This looks like sunburn or other environmental stress on the basil. Were these plants put in the ground in the past two weeks or so? Usually when we plant basil that’s fresh from either our indoor growing areas or a local garden center the plants haven’t seen “the real world” yet and often become sun/wind/cold burned. Usually this only happens on the older foliage and the plants grow out of it. Occasionally you see the same browning of leaves if a fertilizer solution hits the foliage, or if hot water from a sun-heated hose splashes on the plants so be careful about both of those. These plants should be fine as they continue to grow provided you don’t have the dreaded “downy mildew of basil” disease in your area. Give them a mild, organic liquid feed such as a seaweed/fish emulsion. They probably won’t take off with growth until it gets warmer, however. Basil often sulks until the NIGHT TIME temperatures are reliably above 55 degrees and the days much, much warmer.
You might want to pull some of this mulch away from the stems of the plants – most plants don’t do so well with mulch right up against their stems – they like some “breathing room.”