirteen heat flow sites in various rock types for the study area is 3.26±0.30±0.10 W m−1°C−1 (giving a present-day heat flow of 83 mW m-2), the thermochronologic study site is located specifically in plutonic-granitic rocks [6]. As such, it is suggested that the mean thermal conductivity value for the seven deep tests greater than 500 m in plutonic rocks across the southern Basin and Range in Arizona best estimates the thermal conductivity at the thermochronologic study location (discussed above). The mean thermal conductivity value for the seven plutonic rock heat flow sites is 3.04±0.44±0.17 W m−1°C−1⁠. It follows the pre-Neogene heat flow is ~47±5 mW m-2, using the statistical calculation for (⁠15.5±1.5°C km−1×3.04±0.17 W m−1°C−1⁠). Therefore, it is proposed that the near-surface heat flow at the study site in the southern Basin and Range has changed from ~47 to ~83 mW m-2 over the past ~25 Myr; the change in the heat flow at the Four Corners area over the same time interval is ~47 to ~64 mW m-2 [1].Using Equation (1) implies that lithosphere temperatures have been relatively constant for an extended geologic time, which may be hypothesized as a limiting approximation for the pre-Neogene (~25 Ma) initial time condition but will be further discussed for the present. As well as model considerations, the thermal conductivity and radiogenic distributions are always somewhat speculative. Sass et al. [13] discuss nonlinear variability of radiogenic heat production Ao vs. heat flow in the Basin and Range resulting from extension and magmatism. Many models of crustal radiogenic distribution (~500, [18]), as well as limited data, contribute to calculated lithosphere temperature uncertainty. The lithosphere thermal conductivity values in the present study are estimated to 200 km depth from average rock type values weighted by the appropriate interval length proportional to the total depth considered (rock type given and extrapolated from [10]; thermal conductivity values from [19]). Interestingly, temperature and pressure effects on mafic lithosphere rock thermal conductivity appear small compared to variation within rock type at room temperature [20].Figure 2 shows the calculated lithosphere temperatures (using Equation (1)) for heat flows of 47 and 83 mW m-2. The temperatures related to 47 mW m-2 estimate the initial temperature distribution proposed for the present study site at about 25 Ma or just before the start of extension. The LAB is suggested to occur at 1300°C or 1573 K [18, 21]. From Figure 2, the LAB appears to occur at ~184 km depth corresponding to near-surface heat flow of 47 mW m-2.As mentioned above, the calculated temperature distribution is sensitive to the near-surface radiogenic heat concentration (⁠Ao in Equation (1)). For example, a valve of 2.6 μW m-3, instead of 2.3 μW m-3, would indicate 1300°C at about 210 km depth for a heat flow of 47 mW m-2, instead of 184 km. In both cases, the preextension lithosphere is suggested to be quite deep, generally consistent with (and possibly the same as) estimates of LAB depths for early Proterozoic areas (200-220 km, [18]; the study site is located in a Paleo-Proterozoic area, [22]). If Ao equals 2.1 μW m-3 [13] instead of 2.3 μW m-3 as used above, the lithosphere thickness using Equation (1) would still be quite deep for a heat flow of 47 mW m-2, 170 km.Figure 2 also shows the calculated present-day temperature-depth profile derived from Equation (1) for a surface heat flow of 83 mW m-2; the LAB at 1300°C is at ~70 km depth (⁠Ao=2.3 mW m−3⁠; for Ao equal to 2.1 μW m-3, the lithosphere thickness is ~68 km using Equation (1)). This estimate of the LAB is in quite good agreement with seismic study estimates of the LAB for the study location (~70-72 km; [23, 24]). Agreement of the LAB depth between several seismic studies and the steady-state conduction temperature model suggests that advection of heat is moving the temperature distribution much faster than conduction alone. Lachenbruch and Sass [17] also present a steady-state thermal model of lithosphere extension by mantle stretching and crustal intrusion; the predicted LAB is the same as above. Agreement of the LAB depth between