A DEEP SEISMIC-REFLECTION PROFILE ACROSS THE EXTINCT MID-LABRADOR SEASPREADING CENTER

In 1990 a deep multichannel seismic reflection line was shot along a f low line across the Labrador Sea. The results from the central portion of this line between magnetic anomalies 25 and across the extinct cen tral ridge are described here. Spreading rates in this part of the Lab rador Sea are very low, from 10 mm/yr to less than 3 mm/yr, so that th e line provides a unique opportunity to examine the relationship betwe en very slow spreading and crustal structure. A clear division is obse rved between two types of crust. The oldest crust, between chrons 21 a nd 25, which formed at a mean half spreading rate of 10 mm/yr, exhibit s smoothly undulating basement with only minor normal faulting. This r egion shares many of the reflection characteristics of North Atlantic crust formed at moderate to low spreading rates. In contrast, the youn ger central region, between chrons 21 and 13, which formed at a mean h alf spreading rate of 3 mm/yr, displays evidence of intense normal fau lting of the crust, giving a total extension of about 70%. Inward faci ng normal faults on both sides of the extinct ridge, with large offset s, many of which extend to lower crust or Moho depths, dominate the se ismic section. The axial region is characterized by a deep, fault-boun ded, median valley. These results suggest that mechanical extension pl ays a more important role in seafloor spreading at low spreading rates than previously documented. Integration of the reflection data with p revious refraction measurements and with gravity modeling of the regio n shows variations in crustal thickness which can be correlated with s preading rates. The region formed at a mean spreading rate of 10 mm/yr , where about 15% extension is observed, exhibits slightly thinner tha n normal crust (4.8 km or less versus a normal thickness of about 7 km ). At a lower spreading rate of 3 mm/yr across the axial region where extension is about 70%, an average crustal thickness of 3 km is obtain ed. Thus lower spreading rates are associated with regions of thinner crust and greater amounts of extension. While many studies suggest tha t thin crust at slow spreading rates may result from a reduced magma s upply, this study suggests that extension is at least equally importan t and may be responsible for most of the variations in crustal thickne ss. The increased cooling of young oceanic lithosphere formed at these very low spreading rates (approximately 3 mm/yr) may have amplified b rittle failure in response to plate separation. However, the timing of extension is still uncertain and some of it may be related to postext inction tectonics and not to the spreading process. The role of extens ion as a control on crustal thickness needs to be considered further i n studies of crustal generation and magmatism at slow spreading ridges . Extension will decrease the importance of magmatism in generating th in oceanic crust and will favor models of magmatic processes which pro duce thicker crust.