The crustal structure beneath the northwest fjords, Iceland, from receiver functions and surface waves

Five broad-band seismic stations were operated in the northwest fjords area of Iceland from 1996 to 1998 as part of the Iceland Hotspot project. The structures of the upper 35  km or so beneath these stations were determined by the modelling and joint inversion of receiver functions and regional surface wave phase velocities. More than 40 teleseismic events and a few regional events containing high-quality surface wave trains were used. Although the middle period passband of the seismograms is corrupted by oceanic microseismic noise, which hinders the interpretation of structural details, the inversions reveal the overall features. Many profiles obtained exhibit large velocity gradients in the upper 5  km or so, smaller zero gradients below this, and, at ~23  km depth, a zone 2–4  km thick with higher velocity gradients. The two shallower intervals are fairly consistent with the 'upper' and 'lower' crust, defined by Flovenz (1980 ). The deep zone of enhanced velocity gradient seems to correspond to the sharp reflector first reported by Bjarnason et al . (1993 ) and identified by them as the 'Moho'. However, this type of structure is not ubiquitous beneath the northwest fjords area. The distinctiveness of the three intervals is variable, and in some cases a structure with velocity gradient increasing smoothly with depth is observed. We term these two end-members structures of the first and second types respectively. Structures of the second type correlate with older areas. Substantial variation in fundamental structure is to be expected in Iceland because of the great geological heterogeneity there.     

Lower crustal involvement in upper crustal thrusting

Summary. Basement structures mapped in the Devonian Adavale Basin, eastern Australia, indicate two styles of lower-crustal involvement in the formation of upper-crustal structures. The first style is typified by thrust features in the upper-crustal sedimentary section and basement, a response to lower-crustal shortening over a wide area. The second style includes lower-crustal thrusting and thickening in a limited region, with associated uplift of the upper crust. These two styles suggest that the upper and lower crust were mechanically decoupled during Palaeozoic compressive episodes.     

Older crust underlies Iceland

The oldest rocks outcropping in northwest Iceland are ∼16 Myr old and in east Iceland ∼13 Myr. The full plate spreading rate in this region during the Cenozoic has been ∼2 cm a⁻¹, and thus these rocks are expected to be separated by ∼290 km. They are, however, ∼500 km apart. The conclusion is inescapable that an expanse of older crust ∼210 km wide underlies Iceland, submerged beneath younger lavas. This conclusion is independent of any considerations regarding spreading ridge migrations, jumps, the simultaneous existence of multiple active ridges, three-dimensionality, or subsidence of the lava pile. Such complexities bear on the distribution and age of the older crust, but not on its existence or its width. If it is entirely oceanic its maximum age is most likely 26–37 Ma. It is at least 150 km in north–south extent, but may taper and extend beneath south Iceland. Part of it might be continental—a southerly extension of the Jan Mayen microcontinent. This older crust contributes significantly to crustal thickness beneath Iceland and the ∼40 km local thickness measured seismically is thus probably an overestimate of present-day steady-state crustal production at Iceland.     

Some Unusual Ground-Water Phenomena in Iceland

The general mode of occurrence of ground water in Iceland is much like that encountered in the Hawaiian Islands, fully described by Meinzer¹, Stearns and Clark, and Stearns and Vaksvik. But in Iceland the active volcanism has induced some peculiar changes in the hydrology that are well worth special mentioning.