the two thermal models likely results from different thermal conductivity and Ao estimates. From these estimates of heat flow ~25 Ma and present day, considerable thinning of the lithosphere in the southern Basin and Range of Arizona over the past 25 Myr is suggested, >~100 km (⁠~114 km=184–70 km⁠). This is somewhat larger than predicted for the Four Corners region in the Colorado Plateau (~100 km; [1]).In an area of about 50 km in radius centered at 111°W and 32°N, just south of the study location, there are 19 heat flow values [7, 13]. Considering the data within a 99.9% confidence level and applying a small Neogene erosion correction as discussed above, the average heat flow is ~87 mW m-2. The resulting LAB calculated as above is 65 km. From seismic studies, the LAB is also ~65 km [23, 24]. Although we have no preextension thermal gradient estimate at this second location, the elevated present-day heat flow data correlating with a relatively thin lithosphere supports the notion that thinning of the lithosphere relates to a thermal source which may be the fundamental cause for extensional tectonic development of the southern Basin and Range.Why the steady-state conduction thermal model (Equation (1)) appears to present accurate estimates of the present LAB depth is rather enigmatic (of course more study sites are needed). Lachenbruch et al. [25] suggest that various advection phenomena during southern Basin and Range extension followed by conductive cooling may be consistent with present heat flow in the southern Basin and Range. The 83 mW m-2 temperature profile represents steady-state conduction (Figure 2). Advection processes in the asthenosphere and mantle lithosphere that would bring temperatures of 1300°C from ~184 km depth (25 Ma LAB estimate) to ~70 km depth (present LAB estimate, Figure 2) beginning ~25 Ma must be accompanied by additional advection processes in the mantle lithosphere which bring warmer temperatures closer to the earth’s surface than 70 km. This is because the conduction time constant for 70 km is ~39 Myr (the conduction time constant is the time it takes for the near-surface temperatures to reach within 1/eth of steady state assuming a step continuous temperature increase). If conduction were the sole means of heat transport from 70 km to the surface, while 1300°C was initiated and maintained at 70 km depth 25 Ma (possible maximum boundary conditions), the near-surface temperature (heat flow) would only have increased ~47% of the equilibrium increase in heat flow or about 17 mW m-2 for the present case (⁠0.47×83−47 mW m−2⁠).From another perspective, it may be noted from Figure 2 that steady-state conduction temperatures at the base of the present crust (83 mW m-2, ~28.5 km depth; [10]) are the same as those steady-state conduction temperatures at 82 km for heat flow of 47 mW m-2 25 Ma. This involves advection to establish Moho temperatures compatible with surface heat flow of 83 mW m-2. Heat delivered and temperatures maintained at the base of the 28.5 km thick crust can provide near-surface almost steady-state temperatures after 25 Myr because the conduction time constant for 28.5 km is ~6.4 Myr resulting in a heat flow increase that is 98% of an equilibrium value.Pre-Neogene (25 Ma) crustal thickness estimates depend on Neogene extension estimates and present crustal thickness. The characteristic crustal thickness for the southern Basin and Range of Arizona is 28.2 ± 0.5 km, at the study site I estimate ~28.5 km [10]. Extension over the entire southern Basin and Range from ~25 Ma (El Paso TX to the North American Plate margin) is ~57% (change in width/initial width; [9]); the ratio of the present width to the initial width is then 1.57. If the cross section area of the crust along cross section remains constant during extension ([10], suggest from seismic data a simple extension of the upper and lower crust), then the crustal thickness ratio after and before extension is the inverse of the width ratio (or d/28.2=1.57⁠, where the initial crustal thickness d=44.3 km⁠). If the crustal thickness at the study location is 28.5 km, then the initial crustal thickness is 44.7 km (I use an average of ~44.5 km). The