Deep structure of the central Andes

In 1996, geoscientists working on the Andean Continental Research Project (ANCORP '96) completed a

400-km-long seismic reflection profile across the central Andes at 21°S latitude (Fig. 10.6). This profile, together with the results of geologic (Allmendinger et al., 1997; McQuarrie, 2002) and other geophysical studies (Patzwahl et al., 1999; Beck & Zandt, 2002), forms part of a >1000-km-long transect between the Pacific coast and the Brazilian craton (Fig. 10.7). Below the central Andean forearc, the seismic reflection profile shows east-dipping (~20°) packages of reflectors that mark the top of the subducting Nazca plate (ANCORP Working Group, 2003). Above and parallel to the slab are thick, highly reflective zones that indicate the presence of trapped fluids and sheared, hydrated mantle at the top of descending slab. Some diffuse seismicity in this region is probably related to dehydration embrittlement (Section 9.4). Sub-horizontal reflectors below the Coastal Cordillera may represent ancient intrusions that were emplaced during Mesozoic arc magmatism.

East of the forearc, converted (compressional-to-shear) teleseismic waves indicate that crustal thickness increases from about 35 km to some 70 km beneath the Western Cordillera and Altiplano (Yuan et Beck & Zandt, 2002). Crustal thickness also varies along the strike of the orogen, reaching a maximum of 75 km under the northern Altiplano and a minimum of 50 km under the Puna Plateau (Yuan et al., 2000, 2002). The lithosphere is 100-150 km thick below the Altiplano (Whitman et al., 1996) and several tens of kilometers thinner beneath the Puna. Lithospheric thinning beneath this latter segment explains the high elevation (~4 km) of the Puna above a relatively shallow Moho. The southward transition from thin-skinned to thick-skinned thrust faulting in this same region (Fig. 10.5) suggests that the removal of excess mantle lithosphere accommodates the westward underthrusting of the Brazilian Shield (McQuarrie et al., 2005).

Across the ANCORP '96 seismic reflection profile (Fig. 10.6), a distinct Moho is conspicuously absent. A broad transitional zone of weak reflectivity occurs at its expected depth. The cause of this diffuse character of the boundary appears to be related to active fluid-assisted processes, including the hydration of mantle rocks and the emplacement of magma under and into the lower crust. Most of the reflectivity across the profile is linked to petrologic processes involving the release, trapping, and/or consumption of fluids (ANCORP Working Group, 2003). These processes have produced a seismic reflection profile whose character contrasts with those collected across fossil mountain belts (Figs 10.33b, 10.34b, 11.15b,c) where seismic

CINCA 1 ANCORP i

(a) W Trench Coast Coastal Long. valley Precordillera West. Cordillera Altiplano e

140 120 100 80 60 40 20 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360

Distance from coast (km)

Intruded forarc cri Jurassic ' . —'underplating?

alternative interpretation

Atacama Fault 70°W

West Fissure

69°W Volcanic Front

Phanerozoic sediments 68°W and volcanics

Uyuni-Kenayani Fault E

QBBS^^ Transparent

Fluid or melt and Thickened, partially molten backarc crust ^ ^ andesite intrusions 700-1000° Shear fabric?

t^r^lFluid trap above slab

Transitional Moho

Continental mantle

i f

Distance from coast (km)

Figure 10.6 foj Seismic results and (b) interpretation of depth-migrated ANCORP reflection data, including onshore wide-angle and receiver function results O

of Yuan et al. (2000) merged with offshore results of CINCA experiment (Patzwahl et al., 1999) (image provided by O. Oncken and modified from ANCORP Working Group, 2003, by permission of the American Geophysical Union. Copyright © 2003 American Geophysical Union). Location of section shown in Fig. 10.1a. Inset in (b) shows an alternative interpretation of the slab geometry and seismicity. ALVZ, Altiplano low velocity zone; QBBS, Quebrada Blanca Bright to vo m

Edge of the Brazilian Lithosphere

Local compensation Weak lower crust

Edge of the Brazilian Lithosphere

Local compensation Weak lower crust

Change in shear wave splitting direction

Figure 10.7 Lithospheric-scale cross-section of the Central Andes at latitudes 18-20°S showing interpretations of the crust and mantle structure (image provided by N. McQuarrie and modified from McQuarrie et al., 2005, with permission from Elsevier). Fast upper mantle P-wave velocities (dark gray) and slow P-wave velocities (white and gray shades) are shown. White waves, crustal low-velocity zones. Cross-section shows basement thrust sheets beneath the Eastern Cordillera, cover rocks are white. White stars are data recording stations.

Change in shear wave splitting direction

Figure 10.7 Lithospheric-scale cross-section of the Central Andes at latitudes 18-20°S showing interpretations of the crust and mantle structure (image provided by N. McQuarrie and modified from McQuarrie et al., 2005, with permission from Elsevier). Fast upper mantle P-wave velocities (dark gray) and slow P-wave velocities (white and gray shades) are shown. White waves, crustal low-velocity zones. Cross-section shows basement thrust sheets beneath the Eastern Cordillera, cover rocks are white. White stars are data recording stations.

reflection profiles can be interpreted only in terms of structure and lithologic contrasts.

Within the crust, seismic velocities indicate the presence of a 15- to 20-km-thick zone of low seismic wave speeds at depths of 14-30 km beneath the Western Cordillera and Altiplano-Puna (Yuan et al., 2000, 2002). Average crustal Vp/Vs ratios of 1.77 beneath the plateau and peak values of 1.80-1.85 beneath the active volcanic arc suggest the presence of high crustal temperatures and widespread intra-crustal melting. Patches of high amplitude reflections and zones of diffuse reflectivity at some 20-30 km depth beneath the Precordillera and Altiplano suggest the presence of fluids rising through deep fault zones (ANCORP Working Group, 2003). One of the largest of these patches is the Quebrada Blanca Bright Spot (Fig. 10.6). Other low velocity zones and bright reflectors occur at mid-crustal depths beneath the Eastern Cordillera and backarc region. The presence of these features helps to explain differences in crustal thickness and in the altitude of the Altiplano and

Puna. Widespread fluid transport and partial melting of the middle and lower crust during Neogene shortening and plateau growth appears to have weakened the crust sufficiently to allow it to flow (Gerbault et al., 2005). Similar features beneath the Tibetan Plateau (Section 10.4.5) suggest that orogenic plateaux in general involve very weak crust.

East of the Altiplano-Puna, receiver function determinations show a decrease in crustal thickness from 60 to 74 km beneath the Eastern Cordillera to about 30 km beneath the Chaco plain (Yuan et al., 2000; Beck & Zandt, 2002). Mantle tomography images indicate that zones of low wave speed at 30 km depth extend through the lithosphere beneath Los Frailes ignimbrite field (Fig. 10.7), suggesting that the volcanism is rooted in the mantle and that the mantle lithosphere in this region has been altered or removed (Myers et al., 1998). East of this low velocity zone, high seismic velocities in the shallow mantle, high Q (Section 9.4), and a change in the fast direction of shear-wave anisotropies suggest the presence of thick, strong, cold lithosphere of the Brazilian Shield (Polet et al., 2000). Bouguer gravity anomalies show that lithospheric flexure (Sections 2.11.4, 10.3.2) supports part of the Eastern Cordillera and the sub-Andean zone (Watts et al., 1995). Relationships between surface elevations and crustal thickness (Yuan et al., 2002) indicate a lithospheric thickness of 130-150 km beneath the sub-Andean belt and much thicker mantle lithosphere farther east (Fig. 10.7).

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