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HEAT FLOW TRANSECTS ACROSS THE CASCADES AND ANDES MAGMATIC ARCS Will Gosnold University of North Dakota, USA Valiya M. Hamza Observatorio Nacional, Rio de Janeiro, Brazil Overview • HFD data • Data treatment • Characteristics of each HFD profile • Tectonic history • Tectonic models The Global Heat Flow Database of the International Heat Flow Commission Click on the globe to enter Global Heat Flow ( mW m^2) : 0-40 violet, 40-80 blue, 80-120 green, 120-240 yellow, 240+ red www.heatflow.und.edu Continents and Oceans Antarctica Australia xls or ASCII North America xls or ASCII South America xls or ASCII Eastern North Pacific Western North Pacific Western South Pacific North Atlantic Ocean xls or ASCII Indian Ocean Africa xls or ASCII Asia xls or ASCII Europe xls or ASCII Eastern South Pacific Mediterranean area seas www.heatflow.und.edu Oceanic References Global in RTF format Continental Data xls or ASCII Marine Data xls or ASCII Countries North America & South America Argentina xls or ASCII Bermuda xls or ASCII Bolivia xls or ASCII Brazil xls or ASCII Canada xls or ASCII Chile xls or ASCII Columbia xls or ASCII Cuba xls or ASCII Ecuador xls or ASCII Mexico xls or ASCII Panama xls or ASCII Peru xls or ASCII Puerto Rico xls or ASCII USA xls or ASCII www.heatflow.und.edu Residual Heat Flow Density contour map of South America (Hamza et al., 2005) Locations of active volcanoes (red triangles) and heat flow sites in South America. Light blue sites are in low-angle subduction area; purple sites are in the high angle subduction area. 400 mW m-2 350 300 Unsmoothed HFD 250 Smoothed HFD 200 150 100 50 0 0 1000 2000 3000 4000 5000 Distance (km) The data were smoothed using a 10-point running mean of heat flow density vs. distance from the volcanic front. 6000 In the Andes region of steep subduction, HFD increases sharply to >250 mW m-2 at the volcanic front and the high HFD region extends about 200 km behind the VF. At 300 km behind the VF, HFD has declined to 60 mW m-2. HFD is relatively stable in the back arc basins as well as in Precambrian regions to the east, with values in the range of 60 to 80 mW m-2. In the Andes region with sub-horizontal subduction, the transition from magmatic arc to craton is indistinguishable from normal crustal HFD variability due to age and radioactive heat production. The transect across the Cascades between latitudes 45 N to 49 N shows a narrow band of high HFD over the volcanic arc followed by a gradual increase in HFD from 60 mW m-2 to 80 mW m-2 over a distance of about 800 km. In the region of the Idaho Batholith, between 800 km and 1150 km, HFD is about 80 mW m-2. mW m-2 200 Andes high angle Andes low angle 150 100 50 0 0 100 200 300 400 500 600 km • The zone of high HFD is about 300 km wide in the steeply subducting section of the Andean arc and <100 km wide in the Cascade arc. HFD variability in the flat subduction zone is indistinguishable from variability due to crustal age and radioactive heat production. Tectonic scheme for Altiplano-Puna Volcanic Complex • 30 ma - Crustal doubling • 13 ma – Delamination event • 10 ma – Initiation of pulsed emplacement of upper to mid-crustal magmas which fueled eruption of at least 30,000 km3 of ignimbrites • 10 ma – emplacement of 1,500 km3 • 8 ma – emplacement of 2,500 km3 • 6 ma – emplacement of 5,400 km3 • 4 ma – emplacement of 10,000 km3 0 -20000 meters -40000 -60000 0 50000 100000 150000 200000 250000 300000 200000 250000 300000 meters 0 -20000 meters -40000 -60000 0 50000 100000 150000 meters Temperature contours 13 my after delamination. Magma emplacement in the upper crust is necessary to account for observed heat flow. 1800 90 1600 80 1400 70 1200 60 1000 50 800 40 600 30 Initial Tz 400 20 Tz at present Initial HFD profile 200 10 HFD vs depth at present 0 0 10 20 30 40 Depth (km) 50 60 0 70 mW m-2 Deg C Can we detect the delamination event at 13 ma? NO. 1800 90 1600 80 1400 70 1200 60 1000 50 800 40 600 30 Initial Tz 400 20 Tz at present Initial HFD profile 200 10 HFD vs depth at present 0 0 10 20 30 40 Depth (km) 50 60 0 70 mW m-2 Deg C Can we detect the delamination event at 13 ma? Plate rollback with counter flow at subduction velocity Depth (m) 0 -50000 -100000 -150000 0 100000 200000 300000 400000 500000 600000 Distance from trench Plate rollback with counter flow at half subduction velocity Depth (m) 0 -50000 -100000 -150000 0 100000 200000 300000 400000 500000 600000 Plate rollback with no counter flow Depth (m) 0 -50000 -100000 -150000 0 100000 200000 300000 400000 500000 600000 Andes Surface HFD and Intrusion Models Cascades HFD profile Locations of active volcanoes (red triangles) and HFD sites in South America. Light blue sites are in low-angle subduction area; purple sites are in the high angle subduction area. Locations of HFD sites (purple circles) and active Volcanoes (red triangles) in Cascade range. 200 Andes high angle Andes low angle Cascades mW m-2 150 100 50 0 0 1000 2000 3000 4000 5000 km Conclusions In all cases, the high HFD belt coincides with the zone of active volcanism and the amplitude of the HFD anomaly appears to correlate with the angle of subduction. Variable width of the high HFD zones is interpreted to be related to differences in thickness and composition of the local crust, and to the duration of subduction. • The sharpness of changes in HFD is due to magma emplacement within the past 4 my. • The overprint of near surface magmatism masks the thermal effects of delamination. Special acknowledgement of volcanologist Shanaka deSilva for consultation on volcano-tectonic history. 0 -20000 -40000 -60000 0 50000 100000 150000 200000 250000 300000 50000 100000 150000 200000 250000 300000 0 -20000 -40000 -60000 0 0 6 my after delamination -20000 -40000 -60000 0 50000 100000 150000 200000 250000 300000 0 50000 100000 150000 200000 250000 300000 0 10 my after delamination -20000 -40000 -60000