crustal Neogene thinning is ~36% (~44.5-28.5/44.5). This compares to lithosphere thinning of ~62% (⁠184−70/184⁠). The ratio of the pre-Neogene crustal thickness 25 Ma to the present crustal thickness is estimated to be 1.57=α⁠, whereas the ratio of the pre-Neogene mantle lithosphere thickness 25 Ma to the present mantle lithosphere thickness is 3.4 (⁠β=184−44.5/70−28.5⁠). Mantle lithosphere thinning would be about 70%. The results are comparable to rifted regions in the Salton Trough and in the Ethiopian Rift/Afar region (⁠α≤2⁠, β=2 to 3⁠, [26]).It is noted that large areas of the northern Basin and Range across Nevada have a greater crustal thickness and have experienced greater Neogene extension, than the southern Basin and Range (~37 km vs. ~28 km and ~ 100% vs. ~57%; [9, 27]). The preextension crustal thickness in these areas for the present model (~74 km) would be quite large even compared with much of the present southern Rocky Mountains’ crustal thickness (48 – 50 km; [27]). This suggests that additional processes contributed to crustal thickness and crustal extension in these regions of the northern Basin and Range, e.g., possibly extensive crustal under plating and sill and dike formation.The above calculations suggest much greater thinning in the mantle lithosphere than in the crust for the southern Basin and Range. If a given cross section area of the mantle lithosphere remains the same during the thinning process, then as the mantle lithosphere thins, it should extend much more than the crust extends; alternatively, additional vertical advection in the mantle lithosphere accompanies extension and contributes to its thinning.One may estimate the immense heat input to the lithosphere at 70 km accompanying the LAB change from 184 to 70 km depth and the heat flow increase from 47 to 83 mW m-2 by subtracting the area under the 47 mW m-2 temperature-depth curve, from the surface to 70 km, from the corresponding area under the 83 mW m-2 curve (Figure 2); i.e., ΔQ change in heat=Δ integrals of Tdz from 0 to 70 km×c×ρ⁠, c is specific heat and ρ is density. Bashir et al. [10] suggest Vs/Vp and R data for the southern Basin and Range in Arizona, referenced to a typical cratonic region, indicate simple extension of both the upper and lower crusts (⁠R is amplitude of P to S converted wave). Therefore, as a first-order approximation allow the pre-Neogene (~ 25 Ma) upper and lower crust thicknesses to have the same ratio as today (upper crust/lower crust ~1.5, but does vary along profile, [10]). With the pre-Neogene (~ 25 Ma) thickness of 44.5 km (above), the upper and lower crustal thickness estimates are ~26.7 and 17.8 km. The three-layer model after Bashir et al. [10] is then adopted (the heat calculations depend mainly on the change in the upper mantle thickness). The pre-Neogene (~ 25 Ma) structure to 70 km is then crust 44.5 km-26.7 km granitic-17.8 km basalti, and upper mantle-25.5 km peridotite; and present structure to 70 km is 17.2 km-granitic, 11.3 km-basaltic, and 41.5 km-peridotite (approximately after [10]). With densities of 2.75, 2.87, and 3.25 kg m-3, respectively, and specific heats of 1000, 1100, and 1300 J kg-1°C-1, respectively ([10]: [28]), the heat input to the LAB at 70 km is ~11.2×1013 J m−2⁠.To better illustrate the magnitude of this amount of heat, compare with a surface basalt flow of height “z”, the associated heat is Q=ρ×c×T×z+L×ρ×z⁠. Choosing density and specific heat as above, temperature T=1250°C⁠, and latent heat of solidification L=4×105 J kg−1 [29], the amount of heat per horizontal square meter in the basalt flow is ~4.62×109 J m−3×z⁠. Equating to the amount of heat input at 70 km when heat flow is increased from 47 to 83 mW m-2, the estimate z is ~2.43×104 m⁠, or 24.3 km, a remarkable height.Proceeding as above, consider the heat input into the new crust at 28.5 km as heat flow changes from 47 to 83 mW m-2. The calculated heat input at 28.5 km is ~16.6×1012 J m−2⁠, equivalent to ~3.6 km of basalt flow. As heat flow changes from 47 to 83 mW m-2, heat input into the mantle lithosphere would be ~9.54×1013 J m−2 (⁠11.2×1013–1.66×1013⁠), or about 5.75 times the heat input into the crust.Heat flow and seismic