Extension Without a Trench
A geoscience team maps a 900 km-long continental region, Region X. GPS stations spanning 60 km across a north–south line record extension of 14 ± 1 mm/yr. Earthquakes cluster at 3–12 km depth with normal-fault focal mechanisms; none occur below 25 km. Basalt flows dated 0.2–1.1 Ma cover the central valley, and heat flow averages 95 mW/m². There is no trench and no andesitic arc. Two hundred kilometers east, Region Y shows 8 mm/yr of convergence, earthquakes to 180 km depth, and active andesite volcanoes. A geothermal company proposes a 40 MW plant in Region X and asks the team to rank a valley-floor site near an open fissure swarm against a site 15 km off-axis on 4 Ma volcanic rock.
1. Determine the plate boundary type at Region X and justify it with at least three independent lines of evidence. Then explain why the absence of a trench is not evidence against your interpretation.
Read the worked answer
Boundary type: continental rift (divergent), not subduction.
Independent lines of evidence:
- Kinematics: 14 ± 1 mm/yr extension across a 60 km profile is a real strain rate: 14 mm/yr ÷ 60,000 m ≈ 2.3 × 10⁻⁷ yr⁻¹.
- Seismicity: normal-fault mechanisms at 3–12 km match upper-crustal stretching. Subduction zones produce thrust mechanisms and earthquakes down to 100–700 km; none here are deeper than 25 km.
- Magmatism: 0.2–1.1 Ma basalt with no andesite fits decompression melting of extending mantle, not flux melting above a slab.
- Heat flow: 95 mW/m² is well above the ~65 mW/m² global average, consistent with thinned lithosphere.
Why no trench: Trenches are a seafloor expression of subduction. In continental rifts, extension produces a fault-bounded valley and basin fill, not a trench. Absence of a trench therefore does not argue against divergence - it argues against a submarine subduction setting, which the other data already exclude.
2. Region X lies only 200 km from the subduction zone at Region Y. Does this proximity support a back-arc interpretation? State observations that would support it and observations that would falsify it.
Read the worked answer
Region Y is a subduction zone: convergence at 8 mm/yr, earthquakes to 180 km, and andesite. Region X, only 200 km away, is extending. That geometry is consistent with a back-arc rift above the Y slab, but proximity alone does not prove it - X could be an unrelated continental rift that happens to sit nearby.
Observations that would support back-arc: arc volcanism and extension overlapping in time; a slab imaged beneath both regions on seismic tomography; and a geochemical gradient from arc andesite at Y to more mid-ocean-ridge-like basalt at X.
Observations that would falsify it: extension in X beginning many millions of years before or after arc activity at Y, no slab beneath X, or rift basalts carrying an enriched continental-lithosphere signature unrelated to subduction.
Prediction: 14 mm/yr over 5 Myr is about 70 km of extension, enough to thin the crust and possibly localize seafloor spreading. The test is whether magnetic anomalies or marine sediments appear along the rift axis; today the evidence supports rifting, not yet oceanic crust.
3. Recommend which site the geothermal company should develop first, and justify the choice using hazard, resource, and cost reasoning.
Read the worked answer
Recommendation: stage the project, with exploration toward the fissure swarm and production wells off-axis.
- Valley floor near the fissure swarm: highest heat flow and shallowest resource, so cheapest drilling. But it carries volcanic, fault-rupture, and hydrothermal hazard, and the 0.2–1.1 Ma basalts show magma can reach the surface. A 40 MW plant is a 20–30 year investment; designing for surface rupture and ash is expensive.
- Off-axis 4 Ma rock: lower gradient, so deeper and costlier wells, but lower volcanic and fault hazard and easier permitting.
Plan: drill slim gradient holes at both sites to measure temperature gradient and permeability, then site production wells off-axis and drill directionally toward the swarm. Require seismic and gas monitoring, and design the plant so it can be isolated from a fissure. Choose the valley floor only if the measured gradient is high enough to offset the hazard costs.