data integrated with thermochronologic paleogeotemperature gradient data provide an estimate of considerable lithosphere thinning (~114 km or ~ 62%) over the past ~25 Myr at the study location in the eastern southern Basin and Range of Arizona. The difference between crustal thinning and mantle lithosphere thinning proposed from the above calculations, as well as the conduction time constants, suggests considerable vertical upward advective heat transfer to thin the mantle lithosphere. The study location is an amagmatic area with considerable extension, but not in a metamorphic core complex, has a heat flow almost the same as the southern Basin and Range average heat flow (83 vs. 82 mW m-2), and a crustal thickness essentially the same as that characterizing the southern Basin and Range in Arizona (⁠28.2±0.5 vs. ~28.5 km; [5, 10, 13]). The change from subduction to divergence between the North American and Pacific plate boundary is followed by the beginning of extension across the southern Basin and Range and at the study area as well [5, 6, 9, 30]. For these reasons, I suggest the study area as a potential location for first-order representation of geothermal and extensional processes in many areas over the southern Basin and Range of Arizona.Figure 3 shows depth estimates from several studies for the Moho and LAB along 33°N across southern Arizona. The LAB shallows dramatically west of 114°W approaching the Salton Sea. East of 114°W, the LAB depth shallows gradually to 110°W and then deepens somewhat to 109°W. The depth of 70 km at 111°W (the study location) appears to be an approximate average depth east of 114°W, although the LAB depths do vary along the profile east of 114°W about 10 to 16 km depending on the study. It is noted that the depth estimates are ± several km while interpolating the color-coded maps for LAB depth. Further study may better correlate heat flow with the LAB; this has been done in the present study for another site near 111°W and 32°N (above). The crustal depth estimates shallow approaching 115°W and are within several km of one another eastward to 111°W at which point depths appear to diverge going eastward. The study by Bashir et al. [10] shows only a couple km difference in crustal depths east of 114°W along 33°N; the map by Gilbert [27] shows rather uniform crustal thickness of about 28 km across the entire southern Basin and Range of Arizona.To further examine lithosphere thinning in the southern Basin and Range, consider a model where mantle lithosphere extension across the southern Basin and Range is limited by the western boundary of the North American plate and the eastern boundary of the Pacific Plate and is therefore the same as the crustal extension. One may calculate potential thinning of mantle lithosphere due to extension or spreading for a straightforward geometric model versus separate vertically upward advection not explicitly a component of lithosphere extension (extension or spreading and thinning the mantle lithosphere transports heat both horizontally and vertically, see [17], their Figure 9-8). As before, allow the model cross sectional area along the cross section to remain constant during mantle lithosphere extension and the LAB temperature to also remain constant. In this model, the upward advection associated with mantle lithosphere spreading results from the LAB rising with mantle lithosphere thinning inversely proportional to extension. As mentioned before, overall east–west extension across the southern Basin and Range during the past ~30 to 25 Myr is ~57% (El Paso, TX to the North American plate margin; from [9]). Of course this is an average, but from Figure 3 and previous discussions, it would appear the crustal and LAB depths, as well as extension and heat flow, at the study site are reasonable average approximations across the southern Basin and Range in Arizona. If extension was 57%, the lithosphere thickness after extension may be calculated as the initial depth divided by final depth z⁠: (⁠184/z=1.57⁠) giving the LAB depth z as 117 km. This represents a first-order estimate, as per the present model, of the effect of vertical heat transport associated with mantle lithosphere extension and thinning. As such, in the present model, additional processes advecting heat vertically upward and involved with lithosphere thinning are necessary to account for the LAB at 1300°C to move an additional ~47 km upward from ~117 km to 70 km (Figure 2). Some of the fundamental advection mechanisms suggested are a rising primary mantle plume inducing convection cells, delaminating parts of the mantle with associated upward advection, and magma movement along conduits (a few references are, respectively, [3, 11, 12]).The same basic geometric model can be applied to the crust and upper mantle lithosphere. If crustal extension and thinning occurred in such a manner as to preserve Moho temperatures, then preextension temperatures at ~44.5 km (429°C for 47 mW m-2) would be brought to ~28.5 km. However, the present heat flow suggests a temperature of ~656°C at 28.5 km; this requires additional heat transfer involving advection in the thinning mantle lithosphere (Figure 2).Seismic studies of rifted regions in southern California suggest that mechanisms of lithosphere deformation and strain accommodation are unclear; however, a lack of systematic offset between the lower lithosphere and crustal surface deformation agrees with the notion of lithosphere symmetric extension [26]. Colocation of crustal surface rifting evidence and a flat and shallow LAB suggests that deep lithosphere extension and crustal extension are related [26].Increased elevation and crustal thickness as well as lithosphere thinning can provide deviatoric stresses promoting southern Basin and Range extension; lithosphere thinning with asthenosphere replacement can also increase elevation [30]. Lithosphere thinning warms the remaining lithosphere which decreases its viscosity allowing for easier deformation. Thinning of lithosphere thickness by over 100 km at the study location should provide considerable deviatoric stresses and initially preextension elevation increase [30]. The change from a convergent to divergent plate boundary just beginning ~30 Ma [9] appears to have allowed deviatoric tension to proceed with extension.Yuen and Fleitout [12] suggest that a primary thermal plume causes strong small-scale convection that thins the lithosphere and reduces viscosity. Their model predicts observed rapid rates of uplift and lithosphere thinning. Lachenbruch et al. [25] suggest that the history of extension in the southern Basin and Range (occurring mainly between 28 and 16 Ma; [30]) indicates advection rapidly changing lithosphere and crustal temperatures to promote extension, whereas tectonic quiescence post 16 Ma and present relatively high heat flow suggest the absence of advection and gradual cooling by conduction. Van Wijk et al. [31] discuss small-scale convection at the boundary of Colorado Plateau and the Basin and Range driven by a lithosphere step between the two regions. Schmandt and Lin [32] show negative velocity perturbations (dVp/Vp and dVs/Vs) at 75 and 200 km depth over almost all of the southern Basin and Range, suggesting warmer temperatures at considerable depth over a very large region.The above discussions indicate mantle lithosphere thinning results because of both mantle lithosphere extension and additional vertical heat advection; but mantle lithosphere extension is in turn dependent on lithosphere thinning. A thick crust and a thinning lithosphere result in a potential energy configuration most likely to develop deviatoric stresses [30]. Both the crust and the lithosphere in the southern Basin and Range of Arizona were much thicker ~30-~25 Ma (~44.5 and ~ 184 km, respectively, as calculated before) when the Pacific and North American plates just began to diverge. Because a thick crust is conducive for deviatoric stress, it would seem reasonable that lithosphere thinning began with thermal erosion near the LAB caused by a large primary thermal source. The seismic data indicating warmer conditions to depths of at least 200 km under almost all of the southern Basin and Range suggest a very large thermal anomaly at depth ultimately associated with a primary thermal upwelling or plume, subsequent convection to contribute to lithosphere thinning as well as magma movement along conduits is also likely occur [11, 12, 32].With the thick crust and divergent plate motion, the thinning mantle lithosphere may promote sufficient deviatoric stress to initiate extension. One may suggest that a feedback mechanism could have been operating in the southern Basin and Range. As the mantle lithosphere continues to thin, more extensional stress is generated. However, as the crust extends and thins, extensional stress is decreased. A decrease in thermal activity associated with mantle lithosphere thinning may have brought about a balance between decreasing stresses generated by crustal thinning and increasing stresses generated by mantle lithosphere thinning, arriving at a cessation in southern Basin and Range extension.Processes that may produce the present lithosphere depth as well as the estimated Moho temperature (Equation (1)) would establish the present LAB by asthenosphere and mantle lithosphere advection and lithosphere extension; upward advection in the thinned mantle lithosphere would establish a new elevated shallower Moho temperature. The crustal temperatures could respond conductively rather quickly to the new Moho temperature (relatively short time constant as discussed above). The high thermal gradient just below the Moho caused by advection may supply heat conductively to maintain Moho temperature after advection and extension stop (after [25]). Crustal intrusion and sill formation have not presently been included. Temperatures shown in Figure 2 between the present Moho and LAB are probably not linear as shown but are instead convex upward as per temperature profiles showing upward fluid movement across a layer. Long-term thermal relaxation should approach temperatures shown in Figure 2 as long as the LAB remains at 70 km.The increase in heat flow sometime during the past ~25 Myr as measured at the study site in the southern Basin and Range of Arizona identifies thinning of the lithosphere caused by thermal phenomena as a probable fundamental driving force of southern Basin and Range extension. The straightforward model presented above suggests lithosphere thinning may be accomplished by both extension and additional thermal advection, while extension is allowed by the change to divergent plate motion.There is no conflict of interest with respect to the present manuscript.I thank S. Roeske and other reviewers for many helpful suggestions to improve the manuscript.

中文翻译：

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南部东部盆地和亚利桑那州范围内某个地区的热流数据有助于分析新近系岩石圈变薄大于100 Km

检查了热流数据和热年代学得出的古温度梯度数据，以计算出约25 Ma的热流，目前，对于亚利桑那州图森以北的盆地南部和山脉地区，都可以计算出热流。估计表面热流的增加是从大约25 Ma到现在。从〜47变为〜83 mW m-2。稳态传导温度与深度的关系曲线提供了当前和约25 Ma岩石圈厚度的估计值。当前热流的不同传热模型预测了当前LAB深度，该深度与地震研究一致。根据这些温度曲线，建议在新近纪期间将岩石圈的厚度从184 km减小到70 km。由热现象引起的地幔岩石圈变薄可能是盆地南部和范围扩展的基本动力。由于地幔岩石圈的厚度可能比地壳薄得多，因此显示出额外的垂直对流，例如软流圈羽流，地幔岩石圈的部分分层，对流单元和沿管道上升的岩浆，增加了地壳的垂直对流分量。上地幔岩石圈扩展。有趣的是，即使新近纪构造发育差异很大，即没有新近纪，热流25 Ma，岩石圈厚度25 Ma和新近纪岩石圈变薄的情况在科罗拉多高原的四个角区域和南部盆地和山脉都有些相似。与科罗拉多盆地南部和山脉地区约57％的面积相比，科罗拉多高原地区的面积扩展幅度最大。在这两个地区，新近纪岩石圈变薄现象可能有所不同。在美国西部不同地质省份中获得以数据为依据的岩石圈厚度约25 Ma和新近纪岩石圈变薄的估计，可能为新近纪构造的地质模型提供有价值的边界条件。可以与当前热流数据一起考虑提供与过去地质时间相关的地下温度估计的地热信息，以估计相关过去的岩石圈厚度以及在相应地质间隔内岩石圈变薄的情况。例如，通过矿物学和热流研究估算了科罗拉多高原四个角区域的热流约为25 Ma [1，2]，并建议该地区的新近纪岩石圈变薄了约100 km。与本研究相同的热流温度深度表达式 在约25 Ma处使用上地幔深度-温度估算值与约70 km和约140 km深度有关的矿物学数据一致[1，2]。通过该分析，可以估计25 Ma的热流约为47 mW m-2，因此可以估算岩石圈厚度[1]。热流分析还根据科罗拉多高原目前的热流数据提供了当前岩石圈厚度的估计，这与地震研究一致[1]。科罗拉多高原提议的岩石圈减薄是由于地幔分层过程或上地幔层分离[1，3]。有趣的是，Axen等。[4]为拉拉蒙构造提供了一种截然不同的机制，其中平板俯冲作用在美国西部的大片区域上掀开了20-50公里的低陆岩石圈。在本研究中，我采用热年代学推导的近地表地热梯度，该梯度相对于盆地南部和亚利桑那州图森以北约87 km范围内（托尔蒂利亚山脉的Grayback正常断层； ，6]；图1）。扩展被认为已经开始了〜25 Ma或25-20 Ma [5，6]。该位置非常独特，因为该区域还提供了十多个当前的热流量测量值。与在四个角落地区进行的研究不同，近地表古地热梯度与适当的热导率数据（来自[7，8]）相结合，可以估算盆地南部和山脉中研究位置的延伸前热流。使用导出的约25 Ma的热流量估算值，计算了同期岩石圈厚度。从研究区域目前的热流量数据，还可以计算出目前的岩石圈厚度，这与岩石圈深度的地震研究相吻合。这些计算表明，过去约25 Myr的热流量显着增加可能与岩石圈厚度的大幅减少有关。本盆地南部和山脉的本研究区经历了相当大的新近纪伸展和地壳变薄，而四角区科罗拉多高原的一部分尚未经历新近纪扩展[9，10]。在这项研究中，将显示这两个非常不同的地质区域具有相近的新近纪前（25 Ma）热流和相当相似的新近纪前（25 Ma）岩石圈厚度，以及大量的新近纪岩石圈变薄。岩石圈分离被认为是四个角落地区新近纪岩石圈变薄的过程，可能发生在南部盆地和山脉。然而，对于南部盆地和山脉的发展，可能还存在其他平流机制，例如初级热羽，对流单元和沿管道的岩浆运移[11，12]。地壳和地幔岩石圈扩展被认为表明了南部盆地和山脉上地幔软流圈和岩石圈对流的必要性。本研究将为亚利桑那州南部盆地和新近纪岩石圈变薄的数值估算。通过热年代学方法确定的近地热梯度与研究区（正好在地壳伸展开始约25 Ma之前）有关的新近纪前期（图1）相当低（⁠17±5°C km−1⁠，[5]；14±7°Ckm-1⁠，[6]）。这两个值的平均值为15.5±2.1±1.5°C km-1（平均值±标准偏差±平均值的标准误差；更多的研究值可能会提供更确定的平均值并降低平均值的不确定性）。尽管研究位置已经高度扩展，但该位置不在变质核心复合物中[6，13]。研究区目前的地壳厚度约为28.5 km，几乎与亚利桑那州南部盆地和山脉的特征性薄壳28.2±0.5 km相同[10]。与位于科罗拉多河下游地区主要扩展区域的另外两个古地温梯度研究地点（具有较高的计算温度梯度）相反，该地点是岩浆环境[5，14]。因此，建议研究地点是发生在南部盆地南部和亚利桑那范围内许多地区的新近纪地质发育的相当有代表性的地点。在研究地点约50 km以内的地点，目前进行了16次现今的热流量测量值83.1±14.4±3.7 mW m-2 [7，8，13]。包括99.9％置信范围内的值，十三个热流值的平均值为84.5±7.7±2.2 mW m-2（图1）。这个值恰好与深部岩石深部大于500 m的深成岩中亚利桑那州南部盆地和山脉大部分地区的七个热流值的平均值相似（84.5±6.1±2.5 mW m−2⁠; [7] ，8]；图1；在深成岩中进行更深入的测试通常可以观察到更长的传导温度梯度间隔。对于研究地点，建议有〜1 km的新近纪侵蚀，但也受到质疑[15]。因此，研究区域的当前热流值减少了约0-3％（在[16]之后计算），从而使热流约为83 mW m-2。该值与整个盆地南部和山脉范围内大量值的平均值非常相似（82 mWm-2⁠； [13]； 157个值）。因此，研究位置也可能代表了过去25 Myr在南部盆地和亚利桑那州许多地区的地热演化。尽管研究区域各种岩石类型的13个热流场的平均热导率是3.26±0.30±0.10 W m-1°C-1（当前的热流量为83 mW m-2），热年代学研究地点专门位于云母-晚生花岗岩中[6]。因此，建议在横跨南部盆地和亚利桑那州山脉的深部岩石中进行的七个深度测试的平均热导率值大于500 m，可以最好地估计热年代学研究地点的热导率（如上所述）。七个岩生岩热流场的平均热导率值为3.04±0.44±0.17 W m-1°C-1⁠。遵循新近纪前的热流约为47±5 mW m-2，使用的统计计算为（15.5±1.5°C km-1×3.04±0.17 W m-1°C-1⁠）。因此，建议在过去的〜25 Myr期间，南部盆地和山脉研究地点的近地表热流从〜47变为〜83 mW m-2。在相同的时间间隔内，四个角区域的热流变化为〜47至〜64 mW m-2 [1]。使用公式（1）意味着岩石圈温度在延长的地质时间内一直保持相对恒定，可以假设这是前新近纪（约25 Ma）初始时间条件的极限近似值，但目前将对其进行进一步讨论。除了模型方面的考虑之外，热导率和放射源分布总是有些推测。Sass等。[13]讨论了由伸展和岩浆作用引起的辐射生热Ao与盆地和范围内热流的非线性变化。地壳放射成因分布的许多模型（〜500，[18]）以及有限的数据有助于计算岩石圈温度不确定性。本研究中的岩石圈热导率值是根据平均岩石类型值估计的200 km深度，该平均岩石类型值由与所考虑的总深度成比例的适当间隔长度加权（从[10]中给出和推断出的岩石类型；从[19]中得出的热导率值） ）。有趣的是，相比于室温下岩石类型的变化，温度和压力对镁铁质岩石圈岩石热导率的影响似乎很小[20]。图2显示了对于47和83 mW m的热流计算的岩石圈温度（使用公式（1））。 -2。与47 mW m-2相关的温度估计当前研究地点建议的初始温度分布约为25 Ma或即将开始扩展。建议LAB在1300°C或1573 K [18，21]下进行。从图2 LAB似乎发生在约184 km深度处，对应于47 mW m-2的近地表热流。如上所述，计算出的温度分布对近地表放射源热浓度敏感（方程（1）中的Ao） ）。例如，阀为2.6μWm-3而不是2.3μWm-3时，在约210 km深度处指示1300°C，而热流为47 mW m-2，而不是184 km。在这两种情况下，建议伸展前的岩石圈都非常深，通常与元古生代早期地区（200-220 km，[18]）的LAB深度估计值一致（并且可能与之相同）[18]；研究地点位于古-元古代地区，[22]。如果Ao等于2.1μWm-3 [13]而不是上面使用的2.3μWm-3，则对于47 mW m-2的热流（170 km），使用公式（1）计算的岩石圈厚度仍然会很深。图2还示出了对于83mW m-2的表面热流，由等式（1）得出的计算出的当前温度-深度分布；1300°C时的LAB深度约为70 km（Ao = 2.3 mW m-3−；对于等于2.1μWm-3的Ao，使用等式（1），岩石圈厚度为〜68 km）。LAB的估计值与研究地点（〜70-72 km； [23，24]）的LAB的地震研究估计值非常吻合。几个地震研究和稳态传导温度模型之间的LAB深度一致表明，热对流移动温度分布的速度比单独传导快得多。Lachenbruch和Sass [17]也提出了由地幔伸展和地壳侵入引起的岩石圈扩展的稳态热学模型。预测的LAB与上述相同。两种热模型之间的LAB深度一致可能是由于不同的热导率和Ao估计值所致。根据这些约25 Ma的热流估计值，建议在过去的25 Myr内，南部盆地和亚利桑那范围的岩石圈明显变薄，> 100 km（⁠〜114 km = 184–70km⁠） 。这比科罗拉多高原的四个角区域的预测值要大一些（〜100 km； [1]）。在半径为50 km的区域中，以111°W和32°N为中心，就在研究位置的南部，有19个热流值[7，13]。考虑到数据在99.9％的置信度内，并如上所述应用较小的Neogene腐蚀校正，平均热流约为87 mW m-2。如上计算得出的LAB为65 km。根据地震研究，LAB也约为65 km [23，24]。尽管我们在第二个位置没有预伸展的热梯度估算，但与相对较薄的岩石圈相关的当今升高的热流数据支持了这样一个观点，即岩石圈的变薄与热源有关，这可能是伸展构造发展的根本原因。为什么稳态传导热模型（方程（1））似乎可以准确地估计当前LAB的深度，这还是个谜（当然需要更多的研究地点）。Lachenbruch等。[25]认为，在南部盆地和山脉扩展过程中的各种对流现象，然后进行传导冷却，可能与南部盆地和山脉目前的热流一致。83 mW m-2的温度曲线表示稳态传导（图2）。在软流层和地幔岩石圈中的平流过程，将使温度从约184 km（约25 Ma LAB估计）到约70 km深度（目前LAB估计，图2）从约184 Ma的深度开始，将1300°C的温度伴随着额外的对流地幔岩石圈中的过程，使更温暖的温度更接近地球表面，超过70公里。这是因为70 km的传导时间常数约为〜39 Myr（传导时间常数是假设连续不断升高的温度，近地表温度达到稳定状态的1 / eth所花费的时间）。如果传导是从70 km到地面的唯一热传输方式，而在1300°C开始并保持在70 km深度25 Ma（可能的最大边界条件）的情况下，在当前情况下，近地表温度（热流）仅会增加热流平衡增加量的约47％或约17 mW m-2（⁠0.47×83−47 mW m−2⁠）。从图2可以看出，目前地壳底部的稳态传导温度（83 mW m-2，约28.5 km的深度； [10]）与82处的稳态传导温度相同。对于47 mW m-2 25 Ma的热流而言，为km。这涉及平流以建立与83 mW m-2的表面热流兼容的Moho温度。在25 Myr之后，传递的热量和维持在28.5 km厚的地壳底部的温度可以提供近地表几乎处于稳态的温度，因为28.5 km的传导时间常数约为6.4 Myr，导致热流增加了98％。平衡值。新近纪之前（25 Ma）的地壳厚度估计取决于新近纪延伸估计和当前地壳厚度。我估计研究区南部盆地和亚利桑那范围的地壳厚度为28.2±0.5 km，据我估计约为28.5 km [10]。整个南部盆地和范围从〜25 Ma（埃尔帕索TX到北美板块边缘）的延伸为〜57％（宽度/初始宽度的变化； [9]）。那么，当前宽度与初始宽度之比为1.57。如果地壳沿横截面的横截面面积在延伸过程中保持恒定（[10]，从地震数据表明上地壳和下地壳的简单延伸），那么延伸前后的地壳厚度比就是宽度的倒数。比率（或d / 28.2 =1.57⁠，其中初始地壳厚度d = 44.3km⁠）。如果研究地点的地壳厚度为28.5公里，则初始地壳厚度为44.7公里（我平均使用〜44.5公里）。新近纪地壳变薄为〜36％（〜44.5-28.5 / 44.5）。相比之下，岩石圈变薄约62％（⁠184−70 /184⁠）。新近纪前地壳厚度25 Ma与当前地壳厚度之比估计为1.57 =α⁠，而新近纪前地幔岩石层厚度25 Ma与当前地幔岩石层厚度之比为3.4（⁠β= 184-44.5 /70-28.5⁠）。地幔岩石圈变薄约为70％。结果可与索尔顿海槽和埃塞俄比亚裂谷/阿法尔地区的裂谷区相媲美（⁠α≤2⁠，β= 2至3⁠，[26]）。值得注意的是，北部盆地和山脉的大片地区内华达州的地壳厚度更大，新近纪扩展也更大，比南部盆地和山脉（〜37 km对〜28 km和〜100％对〜57％； [9，27]）。对于目前的模型，这些区域的前伸展地壳厚度（〜74 km）将相当大，即使与目前的南部洛矶山脉地壳厚度（48 – 50 km； [27]）相比也是如此。这表明北部盆地和山脉北部这些地区的地壳厚度和地壳扩展的附加过程，例如，在板块下以及基岩和堤坝的形成下可能存在广泛的地壳。以上计算表明，地幔岩石圈的变薄远大于地壳的变薄。为南部盆地和山脉。如果在变薄过程中，地幔岩石圈的给定横截面面积保持不变，则随着地幔岩石圈变薄，其延伸范围应大于地壳的扩展范围；或者，地幔岩石圈中额外的垂直对流伴随着延伸并导致其变薄。有人可能会估计，随着LAB的深度从184改变为70 km，70 km处岩石圈的巨大热量输入，热流从47 mW m-增加到83 mW 2通过从83 mW m-2曲线下的相应面积减去47 mW m-2温度-深度曲线下的面积（从表面到70 km）（图2）；即，热的ΔQ变化= Tdz的Δ积分从0到70 km×c×ρ⁠，c是比热，ρ是密度。Bashir等。[10]建议亚利桑那州南部盆地和山脉的Vs / Vp和R数据参考典型的克拉通地区，表明上地壳和下地壳都简单扩展（⁠R是P到S转换波的振幅）。因此，作为一阶近似值，新近纪前期（约25 Ma）的上，下地壳厚度与今天的比率相同（上地壳/下地壳约1.5，但沿剖面变化[10]）。在新近纪前（约25 Ma）厚度为44.5 km（以上）时，上，下地壳厚度估计为〜26.7和17.8 km。Bashir等人之后的三层模型。然后采用[10]（热量计算主要取决于上地幔厚度的变化）。到70 km的前新近纪（〜25 Ma）构造，地壳44.5 km-26.7 km花岗岩17.8 km basalti，上地幔25.5 km橄榄岩。目前到70 km的构造是砾石17.2 km，玄武岩11.3 km和橄榄岩41.5 km（大约在[10]之后）。密度分别为2.75、2.87和3.25 kg m-3，比热分别为1000、1100和1300 J kg-1°C-1，分别（[10]：[28]），在LAB处70 km处输入的热量为〜11.2×1013 J m−2⁠。为了更好地说明这一热量的大小，请与高度为1的地表玄武岩流进行比较“ z”，相关的热量为Q =ρ×c×T×z + L×ρ×z⁠。如上选择密度和比热，温度T = 1250°C⁠，凝固潜热L = 4×105 J kg-1 [29]，玄武岩流中每平方米的热量为〜4.62× 109 J m−3×z⁠。当热量从47 mW增加到83 mW m-2时，相当于70 km处的热量输入量，估计z为〜2.43×104m⁠，即24.3 km，是一个显着的高度。当热流从47变为83 mW m-2时，在28.5 km处进入新地壳。在28.5 km处计算得出的热量输入为〜16.6×1012 J m−2⁠，相当于玄武岩流的〜3.6 km。当热流从47 mW m-2变为83 mW m-2时，输入到地幔岩石圈的热量约为〜9.54×1013 J m-2（⁠11.2×1013–1.66×1013⁠），大约是输入到地幔岩石圈中的热量的5.75倍。热流和地震数据与热年代学古地温梯度数据相结合，提供了在南部盆地东部和亚利桑那范围内的研究地点过去25 Myr内岩石圈明显变薄的估计（〜114 km或〜62％）。由上述计算得出的地壳变薄与地幔岩石圈变薄之间的差异以及传导时间常数表明，相当大的垂直向上对流换热使地幔岩石圈变薄。研究地点是一个具有相当大扩展性的岩浆区域，但不在变质岩心中，热流几乎与南部盆地和山脉平均热量流相同（83 vs. 82 mW m-2），地壳厚度与亚利桑那州南部盆地和山脉的特征基本相同（⁠28.2±0.5 vs 〜28.5公里； [5，10，13]）。北美板块和太平洋板块边界从俯冲到发散的变化，随后开始延伸到南部盆地和山脉以及研究区域[5，6，9，30]。出于这些原因，我建议将研究区域作为一阶表示地热和伸展过程的潜在位置，该区域位于南部盆地和亚利桑那州范围内的许多地区。图3显示了Moho和LAB沿33项研究的深度估计°N，横跨亚利桑那州南部。LAB在向索尔顿海（Salton Sea）处向西114°W处急剧变浅。在114°W以东，LAB深度逐渐变浅至110°W，然后再加深至109°W。尽管LAB的深度确实沿114°W以东的剖面变化约10至16 km（根据研究），但111°W（研究位置）处70 km的深度似乎是114°W以东的大约平均深度。 。注意，在对LAB深度的颜色编码图进行插值时，深度估计为±几公里。进一步的研究可能会更好地将热流与LAB相关联；在本研究中，已经针对西经111°W和北经32°（上图）的另一个地点完成了这项工作。地壳深度估计较浅，接近115°W，并且彼此之间相距数公里，即向东至111°W，在该点深度似乎向东扩散。Bashir等人的研究。文献[10]显示，北纬114°W以北33°处地壳深度仅相差几公里；Gilbert [27]的地图显示整个南部盆地和亚利桑那山脉的地壳厚度相当均匀，约为28 km。要进一步研究南部盆地和山脉的岩石圈变薄，可以考虑一个模型，其中地幔岩石圈在整个南部盆地和范围受北美板块的西边界和太平洋板块的东边界的限制，因此与地壳扩展相同。对于简单的几何模型，人们可能会计算出由于伸展或扩散引起的地幔岩石圈潜在变薄，而不是单独的垂直向上的对流，而不是岩石圈扩展的明显组成部分（对地幔岩石圈的扩展或扩散和变薄在水平和垂直方向上都传热，参见[17] ，其图9-8）。和以前一样，允许在地幔岩石圈伸展期间沿横截面的模型横截面面积保持恒定，并且LAB温度也保持恒定。在该模型中，与地幔岩石圈扩展有关的向上对流是由于LAB上升，而地幔岩石圈变薄与伸展成反比。如前所述，过去约30到25 Myr，整个南部盆地和山脉的东西向总体延伸约为〜57％（德克萨斯州埃尔帕索至北美板块边缘；来自[9]）。当然，这是一个平均值，但是从图3和先前的讨论中可以看出，研究地点的地壳和LAB深度以及延伸和热流是整个南部盆地和亚利桑那山脉的合理平均近似值。如果扩展率为57％，延伸后的岩石圈厚度可以用初始深度除以最终深度z来计算：（⁠184/ z =1.57⁠）得出LAB深度z为117 km。根据本模型，这表示与地幔岩石圈扩展和变薄有关的垂直传热作用的一阶估计。因此，在本模型中，为了使LAB在1300°C向上移动约47 km从约117 km到70 km，必须进行垂直向上向上加热并涉及岩石圈变薄的附加过程。建议的一些基本对流机制是上升的主要地幔羽诱导对流单元，伴随着向上的对流使地幔部分分层，以及沿管道的岩浆运动（分别有几个参考文献[3，11，12]）。可以将相同的基本几何模型应用于地壳和上地幔岩石圈。如果以保持Moho温度的方式发生地壳扩展和变薄，那么约44.5 km（47 mW m-2的429°C）的预扩展温度将达到〜28.5 km。然而，目前的热流表明28.5 km处的温度约为656°C。加利福尼亚南部南部裂谷区的地震研究表明，岩石圈变形和应变适应的机制尚不清楚；这需要在稀薄的地幔岩石圈中进行平流的附加热传递（图2）。然而，下部岩石圈和地壳表面形变之间缺乏系统的偏移，与岩石圈对称扩展的观点相吻合[26]。地壳表面裂谷证据和平坦浅层的LAB的共置表明，岩石圈的深层延伸和地壳伸展是相关的[26]。海拔和地壳厚度的增加以及岩石圈的变薄可以提供偏应力，促进南部盆地和山脉的扩展。岩石圈变薄和软流圈替换也可以增加海拔[30]。岩石圈变薄会使剩余的岩石圈变暖，这会降低其黏度，从而易于变形。在研究地点将岩石圈厚度减薄100 km以上，应能提供相当大的偏应力，并在开始时就增加伸展前的高度[30]。从大约30 Ma开始由收敛的板块边界向发散的板块边界的变化[9]似乎已经允许偏斜张力继续进行。Yuen和Fleitout [12]提出，一次热羽流会引起强烈的小尺度对流，从而使岩石圈变薄并降低粘度。他们的模型预测观测到的隆升和岩石圈变薄的快速速度。Lachenbruch等。[25]认为南部盆地和山脉伸展的历史（主要发生在28和16 Ma之间； [30]）表明对流迅速改变岩石圈和地壳温度以促进伸展，而构造静止在16 Ma之后并相对较高热流表明没有对流和通过传导逐渐冷却。Van Wijk等。[31]讨论了由两个区域之间的岩石圈台阶驱动的科罗拉多高原边界与盆地和山脉边界的小尺度对流。Schmandt和Lin [32]在几乎整个南部盆地和山脉的75和200 km深度处显示出负速度扰动（dVp / Vp和dVs / Vs），这表明在很大的区域内相当深的温度较高。表明由于地幔岩石圈扩展和附加的垂直热平流作用，地幔岩石圈变薄的结果；但是地幔岩石圈的扩展又取决于岩石圈的变薄。厚的地壳和变薄的岩石圈导致最有可能产生偏应力的势能构型[30]。当太平洋板块和北美板块刚开始发散时，南部盆地和亚利桑那范围内的地壳和岩石圈的厚度都厚于约30-〜25 Ma（分别为〜44.5和184 km，如先前计算）。由于厚的地壳有利于偏应力，岩石圈变薄始于LAB附近由大的主要热源引起的热侵蚀，这似乎是合理的。地震数据表明在几乎所有南部盆地和山脉下至少200 km处都有较暖的条件，表明该深度的热异常非常大，最终与一次热上升或羽流有关，随后的对流作用导致岩石圈变薄以及沿管道的岩浆运动也很可能发生[11，12，32]。随着地壳的厚实和板块的发散运动，地幔岩石圈变薄可能会促进偏斜应力，从而开始伸展。可能有人建议，在盆地南部和山脉可以使用一种反馈机制。随着地幔岩石圈继续变薄，产生更多的拉伸应力。但是，随着外壳的扩展和变薄，拉伸应力会降低。与地幔岩石层变薄有关的热活动的减少可能在地壳变薄产生的应力减少和地幔岩石层变薄产生的应力增加之间达到平衡，从而达到南部盆地的停止和范围扩展的目的。深度以及估计的莫霍面温度（等式（1））将通过软流圈和地幔岩石圈对流以及岩石圈扩展来建立当前的LAB；在变薄的地幔岩石圈中向上平流将建立新的更高的浅莫霍温度。地壳温度可以很快地对新的Moho温度做出导电响应（如上所述，时间常数相对较短）。由对流引起的莫霍面正下方的高热梯度可能会传导热量，以保持对流和延伸停止（在[25]之后）以保持莫霍面温度。目前还不包括地壳侵入和下陷形成。在当前的Moho和LAB之间的图2中所示的温度可能不是如图所示的线性，而是根据温度曲线显示的是向上凸的，该温度曲线显示了整个层中流体的向上运动。只要LAB保持在70 km，长期的热弛豫应接近图2所示的温度。在南部盆地和亚利桑那范围内的研究现场测得，过去约25 Myr的某个时间的热流增加表明，由热现象引起的岩石圈变薄是南部盆地和范围扩展的可能基本动力。上面介绍的简单模型表明岩石圈变薄可以通过扩展和附加的热平流来完成，而扩展可以通过改变发散的板块运动来实现。与本文不存在利益冲突。感谢S.Roeske和其他审稿人对改进稿件的许多有用建议。